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rfc:rfc4695

Network Working Group J. Lazzaro Request for Comments: 4695 J. Wawrzynek Category: Standards Track UC Berkeley

                                                         November 2006
                    RTP Payload Format for MIDI

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2006).

Abstract

 This memo describes a Real-time Transport Protocol (RTP) payload
 format for the MIDI (Musical Instrument Digital Interface) command
 language.  The format encodes all commands that may legally appear on
 a MIDI 1.0 DIN cable.  The format is suitable for interactive
 applications (such as network musical performance) and content-
 delivery applications (such as file streaming).  The format may be
 used over unicast and multicast UDP and TCP, and it defines tools for
 graceful recovery from packet loss.  Stream behavior, including the
 MIDI rendering method, may be customized during session setup.  The
 format also serves as a mode for the mpeg4-generic format, to support
 the MPEG 4 Audio Object Types for General MIDI, Downloadable Sounds
 Level 2, and Structured Audio.

Table of Contents

 1. Introduction ....................................................4
    1.1. Terminology ................................................5
    1.2. Bitfield Conventions .......................................6
 2. Packet Format ...................................................6
    2.1. RTP Header .................................................7
    2.2. MIDI Payload ..............................................11
 3. MIDI Command Section ...........................................12
    3.1.  Timestamps ...............................................14
    3.2.  Command Coding ...........................................16

Lazzaro & Wawrzynek Standards Track [Page 1] RFC 4695 RTP Payload Format for MIDI November 2006

 4. The Recovery Journal System ....................................22
 5. Recovery Journal Format ........................................24
 6. Session Description Protocol ...................................28
    6.1. Session Descriptions for Native Streams ...................29
    6.2. Session Descriptions for mpeg4-generic Streams ............30
    6.3. Parameters ................................................33
 7. Extensibility ..................................................34
 8. Congestion Control .............................................35
 9. Security Considerations ........................................35
 10. Acknowledgements ..............................................36
 11. IANA Considerations ...........................................37
    11.1. rtp-midi Media Type Registration .........................37
         11.1.1. Repository Request for "audio/rtp-midi" ...........40
    11.2. mpeg4-generic Media Type Registration ....................41
         11.2.1. Repository Request for Mode rtp-midi for
                 mpeg4-generic .....................................44
    11.3. asc Media Type Registration ..............................46
 A. The Recovery Journal Channel Chapters ..........................48
    A.1. Recovery Journal Definitions ..............................48
    A.2. Chapter P: MIDI Program Change ............................52
    A.3. Chapter C: MIDI Control Change ............................53
         A.3.1. Log Inclusion Rules ................................54
         A.3.2. Controller Log Format ..............................55
         A.3.3. Log List Coding Rules ..............................57
         A.3.4. The Parameter System ...............................60
    A.4. Chapter M: MIDI Parameter System ..........................62
         A.4.1. Log Inclusion Rules ................................64
         A.4.2. Log Coding Rules ...................................65
               A.4.2.1. The Value Tool .............................67
               A.4.2.2. The Count Tool .............................70
    A.5. Chapter W: MIDI Pitch Wheel ...............................71
    A.6. Chapter N: MIDI NoteOff and NoteOn ........................71
         A.6.1. Header Structure ...................................73
         A.6.2. Note Structures ....................................74
    A.7. Chapter E: MIDI Note Command Extras .......................75
         A.7.1. Note Log Format ....................................76
         A.7.2. Log Inclusion Rules ................................76
    A.8. Chapter T: MIDI Channel Aftertouch ........................77
    A.9. Chapter A: MIDI Poly Aftertouch ...........................78
 B. The Recovery Journal System Chapters ...........................79
    B.1. System Chapter D: Simple System Commands ..................79
         B.1.1. Undefined System Commands ..........................80
    B.2. System Chapter V: Active Sense Command ....................83
    B.3. System Chapter Q: Sequencer State Commands ................83
         B.3.1. Non-compliant Sequencers ...........................85
    B.4. System Chapter F: MIDI Time Code Tape Position ............86
         B.4.1. Partial Frames .....................................88

Lazzaro & Wawrzynek Standards Track [Page 2] RFC 4695 RTP Payload Format for MIDI November 2006

    B.5. System Chapter X: System Exclusive ........................89
         B.5.1. Chapter Format .....................................90
         B.5.2. Log Inclusion Semantics ............................92
         B.5.3. TCOUNT and COUNT Fields ............................95
 C. Session Configuration Tools ....................................95
    C.1. Configuration Tools: Stream Subsetting ....................97
    C.2. Configuration Tools: The Journalling System ..............101
         C.2.1. The j_sec Parameter ...............................102
         C.2.2. The j_update Parameter ............................103
               C.2.2.1. The anchor Sending Policy .................104
               C.2.2.2. The closed-loop Sending Policy ............104
               C.2.2.3. The open-loop Sending Policy ..............108
         C.2.3. Recovery Journal Chapter Inclusion Parameters .....110
    C.3. Configuration Tools: Timestamp Semantics .................115
         C.3.1. The comex Algorithm ...............................115
         C.3.2. The async Algorithm ...............................116
         C.3.3. The buffer Algorithm ..............................117
    C.4. Configuration Tools: Packet Timing Tools .................118
         C.4.1. Packet Duration Tools .............................119
         C.4.2. The guardtime Parameter ...........................120
    C.5. Configuration Tools: Stream Description ..................121
    C.6. Configuration Tools: MIDI Rendering ......................128
         C.6.1. The multimode Parameter ...........................129
         C.6.2. Renderer Specification ............................129
         C.6.3. Renderer Initialization ...........................131
         C.6.4. MIDI Channel Mapping ..............................133
               C.6.4.1. The smf_info Parameter ....................134
               C.6.4.2. The smf_inline, smf_url, and smf_cid
                        Parameters ................................136
               C.6.4.3. The chanmask Parameter ....................136
         C.6.5. The audio/asc Media Type ..........................137
    C.7. Interoperability .........................................139
         C.7.1. MIDI Content Streaming Applications ...............139
         C.7.2. MIDI Network Musical Performance Applications .....142
 D. Parameter Syntax Definitions ..................................150
 E. A MIDI Overview for Networking Specialists ....................156
    E.1. Commands Types ...........................................159
    E.2. Running Status ...........................................159
    E.3. Command Timing ...........................................160
    E.4. AudioSpecificConfig Templates for MMA Renderers ..........160
 References .......................................................165
 Normative References .............................................165
 Informative References ...........................................166

Lazzaro & Wawrzynek Standards Track [Page 3] RFC 4695 RTP Payload Format for MIDI November 2006

1. Introduction

 The Internet Engineering Task Force (IETF) has developed a set of
 focused tools for multimedia networking ([RFC3550] [RFC4566]
 [RFC3261] [RFC2326]).  These tools can be combined in different ways
 to support a variety of real-time applications over Internet Protocol
 (IP) networks.
 For example, a telephony application might use the Session Initiation
 Protocol (SIP, [RFC3261]) to set up a phone call.  Call setup would
 include negotiations to agree on a common audio codec [RFC3264].
 Negotiations would use the Session Description Protocol (SDP,
 [RFC4566]) to describe candidate codecs.
 After a call is set up, audio data would flow between the parties
 using the Real Time Protocol (RTP, [RFC3550]) under any applicable
 profile (for example, the Audio/Visual Profile (AVP, [RFC3551])).
 The tools used in this telephony example (SIP, SDP, RTP) might be
 combined in a different way to support a content streaming
 application, perhaps in conjunction with other tools, such as the
 Real Time Streaming Protocol (RTSP, [RFC2326]).
 The MIDI (Musical Instrument Digital Interface) command language
 [MIDI] is widely used in musical applications that are analogous to
 the examples described above.  On stage and in the recording studio,
 MIDI is used for the interactive remote control of musical
 instruments, an application similar in spirit to telephony.  On web
 pages, Standard MIDI Files (SMFs, [MIDI]) rendered using the General
 MIDI standard [MIDI] provide a low-bandwidth substitute for audio
 streaming.
 This memo is motivated by a simple premise: if MIDI performances
 could be sent as RTP streams that are managed by IETF session tools,
 a hybridization of the MIDI and IETF application domains may occur.
 For example, interoperable MIDI networking may foster network music
 performance applications, in which a group of musicians, located at
 different physical locations, interact over a network to perform as
 they would if they were located in the same room [NMP].  As a second
 example, the streaming community may begin to use MIDI for low-
 bitrate audio coding, perhaps in conjunction with normative sound
 synthesis methods [MPEGSA].
 To enable MIDI applications to use RTP, this memo defines an RTP
 payload format and its media type.  Sections 2-5 and Appendices A-B
 define the RTP payload format.  Section 6 and Appendices C-D define
 the media types identifying the payload format, the parameters needed
 for configuration, and how the parameters are utilized in SDP.

Lazzaro & Wawrzynek Standards Track [Page 4] RFC 4695 RTP Payload Format for MIDI November 2006

 Appendix C also includes interoperability guidelines for the example
 applications described above: network musical performance using SIP
 (Appendix C.7.2) and content-streaming using RTSP (Appendix C.7.1).
 Another potential application area for RTP MIDI is MIDI networking
 for professional audio equipment and electronic musical instruments.
 We do not offer interoperability guidelines for this application in
 this memo.  However, RTP MIDI has been designed with stage and studio
 applications in mind, and we expect that efforts to define a stage
 and studio framework will rely on RTP MIDI for MIDI transport
 services.
 Some applications may require MIDI media delivery at a certain
 service quality level (latency, jitter, packet loss, etc).  RTP
 itself does not provide service guarantees.  However, applications
 may use lower-layer network protocols to configure the quality of the
 transport services that RTP uses.  These protocols may act to reserve
 network resources for RTP flows [RFC2205] or may simply direct RTP
 traffic onto a dedicated "media network" in a local installation.
 Note that RTP and the MIDI payload format do provide tools that
 applications may use to achieve the best possible real-time
 performance at a given service level.
 This memo normatively defines the syntax and semantics of the MIDI
 payload format.  However, this memo does not define algorithms for
 sending and receiving packets.  An ancillary document [RFC4696]
 provides informative guidance on algorithms.  Supplemental
 information may be found in related conference publications [NMP]
 [GRAME].
 Throughout this memo, the phrase "native stream" refers to a stream
 that uses the rtp-midi media type.  The phrase "mpeg4-generic stream"
 refers to a stream that uses the mpeg4-generic media type (in mode
 rtp-midi) to operate in an MPEG 4 environment [RFC3640].  Section 6
 describes this distinction in detail.

1.1. Terminology

 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
 [RFC2119].

Lazzaro & Wawrzynek Standards Track [Page 5] RFC 4695 RTP Payload Format for MIDI November 2006

1.2. Bitfield Conventions

 In this document, the packet bitfields that share a common name often
 have identical semantics.  As most of these bitfields appear in
 Appendices A-B, we define the common bitfield names in Appendix A.1.
 However, a few of these common names also appear in the main text of
 this document.  For convenience, we list these definitions below:
   o R flag bit.  R flag bits are reserved for future use.  Senders
     MUST set R bits to 0.  Receivers MUST ignore R bit values.
   o LENGTH field.  All fields named LENGTH (as distinct from LEN)
     code the number of octets in the structure that contains it,
     including the header it resides in and all hierarchical levels
     below it.  If a structure contains a LENGTH field, a receiver
     MUST use the LENGTH field value to advance past the structure
     during parsing, rather than use knowledge about the internal
     format of the structure.

2. Packet Format

 In this section, we introduce the format of RTP MIDI packets.  The
 description includes some background information on RTP, for the
 benefit of MIDI implementors new to IETF tools.  Implementors should
 consult [RFC3550] for an authoritative description of RTP.
 This memo assumes that the reader is familiar with MIDI syntax and
 semantics.  Appendix E provides a MIDI overview, at a level of detail
 sufficient to understand most of this memo.  Implementors should
 consult [MIDI] for an authoritative description of MIDI.
 The MIDI payload format maps a MIDI command stream (16 voice channels
 + systems) onto an RTP stream.  An RTP media stream is a sequence of
 logical packets that share a common format.  Each packet consists of
 two parts: the RTP header and the MIDI payload.  Figure 1 shows this
 format (vertical space delineates the header and payload).
 We describe RTP packets as "logical" packets to highlight the fact
 that RTP itself is not a network-layer protocol.  Instead, RTP
 packets are mapped onto network protocols (such as unicast UDP,
 multicast UDP, or TCP) by an application [ALF].  The interleaved mode
 of the Real Time Streaming Protocol (RTSP, [RFC2326]) is an example
 of an RTP mapping to TCP transport, as is [RFC4571].

Lazzaro & Wawrzynek Standards Track [Page 6] RFC 4695 RTP Payload Format for MIDI November 2006

2.1. RTP Header

 [RFC3550] provides a complete description of the RTP header fields.
 In this section, we clarify the role of a few RTP header fields for
 MIDI applications.  All fields are coded in network byte order (big-
 endian).
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | V |P|X|  CC   |M|     PT      |        Sequence number        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Timestamp                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             SSRC                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     MIDI command section ...                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Journal section ...                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 1 -- Packet format
 The behavior of the 1-bit M field depends on the media type of the
 stream.  For native streams, the M bit MUST be set to 1 if the MIDI
 command section has a non-zero LEN field, and MUST be set to 0
 otherwise.  For mpeg4-generic streams, the M bit MUST be set to 1 for
 all packets (to conform to [RFC3640]).
 In an RTP MIDI stream, the 16-bit sequence number field is
 initialized to a randomly chosen value and is incremented by one
 (modulo 2^16) for each packet sent in the stream.  A related
 quantity, the 32-bit extended packet sequence number, may be computed
 by tracking rollovers of the 16-bit sequence number.  Note that
 different receivers of the same stream may compute different extended
 packet sequence numbers, depending on when the receiver joined the
 session.
 The 32-bit timestamp field sets the base timestamp value for the
 packet.  The payload codes MIDI command timing relative to this
 value.  The timestamp units are set by the clock rate parameter.  For
 example, if the clock rate has a value of 44100 Hz, two packets whose
 base timestamp values differ by 2 seconds have RTP timestamp fields
 that differ by 88200.

Lazzaro & Wawrzynek Standards Track [Page 7] RFC 4695 RTP Payload Format for MIDI November 2006

 Note that the clock rate parameter is not encoded within each RTP
 MIDI packet.  A receiver of an RTP MIDI stream becomes aware of the
 clock rate as part of the session setup process.  For example, if a
 session management tool uses the Session Description Protocol (SDP,
 [RFC4566]) to describe a media session, the clock rate parameter is
 set using the rtpmap attribute.  We show examples of session setup in
 Section 6.
 For RTP MIDI streams destined to be rendered into audio, the clock
 rate SHOULD be an audio sample rate of 32 KHz or higher.  This
 recommendation is due to the sensitivity of human musical perception
 to small timing errors in musical note sequences, and due to the
 timbral changes that occur when two near-simultaneous MIDI NoteOns
 are rendered with a different timing than that desired by the content
 author due to clock rate quantization.  RTP MIDI streams that are not
 destined for audio rendering (such as MIDI streams that control stage
 lighting) MAY use a lower clock rate but SHOULD use a clock rate high
 enough to avoid timing artifacts in the application.
 For RTP MIDI streams destined to be rendered into audio, the clock
 rate SHOULD be chosen from rates in common use in professional audio
 applications or in consumer audio distribution.  At the time of this
 writing, these rates include 32 KHz, 44.1 KHz, 48 KHz, 64 KHz, 88.2
 KHz, 96 KHz, 176.4 KHz, and 192 KHz.  If the RTP MIDI session is a
 part of a synchronized media session that includes another (non-MIDI)
 RTP audio stream with a clock rate of 32 KHz or higher, the RTP MIDI
 stream SHOULD use a clock rate that matches the clock rate of the
 other audio stream.  However, if the RTP MIDI stream is destined to
 be rendered into audio, the RTP MIDI stream SHOULD NOT use a clock
 rate lower than 32 KHz, even if this second stream has a clock rate
 less than 32 KHz.
 Timestamps of consecutive packets do not necessarily increment at a
 fixed rate, because RTP MIDI packets are not necessarily sent at a
 fixed rate.  The degree of packet transmission regularity reflects
 the underlying application dynamics.  Interactive applications may
 vary the packet sending rate to track the gestural rate of a human
 performer, whereas content-streaming applications may send packets at
 a fixed rate.
 Therefore, the timestamps for two sequential RTP packets may be
 identical, or the second packet may have a timestamp arbitrarily
 larger than the first packet (modulo 2^32).  Section 3 places
 additional restrictions on the RTP timestamps for two sequential RTP
 packets, as does the guardtime parameter (Appendix C.4.2).
 We use the term "media time" to denote the temporal duration of the
 media coded by an RTP packet.  The media time coded by a packet is

Lazzaro & Wawrzynek Standards Track [Page 8] RFC 4695 RTP Payload Format for MIDI November 2006

 computed by subtracting the last command timestamp in the MIDI
 command section from the RTP timestamp (modulo 2^32).  If the MIDI
 list of the MIDI command section of a packet is empty, the media time
 coded by the packet is 0 ms.  Appendix C.4.1 discusses media time
 issues in detail.
 We now define RTP session semantics, in the context of sessions
 specified using the session description protocol [RFC4566].  A
 session description media line ("m=") specifies an RTP session.  An
 RTP session has an independent space of 2^32 synchronization sources.
 Synchronization source identifiers are coded in the SSRC header field
 of RTP session packets.  The payload types that may appear in the PT
 header field of RTP session packets are listed at the end of the
 media line.
 Several RTP MIDI streams may appear in an RTP session.  Each stream
 is distinguished by a unique SSRC value and has a unique sequence
 number and RTP timestamp space.  Multiple streams in the RTP session
 may be sent by a single party.  Multiple parties may send streams in
 the RTP session.  An RTP MIDI stream encodes data for a single MIDI
 command name space (16 voice channels + Systems).
 Streams in an RTP session may use different payload types, or they
 may use the same payload type.  However, each party may send, at
 most, one RTP MIDI stream for each payload type mapped to an RTP MIDI
 payload format in an RTP session.  Recall that dynamic binding of
 payload type numbers in [RFC4566] lets a party map many payload type
 numbers to the RTP MIDI payload format; thus a party may send many
 RTP MIDI streams in a single RTP session.  Pairs of streams (unicast
 or multicast) that communicate between two parties in an RTP session
 and that share a payload type have the same association as a MIDI
 cable pair that cross-connects two devices in a MIDI 1.0 DIN network.
 The RTP session architecture described above is efficient in its use
 of network ports, as one RTP session (using a port pair per party)
 supports the transport of many MIDI name spaces (16 MIDI channels +
 systems).  We define tools for grouping and labelling MIDI name
 spaces across streams and sessions in Appendix C.5 of this memo.
 The RTP header timestamps for each stream in an RTP session have
 separately and randomly chosen initialization values.  Receivers use
 the timing fields encoded in the RTP control protocol (RTCP,
 [RFC3550]) sender reports to synchronize the streams sent by a party.
 The SSRC values for each stream in an RTP session are also separately
 and randomly chosen, as described in [RFC3550].  Receivers use the
 CNAME field encoded in RTCP sender reports to verify that streams
 were sent by the same party, and to detect SSRC collisions, as
 described in [RFC3550].

Lazzaro & Wawrzynek Standards Track [Page 9] RFC 4695 RTP Payload Format for MIDI November 2006

 In some applications, a receiver renders MIDI commands into audio (or
 into control actions, such as the rewind of a tape deck or the
 dimming of stage lights).  In other applications, a receiver presents
 a MIDI stream to software programs via an Application Programmer
 Interface (API).  Appendix C.6 defines session configuration tools to
 specify what receivers should do with a MIDI command stream.
 If a multimedia session uses different RTP MIDI streams to send
 different classes of media, the streams MUST be sent over different
 RTP sessions.  For example, if a multimedia session uses one MIDI
 stream for audio and a second MIDI stream to control a lighting
 system, the audio and lighting streams MUST be sent over different
 RTP sessions, each with its own media line.
 Session description tools defined in Appendix C.5 let a sending party
 split a single MIDI name space (16 voice channels + systems) over
 several RTP MIDI streams.  Split transport of a MIDI command stream
 is a delicate task, because correct command stream reconstruction by
 a receiver depends on exact timing synchronization across the
 streams.
 To support split name spaces, we define the following requirements:
   o  A party MUST NOT send several RTP MIDI streams that share a MIDI
      name space in the same RTP session.  Instead, each stream MUST
      be sent from a different RTP session.
   o  If several RTP MIDI streams sent by a party share a MIDI name
      space, all streams MUST use the same SSRC value and MUST use the
      same randomly chosen RTP timestamp initialization value.
 These rules let a receiver identify streams that share a MIDI name
 space (by matching SSRC values) and also let a receiver accurately
 reconstruct the source MIDI command stream (by using RTP timestamps
 to interleave commands from the two streams).  Care MUST be taken by
 senders to ensure that SSRC changes due to collisions are reflected
 in both streams.  Receivers MUST regularly examine the RTCP CNAME
 fields associated with the linked streams, to ensure that the assumed
 link is legitimate and not the result of an SSRC collision by another
 sender.
 Except for the special cases described above, a party may send many
 RTP MIDI streams in the same session.  However, it is sometimes
 advantageous for two RTP MIDI streams to be sent over different RTP
 sessions.  For example, two streams may need different values for RTP
 session-level attributes (such as the sendonly and recvonly
 attributes).  As a second example, two RTP sessions may be needed to
 send two unicast streams in a multimedia session that originate on

Lazzaro & Wawrzynek Standards Track [Page 10] RFC 4695 RTP Payload Format for MIDI November 2006

 different computers (with different IP numbers).  Two RTP sessions
 are needed in this case because transport addresses are specified on
 the RTP-session or multimedia-session level, not on a payload type
 level.
 On a final note, in some uses of MIDI, parties send bidirectional
 traffic to conduct transactions (such as file exchange).  These
 commands were designed to work over MIDI 1.0 DIN cable networks may
 be configured in a multicast topology, which use pure "party-line"
 signalling.  Thus, if a multimedia session ensures a multicast
 connection between all parties, bidirectional MIDI commands will work
 without additional support from the RTP MIDI payload format.

2.2. MIDI Payload

 The payload (Figure 1) MUST begin with the MIDI command section.  The
 MIDI command section codes a (possibly empty) list of timestamped
 MIDI commands, and provides the essential service of the payload
 format.
 The payload MAY also contain a journal section.  The journal section
 provides resiliency by coding the recent history of the stream.  A
 flag in the MIDI command section codes the presence of a journal
 section in the payload.
 Section 3 defines the MIDI command section.  Sections 4-5 and
 Appendices A-B define the recovery journal, the default format for
 the journal section.  Here, we describe how these payload sections
 operate in a stream in an RTP session.
 The journalling method for a stream is set at the start of a session
 and MUST NOT be changed thereafter.  A stream may be set to use the
 recovery journal, to use an alternative journal format (none are
 defined in this memo), or not to use a journal.
 The default journalling method of a stream is inferred from its
 transport type.  Streams that use unreliable transport (such as UDP)
 default to using the recovery journal.  Streams that use reliable
 transport (such as TCP) default to not using a journal.  Appendix
 C.2.1 defines session configuration tools for overriding these
 defaults.  For all types of transport, a sender MUST transmit an RTP
 packet stream with consecutive sequence numbers (modulo 2^16).
 If a stream uses the recovery journal, every payload in the stream
 MUST include a journal section.  If a stream does not use
 journalling, a journal section MUST NOT appear in a stream payload.
 If a stream uses an alternative journal format, the specification for
 the journal format defines an inclusion policy.

Lazzaro & Wawrzynek Standards Track [Page 11] RFC 4695 RTP Payload Format for MIDI November 2006

 If a stream is sent over UDP transport, the Maximum Transmission Unit
 (MTU) of the underlying network limits the practical size of the
 payload section (for example, an Ethernet MTU is 1500 octets), for
 applications where predictable and minimal packet transmission
 latency is critical.  A sender SHOULD NOT create RTP MIDI UDP packets
 whose size exceeds the MTU of the underlying network.  Instead, the
 sender SHOULD take steps to keep the maximum packet size under the
 MTU limit.
 These steps may take many forms.  The default closed-loop recovery
 journal sending policy (defined in Appendix C.2.2.2) uses RTP control
 protocol (RTCP, [RFC3550]) feedback to manage the RTP MIDI packet
 size.  In addition, Section 3.2 and Appendix B.5.2 provide specific
 tools for managing the size of packets that code MIDI System
 Exclusive (0xF0) commands.  Appendix C.5 defines session
 configuration tools that may be used to split a dense MIDI name space
 into several UDP streams (each sent in a different RTP session, per
 Section 2.1) so that the payload fits comfortably into an MTU.
 Another option is to use TCP.  Section 4.3 of [RFC4696] provides
 non-normative advice for packet size management.

3. MIDI Command Section

 Figure 2 shows the format of the MIDI command section.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |B|J|Z|P|LEN... |  MIDI list ...                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 2 -- MIDI command section
 The MIDI command section begins with a variable-length header.
 The header field LEN codes the number of octets in the MIDI list that
 follow the header.  If the header flag B is 0, the header is one
 octet long, and LEN is a 4-bit field, supporting a maximum MIDI list
 length of 15 octets.
 If B is 1, the header is two octets long, and LEN is a 12-bit field,
 supporting a maximum MIDI list length of 4095 octets.  LEN is coded
 in network byte order (big-endian): the 4 bits of LEN that appear in
 the first header octet code the most significant 4 bits of the 12-bit
 LEN value.
 A LEN value of 0 is legal, and it codes an empty MIDI list.

Lazzaro & Wawrzynek Standards Track [Page 12] RFC 4695 RTP Payload Format for MIDI November 2006

 If the J header bit is set to 1, a journal section MUST appear after
 the MIDI command section in the payload.  If the J header bit is set
 to 0, the payload MUST NOT contain a journal section.
 We define the semantics of the P header bit in Section 3.2.
 If the LEN header field is nonzero, the MIDI list has the structure
 shown in Figure 3.
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Delta Time 0     (1-4 octets long, or 0 octets if Z = 1)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  MIDI Command 0   (1 or more octets long)                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Delta Time 1     (1-4 octets long)                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  MIDI Command 1   (1 or more octets long)                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              ...                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Delta Time N     (1-4 octets long)                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  MIDI Command N   (0 or more octets long)                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 3 -- MIDI list structure
 If the header flag Z is 1, the MIDI list begins with a complete MIDI
 command (coded in the MIDI Command 0 field, in Figure 3) preceded by
 a delta time (coded in the Delta Time 0 field).  If Z is 0, the Delta
 Time 0 field is not present in the MIDI list, and the command coded
 in the MIDI Command 0 field has an implicit delta time of 0.
 The MIDI list structure may also optionally encode a list of N
 additional complete MIDI commands, each coded in a MIDI Command K
 field.  Each additional command MUST be preceded by a Delta Time K
 field, which codes the command's delta time.  We discuss exceptions
 to the "command fields code complete MIDI commands" rule in Section
 3.2.
 The final MIDI command field (i.e., the MIDI Command N field, shown
 in Figure 3) in the MIDI list MAY be empty.  Moreover, a MIDI list
 MAY consist a single delta time (encoded in the Delta Time 0 field)
 without an associated command (which would have been encoded in the
 MIDI Command 0 field).  These rules enable MIDI coding features that
 are explained in Section 3.1.  We delay the explanations because an
 understanding of RTP MIDI timestamps is necessary to describe the
 features.

Lazzaro & Wawrzynek Standards Track [Page 13] RFC 4695 RTP Payload Format for MIDI November 2006

3.1. Timestamps

 In this section, we describe how RTP MIDI encodes a timestamp for
 each MIDI list command.  Command timestamps have the same units as
 RTP packet header timestamps (described in Section 2.1 and
 [RFC3550]).  Recall that RTP timestamps have units of seconds, whose
 scaling is set during session configuration (see Section 6.1 and
 [RFC4566]).
 As shown in Figure 3, the MIDI list encodes time using a compact
 delta-time format.  The RTP MIDI delta time syntax is a modified form
 of the MIDI File delta time syntax [MIDI].  RTP MIDI delta times use
 1-4 octet fields to encode 32-bit unsigned integers.  Figure 4 shows
 the encoded and decoded forms of delta times.  Note that delta time
 values may be legally encoded in multiple formats; for example, there
 are four legal ways to encode the zero delta time (0x00, 0x8000,
 0x808000, 0x80808000).
 RTP MIDI uses delta times to encode a timestamp for each MIDI
 command.  The timestamp for MIDI Command K is the summation (modulo
 2^32) of the RTP timestamp and decoded delta times 0 through K.  This
 cumulative coding technique, borrowed from MIDI File delta time
 coding, is efficient because it reduces the number of multi-octet
 delta times.
 All command timestamps in a packet MUST be less than or equal to the
 RTP timestamp of the next packet in the stream (modulo 2^32).
 This restriction ensures that a particular RTP MIDI packet in a
 stream is uniquely responsible for encoding time starting at the
 moment after the RTP timestamp encoded in the RTP packet header, and
 ending at the moment before the final command timestamp encoded in
 the MIDI list.  The "moment before" and "moment after" qualifiers
 acknowledge the "less than or equal" semantics (as opposed to
 "strictly less than") in the sentence above this paragraph.
 Note that it is possible to "pad" the end of an RTP MIDI packet with
 time that is guaranteed to be void of MIDI commands, by setting the
 "Delta Time N" field of the MIDI list to the end of the void time,
 and by omitting its corresponding "MIDI Command N" field (a syntactic
 construction the preamble of Section 3 expressly made legal).
 In addition, it is possible to code an RTP MIDI packet to express
 that a period of time in the stream is void of MIDI commands.  The
 RTP timestamp in the header would code the start of the void time.
 The MIDI list of this packet would consist of a "Delta Time 0" field

Lazzaro & Wawrzynek Standards Track [Page 14] RFC 4695 RTP Payload Format for MIDI November 2006

 that coded the end of the void time.  No other fields would be
 present in the MIDI list (a syntactic construction the preamble of
 Section 3 also expressly made legal).
 By default, a command timestamp indicates the execution time for the
 command.  The difference between two timestamps indicates the time
 delay between the execution of the commands.  This difference may be
 zero, coding simultaneous execution.  In this memo, we refer to this
 interpretation of timestamps as "comex" (COMmand EXecution)
 semantics.  We formally define comex semantics in Appendix C.3.
 The comex interpretation of timestamps works well for transcoding a
 Standard MIDI File (SMF) into an RTP MIDI stream, as SMFs code a
 timestamp for each MIDI command stored in the file.  To transcode an
 SMF that uses metric time markers, use the SMF tempo map (encoded in
 the SMF as meta-events) to convert metric SMF timestamp units into
 seconds-based RTP timestamp units.
 The comex interpretation also works well for MIDI hardware
 controllers that are coding raw sensor data directly onto an RTP MIDI
 stream.  Note that this controller design is preferable to a design
 that converts raw sensor data into a MIDI 1.0 cable command stream
 and then transcodes the stream onto an RTP MIDI stream.
 The comex interpretation of timestamps is usually not the best
 timestamp interpretation for transcoding a MIDI source that uses
 implicit command timing (such as MIDI 1.0 DIN cables) into an RTP
 MIDI stream.  Appendix C.3 defines alternatives to comex semantics
 and describes session configuration tools for selecting the timestamp
 interpretation semantics for a stream.

Lazzaro & Wawrzynek Standards Track [Page 15] RFC 4695 RTP Payload Format for MIDI November 2006

      One-Octet Delta Time:
         Encoded form: 0ddddddd
         Decoded form: 00000000 00000000 00000000 0ddddddd
      Two-Octet Delta Time:
         Encoded form: 1ccccccc 0ddddddd
         Decoded form: 00000000 00000000 00cccccc cddddddd
      Three-Octet Delta Time:
         Encoded form: 1bbbbbbb 1ccccccc 0ddddddd
         Decoded form: 00000000 000bbbbb bbcccccc cddddddd
      Four-Octet Delta Time:
         Encoded form: 1aaaaaaa 1bbbbbbb 1ccccccc 0ddddddd
         Decoded form: 0000aaaa aaabbbbb bbcccccc cddddddd
                Figure 4 -- Decoding delta time formats

3.2. Command Coding

 Each non-empty MIDI Command field in the MIDI list codes one of the
 MIDI command types that may legally appear on a MIDI 1.0 DIN cable.
 Standard MIDI File meta-events do not fit this definition and MUST
 NOT appear in the MIDI list.  As a rule, each MIDI Command field
 codes a complete command, in the binary command format defined in
 [MIDI].  In the remainder of this section, we describe exceptions to
 this rule.
 The first MIDI channel command in the MIDI list MUST include a status
 octet.  Running status coding, as defined in [MIDI], MAY be used for
 all subsequent MIDI channel commands in the list.  As in [MIDI],
 System Common and System Exclusive messages (0xF0 ... 0xF7) cancel
 the running status state, but System Real-time messages (0xF8 ...
 0xFF) do not affect the running status state.  All System commands in
 the MIDI list MUST include a status octet.
 As we note above, the first channel command in the MIDI list MUST
 include a status octet.  However, the corresponding command in the
 original MIDI source data stream might not have a status octet (in
 this case, the source would be coding the command using running
 status).  If the status octet of the first channel command in the
 MIDI list does not appear in the source data stream, the P (phantom)
 header bit MUST be set to 1.  In all other cases, the P bit MUST be
 set to 0.

Lazzaro & Wawrzynek Standards Track [Page 16] RFC 4695 RTP Payload Format for MIDI November 2006

 Note that the P bit describes the MIDI source data stream, not the
 MIDI list encoding; regardless of the state of the P bit, the MIDI
 list MUST include the status octet.
 As receivers MUST be able to decode running status, sender
 implementors should feel free to use running status to improve
 bandwidth efficiency.  However, senders SHOULD NOT introduce timing
 jitter into an existing MIDI command stream through an inappropriate
 use or removal of running status coding.  This warning primarily
 applies to senders whose RTP MIDI streams may be transcoded onto a
 MIDI 1.0 DIN cable [MIDI] by the receiver: both the timestamps and
 the command coding (running status or not) must comply with the
 physical restrictions of implicit time coding over a slow serial
 line.
 On a MIDI 1.0 DIN cable [MIDI], a System Real-time command may be
 embedded inside of another "host" MIDI command.  This syntactic
 construction is not supported in the payload format: a MIDI Command
 field in the MIDI list codes exactly one MIDI command (partially or
 completely).
 To encode an embedded System Real-time command, senders MUST extract
 the command from its host and code it in the MIDI list as a separate
 command.  The host command and System Real-time command SHOULD appear
 in the same MIDI list.  The delta time of the System Real-time
 command SHOULD result in a command timestamp that encodes the System
 Real-time command placement in its original embedded position.
 Two methods are provided for encoding MIDI System Exclusive (SysEx)
 commands in the MIDI list.  A SysEx command may be encoded in a MIDI
 Command field verbatim: a 0xF0 octet, followed by an arbitrary number
 of data octets, followed by a 0xF7 octet.
 Alternatively, a SysEx command may be encoded as multiple segments.
 The command is divided into two or more SysEx command segments; each
 segment is encoded in its own MIDI Command field in the MIDI list.
 The payload format supports segmentation in order to encode SysEx
 commands that encode information in the temporal pattern of data
 octets.  By encoding these commands as a series of segments, each
 data octet may be associated with a distinct delta time.
 Segmentation also supports the coding of large SysEx commands across
 several packets.
 To segment a SysEx command, first partition its data octet list into
 two or more sublists.  The last sublist MAY be empty (i.e., contain
 no octets); all other sublists MUST contain at least one data octet.
 To complete the segmentation, add the status octets defined in Figure

Lazzaro & Wawrzynek Standards Track [Page 17] RFC 4695 RTP Payload Format for MIDI November 2006

 5 to the head and tail of the first, last, and any "middle" sublists.
 Figure 6 shows example segmentations of a SysEx command.
 A sender MAY cancel a segmented SysEx command transmission that is in
 progress, by sending the "cancel" sublist shown in Figure 5.  A
 "cancel" sublist MAY follow a "first" or "middle" sublist in the
 transmission, but MUST NOT follow a "last" sublist.  The cancel MUST
 be empty (thus, 0xF7 0xF4 is the only legal cancel sublist).
 The cancellation feature is needed because Appendix C.1 defines
 configuration tools that let session parties exclude certain SysEx
 commands in the stream.  Senders that transcode a MIDI source onto an
 RTP MIDI stream under these constraints have the responsibility of
 excluding undesired commands from the RTP MIDI stream.
 The cancellation feature lets a sender start the transmission of a
 command before the MIDI source has sent the entire command.  If a
 sender determines that the command whose transmission is in progress
 should not appear on the RTP stream, it cancels the command.  Without
 a method for cancelling a SysEx command transmission, senders would
 be forced to use a high-latency store-and-forward approach to
 transcoding SysEx commands onto RTP MIDI packets, in order to
 validate each SysEx command before transmission.
 The recommended receiver reaction to a cancellation depends on the
 capabilities of the receiver.  For example, a sound synthesizer that
 is directly parsing RTP MIDI packets and rendering them to audio will
 be aware of the fact that SysEx commands may be cancelled in RTP
 MIDI.  These receivers SHOULD detect a SysEx cancellation in the MIDI
 list and act as if they had never received the SysEx command.
 As a second example, a synthesizer may be receiving MIDI data from an
 RTP MIDI stream via a MIDI DIN cable (or a software API emulation of
 a MIDI DIN cable).  In this case, an RTP-MIDI-aware system receives
 the RTP MIDI stream and transcodes it onto the MIDI DIN cable (or its
 emulation).  Upon the receipt of the cancel sublist, the RTP-MIDI-
 aware transcoder might have already sent the first part of the SysEx
 command on the MIDI DIN cable to the receiver.
 Unfortunately, the MIDI DIN cable protocol cannot directly code
 "cancel SysEx in progress" semantics.  However, MIDI DIN cable
 receivers begin SysEx processing after the complete command arrives.
 The receiver checks to see if it recognizes the command (coded in the
 first few octets) and then checks to see if the command is the
 correct length.  Thus, in practice, a transcoder can cancel a SysEx
 command by sending an 0xF7 to (prematurely) end the SysEx command --
 the receiver will detect the incorrect command length and discard the
 command.

Lazzaro & Wawrzynek Standards Track [Page 18] RFC 4695 RTP Payload Format for MIDI November 2006

 Appendix C.1 defines configuration tools that may be used to prohibit
 SysEx command cancellation.
 The relative ordering of SysEx command segments in a MIDI list must
 match the relative ordering of the sublists in the original SysEx
 command.  By default, commands other than System Real-time MIDI
 commands MUST NOT appear between SysEx command segments (Appendix C.1
 defines configuration tools to change this default, to let other
 commands types appear between segments).  If the command segments of
 a SysEx command are placed in the MIDI lists of two or more RTP
 packets, the segment ordering rules apply to the concatenation of all
 affected MIDI lists.
  1. ———————————————————-

| Sublist Position | Head Status Octet | Tail Status Octet |

       |-----------------------------------------------------------|
       |    first         |       0xF0         |       0xF0        |
       |-----------------------------------------------------------|
       |    middle        |       0xF7         |       0xF0        |
       |-----------------------------------------------------------|
       |    last          |       0xF7         |       0xF7        |
       |-----------------------------------------------------------|
       |    cancel        |       0xF7         |       0xF4        |
        -----------------------------------------------------------
             Figure 5 -- Command segmentation status octets
 [MIDI] permits 0xF7 octets that are not part of a (0xF0, 0xF7) pair
 to appear on a MIDI 1.0 DIN cable.  Unpaired 0xF7 octets have no
 semantic meaning in MIDI, apart from cancelling running status.
 Unpaired 0xF7 octets MUST NOT appear in the MIDI list of the MIDI
 Command section.  We impose this restriction to avoid interference
 with the command segmentation coding defined in Figure 5.
 SysEx commands carried on a MIDI 1.0 DIN cable may use the "dropped
 0xF7" construction [MIDI].  In this coding method, the 0xF7 octet is
 dropped from the end of the SysEx command, and the status octet of
 the next MIDI command acts both to terminate the SysEx command and
 start the next command.  To encode this construction in the payload
 format, follow these steps:
   o  Determine the appropriate delta times for the SysEx command and
      the command that follows the SysEx command.
   o  Insert the "dropped" 0xF7 octet at the end of the SysEx command,
      to form the standard SysEx syntax.

Lazzaro & Wawrzynek Standards Track [Page 19] RFC 4695 RTP Payload Format for MIDI November 2006

   o  Code both commands into the MIDI list using the rules above.
   o  Replace the 0xF7 octet that terminates the verbatim SysEx
      encoding or the last segment of the segmented SysEx encoding
      with a 0xF5 octet.  This substitution informs the receiver of
      the original dropped 0xF7 coding.
 [MIDI] reserves the undefined System Common commands 0xF4 and 0xF5
 and the undefined System Real-time commands 0xF9 and 0xFD for future
 use.  By default, undefined commands MUST NOT appear in a MIDI
 Command field in the MIDI list, with the exception of the 0xF5 octets
 used to code the "dropped 0xF7" construction and the 0xF4 octets used
 by SysEx "cancel" sublists.
 During session configuration, a stream may be customized to transport
 undefined commands (Appendix C.1).  For this case, we now define how
 senders encode undefined commands in the MIDI list.
 An undefined System Real-time command MUST be coded using the System
 Real-time rules.
 If the undefined System Common commands are put to use in a future
 version of [MIDI], the command will begin with an 0xF4 or 0xF5 status
 octet, followed by an arbitrary number of data octets (i.e., zero or
 more data bytes).  To encode these commands, senders MUST terminate
 the command with an 0xF7 octet and place the modified command into
 the MIDI Command field.
 Unfortunately, non-compliant uses of the undefined System Common
 commands may appear in MIDI implementations.  To model these
 commands, we assume that the command begins with an 0xF4 or 0xF5
 status octet, followed by zero or more data octets, followed by zero
 or more trailing 0xF7 status octets.  To encode the command, senders
 MUST first remove all trailing 0xF7 status octets from the command.
 Then, senders MUST terminate the command with an 0xF7 octet and place
 the modified command into the MIDI Command field.
 Note that we include the trailing octets in our model as a cautionary
 measure: if such commands appeared in a non-compliant use of an
 undefined System Common command, an RTP MIDI encoding of the command
 that did not remove trailing octets could be mistaken for an encoding
 of "middle" or "last" sublist of a segmented SysEx commands (Figure
 5) under certain packet loss conditions.

Lazzaro & Wawrzynek Standards Track [Page 20] RFC 4695 RTP Payload Format for MIDI November 2006

        Original SysEx command:
            0xF0 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0xF7
        A two-segment segmentation:
            0xF0 0x01 0x02 0x03 0x04 0xF0
            0xF7 0x05 0x06 0x07 0x08 0xF7
        A different two-segment segmentation:
            0xF0 0x01 0xF0
            0xF7 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0xF7
        A three-segment segmentation:
            0xF0 0x01 0x02 0xF0
            0xF7 0x03 0x04 0xF0
            0xF7 0x05 0x06 0x07 0x08 0xF7
       The segmentation with the largest number of segments:
            0xF0 0x01 0xF0
            0xF7 0x02 0xF0
            0xF7 0x03 0xF0
            0xF7 0x04 0xF0
            0xF7 0x05 0xF0
            0xF7 0x06 0xF0
            0xF7 0x07 0xF0
            0xF7 0x08 0xF0
            0xF7 0xF7
                   Figure 6 -- Example segmentations

Lazzaro & Wawrzynek Standards Track [Page 21] RFC 4695 RTP Payload Format for MIDI November 2006

4. The Recovery Journal System

 The recovery journal is the default resiliency tool for unreliable
 transport.  In this section, we normatively define the roles that
 senders and receivers play in the recovery journal system.
 MIDI is a fragile code.  A single lost command in a MIDI command
 stream may produce an artifact in the rendered performance.  We
 normatively classify rendering artifacts into two categories:
   o Transient artifacts.  Transient artifacts produce immediate but
     short-term glitches in the performance.  For example, a lost
     NoteOn (0x9) command produces a transient artifact: one note
     fails to play, but the artifact does not extend beyond the end of
     that note.
   o Indefinite artifacts.  Indefinite artifacts produce long-lasting
     errors in the rendered performance.  For example, a lost NoteOff
     (0x8) command may produce an indefinite artifact: the note that
     should have been ended by the lost NoteOff command may sustain
     indefinitely.  As a second example, the loss of a Control Change
     (0xB) command for controller number 7 (Channel Volume) may
     produce an indefinite artifact: after the loss, all notes on the
     channel may play too softly or too loudly.
 The purpose of the recovery journal system is to satisfy the recovery
 journal mandate: the MIDI performance rendered from an RTP MIDI
 stream sent over unreliable transport MUST NOT contain indefinite
 artifacts.
 The recovery journal system does not use packet retransmission to
 satisfy this mandate.  Instead, each packet includes a special
 section, called the recovery journal.
 The recovery journal codes the history of the stream, back to an
 earlier packet called the checkpoint packet.  The range of coverage
 for the journal is called the checkpoint history.  The recovery
 journal codes the information necessary to recover from the loss of
 an arbitrary number of packets in the checkpoint history.  Appendix
 A.1 normatively defines the checkpoint packet and the checkpoint
 history.
 When a receiver detects a packet loss, it compares its own knowledge
 about the history of the stream with the history information coded in
 the recovery journal of the packet that ends the loss event.  By
 noting the differences in these two versions of the past, a receiver
 is able to transform all indefinite artifacts in the rendered

Lazzaro & Wawrzynek Standards Track [Page 22] RFC 4695 RTP Payload Format for MIDI November 2006

 performance into transient artifacts, by executing MIDI commands to
 repair the stream.
 We now state the normative role for senders in the recovery journal
 system.
 Senders prepare a recovery journal for every packet in the stream.
 In doing so, senders choose the checkpoint packet identity for the
 journal.  Senders make this choice by applying a sending policy.
 Appendix C.2.2 normatively defines three sending policies: "closed-
 loop", "open-loop", and "anchor".
 By default, senders MUST use the closed-loop sending policy.  If the
 session description overrides this default policy, by using the
 parameter j_update defined in Appendix C.2.2, senders MUST use the
 specified policy.
 After choosing the checkpoint packet identity for a packet, the
 sender creates the recovery journal.  By default, this journal MUST
 conform to the normative semantics in Section 5 and Appendices A-B in
 this memo.  In Appendix C.2.3, we define parameters that modify the
 normative semantics for recovery journals.  If the session
 description uses these parameters, the journal created by the sender
 MUST conform to the modified semantics.
 Next, we state the normative role for receivers in the recovery
 journal system.
 A receiver MUST detect each RTP sequence number break in a stream.
 If the sequence number break is due to a packet loss event (as
 defined in [RFC3550]), the receiver MUST repair all indefinite
 artifacts in the rendered MIDI performance caused by the loss.  If
 the sequence number break is due to an out-of-order packet (as
 defined in [RFC3550]), the receiver MUST NOT take actions that
 introduce indefinite artifacts (ignoring the out-of-order packet is a
 safe option).
 Receivers take special precautions when entering or exiting a
 session.  A receiver MUST process the first received packet in a
 stream as if it were a packet that ends a loss event.  Upon exiting a
 session, a receiver MUST ensure that the rendered MIDI performance
 does not end with indefinite artifacts.
 Receivers are under no obligation to perform indefinite artifact
 repairs at the moment a packet arrives.  A receiver that uses a
 playout buffer may choose to wait until the moment of rendering
 before processing the recovery journal, as the "lost" packet may be a
 late packet that arrives in time to use.

Lazzaro & Wawrzynek Standards Track [Page 23] RFC 4695 RTP Payload Format for MIDI November 2006

 Next, we state the normative role for the creator of the session
 description in the recovery journal system.  Depending on the
 application, the sender, the receivers, and other parties may take
 part in creating or approving the session description.
 A session description that specifies the default closed-loop sending
 policy and the default recovery journal semantics satisfies the
 recovery journal mandate.  However, these default behaviors may not
 be appropriate for all sessions.  If the creators of a session
 description use the parameters defined in Appendix C.2 to override
 these defaults, the creators MUST ensure that the parameters define a
 system that satisfies the recovery journal mandate.
 Finally, we note that this memo does not specify sender or receiver
 recovery journal algorithms.  Implementations are free to use any
 algorithm that conforms to the requirements in this section.  The
 non-normative [RFC4696] discusses sender and receiver algorithm
 design.

5. Recovery Journal Format

 This section introduces the structure of the recovery journal and
 defines the bitfields of recovery journal headers.  Appendices A-B
 complete the bitfield definition of the recovery journal.
 The recovery journal has a three-level structure:
   o Top-level header.
   o Channel and system journal headers.  These headers encode
     recovery information for a single voice channel (channel journal)
     or for all systems commands (system journal).
   o Chapters.  Chapters describe recovery information for a single
     MIDI command type.
 Figure 7 shows the top-level structure of the recovery journal.  The
 recovery journals consists of a 3-octet header, followed by an
 optional system journal (labeled S-journal in Figure 7) and an
 optional list of channel journals.  Figure 8 shows the recovery
 journal header format.

Lazzaro & Wawrzynek Standards Track [Page 24] RFC 4695 RTP Payload Format for MIDI November 2006

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Recovery journal header            | S-journal ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Channel journals ...                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 7 -- Top-level recovery journal format
            0                   1                   2
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |S|Y|A|H|TOTCHAN|   Checkpoint Packet Seqnum    |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 8 -- Recovery journal header
 If the Y header bit is set to 1, the system journal appears in the
 recovery journal, directly following the recovery journal header.
 If the A header bit is set to 1, the recovery journal ends with a
 list of (TOTCHAN + 1) channel journals (the 4-bit TOTCHAN header
 field is interpreted as an unsigned integer).
 A MIDI channel MAY be represented by (at most) one channel journal in
 a recovery journal.  Channel journals MUST appear in the recovery
 journal in ascending channel-number order.
 If A and Y are both zero, the recovery journal only contains its 3-
 octet header and is considered to be an "empty" journal.
 The S (single-packet loss) bit appears in most recovery journal
 structures, including the recovery journal header.  The S bit helps
 receivers efficiently parse the recovery journal in the common case
 of the loss of a single packet.  Appendix A.1 defines S bit
 semantics.
 The H bit indicates if MIDI channels in the stream have been
 configured to use the enhanced Chapter C encoding (Appendix A.3.3).
 By default, the payload format does not use enhanced Chapter C
 encoding.  In this default case, the H bit MUST be set to 0 for all
 packets in the stream.

Lazzaro & Wawrzynek Standards Track [Page 25] RFC 4695 RTP Payload Format for MIDI November 2006

 If the stream has been configured so that controller numbers for one
 or more MIDI channels use enhanced Chapter C encoding, the H bit MUST
 be set to 1 in all packets in the stream.  In Appendix C.2.3, we show
 how to configure a stream to use enhanced Chapter C encoding.
 The 16-bit Checkpoint Packet Seqnum header field codes the sequence
 number of the checkpoint packet for this journal, in network byte
 order (big-endian).  The choice of the checkpoint packet sets the
 depth of the checkpoint history for the journal (defined in Appendix
 A.1).
 Receivers may use the Checkpoint Packet Seqnum field of the packet
 that ends a loss event to verify that the journal checkpoint history
 covers the entire loss event.  The checkpoint history covers the loss
 event if the Checkpoint Packet Seqnum field is less than or equal to
 one plus the highest RTP sequence number previously received on the
 stream (modulo 2^16).
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S| CHAN  |H|      LENGTH       |P|C|M|W|N|E|T|A|  Chapters ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure 9 -- Channel journal format
 Figure 9 shows the structure of a channel journal: a 3-octet header,
 followed by a list of leaf elements called channel chapters.  A
 channel journal encodes information about MIDI commands on the MIDI
 channel coded by the 4-bit CHAN header field.  Note that CHAN uses
 the same bit encoding as the channel nibble in MIDI Channel Messages
 (the cccc field in Figure E.1 of Appendix E).
 The 10-bit LENGTH field codes the length of the channel journal.  The
 semantics for LENGTH fields are uniform throughout the recovery
 journal, and are defined in Appendix A.1.
 The third octet of the channel journal header is the Table of
 Contents (TOC) of the channel journal.  The TOC is a set of bits that
 encode the presence of a chapter in the journal.  Each chapter
 contains information about a certain class of MIDI channel command:
    o  Chapter P: MIDI Program Change (0xC)
    o  Chapter C: MIDI Control Change (0xB)
    o  Chapter M: MIDI Parameter System (part of 0xB)
    o  Chapter W: MIDI Pitch Wheel (0xE)
    o  Chapter N: MIDI NoteOff (0x8), NoteOn (0x9)
    o  Chapter E: MIDI Note Command Extras (0x8, 0x9)

Lazzaro & Wawrzynek Standards Track [Page 26] RFC 4695 RTP Payload Format for MIDI November 2006

    o  Chapter T: MIDI Channel Aftertouch (0xD)
    o  Chapter A: MIDI Poly Aftertouch (0xA)
 Chapters appear in a list following the header, in order of their
 appearance in the TOC.  Appendices A.2-9 describe the bitfield format
 for each chapter, and define the conditions under which a chapter
 type MUST appear in the recovery journal.  If any chapter types are
 required for a channel, an associated channel journal MUST appear in
 the recovery journal.
 The H bit indicates if controller numbers on a MIDI channel have been
 configured to use the enhanced Chapter C encoding (Appendix A.3.3).
 By default, controller numbers on a MIDI channel do not use enhanced
 Chapter C encoding.  In this default case, the H bit MUST be set to 0
 for all channel journal headers for the channel in the recovery
 journal, for all packets in the stream.
 However, if at least one controller number for a MIDI channel has
 been configured to use the enhanced Chapter C encoding, the H bit for
 its channel journal MUST be set to 1, for all packets in the stream.
 In Appendix C.2.3, we show how to configure a controller number to
 use enhanced Chapter C encoding.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|D|V|Q|F|X|      LENGTH       |  System chapters ...          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure 10 -- System journal format
 Figure 10 shows the structure of the system journal: a 2-octet
 header, followed by a list of system chapters.  Each chapter codes
 information about a specific class of MIDI Systems command:
    o  Chapter D: Song Select (0xF3), Tune Request (0xF6), Reset
                  (0xFF), undefined System commands (0xF4, 0xF5, 0xF9,
                  0xFD)
    o  Chapter V: Active Sense (0xFE)
    o  Chapter Q: Sequencer State (0xF2, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC)
    o  Chapter F: MTC Tape Position (0xF1, 0xF0 0x7F 0xcc 0x01 0x01)
    o  Chapter X: System Exclusive (all other 0xF0)
 The 10-bit LENGTH field codes the size of the system journal and
 conforms to semantics described in Appendix A.1.

Lazzaro & Wawrzynek Standards Track [Page 27] RFC 4695 RTP Payload Format for MIDI November 2006

 The D, V, Q, F, and X header bits form a Table of Contents (TOC) for
 the system journal.  A TOC bit that is set to 1 codes the presence of
 a chapter in the journal.  Chapters appear in a list following the
 header, in the order of their appearance in the TOC.
 Appendix B describes the bitfield format for the system chapters and
 defines the conditions under which a chapter type MUST appear in the
 recovery journal.  If any system chapter type is required to appear
 in the recovery journal, the system journal MUST appear in the
 recovery journal.

6. Session Description Protocol

 RTP does not perform session management.  Instead, RTP works together
 with session management tools, such as the Session Initiation
 Protocol (SIP, [RFC3261]) and the Real Time Streaming Protocol (RTSP,
 [RFC2326]).
 RTP payload formats define media type parameters for use in session
 management (for example, this memo defines "rtp-midi" as the media
 type for native RTP MIDI streams).
 In most cases, session management tools use the media type parameters
 via another standard, the Session Description Protocol (SDP,
 [RFC4566]).
 SDP is a textual format for specifying session descriptions.  Session
 descriptions specify the network transport and media encoding for RTP
 sessions.  Session management tools coordinate the exchange of
 session descriptions between participants ("parties").
 Some session management tools use SDP to negotiate details of media
 transport (network addresses, ports, etc.).  We refer to this use of
 SDP as "negotiated usage".  One example of negotiated usage is the
 Offer/Answer protocol ([RFC3264] and Appendix C.7.2 in this memo) as
 used by SIP.
 Other session management tools use SDP to declare the media encoding
 for the session but use other techniques to negotiate network
 transport.  We refer to this use of SDP as "declarative usage".  One
 example of declarative usage is RTSP ([RFC2326] and Appendix C.7.1 in
 this memo).
 Below, we show session description examples for native (Section 6.1)
 and mpeg4-generic (Section 6.2) streams.  In Section 6.3, we
 introduce session configuration tools that may be used to customize
 streams.

Lazzaro & Wawrzynek Standards Track [Page 28] RFC 4695 RTP Payload Format for MIDI November 2006

6.1. Session Descriptions for Native Streams

 The session description below defines a unicast UDP RTP session (via
 a media ("m=") line) whose sole payload type (96) is mapped to a
 minimal native RTP MIDI stream.
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP4 192.0.2.94
 a=rtpmap:96 rtp-midi/44100
 The rtpmap attribute line uses the "rtp-midi" media type to specify
 an RTP MIDI native stream.  The clock rate specified on the rtpmap
 line (in the example above, 44100 Hz) sets the scaling for the RTP
 timestamp header field (see Section 2.1, and also [RFC3550]).
 Note that this document does not specify a default clock rate value
 for RTP MIDI.  When RTP MIDI is used with SDP, parties MUST use the
 rtpmap line to communicate the clock rate.  Guidance for selecting
 the RTP MIDI clock rate value appears in Section 2.1.
 We consider the RTP MIDI stream shown above to be "minimal" because
 the session description does not customize the stream with
 parameters.  Without such customization, a native RTP MIDI stream has
 these characteristics:
   1. If the stream uses unreliable transport (unicast UDP, multicast
      UDP, etc.), the recovery journal system is in use, and the RTP
      payload contains both the MIDI command section and the journal
      section.  If the stream uses reliable transport (such as TCP),
      the stream does not use journalling, and the payload contains
      only the MIDI command section (Section 2.2).
   2. If the stream uses the recovery journal system, the recovery
      journal system uses the default sending policy and the default
      journal semantics (Section 4).
   3. In the MIDI command section of the payload, command timestamps
      use the default "comex" semantics (Section 3).
   4. The recommended temporal duration ("media time") of an RTP
      packet ranges from 0 to 200 ms, and the RTP timestamp difference
      between sequential packets in the stream may be arbitrarily
      large (Section 2.1).

Lazzaro & Wawrzynek Standards Track [Page 29] RFC 4695 RTP Payload Format for MIDI November 2006

   5. If more than one minimal rtp-midi stream appears in a session,
      the MIDI name spaces for these streams are independent: channel
      1 in the first stream does not reference the same MIDI channel
      as channel 1 in the second stream (see Appendix C.5 for a
      discussion of the independence of minimal rtp-midi streams).
   6. The rendering method for the stream is not specified.  What the
      receiver "does" with a minimal native MIDI stream is "out of
      scope" of this memo.  For example, in content creation
      environments, a user may manually configure client software to
      render the stream with a specific software package.
 As in standard in RTP, RTP sessions managed by SIP are sendrecv by
 default (parties send and receive MIDI), and RTP sessions managed by
 RTSP are recvonly by default (server sends and client receives).
 In sendrecv RTP MIDI sessions for the session description shown
 above, the 16 voice channel + systems MIDI name space is unique for
 each sender.  Thus, in a two-party session, the voice channel 0 sent
 by one party is distinct from the voice channel 0 sent by the other
 party.
 This behavior corresponds to what occurs when two MIDI 1.0 DIN
 devices are cross-connected with two MIDI cables (one cable routing
 MIDI Out from the first device into MIDI In of the second device, a
 second cable routing MIDI In from the first device into MIDI Out of
 the second device).  We define this "association" formally in Section
 2.1.
 MIDI 1.0 DIN networks may be configured in a "party-line" multicast
 topology.  For these networks, the MIDI protocol itself provides
 tools for addressing specific devices in transactions on a multicast
 network, and for device discovery.  Thus, apart from providing a 1-
 to-many forward path and a many-to-1 reverse path, IETF protocols do
 not need to provide any special support for MIDI multicast
 networking.

6.2. Session Descriptions for mpeg4-generic Streams

 An mpeg4-generic [RFC3640] RTP MIDI stream uses an MPEG 4 Audio
 Object Type to render MIDI into audio.  Three Audio Object Types
 accept MIDI input:
   o General MIDI (Audio Object Type ID 15), based on the General MIDI
     rendering standard [MIDI].
   o Wavetable Synthesis (Audio Object Type ID 14), based on the
     Downloadable Sounds Level 2 (DLS 2) rendering standard [DLS2].

Lazzaro & Wawrzynek Standards Track [Page 30] RFC 4695 RTP Payload Format for MIDI November 2006

   o Main Synthetic (Audio Object Type ID 13), based on Structured
     Audio and the programming language SAOL [MPEGSA].
 The primary service of an mpeg4-generic stream is to code Access
 Units (AUs).  We define the mpeg4-generic RTP MIDI AU as the MIDI
 payload shown in Figure 1 of Section 2.1 of this memo: a MIDI command
 section optionally followed by a journal section.
 Exactly one RTP MIDI AU MUST be mapped to one mpeg4-generic RTP MIDI
 packet.  The mpeg4-generic options for placing several AUs in an RTP
 packet MUST NOT be used with RTP MIDI.  The mpeg4-generic options for
 fragmenting and interleaving AUs MUST NOT be used with RTP MIDI.  The
 mpeg4-generic RTP packet payload (Figure 1 in [RFC3640]) MUST contain
 empty AU Header and Auxiliary sections.  These rules yield mpeg4-
 generic packets that are structurally identical to native RTP MIDI
 packets, an essential property for the correct operation of the
 payload format.
 The session description that follows defines a unicast UDP RTP
 session (via a media ("m=") line) whose sole payload type (96) is
 mapped to a minimal mpeg4-generic RTP MIDI stream.  This example uses
 the General MIDI Audio Object Type under Synthesis Profile @ Level 2.
 v=0
 o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP6 2001:DB80::7F2E:172A:1E24
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
 config=7A0A0000001A4D546864000000060000000100604D54726B0000
 000600FF2F000
 (The a=fmtp line has been wrapped to fit the page to accommodate memo
 formatting restrictions; it comprises a single line in SDP.)
 The fmtp attribute line codes the four parameters (streamtype, mode,
 profile-level-id, and config) that are required in all mpeg4-generic
 session descriptions [RFC3640].  For RTP MIDI streams, the streamtype
 parameter MUST be set to 5, the "mode" parameter MUST be set to
 "rtp-midi", and the "profile-level-id" parameter MUST be set to the
 MPEG-4 Profile Level for the stream.  For the Synthesis Profile,
 legal profile-level-id values are 11, 12, and 13, coding low (11),
 medium (12), or high (13) decoder computational complexity, as
 defined by MPEG conformance tests.

Lazzaro & Wawrzynek Standards Track [Page 31] RFC 4695 RTP Payload Format for MIDI November 2006

 In a minimal RTP MIDI session description, the config value MUST be a
 hexadecimal encoding [RFC3640] of the AudioSpecificConfig data block
 [MPEGAUDIO] for the stream.  AudioSpecificConfig encodes the Audio
 Object Type for the stream and also encodes initialization data (SAOL
 programs, DLS 2 wave tables, etc.).  Standard MIDI Files encoded in
 AudioSpecificConfig in a minimal session description MUST be ignored
 by the receiver.
 Receivers determine the rendering algorithm for the session by
 interpreting the first 5 bits of AudioSpecificConfig as an unsigned
 integer that codes the Audio Object Type.  In our example above, the
 leading config string nibbles "7A" yield the Audio Object Type 15
 (General MIDI).  In Appendix E.4, we derive the config string value
 in the session description shown above; the starting point of the
 derivation is the MPEG bitstreams defined in [MPEGSA] and
 [MPEGAUDIO].
 We consider the stream to be "minimal" because the session
 description does not customize the stream through the use of
 parameters, other than the 4 required mpeg4-generic parameters
 described above.  In Section 6.1, we describe the behavior of a
 minimal native stream, as a numbered list of characteristics.  Items
 1-4 on that list also describe the minimal mpeg4-generic stream, but
 items 5 and 6 require restatements, as listed below:
   5. If more than one minimal mpeg4-generic stream appears in a
      session, each stream uses an independent instance of the Audio
      Object Type coded in the config parameter value.
   6. A minimal mpeg4-generic stream encodes the AudioSpecificConfig
      as an inline hexadecimal constant.  If a session description is
      sent over UDP, it may be impossible to transport large
      AudioSpecificConfig blocks within the Maximum Transmission Size
      (MTU) of the underlying network (for Ethernet, the MTU is 1500
      octets).  In some cases, the AudioSpecificConfig block may
      exceed the maximum size of the UDP packet itself.
 The comments in Section 6.1 on SIP and RTSP stream directional
 defaults, sendrecv MIDI channel usage, and MIDI 1.0 DIN multicast
 networks also apply to mpeg4-generic RTP MIDI sessions.
 In sendrecv sessions, each party's session description MUST use
 identical values for the mpeg4-generic parameters (including the
 required streamtype, mode, profile-level-id, and config parameters).
 As a consequence, each party uses an identically configured MPEG 4
 Audio Object Type to render MIDI commands into audio.  The preamble
 to Appendix C discusses a way to create "virtual sendrecv" sessions
 that do not have this restriction.

Lazzaro & Wawrzynek Standards Track [Page 32] RFC 4695 RTP Payload Format for MIDI November 2006

6.3. Parameters

 This section introduces parameters for session configuration for RTP
 MIDI streams.  In session descriptions, parameters modify the
 semantics of a payload type.  Parameters are specified on an fmtp
 attribute line.  See the session description example in Section 6.2
 for an example of a fmtp attribute line.
 The parameters add features to the minimal streams described in
 Sections 6.1-2, and support several types of services:
   o  Stream subsetting.  By default, all MIDI commands that are legal
      to appear on a MIDI 1.0 DIN cable may appear in an RTP MIDI
      stream.  The cm_unused parameter overrides this default by
      prohibiting certain commands from appearing in the stream.  The
      cm_used parameter is used in conjunction with cm_unused, to
      simplify the specification of complex exclusion rules.  We
      describe cm_unused and cm_used in Appendix C.1.
   o  Journal customization.  The j_sec and j_update parameters
      configure the use of the journal section.  The ch_default,
      ch_never, and ch_anchor parameters configure the semantics of
      the recovery journal chapters.  These parameters are described
      in Appendix C.2 and override the default stream behaviors 1 and
      2, listed in Section 6.1 and referenced in Section 6.2.
   o  MIDI command timestamp semantics.  The tsmode, octpos, mperiod,
      and linerate parameters customize the semantics of timestamps in
      the MIDI command section.  These parameters let RTP MIDI
      accurately encode the implicit time coding of MIDI 1.0 DIN
      cables.  These parameters are described in Appendix C.3 and
      override default stream behavior 3, listed in Section 6.1 and
      referenced in Section 6.2
   o  Media time.  The rtp_ptime and rtp_maxptime parameters define
      the temporal duration ("media time") of an RTP MIDI packet.  The
      guardtime parameter sets the minimum sending rate of stream
      packets.  These parameters are described in Appendix C.4 and
      override default stream behavior 4, listed in Section 6.1 and
      referenced in Section 6.2.
   o  Stream description.  The musicport parameter labels the MIDI
      name space of RTP streams in a multimedia session.  Musicport is
      described in Appendix C.5.  The musicport parameter overrides
      default stream behavior 5, in Sections 6.1 and 6.2.

Lazzaro & Wawrzynek Standards Track [Page 33] RFC 4695 RTP Payload Format for MIDI November 2006

   o  MIDI rendering.  Several parameters specify the MIDI rendering
      method of a stream.  These parameters are described in Appendix
      C.6 and override default stream behavior 6, in Sections 6.1 and
      6.2.
 In Appendix C.7, we specify interoperability guidelines for two RTP
 MIDI application areas: content-streaming using RTSP (Appendix C.7.1)
 and network musical performance using SIP (Appendix C.7.2).

7. Extensibility

 The payload format defined in this memo exclusively encodes all
 commands that may legally appear on a MIDI 1.0 DIN cable.
 Many worthy uses of MIDI over RTP do not fall within the narrow scope
 of the payload format.  For example, the payload format does not
 support the direct transport of Standard MIDI File (SMF) meta-event
 and metric timing data.  As a second example, the payload format does
 not define transport tools for user-defined commands (apart from
 tools to support System Exclusive commands [MIDI]).
 The payload format does not provide an extension mechanism to support
 new features of this nature, by design.  Instead, we encourage the
 development of new payload formats for specialized musical
 applications.  The IETF session management tools [RFC3264] [RFC2326]
 support codec negotiation, to facilitate the use of new payload
 formats in a backward-compatible way.
 However, the payload format does provide several extensibility tools,
 which we list below:
   o  Journalling.  As described in Appendix C.2, new token values for
      the j_sec and j_update parameters may be defined in IETF
      standards-track documents.  This mechanism supports the design
      of new journal formats and the definition of new journal sending
      policies.
   o  Rendering.  The payload format may be extended to support new
      MIDI renderers (Appendix C.6.2).  Certain general aspects of the
      RTP MIDI rendering process may also be extended, via the
      definition of new token values for the render (Appendix C.6) and
      smf_info (Appendix C.6.4.1) parameters.
   o  Undefined commands.  [MIDI] reserves 4 MIDI System commands for
      future use (0xF4, 0xF5, 0xF9, 0xFD).  If updates to [MIDI]
      define the reserved commands, IETF standards-track documents may
      be defined to provide resiliency support for the commands.

Lazzaro & Wawrzynek Standards Track [Page 34] RFC 4695 RTP Payload Format for MIDI November 2006

      Opaque LEGAL fields appear in System Chapter D for this purpose
      (Appendix B.1.1).
 A final form of extensibility involves the inclusion of the payload
 format in framework documents.  Framework documents describe how to
 combine protocols to form a platform for interoperable applications.
 For example, a stage and studio framework might define how to use SIP
 [RFC3261], RTSP [RFC2326], SDP [RFC4566], and RTP [RFC3550] to
 support media networking for professional audio equipment and
 electronic musical instruments.

8. Congestion Control

 The RTP congestion control requirements defined in [RFC3550] apply to
 RTP MIDI sessions, and implementors should carefully read the
 congestion control section in [RFC3550].  As noted in [RFC3550], all
 transport protocols used on the Internet need to address congestion
 control in some way, and RTP is not an exception.
 In addition, the congestion control requirements defined in [RFC3551]
 applies to RTP MIDI sessions run under applicable profiles.  The
 basic congestion control requirement defined in [RFC3551] is that RTP
 sessions that use UDP transport should monitor packet loss (via RTCP
 or other means) to ensure that the RTP stream competes fairly with
 TCP flows that share the network.
 Finally, RTP MIDI has congestion control issues that are unique for
 an audio RTP payload format.  In applications such as network musical
 performance [NMP], the packet rate is linked to the gestural rate of
 a human performer.  Senders MUST monitor the MIDI command source for
 patterns that result in excessive packet rates and take actions
 during RTP transcoding to reduce the RTP packet rate.  [RFC4696]
 offers implementation guidance on this issue.

9. Security Considerations

 Implementors should carefully read the Security Considerations
 sections of the RTP [RFC3550], AVP [RFC3551], and other RTP profile
 documents, as the issues discussed in these sections directly apply
 to RTP MIDI streams.  Implementors should also review the Secure
 Real-time Transport Protocol (SRTP, [RFC3711]), an RTP profile that
 addresses the security issues discussed in [RFC3550] and [RFC3551].
 Here, we discuss security issues that are unique to the RTP MIDI
 payload format.
 When using RTP MIDI, authentication of incoming RTP and RTCP packets
 is RECOMMENDED.  Per-packet authentication may be provided by SRTP or

Lazzaro & Wawrzynek Standards Track [Page 35] RFC 4695 RTP Payload Format for MIDI November 2006

 by other means.  Without the use of authentication, attackers could
 forge MIDI commands into an ongoing stream, damaging speakers and
 eardrums.  An attacker could also craft RTP and RTCP packets to
 exploit known bugs in the client and take effective control of a
 client machine.
 Session management tools (such as SIP [RFC3261]) SHOULD use
 authentication during the transport of all session descriptions
 containing RTP MIDI media streams.  For SIP, the Security
 Considerations section in [RFC3261] provides an overview of possible
 authentication mechanisms.  RTP MIDI session descriptions should use
 authentication because the session descriptions may code
 initialization data using the parameters described in Appendix C.  If
 an attacker inserts bogus initialization data into a session
 description, he can corrupt the session or forge an client attack.
 Session descriptions may also code renderer initialization data by
 reference, via the url (Appendix C.6.3) and smf_url (Appendix
 C.6.4.2) parameters.  If the coded URL is spoofed, both session and
 client are open to attack, even if the session description itself is
 authenticated.  Therefore, URLs specified in url and smf_url
 parameters SHOULD use [RFC2818].
 Section 2.1 allows streams sent by a party in two RTP sessions to
 have the same SSRC value and the same RTP timestamp initialization
 value, under certain circumstances.  Normally, these values are
 randomly chosen for each stream in a session, to make plaintext
 guessing harder to do if the payloads are encrypted.  Thus, Section
 2.1 weakens this aspect of RTP security.

10. Acknowledgements

 We thank the networking, media compression, and computer music
 community members who have commented or contributed to the effort,
 including Kurt B, Cynthia Bruyns, Steve Casner, Paul Davis, Robin
 Davies, Joanne Dow, Tobias Erichsen, Nicolas Falquet, Dominique
 Fober, Philippe Gentric, Michael Godfrey, Chris Grigg, Todd Hager,
 Michel Jullian, Phil Kerr, Young-Kwon Lim, Jessica Little, Jan van
 der Meer, Colin Perkins, Charlie Richmond, Herbie Robinson, Larry
 Rowe, Eric Scheirer, Dave Singer, Martijn Sipkema, William Stewart,
 Kent Terry, Magnus Westerlund, Tom White, Jim Wright, Doug Wyatt, and
 Giorgio Zoia.  We also thank the members of the San Francisco Bay
 Area music and audio community for creating the context for the work,
 including Don Buchla, Chris Chafe, Richard Duda, Dan Ellis, Adrian
 Freed, Ben Gold, Jaron Lanier, Roger Linn, Richard Lyon, Dana Massie,
 Max Mathews, Keith McMillen, Carver Mead, Nelson Morgan, Tom
 Oberheim, Malcolm Slaney, Dave Smith, Julius Smith, David Wessel, and
 Matt Wright.

Lazzaro & Wawrzynek Standards Track [Page 36] RFC 4695 RTP Payload Format for MIDI November 2006

11. IANA Considerations

 This section makes a series of requests to IANA.  The IANA has
 completed registration/assignments of the below requests.
 The sub-sections that follow hold the actual, detailed requests.  All
 registrations in this section are in the IETF tree and follow the
 rules of [RFC4288] and [RFC3555], as appropriate.
 In Section 11.1, we request the registration of a new media type:
 "audio/rtp-midi".  Paired with this request is a request for a
 repository for new values for several parameters associated with
 "audio/rtp-midi".  We request this repository in Section 11.1.1.
 In Section 11.2, we request the registration of a new value ("rtp-
 midi") for the "mode" parameter of the "mpeg4-generic" media type.
 The "mpeg4-generic" media type is defined in [RFC3640], and [RFC3640]
 defines a repository for the "mode" parameter.  However, we believe
 we are the first to request the registration of a "mode" value, so we
 believe the registry for "mode" has not yet been created by IANA.
 Paired with our "mode" parameter value request for "mpeg4-generic" is
 a request for a repository for new values for several parameters we
 have defined for use with the "rtp-midi" mode value.  We request this
 repository in Section 11.2.1.
 In Section 11.3, we request the registration of a new media type:
 "audio/asc".  No repository request is associated with this request.

11.1. rtp-midi Media Type Registration

 This section requests the registration of the "rtp-midi" subtype for
 the "audio" media type.  We request the registration of the
 parameters listed in the "optional parameters" section below (both
 the "non-extensible parameters" and the "extensible parameters"
 lists).  We also request the creation of repositories for the
 "extensible parameters"; the details of this request appear in
 Section 11.1.1, below.
 Media type name:
     audio
 Subtype name:
     rtp-midi

Lazzaro & Wawrzynek Standards Track [Page 37] RFC 4695 RTP Payload Format for MIDI November 2006

 Required parameters:
     rate: The RTP timestamp clock rate.  See Sections 2.1 and 6.1
     for usage details.
 Optional parameters:
     Non-extensible parameters:
        ch_anchor:    See Appendix C.2.3 for usage details.
        ch_default:   See Appendix C.2.3 for usage details.
        ch_never:     See Appendix C.2.3 for usage details.
        cm_unused:    See Appendix C.1 for usage details.
        cm_used:      See Appendix C.1 for usage details.
        chanmask:     See Appendix C.6.4.3 for usage details.
        cid:          See Appendix C.6.3 for usage details.
        guardtime:    See Appendix C.4.2 for usage details.
        inline:       See Appendix C.6.3 for usage details.
        linerate:     See Appendix C.3 for usage details.
        mperiod:      See Appendix C.3 for usage details.
        multimode:    See Appendix C.6.1 for usage details.
        musicport:    See Appendix C.5 for usage details.
        octpos:       See Appendix C.3 for usage details.
        rinit:        See Appendix C.6.3 for usage details.
        rtp_maxptime: See Appendix C.4.1 for usage details.
        rtp_ptime:    See Appendix C.4.1 for usage details.
        smf_cid:      See Appendix C.6.4.2 for usage details.
        smf_inline:   See Appendix C.6.4.2 for usage details.
        smf_url:      See Appendix C.6.4.2 for usage details.
        tsmode:       See Appendix C.3 for usage details.
        url:          See Appendix C.6.3 for usage details.
     Extensible parameters:
        j_sec:        See Appendix C.2.1 for usage details.  See
                      Section 11.1.1 for repository details.
        j_update:     See Appendix C.2.2 for usage details.  See
                      Section 11.1.1 for repository details.
        render:       See Appendix C.6 for usage details.  See
                      Section 11.1.1 for repository details.
        subrender:    See Appendix C.6.2 for usage details.  See
                      Section 11.1.1 for repository details.
        smf_info:     See Appendix C.6.4.1 for usage details.  See
                      Section 11.1.1 for repository details.
 Encoding considerations:
     The format for this type is framed and binary.

Lazzaro & Wawrzynek Standards Track [Page 38] RFC 4695 RTP Payload Format for MIDI November 2006

 Restrictions on usage:
     This type is only defined for real-time transfers of MIDI
     streams via RTP.  Stored-file semantics for rtp-midi may
     be defined in the future.
 Security considerations:
     See Section 9 of this memo.
 Interoperability considerations:
     None.
 Published specification:
     This memo and [MIDI] serve as the normative specification.  In
     addition, references [NMP], [GRAME], and [RFC4696] provide
     non-normative implementation guidance.
 Applications that use this media type:
     Audio content-creation hardware, such as MIDI controller piano
     keyboards and MIDI audio synthesizers.  Audio content-creation
     software, such as music sequencers, digital audio workstations,
     and soft synthesizers.  Computer operating systems, for network
     support of MIDI Application Programmer Interfaces.  Content
     distribution servers and terminals may use this media type for
     low bit-rate music coding.
 Additional information:
     None.
 Person & email address to contact for further information:
     John Lazzaro <lazzaro@cs.berkeley.edu>
 Intended usage:
     COMMON.
 Author:
     John Lazzaro <lazzaro@cs.berkeley.edu>

Lazzaro & Wawrzynek Standards Track [Page 39] RFC 4695 RTP Payload Format for MIDI November 2006

 Change controller:
     IETF Audio/Video Transport Working Group delegated
     from the IESG.

11.1.1. Repository Request for "audio/rtp-midi"

 For the "rtp-midi" subtype, we request the creation of repositories
 for extensions to the following parameters (which are those listed as
 "extensible parameters" in Section 11.1).
    j_sec:
       Registrations for this repository may only occur
       via an IETF standards-track document.  Appendix C.2.1
       of this memo describes appropriate registrations for this
       repository.
       Initial values for this repository appear below:
       "none":  Defined in Appendix C.2.1 of this memo.
       "recj":  Defined in Appendix C.2.1 of this memo.
    j_update:
       Registrations for this repository may only occur
       via an IETF standards-track document.  Appendix C.2.2
       of this memo describes appropriate registrations for this
       repository.
       Initial values for this repository appear below:
       "anchor":  Defined in Appendix C.2.2 of this memo.
       "open-loop":  Defined in Appendix C.2.2 of this memo.
       "closed-loop":  Defined in Appendix C.2.2 of this memo.
    render:
       Registrations for this repository MUST include a
       specification of the usage of the proposed value.
       See text in the preamble of Appendix C.6 for details
       (the paragraph that begins "Other render token ...").

Lazzaro & Wawrzynek Standards Track [Page 40] RFC 4695 RTP Payload Format for MIDI November 2006

       Initial values for this repository appear below:
       "unknown":  Defined in Appendix C.6 of this memo.
       "synthetic":  Defined in Appendix C.6 of this memo.
       "api":  Defined in Appendix C.6 of this memo.
       "null":  Defined in Appendix C.6 of this memo.
    subrender:
       Registrations for this repository MUST include a
       specification of the usage of the proposed value.
       See text Appendix C.6.2 for details (the paragraph
       that begins "Other subrender token ...").
       Initial values for this repository appear below:
       "default":  Defined in Appendix C.6.2 of this memo.
    smf_info:
       Registrations for this repository MUST include a
       specification of the usage of the proposed value.
       See text in Appendix C.6.4.1 for details (the
       paragraph that begins "Other smf_info token ...").
       Initial values for this repository appear below:
       "ignore":  Defined in Appendix C.6.4.1 of this memo.
       "sdp_start":  Defined in Appendix C.6.4.1 of this memo.
       "identity":  Defined in Appendix C.6.4.1 of this memo.

11.2. mpeg4-generic Media Type Registration

 This section requests the registration of the "rtp-midi" value for
 the "mode" parameter of the "mpeg4-generic" media type.  The "mpeg4-
 generic" media type is defined in [RFC3640], and [RFC3640] defines a
 repository for the "mode" parameter.  We are registering mode rtp-
 midi to support the MPEG Audio codecs [MPEGSA] that use MIDI.
 In conjunction with this registration request, we request the
 registration of the parameters listed in the "optional parameters"
 section below (both the "non-extensible parameters" and the
 "extensible parameters" lists).  We also request the creation of
 repositories for the "extensible parameters"; the details of this
 request appear in Appendix 11.2.1, below.

Lazzaro & Wawrzynek Standards Track [Page 41] RFC 4695 RTP Payload Format for MIDI November 2006

 Media type name:
     audio
 Subtype name:
     mpeg4-generic
 Required parameters:
     The "mode" parameter is required by [RFC3640].  [RFC3640]
     requests a repository for "mode", so that new values for mode
     may be added.  We request that the value "rtp-midi" be
     added to the "mode" repository.
     In mode rtp-midi, the mpeg4-generic parameter rate is
     a required parameter.  Rate specifies the RTP timestamp
     clock rate.  See Sections 2.1 and 6.2 for usage details
     of rate in mode rtp-midi.
 Optional parameters:
     We request registration of the following parameters
     for use in mode rtp-midi for mpeg4-generic.
     Non-extensible parameters:
        ch_anchor:    See Appendix C.2.3 for usage details.
        ch_default:   See Appendix C.2.3 for usage details.
        ch_never:     See Appendix C.2.3 for usage details.
        cm_unused:    See Appendix C.1 for usage details.
        cm_used:      See Appendix C.1 for usage details.
        chanmask:     See Appendix C.6.4.3 for usage details.
        cid:          See Appendix C.6.3 for usage details.
        guardtime:    See Appendix C.4.2 for usage details.
        inline:       See Appendix C.6.3 for usage details.
        linerate:     See Appendix C.3 for usage details.
        mperiod:      See Appendix C.3 for usage details.
        multimode:    See Appendix C.6.1 for usage details.
        musicport:    See Appendix C.5 for usage details.
        octpos:       See Appendix C.3 for usage details.
        rinit:        See Appendix C.6.3 for usage details.
        rtp_maxptime: See Appendix C.4.1 for usage details.
        rtp_ptime:    See Appendix C.4.1 for usage details.
        smf_cid:      See Appendix C.6.4.2 for usage details.
        smf_inline:   See Appendix C.6.4.2 for usage details.

Lazzaro & Wawrzynek Standards Track [Page 42] RFC 4695 RTP Payload Format for MIDI November 2006

        smf_url:      See Appendix C.6.4.2 for usage details.
        tsmode:       See Appendix C.3 for usage details.
        url:          See Appendix C.6.3 for usage details.
     Extensible parameters:
        j_sec:        See Appendix C.2.1 for usage details.  See
                      Section 11.2.1 for repository details.
        j_update:     See Appendix C.2.2 for usage details.  See
                      Section 11.2.1 for repository details.
        render:       See Appendix C.6 for usage details.  See
                      Section 11.2.1 for repository details.
        subrender:    See Appendix C.6.2 for usage details.  See
                      Section 11.2.1 for repository details.
        smf_info:     See Appendix C.6.4.1 for usage details.  See
                      Section 11.2.1 for repository details.
 Encoding considerations:
     The format for this type is framed and binary.
 Restrictions on usage:
     Only defined for real-time transfers of audio/mpeg4-generic
     RTP streams with mode=rtp-midi.
 Security considerations:
     See Section 9 of this memo.
 Interoperability considerations:
     Except for the marker bit (Section 2.1), the packet formats
     for audio/rtp-midi and audio/mpeg4-generic (mode rtp-midi)
     are identical.  The formats differ in use: audio/mpeg4-generic
     is for MPEG work, and audio/rtp-midi is for all other work.
 Published specification:
     This memo, [MIDI], and [MPEGSA] are the normative references.
     In addition, references [NMP], [GRAME], and [RFC4696] provide
     non-normative implementation guidance.
 Applications that use this media type:
     MPEG 4 servers and terminals that support [MPEGSA].

Lazzaro & Wawrzynek Standards Track [Page 43] RFC 4695 RTP Payload Format for MIDI November 2006

 Additional information:
     None.
 Person & email address to contact for further information:
     John Lazzaro <lazzaro@cs.berkeley.edu>
 Intended usage:
     COMMON.
 Author:
     John Lazzaro <lazzaro@cs.berkeley.edu>
 Change controller:
     IETF Audio/Video Transport Working Group delegated
     from the IESG.

11.2.1. Repository Request for Mode rtp-midi for mpeg4-generic

 For mode rtp-midi of the mpeg4-generic subtype, we request the
 creation of repositories for extensions to the following parameters
 (which are those listed as "extensible parameters" in Section 11.2).
    j_sec:
       Registrations for this repository may only occur
       via an IETF standards-track document.  Appendix C.2.1
       of this memo describes appropriate registrations for this
       repository.
       Initial values for this repository appear below:
       "none":  Defined in Appendix C.2.1 of this memo.
       "recj":  Defined in Appendix C.2.1 of this memo.
    j_update:
       Registrations for this repository may only occur
       via an IETF standards-track document.  Appendix C.2.2
       of this memo describes appropriate registrations for this
       repository.

Lazzaro & Wawrzynek Standards Track [Page 44] RFC 4695 RTP Payload Format for MIDI November 2006

       Initial values for this repository appear below:
       "anchor":  Defined in Appendix C.2.2 of this memo.
       "open-loop":  Defined in Appendix C.2.2 of this memo.
       "closed-loop":  Defined in Appendix C.2.2 of this memo.
    render:
       Registrations for this repository MUST include a
       specification of the usage of the proposed value.
       See text in the preamble of Appendix C.6 for details
       (the paragraph that begins "Other render token ...").
       Initial values for this repository appear below:
       "unknown":  Defined in Appendix C.6 of this memo.
       "synthetic":  Defined in Appendix C.6 of this memo.
       "null":  Defined in Appendix C.6 of this memo.
    subrender:
       Registrations for this repository MUST include a
       specification of the usage of the proposed value.
       See text Appendix C.6.2 for details (the paragraph
       that begins "Other subrender token ..." and
       subsequent paragraphs).  Note that the text in
       Appendix C.6.2 contains restrictions on subrender
       registrations for mpeg4-generic ("Registrations
       for mpeg4-generic subrender values ...").
       Initial values for this repository appear below:
       "default":  Defined in Appendix C.6.2 of this memo.
    smf_info:
       Registrations for this repository MUST include a
       specification of the usage of the proposed value.
       See text in Appendix C.6.4.1 for details (the
       paragraph that begins "Other smf_info token ...").
       Initial values for this repository appear below:
       "ignore":  Defined in Appendix C.6.4.1 of this memo.
       "sdp_start":  Defined in Appendix C.6.4.1 of this memo.
       "identity":  Defined in Appendix C.6.4.1 of this memo.

Lazzaro & Wawrzynek Standards Track [Page 45] RFC 4695 RTP Payload Format for MIDI November 2006

11.3. asc Media Type Registration

 This section registers "asc" as a subtype for the "audio" media type.
 We register this subtype to support the remote transfer of the
 "config" parameter of the mpeg4-generic media type [RFC3640] when it
 is used with mpeg4-generic mode rtp-midi (registered in Appendix 11.2
 above).  We explain the mechanics of using "audio/asc" to set the
 config parameter in Section 6.2 and Appendix C.6.5 of this document.
 Note that this registration is a new subtype registration and is not
 an addition to a repository defined by MPEG-related memos (such as
 [RFC3640]).  Also note that this request for "audio/asc" does not
 register parameters, and does not request the creation of a
 repository.
 Media type name:
     audio
 Subtype name:
     asc
 Required parameters:
     None.
 Optional parameters:
     None.
 Encoding considerations:
     The native form of the data object is binary data,
     zero-padded to an octet boundary.
 Restrictions on usage:
     This type is only defined for data object (stored file)
     transfer.  The most common transports for the type are
     HTTP and SMTP.
 Security considerations:
     See Section 9 of this memo.

Lazzaro & Wawrzynek Standards Track [Page 46] RFC 4695 RTP Payload Format for MIDI November 2006

 Interoperability considerations:
     None.
 Published specification:
     The audio/asc data object is the AudioSpecificConfig
     binary data structure, which is normatively defined in
     [MPEGAUDIO].
 Applications that use this media type:
     MPEG 4 Audio servers and terminals that support
     audio/mpeg4-generic RTP streams for mode rtp-midi.
 Additional information:
     None.
 Person & email address to contact for further information:
     John Lazzaro <lazzaro@cs.berkeley.edu>
 Intended usage:
     COMMON.
 Author:
     John Lazzaro <lazzaro@cs.berkeley.edu>
 Change controller:
     IETF Audio/Video Transport Working Group delegated
     from the IESG.

Lazzaro & Wawrzynek Standards Track [Page 47] RFC 4695 RTP Payload Format for MIDI November 2006

A. The Recovery Journal Channel Chapters

A.1. Recovery Journal Definitions

 This appendix defines the terminology and the coding idioms that are
 used in the recovery journal bitfield descriptions in Section 5
 (journal header structure), Appendices A.2 to A.9 (channel journal
 chapters) and Appendices B.1 to B.5 (system journal chapters).
 We assume that the recovery journal resides in the journal section of
 an RTP packet with sequence number I ("packet I") and that the
 Checkpoint Packet Seqnum field in the top-level recovery journal
 header refers to a previous packet with sequence number C (an
 exception is the self-referential C = I case).  Unless stated
 otherwise, algorithms are assumed to use modulo 2^16 arithmetic for
 calculations on 16-bit sequence numbers and modulo 2^32 arithmetic
 for calculations on 32-bit extended sequence numbers.
 Several bitfield coding idioms appear throughout the recovery journal
 system, with consistent semantics.  Most recovery journal elements
 begin with an "S" (Single-packet loss) bit.  S bits are designed to
 help receivers efficiently parse through the recovery journal
 hierarchy in the common case of the loss of a single packet.
 As a rule, S bits MUST be set to 1.  However, an exception applies if
 a recovery journal element in packet I encodes data about a command
 stored in the MIDI command section of packet I - 1.  In this case,
 the S bit of the recovery journal element MUST be set to 0.  If a
 recovery journal element has its S bit set to 0, all higher-level
 recovery journal elements that contain it MUST also have S bits that
 are set to 0, including the top-level recovery journal header.
 Other consistent bitfield coding idioms are described below:
   o R flag bit.  R flag bits are reserved for future use.  Senders
     MUST set R bits to 0.  Receivers MUST ignore R bit values.
   o LENGTH field.  All fields named LENGTH (as distinct from LEN)
     code the number of octets in the structure that contains it,
     including the header it resides in and all hierarchical levels
     below it.  If a structure contains a LENGTH field, a receiver
     MUST use the LENGTH field value to advance past the structure
     during parsing, rather than use knowledge about the internal
     format of the structure.

Lazzaro & Wawrzynek Standards Track [Page 48] RFC 4695 RTP Payload Format for MIDI November 2006

 We now define normative terms used to describe recovery journal
 semantics.
   o Checkpoint history.  The checkpoint history of a recovery journal
     is the concatenation of the MIDI command sections of packets C
     through I - 1.  The final command in the MIDI command section for
     packet I - 1 is considered the most recent command; the first
     command in the MIDI command section for packet C is the oldest
     command.  If command X is less recent than command Y, X is
     considered to be "before Y".  A checkpoint history with no
     commands is considered to be empty.  The checkpoint history never
     contains the MIDI command section of packet I (the packet
     containing the recovery journal), so if C == I, the checkpoint
     history is empty by definition.
   o Session history.  The session history of a recovery journal is
     the concatenation of MIDI command sections from the first packet
     of the session up to packet I - 1.  The definitions of command
     recency and history emptiness follow those in the checkpoint
     history.  The session history never contains the MIDI command
     section of packet I, and so the session history of the first
     packet in the session is empty by definition.
   o Finished/unfinished commands.  If all octets of a MIDI command
     appear in the session history, the command is defined as being
     finished.  If some but not all octets of a command appear in the
     session history, the command is defined as being unfinished.
     Unfinished commands occur if segments of a SysEx command appear
     in several RTP packets.  For example, if a SysEx command is coded
     as 3 segments, with segment 1 in packet K, segment 2 in packet K
     + 1, and segment 3 in packet K + 2, the session histories for
     packets K + 1 and K + 2 contain unfinished versions of the
     command.  A session history contains a finished version of a
     cancelled SysEx command if the history contains the cancel
     sublist for the command.
   o Reset State commands.  Reset State (RS) commands reset renderers
     to an initialized "powerup" condition.  The RS commands are:
     System Reset (0xFF), General MIDI System Enable (0xF0 0x7E 0xcc
     0x09 0x01 0xF7), General MIDI 2 System Enable (0xF0 0x7E 0xcc
     0x09 0x03 0xF7), General MIDI System Disable (0xF0 0x7E 0xcc 0x09
     0x00 0xF7), Turn DLS On (0xF0 0x7E 0xcc 0x0A 0x01 0xF7), and Turn
     DLS Off (0xF0 0x7E 0xcc 0x0A 0x02 0xF7).  Registrations of
     subrender parameter token values (Appendix C.6.2) and IETF
     standards-track documents MAY specify additional RS commands.
   o Active commands.  Active command are MIDI commands that do not
     appear before a Reset State command in the session history.

Lazzaro & Wawrzynek Standards Track [Page 49] RFC 4695 RTP Payload Format for MIDI November 2006

   o N-active commands.  N-active commands are MIDI commands that do
     not appear before one of the following commands in the session
     history:  MIDI Control Change numbers 123-127 (numbers with All
     Notes Off semantics) or 120 (All Sound Off), and any Reset State
     command.
   o C-active commands.  C-active commands are MIDI commands that do
     not appear before one of the following commands in the session
     history:  MIDI Control Change number 121 (Reset All Controllers)
     and any Reset State command.
   o Oldest-first ordering rule.  Several recovery journal chapters
     contain a list of elements, where each element is associated with
     a MIDI command that appears in the session history.  In most
     cases, the chapter definition requires that list elements be
     ordered in accordance with the "oldest-first ordering rule".
     Below, we normatively define this rule:
     Elements associated with the most recent command in the session
     history coded in the list MUST appear at the end of the list.
     Elements associated with the oldest command in the session
     history coded in the list MUST appear at the start of the list.
     All other list elements MUST be arranged with respect to these
     boundary elements, to produce a list ordering that strictly
     reflects the relative session history recency of the commands
     coded by the elements in the list.
   o Parameter system.  A MIDI feature that provides two sets of
     16,384 parameters to expand the 0-127 controller number space.
     The Registered Parameter Names (RPN) system and the Non-
     Registered Parameter Names (NRPN) system each provides 16,384
     parameters.
   o Parameter system transaction.  The value of RPNs and NRPNs are
     changed by a series of Control Change commands that form a
     parameter system transaction.  A canonical transaction begins
     with two Control Change commands to set the parameter number
     (controller numbers 99 and 98 for NRPNs, controller numbers 101
     and 100 for RPNs).  The transaction continues with an arbitrary
     number of Data Entry (controller numbers 6 and 38), Data
     Increment (controller number 96), and Data Decrement (controller
     number 97) Control Change commands to set the parameter value.
     The transaction ends with a second pair of (99, 98) or (101, 100)
     Control Change commands that specify the null parameter (MSB
     value 0x7F, LSB value 0x7F).

Lazzaro & Wawrzynek Standards Track [Page 50] RFC 4695 RTP Payload Format for MIDI November 2006

     Several variants of the canonical transaction sequence are
     possible.  Most commonly, the terminal pair of (99, 98) or (101,
     100) Control Change commands may specify a parameter other than
     the null parameter.  In this case, the command pair terminates
     the first transaction and starts a second transaction.  The
     command pair is considered to be a part of both transactions.
     This variant is legal and recommended in [MIDI].  We refer to
     this variant as a "type 1 variant".
     Less commonly, the MSB (99 or 101) or LSB (98 or 100) command of
     a (99, 98) or (101, 100) Control Change pair may be omitted.
     If the MSB command is omitted, the transaction uses the MSB value
     of the most recent C-active Control Change command for controller
     number 99 or 101 that appears in the session history.  We refer
     to this variant as a "type 2 variant".
     If the LSB command is omitted, the LSB value 0x00 is assumed.  We
     refer to this variant as a "type 3 variant".  The type 2 and type
     3 variants are defined as legal, but are not recommended, in
     [MIDI].
     System real-time commands may appear at any point during a
     transaction (even between octets of individual commands in the
     transaction).  More generally, [MIDI] does not forbid the
     appearance of unrelated MIDI commands during an open transaction.
     As a rule, these commands are considered to be "outside" the
     transaction and do not affect the status of the transaction in
     any way.  Exceptions to this rule are commands whose semantics
     act to terminate transactions:  Reset State commands, and Control
     Change (0xB) for controller number 121 (Reset All Controllers)
     [RP015].
   o Initiated parameter system transaction.  A canonical parameter
     system transaction whose (99, 98) or (101, 100) initial Control
     Change command pair appears in the session history is considered
     to be an initiated parameter system transaction.  This definition
     also holds for type 1 variants.  For type 2 variants (dropped
     MSB), a transaction whose initial LSB Control Change command
     appears in the session history is an initiated transaction.  For
     type 3 variants (dropped LSB), a transaction is considered to be
     initiated if at least one transaction command follows the initial
     MSB (99 or 101) Control Change command in the session history.
     The completion of a transaction does not nullify its "initiated"
     status.

Lazzaro & Wawrzynek Standards Track [Page 51] RFC 4695 RTP Payload Format for MIDI November 2006

   o Session history reference counts.  Several recovery journal
     chapters include a reference count field, which codes the total
     number of commands of a type that appear in the session history.
     Examples include the Reset and Tune Request command logs (Chapter
     D, Appendix B.1) and the Active Sense command (Chapter V,
     Appendix B.2).  Upon the detection of a loss event, reference
     count fields let a receiver deduce if any instances of the
     command have been lost, by comparing the journal reference count
     with its own reference count.  Thus, a reference count field
     makes sense, even for command types in which knowing the NUMBER
     of lost commands is irrelevant (as is true with all of the
     example commands mentioned above).
 The chapter definitions in Appendices A.2 to A.9 and B.1 to B.5
 reflect the default recovery journal behavior.  The ch_default,
 ch_never, and ch_anchor parameters modify these definitions, as
 described in Appendix C.2.3.
 The chapter definitions specify if data MUST be present in the
 journal.  Senders MAY also include non-required data in the journal.
 This optional data MUST comply with the normative chapter definition.
 For example, if a chapter definition states that a field codes data
 from the most recent active command in the session history, the
 sender MUST NOT code inactive commands or older commands in the
 field.
 Finally, we note that a channel journal only encodes information
 about MIDI commands appearing on the MIDI channel the journal
 protects.  All references to MIDI commands in Appendices A.2 to A.9
 should be read as "MIDI commands appearing on this channel."

A.2. Chapter P: MIDI Program Change

 A channel journal MUST contain Chapter P if an active Program Change
 (0xC) command appears in the checkpoint history.  Figure A.2.1 shows
 the format for Chapter P.
              0                   1                   2
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |S|   PROGRAM   |B|   BANK-MSB  |X|  BANK-LSB   |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.2.1 -- Chapter P format
 The chapter has a fixed size of 24 bits.  The PROGRAM field indicates
 the data value of the most recent active Program Change command in
 the session history.  By default, the B, BANK-MSB, X, and BANK-LSB

Lazzaro & Wawrzynek Standards Track [Page 52] RFC 4695 RTP Payload Format for MIDI November 2006

 fields MUST be set to 0.  Below, we define exceptions to this default
 condition.
 If an active Control Change (0xB) command for controller number 0
 (Bank Select MSB) appears before the Program Change command in the
 session history, the B bit MUST be set to 1, and the BANK-MSB field
 MUST code the data value of the Control Change command.
 If B is set to 1, the BANK-LSB field MUST code the data value of the
 most recent Control Change command for controller number 32 (Bank
 Select LSB) that preceded the Program Change command coded in the
 PROGRAM field and followed the Control Change command coded in the
 BANK-MSB field.  If no such Control Change command exists, the BANK-
 LSB field MUST be set to 0.
 If B is set to 1, and if a Control Change command for controller
 number 121 (Reset All Controllers) appears in the MIDI stream between
 the Control Change command coded by the BANK-MSB field and the
 Program Change command coded by the PROGRAM field, the X bit MUST be
 set to 1.
 Note that [RP015] specifies that Reset All Controllers does not reset
 the values of controller numbers 0 (Bank Select MSB) and 32 (Bank
 Select LSB).  Thus, the X bit does not effect how receivers will use
 the BANK-LSB and BANK-MSB values when recovering from a lost Program
 Change command.  The X bit serves to aid recovery in MIDI
 applications where controller numbers 0 and 32 are used in a non-
 standard way.

A.3. Chapter C: MIDI Control Change

 Figure A.3.1 shows the format for Chapter C.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|     LEN     |S|   NUMBER    |A|  VALUE/ALT  |S|   NUMBER    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |A|  VALUE/ALT  |  ....                                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure A.3.1 -- Chapter C format
 The chapter consists of a 1-octet header, followed by a variable
 length list of 2-octet controller logs.  The list MUST contain at
 least one controller log.  The 7-bit LEN field codes the number of
 controller logs in the list, minus one.  We define the semantics of
 the controller log fields in Appendix A.3.2.

Lazzaro & Wawrzynek Standards Track [Page 53] RFC 4695 RTP Payload Format for MIDI November 2006

 A channel journal MUST contain Chapter C if the rules defined in this
 appendix require that one or more controller logs appear in the list.

A.3.1. Log Inclusion Rules

 A controller log encodes information about a particular Control
 Change command in the session history.
 In the default use of the payload format, list logs MUST encode
 information about the most recent active command in the session
 history for a controller number.  Logs encoding earlier commands MUST
 NOT appear in the list.
 Also, as a rule, the list MUST contain a log for the most recent
 active command for a controller number that appears in the checkpoint
 history.  Below, we define exceptions to this rule:
   o  MIDI streams may transmit 14-bit controller values using paired
      Most Significant Byte (MSB, controller numbers 0-31, 99, 101)
      and Least Significant Byte (LSB, controller numbers 32-63, 98,
      100) Control Change commands [MIDI].
      If the most recent active Control Change command in the session
      history for a 14-bit controller pair uses the MSB number,
      Chapter C MAY omit the controller log for the most recent active
      Control Change command for the associated LSB number, as the
      command ordering makes this LSB value irrelevant.  However, this
      exception MUST NOT be applied if the sender is not certain that
      the MIDI source uses 14-bit semantics for the controller number
      pair.  Note that some MIDI sources ignore 14-bit controller
      semantics and use the LSB controller numbers as independent 7-
      bit controllers.
   o  If active Control Change commands for controller numbers 0 (Bank
      Select MSB) or 32 (Bank Select LSB) appear in the checkpoint
      history, and if the command instances are also coded in the
      BANK-MSB and BANK-LSB fields of the Chapter P (Appendix A.2),
      Chapter C MAY omit the controller logs for the commands.
   o  Several controller number pairs are defined to be mutually
      exclusive.  Controller numbers 124 (Omni Off) and 125 (Omni On)
      form a mutually exclusive pair, as do controller numbers 126
      (Mono) and 127 (Poly).
      If active Control Change commands for one or both members of a
      mutually exclusive pair appear in the checkpoint history, a log
      for the controller number of the most recent command for the
      pair in the checkpoint history MUST appear in the controller

Lazzaro & Wawrzynek Standards Track [Page 54] RFC 4695 RTP Payload Format for MIDI November 2006

      list.  However, the list MAY omit the controller log for the
      most recent active command for the other number in the pair.
      If active Control Change commands for one or both members of a
      mutually exclusive pair appear in the session history, and if a
      log for the controller number of the most recent command for the
      pair does not appear in the controller list, a log for the most
      recent command for the other number of the pair MUST NOT appear
      in the controller list.
   o  If an active Control Change command for controller number 121
      (Reset All Controllers) appears in the session history, the
      controller list MAY omit logs for Control Change commands that
      precede the Reset All Controllers command in the session
      history, under certain conditions.
      Namely, a log MAY be omitted if the sender is certain that a
      command stream follows the Reset All Controllers semantics
      defined in [RP015], and if the log codes a controller number for
      which [RP015] specifies a reset value.
      For example, [RP015] specifies that controller number 1
      (Modulation Wheel) is reset to the value 0, and thus a
      controller log for Modulation Wheel MAY be omitted from the
      controller log list.  In contrast, [RP015] specifies that
      controller number 7 (Channel Volume) is not reset, and thus a
      controller log for Channel Volume MUST NOT be omitted from the
      controller log list.
   o  Appendix A.3.4 defines exception rules for the MIDI Parameter
      System controller numbers 6, 38, and 96-101.

A.3.2. Controller Log Format

 Figure A.3.2 shows the controller log structure of Chapter C.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |S|    NUMBER   |A|  VALUE/ALT  |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure A.3.2 -- Chapter C controller log
 The 7-bit NUMBER field identifies the controller number of the coded
 command.  The 7-bit VALUE/ALT field codes recovery information for
 the command.  The A bit sets the format of the VALUE/ALT field.

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 A log encodes recovery information using one of the following tools:
 the value tool, the toggle tool, or the count tool.
 A log uses the value tool if the A bit is set to 0.  The value tool
 codes the 7-bit data value of a command in the VALUE/ALT field.  The
 value tool works best for controllers that code a continuous
 quantity, such as number 1 (Modulation Wheel).
 The A bit is set to 1 to code the toggle or count tool.  These tools
 work best for controllers that code discrete actions.  Figure A.3.3
 shows the controller log for these tools.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |S|    NUMBER   |1|T|    ALT    |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure A.3.3 -- Controller log for ALT tools
 A log uses the toggle tool if the T bit is set to 0.  A log uses the
 count tool if the T bit is set to 1.  Both methods use the 6-bit ALT
 field as an unsigned integer.
 The toggle tool works best for controllers that act as on/off
 switches, such as 64 (Damper Pedal (Sustain)).  These controllers
 code the "off" state with control values 0-63 and the "on" state with
 64-127.
 For the toggle tool, the ALT field codes the total number of toggles
 (off->on and on->off) due to Control Change commands in the session
 history, up to and including a toggle caused by the command coded by
 the log.  The toggle count includes toggles caused by Control Change
 commands for controller number 121 (Reset All Controllers).
 Toggle counting is performed modulo 64.  The toggle count is reset at
 the start of a session, and whenever a Reset State command (Appendix
 A.1) appears in the session history.  When these reset events occur,
 the toggle count for a controller is set to 0 (for controllers whose
 default value is 0-63) or 1 (for controllers whose default value is
 64-127).
 The Damper Pedal (Sustain) controller illustrates the benefits of the
 toggle tool over the value tool for switch controllers.  As often
 used in piano applications, the "on" state of the controller lets
 notes resonate, while the "off" state immediately damps notes to
 silence.  The loss of the "off" command in an "on->off->on" sequence
 results in ringing notes that should have been damped silent.  The

Lazzaro & Wawrzynek Standards Track [Page 56] RFC 4695 RTP Payload Format for MIDI November 2006

 toggle tool lets receivers detect this lost "off" command, but the
 value tool does not.
 The count tool is conceptually similar to the toggle tool.  For the
 count tool, the ALT field codes the total number of Control Change
 commands in the session history, up to and including the command
 coded by the log.  Command counting is performed modulo 64.  The
 command count is set to 0 at the start of the session and is reset to
 0 whenever a Reset State command (Appendix A.1) appears in the
 session history.
 Because the count tool ignores the data value, it is a good match for
 controllers whose controller value is ignored, such as number 123
 (All Notes Off).  More generally, the count tool may be used to code
 a (modulo 64) identification number for a command.

A.3.3. Log List Coding Rules

 In this section, we describe the organization of controller logs in
 the Chapter C log list.
 A log encodes information about a particular Control Change command
 in the session history.  In most cases, a command SHOULD be coded by
 a single tool (and, thus, a single log).  If a number is coded with a
 single tool and this tool is the count tool, recovery Control Change
 commands generated by a receiver SHOULD use the default control value
 for the controller.
 However, a command MAY be coded by several tool types (and, thus,
 several logs, each using a different tool).  This technique may
 improve recovery performance for controllers with complex semantics,
 such as controller number 84 (Portamento Control) or controller
 number 121 (Reset All Controllers) when used with a non-zero data
 octet (with the semantics described in [DLS2]).
 If a command is encoded by multiple tools, the logs MUST be placed in
 the list in the following order: count tool log (if any), followed by
 value tool log (if any), followed by toggle tool log (if any).
 The Chapter C log list MUST obey the oldest-first ordering rule
 (defined in Appendix A.1).  Note that this ordering preserves the
 information necessary for the recovery of 14-bit controller values,
 without precluding the use of MSB and LSB controller pairs as
 independent 7-bit controllers.

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 In the default use of the payload format, all logs that appear in the
 list for a controller number encode information about one Control
 Change command -- namely, the most recent active Control Change
 command in the session history for the number.
 This coding scheme provides good recovery performance for the
 standard uses of Control Change commands defined in [MIDI].  However,
 not all MIDI applications restrict the use of Control Change commands
 to those defined in [MIDI].
 For example, consider the common MIDI encoding of rotary encoders
 ("infinite" rotation knobs).  The mixing console MIDI convention
 defined in [LCP] codes the position of rotary encoders as a series of
 Control Change commands.  Each command encodes a relative change of
 knob position from the last update (expressed as a clockwise or
 counter-clockwise knob turning angle).
 As the knob position is encoded incrementally over a series of
 Control Change commands, the best recovery performance is obtained if
 the log list encodes all Control Change commands for encoder
 controller numbers that appear in the checkpoint history, not only
 the most recent command.
 To support application areas that use Control Change commands in this
 way, Chapter C may be configured to encode information about several
 Control Change commands for a controller number.  We use the term
 "enhanced" to describe this encoding method, which we describe below.
 In Appendix C.2.3, we show how to configure a stream to use enhanced
 Chapter C encoding for specific controller numbers.  In Section 5 in
 the main text, we show how the H bits in the recovery journal header
 (Figure 8) and in the channel journal header (Figure 9) indicate the
 use of enhanced Chapter C encoding.
 Here, we define how to encode a Chapter C log list that uses the
 enhanced encoding method.
 Senders that use the enhanced encoding method for a controller number
 MUST obey the rules below.  These rules let a receiver determine
 which logs in the list correspond to lost commands.  Note that these
 rules override the exceptions listed in Appendix A.3.1.
   o  If N commands for a controller number are encoded in the list,
      the commands MUST be the N most recent commands for the
      controller number in the session history.  For example, for N =
      2, the sender MUST encode the most recent command and the second
      most recent command, not the most recent command and the third
      most recent command.

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   o  If a controller number uses enhanced encoding, the encoding of
      the least-recent command for the controller number in the log
      list MUST include a count tool log.  In addition, if commands
      are encoded for the controller number whose logs have S bits set
      to 0, the encoding of the least-recent command with S = 0 logs
      MUST include a count tool log.
      The count tool is OPTIONAL for the other commands for the
      controller number encoded in the list, as a receiver is able to
      efficiently deduce the count tool value for these commands, for
      both single-packet and multi-packet loss events.
   o  The use of the value and toggle tools MUST be identical for all
      commands for a controller number encoded in the list.  For
      example, a value tool log either MUST appear for all commands
      for the controller number coded in the list, or alternatively,
      value tool logs for the controller number MUST NOT appear in the
      list.  Likewise, a toggle tool log either MUST appear for all
      commands for the controller number coded in the list, or
      alternatively, toggle tool logs for the controller number MUST
      NOT appear in the list.
   o  If a command is encoded by multiple tools, the logs MUST be
      placed in the list in the following order: count tool log (if
      any), followed by value tool log (if any), followed by toggle
      tool log (if any).
 These rules permit a receiver recovering from a packet loss to use
 the count tool log to match the commands encoded in the list with its
 own history of the stream, as we describe below.  Note that the text
 below describes a non-normative algorithm; receivers are free to use
 any algorithm to match its history with the log list.
 In a typical implementation of the enhanced encoding method, a
 receiver computes and stores count, value, and toggle tool data field
 values for the most recent Control Change command it has received for
 a controller number.
 After a loss event, a receiver parses the Chapter C list and
 processes list logs for a controller number that uses enhanced
 encoding as follows.
 The receiver compares the count tool ALT field for the least-recent
 command for the controller number in the list against its stored
 count data for the controller number, to determine if recovery is
 necessary for the command coded in the list.  The value and toggle
 tool logs (if any) that directly follow the count tool log are
 associated with this least-recent command.

Lazzaro & Wawrzynek Standards Track [Page 59] RFC 4695 RTP Payload Format for MIDI November 2006

 To check more-recent commands for the controller, the receiver
 detects additional value and/or toggle tool logs for the controller
 number in the list and infers count tool data for the command coded
 by these logs.  This inferred data is used to determine if recovery
 is necessary for the command coded by the value and/or toggle tool
 logs.
 In this way, a receiver is able to execute only lost commands,
 without executing a command twice.  While recovering from a single
 packet loss, a receiver may skip through S = 1 logs in the list, as
 the first S = 0 log for an enhanced controller number is always a
 count tool log.
 Note that the requirements in Appendix C.2.2.2 for protective sender
 and receiver actions during session startup for multicast operation
 are of particular importance for enhanced encoding, as receivers need
 to initialize its count tool data structures with recovery journal
 data in order to match commands correctly after a loss event.
 Finally, we note in passing that in some applications of rotary
 encoders, a good user experience may be possible without the use of
 enhanced encoding.  These applications are distinguished by visual
 feedback of encoding position that is driven by the post-recovery
 rotary encoding stream, and relatively low packet loss.  In these
 domains, recovery performance may be acceptable for rotary encoders
 if the log list encodes only the most recent command, if both count
 and value logs appear for the command.

A.3.4. The Parameter System

 Readers may wish to review the Appendix A.1 definitions of "parameter
 system", "parameter system transaction", and "initiated parameter
 system transaction" before reading this section.
 Parameter system transactions update a MIDI Registered Parameter
 Number (RPN) or Non-Registered Parameter Number (NRPN) value.  A
 parameter system transaction is a sequence of Control Change commands
 that may use the following controllers numbers:
   o  Data Entry MSB (6)
   o  Data Entry LSB (38)
   o  Data Increment (96)
   o  Data Decrement (97)
   o  Non-Registered Parameter Number (NRPN) LSB (98)
   o  Non-Registered Parameter Number (NRPN) MSB (99)
   o  Registered Parameter Number (RPN) LSB (100)
   o  Registered Parameter Number (RPN) MSB (101)

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 Control Change commands that are a part of a parameter system
 transaction MUST NOT be coded in Chapter C controller logs.  Instead,
 these commands are coded in Chapter M, the MIDI Parameter chapter
 defined in Appendix A.4.
 However, Control Change commands that use the listed controllers as
 general-purpose controllers (i.e., outside of a parameter system
 transaction) MUST NOT be coded in Chapter M.
 Instead, the controllers are coded in Chapter C controller logs.  The
 controller logs follow the coding rules stated in Appendix A.3.2 and
 A.3.3.  The rules for coding paired LSB and MSB controllers, as
 defined in Appendix A.3.1, apply to the pairs (6, 38), (99, 98), and
 (101, 100) when coded in Chapter C.
 If active Control Change commands for controller numbers 6, 38, or
 96-101 appear in the checkpoint history, and these commands are used
 as general-purpose controllers, the most recent general-purpose
 command instance for these controller numbers MUST appear as entries
 in the Chapter C controller list.
 MIDI syntax permits a source to use controllers 6, 38, 96, and 97 as
 parameter-system controllers and general-purpose controllers in the
 same stream.  An RTP MIDI sender MUST deduce the role of each Control
 Change command for these controller numbers by noting the placement
 of the command in the stream and MUST use this information to code
 the command in Chapter C or Chapter M, as appropriate.
 Specifically, active Control Change commands for controllers 6, 38,
 96, and 97 act in a general-purpose way when
   o  no active Control Change commands that set an RPN or NRPN
      parameter number appear in the session history, or
   o  the most recent active Control Change commands in the session
      history that set an RPN or NRPN parameter number code the null
      parameter (MSB value 0x7F, LSB value 0x7F), or
   o  a Control Change command for controller number 121 (Reset All
      Controllers) appears more recently in the session history than
      all active Control Change commands that set an RPN or NRPN
      parameter number (see [RP015] for details).
 Finally, we note that a MIDI source that follows the recommendations
 of [MIDI] exclusively uses numbers 98-101 as parameter system
 controllers.  Alternatively, a MIDI source may exclusively use 98-101
 as general-purpose controllers and lose the ability perform parameter
 system transactions in a stream.

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 In the language of [MIDI], the general-purpose use of controllers
 98-101 constitutes a non-standard controller assignment.  As most
 real-world MIDI sources use the standard controller assignment for
 controller numbers 98-101, an RTP MIDI sender SHOULD assume these
 controllers act as parameter system controllers, unless it knows that
 a MIDI source uses controller numbers 98-101 in a general-purpose
 way.

A.4. Chapter M: MIDI Parameter System

 Readers may wish to review the Appendix A.1 definitions for
 "C-active", "parameter system", "parameter system transaction", and
 "initiated parameter system transaction" before reading this
 appendix.
 Chapter M protects parameter system transactions for Registered
 Parameter Number (RPN) and Non-Registered Parameter Number (NRPN)
 values.  Figure A.4.1 shows the format for Chapter M.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|P|E|U|W|Z|      LENGTH       |Q|  PENDING    |  Log list ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure A.4.1 -- Top-level Chapter M format
 Chapter M begins with a 2-octet header.  If the P header bit is set
 to 1, a 1-octet field follows the header, coding the 7-bit PENDING
 value and its associated Q bit.
 The 10-bit LENGTH field codes the size of Chapter M and conforms to
 semantics described in Appendix A.1.
 Chapter M ends with a list of zero or more variable-length parameter
 logs.  Appendix A.4.2 defines the bitfield format of a parameter log.
 Appendix A.4.1 defines the inclusion semantics of the log list.
 A channel journal MUST contain Chapter M if the rules defined in
 Appendix A.4.1 require that one or more parameter logs appear in the
 list.
 A channel journal also MUST contain Chapter M if the most recent
 C-active Control Change command involved in a parameter system
 transaction in the checkpoint history is
   o  an RPN MSB (101) or NRPN MSB (99) controller, or

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   o  an RPN LSB (100) or NRPN LSB (98) controller that completes the
      coding of the null parameter (MSB value 0x7F, LSB value 0x7F).
 This rule provides loss protection for partially transmitted
 parameter numbers and for the null parameter numbers.
 If the most recent C-active Control Change command involved in a
 parameter system transaction in the session history is for the RPN
 MSB or NRPN MSB controller, the P header bit MUST be set to 1, and
 the PENDING field (and its associated Q bit) MUST follow the Chapter
 M header.  Otherwise, the P header bit MUST be set to 0, and the
 PENDING field and Q bit MUST NOT appear in Chapter M.
 If PENDING codes an NRPN MSB, the Q bit MUST be set to 1.  If PENDING
 codes an RPN MSB, the Q bit MUST be set to 0.
 The E header bit codes the current transaction state of the MIDI
 stream.  If E = 1, an initiated transaction is in progress.  Below,
 we define the rules for setting the E header bit:
   o  If no C-active parameter system transaction Control Change
      commands appear in the session history, the E bit MUST be set to
      0.
   o  If the P header bit is set to 1, the E bit MUST be set to 0.
   o  If the most recent C-active parameter system transaction Control
      Change command in the session history is for the NRPN LSB or RPN
      LSB controller number, and if this command acts to complete the
      coding of the null parameter (MSB value 0x7F, LSB value 0x7F),
      the E bit MUST be set to 0.
   o  Otherwise, an initiated transaction is in progress, and the E
      bit MUST be set to 1.
 The U, W, and Z header bits code properties that are shared by all
 parameter logs in the list.  If these properties are set, parameter
 logs may be coded with improved efficiency (we explain how in A.4.1).
 By default, the U, W, and Z bits MUST be set to 0.  If all parameter
 logs in the list code RPN parameters, the U bit MAY be set to 1.  If
 all parameter logs in the list code NRPN parameters, the W bit MAY be
 set to 1.  If the parameter numbers of all RPN and NRPN logs in the
 list lie in the range 0-127 (and thus have an MSB value of 0), the Z
 bit MAY be set to 1.

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 Note that C-active semantics appear in the preceding paragraphs
 because [RP015] specifies that pending Parameter System transactions
 are closed by a Control Change command for controller number 121
 (Reset All Controllers).

A.4.1. Log Inclusion Rules

 Parameter logs code recovery information for a specific RPN or NRPN
 parameter.
 A parameter log MUST appear in the list if an active Control Change
 command that forms a part of an initiated transaction for the
 parameter appears in the checkpoint history.
 An exception to this rule applies if the checkpoint history only
 contains transaction Control Change commands for controller numbers
 98-101 that act to terminate the transaction.  In this case, a log
 for the parameter MAY be omitted from the list.
 A log MAY appear in the list if an active Control Change command that
 forms a part of an initiated transaction for the parameter appears in
 the session history.  Otherwise, a log for the parameter MUST NOT
 appear in the list.
 Multiple logs for the same RPN or NRPN parameter MUST NOT appear in
 the log list.
 The parameter log list MUST obey the oldest-first ordering rule
 (defined in Appendix A.1), with the phrase "parameter transaction"
 replacing the word "command" in the rule definition.
 Parameter logs associated with the RPN or NRPN null parameter (LSB =
 0x7F, MSB = 0x7F) MUST NOT appear in the log list.  Chapter M uses
 the E header bit (Figure A.4.1) and the log list ordering rules to
 code null parameter semantics.
 Note that "active" semantics (rather than "C-active" semantics)
 appear in the preceding paragraphs because [RP015] specifies that
 pending Parameter System transactions are not reset by a Control
 Change command for controller number 121 (Reset All Controllers).
 However, the rule that follows uses C-active semantics, because it
 concerns the protection of the transaction system itself, and [RP015]
 specifies that Reset All Controllers acts to close a transaction in
 progress.

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 In most cases, parameter logs for RPN and NRPN parameters that are
 assigned to the ch_never parameter (Appendix C.2.3) MAY be omitted
 from the list.  An exception applies if
   o  the log codes the most recent initiated transaction in the
      session history, and
   o  a C-active command that forms a part of the transaction appears
      in the checkpoint history, and
   o  the E header bit for the top-level Chapter M header (Figure
      A.4.1) is set to 1.
 In this case, a log for the parameter MUST appear in the list.  This
 log informs receivers recovering from a loss that a transaction is in
 progress, so that the receiver is able to correctly interpret RPN or
 NRPN Control Change commands that follow the loss event.

A.4.2. Log Coding Rules

 Figure A.4.2 shows the parameter log structure of Chapter M.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|  PNUM-LSB   |Q|  PNUM-MSB   |J|K|L|M|N|T|V|R|   Fields ...  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.4.2 -- Parameter log format
 The log begins with a header, whose default size (as shown in Figure
 A.4.2) is 3 octets.  If the Q header bit is set to 0, the log encodes
 an RPN parameter.  If Q = 1, the log encodes an NRPN parameter.  The
 7-bit PNUM-MSB and PNUM-LSB fields code the parameter number and
 reflect the Control Change command data values for controllers 99 and
 98 (for NRPNs) or 101 and 100 (for RPNs).
 The J, K, L, M, and N header bits form a Table of Contents (TOC) for
 the log and signal the presence of fixed-sized fields that follow the
 header.  A header bit that is set to 1 codes the presence of a field
 in the log.  The ordering of fields in the log follows the ordering
 of the header bits in the TOC.  Appendices A.4.2.1-2 define the
 fields associated with each TOC header bit.
 The T and V header bits code information about the parameter log but
 are not part of the TOC.  A set T or V bit does not signal the
 presence of any parameter log field.

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 If the rules in Appendix A.4.1 state that a log for a given parameter
 MUST appear in Chapter M, the log MUST code sufficient information to
 protect the parameter from the loss of active parameter transaction
 Control Change commands in the checkpoint history.
 This rule does not apply if the parameter coded by the log is
 assigned to the ch_never parameter (Appendix C.2.3).  In this case,
 senders MAY choose to set the J, K, L, M, and N TOC bits to 0, coding
 a parameter log with no fields.
 Note that logs to protect parameters that are assigned to ch_never
 are REQUIRED under certain conditions (see Appendix A.4.1).  The
 purpose of the log is to inform receivers recovering from a loss that
 a transaction is in progress, so that the receiver is able to
 correctly interpret RPN or NRPN Control Change commands that follow
 the loss event.
 Parameter logs provide two tools for parameter protection: the value
 tool and the count tool.  Depending on the semantics of the
 parameter, senders may use either tool, both tools, or neither tool
 to protect a given parameter.
 The value tool codes information a receiver may use to determine the
 current value of an RPN or NRPN parameter.  If a parameter log uses
 the value tool, the V header bit MUST be set to 1, and the semantics
 defined in Appendices A.4.2.1 for setting the J, K, L, and M TOC bits
 MUST be followed.  If a parameter log does not use the value tool,
 the V bit MUST be set to 0, and the J, K, L, and M TOC bits MUST also
 be set to 0.
 The count tool codes the number of transactions for an RPN or NRPN
 parameter.  If a parameter log uses the count tool, the T header bit
 MUST be set to 1, and the semantics defined in Appendices A.4.2.2 for
 setting the N TOC bit MUST be followed.  If a parameter log does not
 use the count tool, the T bit and the N TOC bit MUST be set to 0.
 Note that V and T are set if the sender uses value (V) or count (T)
 tool for the log on an ongoing basis.  Thus, V may be set even if J =
 K = L = M = 0, and T may be set even if N = 0.
 In many cases, all parameters coded in the log list are of one type
 (RPN and NRPN), and all parameter numbers lie in the range 0-127.  As
 described in Appendix A.4.1, senders MAY signal this condition by
 setting the top-level Chapter M header bit Z to 1 (to code the
 restricted range) and by setting the U or W bit to 1 (to code the
 parameter type).

Lazzaro & Wawrzynek Standards Track [Page 66] RFC 4695 RTP Payload Format for MIDI November 2006

 If the top-level Chapter M header codes Z = 1 and either U = 1 or
 W = 1, all logs in the parameter log list MUST use a modified header
 format.  This modification deletes bits 8-15 of the bitfield shown in
 Figure A.4.2, to yield a 2-octet header.  The values of the deleted
 PNUM-MSB and Q fields may be inferred from the U, W, and Z bit
 values.

A.4.2.1. The Value Tool

 The value tool uses several fields to track the value of an RPN or
 NRPN parameter.
 The J TOC bit codes the presence of the octet shown in Figure A.4.3
 in the field list.
                           0
                           0 1 2 3 4 5 6 7
                          +-+-+-+-+-+-+-+-+
                          |X|  ENTRY-MSB  |
                          +-+-+-+-+-+-+-+-+
                    Figure A.4.3 -- ENTRY-MSB field
 The 7-bit ENTRY-MSB field codes the data value of the most recent
 active Control Change command for controller number 6 (Data Entry
 MSB) in the session history that appears in a transaction for the log
 parameter.
 The X bit MUST be set to 1 if the command coded by ENTRY-MSB precedes
 the most recent Control Change command for controller 121 (Reset All
 Controllers) in the session history.  Otherwise, the X bit MUST be
 set to 0.
 A parameter log that uses the value tool MUST include the ENTRY-MSB
 field if an active Control Change command for controller number 6
 appears in the checkpoint history.
 Note that [RP015] specifies that Control Change commands for
 controller 121 (Reset All Controllers) do not reset RPN and NRPN
 values, and thus the X bit would not play a recovery role for MIDI
 systems that comply with [RP015].
 However, certain renderers (such as DLS 2 [DLS2]) specify that
 certain RPN values are reset for some uses of Reset All Controllers.
 The X bit (and other bitfield features of this nature in this
 appendix) plays a role in recovery for renderers of this type.

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 The K TOC bit codes the presence of the octet shown in Figure A.4.4
 in the field list.
                           0
                           0 1 2 3 4 5 6 7
                          +-+-+-+-+-+-+-+-+
                          |X|  ENTRY-LSB  |
                          +-+-+-+-+-+-+-+-+
                    Figure A.4.4 -- ENTRY-LSB field
 The 7-bit ENTRY-LSB field codes the data value of the most recent
 active Control Change command for controller number 38 (Data Entry
 LSB) in the session history that appears in a transaction for the log
 parameter.
 The X bit MUST be set to 1 if the command coded by ENTRY-LSB precedes
 the most recent Control Change command for controller 121 (Reset All
 Controllers) in the session history.  Otherwise, the X bit MUST be
 set to 0.
 As a rule, a parameter log that uses the value tool MUST include the
 ENTRY-LSB field if an active Control Change command for controller
 number 38 appears in the checkpoint history.  However, the ENTRY-LSB
 field MUST NOT appear in a parameter log if the Control Change
 command associated with the ENTRY-LSB precedes a Control Change
 command for controller number 6 (Data Entry MSB) that appears in a
 transaction for the log parameter in the session history.
 The L TOC bit codes the presence of the octets shown in Figure A.4.5
 in the field list.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |G|X|       A-BUTTON            |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure A.4.5 -- A-BUTTON field
 The 14-bit A-BUTTON field codes a count of the number of active
 Control Change commands for controller numbers 96 and 97 (Data
 Increment and Data Decrement) in the session history that appear in a
 transaction for the log parameter.

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 The M TOC bit codes the presence of the octets shown in Figure A.4.6
 in the field list.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |G|R|       C-BUTTON            |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure A.4.6 -- C-BUTTON field
 The 14-bit C-BUTTON field has semantics identical to A-BUTTON, except
 that Data Increment and Data Decrement Control Change commands that
 precede the most recent Control Change command for controller 121
 (Reset All Controllers) in the session history are not counted.
 For both A-BUTTON and C-BUTTON, Data Increment and Data Decrement
 Control Change commands are not counted if they precede Control
 Changes commands for controller numbers 6 (Data Entry MSB) or 38
 (Data Entry LSB) that appear in a transaction for the log parameter
 in the session history.
 The A-BUTTON and C-BUTTON fields are interpreted as unsigned
 integers, and the G bit associated the field codes the sign of the
 integer (G = 0 for positive or zero, G = 1 for negative).
 To compute and code the count value, initialize the count value to 0,
 add 1 for each qualifying Data Increment command, and subtract 1 for
 each qualifying Data Decrement command.  After each add or subtract,
 limit the count magnitude to 16383.  The G bit codes the sign of the
 count, and the A-BUTTON or C-BUTTON field codes the count magnitude.
 For the A-BUTTON field, if the most recent qualified Data Increment
 or Data Decrement command precedes the most recent Control Change
 command for controller 121 (Reset All Controllers) in the session
 history, the X bit associated with A-BUTTON field MUST be set to 1.
 Otherwise, the X bit MUST be set to 0.
 A parameter log that uses the value tool MUST include the A-BUTTON
 and C-BUTTON fields if an active Control Change command for
 controller numbers 96 or 97 appears in the checkpoint history.
 However, to improve coding efficiency, this rule has several
 exceptions:
   o  If the log includes the A-BUTTON field, and if the X bit of the
      A-BUTTON field is set to 1, the C-BUTTON field (and its
      associated R and G bits) MAY be omitted from the log.

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   o  If the log includes the A-BUTTON field, and if the A-BUTTON and
      C-BUTTON fields (and their associated G bits) code identical
      values, the C-BUTTON field (and its associated R and G bits) MAY
      be omitted from the log.

A.4.2.2. The Count Tool

 The count tool tracks the number of transactions for an RPN or NRPN
 parameter.  The N TOC bit codes the presence of the octet shown in
 Figure A.4.7 in the field list.
                        0
                        0 1 2 3 4 5 6 7
                       +-+-+-+-+-+-+-+-+
                       |X|    COUNT    |
                       +-+-+-+-+-+-+-+-+
                   Figure A.4.7 -- COUNT field
 The 7-bit COUNT codes the number of initiated transactions for the
 log parameter that appear in the session history.  Initiated
 transactions are counted if they contain one or more active Control
 Change commands, including commands for controllers 98-101 that
 initiate the parameter transaction.
 If the most recent counted transaction precedes the most recent
 Control Change command for controller 121 (Reset All Controllers) in
 the session history, the X bit associated with the COUNT field MUST
 be set to 1.  Otherwise, the X bit MUST be set to 0.
 Transaction counting is performed modulo 128.  The transaction count
 is set to 0 at the start of a session and is reset to 0 whenever a
 Reset State command (Appendix A.1) appears in the session history.
 A parameter log that uses the count tool MUST include the COUNT field
 if an active command that increments the transaction count (modulo
 128) appears in the checkpoint history.

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A.5. Chapter W: MIDI Pitch Wheel

 A channel journal MUST contain Chapter W if a C-active MIDI Pitch
 Wheel (0xE) command appears in the checkpoint history.  Figure A.5.1
 shows the format for Chapter W.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |S|     FIRST   |R|    SECOND   |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.5.1 -- Chapter W format
 The chapter has a fixed size of 16 bits.  The FIRST and SECOND fields
 are the 7-bit values of the first and second data octets of the most
 recent active Pitch Wheel command in the session history.
 Note that Chapter W encodes C-active commands and thus does not
 encode active commands that are not C-active (see the second-to-last
 paragraph of Appendix A.1 for an explanation of chapter inclusion
 text in this regard).
 Chapter W does not encode "active but not C-active" commands because
 [RP015] declares that Control Change commands for controller number
 121 (Reset All Controllers) act to reset the Pitch Wheel value to 0.
 If Chapter W encoded "active but not C-active" commands, a repair
 operation following a Reset All Controllers command could incorrectly
 repair the stream with a stale Pitch Wheel value.

A.6. Chapter N: MIDI NoteOff and NoteOn

 In this appendix, we consider NoteOn commands with zero velocity to
 be NoteOff commands.  Readers may wish to review the Appendix A.1
 definition of "N-active commands" before reading this appendix.
 Chapter N completely protects note commands in streams that alternate
 between NoteOn and NoteOff commands for a particular note number.
 However, in rare applications, multiple overlapping NoteOn commands
 may appear for a note number.  Chapter E, described in Appendix A.7,
 augments Chapter N to completely protect these streams.
 A channel journal MUST contain Chapter N if an N-active MIDI NoteOn
 (0x9) or NoteOff (0x8) command appears in the checkpoint history.
 Figure A.6.1 shows the format for Chapter N.

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |B|     LEN     |  LOW  | HIGH  |S|   NOTENUM   |Y|  VELOCITY   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|   NOTENUM   |Y|  VELOCITY   |             ....              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    OFFBITS    |    OFFBITS    |     ....      |    OFFBITS    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.6.1 -- Chapter N format
 Chapter N consists of a 2-octet header, followed by at least one of
 the following data structures:
    o A list of note logs to code NoteOn commands.
    o A NoteOff bitfield structure to code NoteOff commands.
 We define the header bitfield semantics in Appendix A.6.1.  We define
 the note log semantics and the NoteOff bitfield semantics in Appendix
 A.6.2.
 If one or more N-active NoteOn or NoteOff commands in the checkpoint
 history reference a note number, the note number MUST be coded in
 either the note log list or the NoteOff bitfield structure.
 The note log list MUST contain an entry for all note numbers whose
 most recent checkpoint history appearance is in an N-active NoteOn
 command.  The NoteOff bitfield structure MUST contain a set bit for
 all note numbers whose most recent checkpoint history appearance is
 in an N-active NoteOff command.
 A note number MUST NOT be coded in both structures.
 All note logs and NoteOff bitfield set bits MUST code the most recent
 N-active NoteOn or NoteOff reference to a note number in the session
 history.
 The note log list MUST obey the oldest-first ordering rule (defined
 in Appendix A.1).

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A.6.1. Header Structure

 The header for Chapter N, shown in Figure A.6.2, codes the size of
 the note list and bitfield structures.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |B|     LEN     |  LOW  | HIGH  |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.6.2 -- Chapter N header
 The LEN field, a 7-bit integer value, codes the number of 2-octet
 note logs in the note list.  Zero is a valid value for LEN and codes
 an empty note list.
 The 4-bit LOW and HIGH fields code the number of OFFBITS octets that
 follow the note log list.  LOW and HIGH are unsigned integer values.
 If LOW <= HIGH, there are (HIGH - LOW + 1) OFFBITS octets in the
 chapter.  The value pairs (LOW = 15, HIGH = 0) and (LOW = 15, HIGH =
 1) code an empty NoteOff bitfield structure (i.e., no OFFBITS
 octets).  Other (LOW > HIGH) value pairs MUST NOT appear in the
 header.
 The B bit provides S-bit functionality (Appendix A.1) for the NoteOff
 bitfield structure.  By default, the B bit MUST be set to 1.
 However, if the MIDI command section of the previous packet (packet I
 - 1, with I as defined in Appendix A.1) includes a NoteOff command
 for the channel, the B bit MUST be set to 0.  If the B bit is set to
 0, the higher-level recovery journal elements that contain Chapter N
 MUST have S bits that are set to 0, including the top-level journal
 header.
 The LEN value of 127 codes a note list length of 127 or 128 note
 logs, depending on the values of LOW and HIGH.  If LEN = 127, LOW =
 15, and HIGH = 0, the note list holds 128 note logs, and the NoteOff
 bitfield structure is empty.  For other values of LOW and HIGH, LEN =
 127 codes that the note list contains 127 note logs.  In this case,
 the chapter has (HIGH - LOW + 1) NoteOff OFFBITS octets if LOW <=
 HIGH and has no OFFBITS octets if LOW = 15 and HIGH = 1.

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A.6.2. Note Structures

 Figure A.6.3 shows the 2-octet note log structure.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |S|   NOTENUM   |Y|  VELOCITY   |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure A.6.3 -- Chapter N note log
 The 7-bit NOTENUM field codes the note number for the log.  A note
 number MUST NOT be represented by multiple note logs in the note
 list.
 The 7-bit VELOCITY field codes the velocity value for the most recent
 N-active NoteOn command for the note number in the session history.
 Multiple overlapping NoteOns for a given note number may be coded
 using Chapter E, as discussed in Appendix A.7.
 VELOCITY is never zero; NoteOn commands with zero velocity are coded
 as NoteOff commands in the NoteOff bitfield structure.
 The note log does not code the execution time of the NoteOn command.
 However, the Y bit codes a hint from the sender about the NoteOn
 execution time.  The Y bit codes a recommendation to play (Y = 1) or
 skip (Y = 0) the NoteOn command recovered from the note log.  See
 Section 4.2 of [RFC4696] for non-normative guidance on the use of the
 Y bit.
 Figure A.6.1 shows the NoteOff bitfield structure, as the list of
 OFFBITS octets at the end of the chapter.  A NoteOff OFFBITS octet
 codes NoteOff information for eight consecutive MIDI note numbers,
 with the most-significant bit representing the lowest note number.
 The most-significant bit of the first OFFBITS octet codes the note
 number 8*LOW; the most-significant bit of the last OFFBITS octet
 codes the note number 8*HIGH.
 A set bit codes a NoteOff command for the note number.  In the most
 efficient coding for the NoteOff bitfield structure, the first and
 last octets of the structure contain at least one set bit.  Note that
 Chapter N does not code NoteOff velocity data.
 Note that in the general case, the recovery journal does not code the
 relative placement of a NoteOff command and a Change Control command
 for controller 64 (Damper Pedal (Sustain)).  In many cases, a
 receiver processing a loss event may deduce this relative placement

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 from the history of the stream and thus determine if a NoteOff note
 is sustained by the pedal.  If such a determination is not possible,
 receivers SHOULD err on the side of silencing pedal sustains, as
 erroneously sustained notes may produce unpleasant (albeit transient)
 artifacts.

A.7. Chapter E: MIDI Note Command Extras

 Readers may wish to review the Appendix A.1 definition of "N-active
 commands" before reading this appendix.  In this appendix, a NoteOn
 command with a velocity of 0 is considered to be a NoteOff command
 with a release velocity value of 64.
 Chapter E encodes recovery information about MIDI NoteOn (0x9) and
 NoteOff (0x8) command features that rarely appear in MIDI streams.
 Receivers use Chapter E to reduce transient artifacts for streams
 where several NoteOn commands appear for a note number without an
 intervening NoteOff.  Receivers also use Chapter E to reduce
 transient artifacts for streams that use NoteOff release velocity.
 Chapter E supplements the note information coded in Chapter N
 (Appendix A.6).
 Figure A.7.1 shows the format for Chapter E.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|     LEN     |S|   NOTENUM   |V|  COUNT/VEL  |S|  NOTENUM    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |V|  COUNT/VEL  |  ....                                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure A.7.1 -- Chapter E format
 The chapter consists of a 1-octet header, followed by a variable-
 length list of 2-octet note logs.  Appendix A.7.1 defines the
 bitfield format for a note log.
 The log list MUST contain at least one note log.  The 7-bit LEN
 header field codes the number of note logs in the list, minus one.  A
 channel journal MUST contain Chapter E if the rules defined in this
 appendix require that one or more note logs appear in the list.  The
 note log list MUST obey the oldest-first ordering rule (defined in
 Appendix A.1).

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A.7.1. Note Log Format

 Figure A.7.2 reproduces the note log structure of Chapter E.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |S|   NOTENUM   |V|  COUNT/VEL  |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.7.2 -- Chapter E note log
 A note log codes information about the MIDI note number coded by the
 7-bit NOTENUM field.  The nature of the information depends on the
 value of the V flag bit.
 If the V bit is set to 1, the COUNT/VEL field codes the release
 velocity value for the most recent N-active NoteOff command for the
 note number that appears in the session history.
 If the V bit is set to 0, the COUNT/VEL field codes a reference count
 of the number of NoteOn and NoteOff commands for the note number that
 appear in the session history.
 The reference count is set to 0 at the start of the session.  NoteOn
 commands increment the count by 1.  NoteOff commands decrement the
 count by 1.  However, a decrement that generates a negative count
 value is not performed.
 If the reference count is in the range 0-126, the 7-bit COUNT/VEL
 field codes an unsigned integer representation of the count.  If the
 count is greater than or equal to 127, COUNT/VEL is set to 127.
 By default, the count is reset to 0 whenever a Reset State command
 (Appendix A.1) appears in the session history, and whenever MIDI
 Control Change commands for controller numbers 123-127 (numbers with
 All Notes Off semantics) or 120 (All Sound Off) appear in the session
 history.

A.7.2. Log Inclusion Rules

 If the most recent N-active NoteOn or NoteOff command for a note
 number in the checkpoint history is a NoteOff command with a release
 velocity value other than 64, a note log whose V bit is set to 1 MUST
 appear in Chapter E for the note number.
 If the most recent N-active NoteOn or NoteOff command for a note
 number in the checkpoint history is a NoteOff command, and if the

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 reference count for the note number is greater than 0, a note log
 whose V bit is set to 0 MUST appear in Chapter E for the note number.
 If the most recent N-active NoteOn or NoteOff command for a note
 number in the checkpoint history is a NoteOn command, and if the
 reference count for the note number is greater than 1, a note log
 whose V bit is set to 0 MUST appear in Chapter E for the note number.
 At most, two note logs MAY appear in Chapter E for a note number: one
 log whose V bit is set to 0, and one log whose V bit is set to 1.
 Chapter E codes a maximum of 128 note logs.  If the log inclusion
 rules yield more than 128 REQUIRED logs, note logs whose V bit is set
 to 1 MUST be dropped from Chapter E in order to reach the 128-log
 limit.  Note logs whose V bit is set to 0 MUST NOT be dropped.
 Most MIDI streams do not use NoteOn and NoteOff commands in ways that
 would trigger the log inclusion rules.  For these streams, Chapter E
 would never be REQUIRED to appear in a channel journal.
 The ch_never parameter (Appendix C.2.3) may be used to configure the
 log inclusion rules for Chapter E.

A.8. Chapter T: MIDI Channel Aftertouch

 A channel journal MUST contain Chapter T if an N-active and C-active
 MIDI Channel Aftertouch (0xD) command appears in the checkpoint
 history.  Figure A.8.1 shows the format for Chapter T.
                           0
                           0 1 2 3 4 5 6 7
                          +-+-+-+-+-+-+-+-+
                          |S|   PRESSURE  |
                          +-+-+-+-+-+-+-+-+
                    Figure A.8.1 -- Chapter T format
 The chapter has a fixed size of 8 bits.  The 7-bit PRESSURE field
 holds the pressure value of the most recent N-active and C-active
 Channel Aftertouch command in the session history.
 Chapter T only encodes commands that are C-active and N-active.  We
 define a C-active restriction because [RP015] declares that a Control
 Change command for controller 121 (Reset All Controllers) acts to
 reset the channel pressure to 0 (see the discussion at the end of
 Appendix A.5 for a more complete rationale).

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 We define an N-active restriction on the assumption that aftertouch
 commands are linked to note activity, and thus Channel Aftertouch
 commands that are not N-active are stale and should not be used to
 repair a stream.

A.9. Chapter A: MIDI Poly Aftertouch

 A channel journal MUST contain Chapter A if a C-active Poly
 Aftertouch (0xA) command appears in the checkpoint history.  Figure
 A.9.1 shows the format for Chapter A.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 8 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|    LEN      |S|   NOTENUM   |X|  PRESSURE   |S|   NOTENUM   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|  PRESSURE   |  ....                                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure A.9.1 -- Chapter A format
 The chapter consists of a 1-octet header, followed by a variable-
 length list of 2-octet note logs.  A note log MUST appear for a note
 number if a C-active Poly Aftertouch command for the note number
 appears in the checkpoint history.  A note number MUST NOT be
 represented by multiple note logs in the note list.  The note log
 list MUST obey the oldest-first ordering rule (defined in Appendix
 A.1).
 The 7-bit LEN field codes the number of note logs in the list, minus
 one.  Figure A.9.2 reproduces the note log structure of Chapter A.
                     0                   1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |S|   NOTENUM   |X|  PRESSURE   |
                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure A.9.2 -- Chapter A note log
 The 7-bit PRESSURE field codes the pressure value of the most recent
 C-active Poly Aftertouch command in the session history for the MIDI
 note number coded in the 7-bit NOTENUM field.
 As a rule, the X bit MUST be set to 0.  However, the X bit MUST be
 set to 1 if the command coded by the log appears before one of the
 following commands in the session history: MIDI Control Change

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 numbers 123-127 (numbers with All Notes Off semantics) or 120 (All
 Sound Off).
 We define C-active restrictions for Chapter A because [RP015]
 declares that a Control Change command for controller 121 (Reset All
 Controllers) acts to reset the polyphonic pressure to 0 (see the
 discussion at the end of Appendix A.5 for a more complete rationale).

B. The Recovery Journal System Chapters

B.1. System Chapter D: Simple System Commands

 The system journal MUST contain Chapter D if an active MIDI Reset
 (0xFF), MIDI Tune Request (0xF6), MIDI Song Select (0xF3), undefined
 MIDI System Common (0xF4 and 0xF5), or undefined MIDI System Real-
 time (0xF9 and 0xFD) command appears in the checkpoint history.
 Figure B.1.1 shows the variable-length format for Chapter D.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|B|G|H|J|K|Y|Z|  Command logs ...                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure B.1.1 -- System Chapter D format
 The chapter consists of a 1-octet header, followed by one or more
 command logs.  Header flag bits indicate the presence of command logs
 for the Reset (B = 1), Tune Request (G = 1), Song Select (H = 1),
 undefined System Common 0xF4 (J = 1), undefined System Common 0xF5 (K
 = 1), undefined System Real-time 0xF9 (Y = 1), or undefined System
 Real-time 0xFD (Z = 1) commands.
 Command logs appear in a list following the header, in the order that
 the flag bits appear in the header.
 Figure B.1.2 shows the 1-octet command log format for the Reset and
 Tune Request commands.
                          0
                          0 1 2 3 4 5 6 7
                         +-+-+-+-+-+-+-+-+
                         |S|    COUNT    |
                         +-+-+-+-+-+-+-+-+
           Figure B.1.2 -- Command log for Reset and Tune Request

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 Chapter D MUST contain the Reset command log if an active Reset
 command appears in the checkpoint history.  The 7-bit COUNT field
 codes the total number of Reset commands (modulo 128) present in the
 session history.
 Chapter D MUST contain the Tune Request command log if an active Tune
 Request command appears in the checkpoint history.  The 7-bit COUNT
 field codes the total number of Tune Request commands (modulo 128)
 present in the session history.
 For these commands, the COUNT field acts as a reference count.  See
 the definition of "session history reference counts" in Appendix A.1
 for more information.
 Figure B.1.3 shows the 1-octet command log format for the Song Select
 command.
                             0
                             0 1 2 3 4 5 6 7
                            +-+-+-+-+-+-+-+-+
                            |S|    VALUE    |
                            +-+-+-+-+-+-+-+-+
               Figure B.1.3 -- Song Select command log format
 Chapter D MUST contain the Song Select command log if an active Song
 Select command appears in the checkpoint history.  The 7-bit VALUE
 field codes the song number of the most recent active Song Select
 command in the session history.

B.1.1. Undefined System Commands

 In this section, we define the Chapter D command logs for the
 undefined System commands.  [MIDI] reserves the undefined System
 commands 0xF4, 0xF5, 0xF9, and 0xFD for future use.  At the time of
 this writing, any MIDI command stream that uses these commands is
 non-compliant with [MIDI].  However, future versions of [MIDI] may
 define these commands, and a few products do use these commands in a
 non-compliant manner.

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 Figure B.1.4 shows the variable-length command log format for the
 undefined System Common commands (0xF4 and 0xF5).
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|C|V|L|DSZ|      LENGTH       |    COUNT      |  VALUE ...    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  LEGAL ...                                                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Figure B.1.4 -- Undefined System Common command log format
 The command log codes a single command type (0xF4 or 0xF5, not both).
 Chapter D MUST contain a command log if an active 0xF4 command
 appears in the checkpoint history and MUST contain an independent
 command log if an active 0xF5 command appears in the checkpoint
 history.
 Chapter D consists of a two-octet header followed by a variable
 number of data fields.  Header flag bits indicate the presence of the
 COUNT field (C = 1), the VALUE field (V = 1), and the LEGAL field (L
 = 1).  The 10-bit LENGTH field codes the size of the command log and
 conforms to semantics described in Appendix A.1.
 The 2-bit DSZ field codes the number of data octets in the command
 instance that appears most recently in the session history.  If DSZ =
 0-2, the command has 0-2 data octets.  If DSZ = 3, the command has 3
 or more command data octets.
 We now define the default rules for the use of the COUNT, VALUE, and
 LEGAL fields.  The session configuration tools defined in Appendix
 C.2.3 may be used to override this behavior.
 By default, if the DSZ field is set to 0, the command log MUST
 include the COUNT field.  The 8-bit COUNT field codes the total
 number of commands of the type coded by the log (0xF4 or 0xF5)
 present in the session history, modulo 256.
 By default, if the DSZ field is set to 1-3, the command log MUST
 include the VALUE field.  The variable-length VALUE field codes a
 verbatim copy the data octets for the most recent use of the command
 type coded by the log (0xF4 or 0xF5) in the session history.  The
 most-significant bit of the final data octet MUST be set to 1, and
 the most-significant bit of all other data octets MUST be set to 0.

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 The LEGAL field is reserved for future use.  If an update to [MIDI]
 defines the 0xF4 or 0xF5 command, an IETF standards-track document
 may define the LEGAL field.  Until such a document appears, senders
 MUST NOT use the LEGAL field, and receivers MUST use the LENGTH field
 to skip over the LEGAL field.  The LEGAL field would be defined by
 the IETF if the semantics of the new 0xF4 or 0xF5 command could not
 be protected from packet loss via the use of the COUNT and VALUE
 fields.
 Figure B.1.5 shows the variable-length command log format for the
 undefined System Real-time commands (0xF9 and 0xFD).
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|C|L| LENGTH  |     COUNT     |  LEGAL ...                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Figure B.1.5 -- Undefined System Real-time command log format
 The command log codes a single command type (0xF9 or 0xFD, not both).
 Chapter D MUST contain a command log if an active 0xF9 command
 appears in the checkpoint history and MUST contain an independent
 command log if an active 0xFD command appears in the checkpoint
 history.
 Chapter D consists of a one-octet header followed by a variable
 number of data fields.  Header flag bits indicate the presence of the
 COUNT field (C = 1) and the LEGAL field (L = 1).  The 5-bit LENGTH
 field codes the size of the command log and conforms to semantics
 described in Appendix A.1.
 We now define the default rules for the use of the COUNT and LEGAL
 fields.  The session configuration tools defined in Appendix C.2.3
 may be used to override this behavior.
 The 8-bit COUNT field codes the total number of commands of the type
 coded by the log present in the session history, modulo 256.  By
 default, the COUNT field MUST be present in the command log.
 The LEGAL field is reserved for future use.  If an update to [MIDI]
 defines the 0xF9 or 0xFD command, an IETF standards-track document
 may define the LEGAL field to protect the command.  Until such a
 document appears, senders MUST NOT use the LEGAL field, and receivers
 MUST use the LENGTH field to skip over the LEGAL field.  The LEGAL
 field would be defined by the IETF if the semantics of the new 0xF9
 or 0xFD command could not be protected from packet loss via the use
 of the COUNT field.

Lazzaro & Wawrzynek Standards Track [Page 82] RFC 4695 RTP Payload Format for MIDI November 2006

 Finally, we note that some non-standard uses of the undefined System
 Real-time commands act to implement non-compliant variants of the
 MIDI sequencer system.  In Appendix B.3.1, we describe resiliency
 tools for the MIDI sequencer system that provide some protection in
 this case.

B.2. System Chapter V: Active Sense Command

 The system journal MUST contain Chapter V if an active MIDI Active
 Sense (0xFE) command appears in the checkpoint history.  Figure B.2.1
 shows the format for Chapter V.
                             0
                             0 1 2 3 4 5 6 7
                            +-+-+-+-+-+-+-+-+
                            |S|    COUNT    |
                            +-+-+-+-+-+-+-+-+
                   Figure B.2.1 -- System Chapter V format
 The 7-bit COUNT field codes the total number of Active Sense commands
 (modulo 128) present in the session history.  The COUNT field acts as
 a reference count.  See the definition of "session history reference
 counts" in Appendix A.1 for more information.

B.3. System Chapter Q: Sequencer State Commands

 This appendix describes Chapter Q, the system chapter for the MIDI
 sequencer commands.
 The system journal MUST contain Chapter Q if an active MIDI Song
 Position Pointer (0xF2), MIDI Clock (0xF8), MIDI Start (0xFA), MIDI
 Continue (0xFB), or MIDI Stop (0xFC) command appears in the
 checkpoint history, and if the rules defined in this appendix require
 a change in the Chapter Q bitfield contents because of the command
 appearance.
 Figure B.3.1 shows the variable-length format for Chapter Q.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|N|D|C|T| TOP |            CLOCK              | TIMETOOLS ... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              ...              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure B.3.1 -- System Chapter Q format

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 Chapter Q consists of a 1-octet header followed by several optional
 fields, in the order shown in Figure B.3.1.
 Header flag bits signal the presence of the 16-bit CLOCK field (C =
 1) and the 24-bit TIMETOOLS field (T = 1).  The 3-bit TOP header
 field is interpreted as an unsigned integer, as are CLOCK and
 TIMETOOLS.  We describe the TIMETOOLS field in Appendix B.3.1.
 Chapter Q encodes the most recent state of the sequencer system.
 Receivers use the chapter to re-synchronize the sequencer after a
 packet loss episode.  Chapter fields encode the on/off state of the
 sequencer, the current position in the song, and the downbeat.
 The N header bit encodes the relative occurrence of the Start, Stop,
 and Continue commands in the session history.  If an active Start or
 Continue command appears most recently, the N bit MUST be set to 1.
 If an active Stop appears most recently, or if no active Start, Stop,
 or Continue commands appear in the session history, the N bit MUST be
 set to 0.
 The C header flag, the TOP header field, and the CLOCK field act to
 code the current position in the sequence:
   o  If C = 1, the 3-bit TOP header field and the 16-bit CLOCK field
      are combined to form the 19-bit unsigned quantity 65536*TOP +
      CLOCK.  This value encodes the song position in units of MIDI
      Clocks (24 clocks per quarter note), modulo 524288.  Note that
      the maximum song position value that may be coded by the Song
      Position Pointer command is 98303 clocks (which may be coded
      with 17 bits), and that MIDI-coded songs are generally
      constructed to avoid durations longer than this value.  However,
      the 19-bit size may be useful for real-time applications, such
      as a drum machine MIDI output that is sending clock commands for
      long periods of time.
   o  If C = 0, the song position is the start of the song.  The C = 0
      position is identical to the position coded by C = 1, TOP = 0,
      and CLOCK = 0, for the case where the song position is less than
      524288 MIDI clocks.  In certain situations (defined later in
      this section), normative text may require the C = 0 or the C =
      1, TOP = 0, CLOCK = 0 encoding of the start of the song.
 The C, TOP, and CLOCK fields MUST be set to code the current song
 position, for both N = 0 and N = 1 conditions.  If C = 0, the TOP
 field MUST be set to 0.  See [MIDI] for a precise definition of a
 song position.

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 The D header bit encodes information about the downbeat and acts to
 qualify the song position coded by the C, TOP, and CLOCK fields.
 If the D bit is set to 1, the song position represents the most
 recent position in the sequence that has played.  If D = 1, the next
 Clock command (if N = 1) or the next (Continue, Clock) pair (if
 N = 0) acts to increment the song position by one clock, and to play
 the updated position.
 If the D bit is set to 0, the song position represents a position in
 the sequence that has not yet been played.  If D = 0, the next Clock
 command (if N = 1) or the next (Continue, Clock) pair (if N = 0) acts
 to play the point in the song coded by the song position.  The song
 position is not incremented.
 An example of a stream that uses D = 0 coding is one whose most
 recent sequence command is a Start or Song Position Pointer command
 (both N = 1 conditions).  However, it is also possible to construct
 examples where D = 0 and N = 0.  A Start command immediately followed
 by a Stop command is coded in Chapter Q by setting C = 0, D = 0,
 N = 0, TOP = 0.
 If N = 1 (coding Start or Continue), D = 0 (coding that the downbeat
 has yet to be played), and the song position is at the start of the
 song, the C = 0 song position encoding MUST be used if a Start
 command occurs more recently than a Continue command in the session
 history, and the C = 1, TOP = 0, CLOCK = 0 song position encoding
 MUST be used if a Continue command occurs more recently than a Start
 command in the session history.

B.3.1. Non-compliant Sequencers

 The Chapter Q description in this appendix assumes that the sequencer
 system counts off time with Clock commands, as mandated in [MIDI].
 However, a few non-compliant products do not use Clock commands to
 count off time, but instead use non-standard methods.
 Chapter Q uses the TIMETOOLS field to provide resiliency support for
 these non-standard products.  By default, the TIMETOOLS field MUST
 NOT appear in Chapter Q, and the T header bit MUST be set to 0.  The
 session configuration tools described in Appendix C.2.3 may be used
 to select TIMETOOLS coding.

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 Figure B.3.2 shows the format of the 24-bit TIMETOOLS field.
              0                   1                   2
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                   TIME                        |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure B.3.2 -- TIMETOOLS format
 The TIME field is a 24-bit unsigned integer quantity, with units of
 milliseconds.  TIME codes an additive correction term for the song
 position coded by the TOP, CLOCK, and C fields.  TIME is coded in
 network byte order (big-endian).
 A receiver computes the correct song position by converting TIME into
 units of MIDI clocks and adding it to 65536*TOP + CLOCK (assuming
 C = 1).  Alternatively, a receiver may convert 65536*TOP + CLOCK into
 milliseconds (assuming C = 1) and add it to TIME.  The downbeat (D
 header bit) semantics defined in Appendix B.3 apply to the corrected
 song position.

B.4. System Chapter F: MIDI Time Code Tape Position

 This appendix describes Chapter F, the system chapter for the MIDI
 Time Code (MTC) commands.  Readers may wish to review the Appendix
 A.1 definition of "finished/unfinished commands" before reading this
 appendix.
 The system journal MUST contain Chapter F if an active System Common
 Quarter Frame command (0xF1) or an active finished System Exclusive
 (Universal Real Time) MTC Full Frame command (F0 7F cc 01 01 hr mn sc
 fr F7) appears in the checkpoint history.  Otherwise, the system
 journal MUST NOT contain Chapter F.
 Figure B.4.1 shows the variable-length format for Chapter F.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|C|P|Q|D|POINT|  COMPLETE ...                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     ...       |  PARTIAL  ...                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     ...       |
    +-+-+-+-+-+-+-+-+
                  Figure B.4.1 -- System Chapter F format

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 Chapter F holds information about recent MTC tape positions coded in
 the session history.  Receivers use Chapter F to re-synchronize the
 MTC system after a packet loss episode.
 Chapter F consists of a 1-octet header followed by several optional
 fields, in the order shown in Figure B.4.1.  The C and P header bits
 form a Table of Contents (TOC) and signal the presence of the 32-bit
 COMPLETE field (C = 1) and the 32-bit PARTIAL field (P = 1).
 The Q header bit codes information about the COMPLETE field format.
 If Chapter F does not contain a COMPLETE field, Q MUST be set to 0.
 The D header bit codes the tape movement direction.  If the tape is
 moving forward, or if the tape direction is indeterminate, the D bit
 MUST be set to 0.  If the tape is moving in the reverse direction,
 the D bit MUST be set to 1.  In most cases, the ordering of commands
 in the session history clearly defines the tape direction.  However,
 a few command sequences have an indeterminate direction (such as a
 session history consisting of one Full Frame command).
 The 3-bit POINT header field is interpreted as an unsigned integer.
 Appendix B.4.1 defines how the POINT field codes information about
 the contents of the PARTIAL field.  If Chapter F does not contain a
 PARTIAL field, POINT MUST be set to 7 (if D = 0) or 0 (if D = 1).
 Chapter F MUST include the COMPLETE field if an active finished Full
 Frame command appears in the checkpoint history, or if an active
 Quarter Frame command that completes the encoding of a frame value
 appears in the checkpoint history.
 The COMPLETE field encodes the most recent active complete MTC frame
 value that appears in the session history.  This frame value may take
 the form of a series of 8 active Quarter Frame commands (0xF1 0x0n
 through 0xF1 0x7n for forward tape movement, 0xF1 0x7n through 0xF1
 0x0n for reverse tape movement) or may take the form of an active
 finished Full Frame command.
 If the COMPLETE field encodes a Quarter Frame command series, the Q
 header bit MUST be set to 1, and the COMPLETE field MUST have the
 format shown in Figure B.4.2.  The 4-bit fields MT0 through MT7 code
 the data (lower) nibble for the Quarter Frame commands for Message
 Type 0 through Message Type 7 [MIDI].  These nibbles encode a
 complete frame value, in addition to fields reserved for future use
 by [MIDI].

Lazzaro & Wawrzynek Standards Track [Page 87] RFC 4695 RTP Payload Format for MIDI November 2006

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  MT0  |  MT1  |  MT2  |  MT3  |  MT4  |  MT5  |  MT6  |  MT7  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure B.4.2 -- COMPLETE field format, Q = 1
 In this usage, the frame value encoded in the COMPLETE field MUST be
 offset by 2 frames (relative to the frame value encoded in the
 Quarter Frame commands) if the frame value codes a 0xF1 0x0n through
 0xF1 0x7n command sequence.  This offset compensates for the two-
 frame latency of the Quarter Frame encoding for forward tape
 movement.  No offset is applied if the frame value codes a 0xF1 0x7n
 through 0xF1 0x0n Quarter Frame command sequence.
 The most recent active complete MTC frame value may alternatively be
 encoded by an active finished Full Frame command.  In this case, the
 Q header bit MUST be set to 0, and the COMPLETE field MUST have
 format shown in Figure B.4.3.  The HR, MN, SC, and FR fields
 correspond to the hr, mn, sc, and fr data octets of the Full Frame
 command.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      HR       |      MN       |      SC       |      FR       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure B.4.3 -- COMPLETE field format, Q = 0

B.4.1. Partial Frames

 The most recent active session history command that encodes MTC frame
 value data may be a Quarter Frame command other than a forward-moving
 0xF1 0x7n command (which completes a frame value for forward tape
 movement) or a reverse-moving 0xF1 0x1n command (which completes a
 frame value for reverse tape movement).
 We consider this type of Quarter Frame command to be associated with
 a partial frame value.  The Quarter Frame sequence that defines a
 partial frame value MUST either start at Message Type 0 and increment
 contiguously to an intermediate Message Type less than 7, or start at
 Message Type 7 and decrement contiguously to an intermediate Message
 type greater than 0.  A Quarter Frame command sequence that does not
 follow this pattern is not associated with a partial frame value.

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 Chapter F MUST include a PARTIAL field if the most recent active
 command in the checkpoint history that encodes MTC frame value data
 is a Quarter Frame command that is associated with a partial frame
 value.  Otherwise, Chapter F MUST NOT include a PARTIAL field.
 The partial frame value consists of the data (lower) nibbles of the
 Quarter Frame command sequence.  The PARTIAL field codes the partial
 frame value, using the format shown in Figure B.4.2.  Message Type
 fields that are not associated with a Quarter Frame command MUST be
 set to 0.
 The POINT header field indicates the Message Type fields in the
 PARTIAL field code valid data.  If P = 1, the POINT field MUST encode
 the unsigned integer value formed by the lower 3 bits of the upper
 nibble of the data value of the most recent active Quarter Frame
 command in the session history.  If D = 0 and P = 1, POINT MUST take
 on a value in the range 0-6.  If D = 1 and P = 1, POINT MUST take on
 a value in the range 1-7.
 If D = 0, MT fields (Figure B.4.2) in the inclusive range from 0 up
 to and including the POINT value encode the partial frame value.  If
 D = 1, MT fields in the inclusive range from 7 down to and including
 the POINT value encode the partial frame value.  Note that, unlike
 the COMPLETE field encoding, senders MUST NOT add a 2-frame offset to
 the partial frame value encoded in PARTIAL.
 For the default semantics, if a recovery journal contains Chapter F,
 and if the session history codes a legal [MIDI] series of Quarter
 Frame and Full Frame commands, the chapter always contains a COMPLETE
 or a PARTIAL field (and may contain both fields).  Thus, a one-octet
 Chapter F (C = P = 0) always codes the presence of an illegal command
 sequence in the session history (under some conditions, the C = 1,
 P = 0 condition may also code the presence of an illegal command
 sequence).  The illegal command sequence conditions are transient in
 nature and usually indicate that a Quarter Frame command sequence
 began with an intermediate Message Type.

B.5. System Chapter X: System Exclusive

 This appendix describes Chapter X, the system chapter for MIDI System
 Exclusive (SysEx) commands (0xF0).  Readers may wish to review the
 Appendix A.1 definition of "finished/unfinished commands" before
 reading this appendix.
 Chapter X consists of a list of one or more command logs.  Each log
 in the list codes information about a specific finished or unfinished
 SysEx command that appears in the session history.  The system

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 journal MUST contain Chapter X if the rules defined in Appendix B.5.2
 require that one or more logs appear in the list.
 The log list is not preceded by a header.  Instead, each log
 implicitly encodes its own length.  Given the length of the N'th list
 log, the presence of the (N+1)'th list log may be inferred from the
 LENGTH field of the system journal header (Figure 10 in Section 5 of
 the main text).  The log list MUST obey the oldest-first ordering
 rule (defined in Appendix A.1).

B.5.1. Chapter Format

 Figure B.5.1 shows the bitfield format for the Chapter X command log.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|T|C|F|D|L|STA|    TCOUNT     |     COUNT     |  FIRST ...    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  DATA ...                                                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure B.5.1 -- Chapter X command log format
 A Chapter X command log consists of a 1-octet header, followed by the
 optional TCOUNT, COUNT, FIRST, and DATA fields.
 The T, C, F, and D header bits act as a Table of Contents (TOC) for
 the log.  If T is set to 1, the 1-octet TCOUNT field appears in the
 log.  If C is set to 1, the 1-octet COUNT field appears in the log.
 If F is set to 1, the variable-length FIRST field appears in the log.
 If D is set to 1, the variable-length DATA field appears in the log.
 The L header bit sets the coding tool for the log.  We define the log
 coding tools in Appendix B.5.2.
 The STA field codes the status of the command coded by the log.  The
 2-bit STA value is interpreted as an unsigned integer.  If STA is 0,
 the log codes an unfinished command.  Non-zero STA values code
 different classes of finished commands.  An STA value of 1 codes a
 cancelled command, an STA value of 2 codes a command that uses the
 "dropped F7" construction, and an STA value of 3 codes all other
 finished commands.  Section 3.2 in the main text describes cancelled
 and "dropped F7" commands.
 The S bit (Appendix A.1) of the first log in the list acts as the S
 bit for Chapter X.  For the other logs in the list, the S bit refers

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 to the log itself.  The value of the "phantom" S bit associated with
 the first log is defined by the following rules:
   o  If the list codes one log, the phantom S-bit value is the same
      as the Chapter X S-bit value.
   o  If the list codes multiple logs, the phantom S-bit value is the
      logical OR of the S-bit value of the first and second command
      logs in the list.
 In all other respects, the S bit follows the semantics defined in
 Appendix A.1.
 The FIRST field (present if F = 1) encodes a variable-length unsigned
 integer value that sets the coverage of the DATA field.
 The FIRST field (present if F = 1) encodes a variable-length unsigned
 integer value that specifies which SysEx data bytes are encoded in
 the DATA field of the log.  The FIRST field consists of an octet
 whose most-significant bit is set to 0, optionally preceded by one or
 more octets whose most-significant bit is set to 1.  The algorithm
 shown in Figure B.5.2 decodes this format into an unsigned integer,
 to yield the value dec(FIRST).  FIRST uses a variable-length encoding
 because dec(FIRST) references a data octet in a SysEx command, and a
 SysEx command may contain an arbitrary number of data octets.
      One-Octet FIRST value:
         Encoded form: 0ddddddd
         Decoded form: 00000000 00000000 00000000 0ddddddd
      Two-Octet FIRST value:
         Encoded form: 1ccccccc 0ddddddd
         Decoded form: 00000000 00000000 00cccccc cddddddd
      Three-Octet FIRST value:
         Encoded form: 1bbbbbbb 1ccccccc 0ddddddd
         Decoded form: 00000000 000bbbbb bbcccccc cddddddd
      Four-Octet FIRST value:
         Encoded form: 1aaaaaaa 1bbbbbbb 1ccccccc 0ddddddd
         Decoded form: 0000aaaa aaabbbbb bbcccccc cddddddd
              Figure B.5.2 -- Decoding FIRST field formats

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 The DATA field (present if D = 1) encodes a modified version of the
 data octets of the SysEx command coded by the log.  Status octets
 MUST NOT be coded in the DATA field.
 If F = 0, the DATA field begins with the first data octet of the
 SysEx command and includes all subsequent data octets for the command
 that appear in the session history.  If F = 1, the DATA field begins
 with the (dec(FIRST) + 1)'th data octet of the SysEx command and
 includes all subsequent data octets for the command that appear in
 the session history.  Note that the word "command" in the
 descriptions above refers to the original SysEx command as it appears
 in the source MIDI data stream, not to a particular MIDI list SysEx
 command segment.
 The length of the DATA field is coded implicitly, using the most-
 significant bit of each octet.  The most-significant bit of the final
 octet of the DATA field MUST be set to 1.  The most-significant bit
 of all other DATA octets MUST be set to 0.  This coding method relies
 on the fact that the most-significant bit of a MIDI data octet is 0
 by definition.  Apart from this length-coding modification, the DATA
 field encodes a verbatim copy of all data octets it encodes.

B.5.2. Log Inclusion Semantics

 Chapter X offers two tools to protect SysEx commands: the "recency"
 tool and the "list" tool.  The tool definitions use the concept of
 the "SysEx type" of a command, which we now define.
 Each SysEx command instance in a session, excepting MTC Full Frame
 commands, is said to have a "SysEx type".  Types are used in equality
 comparisons: two SysEx commands in a session are said to have "the
 same SysEx type" or "different SysEx types".
 If efficiency is not a concern, a sender may follow a simple typing
 rule: every SysEx command in the session history has a different
 SysEx type, and thus no two commands in the session have the same
 type.
 To improve efficiency, senders MAY implement exceptions to this rule.
 These exceptions declare that certain sets of SysEx command instances
 have the same SysEx type.  Any command not covered by an exception
 follows the simple rule.  We list exceptions below:
   o  All commands with identical data octet fields (same number of
      data octets, same value for each data octet) have the same type.
      This rule MUST be applied to all SysEx commands in the session,
      or not at all.  Note that the implementation of this exception
      requires no sender knowledge of the format and semantics of the

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      SysEx commands in the stream, merely the ability to count and
      compare octets.
   o  Two instances of the same command whose semantics set or report
      the value of the same "parameter" have the same type.  The
      implementation of this exception requires specific knowledge of
      the format and semantics of SysEx commands.  In practice, a
      sender implementation chooses to support this exception for
      certain classes of commands (such as the Universal System
      Exclusive commands defined in [MIDI]).  If a sender supports
      this exception for a particular command in a class (for example,
      the Universal Real Time System Exclusive message for Master
      Volume, F0 F7 cc 04 01 vv vv F7, defined in [MIDI]), it MUST
      support the exception to all instances of this particular
      command in the session.
 We now use this definition of "SysEx type" to define the "recency"
 tool and the "list" tool for Chapter X.
 By default, the Chapter X log list MUST code sufficient information
 to protect the rendered MIDI performance from indefinite artifacts
 caused by the loss of all finished or unfinished active SysEx
 commands that appear in the checkpoint history (excluding finished
 MTC Full Frame commands, which are coded in Chapter F (Appendix
 B.4)).
 To protect a command of a specific SysEx type with the recency tool,
 senders MUST code a log in the log list for the most recent finished
 active instance of the SysEx type that appears in the checkpoint
 history.  Additionally, if an unfinished active instance of the SysEx
 type appears in the checkpoint history, senders MUST code a log in
 the log list for the unfinished command instance.  The L header bit
 of both command logs MUST be set to 0.
 To protect a command of a specific SysEx type with the list tool,
 senders MUST code a log in the Chapter X log list for each finished
 or unfinished active instance of the SysEx type that appears in the
 checkpoint history.  The L header bit of list tool command logs MUST
 be set to 1.
 As a rule, a log REQUIRED by the list or recency tool MUST include a
 DATA field that codes all data octets that appear in the checkpoint
 history for the SysEx command instance associated with the log.  The
 FIRST field MAY be used to configure a DATA field that minimally
 meets this requirement.

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 An exception to this rule applies to cancelled commands (defined in
 Section 3.2).  REQUIRED command logs associated with cancelled
 commands MAY be coded with no DATA field.  However, if DATA appears
 in the log, DATA MUST code all data octets that appear in the
 checkpoint history for the command associated with the log.
 As defined by the preceding text in this section, by default all
 finished or unfinished active SysEx commands that appear in the
 checkpoint history (excluding finished MTC Full Frame commands) MUST
 be protected by the list tool or the recency tool.
 For some MIDI source streams, this default yields a Chapter X whose
 size is too large.  For example, imagine that a sender begins to
 transcode a SysEx command with 10,000 data octets onto a UDP RTP
 stream "on the fly", by sending SysEx command segments as soon as
 data octets are delivered by the MIDI source.  After 1000 octets have
 been sent, the expansion of Chapter X yields an RTP packet that is
 too large to fit in the Maximum Transmission Unit (MTU) for the
 stream.
 In this situation, if a sender uses the closed-loop sending policy
 for SysEx commands, the RTP packet size may always be capped by
 stalling the stream.  In a stream stall, once the packet reaches a
 maximum size, the sender refrains from sending new packets with non-
 empty MIDI Command Sections until receiver feedback permits the
 trimming of Chapter X.  If the stream permits arbitrary commands to
 appear between SysEx segments (selectable during configuration using
 the tools defined in Appendix C.1), the sender may stall the SysEx
 segment stream but continue to code other commands in the MIDI list.
 Stalls are a workable but sub-optimal solution to Chapter X size
 issues.  As an alternative to stalls, senders SHOULD take preemptive
 action during session configuration to reduce the anticipated size of
 Chapter X, using the methods described below:
   o  Partitioned transport.  Appendix C.5 provides tools for sending
      a MIDI name space over several RTP streams.  Senders may use
      these tools to map a MIDI source into a low-latency UDP RTP
      stream (for channel commands and short SysEx commands) and a
      reliable [RFC4571] TCP stream (for bulk-data SysEx commands).
      The cm_unused and cm_used parameters (Appendix C.1) may be used
      to communicate the nature of the SysEx command partition.  As
      TCP is reliable, the RTP MIDI TCP stream would not use the
      recovery journal.  To minimize transmission latency for short
      SysEx commands, senders may begin segmental transmission for all
      SysEx commands over the UDP stream and then cancel the UDP
      transmission of long commands (using tools described in Section
      3.2) and resend the commands over the TCP stream.

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   o  Selective protection.  Journal protection may not be necessary
      for all SysEx commands in a stream.  The ch_never parameter
      (Appendix C.2) may be used to communicate which SysEx commands
      are excluded from Chapter X.

B.5.3. TCOUNT and COUNT Fields

 If the T header bit is set to 1, the 8-bit TCOUNT field appears in
 the command log.  If the C header bit is set to 1, the 8-bit COUNT
 field appears in the command log.  TCOUNT and COUNT are interpreted
 as unsigned integers.
 The TCOUNT field codes the total number of SysEx commands of the
 SysEx type coded by the log that appear in the session history, at
 the moment after the (finished or unfinished) command coded by the
 log enters the session history.
 The COUNT field codes the total number of SysEx commands that appear
 in the session history, excluding commands that are excluded from
 Chapter X via the ch_never parameter (Appendix C.2), at the moment
 after the (finished or unfinished) command coded by the log enters
 the session history.
 Command counting for TCOUNT and COUNT uses modulo-256 arithmetic.
 MTC Full Frame command instances (Appendix B.4) are included in
 command counting if the TCOUNT and COUNT definitions warrant their
 inclusion, as are cancelled commands (Section 3.2).
 Senders use the TCOUNT and COUNT fields to track the identity and
 (for TCOUNT) the sequence position of a command instance.  Senders
 MUST use the TCOUNT or COUNT fields if identity or sequence
 information is necessary to protect the command type coded by the
 log.
 If a sender uses the COUNT field in a session, the final command log
 in every Chapter X in the stream MUST code the COUNT field.  This
 rule lets receivers resynchronize the COUNT value after a packet
 loss.

C. Session Configuration Tools

 In Sections 6.1-2 of the main text, we show session descriptions for
 minimal native and mpeg4-generic RTP MIDI streams.  Minimal streams
 lack the flexibility to support some applications.  In this appendix,
 we describe how to customize stream behavior through the use of the
 payload format parameters.

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 The appendix begins with 6 sections, each devoted to parameters that
 affect a particular aspect of stream behavior:
   o  Appendix C.1 describes the stream subsetting system (cm_unused
      and cm_used).
   o  Appendix C.2 describes the journalling system (ch_anchor,
      ch_default, ch_never, j_sec, j_update).
   o  Appendix C.3 describes MIDI command timestamp semantics
      (linerate, mperiod, octpos, tsmode).
   o  Appendix C.4 describes the temporal duration ("media time") of
      an RTP MIDI packet (guardtime, rtp_maxptime, rtp_ptime).
   o  Appendix C.5 concerns stream description (musicport).
   o  Appendix C.6 describes MIDI rendering (chanmask, cid, inline,
      multimode, render, rinit, subrender, smf_cid, smf_info,
      smf_inline, smf_url, url).
 The parameters listed above may optionally appear in session
 descriptions of RTP MIDI streams.  If these parameters are used in an
 SDP session description, the parameters appear on an fmtp attribute
 line.  This attribute line applies to the payload type associated
 with the fmtp line.
 The parameters listed above add extra functionality ("features") to
 minimal RTP MIDI streams.  In Appendix C.7, we show how to use these
 features to support two classes of applications: content-streaming
 using RTSP (Appendix C.7.1) and network musical performance using SIP
 (Appendix C.7.2).
 The participants in a multimedia session MUST share a common view of
 all of the RTP MIDI streams that appear in an RTP session, as defined
 by a single media (m=) line.  In some RTP MIDI applications, the
 "common view" restriction makes it difficult to use sendrecv streams
 (all parties send and receive), as each party has its own
 requirements.  For example, a two-party network musical performance
 application may wish to customize the renderer on each host to match
 the CPU performance of the host [NMP].
 We solve this problem by using two RTP MIDI streams -- one sendonly,
 one recvonly -- in lieu of one sendrecv stream.  The data flows in
 the two streams travel in opposite directions, to control receivers
 configured to use different renderers.  In the third example in
 Appendix C.5, we show how the musicport parameter may be used to
 define virtual sendrecv streams.

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 As a general rule, the RTP MIDI protocol does not handle parameter
 changes during a session well, because the parameters describe
 heavyweight or stateful configuration that is not easily changed once
 a session has begun.  Thus, parties SHOULD NOT expect that parameter
 change requests during a session will be accepted by other parties.
 However, implementors SHOULD support in-session parameter changes
 that are easy to handle (for example, the guardtime parameter defined
 in Appendix C.4) and SHOULD be capable of accepting requests for
 changes of those parameters, as received by its session management
 protocol (for example, re-offers in SIP [RFC3264]).
 Appendix D defines the Augmented Backus-Naur Form (ABNF, [RFC4234])
 syntax for the payload parameters.  Section 11 provides information
 to the Internet Assigned Numbers Authority (IANA) on the media types
 and parameters defined in this document.
 Appendix C.6.5 defines the media type "audio/asc", a stored object
 for initializing mpeg4-generic renderers.  As described in Appendix
 C.6, the audio/asc media type is assigned to the "rinit" parameter to
 specify an initialization data object for the default mpeg4-generic
 renderer.  Note that RTP stream semantics are not defined for
 "audio/asc".  Therefore, the "asc" subtype MUST NOT appear on the
 rtpmap line of a session description.

C.1. Configuration Tools: Stream Subsetting

 As defined in Section 3.2 in the main text, the MIDI list of an RTP
 MIDI packet may encode any MIDI command that may legally appear on a
 MIDI 1.0 DIN cable.
 In this appendix, we define two parameters (cm_unused and cm_used)
 that modify this default condition, by excluding certain types of
 MIDI commands from the MIDI list of all packets in a stream.  For
 example, if a multimedia session partitions a MIDI name space into
 two RTP MIDI streams, the parameters may be used to define which
 commands appear in each stream.
 In this appendix, we define a simple language for specifying MIDI
 command types.  If a command type is assigned to cm_unused, the
 commands coded by the string MUST NOT appear in the MIDI list.  If a
 command type is assigned to cm_used, the commands coded by the string
 MAY appear in the MIDI list.
 The parameter list may code multiple assignments to cm_used and
 cm_unused.  Assignments have a cumulative effect and are applied in
 the order of appearance in the parameter list.  A later assignment of
 a command type to the same parameter expands the scope of the earlier
 assignment.  A later assignment of a command type to the opposite

Lazzaro & Wawrzynek Standards Track [Page 97] RFC 4695 RTP Payload Format for MIDI November 2006

 parameter cancels (partially or completely) the effect of an earlier
 assignment.
 To initialize the stream subsetting system, "implicit" assignments to
 cm_unused and cm_used are processed before processing the actual
 assignments that appear in the parameter list.  The System Common
 undefined commands (0xF4, 0xF5) and the System Real-Time Undefined
 commands (0xF9, 0xFD) are implicitly assigned to cm_unused.  All
 other command types are implicitly assigned to cm_used.
 Note that the implicit assignments code the default behavior of an
 RTP MIDI stream as defined in Section 3.2 in the main text (namely,
 that all commands that may legally appear on a MIDI 1.0 DIN cable may
 appear in the stream).  Also note that assignments of the System
 Common undefined commands (0xF4, 0xF5) apply to the use of these
 commands in the MIDI source command stream, not the special use of
 0xF4 and 0xF5 in SysEx segment encoding defined in Section 3.2 in the
 main text.
 As a rule, parameter assignments obey the following syntax (see
 Appendix D for ABNF):
   <parameter> = [channel list]<command-type list>[field list]
 The command-type list is mandatory; the channel and field lists are
 optional.
 The command-type list specifies the MIDI command types for which the
 parameter applies.  The command-type list is a concatenated sequence
 of one or more of the letters (ABCFGHJKMNPQTVWXYZ).  The letters code
 the following command types:
    o  A: Poly Aftertouch (0xA)
    o  B: System Reset (0xFF)
    o  C: Control Change (0xB)
    o  F: System Time Code (0xF1)
    o  G: System Tune Request (0xF6)
    o  H: System Song Select (0xF3)
    o  J: System Common Undefined (0xF4)
    o  K: System Common Undefined (0xF5)
    o  N: NoteOff (0x8), NoteOn (0x9)
    o  P: Program Change (0xC)
    o  Q: System Sequencer (0xF2, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC)
    o  T: Channel Aftertouch (0xD)
    o  V: System Active Sense (0xFE)
    o  W: Pitch Wheel (0xE)

Lazzaro & Wawrzynek Standards Track [Page 98] RFC 4695 RTP Payload Format for MIDI November 2006

    o  X: SysEx (0xF0)
    o  Y: System Real-Time Undefined (0xF9)
    o  Z: System Real-Time Undefined (0xFD)
 In addition to the letters above, the letter M may also appear in the
 command-type list.  The letter M refers to the MIDI parameter system
 (see definition in Appendix A.1 and in [MIDI]).  An assignment of M
 to cm_unused codes that no RPN or NRPN transactions may appear in the
 MIDI list.
 Note that if cm_unused is assigned the letter M, Control Change (0xB)
 commands for the controller numbers in the standard controller
 assignment might still appear in the MIDI list.  For an explanation,
 see Appendix A.3.4 for a discussion of the "general-purpose" use of
 parameter system controller numbers.
 In the text below, rules that apply to "MIDI voice channel commands"
 also apply to the letter M.
 The letters in the command-type list MUST be uppercase and MUST
 appear in alphabetical order.  Letters other than
 (ABCFGHJKMNPQTVWXYZ) that appear in the list MUST be ignored.
 For MIDI voice channel commands, the channel list specifies the MIDI
 channels for which the parameter applies.  If no channel list is
 provided, the parameter applies to all MIDI channels (0-15).  The
 channel list takes the form of a list of channel numbers (0 through
 15) and dash-separated channel number ranges (i.e., 0-5, 8-12, etc).
 Dots (i.e., "." characters) separate elements in the channel list.
 Recall that System commands do not have a MIDI channel associated
 with them.  Thus, for most command-type letters that code System
 commands (B, F, G, H, J, K, Q, V, Y, and Z), the channel list is
 ignored.
 For the command-type letter X, the appearance of certain numbers in
 the channel list codes special semantics.
   o  The digit 0 codes that SysEx "cancel" sublists (Section 3.2 in
      the main text) MUST NOT appear in the MIDI list.
   o  The digit 1 codes that cancel sublists MAY appear in the MIDI
      list (the default condition).
   o  The digit 2 codes that commands other than System Real-time MIDI
      commands MUST NOT appear between SysEx command segments in the
      MIDI list (the default condition).

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   o  The digit 3 codes that any MIDI command type may appear between
      SysEx command segments in the MIDI list, with the exception of
      the segmented encoding of a second SysEx command (verbatim SysEx
      commands are OK).
 For command-type X, the channel list MUST NOT contain both digits 0
 and 1, and it MUST NOT contain both digits 2 and 3.  For command-type
 X, channel list numbers other than the numbers defined above are
 ignored.  If X does not have a channel list, the semantics marked
 "the default condition" in the list above apply.
 The syntax for field lists in a parameter assignment follows the
 syntax for channel lists.  If no field list is provided, the
 parameter applies to all controller or note numbers.
 For command-type C (Control Change), the field list codes the
 controller numbers (0-255) for which the parameter applies.
 For command-type M (Parameter System), the field list codes the
 Registered Parameter Numbers (RPNs) and Non-Registered Parameter
 Numbers (NRPNs) for which the parameter applies.  The number range
 0-16383 specifies RPNs, the number range 16384-32767 specifies NRPNs
 (16384 corresponds to NRPN 0, 32767 corresponds to NRPN 16383).
 For command-types N (NoteOn and NoteOff) and A (Poly Aftertouch), the
 field list codes the note numbers for which the parameter applies.
 For command-types J and K (System Common Undefined), the field list
 consists of a single digit, which specifies the number of data octets
 that follow the command octet.
 For command-type X (SysEx), the field list codes the number of data
 octets that may appear in a SysEx command.  Thus, the field list
 0-255 specifies SysEx commands with 255 or fewer data octets, the
 field list 256-4294967295 specifies SysEx commands with more than 255
 data octets but excludes commands with 255 or fewer data octets, and
 the field list 0 excludes all commands.
 A secondary parameter assignment syntax customizes command-type X
 (see Appendix D for complete ABNF):
   <parameter> = "__" <h-list> ["_" <h-list>] "__"
 The assignment defines the class of SysEx commands that obeys the
 semantics of the assigned parameter.  The command class is specified
 by listing the permitted values of the first N data octets that
 follow the SysEx 0xF0 command octet.  Any SysEx command whose first N
 data octets match the list is a member of the class.

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 Each <h-list> defines a data octet of the command, as a dot-separated
 (".") list of one or more hexadecimal constants (such as "7F") or
 dash-separated hexadecimal ranges (such as "01-1F").  Underscores
 ("_") separate each <h-list>.  Double-underscores ("__") delineate
 the data octet list.
 Using this syntax, each assignment specifies a single SysEx command
 class.  Session descriptions may use several assignments to cm_used
 and cm_unused to specify complex behaviors.
 The example session description below illustrates the use of the
 stream subsetting parameters:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP6 2001:DB80::7F2E:172A:1E24
 a=rtpmap:96 rtp-midi/44100
 a=fmtp:96 cm_unused=ACGHJKNMPTVWXYZ; cm_used=__7F_00-7F_01_01__
 The session description configures the stream for use in clock
 applications.  All voice channels are unused, as are all System
 Commands except those used for MIDI Time Code (command-type F, and
 the Full Frame SysEx command that is matched by the string assigned
 to cm_used), the System Sequencer commands (command-type Q), and
 System Reset (command-type B).

C.2. Configuration Tools: The Journalling System

 In this appendix, we define the payload format parameters that
 configure stream journalling and the recovery journal system.
 The j_sec parameter (Appendix C.2.1) sets the journalling method for
 the stream.  The j_update parameter (Appendix C.2.2) sets the
 recovery journal sending policy for the stream.  Appendix C.2.2 also
 defines the sending policies of the recovery journal system.
 Appendix C.2.3 defines several parameters that modify the recovery
 journal semantics.  These parameters change the default recovery
 journal semantics as defined in Section 5 and Appendices A-B.
 The journalling method for a stream is set at the start of a session
 and MUST NOT be changed thereafter.  This requirement forbids changes
 to the j_sec parameter once a session has begun.

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 A related requirement, defined in the appendix sections below,
 forbids the acceptance of parameter values that would violate the
 recovery journal mandate.  In many cases, a change in one of the
 parameters defined in this appendix during an ongoing session would
 result in a violation of the recovery journal mandate for an
 implementation; in this case, the parameter change MUST NOT be
 accepted.

C.2.1. The j_sec Parameter

 Section 2.2 defines the default journalling method for a stream.
 Streams that use unreliable transport (such as UDP) default to using
 the recovery journal.  Streams that use reliable transport (such as
 TCP) default to not using a journal.
 The parameter j_sec may be used to override this default.  This memo
 defines two symbolic values for j_sec: "none", to indicate that all
 stream payloads MUST NOT contain a journal section, and "recj", to
 indicate that all stream payloads MUST contain a journal section that
 uses the recovery journal format.
 For example, the j_sec parameter might be set to "none" for a UDP
 stream that travels between two hosts on a local network that is
 known to provide reliable datagram delivery.
 The session description below configures a UDP stream that does not
 use the recovery journal:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP4 192.0.2.94
 a=rtpmap:96 rtp-midi/44100
 a=fmtp:96 j_sec=none
 Other IETF standards-track documents may define alternative journal
 formats.  These documents MUST define new symbolic values for the
 j_sec parameter to signal the use of the format.
 Parties MUST NOT accept a j_sec value that violates the recovery
 journal mandate (see Section 4 for details).  If a session
 description uses a j_sec value unknown to the recipient, the
 recipient MUST NOT accept the description.

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 Special j_sec issues arise when sessions are managed by session
 management tools (like RTSP, [RFC2326]) that use SDP for "declarative
 usage" purposes (see the preamble of Section 6 for details).  For
 these session management tools, SDP does not code transport details
 (such as UDP or TCP) for the session.  Instead, server and client
 negotiate transport details via other means (for RTSP, the SETUP
 method).
 In this scenario, the use of the j_sec parameter may be ill-advised,
 as the creator of the session description may not yet know the
 transport type for the session.  In this case, the session
 description SHOULD configure the journalling system using the
 parameters defined in the remainder of Appendix C.2, but it SHOULD
 NOT use j_sec to set the journalling status.  Recall that if j_sec
 does not appear in the session description, the default method for
 choosing the journalling method is in effect (no journal for reliable
 transport, recovery journal for unreliable transport).
 However, in declarative usage situations where the creator of the
 session description knows that journalling is always required or
 never required, the session description SHOULD use the j_sec
 parameter.

C.2.2. The j_update Parameter

 In Section 4, we use the term "sending policy" to describe the method
 a sender uses to choose the checkpoint packet identity for each
 recovery journal in a stream.  In the sub-sections that follow, we
 normatively define three sending policies: anchor, closed-loop, and
 open-loop.
 As stated in Section 4, the default sending policy for a stream is
 the closed-loop policy.  The j_update parameter may be used to
 override this default.
 We define three symbolic values for j_update: "anchor", to indicate
 that the stream uses the anchor sending policy, "open-loop", to
 indicate that the stream uses the open-loop sending policy, and
 "closed-loop", to indicate that the stream uses the closed-loop
 sending policy.  See Appendix C.2.3 for examples session descriptions
 that use the j_update parameter.
 Parties MUST NOT accept a j_update value that violates the recovery
 journal mandate (Section 4).
 Other IETF standards-track documents may define additional sending
 policies for the recovery journal system.  These documents MUST
 define new symbolic values for the j_update parameter to signal the

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 use of the new policy.  If a session description uses a j_update
 value unknown to the recipient, the recipient MUST NOT accept the
 description.

C.2.2.1. The anchor Sending Policy

 In the anchor policy, the sender uses the first packet in the stream
 as the checkpoint packet for all packets in the stream.  The anchor
 policy satisfies the recovery journal mandate (Section 4), as the
 checkpoint history always covers the entire stream.
 The anchor policy does not require the use of the RTP control
 protocol (RTCP, [RFC3550]) or other feedback from receiver to sender.
 Senders do not need to take special actions to ensure that received
 streams start up free of artifacts, as the recovery journal always
 covers the entire history of the stream.  Receivers are relieved of
 the responsibility of tracking the changing identity of the
 checkpoint packet, because the checkpoint packet never changes.
 The main drawback of the anchor policy is bandwidth efficiency.
 Because the checkpoint history covers the entire stream, the size of
 the recovery journals produced by this policy usually exceeds the
 journal size of alternative policies.  For single-channel MIDI data
 streams, the bandwidth overhead of the anchor policy is often
 acceptable (see Appendix A.4 of [NMP]).  For dense streams, the
 closed-loop or open-loop policies may be more appropriate.

C.2.2.2. The closed-loop Sending Policy

 The closed-loop policy is the default policy of the recovery journal
 system.  For each packet in the stream, the policy lets senders
 choose the smallest possible checkpoint history that satisfies the
 recovery journal mandate.  As smaller checkpoint histories generally
 yield smaller recovery journals, the closed-loop policy reduces the
 bandwidth of a stream, relative to the anchor policy.
 The closed-loop policy relies on feedback from receiver to sender.
 The policy assumes that a receiver periodically informs the sender of
 the highest sequence number it has seen so far in the stream, coded
 in the 32-bit extension format defined in [RFC3550].  For RTCP,
 receivers transmit this information in the Extended Highest Sequence
 Number Received (EHSNR) field of Receiver Reports.  RTCP Sender or
 Receiver Reports MUST be sent by any participant in a session with
 closed loop sending policy, unless another feedback mechanism has
 been agreed upon.

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 The sender may safely use receiver sequence number feedback to guide
 checkpoint history management, because Section 4 requires that
 receivers repair indefinite artifacts whenever a packet loss event
 occur.
 We now normatively define the closed-loop policy.  At the moment a
 sender prepares an RTP packet for transmission, the sender is aware
 of R >= 0 receivers for the stream.  Senders may become aware of a
 receiver via RTCP traffic from the receiver, via RTP packets from a
 paired stream sent by the receiver to the sender, via messages from a
 session management tool, or by other means.  As receivers join and
 leave a session, the value of R changes.
 Each known receiver k (1 <= k <= R) is associated with a 32-bit
 extended packet sequence number M(k), where the extension reflects
 the sequence number rollover count of the sender.
 If the sender has received at least one feedback report from receiver
 k, M(k) is the most recent report of the highest RTP packet sequence
 number seen by the receiver, normalized to reflect the rollover count
 of the sender.
 If the sender has not received a feedback report from the receiver,
 M(k) is the extended sequence number of the last packet the sender
 transmitted before it became aware of the receiver.  If the sender
 became aware of this receiver before it sent the first packet in the
 stream, M(k) is the extended sequence number of the first packet in
 the stream.
 Given this definition of M(), we now state the closed-loop policy.
 When preparing a new packet for transmission, a sender MUST choose a
 checkpoint packet with extended sequence number N, such that M(k) >=
 (N - 1) for all k, 1 <= k <= R, where R >= 1.  The policy does not
 restrict sender behavior in the R == 0 (no known receivers) case.
 Under the closed-loop policy as defined above, a sender may transmit
 packets whose checkpoint history is shorter than the session history
 (as defined in Appendix A.1).  In this event, a new receiver that
 joins the stream may experience indefinite artifacts.
 For example, if a Control Change (0xB) command for Channel Volume
 (controller number 7) was sent early in a stream, and later a new
 receiver joins the session, the closed-loop policy may permit all
 packets sent to the new receiver to use a checkpoint history that
 does not include the Channel Volume Control Change command.  As a
 result, the new receiver experiences an indefinite artifact, and
 plays all notes on a channel too loudly or too softly.

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 To address this issue, the closed-loop policy states that whenever a
 sender becomes aware of a new receiver, the sender MUST determine if
 the receiver would be subject to indefinite artifacts under the
 closed-loop policy.  If so, the sender MUST ensure that the receiver
 starts the session free of indefinite artifacts.
 For example, to solve the Channel Volume issue described above, the
 sender may code the current state of the Channel Volume controller
 numbers in the recovery journal Chapter C, until it receives the
 first RTCP RR report that signals that a packet containing this
 Chapter C has been received.
 In satisfying this requirement, senders MAY infer the initial MIDI
 state of the receiver from the session description.  For example, the
 stream example in Section 6.2 has the initial state defined in [MIDI]
 for General MIDI.
 In a unicast RTP session, a receiver may safely assume that the
 sender is aware of its presence of a receiver from the first packet
 sent in the RTP stream.  However, in other types of RTP sessions
 (multicast, conference focus, RTP translator/mixer), a receiver is
 often not able to determine if the sender is initially aware of its
 presence as a receiver.
 To address this issue, the closed-loop policy states that if a
 receiver participates in a session where it may have access to a
 stream whose sender is not aware of the receiver, the receiver MUST
 take actions to ensure that its rendered MIDI performance does not
 contain indefinite artifacts.  These protections will be necessarily
 incomplete.  For example, a receiver may monitor the Checkpoint
 Packet Seqnum for uncovered loss events, and "err on the side of
 caution" with respect to handling stuck notes due to lost MIDI
 NoteOff commands, but the receiver is not able to compensate for the
 lack of Channel Volume initialization data in the recovery journal.
 The receiver MUST NOT discontinue these protective actions until it
 is certain that the sender is aware of its presence.  If a receiver
 is not able to ascertain sender awareness, the receiver MUST continue
 these protective actions for the duration of the session.
 Note that in a multicast session where all parties are expected to
 send and receive, the reception of RTCP receiver reports from the
 sender about the RTP stream a receiver is multicasting is evidence of
 the sender's awareness that the RTP stream multicast by the sender is
 being monitored by the receiver.  Receivers may also obtain sender
 awareness evidence from session management tools, or by other means.
 In practice, ongoing observation of the Checkpoint Packet Seqnum to
 determine if the sender is taking actions to prevent loss events for

Lazzaro & Wawrzynek Standards Track [Page 106] RFC 4695 RTP Payload Format for MIDI November 2006

 a receiver is a good indication of sender awareness, as is the sudden
 appearance of recovery journal chapters with numerous Control Change
 controller data that was not foreshadowed by recent commands coded in
 the MIDI list shortly after sending an RTCP RR.
 The final set of normative closed-loop policy requirements concern
 how senders and receivers handle unplanned disruptions of RTCP
 feedback from a receiver to a sender.  By "unplanned", we refer to
 disruptions that are not due to the signalled termination of an RTP
 stream, via an RTCP BYE or via session management tools.
 As defined earlier in this section, the closed-loop policy states
 that a sender MUST choose a checkpoint packet with extended sequence
 number N, such that M(k) >= (N - 1) for all k, 1 <= k <= R, where R
 >= 1.  If the sender has received at least one feedback report from
 receiver k, M(k) is the most recent report of the highest RTP packet
 sequence number seen by the receiver, normalized to reflect the
 rollover count of the sender.
 If this receiver k stops sending feedback to the sender, the M(k)
 value used by the sender reflects the last feedback report from the
 receiver.  As time progresses without feedback from receiver k, this
 fixed M(k) value forces the sender to increase the size of the
 checkpoint history, and thus increases the bandwidth of the stream.
 At some point, the sender may need to take action in order to limit
 the bandwidth of the stream.  In most envisioned uses of RTP MIDI,
 long before this point is reached, the SSRC time-out mechanism
 defined in [RFC3550] will remove the uncooperative receiver from the
 session (note that the closed-loop policy does not suggest or require
 any special sender behavior upon an SSRC time-out, other than the
 sender actions related to changing R, described earlier in this
 section).
 However, in rare situations, the bandwidth of the stream (due to a
 lack of feedback reports from the sender) may become too large to
 continue sending the stream to the receiver before the SSRC time-out
 occurs for the receiver.  In this case, the closed-loop policy states
 that the sender should invoke the SSRC time-out for the receiver
 early.
 We now discuss receiver responsibilities in the case of unplanned
 disruptions of RTCP feedback from receiver to sender.
 In the unicast case, if a sender invokes the SSRC time-out mechanism
 for a receiver, the receiver stops receiving packets from the sender.
 The sender behavior imposed by the guardtime parameter (Appendix

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 C.4.2) lets the receiver conclude that an SSRC time-out has occurred
 in a reasonable time period.
 In this case of a time-out, a receiver MUST keep sending RTCP
 feedback, in order to re-establish the RTP flow from the sender.
 Unless the receiver expects a prompt recovery of the RTP flow, the
 receiver MUST take actions to ensure that the rendered MIDI
 performance does not exhibit "very long transient artifacts" (for
 example, by silencing NoteOns to prevent stuck notes) while awaiting
 reconnection of the flow.
 In the multicast case, if a sender invokes the SSRC time-out
 mechanism for a receiver, the receiver may continue to receive
 packets, but the sender will no longer be using the M(k) feedback
 from the receiver to choose each checkpoint packet.  If the receiver
 does not have additional information that precludes an SSRC time-out
 (such as RTCP Receiver Reports from the sender about an RTP stream
 the receiver is multicasting back to the sender), the receiver MUST
 monitor the Checkpoint Packet Seqnum to detect an SSRC time-out.  If
 an SSRC time-out is detected, the receiver MUST follow the
 instructions for SSRC time-outs described for the unicast case above.
 Finally, we note that the closed-loop policy is suitable for use in
 RTP/RTCP sessions that use multicast transport.  However, aspects of
 the closed-loop policy do not scale well to sessions with large
 numbers of participants.  The sender state scales linearly with the
 number of receivers, as the sender needs to track the identity and
 M(k) value for each receiver k.  The average recovery journal size is
 not independent of the number of receivers, as the RTCP reporting
 interval backoff slows down the rate of a full update of M(k) values.
 The backoff algorithm may also increase the amount of ancillary state
 used by implementations of the normative sender and receiver
 behaviors defined in Section 4.

C.2.2.3. The open-loop Sending Policy

 The open-loop policy is suitable for sessions that are not able to
 implement the receiver-to-sender feedback required by the closed-loop
 policy, and that are also not able to use the anchor policy because
 of bandwidth constraints.
 The open-loop policy does not place constraints on how a sender
 chooses the checkpoint packet for each packet in the stream.  In the
 absence of such constraints, a receiver may find that the recovery
 journal in the packet that ends a loss event has a checkpoint history
 that does not cover the entire loss event.  We refer to loss events
 of this type as uncovered loss events.

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 To ensure that uncovered loss events do not compromise the recovery
 journal mandate, the open-loop policy assigns specific recovery tasks
 to senders, receivers, and the creators of session descriptions.  The
 underlying premise of the open-loop policy is that the indefinite
 artifacts produced during uncovered loss events fall into two
 classes.
 One class of artifacts is recoverable indefinite artifacts.
 Receivers are able to repair recoverable artifacts that occur during
 an uncovered loss event without intervention from the sender, at the
 potential cost of unpleasant transient artifacts.
 For example, after an uncovered loss event, receivers are able to
 repair indefinite artifacts due to NoteOff (0x8) commands that may
 have occurred during the loss event, by executing NoteOff commands
 for all active NoteOns commands.  This action causes a transient
 artifact (a sudden silent period in the performance), but ensures
 that no stuck notes sound indefinitely.  We refer to MIDI commands
 that are amenable to repair in this fashion as recoverable MIDI
 commands.
 A second class of artifacts is unrecoverable indefinite artifacts.
 If this class of artifact occurs during an uncovered loss event, the
 receiver is not able to repair the stream.
 For example, after an uncovered loss event, receivers are not able to
 repair indefinite artifacts due to Control Change (0xB) Channel
 Volume (controller number 7) commands that have occurred during the
 loss event.  A repair is impossible because the receiver has no way
 of determining the data value of a lost Channel Volume command.  We
 refer to MIDI commands that are fragile in this way as unrecoverable
 MIDI commands.
 The open-loop policy does not specify how to partition the MIDI
 command set into recoverable and unrecoverable commands.  Instead, it
 assumes that the creators of the session descriptions are able to
 come to agreement on a suitable recoverable/unrecoverable MIDI
 command partition for an application.
 Given these definitions, we now state the normative requirements for
 the open-loop policy.
 In the open-loop policy, the creators of the session description MUST
 use the ch_anchor parameter (defined in Appendix C.2.3) to protect
 all unrecoverable MIDI command types from indefinite artifacts, or
 alternatively MUST use the cm_unused parameter (defined in Appendix

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 C.1) to exclude the command types from the stream.  These options act
 to shield command types from artifacts during an uncovered loss
 event.
 In the open-loop policy, receivers MUST examine the Checkpoint Packet
 Seqnum field of the recovery journal header after every loss event,
 to check if the loss event is an uncovered loss event.  Section 5
 shows how to perform this check.  If an uncovered loss event has
 occurred, a receiver MUST perform indefinite artifact recovery for
 all MIDI command types that are not shielded by ch_anchor and
 cm_unused parameter assignments in the session description.
 The open-loop policy does not place specific constraints on the
 sender.  However, the open-loop policy works best if the sender
 manages the size of the checkpoint history to ensure that uncovered
 losses occur infrequently, by taking into account the delay and loss
 characteristics of the network.  Also, as each checkpoint packet
 change incurs the risk of an uncovered loss, senders should only move
 the checkpoint if it reduces the size of the journal.

C.2.3. Recovery Journal Chapter Inclusion Parameters

 The recovery journal chapter definitions (Appendices A-B) specify
 under what conditions a chapter MUST appear in the recovery journal.
 In most cases, the definition states that if a certain command
 appears in the checkpoint history, a certain chapter type MUST appear
 in the recovery journal to protect the command.
 In this section, we describe the chapter inclusion parameters.  These
 parameters modify the conditions under which a chapter appears the
 journal.  These parameters are essential to the use of the open-loop
 policy (Appendix C.2.2.3) and may also be used to simplify
 implementations of the closed-loop (Appendix C.2.2.2) and anchor
 (Appendix C.2.2.1) policies.
 Each parameter represents a type of chapter inclusion semantics.  An
 assignment to a parameter declares which chapters (or chapter
 subsets) obey the inclusion semantics.  We describe the assignment
 syntax for these parameters later in this section.
 A party MUST NOT accept chapter inclusion parameter values that
 violate the recovery journal mandate (Section 4).  All assignments of
 the subsetting parameters (cm_used and cm_unused) MUST precede the
 first assignment of a chapter inclusion parameter in the parameter
 list.

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 Below, we normatively define the semantics of the chapter inclusion
 parameters.  For clarity, we define the action of parameters on
 complete chapters.  If a parameter is assigned a subset of a chapter,
 the definition applies only to the chapter subset.
   o  ch_never.  A chapter assigned to the ch_never parameter MUST NOT
      appear in the recovery journal (Appendix A.4.1-2 defines
      exceptions to this rule for Chapter M).  To signal the exclusion
      of a chapter from the journal, an assignment to ch_never MUST be
      made, even if the commands coded by the chapter are assigned to
      cm_unused.  This rule simplifies the handling of commands types
      that may be coded in several chapters.
   o  ch_default.  A chapter assigned to the ch_default parameter MUST
      follow the default semantics for the chapter, as defined in
      Appendices A-B.
   o  ch_anchor.  A chapter assigned to the ch_anchor MUST obey a
      modified version of the default chapter semantics.  In the
      modified semantics, all references to the checkpoint history are
      replaced with references to the session history, and all
      references to the checkpoint packet are replaced with references
      to the first packet sent in the stream.
 Parameter assignments obey the following syntax (see Appendix D for
 ABNF):
   <parameter> = [channel list]<chapter list>[field list]
 The chapter list is mandatory; the channel and field lists are
 optional.  Multiple assignments to parameters have a cumulative
 effect and are applied in the order of parameter appearance in a
 media description.
 To determine the semantics of a list of chapter inclusion parameter
 assignments, we begin by assuming an implicit assignment of all
 channel and system chapters to the ch_default parameter, with the
 default values for the channel list and field list for each chapter
 that are defined below.
 We then interpret the semantics of the actual parameter assignments,
 using the rules below.
 A later assignment of a chapter to the same parameter expands the
 scope of the earlier assignment.  In most cases, a later assignment
 of a chapter to a different parameter cancels (partially or
 completely) the effect of an earlier assignment.

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 The chapter list specifies the channel or system chapters for which
 the parameter applies.  The chapter list is a concatenated sequence
 of one or more of the letters corresponding to the chapter types
 (ACDEFMNPQTVWX).  In addition, the list may contain one or more of
 the letters for the sub-chapter types (BGHJKYZ) of System Chapter D.
 The letters in a chapter list MUST be uppercase and MUST appear in
 alphabetical order.  Letters other than (ABCDEFGHJKMNPQTVWXYZ) that
 appear in the chapter list MUST be ignored.
 The channel list specifies the channel journals for which this
 parameter applies; if no channel list is provided, the parameter
 applies to all channel journals.  The channel list takes the form of
 a list of channel numbers (0 through 15) and dash-separated channel
 number ranges (i.e., 0-5, 8-12, etc.).  Dots (i.e., "." characters)
 separate elements in the channel list.
 Several of the systems chapters may be configured to have special
 semantics.  Configuration occurs by specifying a channel list for the
 systems channel, using the coding described below (note that MIDI
 Systems commands do not have a "channel", and thus the original
 purpose of the channel list does not apply to systems chapters).  The
 expression "the digit N" in the text below refers to the inclusion of
 N as a "channel" in the channel list for a systems chapter.
 For the J and K Chapter D sub-chapters (undefined System Common), the
 digit 0 codes that the parameter applies to the LEGAL field of the
 associated command log (Figure B.1.4 of Appendix B.1), the digit 1
 codes that the parameter applies to the VALUE field of the command
 log, and the digit 2 codes that the parameter applies to the COUNT
 field of the command log.
 For the Y and Z Chapter D sub-chapters (undefined System Real-time),
 the digit 0 codes that the parameter applies to the LEGAL field of
 the associated command log (Figure B.1.5 of Appendix B.1) and the
 digit 1 codes that the parameter applies to the COUNT field of the
 command log.
 For Chapter Q (Sequencer State Commands), the digit 0 codes that the
 parameter applies to the default Chapter Q definition, which forbids
 the TIME field.  The digit 1 codes that the parameter applies to the
 optional Chapter Q definition, which supports the TIME field.
 The syntax for field lists follows the syntax for channel lists.  If
 no field list is provided, the parameter applies to all controller or
 note numbers.  For Chapter C, if no field list is provided, the
 controller numbers do not use enhanced Chapter C encoding (Appendix
 A.3.3).

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 For Chapter C, the field list may take on values in the range 0 to
 255.  A field value X in the range 0-127 refers to a controller
 number X, and indicates that the controller number does not use
 enhanced Chapter C encoding.  A field value X in the range 128-255
 refers to a controller number "X minus 128" and indicates the
 controller number does use the enhanced Chapter C encoding.
 Assignments made to configure the Chapter C encoding method for a
 controller number MUST be made to the ch_default or ch_anchor
 parameters, as assignments to ch_never act to exclude the number from
 the recovery journal (and thus the indicated encoding method is
 irrelevant).
 A Chapter C field list MUST NOT encode conflicting information about
 the enhanced encoding status of a particular controller number.  For
 example, values 0 and 128 MUST NOT both be coded by a field list.
 For Chapter M, the field list codes the Registered Parameter Numbers
 (RPNs) and Non-Registered Parameter Numbers (NRPNs) for which the
 parameter applies.  The number range 0-16383 specifies RPNs, the
 number range 16384-32767 specifies NRPNs (16384 corresponds to NRPN
 0, 32767 corresponds to NRPN 16383).
 For Chapters N and A, the field list codes the note numbers for which
 the parameter applies.  The note number range specified for Chapter N
 also applies to Chapter E.
 For Chapter E, the digit 0 codes that the parameter applies to
 Chapter E note logs whose V bit is set to 0, and the digit 1 codes
 that the parameter applies to note logs whose V bit is set to 1.
 For Chapter X, the field list codes the number of data octets that
 may appear in a SysEx command that is coded in the chapter.  Thus,
 the field list 0-255 specifies SysEx commands with 255 or fewer data
 octets, the field list 256-4294967295 specifies SysEx commands with
 more than 255 data octets but excludes commands with 255 or fewer
 data octets, and the field list 0 excludes all commands.
 A secondary parameter assignment syntax customizes Chapter X (see
 Appendix D for complete ABNF):
   <parameter> = "__" <h-list> ["_" <h-list>] "__"
 The assignment defines a class of SysEx commands whose Chapter X
 coding obeys the semantics of the assigned parameter.  The command
 class is specified by listing the permitted values of the first N

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 data octets that follow the SysEx 0xF0 command octet.  Any SysEx
 command whose first N data octets match the list is a member of the
 class.
 Each <h-list> defines a data octet of the command, as a dot-separated
 (".") list of one or more hexadecimal constants (such as "7F") or
 dash-separated hexadecimal ranges (such as "01-1F").  Underscores
 ("_") separate each <h-list>.  Double-underscores ("__") delineate
 the data octet list.
 Using this syntax, each assignment specifies a single SysEx command
 class.  Session descriptions may use several assignments to the same
 (or different) parameters to specify complex Chapter X behaviors.
 The ordering behavior of multiple assignments follows the guidelines
 for chapter parameter assignments described earlier in this section.
 The example session description below illustrates the use of the
 chapter inclusion parameters:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP6 2001:DB80::7F2E:172A:1E24
 a=rtpmap:96 rtp-midi/44100
 a=fmtp:96 j_update=open-loop; cm_unused=ABCFGHJKMQTVWXYZ;
 cm_used=__7E_00-7F_09_01.02.03__;
 cm_used=__7F_00-7F_04_01.02__; cm_used=C7.64;
 ch_never=ABCDEFGHJKMQTVWXYZ; ch_never=4.11-13N;
 ch_anchor=P; ch_anchor=C7.64;
 ch_anchor=__7E_00-7F_09_01.02.03__;
 ch_anchor=__7F_00-7F_04_01.02__
 (The a=fmtp line has been wrapped to fit the page to accommodate
  memo formatting restrictions; it comprises a single line in SDP.)
 The j_update parameter codes that the stream uses the open-loop
 policy.  Most MIDI command-types are assigned to cm_unused and thus
 do not appear in the stream.  As a consequence, the assignments to
 the first ch_never parameter reflect that most chapters are not in
 use.
 Chapter N for several MIDI channels is assigned to ch_never.  Chapter
 N for MIDI channels other than 4, 11, 12, and 13 may appear in the
 recovery journal, using the (default) ch_default semantics.  In
 practice, this assignment pattern would reflect knowledge about a
 resilient rendering method in use for the excluded channels.

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 The MIDI Program Change command and several MIDI Control Change
 controller numbers are assigned to ch_anchor.  Note that the ordering
 of the ch_anchor chapter C assignment after the ch_never command acts
 to override the ch_never assignment for the listed controller numbers
 (7 and 64).
 The assignment of command-type X to cm_unused excludes most SysEx
 commands from the stream.  Exceptions are made for General MIDI
 System On/Off commands and for the Master Volume and Balance
 commands, via the use of the secondary assignment syntax.  The
 cm_used assignment codes the exception, and the ch_anchor assignment
 codes how these commands are protected in Chapter X.

C.3. Configuration Tools: Timestamp Semantics

 The MIDI command section of the payload format consists of a list of
 commands, each with an associated timestamp.  The semantics of
 command timestamps may be set during session configuration, using the
 parameters we describe in this section
 The parameter "tsmode" specifies the timestamp semantics for a
 stream.  The parameter takes on one of three token values: "comex",
 "async", or "buffer".
 The default "comex" value specifies that timestamps code the
 execution time for a command (Appendix C.3.1) and supports the
 accurate transcoding Standard MIDI Files (SMFs, [MIDI]).  The "comex"
 value is also RECOMMENDED for new MIDI user-interface controller
 designs.  The "async" value specifies an asynchronous timestamp
 sampling algorithm for time-of-arrival sources (Appendix C.3.2).  The
 "buffer" value specifies a synchronous timestamp sampling algorithm
 (Appendix C.3.3) for time-of-arrival sources.
 Ancillary parameters MAY follow tsmode in a media description.  We
 define these parameters in Appendices C.3.2-3 below.

C.3.1. The comex Algorithm

 The default "comex" (COMmand EXecution) tsmode value specifies the
 execution time for the command.  With comex, the difference between
 two timestamps indicates the time delay between the execution of the
 commands.  This difference may be zero, coding simultaneous
 execution.
 The comex interpretation of timestamps works well for transcoding a
 Standard MIDI File (SMF, [MIDI]) into an RTP MIDI stream, as SMFs
 code a timestamp for each MIDI command stored in the file.  To
 transcode an SMF that uses metric time markers, use the SMF tempo map

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 (encoded in the SMF as meta-events) to convert metric SMF timestamp
 units into seconds-based RTP timestamp units.
 New MIDI controller designs (piano keyboard, drum pads, etc.) that
 support RTP MIDI and that have direct access to sensor data SHOULD
 use comex interpretation for timestamps, so that simultaneous
 gestural events may be accurately coded by RTP MIDI.
 Comex is a poor choice for transcoding MIDI 1.0 DIN cables [MIDI],
 for a reason that we will now explain.  A MIDI DIN cable is an
 asynchronous serial protocol (320 microseconds per MIDI byte).  MIDI
 commands on a DIN cable are not tagged with timestamps.  Instead,
 MIDI DIN receivers infer command timing from the time of arrival of
 the bytes.  Thus, two two-byte MIDI commands that occur at a source
 simultaneously are encoded on a MIDI 1.0 DIN cable with a 640
 microsecond time offset.  A MIDI DIN receiver is unable to tell if
 this time offset existed in the source performance or is an artifact
 of the serial speed of the cable.  However, the RTP MIDI comex
 interpretation of timestamps declares that a timestamp offset between
 two commands reflects the timing of the source performance.
 This semantic mismatch is the reason that comex is a poor choice for
 transcoding MIDI DIN cables.  Note that the choice of the RTP
 timestamp rate (Section 6.1-2 in the main text) cannot fix this
 inaccuracy issue.  In the sections that follow, we describe two
 alternative timestamp interpretations ("async" and "buffer") that are
 a better match to MIDI 1.0 DIN cable timing, and to other MIDI time-
 of-arrival sources.
 The "octpos", "linerate", and "mperiod" ancillary parameters (defined
 below) SHOULD NOT be used with comex.

C.3.2. The async Algorithm

 The "async" tsmode value specifies the asynchronous sampling of a
 MIDI time-of-arrival source.  In asynchronous sampling, the moment an
 octet is received from a source, it is labelled with a wall-clock
 time value.  The time value has RTP timestamp units.
 The "octpos" ancillary parameter defines how RTP command timestamps
 are derived from octet time values.  If octpos has the token value
 "first", a timestamp codes the time value of the first octet of the
 command.  If octpos has the token value "last", a timestamp codes the
 time value of the last octet of the command.  If the octpos parameter
 does not appear in the media description, the sender does not know
 which octet of the command the timestamp references (for example, the
 sender may be relying on an operating system service that does not
 specify this information).

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 The octpos semantics refer to the first or last octet of a command as
 it appears on a time-of-arrival MIDI source, not as it appears in an
 RTP MIDI packet.  This distinction is significant because the RTP
 coding may contain octets that are not present in the source.  For
 example, the status octet of the first MIDI command in a packet may
 have been added to the MIDI stream during transcoding, to comply with
 the RTP MIDI running status requirements (Section 3.2).
 The "linerate" ancillary parameter defines the timespan of one MIDI
 octet on the transmission medium of the MIDI source to be sampled
 (such as a MIDI 1.0 DIN cable).  The parameter has units of
 nanoseconds, and takes on integral values.  For MIDI 1.0 DIN cables,
 the correct linerate value is 320000 (this value is also the default
 value for the parameter).
 We now show a session description example for the async algorithm.
 Consider a sender that is transcoding a MIDI 1.0 DIN cable source
 into RTP.  The sender runs on a computing platform that assigns time
 values to every incoming octet of the source, and the sender uses the
 time values to label the first octet of each command in the RTP
 packet.  This session description describes the transcoding:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP4 192.0.2.94
 a=rtpmap:96 rtp-midi/44100
 a=sendonly
 a=fmtp:96 tsmode=async; linerate=320000; octpos=first

C.3.3. The buffer Algorithm

 The "buffer" tsmode value specifies the synchronous sampling of a
 MIDI time-of-arrival source.
 In synchronous sampling, octets received from a source are placed in
 a holding buffer upon arrival.  At periodic intervals, the RTP sender
 examines the buffer.  The sender removes complete commands from the
 buffer and codes those commands in an RTP packet.  The command
 timestamp codes the moment of buffer examination, expressed in RTP
 timestamp units.  Note that several commands may have the same
 timestamp value.
 The "mperiod" ancillary parameter defines the nominal periodic
 sampling interval.  The parameter takes on positive integral values
 and has RTP timestamp units.

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 The "octpos" ancillary parameter, defined in Appendix C.3.1 for
 asynchronous sampling, plays a different role in synchronous
 sampling.  In synchronous sampling, the parameter specifies the
 timestamp semantics of a command whose octets span several sampling
 periods.
 If octpos has the token value "first", the timestamp reflects the
 arrival period of the first octet of the command.  If octpos has the
 token value "last", the timestamp reflects the arrival period of the
 last octet of the command.  The octpos semantics refer to the first
 or last octet of the command as it appears on a time-of-arrival
 source, not as it appears in the RTP packet.
 If the octpos parameter does not appear in the media description, the
 timestamp MAY reflect the arrival period of any octet of the command;
 senders use this option to signal a lack of knowledge about the
 timing details of the buffering process at sub-command granularity.
 We now show a session description example for the buffer algorithm.
 Consider a sender that is transcoding a MIDI 1.0 DIN cable source
 into RTP.  The sender runs on a computing platform that places source
 data into a buffer upon receipt.  The sender polls the buffer 1000
 times a second, extracts all complete commands from the buffer, and
 places the commands in an RTP packet.  This session description
 describes the transcoding:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP6 2001:DB80::7F2E:172A:1E24
 a=rtpmap:96 rtp-midi/44100
 a=sendonly
 a=fmtp:96 tsmode=buffer; linerate=320000; octpos=last; mperiod=44
 The mperiod value of 44 is derived by dividing the clock rate
 specified by the rtpmap attribute (44100 Hz) by the 1000 Hz buffer
 sampling rate and rounding to the nearest integer.  Command
 timestamps might not increment by exact multiples of 44, as the
 actual sampling period might not precisely match the nominal mperiod
 value.

C.4. Configuration Tools: Packet Timing Tools

 In this appendix, we describe session configuration tools for
 customizing the temporal behavior of MIDI stream packets.

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C.4.1. Packet Duration Tools

 Senders control the granularity of a stream by setting the temporal
 duration ("media time") of the packets in the stream.  Short media
 times (20 ms or less) often imply an interactive session.  Longer
 media times (100 ms or more) usually indicate a content streaming
 session.  The RTP AVP profile [RFC3551] recommends audio packet media
 times in a range from 0 to 200 ms.
 By default, an RTP receiver dynamically senses the media time of
 packets in a stream and chooses the length of its playout buffer to
 match the stream.  A receiver typically sizes its playout buffer to
 fit several audio packets and adjusts the buffer length to reflect
 the network jitter and the sender timing fidelity.
 Alternatively, the packet media time may be statically set during
 session configuration.  Session descriptions MAY use the RTP MIDI
 parameter "rtp_ptime" to set the recommended media time for a packet.
 Session descriptions MAY also use the RTP MIDI parameter
 "rtp_maxptime" to set the maximum media time for a packet permitted
 in a stream.  Both parameters MAY be used together to configure a
 stream.
 The values assigned to the rtp_ptime and rtp_maxptime parameters have
 the units of the RTP timestamp for the stream, as set by the rtpmap
 attribute (see Section 6.1).  Thus, if rtpmap sets the clock rate of
 a stream to 44100 Hz, a maximum packet media time of 10 ms is coded
 by setting rtp_maxptime=441.  As stated in the Appendix C preamble,
 the senders and receivers of a stream MUST agree on common values for
 rtp_ptime and rtp_maxptime if the parameters appear in the media
 description for the stream.
 0 ms is a reasonable media time value for MIDI packets and is often
 used in low-latency interactive applications.  In a packet with a 0
 ms media time, all commands execute at the instant they are coded by
 the packet timestamp.  The session description below configures all
 packets in the stream to have 0 ms media time:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP4 192.0.2.94
 a=rtpmap:96 rtp-midi/44100
 a=fmtp:96 rtp_ptime=0; rtp_maxptime=0

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 The session attributes ptime and maxptime [RFC4566] MUST NOT be used
 to configure an RTP MIDI stream.  Sessions MUST use rtp_ptime in lieu
 of ptime and MUST use rtp_maxptime in lieu of maxptime.  RTP MIDI
 defines its own parameters for media time configuration because 0 ms
 values for ptime and maxptime are forbidden by [RFC3264] but are
 essential for certain applications of RTP MIDI.
 See the Appendix C.7 examples for additional discussion about using
 rtp_ptime and rtp_maxptime for session configuration.

C.4.2. The guardtime Parameter

 RTP permits a sender to stop sending audio packets for an arbitrary
 period of time during a session.  When sending resumes, the RTP
 sequence number series continues unbroken, and the RTP timestamp
 value reflects the media time silence gap.
 This RTP feature has its roots in telephony, but it is also well
 matched to interactive MIDI sessions, as players may fall silent for
 several seconds during (or between) songs.
 Certain MIDI applications benefit from a slight enhancement to this
 RTP feature.  In interactive applications, receivers may use on-line
 network models to guide heuristics for handling lost and late RTP
 packets.  These models may work poorly if a sender ceases packet
 transmission for long periods of time.
 Session descriptions may use the parameter "guardtime" to set a
 minimum sending rate for a media session.  The value assigned to
 guardtime codes the maximum separation time between two sequential
 packets, as expressed in RTP timestamp units.
 Typical guardtime values are 500-2000 ms.  This value range is not a
 normative bound, and parties SHOULD be prepared to process values
 outside this range.
 The congestion control requirements for sender implementations
 (described in Section 8 and [RFC3550]) take precedence over the
 guardtime parameter.  Thus, if the guardtime parameter requests a
 minimum sending rate, but sending at this rate would violate the
 congestion control requirements, senders MUST ignore the guardtime
 parameter value.  In this case, senders SHOULD use the lowest minimum
 sending rate that satisfies the congestion control requirements.

Lazzaro & Wawrzynek Standards Track [Page 120] RFC 4695 RTP Payload Format for MIDI November 2006

 Below, we show a session description that uses the guardtime
 parameter.
 v=0
 o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP6 2001:DB80::7F2E:172A:1E24
 a=rtpmap:96 rtp-midi/44100
 a=fmtp:96 guardtime=44100; rtp_ptime=0; rtp_maxptime=0

C.5. Configuration Tools: Stream Description

 As we discussed in Section 2.1, a party may send several RTP MIDI
 streams in the same RTP session, and several RTP sessions that carry
 MIDI may appear in a multimedia session.
 By default, the MIDI name space (16 channels + systems) of each RTP
 stream sent by a party in a multimedia session is independent.  By
 independent, we mean three distinct things:
   o  If a party sends two RTP MIDI streams (A and B), MIDI voice
      channel 0 in stream A is a different "channel 0" than MIDI voice
      channel 0 in stream B.
   o  MIDI voice channel 0 in stream B is not considered to be
      "channel 16" of a 32-channel MIDI voice channel space whose
      "channel 0" is channel 0 of stream A.
   o  Streams sent by different parties over different RTP sessions,
      or over the same RTP session but with different payload type
      numbers, do not share the association that is shared by a MIDI
      cable pair that cross-connects two devices in a MIDI 1.0 DIN
      network.  By default, this association is only held by streams
      sent by different parties in the same RTP session that use the
      same payload type number.
 In this appendix, we show how to express that specific RTP MIDI
 streams in a multimedia session are not independent but instead are
 related in one of the three ways defined above.  We use two tools to
 express these relations:
   o  The musicport parameter.  This parameter is assigned a non-
      negative integer value between 0 and 4294967295.  It appears in
      the fmtp lines of payload types.

Lazzaro & Wawrzynek Standards Track [Page 121] RFC 4695 RTP Payload Format for MIDI November 2006

   o  The FID grouping attribute [RFC3388] signals that several RTP
      sessions in a multimedia session are using the musicport
      parameter to express an inter-session relationship.
 If a multimedia session has several payload types whose musicport
 parameters are assigned the same integer value, streams using these
 payload types share an "identity relationship" (including streams
 that use the same payload type).  Streams in an identity relationship
 share two properties:
   o  Identity relationship streams sent by the same party target the
      same MIDI name space.  Thus, if streams A and B share an
      identity relationship, voice channel 0 in stream A is the same
      "channel 0" as voice channel 0 in stream B.
   o  Pairs of identity relationship streams that are sent by
      different parties share the association that is shared by a MIDI
      cable pair that cross-connects two devices in a MIDI 1.0 DIN
      network.
 A party MUST NOT send two RTP MIDI streams that share an identity
 relationship in the same RTP session.  Instead, each stream MUST be
 in a separate RTP session.  As explained in Section 2.1, this
 restriction is necessary to support the RTP MIDI method for the
 synchronization of streams that share a MIDI name space.
 If a multimedia session has several payload types whose musicport
 parameters are assigned sequential values (i.e., i, i+1, ... i+k),
 the streams using the payload types share an "ordered relationship".
 For example, if payload type A assigns 2 to musicport and payload
 type B assigns 3 to musicport, A and B are in an ordered
 relationship.
 Streams in an ordered relationship that are sent by the same party
 are considered by renderers to form a single larger MIDI space.  For
 example, if stream A has a musicport value of 2 and stream B has a
 musicport value of 3, MIDI voice channel 0 in stream B is considered
 to be voice channel 16 in the larger MIDI space formed by the
 relationship.  Note that it is possible for streams to participate in
 both an identity relationship and an ordered relationship.
 We now state several rules for using musicport:
   o  If streams from several RTP sessions in a multimedia session use
      the musicport parameter, the RTP sessions MUST be grouped using
      the FID grouping attribute defined in [RFC3388].

Lazzaro & Wawrzynek Standards Track [Page 122] RFC 4695 RTP Payload Format for MIDI November 2006

   o  An ordered or identity relationship MUST NOT contain both native
      RTP MIDI streams and mpeg4-generic RTP MIDI streams.  An
      exception applies if a relationship consists of sendonly and
      recvonly (but not sendrecv) streams.  In this case, the sendonly
      streams MUST NOT contain both types of streams, and the recvonly
      streams MUST NOT contain both types of streams.
   o  It is possible to construct identity relationships that violate
      the recovery journal mandate (for example, sending NoteOns for a
      voice channel on stream A and NoteOffs for the same voice
      channel on stream B).  Parties MUST NOT generate (or accept)
      session descriptions that exhibit this flaw.
   o  Other payload formats MAY define musicport media type
      parameters.  Formats would define these parameters so that their
      sessions could be bundled into RTP MIDI name spaces.  The
      parameter definitions MUST be compatible with the musicport
      semantics defined in this appendix.
 As a rule, at most one payload type in a relationship may specify a
 MIDI renderer.  An exception to the rule applies to relationships
 that contain sendonly and recvonly streams but no sendrecv streams.
 In this case, one sendonly session and one recvonly session may each
 define a renderer.
 Renderer specification in a relationship may be done using the tools
 described in Appendix C.6.  These tools work for both native streams
 and mpeg4-generic streams.  An mpeg4-generic stream that uses the
 Appendix C.6 tools MUST set all "config" parameters to the empty
 string ("").
 Alternatively, for mpeg4-generic streams, renderer specification may
 be done by setting one "config" parameter in the relationship to the
 renderer configuration string, and all other config parameters to the
 empty string ("").
 We now define sender and receiver rules that apply when a party sends
 several streams that target the same MIDI name space.
 Senders MAY use the subsetting parameters (Appendix C.1) to predefine
 the partitioning of commands between streams, or they MAY use a
 dynamic partitioning strategy.
 Receivers that merge identity relationship streams into a single MIDI
 command stream MUST maintain the structural integrity of the MIDI
 commands coded in each stream during the merging process, in the same
 way that software that merges traditional MIDI 1.0 DIN cable flows is

Lazzaro & Wawrzynek Standards Track [Page 123] RFC 4695 RTP Payload Format for MIDI November 2006

 responsible for creating a merged command flow compatible with
 [MIDI].
 Senders MUST partition the name space so that the rendered MIDI
 performance does not contain indefinite artifacts (as defined in
 Section 4).  This responsibility holds even if all streams are sent
 over reliable transport, as different stream latencies may yield
 indefinite artifacts.  For example, stuck notes may occur in a
 performance split over two TCP streams, if NoteOn commands are sent
 on one stream and NoteOff commands are sent on the other.
 Senders MUST NOT split a Registered Parameter Name (RPN) or Non-
 Registered Parameter Name (NRPN) transaction appearing on a MIDI
 channel across multiple identity relationship sessions.  Receivers
 MUST assume that the RPN/NRPN transactions that appear on different
 identity relationship sessions are independent and MUST preserve
 transactional integrity during the MIDI merge.
 A simple way to safely partition voice channel commands is to place
 all MIDI commands for a particular voice channel into the same
 session.  Safe partitioning of MIDI Systems commands may be more
 complicated for sessions that extensively use System Exclusive.
 We now show several session description examples that use the
 musicport parameter.
 Our first session description example shows two RTP MIDI streams that
 drive the same General MIDI decoder.  The sender partitions MIDI
 commands between the streams dynamically.  The musicport values
 indicate that the streams share an identity relationship.

Lazzaro & Wawrzynek Standards Track [Page 124] RFC 4695 RTP Payload Format for MIDI November 2006

 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 a=group:FID 1 2
 c=IN IP4 192.0.2.94
 m=audio 5004 RTP/AVP 96
 a=rtpmap:96 mpeg4-generic/44100
 a=mid:1
 a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
 config=7A0A0000001A4D546864000000060000000100604D54726B0
 000000600FF2F000; musicport=12
 m=audio 5006 RTP/AVP 96
 a=rtpmap:96 mpeg4-generic/44100
 a=mid:2
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; musicport=12
 (The a=fmtp lines have been wrapped to fit the page to accommodate
  memo formatting restrictions; they comprise single lines in SDP.)
 Recall that Section 2.1 defines rules for streams that target the
 same MIDI name space.  Those rules, implemented in the example above,
 require that each stream resides in a separate RTP session, and that
 the grouping mechanisms defined in [RFC3388] signal an inter-session
 relationship.  The "group" and "mid" attribute lines implement this
 grouping mechanism.
 A variant on this example, whose session description is not shown,
 would use two streams in an identity relationship driving the same
 MIDI renderer, each with a different transport type.  One stream
 would use UDP and would be dedicated to real-time messages.  A second
 stream would use TCP [RFC4571] and would be used for SysEx bulk data
 messages.

Lazzaro & Wawrzynek Standards Track [Page 125] RFC 4695 RTP Payload Format for MIDI November 2006

 In the next example, two mpeg4-generic streams form an ordered
 relationship to drive a Structured Audio decoder with 32 MIDI voice
 channels.  Both streams reside in the same RTP session.
 v=0
 o=lazzaro 2520644554 2838152170 IN IP6 first.example.net
 s=Example
 t=0 0
 m=audio 5006 RTP/AVP 96 97
 c=IN IP6 2001:DB80::7F2E:172A:1E24
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=13; musicport=5
 a=rtpmap:97 mpeg4-generic/44100
 a=fmtp:97 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=13; musicport=6; render=synthetic;
 rinit="audio/asc";
 url="http://example.com/cardinal.asc";
 cid="azsldkaslkdjqpwojdkmsldkfpe"
 (The a=fmtp lines have been wrapped to fit the page to accommodate
  memo formatting restrictions; they comprise single lines in SDP.)
 The sequential musicport values for the two sessions establish the
 ordered relationship.  The musicport=5 session maps to Structured
 Audio extended channels range 0-15, the musicport=6 session maps to
 Structured Audio extended channels range 16-31.
 Both config strings are empty.  The configuration data is specified
 by parameters that appear in the fmtp line of the second media
 description.  We define this configuration method in Appendix C.6.

Lazzaro & Wawrzynek Standards Track [Page 126] RFC 4695 RTP Payload Format for MIDI November 2006

 The next example shows two RTP MIDI streams (one recvonly, one
 sendonly) that form a "virtual sendrecv" session.  Each stream
 resides in a different RTP session (a requirement because sendonly
 and recvonly are RTP session attributes).
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 a=group:FID 1 2
 c=IN IP4 192.0.2.94
 m=audio 5004 RTP/AVP 96
 a=sendonly
 a=rtpmap:96 mpeg4-generic/44100
 a=mid:1
 a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
 config=7A0A0000001A4D546864000000060000000100604D54726B0
 000000600FF2F000; musicport=12
 m=audio 5006 RTP/AVP 96
 a=recvonly
 a=rtpmap:96 mpeg4-generic/44100
 a=mid:2
 a=fmtp:96 streamtype=5; mode=rtp-midi; profile-level-id=12;
 config=7A0A0000001A4D546864000000060000000100604D54726B0
 000000600FF2F000; musicport=12
 (The a=fmtp lines have been wrapped to fit the page to accommodate
  memo formatting restrictions; they comprise single lines in SDP.)
 To signal the "virtual sendrecv" semantics, the two streams assign
 musicport to the same value (12).  As defined earlier in this
 section, pairs of identity relationship streams that are sent by
 different parties share the association that is shared by a MIDI
 cable pair that cross-connects two devices in a MIDI 1.0 network.  We
 use the term "virtual sendrecv" because streams sent by different
 parties in a true sendrecv session also have this property.
 As discussed in the preamble to Appendix C, the primary advantage of
 the virtual sendrecv configuration is that each party can customize
 the property of the stream it receives.  In the example above, each
 stream defines its own "config" string that could customize the
 rendering algorithm for each party (in fact, the particular strings
 shown in this example are identical, because General MIDI is not a
 configurable MPEG 4 renderer).

Lazzaro & Wawrzynek Standards Track [Page 127] RFC 4695 RTP Payload Format for MIDI November 2006

C.6. Configuration Tools: MIDI Rendering

 This appendix defines the session configuration tools for rendering.
 The "render" parameter specifies a rendering method for a stream.
 The parameter is assigned a token value that signals the top-level
 rendering class.  This memo defines four token values for render:
 "unknown", "synthetic", "api", and "null":
   o  An "unknown" renderer is a renderer whose nature is unspecified.
      It is the default renderer for native RTP MIDI streams.
   o  A "synthetic" renderer transforms the MIDI stream into audio
      output (or sometimes into stage lighting changes or other
      actions).  It is the default renderer for mpeg4-generic RTP MIDI
      streams.
   o  An "api" renderer presents the command stream to applications
      via an Application Programmer Interface (API).
   o  The "null" renderer discards the MIDI stream.
 The "null" render value plays special roles during Offer/Answer
 negotiations [RFC3264].  A party uses the "null" value in an answer
 to reject an offered renderer.  Note that rejecting a renderer is
 independent from rejecting a payload type (coded by removing the
 payload type from a media line) and rejecting a media stream (coded
 by zeroing the port of a media line that uses the renderer).
 Other render token values MAY be registered with IANA.  The token
 value MUST adhere to the ABNF for render tokens defined in Appendix
 D.  Registrations MUST include a complete specification of parameter
 value usage, similar in depth to the specifications that appear
 throughout Appendix C.6 for "synthetic" and "api" render values.  If
 a party is offered a session description that uses a render token
 value that is not known to the party, the party MUST NOT accept the
 renderer.  Options include rejecting the renderer (using the "null"
 value), the payload type, the media stream, or the session
 description.
 Other parameters MAY follow a render parameter in a parameter list.
 The additional parameters act to define the exact nature of the
 renderer.  For example, the "subrender" parameter (defined in
 Appendix C.6.2) specifies the exact nature of the renderer.
 Special rules apply to using the render parameter in an mpeg4-generic
 stream.  We define these rules in Appendix C.6.5.

Lazzaro & Wawrzynek Standards Track [Page 128] RFC 4695 RTP Payload Format for MIDI November 2006

C.6.1. The multimode Parameter

 A media description MAY contain several render parameters.  By
 default, if a parameter list includes several render parameters, a
 receiver MUST choose exactly one renderer from the list to render the
 stream.  The "multimode" parameter may be used to override this
 default.  We define two token values for multimode: "one" and "all":
   o  The default "one" value requests rendering by exactly one of the
      listed renderers.
   o  The "all" value requests the synchronized rendering of the RTP
      MIDI stream by all listed renderers, if possible.
 If the multimode parameter appears in a parameter list, it MUST
 appear before the first render parameter assignment.
 Render parameters appear in the parameter list in order of decreasing
 priority.  A receiver MAY use the priority ordering to decide which
 renderer(s) to retain in a session.
 If the "offer" in an Offer/Answer-style negotiation [RFC3264]
 contains a parameter list with one or more render parameters, the
 "answer" MUST set the render parameters of all unchosen renderers to
 "null".

C.6.2. Renderer Specification

 The render parameter (Appendix C.6 preamble) specifies, in a broad
 sense, what a renderer does with a MIDI stream.  In this appendix, we
 describe the "subrender" parameter.  The token value assigned to
 subrender defines the exact nature of the renderer.  Thus, "render"
 and "subrender" combine to define a renderer, in the same way as MIME
 types and MIME subtypes combine to define a type of media [RFC2045].
 If the subrender parameter is used for a renderer definition, it MUST
 appear immediately after the render parameter in the parameter list.
 At most one subrender parameter may appear in a renderer definition.
 This document defines one value for subrender: the value "default".
 The "default" token specifies the use of the default renderer for the
 stream type (native or mpeg4-generic).  The default renderer for
 native RTP MIDI streams is a renderer whose nature is unspecified
 (see point 6 in Section 6.1 for details).  The default renderer for
 mpeg4-generic RTP MIDI streams is an MPEG 4 Audio Object Type whose
 ID number is 13, 14, or 15 (see Section 6.2 for details).

Lazzaro & Wawrzynek Standards Track [Page 129] RFC 4695 RTP Payload Format for MIDI November 2006

 If a renderer definition does not use the subrender parameter, the
 value "default" is assumed for subrender.
 Other subrender token values may be registered with IANA.  We now
 discuss guidelines for registering subrender values.
 A subrender value is registered for a specific stream type (native or
 mpeg4-generic) and a specific render value (excluding "null" and
 "unknown").  Registrations for mpeg4-generic subrender values are
 restricted to new MPEG 4 Audio Object Types that accept MIDI input.
 The syntax of the token MUST adhere to the token definition in
 Appendix D.
 For "render=synthetic" renderers, a subrender value registration
 specifies an exact method for transforming the MIDI stream into audio
 (or sometimes into video or control actions, such as stage lighting).
 For standardized renderers, this specification is usually a pointer
 to a standards document, perhaps supplemented by RTP-MIDI-specific
 information.  For commercial products and open-source projects, this
 specification usually takes the form of instructions for interfacing
 the RTP MIDI stream with the product or project software.  A
 "render=synthetic" registration MAY specify additional Reset State
 commands for the renderer (Appendix A.1).
 A "render=api" subrender value registration specifies how an RTP MIDI
 stream interfaces with an API (Application Programmers Interface).
 This specification is usually a pointer to programmer's documentation
 for the API, perhaps supplemented by RTP-MIDI-specific information.
 A subrender registration MAY specify an initialization file (referred
 to in this document as an initialization data object) for the stream.
 The initialization data object MAY be encoded in the parameter list
 (verbatim or by reference) using the coding tools defined in Appendix
 C.6.3.  An initialization data object MUST have a registered
 [RFC4288] media type and subtype [RFC2045].
 For "render=synthetic" renderers, the data object usually encodes
 initialization data for the renderer (sample files, synthesis patch
 parameters, reverberation room impulse responses, etc.).
 For "render=api" renderers, the data object usually encodes data
 about the stream used by the API (for example, for an RTP MIDI stream
 generated by a piano keyboard controller, the manufacturer and model
 number of the keyboard, for use in GUI presentation).

Lazzaro & Wawrzynek Standards Track [Page 130] RFC 4695 RTP Payload Format for MIDI November 2006

 Usually, only one initialization object is encoded for a renderer.
 If a renderer uses multiple data objects, the correct receiver
 interpretation of multiple data objects MUST be defined in the
 subrender registration.
 A subrender value registration may also specify additional
 parameters, to appear in the parameter list immediately after
 subrender.  These parameter names MUST begin with the subrender
 value, followed by an underscore ("_"), to avoid name space
 collisions with future RTP MIDI parameter names (for example, a
 parameter "foo_bar" defined for subrender value "foo").
 We now specify guidelines for interpreting the subrender parameter
 during session configuration.
 If a party is offered a session description that uses a renderer
 whose subrender value is not known to the party, the party MUST NOT
 accept the renderer.  Options include rejecting the renderer (using
 the "null" value), the payload type, the media stream, or the session
 description.
 Receivers MUST be aware of the Reset State commands (Appendix A.1)
 for the renderer specified by the subrender parameter and MUST insure
 that the renderer does not experience indefinite artifacts due to the
 presence (or the loss) of a Reset State command.

C.6.3. Renderer Initialization

 If the renderer for a stream uses an initialization data object, an
 "rinit" parameter MUST appear in the parameter list immediately after
 the "subrender" parameter.  If the renderer parameter list does not
 include a subrender parameter (recall the semantics for "default" in
 Appendix C.6.2), the "rinit" parameter MUST appear immediately after
 the "render" parameter.
 The value assigned to the rinit parameter MUST be the media
 type/subtype [RFC2045] for the initialization data object.  If an
 initialization object type is registered with several media types,
 including audio, the assignment to rinit MUST use the audio media
 type.
 RTP MIDI supports several parameters for encoding initialization data
 objects for renderers in the parameter list: "inline", "url", and
 "cid".
 If the "inline", "url", and/or "cid" parameters are used by a
 renderer, these parameters MUST immediately follow the "rinit"
 parameter.

Lazzaro & Wawrzynek Standards Track [Page 131] RFC 4695 RTP Payload Format for MIDI November 2006

 If a "url" parameter appears for a renderer, an "inline" parameter
 MUST NOT appear.  If an "inline" parameter appears for a renderer, a
 "url" parameter MUST NOT appear.  However, neither "url" or "inline"
 is required to appear.  If neither "url" or "inline" parameters
 follow "rinit", the "cid" parameter MUST follow "rinit".
 The "inline" parameter supports the inline encoding of the data
 object.  The parameter is assigned a double-quoted Base64 [RFC2045]
 encoding of the binary data object, with no line breaks.  Appendix
 E.4 shows an example that constructs an inline parameter value.
 The "url" parameter is assigned a double-quoted string representation
 of a Uniform Resource Locator (URL) for the data object.  The string
 MUST specify a HyperText Transport Protocol URL (HTTP, [RFC2616]).
 HTTP MAY be used over TCP or MAY be used over a secure network
 transport, such as the method described in [RFC2818].  The media
 type/subtype for the data object SHOULD be specified in the
 appropriate HTTP transport header.
 The "cid" parameter supports data object caching.  The parameter is
 assigned a double-quoted string value that encodes a globally unique
 identifier for the data object.
 A cid parameter MAY immediately follow an inline parameter, in which
 case the cid identifier value MUST be associated with the inline data
 object.
 If a url parameter is present, and if the data object for the URL is
 expected to be unchanged for the life of the URL, a cid parameter MAY
 immediately follow the url parameter.  The cid identifier value MUST
 be associated with the data object for the URL.  A cid parameter
 assigned to the same identifier value SHOULD be specified following
 the data object type/subtype in the appropriate HTTP transport
 header.
 If a url parameter is present, and if the data object for the URL is
 expected to change during the life of the URL, a cid parameter MUST
 NOT follow the url parameter.  A receiver interprets the presence of
 a cid parameter as an indication that it is safe to use a cached copy
 of the url data object; the absence of a cid parameter is an
 indication that it is not safe to use a cached copy, as it may
 change.
 Finally, the cid parameter MAY be used without the inline and url
 parameters.  In this case, the identifier references a local or
 distributed catalog of data objects.

Lazzaro & Wawrzynek Standards Track [Page 132] RFC 4695 RTP Payload Format for MIDI November 2006

 In most cases, only one data object is coded in the parameter list
 for each renderer.  For example, the default renderer for mpeg4-
 generic streams uses a single data object (see Appendix C.6.5 for
 example usage).
 However, a subrender registration MAY permit the use of multiple data
 objects for a renderer.  If multiple data objects are encoded for a
 renderer, each object encoding begins with an "rinit" parameter,
 followed by "inline", "url", and/or "cid" parameters.
 Initialization data objects MAY encapsulate a Standard MIDI File
 (SMF).  By default, the SMFs that are encapsulated in a data object
 MUST be ignored by an RTP MIDI receiver.  We define parameters to
 override this default in Appendix C.6.4.
 To end this section, we offer guidelines for registering media types
 for initialization data objects.  These guidelines are in addition to
 the information in [RFC4288] [RFC4289].
 Some initialization data objects are also capable of encoding MIDI
 note information and thus complete audio performances.  These objects
 SHOULD be registered using the "audio" media type, so that the
 objects may also be used for store-and-forward rendering, and
 "application" media type, to support editing tools.  Initialization
 objects without note storage, or initialization objects for non-audio
 renderers, SHOULD be registered only for an "application" media type.

C.6.4. MIDI Channel Mapping

 In this appendix, we specify how to map MIDI name spaces (16 voice
 channels + systems) onto a renderer.
 In the general case:
   o  A session may define an ordered relationship (Appendix C.5) that
      presents more than one MIDI name space to a renderer.
   o  A renderer may accept an arbitrary number of MIDI name spaces,
      or it may expect a specific number of MIDI name spaces.
 A session description SHOULD provide a compatible MIDI name space to
 each renderer in the session.  If a receiver detects that a session
 description has too many or too few MIDI name spaces for a renderer,
 MIDI data from extra stream name spaces MUST be discarded, and extra
 renderer name spaces MUST NOT be driven with MIDI data (except as
 described in Appendix C.6.4.1, below).

Lazzaro & Wawrzynek Standards Track [Page 133] RFC 4695 RTP Payload Format for MIDI November 2006

 If a parameter list defines several renderers and assigns the "all"
 token value to the multimode parameter, the same name space is
 presented to each renderer.  However, the "chanmask" parameter may be
 used to mask out selected voice channels to each renderer.  We define
 "chanmask" and other MIDI management parameters in the sub-sections
 below.

C.6.4.1. The smf_info Parameter

 The smf_info parameter defines the use of the SMFs encapsulated in
 renderer data objects (if any).  The smf_info parameter also defines
 the use of SMFs coded in the smf_inline, smf_url, and smf_cid
 parameters (defined in Appendix C.6.4.2).
 The smf_info parameter describes the "render" parameter that most
 recently precedes it in the parameter list.  The smf_info parameter
 MUST NOT appear in parameter lists that do not use the "render"
 parameter, and MUST NOT appear before the first use of "render" in
 the parameter list.
 We define three token values for smf_info: "ignore", "sdp_start", and
 "identity":
   o  The "ignore" value indicates that the SMFs MUST be discarded.
      This behavior is the default SMF rendering behavior.
   o  The "sdp_start" value codes that SMFs MUST be rendered, and that
      the rendering MUST begin upon the acceptance of the session
      description.  If a receiver is offered a session description
      with a renderer that uses an smf_info parameter set to
      sdp_start, and if the receiver does not support rendering SMFs,
      the receiver MUST NOT accept the renderer associated with the
      smf_info parameter.  Options include rejecting the renderer (by
      setting the "render" parameter to "null"), the payload type, the
      media stream, or the entire session description.
   o  The "identity" value indicates that the SMFs code the identity
      of the renderer.  The value is meant for use with the "unknown"
      renderer (see Appendix C.6 preamble).  The MIDI commands coded
      in the SMF are informational in nature and MUST NOT be presented
      to a renderer for audio presentation.  In typical use, the SMF
      would use SysEx Identity Reply commands (F0 7E nn 06 02, as
      defined in [MIDI]) to identify devices, and use device-specific
      SysEx commands to describe current state of the devices (patch
      memory contents, etc.).
 Other smf_info token values MAY be registered with IANA.  The token
 value MUST adhere to the ABNF for render tokens defined in Appendix

Lazzaro & Wawrzynek Standards Track [Page 134] RFC 4695 RTP Payload Format for MIDI November 2006

 D.  Registrations MUST include a complete specification of parameter
 usage, similar in depth to the specifications that appear in this
 appendix for "sdp_start" and "identity".
 If a party is offered a session description that uses an smf_info
 parameter value that is not known to the party, the party MUST NOT
 accept the renderer associated with the smf_info parameter.  Options
 include rejecting the renderer, the payload type, the media stream,
 or the entire session description.
 We now define the rendering semantics for the "sdp_start" token value
 in detail.
 The SMFs and RTP MIDI streams in a session description share the same
 MIDI name space(s).  In the simple case of a single RTP MIDI stream
 and a single SMF, the SMF MIDI commands and RTP MIDI commands are
 merged into a single name space and presented to the renderer.  The
 indefinite artifact responsibilities for merged MIDI streams defined
 in Appendix C.5 also apply to merging RTP and SMF MIDI data.
 If a payload type codes multiple SMFs, the SMF name spaces are
 presented as an ordered entity to the renderer.  To determine the
 ordering of SMFs for a renderer (which SMF is "first", which is
 "second", etc.), use the following rules:
   o  If the renderer uses a single data object, the order of
      appearance of the SMFs in the object's internal structure
      defines the order of the SMFs (the earliest SMF in the object is
      "first", the next SMF in the object is "second", etc.).
   o  If multiple data objects are encoded for a renderer, the
      appearance of each data object in the parameter list sets the
      relative order of the SMFs encoded in each data object (SMFs
      encoded in parameters that appear earlier in the list are
      ordered before SMFs encoded in parameters that appear later in
      the list).
   o  If SMFs are encoded in data objects parameters and in the
      parameters defined in C.6.4.2, the relative order of the data
      object parameters and C.6.4.2 parameters in the parameter list
      sets the relative order of SMFs (SMFs encoded in parameters that
      appear earlier in the list are ordered before SMFs in parameters
      that appear later in the list).
 Given this ordering of SMFs, we now define the mapping of SMFs to
 renderer name spaces.  The SMF that appears first for a renderer maps
 to the first renderer name space.  The SMF that appears second for a
 renderer maps to the second renderer name space, etc.  If the

Lazzaro & Wawrzynek Standards Track [Page 135] RFC 4695 RTP Payload Format for MIDI November 2006

 associated RTP MIDI streams also form an ordered relationship, the
 first SMF is merged with the first name space of the relationship,
 the second SMF is merged to the second name space of the
 relationship, etc.
 Unless the streams and the SMFs both use MIDI Time Code, the time
 offset between SMF and stream data is unspecified.  This restriction
 limits the use of SMFs to applications where synchronization is not
 critical, such as the transport of System Exclusive commands for
 renderer initialization, or human-SMF interactivity.
 Finally, we note that each SMF in the sdp_start discussion above
 encodes exactly one MIDI name space (16 voice channels + systems).
 Thus, the use of the Device Name SMF meta event to specify several
 MIDI name spaces in an SMF is not supported for sdp_start.

C.6.4.2. The smf_inline, smf_url, and smf_cid Parameters

 In some applications, the renderer data object may not encapsulate
 SMFs, but an application may wish to use SMFs in the manner defined
 in Appendix C.6.4.1.
 The "smf_inline", "smf_url", and "smf_cid" parameters address this
 situation.  These parameters use the syntax and semantics of the
 inline, url, and cid parameters defined in Appendix C.6.3, except
 that the encoded data object is an SMF.
 The "smf_inline", "smf_url", and "smf_cid" parameters belong to the
 "render" parameter that most recently precedes it in the session
 description.  The "smf_inline", "smf_url", and "smf_cid" parameters
 MUST NOT appear in parameter lists that do not use the "render"
 parameter and MUST NOT appear before the first use of "render" in the
 parameter list.  If several "smf_inline", "smf_url", or "smf_cid"
 parameters appear for a renderer, the order of the parameters defines
 the SMF name space ordering.

C.6.4.3. The chanmask Parameter

 The chanmask parameter instructs the renderer to ignore all MIDI
 voice commands for certain channel numbers.  The parameter value is a
 concatenated string of "1" and "0" digits.  Each string position maps
 to a MIDI voice channel number (system channels may not be masked).
 A "1" instructs the renderer to process the voice channel; a "0"
 instructs the renderer to ignore the voice channel.
 The string length of the chanmask parameter value MUST be 16 (for a
 single stream or an identity relationship) or a multiple of 16 (for
 an ordered relationship).

Lazzaro & Wawrzynek Standards Track [Page 136] RFC 4695 RTP Payload Format for MIDI November 2006

 The chanmask parameter describes the "render" parameter that most
 recently precedes it in the session description; chanmask MUST NOT
 appear in parameter lists that do not use the "render" parameter and
 MUST NOT appear before the first use of "render" in the parameter
 list.
 The chanmask parameter describes the final MIDI name spaces presented
 to the renderer.  The SMF and stream components of the MIDI name
 spaces may not be independently masked.
 If a receiver is offered a session description with a renderer that
 uses the chanmask parameter, and if the receiver does not implement
 the semantics of the chanmask parameter, the receiver MUST NOT accept
 the renderer unless the chanmask parameter value contains only "1"s.

C.6.5. The audio/asc Media Type

 In Appendix 11.3, we register the audio/asc media type.  The data
 object for audio/asc is a binary encoding of the AudioSpecificConfig
 data block used to initialize mpeg4-generic streams (Section 6.2 and
 [MPEGAUDIO]).
 An mpeg4-generic parameter list MAY use the render, subrender, and
 rinit parameters with the audio/asc media type for renderer
 configuration.  Several restrictions apply to the use of these
 parameters in mpeg4-generic parameter lists:
   o  An mpeg4-generic media description that uses the render
      parameter MUST assign the empty string ("") to the mpeg4-generic
      "config" parameter.  The use of the streamtype, mode, and
      profile-level-id parameters MUST follow the normative text in
      Section 6.2.
   o  Sessions that use identity or ordered relationships MUST follow
      the mpeg4-generic configuration restrictions in Appendix C.5.
   o  The render parameter MUST be assigned the value "synthetic",
      "unknown", "null", or a render value that has been added to the
      IANA repository for use with mpeg4-generic RTP MIDI streams.
      The "api" token value for render MUST NOT be used.
   o  If a subrender parameter is present, it MUST immediately follow
      the render parameter, and it MUST be assigned the token value
      "default" or assigned a subrender value added to the IANA
      repository for use with mpeg4-generic RTP MIDI streams.  A
      subrender parameter assignment may be left out of the renderer
      configuration, in which case the implied value of subrender is
      the default value of "default".

Lazzaro & Wawrzynek Standards Track [Page 137] RFC 4695 RTP Payload Format for MIDI November 2006

   o  If the render parameter is assigned the value "synthetic" and
      the subrender parameter has the value "default" (assigned or
      implied), the rinit parameter MUST be assigned the value
      "audio/asc", and an AudioSpecificConfig data object MUST be
      encoded using the mechanisms defined in C.6.2-3.  The
      AudioSpecificConfig data MUST encode one of the MPEG 4 Audio
      Object Types defined for use with mpeg4-generic in Section 6.2.
      If the subrender value is other than "default", refer to the
      subrender registration for information on the use of "audio/asc"
      with the renderer.
   o  If the render parameter is assigned the value "null" or
      "unknown", the data object MAY be omitted.
 Several general restrictions apply to the use of the audio/asc media
 type in RTP MIDI:
   o  A native stream MUST NOT assign "audio/asc" to rinit.  The
      audio/asc media type is not intended to be a general-purpose
      container for rendering systems outside of MPEG usage.
   o  The audio/asc media type defines a stored object type; it does
      not define semantics for RTP streams.  Thus, audio/asc MUST NOT
      appear on an rtpmap line of a session description.
 Below, we show session description examples for audio/asc.  The
 session description below uses the inline parameter to code the
 AudioSpecificConfig block for a mpeg4-generic General MIDI stream.
 We derive the value assigned to the inline parameter in Appendix E.4.
 The subrender token value of "default" is implied by the absence of
 the subrender parameter in the parameter list.
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP4 192.0.2.94
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; render=synthetic; rinit="audio/asc";
 inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
 (The a=fmtp line has been wrapped to fit the page to accommodate
  memo formatting restrictions; it comprises a single line in SDP.)

Lazzaro & Wawrzynek Standards Track [Page 138] RFC 4695 RTP Payload Format for MIDI November 2006

 The session description below uses the url parameter to code the
 AudioSpecificConfig block for the same General MIDI stream:
 v=0
 o=lazzaro 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 m=audio 5004 RTP/AVP 96
 c=IN IP4 192.0.2.94
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; render=synthetic; rinit="audio/asc";
 url="http://example.net/oski.asc";
 cid="xjflsoeiurvpa09itnvlduihgnvet98pa3w9utnuighbuk"
 (The a=fmtp line has been wrapped to fit the page to accommodate
  memo formatting restrictions; it comprises a single line in SDP.)

C.7. Interoperability

 In this appendix, we define interoperability guidelines for two
 application areas:
   o  MIDI content-streaming applications.  RTP MIDI is added to
      RTSP-based content-streaming servers, so that viewers may
      experience MIDI performances (produced by a specified client-
      side renderer) in synchronization with other streams (video,
      audio).
   o  Long-distance network musical performance applications.  RTP
      MIDI is added to SIP-based voice chat or videoconferencing
      programs, as an alternative, or as an addition, to audio and/or
      video RTP streams.
 For each application, we define a core set of functionality that all
 implementations MUST implement.
 The applications we address in this section are not an exhaustive
 list of potential RTP MIDI uses.  We expect framework documents for
 other applications to be developed, within the IETF or within other
 organizations.  We discuss other potential application areas for RTP
 MIDI in Section 1 of the main text of this memo.

C.7.1. MIDI Content Streaming Applications

 In content-streaming applications, a user invokes an RTSP client to
 initiate a request to an RTSP server to view a multimedia session.
 For example, clicking on a web page link for an Internet Radio

Lazzaro & Wawrzynek Standards Track [Page 139] RFC 4695 RTP Payload Format for MIDI November 2006

 channel launches an RTSP client that uses the link's RTSP URL to
 contact the RTSP server hosting the radio channel.
 The content may be pre-recorded (for example, on-demand replay of
 yesterday's football game) or "live" (for example, football game
 coverage as it occurs), but in either case the user is usually an
 "audience member" as opposed to a "participant" (as the user would be
 in telephony).
 Note that these examples describe the distribution of audio content
 to an audience member.  The interoperability guidelines in this
 appendix address RTP MIDI applications of this nature, not
 applications such as the transmission of raw MIDI command streams for
 use in a professional environment (recording studio, performance
 stage, etc.).
 In an RTSP session, a client accesses a session description that is
 "declared" by the server, either via the RTSP DESCRIBE method, or via
 other means, such as HTTP or email.  The session description defines
 the session from the perspective of the client.  For example, if a
 media line in the session description contains a non-zero port
 number, it encodes the server's preference for the client's port
 numbers for RTP and RTCP reception.  Once media flow begins, the
 server sends an RTP MIDI stream to the client, which renders it for
 presentation, perhaps in synchrony with video or other audio streams.
 We now define the interoperability text for content-streaming RTSP
 applications.
 In most cases, server interoperability responsibilities are described
 in terms of limits on the "reference" session description a server
 provides for a performance if it has no information about the
 capabilities of the client.  The reference session is a "lowest
 common denominator" session that maximizes the odds that a client
 will be able to view the session.  If a server is aware of the
 capabilities of the client, the server is free to provide a session
 description customized for the client in the DESCRIBE reply.
 Clients MUST support unicast UDP RTP MIDI streams that use the
 recovery journal with the closed-loop or the anchor sending policies.
 Clients MUST be able to interpret stream subsetting and chapter
 inclusion parameters in the session description that qualify the
 sending policies.  Client support of enhanced Chapter C encoding is
 OPTIONAL.
 The reference session description offered by a server MUST send all
 RTP MIDI UDP streams as unicast streams that use the recovery journal
 and the closed-loop or anchor sending policies.  Servers SHOULD use

Lazzaro & Wawrzynek Standards Track [Page 140] RFC 4695 RTP Payload Format for MIDI November 2006

 the stream subsetting and chapter inclusion parameters in the
 reference session description, to simplify the rendering task of the
 client.  Server support of enhanced Chapter C encoding is OPTIONAL.
 Clients and servers MUST support the use of RTSP interleaved mode (a
 method for interleaving RTP onto the RTSP TCP transport).
 Clients MUST be able to interpret the timestamp semantics signalled
 by the "comex" value of the tsmode parameter (i.e., the timestamp
 semantics of Standard MIDI Files [MIDI]).  Servers MUST use the
 "comex" value for the "tsmode" parameter in the reference session
 description.
 Clients MUST be able to process an RTP MIDI stream whose packets
 encode an arbitrary temporal duration ("media time").  Thus, in
 practice, clients MUST implement a MIDI playout buffer.  Clients MUST
 NOT depend on the presence of rtp_ptime, rtp_maxtime, and guardtime
 parameters in the session description in order to process packets,
 but they SHOULD be able to use these parameters to improve packet
 processing.
 Servers SHOULD strive to send RTP MIDI streams in the same way media
 servers send conventional audio streams: a sequence of packets that
 either all code the same temporal duration (non-normative example: 50
 ms packets) or that code one of an integral number of temporal
 durations (non-normative example: 50 ms, 100 ms, 250 ms, or 500 ms
 packets).  Servers SHOULD encode information about the packetization
 method in the rtp_ptime and rtp_maxtime parameters in the session
 description.
 Clients MUST be able to examine the render and subrender parameter,
 to determine if a multimedia session uses a renderer it supports.
 Clients MUST be able to interpret the default "one" value of the
 "multimode" parameter, to identify supported renderers from a list of
 renderer descriptions.  Clients MUST be able to interpret the
 musicport parameter, to the degree that it is relevant to the
 renderers it supports.  Clients MUST be able to interpret the
 chanmask parameter.
 Clients supporting renderers whose data object (as encoded by a
 parameter value for "inline") could exceed 300 octets in size MUST
 support the url and cid parameters and thus must implement the HTTP
 protocol in addition to RTSP.
 Servers MUST specify complete rendering systems for RTP MIDI streams.
 Note that a minimal RTP MIDI native stream does not meet this
 requirement (Section 6.1), as the rendering method for such streams
 is "not specified".

Lazzaro & Wawrzynek Standards Track [Page 141] RFC 4695 RTP Payload Format for MIDI November 2006

 At the time of this memo, the only way for servers to specify a
 complete rendering system is to specify an mpeg4-generic RTP MIDI
 stream in mode rtp-midi (Section 6.2 and C.6.5).  As a consequence,
 the only rendering systems that may be presently used are General
 MIDI [MIDI], DLS 2 [DLS2], or Structured Audio [MPEGSA].  Note that
 the maximum inline value for General MIDI is well under 300 octets
 (and thus clients need not support the "url" parameter), and that the
 maximum inline values for DLS 2 and Structured Audio may be much
 larger than 300 octets (and thus clients MUST support the url
 parameter).
 We anticipate that the owners of rendering systems (both standardized
 and proprietary) will register subrender parameters for their
 renderers.  Once registration occurs, native RTP MIDI sessions may
 use render and subrender (Appendix C.6.2) to specify complete
 rendering systems for RTSP content-streaming multimedia sessions.
 Servers MUST NOT use the sdp_start value for the smf_info parameter
 in the reference session description, as this use would require that
 clients be able to parse and render Standard MIDI Files.
 Clients MUST support mpeg4-generic mode rtp-midi General MIDI (GM)
 sessions, at a polyphony limited by the hardware capabilities of the
 client.  This requirement provides a "lowest common denominator"
 rendering system for content providers to target.  Note that this
 requirement does not force implementors of a non-GM renderer (such as
 DLS 2 or Structured Audio) to add a second rendering engine.
 Instead, a client may satisfy the requirement by including a set of
 voice patches that implement the GM instrument set, and using this
 emulation for mpeg4-generic GM sessions.
 It is RECOMMENDED that servers use General MIDI as the renderer for
 the reference session description, because clients are REQUIRED to
 support it.  We do not require General MIDI as the reference
 renderer, because for normative applications it is an inappropriate
 choice.  Servers using General MIDI as a "lowest common denominator"
 renderer SHOULD use Universal Real-Time SysEx MIP message [SPMIDI] to
 communicate the priority of voices to polyphony-limited clients.

C.7.2. MIDI Network Musical Performance Applications

 In Internet telephony and videoconferencing applications, parties
 interact over an IP network as they would face-to-face.  Good user
 experiences require low end-to-end audio latency and tight
 audiovisual synchronization (for "lip-sync").  The Session Initiation
 Protocol (SIP, [RFC3261]) is used for session management.

Lazzaro & Wawrzynek Standards Track [Page 142] RFC 4695 RTP Payload Format for MIDI November 2006

 In this appendix section, we define interoperability guidelines for
 using RTP MIDI streams in interactive SIP applications.  Our primary
 interest is supporting Network Musical Performances (NMP), where
 musicians in different locations interact over the network as if they
 were in the same room.  See [NMP] for background information on NMP,
 and see [RFC4696] for a discussion of low-latency RTP MIDI
 implementation techniques for NMP.
 Note that the goal of NMP applications is telepresence: the parties
 should hear audio that is close to what they would hear if they were
 in the same room.  The interoperability guidelines in this appendix
 address RTP MIDI applications of this nature, not applications such
 as the transmission of raw MIDI command streams for use in a
 professional environment (recording studio, performance stage, etc.).
 We focus on session management for two-party unicast sessions that
 specify a renderer for RTP MIDI streams.  Within this limited scope,
 the guidelines defined here are sufficient to let applications
 interoperate.  We define the REQUIRED capabilities of RTP MIDI
 senders and receivers in NMP sessions and define how session
 descriptions exchanged are used to set up network musical performance
 sessions.
 SIP lets parties negotiate details of the session, using the
 Offer/Answer protocol [RFC3264].  However, RTP MIDI has so many
 parameters that "blind" negotiations between two parties using
 different applications might not yield a common session
 configuration.
 Thus, we now define a set of capabilities that NMP parties MUST
 support.  Session description offers whose options lie outside the
 envelope of REQUIRED party behavior risk negotiation failure.  We
 also define session description idioms that the RTP MIDI part of an
 offer MUST follow, in order to structure the offer for simpler
 analysis.
 We use the term "offerer" for the party making a SIP offer, and
 "answerer" for the party answering the offer.  Finally, we note that
 unless it is qualified by the adjective "sender" or "receiver", a
 statement that a party MUST support X implies that it MUST support X
 for both sending and receiving.
 If an offerer wishes to define a "sendrecv" RTP MIDI stream, it may
 use a true sendrecv session or the "virtual sendrecv" construction
 described in the preamble to Appendix C and in Appendix C.5.  A true
 sendrecv session indicates that the offerer wishes to participate in
 a session where both parties use identically configured renderers.  A
 virtual sendrecv session indicates that the offerer is willing to

Lazzaro & Wawrzynek Standards Track [Page 143] RFC 4695 RTP Payload Format for MIDI November 2006

 participate in a session where the two parties may be using different
 renderer configurations.  Thus, parties MUST be prepared to see both
 real and virtual sendrecv sessions in an offer.
 Parties MUST support unicast UDP transport of RTP MIDI streams.
 These streams MUST use the recovery journal with the closed-loop or
 anchor sending policies.  These streams MUST use the stream
 subsetting and chapter inclusion parameters to declare the types of
 MIDI commands that will be sent on the stream (for sendonly streams)
 or will be processed (for recvonly streams), including the size
 limits on System Exclusive commands.  Support of enhanced Chapter C
 encoding is OPTIONAL.
 Note that both TCP and multicast UDP support are OPTIONAL.  We make
 TCP OPTIONAL because we expect NMP renderers to rely on data objects
 (signalled by "rinit" and associated parameters) for initialization
 at the start of the session, and only to use System Exclusive
 commands for interactive control during the session.  These
 interactive commands are small enough to be protected via the
 recovery journal mechanism of RTP MIDI UDP streams.
 We now discuss timestamps, packet timing, and packet sending
 algorithms.
 Recall that the tsmode parameter controls the semantics of command
 timestamps in the MIDI list of RTP packets.
 Parties MUST support clock rates of 44.1 kHz, 48 kHz, 88.2 kHz, and
 96 kHz.  Parties MUST support streams using the "comex", "async", and
 "buffer" tsmode values.  Recvonly offers MUST offer the default
 "comex".
 Parties MUST support a wide range of packet temporal durations: from
 rtp_ptime and rtp_maxptime values of 0, to rtp_ptime and rtp_maxptime
 values that code 100 ms.  Thus, receivers MUST be able to implement a
 playout buffer.
 Offers and answers MUST present rtp_ptime, rtp_maxptime, and
 guardtime values that support the latency that users would expect in
 the application, subject to bandwidth constraints.  As senders MUST
 abide by values set for these parameters in a session description, a
 receiver SHOULD use these values to size its playout buffer to
 produce the lowest reliable latency for a session.  Implementers
 should refer to [RFC4696] for information on packet sending
 algorithms for latency-sensitive applications.  Parties MUST be able
 to implement the semantics of the guardtime parameter, for times from
 5 ms to 5000 ms.

Lazzaro & Wawrzynek Standards Track [Page 144] RFC 4695 RTP Payload Format for MIDI November 2006

 We now discuss the use of the render parameter.
 Sessions MUST specify complete rendering systems for all RTP MIDI
 streams.  Note that a minimal RTP MIDI native stream does not meet
 this requirement (Section 6.1), as the rendering method for such
 streams is "not specified".
 At the time this writing, the only way for parties to specify a
 complete rendering system is to specify an mpeg4-generic RTP MIDI
 stream in mode rtp-midi (Section 6.2 and C.6.5).  We anticipate that
 the owners of rendering systems (both standardized and proprietary)
 will register subrender values for their renderers.  Once IANA
 registration occurs, native RTP MIDI sessions may use render and
 subrender (Appendix C.6.2) to specify complete rendering systems for
 SIP network musical performance multimedia sessions.
 All parties MUST support General MIDI (GM) sessions, at a polyphony
 limited by the hardware capabilities of the party.  This requirement
 provides a "lowest common denominator" rendering system, without
 which practical interoperability will be quite difficult.  When using
 GM, parties SHOULD use Universal Real-Time SysEx MIP message [SPMIDI]
 to communicate the priority of voices to polyphony-limited clients.
 Note that this requirement does not force implementors of a non-GM
 renderer (for mpeg4-generic sessions, DLS 2, or Structured Audio) to
 add a second rendering engine.  Instead, a client may satisfy the
 requirement by including a set of voice patches that implement the GM
 instrument set, and using this emulation for mpeg4-generic GM
 sessions.  We require GM support so that an offerer that wishes to
 maximize interoperability may do so by offering GM if its preferred
 renderer is not accepted by the answerer.
 Offerers MUST NOT present several renderers as options in a session
 description by listing several payload types on a media line, as
 Section 2.1 uses this construct to let a party send several RTP MIDI
 streams in the same RTP session.
 Instead, an offerer wishing to present rendering options SHOULD offer
 a single payload type that offers several renderers.  In this
 construct, the parameter list codes a list of render parameters (each
 followed by its support parameters).  As discussed in Appendix C.6.1,
 the order of renderers in the list declares the offerer's preference.
 The "unknown" and "null" values MUST NOT appear in the offer.  The
 answer MUST set all render values except the desired renderer to
 "null".  Thus, "unknown" MUST NOT appear in the answer.

Lazzaro & Wawrzynek Standards Track [Page 145] RFC 4695 RTP Payload Format for MIDI November 2006

 We use SHOULD instead of MUST in the first sentence in the paragraph
 above, because this technique does not work in all situations
 (example:  an offerer wishes to offer both mpeg4-generic renderers
 and native RTP MIDI renderers as options).  In this case, the offerer
 MUST present a series of session descriptions, each offering a single
 renderer, until the answerer accepts a session description.
 Parties MUST support the musicport, chanmask, subrender, rinit, and
 inline parameters.  Parties supporting renderers whose data object
 (as encoded by a parameter value for "inline") could exceed 300
 octets in size MUST support the url and cid parameters and thus must
 implement HTTP protocol.  Note that in mpeg4-generic, General MIDI
 data objects cannot exceed 300 octets, but DLS 2 and Structured Audio
 data objects may.  Support for the other rendering parameters
 (smf_cif, smf_info, smf_inline, smf_url) is OPTIONAL.
 Thus far in this document, our discussion has assumed that the only
 MIDI flows that drive a renderer are the network flows described in
 the session description.  In NMP applications, this assumption would
 require two rendering engines: one for local use by a party, a second
 for the remote party.
 In practice, applications may wish to have both parties share a
 single rendering engine.  In this case, the session description MUST
 use a virtual sendrecv session and MUST use the stream subsetting and
 chapter inclusion parameters to allocate which MIDI channels are
 intended for use by a party.  If two parties are sharing a MIDI
 channels, the application MUST ensure that appropriate MIDI merging
 occurs at the input to the renderer.
 We now discuss the use of (non-MIDI) audio streams in the session.
 Audio streams may be used for two purposes: as a "talkback" channel
 for parties to converse, or as a way to conduct a performance that
 includes MIDI and audio channels.  In the latter case, offers MUST
 use sample rates and the packet temporal durations for the audio and
 MIDI streams that support low-latency synchronized rendering.

Lazzaro & Wawrzynek Standards Track [Page 146] RFC 4695 RTP Payload Format for MIDI November 2006

 We now show an example of an offer/answer exchange in a network
 musical performance application (next page).  Below, we show an offer
 that complies with the interoperability text in this appendix
 section.
 v=0
 o=first 2520644554 2838152170 IN IP4 first.example.net
 s=Example
 t=0 0
 a=group:FID 1 2
 c=IN IP4 192.0.2.94
 m=audio 16112 RTP/AVP 96
 a=recvonly
 a=mid:1
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=2NPTW;
 cm_used=2C0.1.7.10.11.64.121.123; cm_used=2M0.1.2
 cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
 ch_default=2NPTW; ch_default=2C0.1.7.10.11.64.121.123;
 ch_default=2M0.1.2; cm_default=X0-16;
 rtp_ptime=0; rtp_maxptime=0; guardtime=44100;
 musicport=1; render=synthetic; rinit="audio/asc";
 inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
 m=audio 16114 RTP/AVP 96
 a=sendonly
 a=mid:2
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=1NPTW;
 cm_used=1C0.1.7.10.11.64.121.123; cm_used=1M0.1.2
 cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
 ch_default=1NPTW; ch_default=1C0.1.7.10.11.64.121.123;
 ch_default=1M0.1.2; cm_default=X0-16;
 rtp_ptime=0; rtp_maxptime=0; guardtime=44100;
 musicport=1; render=synthetic; rinit="audio/asc";
 inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
 (The a=fmtp lines have been wrapped to fit the page to accommodate
  memo formatting restrictions; it comprises a single line in SDP.)
 The owner line (o=) identifies the session owner as "first".
 The session description defines two MIDI streams: a recvonly stream
 on which "first" receives a performance, and a sendonly stream that
 "first" uses to send a performance.  The recvonly port number encodes
 the ports on which "first" wishes to receive RTP (16112) and RTCP
 (16113) media at IP4 address 192.0.2.94.  The sendonly port number

Lazzaro & Wawrzynek Standards Track [Page 147] RFC 4695 RTP Payload Format for MIDI November 2006

 encodes the port on which "first" wishes to receive RTCP for the
 stream (16115).
 The musicport parameters code that the two streams share and identity
 relationship and thus form a virtual sendrecv stream.
 Both streams are mpeg4-generic RTP MIDI streams that specify a
 General MIDI renderer.  The stream subsetting parameters code that
 the recvonly stream uses MIDI channel 1 exclusively for voice
 commands, and that the sendonly stream uses MIDI channel 2
 exclusively for voice commands.  This mapping permits the application
 software to share a single renderer for local and remote performers.

Lazzaro & Wawrzynek Standards Track [Page 148] RFC 4695 RTP Payload Format for MIDI November 2006

 We now show the answer to the offer.
 v=0
 o=second 2520644554 2838152170 IN IP4 second.example.net
 s=Example
 t=0 0
 a=group:FID 1 2
 c=IN IP4 192.0.2.105
 m=audio 5004 RTP/AVP 96
 a=sendonly
 a=mid:1
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=2NPTW;
 cm_used=2C0.1.7.10.11.64.121.123; cm_used=2M0.1.2
 cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
 ch_default=2NPTW; ch_default=2C0.1.7.10.11.64.121.123;
 ch_default=2M0.1.2; cm_default=X0-16;
 rtp_ptime=0; rtp_maxptime=882; guardtime=44100;
 musicport=1; render=synthetic; rinit="audio/asc";
 inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
 m=audio 5006 RTP/AVP 96
 a=recvonly
 a=mid:2
 a=rtpmap:96 mpeg4-generic/44100
 a=fmtp:96 streamtype=5; mode=rtp-midi; config="";
 profile-level-id=12; cm_unused=ABCFGHJKMNPQTVWXYZ; cm_used=1NPTW;
 cm_used=1C0.1.7.10.11.64.121.123; cm_used=1M0.1.2
 cm_used=X0-16; ch_never=ABCDEFGHJKMNPQTVWXYZ;
 ch_default=1NPTW; ch_default=1C0.1.7.10.11.64.121.123;
 ch_default=1M0.1.2; cm_default=X0-16;
 rtp_ptime=0; rtp_maxptime=0; guardtime=88200;
 musicport=1; render=synthetic; rinit="audio/asc";
 inline="egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA"
 (The a=fmtp lines have been wrapped to fit the page to accommodate
  memo formatting restrictions; they comprise single lines in SDP.)
 The owner line (o=) identifies the session owner as "second".
 The port numbers for both media streams are non-zero; thus, "second"
 has accepted the session description.  The stream marked "sendonly"
 in the offer is marked "recvonly" in the answer, and vice versa,
 coding the different view of the session held by "session".  The IP4
 number (192.0.2.105) and the RTP (5004 and 5006) and RTCP (5005 and
 5007) have been changed by "second" to match its transport wishes.

Lazzaro & Wawrzynek Standards Track [Page 149] RFC 4695 RTP Payload Format for MIDI November 2006

 In addition, "second" has made several parameter changes:
 rtp_maxptime for the sendonly stream has been changed to code 2 ms
 (441 in clock units), and the guardtime for the recvonly stream has
 been doubled.  As these parameter modifications request capabilities
 that are REQUIRED to be implemented by interoperable parties,
 "second" can make these changes with confidence that "first" can
 abide by them.

D. Parameter Syntax Definitions

 In this appendix, we define the syntax for the RTP MIDI media type
 parameters in Augmented Backus-Naur Form (ABNF, [RFC4234]).  When
 using these parameters with SDP, all parameters MUST appear on a
 single fmtp attribute line of an RTP MIDI media description.  For
 mpeg4-generic RTP MIDI streams, this line MUST also include any
 mpeg4-generic parameters (usage described in Section 6.2).  An fmtp
 attribute line may be defined (after [RFC3640]) as:
 ;
 ; SDP fmtp line definition
 ;
 fmtp = "a=fmtp:" token SP param-assign 0*(";" SP param-assign) CRLF
 where <token> codes the RTP payload type.  Note that white space MUST
 NOT appear between the "a=fmtp:" and the RTP payload type.
 We now define the syntax of the parameters defined in Appendix C.
 The definition takes the form of the incremental assembly of the
 <param-assign> token.  See [RFC3640] for the syntax of the
 mpeg4-generic parameters discussed in Section 6.2.
 ;
 ;
 ; top-level definition for all parameters
 ;
 ;
 ;
 ; Parameters defined in Appendix C.1
 param-assign =   ("cm_unused="  (([channel-list] command-type
                                   [f-list]) / sysex-data))
 param-assign =/  ("cm_used="    (([channel-list] command-type
                                   [f-list]) / sysex-data))

Lazzaro & Wawrzynek Standards Track [Page 150] RFC 4695 RTP Payload Format for MIDI November 2006

 ;
 ; Parameters defined in Appendix C.2
 param-assign =/  ("j_sec="      ("none" / "recj" / *ietf-extension))
 param-assign =/  ("j_update="   ("anchor" / "closed-loop" /
                                  "open-loop" / *ietf-extension))
 param-assign =/  ("ch_default=" (([channel-list] chapter-list
                                   [f-list]) / sysex-data))
 param-assign =/  ("ch_never="   (([channel-list] chapter-list
                                   [f-list]) / sysex-data))
 param-assign =/  ("ch_anchor="  (([channel-list] chapter-list
                                   [f-list]) / sysex-data))
 ;
 ; Parameters defined in Appendix C.3
 param-assign =/  ("tsmode="     ("comex" / "async" / "buffer"))
 param-assign =/  ("linerate="    nonzero-four-octet)
 param-assign =/  ("octpos="      ("first" / "last"))
 param-assign =/  ("mperiod="     nonzero-four-octet)
 ;
 ; Parameter defined in Appendix C.4
 param-assign =/  ("guardtime="     nonzero-four-octet)
 param-assign =/  ("rtp_ptime="     four-octet)
 param-assign =/  ("rtp_maxptime="  four-octet)
 ;
 ; Parameters defined in Appendix C.5
 param-assign =/  ("musicport="     four-octet)

Lazzaro & Wawrzynek Standards Track [Page 151] RFC 4695 RTP Payload Format for MIDI November 2006

 ;
 ; Parameters defined in Appendix C.6
 param-assign =/  ("chanmask="     ( 1*( 16( "0" / "1" ) )))
 param-assign =/  ("cid="          double-quote cid-block
                                   double-quote)
 param-assign =/  ("inline="       double-quote base-64-block
                                   double-quote)
 param-assign =/  ("multimode="    ("all" / "one"))
 param-assign =/  ("render="       ("synthetic" / "api" / "null" /
                                    "unknown" / *extension))
 param-assign =/  ("rinit="        mime-type "/" mime-subtype)
 param-assign =/  ("smf_cid="      double-quote cid-block
                                   double-quote)
 param-assign =/  ("smf_info="     ("ignore" / "identity" /
                                   "sdp_start" / *extension))
 param-assign =/  ("smf_inline="   double-quote base-64-block
                                   double-quote)
 param-assign =/  ("smf_url="      double-quote uri-element
                                   double-quote)
 param-assign =/  ("subrender="    ("default" / *extension))
 param-assign =/  ("url="          double-quote uri-element
                                   double-quote)
 ;
 ; list definitions for the cm_ command-type
 ;
 command-type    = command-part1 command-part2 command-part3
 command-part1   = (*1"A") (*1"B") (*1"C") (*1"F") (*1"G") (*1"H")
 command-part2   = (*1"J") (*1"K") (*1"M") (*1"N") (*1"P") (*1"Q")
 command-part3   = (*1"T") (*1"V") (*1"W") (*1"X") (*1"Y") (*1"Z")

Lazzaro & Wawrzynek Standards Track [Page 152] RFC 4695 RTP Payload Format for MIDI November 2006

 ;
 ; list definitions for the ch_ chapter-list
 ;
 chapter-list  =  ch-part1 ch-part2 ch-part3
 ch-part1  = (*1"A") (*1"B") (*1"C") (*1"D") (*1"E") (*1"F") (*1"G")
 ch-part2  = (*1"H") (*1"J") (*1"K") (*1"M") (*1"N") (*1"P") (*1"Q")
 ch-part3  = (*1"T") (*1"V") (*1"W") (*1"X") (*1"Y") (*1"Z")
 ;
 ; list definitions for the ch_ channel-list
 ;
 channel-list       = midi-chan-element *("." midi-chan-element)
 midi-chan-element  = midi-chan / midi-chan-range
 midi-chan-range    = midi-chan "-" midi-chan
                    ; decimal value of left midi-chan
                    ; MUST be strictly less than decimal
                    ; value of right midi-chan
 midi-chan          = %d0-15
 ;
 ; list definitions for the ch_ field list (f-list)
 ;
 f-list             = midi-field-element *("." midi-field-element)
 midi-field-element = midi-field / midi-field-range
 midi-field-range   = midi-field "-" midi-field
                    ;
                    ; decimal value of left midi-field
                    ; MUST be strictly less than decimal
                    ; value of right midi-field
 midi-field         = four-octet
                    ;
                    ; large range accommodates Chapter M
                    ; RPN (0-16383) and NRPN (16384-32767)
                    ; parameters, and Chapter X octet sizes.

Lazzaro & Wawrzynek Standards Track [Page 153] RFC 4695 RTP Payload Format for MIDI November 2006

 ;
 ; definitions for ch_ sysex-data
 ;
 sysex-data         = "__"  h-list *("_" h-list) "__"
 h-list             = hex-field-element *("." hex-field-element)
 hex-field-element  = hex-octet / hex-field-range
 hex-field-range    = hex-octet "-" hex-octet
                    ;
                    ; hexadecimal value of left hex-octet
                    ; MUST be strictly less than hexadecimal
                    ; value of right hex-octet
 hex-octet          = 2("0" / "1" / "2"/ "3" / "4" /
                        "5" / "6" / "7" / "8" / "9" /
                        "A" / "B" / "C" / "D" / "E" / "F")
                    ;
                    ; rewritten version of hex-octet in [RFC2045]
                    ; (page 23).
                    ; note that a-f are not permitted, only A-F.
                    ; hex-octet values MUST NOT exceed 7F.
 ;
 ; definitions for rinit parameter
 ;
 mime-type          = "audio" / "application"
 mime-subtype       = token
                    ;
                    ; See Appendix C.6.2 for registration
                    ; requirements for rinit type/subtypes.
 ;
 ; definitions for base64 encoding
 ; copied from [RFC4566]
 base-64-block      = *base64-unit [base64-pad]
 base64-unit        =  4base64-char
 base64-pad         =  2base64-char "==" / 3base64-char "="
 base64-char        =  %x41-5A / %x61-7A / %x30-39 / "+" / "/"
                    ;  A-Z, a-z, 0-9, "+" and "/"

Lazzaro & Wawrzynek Standards Track [Page 154] RFC 4695 RTP Payload Format for MIDI November 2006

 ;
 ; generic rules
 ;
 ietf-extension     = token
                    ;
                    ; ietf-extension may only be defined in
                    ; standards-track RFCs.
 extension          = token
                    ;
                    ; extension may be defined by filing
                    ; a registration with IANA.
 four-octet         = %d0-4294967295
                    ; unsigned encoding of 32-bits
 nonzero-four-octet = %d1-4294967295
                    ; unsigned encoding of 32-bits, ex-zero
 uri-element        = URI-reference
                    ; as defined in [RFC3986]
 double-quote       = %x22
                    ; the double-quote (") character
 token              =  1*token-char
                    ; copied from [RFC4566]
 token-char         =  %x21 / %x23-27 / %x2A-2B / %x2D-2E /
                       %x30-39 / %x41-5A / %x5E-7E
                    ; copied from [RFC4566]
 cid-block          = 1*cid-char
 cid-char           =  token-char
 cid-char           =/  "@"
 cid-char           =/  ","
 cid-char           =/  ";"
 cid-char           =/  ":"
 cid-char           =/  "\"
 cid-char           =/  "/"
 cid-char           =/  "["
 cid-char           =/  "]"
 cid-char           =/  "?"
 cid-char           =/  "="

Lazzaro & Wawrzynek Standards Track [Page 155] RFC 4695 RTP Payload Format for MIDI November 2006

                    ;
                    ; add back in the tspecials [RFC2045], except for
                    ; double-quote and the non-email safe () <>
                    ; note that "cid" defined above ensures that
                    ; cid-block is enclosed with double-quotes
 ; external references
 ; URI-reference: from [RFC3986]
 ;
 ; End of ABNF
 The mpeg4-generic RTP payload [RFC3640] defines a "mode" parameter
 that signals the type of MPEG stream in use.  We add a new mode
 value, "rtp-midi", using the ABNF rule below:
 ;
 ; mpeg4-generic mode parameter extension
 ;
 mode              =/ "rtp-midi"
                   ; as described in Section 6.2 of this memo

E. A MIDI Overview for Networking Specialists

 This appendix presents an overview of the MIDI standard, for the
 benefit of networking specialists new to musical applications.
 Implementors should consult [MIDI] for a normative description of
 MIDI.
 Musicians make music by performing a controlled sequence of physical
 movements.  For example, a pianist plays by coordinating a series of
 key presses, key releases, and pedal actions.  MIDI represents a
 musical performance by encoding these physical gestures as a sequence
 of MIDI commands.  This high-level musical representation is compact
 but fragile: one lost command may be catastrophic to the performance.
 MIDI commands have much in common with the machine instructions of a
 microprocessor.  MIDI commands are defined as binary elements.
 Bitfields within a MIDI command have a regular structure and a
 specialized purpose.  For example, the upper nibble of the first
 command octet (the opcode field) codes the command type.  MIDI
 commands may consist of an arbitrary number of complete octets, but
 most MIDI commands are 1, 2, or 3 octets in length.

Lazzaro & Wawrzynek Standards Track [Page 156] RFC 4695 RTP Payload Format for MIDI November 2006

     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     |     Channel Voice Messages     |      Bitfield Pattern      |
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | NoteOff (end a note)           | 1000cccc 0nnnnnnn 0vvvvvvv |
     |-------------------------------------------------------------|
     | NoteOn (start a note)          | 1001cccc 0nnnnnnn 0vvvvvvv |
     |-------------------------------------------------------------|
     | PTouch (Polyphonic Aftertouch) | 1010cccc 0nnnnnnn 0aaaaaaa |
     |-------------------------------------------------------------|
     | CControl (Controller Change)   | 1011cccc 0xxxxxxx 0yyyyyyy |
     |-------------------------------------------------------------|
     | PChange (Program Change)       | 1100cccc 0ppppppp          |
     |-------------------------------------------------------------|
     | CTouch (Channel Aftertouch)    | 1101cccc 0aaaaaaa          |
     |-------------------------------------------------------------|
     | PWheel (Pitch Wheel)           | 1110cccc 0xxxxxxx 0yyyyyyy |
      -------------------------------------------------------------
               Figure E.1 -- MIDI Channel Messages

Lazzaro & Wawrzynek Standards Track [Page 157] RFC 4695 RTP Payload Format for MIDI November 2006

     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     |      System Common Messages    |     Bitfield Pattern       |
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | System Exclusive               | 11110000, followed by a    |
     |                                | list of 0xxxxxx octets,    |
     |                                | followed by 11110111       |
     |-------------------------------------------------------------|
     | MIDI Time Code Quarter Frame   | 11110001 0xxxxxxx          |
     |-------------------------------------------------------------|
     | Song Position Pointer          | 11110010 0xxxxxxx 0yyyyyyy |
     |-------------------------------------------------------------|
     | Song Select                    | 11110011 0xxxxxxx          |
     |-------------------------------------------------------------|
     | Undefined                      | 11110100                   |
     |-------------------------------------------------------------|
     | Undefined                      | 11110101                   |
     |-------------------------------------------------------------|
     | Tune Request                   | 11110110                   |
     |-------------------------------------------------------------|
     | System Exclusive End Marker    | 11110111                   |
      -------------------------------------------------------------
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     |    System Realtime Messages    |     Bitfield Pattern       |
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Clock                          | 11111000                   |
     |-------------------------------------------------------------|
     | Undefined                      | 11111001                   |
     |-------------------------------------------------------------|
     | Start                          | 11111010                   |
     |-------------------------------------------------------------|
     | Continue                       | 11111011                   |
     |-------------------------------------------------------------|
     | Stop                           | 11111100                   |
     |-------------------------------------------------------------|
     | Undefined                      | 11111101                   |
     |-------------------------------------------------------------|
     | Active Sense                   | 11111110                   |
     |-------------------------------------------------------------|
     | System Reset                   | 11111111                   |
      -------------------------------------------------------------
                    Figure E.2 -- MIDI System Messages

Lazzaro & Wawrzynek Standards Track [Page 158] RFC 4695 RTP Payload Format for MIDI November 2006

 Figure E.1 and E.2 show the MIDI command family.  There are three
 major classes of commands: voice commands (opcode field values in the
 range 0x8 through 0xE), system common commands (opcode field 0xF,
 commands 0xF0 through 0xF7), and system real-time commands (opcode
 field 0xF, commands 0xF8 through 0xFF).  Voice commands code the
 musical gestures for each timbre in a composition.  Systems commands
 perform functions that usually affect all voice channels, such as
 System Reset (0xFF).

E.1. Commands Types

 Voice commands execute on one of 16 MIDI channels, as coded by its
 4-bit channel field (field cccc in Figure E.1).  In most
 applications, notes for different timbres are assigned to different
 channels.  To support applications that require more than 16
 channels, MIDI systems use several MIDI command streams in parallel,
 to yield 32, 48, or 64 MIDI channels.
 As an example of a voice command, consider a NoteOn command (opcode
 0x9), with binary encoding 1001cccc 0nnnnnnn 0aaaaaaa.  This command
 signals the start of a musical note on MIDI channel cccc.  The note
 has a pitch coded by the note number nnnnnnn, and an onset amplitude
 coded by note velocity aaaaaaa.
 Other voice commands signal the end of notes (NoteOff, opcode 0x8),
 map a specific timbre to a MIDI channel (PChange, opcode 0xC), or set
 the value of parameters that modulate the timbral quality (all other
 voice commands).  The exact meaning of most voice channel commands
 depends on the rendering algorithms the MIDI receiver uses to
 generate sound.  In most applications, a MIDI sender has a model (in
 some sense) of the rendering method used by the receiver.
 System commands perform a variety of global tasks in the stream,
 including "sequencer" playback control of pre-recorded MIDI commands
 (the Song Position Pointer, Song Select, Clock, Start, Continue, and
 Stop messages), SMPTE time code (the MIDI Time Code Quarter Frame
 command), and the communication of device-specific data (the System
 Exclusive messages).

E.2. Running Status

 All MIDI command bitfields share a special structure: the leading bit
 of the first octet is set to 1, and the leading bit of all subsequent
 octets is set to 0.  This structure supports a data compression
 system, called running status [MIDI], that improves the coding
 efficiency of MIDI.

Lazzaro & Wawrzynek Standards Track [Page 159] RFC 4695 RTP Payload Format for MIDI November 2006

 In running status coding, the first octet of a MIDI voice command may
 be dropped if it is identical to the first octet of the previous MIDI
 voice command.  This rule, in combination with a convention to
 consider NoteOn commands with a null third octet as NoteOff commands,
 supports the coding of note sequences using two octets per command.
 Running status coding is only used for voice commands.  The presence
 of a system common message in the stream cancels running status mode
 for the next voice command.  However, system real-time messages do
 not cancel running status mode.

E.3. Command Timing

 The bitfield formats in Figures E.1 and E.2 do not encode the
 execution time for a command.  Timing information is not a part of
 the MIDI command syntax itself; different applications of the MIDI
 command language use different methods to encode timing.
 For example, the MIDI command set acts as the transport layer for
 MIDI 1.0 DIN cables [MIDI].  MIDI cables are short asynchronous
 serial lines that facilitate the remote operation of musical
 instruments and audio equipment.  Timestamps are not sent over a MIDI
 1.0 DIN cable.  Instead, the standard uses an implicit "time of
 arrival" code.  Receivers execute MIDI commands at the moment of
 arrival.
 In contrast, Standard MIDI Files (SMFs, [MIDI]), a file format for
 representing complete musical performances, add an explicit timestamp
 to each MIDI command, using a delta encoding scheme that is optimized
 for statistics of musical performance.  SMF timestamps usually code
 timing using the metric notation of a musical score.  SMF meta-events
 are used to add a tempo map to the file, so that score beats may be
 accurately converted into units of seconds during rendering.

E.4. AudioSpecificConfig Templates for MMA Renderers

 In Section 6.2 and Appendix C.6.5, we describe how session
 descriptions include an AudioSpecificConfig data block to specify a
 MIDI rendering algorithm for mpeg4-generic RTP MIDI streams.
 The bitfield format of AudioSpecificConfig is defined in [MPEGAUDIO].
 StructuredAudioSpecificConfig, a key data structure coded in
 AudioSpecificConfig, is defined in [MPEGSA].
 For implementors wishing to specify Structured Audio renderers, a
 full understanding of [MPEGSA] and [MPEGAUDIO] is essential.
 However, many implementors will limit their rendering options to the
 two MIDI Manufacturers Association renderers that may be specified in

Lazzaro & Wawrzynek Standards Track [Page 160] RFC 4695 RTP Payload Format for MIDI November 2006

 AudioSpecificConfig: General MIDI (GM, [MIDI]) and Downloadable
 Sounds 2 (DLS 2, [DLS2]).
 To aid these implementors, we reproduce the AudioSpecificConfig
 bitfield formats for a GM renderer and a DLS 2 renderer below.  We
 have checked these bitfields carefully and believe they are correct.
 However, we stress that the material below is informative, and that
 [MPEGAUDIO] and [MPEGSA] are the normative definitions for
 AudioSpecificConfig.
 As described in Section 6.2, a minimal mpeg4-generic session
 description encodes the AudioSpecificConfig binary bitfield as a
 hexadecimal string (whose format is defined in [RFC3640]) that is
 assigned to the "config" parameter.  As described in Appendix C.6.3,
 a session description that uses the render parameter encodes the
 AudioSpecificConfig binary bitfield as a Base64-encoded string
 assigned to the "inline" parameter, or in the body of an HTTP URL
 assigned to the "url" parameter.
 Below, we show a simplified binary AudioSpecificConfig bitfield
 format, suitable for sending and receiving GM and DLS 2 data:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | AOTYPE  |FREQIDX|CHANNEL|SACNK|  FILE_BLK 1 (required) ...    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|SACNK|              FILE_BLK 2 (optional) ...                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  ...  |1|SACNK| FILE_BLK N (optional) ...                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|0|        (first "0" bit terminates FILE_BLK list)
    +-+-+
                Figure E.3 -- Simplified AudioSpecificConfig
 The 5-bit AOTYPE field specifies the Audio Object Type as an unsigned
 integer.  The legal values for use with mpeg4-generic RTP MIDI
 streams are "15" (General MIDI), "14" (DLS 2), and "13" (Structured
 Audio).  Thus, receivers that do not support all three mpeg4-generic
 renderers may parse the first 5 bits of an AudioSpecificConfig coded
 in a session description and reject sessions that specify unsupported
 renderers.
 The 4-bit FREQIDX field specifies the sampling rate of the renderer.
 We show the mapping of FREQIDX values to sampling rates in Figure
 E.4.  Senders MUST specify a sampling frequency that matches the RTP
 clock rate, if possible; if not, senders MUST specify the escape

Lazzaro & Wawrzynek Standards Track [Page 161] RFC 4695 RTP Payload Format for MIDI November 2006

 value.  Receivers MUST consult the RTP clock parameter for the true
 sampling rate if the escape value is specified.
                     FREQIDX    Sampling Frequency
                       0x0            96000
                       0x1            88200
                       0x2            64000
                       0x3            48000
                       0x4            44100
                       0x5            32000
                       0x6            24000
                       0x7            22050
                       0x8            16000
                       0x9            12000
                       0xa            11025
                       0xb             8000
                       0xc          reserved
                       0xd          reserved
                       0xe          reserved
                       0xf         escape value
                   Figure E.4 -- FreqIdx encoding
 The 4-bit CHANNEL field specifies the number of audio channels for
 the renderer.  The values 0x1 to 0x5 specify 1 to 5 audio channels;
 the value 0x6 specifies 5+1 surround sound, and the value 0x7
 specifies 7+1 surround sound.  If the rtpmap line in the session
 description specifies one of these formats, CHANNEL MUST be set to
 the corresponding value.  Otherwise, CHANNEL MUST be set to 0x0.
 The CHANNEL field is followed by a list of one or more binary file
 data blocks.  The 3-bit SACNK field (the chunk_type field in class
 StructuredAudioSpecificConfig, defined in [MPEGSA]) specifies the
 type of each data block.
 For General MIDI, only Standard MIDI Files may appear in the list
 (SACNK field value 2).  For DLS 2, only Standard MIDI Files and DLS 2
 RIFF files (SACNK field value 4) may appear.  For both of these file
 types, the FILE_BLK field has the format shown in Figure E.5: a 32-
 bit unsigned integer value (FILE_LEN) coding the number of bytes in
 the SMF or RIFF file, followed by FILE_LEN bytes coding the file
 data.

Lazzaro & Wawrzynek Standards Track [Page 162] RFC 4695 RTP Payload Format for MIDI November 2006

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     FILE_LEN (32-bit, a byte count SMF file or RIFF file)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  FILE_DATA (file contents, a list of FILE_LEN bytes) ...      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure E.5 -- The FILE_BLK field format
 Note that several files may follow CHANNEL field.  The "1" constant
 fields in Figure E.3 code the presence of another file; the "0"
 constant field codes the end of the list.  The final "0" bit in
 Figure E.3 codes the absence of special coding tools (see [MPEGAUDIO]
 for details).  Senders not using these tools MUST append this "0"
 bit; receivers that do not understand these coding tools MUST ignore
 all data following a "1" in this position.
 The StructuredAudioSpecificConfig bitfield structure requires the
 presence of one FILE_BLK.  For mpeg4-generic RTP MIDI use of DLS 2,
 FILE_BLKs MUST code RIFF files or SMF files.  For mpeg4-generic RTP
 MIDI use of General MIDI, FILE_BLKs MUST code SMF files.  By default,
 this SMF will be ignored (Appendix C.6.4.1).  In this default case, a
 GM StructuredAudioSpecificConfig bitfield SHOULD code a FILE_BLK
 whose FILE_LEN is 0, and whose FILE_DATA is empty.
 To complete this appendix, we derive the
 StructuredAudioSpecificConfig that we use in the General MIDI session
 examples in this memo.  Referring to Figure E.3, we note that for GM,
 AOTYPE = 15.  Our examples use a 44,100 Hz sample rate (FREQIDX = 4)
 and are in mono (CHANNEL = 1).  For GM, a single SMF is encoded
 (SACNK = 2), using the SMF shown in Figure E.6 (a 26 byte file).
  1. ——————————————-

| MIDI File = <Header Chunk> <Track Chunk> |

  1. ——————————————-
 <Header Chunk> = <chunk type> <length>     <format> <ntrks> <divsn>
                  4D 54 68 64  00 00 00 06  00 00    00 01   00 60
 <Track Chunk> = <chunk type>  <length>     <delta-time> <end-event>
                 4D 54 72 6B   00 00 00 04  00           FF 2F 00
          Figure E.6 -- SMF file encoded in the example

Lazzaro & Wawrzynek Standards Track [Page 163] RFC 4695 RTP Payload Format for MIDI November 2006

 Placing these constants in binary format into the data structure
 shown in Figure E.3 yields the constant shown in Figure E.7.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 1 1 1 1|0 1 0 0|0 0 0 1|0 1 0|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 0|0 1 0 0|1 1 0 1|0 1 0 1|0 1 0 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 1 1 0|1 0 0 0|0 1 1 0|0 1 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0|0 0 0 0|0 0 0 0|0 1 1 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 1|0 0 0 0|0 0 0 0|0 1 1 0|0 0 0 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 1 0 0|1 1 0 1|0 1 0 1|0 1 0 0|0 1 1 1|0 0 1 0|0 1 1 0|1 0 1 1|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 0 0 0|0 1 1 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0|0 0 0 0|1 1 1 1|1 1 1 1|0 0 1 0|1 1 1 1|0 0 0 0|0 0 0 0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|0|
    +-+-+
          Figure E.7 -- AudioSpecificConfig used in GM examples
 Expressing this bitfield as an ASCII hexadecimal string yields:
    7A0A0000001A4D546864000000060000000100604D54726B0000000600FF2F000
 This string is assigned to the "config" parameter in the minimal
 mpeg4-generic General MIDI examples in this memo (such as the example
 in Section 6.2).  Expressing this string in Base64 [RFC2045] yields:
    egoAAAAaTVRoZAAAAAYAAAABAGBNVHJrAAAABgD/LwAA
 This string is assigned to the "inline" parameter in the General MIDI
 example shown in Appendix C.6.5.

Lazzaro & Wawrzynek Standards Track [Page 164] RFC 4695 RTP Payload Format for MIDI November 2006

References

Normative References

 [MIDI]      MIDI Manufacturers Association.  "The Complete MIDI 1.0
             Detailed Specification", 1996.
 [RFC3550]   Schulzrinne, H., Casner, S., Frederick, R., and V.
             Jacobson, "RTP: A Transport Protocol for Real-Time
             Applications", STD 64, RFC 3550, July 2003.
 [RFC3551]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
             Video Conferences with Minimal Control", STD 65, RFC
             3551, July 2003.
 [RFC3640]   van der Meer, J., Mackie, D., Swaminathan, V., Singer,
             D., and P. Gentric, "RTP Payload Format for Transport of
             MPEG-4 Elementary Streams", RFC 3640, November 2003.
 [MPEGSA]    International Standards Organization.  "ISO/IEC 14496
             MPEG-4", Part 3 (Audio), Subpart 5 (Structured Audio),
             2001.
 [RFC4566]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
             Description Protocol", RFC 4566, July 2006.
 [MPEGAUDIO] International Standards Organization.  "ISO 14496 MPEG-
             4", Part 3 (Audio), 2001.
 [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, November 1996.
 [DLS2]      MIDI Manufacturers Association.  "The MIDI Downloadable
             Sounds Specification", v98.2, 1998.
 [RFC4234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", RFC 4234, October 2005.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3711]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
             Norrman, "The Secure Real-time Transport Protocol
             (SRTP)", RFC 3711, March 2004.

Lazzaro & Wawrzynek Standards Track [Page 165] RFC 4695 RTP Payload Format for MIDI November 2006

 [RFC3264]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
             with Session Description Protocol (SDP)", RFC 3264, June
             2002.
 [RFC3986]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
             Resource Identifier (URI): Generic Syntax", STD 66, RFC
             3986, January 2005.
 [RFC2616]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
             Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
             Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC3388]   Camarillo, G., Eriksson, G., Holler, J., and H.
             Schulzrinne, "Grouping of Media Lines in the Session
             Description Protocol (SDP)", RFC 3388, December 2002.
 [RP015]     MIDI Manufacturers Association.  "Recommended Practice
             015 (RP-015): Response to Reset All Controllers", 11/98.
 [RFC4288]   Freed, N. and J. Klensin, "Media Type Specifications and
             Registration Procedures", BCP 13, RFC 4288, December
             2005.
 [RFC3555]   Casner, S. and P. Hoschka, "MIME Type Registration of RTP
             Payload Formats", RFC 3555, July 2003.

Informative References

 [NMP]       Lazzaro, J. and J. Wawrzynek.  "A Case for Network
             Musical Performance", 11th International Workshop on
             Network and Operating Systems Support for Digital Audio
             and Video (NOSSDAV 2001) June 25-26, 2001, Port
             Jefferson, New York.
 [GRAME]     Fober, D., Orlarey, Y. and S. Letz.  "Real Time Musical
             Events Streaming over Internet", Proceedings of the
             International Conference on WEB Delivering of Music 2001,
             pages 147-154.
 [RFC3261]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., and E.
             Schooler, "SIP: Session Initiation Protocol", RFC 3261,
             June 2002.
 [RFC2326]   Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
             Streaming Protocol (RTSP)", RFC 2326, April 1998.

Lazzaro & Wawrzynek Standards Track [Page 166] RFC 4695 RTP Payload Format for MIDI November 2006

 [ALF]       Clark, D. D. and D. L. Tennenhouse. "Architectural
             considerations for a new generation of protocols",
             SIGCOMM Symposium on Communications Architectures and
             Protocols , (Philadelphia, Pennsylvania), pp. 200--208,
             IEEE, Sept. 1990.
 [RFC4696]   Lazzaro, J. and J. Wawrzynek, "An Implementation Guide
             for RTP MIDI", RFC 4696, November 2006.
 [RFC2205]   Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
             Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
             Functional Specification", RFC 2205, September 1997.
 [RFC4288]   Freed, N. and J. Klensin, "Media Type Specifications and
             Registration Procedures", BCP 13, RFC 4288, December
             2005.
 [RFC4289]   Freed, N. and J. Klensin, "Multipurpose Internet Mail
             Extensions (MIME) Part Four: Registration Procedures",
             BCP 13, RFC 4289, December 2005.
 [RFC4571]   Lazzaro, J. "Framing Real-time Transport Protocol (RTP)
             and RTP Control Protocol (RTCP) Packets over Connection-
             Oriented Transport", RFC 4571, July 2006.
 [RFC2818]   Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [SPMIDI]    MIDI Manufacturers Association.  "Scalable Polyphony
             MIDI, Specification and Device Profiles", Document
             Version 1.0a, 2002.
 [LCP]       Apple Computer. "Logic 7 Dedicated Control Surface
             Support", Appendix B.  Product manual available from
             www.apple.com.

Lazzaro & Wawrzynek Standards Track [Page 167] RFC 4695 RTP Payload Format for MIDI November 2006

Authors' Addresses

 John Lazzaro (corresponding author)
 UC Berkeley
 CS Division
 315 Soda Hall
 Berkeley CA 94720-1776
 EMail: lazzaro@cs.berkeley.edu
 John Wawrzynek
 UC Berkeley
 CS Division
 631 Soda Hall
 Berkeley CA 94720-1776
 EMail: johnw@cs.berkeley.edu

Lazzaro & Wawrzynek Standards Track [Page 168] RFC 4695 RTP Payload Format for MIDI November 2006

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Lazzaro & Wawrzynek Standards Track [Page 169]

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