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rfc:rfc3940

Network Working Group B. Adamson Request for Comments: 3940 NRL Category: Experimental C. Bormann

                                               Universitaet Bremen TZI
                                                            M. Handley
                                                                   UCL
                                                             J. Macker
                                                                   NRL
                                                         November 2004
              Negative-acknowledgment (NACK)-Oriented
                 Reliable Multicast (NORM) Protocol

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 This document describes the messages and procedures of the Negative-
 acknowledgment (NACK) Oriented Reliable Multicast (NORM) protocol.
 This protocol is designed to provide end-to-end reliable transport of
 bulk data objects or streams over generic IP multicast routing and
 forwarding services.  NORM uses a selective, negative acknowledgment
 mechanism for transport reliability and offers additional protocol
 mechanisms to allow for operation with minimal "a priori"
 coordination among senders and receivers.  A congestion control
 scheme is specified to allow the NORM protocol to fairly share
 available network bandwidth with other transport protocols such as
 Transmission Control Protocol (TCP).  It is capable of operating with
 both reciprocal multicast routing among senders and receivers and
 with asymmetric connectivity (possibly a unicast return path) between
 the senders and receivers.  The protocol offers a number of features
 to allow different types of applications or possibly other higher
 level transport protocols to utilize its service in different ways.
 The protocol leverages the use of FEC-based repair and other IETF
 reliable multicast transport (RMT) building blocks in its design.

Adamson, et al. Experimental [Page 1] RFC 3940 NORM Protocol November 2004

Table of Contents

 1.  Introduction and Applicability. . . . . . . . . . . . . . . .   3
     1.1. NORM Delivery Service Model. . . . . . . . . . . . . . .   4
     1.2. NORM Scalability . . . . . . . . . . . . . . . . . . . .   6
     1.3. Environmental Requirements and Considerations. . . . . .   7
 2.  Architecture Definition . . . . . . . . . . . . . . . . . . .   7
     2.1. Protocol Operation Overview. . . . . . . . . . . . . . .   9
     2.2. Protocol Building Blocks . . . . . . . . . . . . . . . .  10
     2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . .  11
 3.  Conformance Statement . . . . . . . . . . . . . . . . . . . .  12
 4.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1. NORM Common Message Header and Extensions. . . . . . . .  14
     4.2. Sender Messages. . . . . . . . . . . . . . . . . . . . .  16
          4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . .  16
          4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . .  24
          4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . .  26
     4.3. Receiver Messages. . . . . . . . . . . . . . . . . . . .  43
          4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . .  43
          4.3.2. NORM_ACK Message. . . . . . . . . . . . . . . . .  50
     4.4. General Purpose Messages . . . . . . . . . . . . . . . .  52
          4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . .  52
 5.  Detailed Protocol Operation . . . . . . . . . . . . . . . . .  52
     5.1. Sender Initialization and Transmission . . . . . . . . .  54
          5.1.1. Object Segmentation Algorithm . . . . . . . . . .  55
     5.2. Receiver Initialization and Reception. . . . . . . . . .  57
     5.3. Receiver NACK Procedure. . . . . . . . . . . . . . . . .  57
     5.4. Sender NACK Processing and Response. . . . . . . . . . .  59
          5.4.1. Sender Repair State Aggregation . . . . . . . . .  60
          5.4.2. Sender FEC Repair Transmission Strategy . . . . .  61
          5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . .  62
          5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation. . . . . .  62
     5.5. Additional Protocol Mechanisms . . . . . . . . . . . . .  63
          5.5.1. Greatest Round-trip Time Collection . . . . . . .  63
          5.5.2. NORM Congestion Control Operation . . . . . . . .  64
          5.5.3. NORM Positive Acknowledgment Procedure. . . . . .  72
          5.5.4. Group Size Estimate . . . . . . . . . . . . . . .  74
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  75
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  75
 8.  Suggested Use . . . . . . . . . . . . . . . . . . . . . . . .  75
 9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  76
 10. References. . . . . . . . . . . . . . . . . . . . . . . . . .  76
     10.1. Normative References. . . . . . . . . . . . . . . . . .  76
     10.2. Informative References. . . . . . . . . . . . . . . . .  77
 11. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  79
     Full Copyright Statement. . . . . . . . . . . . . . . . . . .  80

Adamson, et al. Experimental [Page 2] RFC 3940 NORM Protocol November 2004

1. Introduction and Applicability

 The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM)
 protocol is designed to provide reliable transport of data from one
 or more sender(s) to a group of receivers over an IP multicast
 network.  The primary design goals of NORM are to provide efficient,
 scalable, and robust bulk data (e.g., computer files, transmission of
 persistent data) transfer across possibly heterogeneous IP networks
 and topologies.  The NORM protocol design provides support for
 distributed multicast session participation with minimal coordination
 among senders and receivers.  NORM allows senders and receivers to
 dynamically join and leave multicast sessions at will with minimal
 overhead for control information and timing synchronization among
 participants.  To accommodate this capability, NORM protocol message
 headers contain some common information allowing receivers to easily
 synchronize to senders throughout the lifetime of a reliable
 multicast session.  NORM is designed to be self-adapting to a wide
 range of dynamic network conditions with little or no pre-
 configuration.  The protocol is purposely designed to be tolerant of
 inaccurate timing estimations or lossy conditions that may occur in
 many networks including mobile and wireless.  The protocol is also
 designed to exhibit convergence and efficient operation even in
 situations of heavy packet loss and large queuing or transmission
 delays.
 This document is a product of the IETF RMT WG and follows the
 guidelines provided in RFC 3269 [1].  The key words "MUST", "MUST
 NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
 "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
 interpreted as described in BCP 14, RFC 2119 [2].

Statement of Intent

 This memo contains part of the definitions necessary to fully specify
 a Reliable Multicast Transport protocol in accordance with RFC 2357.
 As per RFC 2357, the use of any reliable multicast protocol in the
 Internet requires an adequate congestion control scheme.
 While waiting for such a scheme to be available, or for an existing
 scheme to be proven adequate, the Reliable Multicast Transport
 working group (RMT) publishes this Request for Comments in the
 "Experimental" category.
 It is the intent of RMT to re-submit this specification as an IETF
 Proposed Standard as soon as the above condition is met.

Adamson, et al. Experimental [Page 3] RFC 3940 NORM Protocol November 2004

1.1. NORM Delivery Service Model

 A NORM protocol instance (NormSession) is defined within the context
 of participants communicating connectionless (e.g., Internet Protocol
 (IP) or User Datagram Protocol (UDP)) packets over a network using
 pre-determined addresses and host port numbers.  Generally, the
 participants exchange packets using an IP multicast group address,
 but unicast transport may also be established or applied as an
 adjunct to multicast delivery.  In the case of multicast, the
 participating NormNodes will communicate using a common IP multicast
 group address and port number that has been chosen via means outside
 the context of the given NormSession.  Other IETF data format and
 protocol standards exist that may be applied to describe and convey
 the required "a priori" information for a specific NormSession (e.g.,
 Session Description Protocol (SDP) [7], Session Announcement Protocol
 (SAP) [8], etc.).
 The NORM protocol design is principally driven by the assumption of a
 single sender transmitting bulk data content to a group of receivers.
 However, the protocol MAY operate with multiple senders within the
 context of a single NormSession.  In initial implementations of this
 protocol, it is anticipated that multiple senders will transmit
 independent of one another and receivers will maintain state as
 necessary for each sender.  However, in future versions of NORM, it
 is possible that some aspects of protocol operation (e.g., round-trip
 time collection) may provide for alternate modes allowing more
 efficient performance for applications requiring multiple senders.
 NORM provides for three types of bulk data content objects
 (NormObjects) to be reliably transported.  These types include:
 1) static computer memory data content (NORM_OBJECT_DATA type),
 2) computer storage files (NORM_OBJECT_FILE type), and
 3) non-finite streams of continuous data content (NORM_OBJECT_STREAM
    type).
 The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is
 simply to provide a "hint" to receivers in NormSessions serving
 multiple types of content as to what type of storage should be
 allocated for received content (i.e., memory or file storage).  Other
 than that distinction, the two are identical, providing for reliable
 transport of finite (but potentially very large) units of content.
 These static data and file services are anticipated to be useful for
 multicast-based cache applications with the ability to reliably
 provide transmission of large quantities of static data.  Other types
 of static data/file delivery services might make use of these

Adamson, et al. Experimental [Page 4] RFC 3940 NORM Protocol November 2004

 transport object types, too.  The use of the NORM_OBJECT_STREAM type
 is at the application's discretion and could be used to carry static
 data or file content also.  The NORM reliable stream service opens up
 additional possibilities such as serialized reliable messaging or
 other unbounded, perhaps dynamically produced content.  The
 NORM_OBJECT_STREAM provides for reliable transport analogous to that
 of the Transmission Control Protocol (TCP), although NORM receivers
 will be able to begin receiving stream content at any point in time.
 The applicability of this feature will depend upon the application.
 The NORM protocol also allows for a small amount of "out-of-band"
 data (sent as NORM_INFO messages) to be attached to the data content
 objects transmitted by the sender.  This readily-available "out-of-
 band" data allows multicast receivers to quickly and efficiently
 determine the nature of the corresponding data, file, or stream bulk
 content being transmitted.  This allows application-level control of
 the receiver node's participation in the current transport activity.
 This also allows the protocol to be flexible with minimal pre-
 coordination among senders and receivers.  The NORM_INFO content is
 designed to be atomic in that its size MUST fit into the payload
 portion of a single NORM message.
 NORM does _not_ provide for global or application-level
 identification of data content within in its message headers.  Note
 the NORM_INFO out-of-band data mechanism could be leveraged by the
 application for this purpose if desired, or identification could
 alternatively be embedded within the data content.  NORM does
 identify transmitted content (NormObjects) with transport identifiers
 that are applicable only while the sender is transmitting and/or
 repairing the given object.  These transport data content identifiers
 (NormTransportIds) are assigned in a monotonically increasing fashion
 by each NORM sender during the course of a NormSession.  Each sender
 maintains its NormTransportId assignments independently so that
 individual NormObjects may be uniquely identified during transport
 with the concatenation of the sender session-unique identifier
 (NormNodeId) and the assigned NormTransportId.  The NormTransportIds
 are assigned from a large, but fixed, numeric space in increasing
 order and may be reassigned during long-lived sessions.  The NORM
 protocol provides mechanisms so that the sender application may
 terminate transmission of data content and inform the group of this
 in an efficient manner.  Other similar protocol control mechanisms
 (e.g., session termination, receiver synchronization, etc.) are
 specified so that reliable multicast application variants may
 construct different, complete bulk transfer communication models to
 meet their goals.

Adamson, et al. Experimental [Page 5] RFC 3940 NORM Protocol November 2004

 To summarize, the NORM protocol provides reliable transport of
 different types of data content (including potentially mixed types).
 The senders enqueue and transmit bulk content in the form of static
 data or files and/or non-finite, ongoing stream types.  NORM senders
 provide for repair transmission of data and/or FEC content in
 response to NACK messages received from the receiver group.
 Mechanisms for "out-of-band" information and other transport control
 mechanisms are specified for use by applications to form complete
 reliable multicast solutions for different purposes.

1.2. NORM Scalability

 Group communication scalability requirements lead to adaptation of
 negative acknowledgment (NACK) based protocol schemes when feedback
 for reliability is required [9].  NORM is a protocol centered around
 the use of selective NACKs to request repairs of missing data.  NORM
 provides for the use of packet-level forward error correction (FEC)
 techniques for efficient multicast repair and optional proactive
 transmission robustness [10].  FEC-based repair can be used to
 greatly reduce the quantity of reliable multicast repair requests and
 repair transmissions [11] in a NACK-oriented protocol.  The principal
 factor in NORM scalability is the volume of feedback traffic
 generated by the receiver set to facilitate reliability and
 congestion control.  NORM uses probabilistic suppression of redundant
 feedback based on exponentially distributed random backoff timers.
 The performance of this type of suppression relative to other
 techniques is described in [12].  NORM dynamically measures the
 group's roundtrip timing status to set its suppression and other
 protocol timers.  This allows NORM to scale well while maintaining
 reliable data delivery transport with low latency relative to the
 network topology over which it is operating.
 Feedback messages can be either multicast to the group at large or
 sent via unicast routing to the sender.  In the case of unicast
 feedback, the sender "advertises" the feedback state to the group to
 facilitate feedback suppression.  In typical Internet environments,
 it is expected that the NORM protocol will readily scale to group
 sizes on the order of tens of thousands of receivers.  A study of the
 quantity of feedback for this type of protocol is described in [13].
 NORM is able to operate with a smaller amount of feedback than a
 single TCP connection, even with relatively large numbers of
 receivers.  Thus, depending upon the network topology, it is possible
 that NORM may scale to larger group sizes.  With respect to computer
 resource usage, the NORM protocol does _not_ require that state be
 kept on all receivers in the group.  NORM senders maintain state only
 for receivers providing explicit congestion control feedback.  NORM
 receivers must maintain state for each active sender.  This may
 constrain the number of simultaneous senders in some uses of NORM.

Adamson, et al. Experimental [Page 6] RFC 3940 NORM Protocol November 2004

1.3. Environmental Requirements and Considerations

 All of the environmental requirements and considerations that apply
 to the RMT NORM Building Block [4] and the RMT FEC Building Block [5]
 also apply to the NORM protocol.
 The NORM protocol SHALL be capable of operating in an end-to-end
 fashion with no assistance from intermediate systems beyond basic IP
 multicast group management, routing, and forwarding services.  While
 the techniques utilized in NORM are principally applicable to "flat"
 end-to-end IP multicast topologies, they could also be applied in the
 sub-levels of hierarchical (e.g., tree-based) multicast distribution
 if so desired.  NORM can make use of reciprocal (among senders and
 receivers) multicast communication under the Any-Source Multicast
 (ASM) model defined in RFC 1112 [3], but SHALL also be capable of
 scalable operation in asymmetric topologies such as Source Specific
 Multicast (SSM) [14] where there may only be unicast routing service
 from the receivers to the sender(s).
 NORM is compatible with IPv4 and IPv6.  Additionally, NORM may be
 used with networks employing Network Address Translation (NAT)
 providing the NAT device supports IP multicast and/or can cache UDP
 traffic source port numbers for remapping feedback traffic from
 receivers to the sender(s).

2. Architecture Definition

 A NormSession is comprised of participants (NormNodes) acting as
 senders and/or receivers.  NORM senders transmit data content in the
 form of NormObjects to the session destination address and the NORM
 receivers attempt to reliably receive the transmitted content using
 negative acknowledgments to request repair.  Each NormNode within a
 NormSession is assumed to have a preselected unique 32-bit identifier
 (NormNodeId).  NormNodes MUST have uniquely assigned identifiers
 within a single NormSession to distinguish  between possible multiple
 senders and to distinguish feedback information from different
 receivers.  There are two reserved NormNodeId values.  A value of
 0x00000000 is considered an invalid NormNodeId value and a value of
 0xffffffff is a "wildcard" NormNodeId.  While the protocol does not
 preclude multiple sender nodes concurrently transmitting within the
 context of a single NORM session (i.e., many-to-many operation), any
 type of interactive coordination among NORM senders is assumed to be
 controlled by the application or higher protocol layer.  There are
 some optional mechanisms specified in this document that can be
 leveraged for such application layer coordination.

Adamson, et al. Experimental [Page 7] RFC 3940 NORM Protocol November 2004

 As previously noted, NORM allows for reliable transmission of three
 different basic types of data content.  The first type is
 NORM_OBJECT_DATA, which is used for static, persistent blocks of data
 content maintained in the sender's application memory storage.  The
 second type is NORM_OBJECT_FILE, which corresponds to data stored in
 the sender's non-volatile file system.  The NORM_OBJECT_DATA and
 NORM_OBJECT_FILE types both represent "NormObjects" of finite but
 potentially very large size.  The third type of data content is
 NORM_OBJECT_STREAM, which corresponds to an ongoing transmission of
 undefined length.  This is analogous to the reliable stream service
 provide by TCP for unicast data transport.  The format of the stream
 content is application-defined and may be byte or message oriented.
 The NORM protocol provides for "flushing" of the stream to expedite
 delivery or possibly enforce application message boundaries.  NORM
 protocol implementations may offer either (or both) in-order delivery
 of the stream data to the receive application or out-of-order (more
 immediate) delivery of received segments of the stream to the
 receiver application.  In either case, NORM sender and receiver
 implementations provide buffering to facilitate repair of the stream
 as it is transported.
 All NormObjects are logically segmented into FEC coding blocks and
 symbols for transmission by the sender.  In NORM, an FEC encoding
 symbol directly corresponds to the payload of NORM_DATA messages or
 "segment".  Note that when systematic FEC codes are used, the payload
 of NORM_DATA messages sent for the first portion of a FEC encoding
 block are source symbols (actual segments of original user data),
 while the remaining symbols for the block consist of parity symbols
 generated by FEC encoding.  These parity symbols are generally sent
 in response to repair requests, but some number may be sent
 proactively at the end each encoding block to increase the robustness
 of transmission.  When non-systematic FEC codes are used, all symbols
 sent consist of FEC encoding parity content.  In this case, the
 receiver must receive a sufficient number of symbols to reconstruct
 (via FEC decoding) the original user data for the given block.  In
 this document, the terms "symbol" and "segment" are used
 interchangeably.
 Transmitted NormObjects are temporarily yet uniquely identified
 within the NormSession context using the given sender's NormNodeId,
 NormInstanceId, and a temporary NormObjectTransportId.  Depending
 upon the implementation, individual NORM senders may manage their
 NormInstanceIds independently, or a common NormInstanceId may be
 agreed upon for all participating nodes within a session if needed as
 a session identifier.  NORM NormObjectTransportId data content
 identifiers are sender-assigned and applicable and valid only during
 a NormObject's actual _transport_ (i.e., for as long as the sender is
 transmitting and providing repair of the indicated NormObject).  For

Adamson, et al. Experimental [Page 8] RFC 3940 NORM Protocol November 2004

 a long-lived session, the NormObjectTransportId field can wrap and
 previously-used identifiers may be re-used.  Note that globally
 unique identification of transported data content is not provided by
 NORM and, if required, must be managed by the NORM application.  The
 individual segments or symbols of the NormObject are further
 identified with FEC payload identifiers which include coding block
 and symbol identifiers.  These are discussed in detail later in this
 document.

2.1. Protocol Operation Overview

 A NORM sender primarily generates messages of type NORM_DATA.  These
 messages carry original data segments or FEC symbols and repair
 segments/symbols for the bulk data/file or stream NormObjects being
 transferred.  By default, redundant FEC symbols are sent only in
 response to receiver repair requests (NACKs) and thus normally little
 or no additional transmission overhead is imposed due to FEC
 encoding.  However, the NORM implementation MAY be optionally
 configured to proactively transmit some amount of redundant FEC
 symbols along with the original content to potentially enhance
 performance (e.g., improved delay) at the cost of additional
 transmission overhead.  This option may be sensible for certain
 network conditions and can allow for robust, asymmetric multicast
 (e.g., unidirectional routing, satellite, cable) [15] with reduced
 receiver feedback, or, in some cases, no feedback.
 A sender message of type NORM_INFO is also defined and is used to
 carry OPTIONAL "out-of-band" context information for a given
 transport object.  A single NORM_INFO message can be associated with
 a NormObject.  Because of its atomic nature, missing NORM_INFO
 messages can be NACKed and repaired with a slightly lower delay
 process than NORM's general FEC-encoded data content.  NORM_INFO may
 serve special purposes for some bulk transfer, reliable multicast
 applications where receivers join the group mid-stream and need to
 ascertain contextual information on the current content being
 transmitted.  The NACK process for NORM_INFO will be described later.
 When the NORM_INFO message type is used, its transmission should
 precede transmission of any NORM_DATA message for the associated
 NormObject.
 The sender also generates messages of type NORM_CMD to assist in
 certain protocol operations such as congestion control, end-of-
 transmission flushing, round trip time estimation, receiver
 synchronization, and optional positive acknowledgment requests or
 application defined commands.  The transmission of NORM_CMD messages
 from the sender is accomplished by one of three different procedures.
 These procedures are: single, best effort unreliable transmission of
 the command; repeated redundant transmissions of the command; and

Adamson, et al. Experimental [Page 9] RFC 3940 NORM Protocol November 2004

 positively-acknowledged commands.  The transmission technique used
 for a given command depends upon the function of the command.
 Several core commands are defined for basic protocol operation.
 Additionally, implementations MAY wish to consider providing the
 OPTIONAL application-defined commands that can take advantage of the
 transmission methodologies available for commands.  This allows for
 application-level session management mechanisms that can make use of
 information available to the underlying NORM protocol engine (e.g.,
 round-trip timing, transmission rate, etc.).
 NORM receivers generate messages of type NORM_NACK or NORM_ACK in
 response to transmissions of data and commands from a sender.  The
 NORM_NACK messages are generated to request repair of detected data
 transmission losses.  Receivers generally detect losses by tracking
 the sequence of transmission from a sender.  Sequencing information
 is embedded in the transmitted data packets and end-of-transmission
 commands from the sender.  NORM_ACK messages are generated in
 response to certain commands transmitted by the sender.  In the
 general (and most scalable) protocol mode, NORM_ACK messages are sent
 only in response to congestion control commands from the sender.  The
 feedback volume of these congestion control NORM_ACK messages is
 controlled using the same timer-based probabilistic suppression
 techniques as for NORM_NACK messages to avoid feedback implosion.  In
 order to meet potential application requirements for positive
 acknowledgment from receivers, other NORM_ACK messages are defined
 and available for use.  All sender and receiver transmissions are
 subject to rate control governed by a peak transmission rate set for
 each participant by the application.  This can be used to limit the
 quantity of multicast data transmitted by the group.  When NORM's
 congestion control algorithm is enabled the rate for senders is
 automatically adjusted.  In some networks, it may be desirable to
 establish minimum and maximum bounds for the rate adjustment
 depending upon the application even when dynamic congestion control
 is enabled.  However, in the case of the general Internet, congestion
 control policy SHALL be observed that is compatible with coexistent
 TCP flows.

2.2. Protocol Building Blocks

 The operation of the NORM protocol is based primarily upon the
 concepts presented in the Nack-Oriented Reliable Multicast (NORM)
 Building Block document [4].  This includes the basic NORM
 architecture and the data transmission, repair, and feedback
 strategies discussed in that document.  Additional reliable multicast
 building blocks are applied in creating the full NORM protocol
 instantiation [16].  NORM also makes use of Forward Error Correction
 encoding techniques for repair messaging and optional transmission
 robustness as described in [10].  NORM uses the FEC Payload ID as

Adamson, et al. Experimental [Page 10] RFC 3940 NORM Protocol November 2004

 specified by the FEC Building Block Document [5].  Additionally, for
 congestion control, this document includes a baseline congestion
 control mechanism (NORM-CC) based on the TCP-Friendly Multicast
 Congestion Control (TFMCC) scheme described in [19].

2.3. Design Tradeoffs

 While the various features of NORM are designed to provide some
 measure of general purpose utility, it is important to emphasize the
 understanding that "no one size fits all" in the reliable multicast
 transport arena.  There are numerous engineering tradeoffs involved
 in reliable multicast transport design and this requires an increased
 awareness of application and network architecture considerations.
 Performance requirements affecting design can include:  group size,
 heterogeneity (e.g., capacity and/or delay), asymmetric delivery,
 data ordering, delivery delay, group dynamics, mobility, congestion
 control, and transport across low capacity connections.  NORM
 contains various parameters to accommodate many of these differing
 requirements.  The NORM protocol and its mechanisms MAY be applied in
 multicast applications outside of bulk data transfer, but there is an
 assumed model of bulk transfer transport service that drives the
 trade-offs that determine the scalability and performance described
 in this document.
 The ability of NORM to provide reliable data delivery is also
 governed by any buffer constraints of the sender and receiver
 applications.  NORM protocol implementations SHOULD be designed to
 operate with the greatest efficiency and robustness possible within
 application-defined buffer constraints.  Buffer requirements for
 reliability, as always, are a function of the delay-bandwidth product
 of the network topology.  NORM performs best when allowed more
 buffering resources than typical point-to-point transport protocols.
 This is because NORM feedback suppression is based upon randomly-
 delayed transmissions from the receiver set, rather than immediately
 transmitted feedback.  There are definitive tradeoffs between buffer
 utilization, group size scalability, and efficiency of performance.
 Large buffer sizes allow the NORM protocol to perform most
 efficiently in large delay-bandwidth topologies and allow for longer
 feedback suppression backoff timeouts.  This yields improved group
 size scalability.  NORM can operate with reduced buffering but at a
 cost of decreased efficiency (lower relative goodput) and reduced
 group size scalability.

Adamson, et al. Experimental [Page 11] RFC 3940 NORM Protocol November 2004

3. Conformance Statement

 This Protocol Instantiation document, in conjunction with the RMT
 Building Block documents of [4] and [5], completely specifies a
 working reliable multicast transport protocol that conforms to the
 requirements described in RFC 2357 [17].
 This document specifies the following message types and mechanisms
 which are REQUIRED in complying NORM protocol implementations:

+——————–+———————————————–+

Message Type Purpose

+——————–+———————————————–+

NORM_DATA Sender message for application data
transmission. Implementations must support
at least one of the NORM_OBJECT_DATA,
NORM_OBJECT_FILE, or NORM_OBJECT_STREAM
delivery services. The use of the NORM FEC
Object Transmission Information header
extension is OPTIONAL with NORM_DATA
messages.

+——————–+———————————————–+

NORM_CMD(FLUSH) Sender command to excite receivers for repair
requests in lieu of ongoing NORM_DATA
transmissions. Note the use of the
NORM_CMD(FLUSH) for positive acknowledgment
of data receipt is OPTIONAL.

+——————–+———————————————–+

NORM_CMD(SQUELCH) Sender command to advertise its current valid
repair window in response to invalid requests
for repair.

+——————–+———————————————–+

NORM_CMD(REPAIR_ADV) Sender command to advertise current repair
(and congestion control state) to group when
unicast feedback messages are detected. Used
to control/suppress excessive receiver
feedback in asymmetric multicast topologies.

+——————–+———————————————–+

NORM_CMD(CC) Sender command used in collection of round
trip timing and congestion control status
from group (this may be OPTIONAL if
alternative congestion control mechanism and
round trip timing collection is used).

+——————–+———————————————–+

NORM_NACK Receiver message used to request repair of
missing transmitted content.

+——————–+———————————————–+

Adamson, et al. Experimental [Page 12] RFC 3940 NORM Protocol November 2004

+——————–+———————————————–+

NORM_ACK Receiver message used to proactively provide
feedback for congestion control purposes.
Also used with the OPTIONAL NORM Positive
Acknowledgment Process.

+——————–+———————————————–+

 This document also describes the following message types and
 associated mechanisms which are OPTIONAL for complying NORM protocol
 implementations:

+———————-+———————————————-+

Message Type Purpose

+———————-+———————————————-+

NORM_INFO Sender message for providing ancillary
context information associated with NORM
transport objects. The use of the NORM FEC
Object Transmission Information header
extension is OPTIONAL with NORM_INFO
messages.

+———————-+———————————————-+

NORM_CMD(EOT) Sender command to indicate it has reached
end-of-transmission and will no longer
respond to repair requests.

+———————-+———————————————-+

NORM_CMD(ACK_REQ) Sender command to support application-
defined, positively acknowledged commands
sent outside of the context of the bulk data
content being transmitted. The NORM Positive
Acknowledgment Procedure associated with this
message type is OPTIONAL.

+———————-+———————————————-+

NORM_CMD(APPLICATION) Sender command containing application-defined
commands sent outside of the context of the
bulk data content being transmitted.

+———————-+———————————————-+

NORM_REPORT Optional message type reserved for
experimental implementations of the NORM
protocol.

+———————-+———————————————-+

4. Message Formats

 As mentioned in Section 2.1, there are two primary classes of NORM
 messages: sender messages and receiver messages.  NORM_CMD,
 NORM_INFO, and NORM_DATA message types are generated by senders of
 data content, and NORM_NACK and NORM_ACK messages generated by
 receivers within a NormSession.  An auxiliary message type of

Adamson, et al. Experimental [Page 13] RFC 3940 NORM Protocol November 2004

 NORM_REPORT is also provided for experimental purposes.  This section
 describes the message formats used by the NORM protocol.  These
 messages and their fields are referenced in the detailed functional
 description of the NORM protocol given in Section 5.  Individual NORM
 messages are designed to be compatible with the MTU limitations of
 encapsulating Internet protocols including IPv4, IPv6, and UDP.  The
 current NORM protocol specification assumes UDP encapsulation and
 leverages the transport features of UDP.  The NORM messages are
 independent of network addresses and can be used in IPv4 and IPv6
 networks.

4.1. NORM Common Message Header and Extensions

 There are some common message fields contained in all NORM message
 types.  Additionally, a header extension mechanism is defined to
 expand the functionality of the NORM protocol without revision to
 this document.  All NORM protocol messages begin with a common header
 with information fields as follows:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version|  type |    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   NORM Common Message Header Format
 The "version" field is a 4-bit value indicating the protocol version
 number.  NORM implementations SHOULD ignore received messages with
 version numbers different from their own.  This number is intended to
 indicate and distinguish upgrades of the protocol which may be non-
 interoperable.  The NORM version number for this specification is 1.
 The message "type" field is a 4-bit value indicating the NORM
 protocol message type.  These types are defined as follows:
         Message     Value
       NORM_INFO       1
       NORM_DATA       2
       NORM_CMD        3
       NORM_NACK       4
       NORM_ACK        5
       NORM_REPORT     6

Adamson, et al. Experimental [Page 14] RFC 3940 NORM Protocol November 2004

 The 8-bit "hdr_len" field indicates the number of 32-bit words that
 comprise the given message's header portion.  This is used to
 facilitate header extensions that may be applied.  The presence of
 header extensions are implied when the "hdr_len" value is greater
 than the base value for the given message "type".
 The "sequence" field is a 16-bit value that is set by the message
 originator as a monotonically increasing number incremented with each
 NORM message transmitted to a given destination address.  A
 "sequence" field number space SHOULD be maintained for messages sent
 to the NormSession group address.  This value can be monitored by
 receiving nodes to detect packet losses in the transmission from a
 sender and used in estimating raw packet loss for congestion control
 purposes.  Note that this value is NOT used in the NORM protocol to
 detect missing reliable data content and does NOT identify the
 application data or FEC payload that may be attached.  With message
 authentication, the "sequence" field may also be leveraged for
 protection from message "replay" attacks, particularly of NORM_NACK
 or other feedback messages.  In this case, the receiver node should
 maintain a monotonically increasing "sequence" field space for each
 destination to which it transmits (this may be multiple destinations
 when unicast feedback is used).  The size of this field is intended
 to be sufficient to allow detection of a reasonable range of packet
 loss within the delay-bandwidth product of expected network
 connections.
 The "source_id" field is a 32-bit value identifying the node that
 sent the message.  A participant's NORM node identifier (NormNodeId)
 can be set according to application needs but unique identifiers must
 be assigned within a single NormSession.  In some cases, use of the
 host IP address or a hash of it can suffice, but alternative
 methodologies for assignment and potential collision resolution of
 node identifiers within a multicast session need to be considered.
 For example, the "source identifier" mechanism defined in the Real-
 Time Protocol (RTP) specification [18] may be applicable to use for
 NORM node identifiers.  At this point in time, the protocol makes no
 assumptions about how these unique identifiers are actually assigned.
 NORM Header Extensions
 When header extensions are applied, they follow the message type's
 base header and precede any payload portion.  There are two formats
 for header extensions, both of which begin with an 8-bit "het"
 (header extension type) field.  One format is provided for variable-
 length extensions with "het" values in the range from 0 through 127.
 The other format is for fixed length (one 32-bit word) extensions
 with "het" values in the range from 128 through 255.  These formats
 are given here:

Adamson, et al. Experimental [Page 15] RFC 3940 NORM Protocol November 2004

    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   het <=127   |      hel      |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                    Header Extension Content                   |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            NORM Variable Length Header Extension Format
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   het >=128   |   reserved    |    Header Extension Content   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         NORM Fixed Length (32-bit) Header Extension Format
 The "Header Extension Content" portion of these header extension
 format is defined for each header extension type defined for NORM
 messages.  Some header extensions are defined within this document
 for NORM baseline FEC and congestion control operations.

4.2. Sender Messages

 NORM sender messages include the NORM_DATA type, the NORM_INFO type,
 and the NORM_CMD type.  NORM_DATA and NORM_INFO messages contain
 application data content while NORM_CMD messages are used for various
 protocol control functions.

4.2.1. NORM_DATA Message

 The NORM_DATA message is expected to be the predominant type
 transmitted by NORM senders.  These messages are used to encapsulate
 segmented data content for objects of type NORM_OBJECT_DATA,
 NORM_OBJECT_FILE, and NORM_OBJECT_STREAM.  NORM_DATA messages may
 contain original or FEC-encoded application data content.
 The format of NORM_DATA messages is comprised of three logical
 portions:  1) a fixed-format NORM_DATA header portion, 2) a FEC
 Payload ID portion with a format dependent upon the FEC encoding
 used, and 3) a payload portion containing source or encoded
 application data content.  Note for objects of type
 NORM_OBJECT_STREAM, the payload portion contains additional fields
 used to appropriately recover stream content.  NORM implementations
 MAY also extend the NORM_DATA header to include a FEC Object

Adamson, et al. Experimental [Page 16] RFC 3940 NORM Protocol November 2004

 Transmission Information (EXT_FTI) header extension.  This allows
 NORM receivers to automatically allocate resources and properly
 perform FEC decoding without the need for pre-configuration or out-
 of-band information.
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=2|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     flags     |    fec_id     |     object_transport_id       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         fec_payload_id                        |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                header_extensions (if applicable)              |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       payload_reserved*       |          payload_len*         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        payload_offset*                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          payload_data*                        |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      NORM_DATA Message Format
  • NOTE: The "payload_reserved", "payload_len" and "payload_offset"

fields are present only for objects of type NORM_OBJECT_STREAM. The

 "payload_len" and "payload_offset" fields allow senders to
 arbitrarily vary the size of NORM_DATA payload segments for streams.
 This allows applications to flush transmitted streams as needed to
 meet unique streaming requirements.  For objects of types
 NORM_OBJECT_FILE and NORM_OBJECT_DATA, these fields are unnecessary
 since the receiver can calculate the payload length and offset
 information from the "fec_payload_id" using the algorithm described
 in Section 5.1.1.  The "payload_reserved" field is kept for
 anticipated future NORM stream control functions.  When systematic
 FEC codes (e.g., "fec_id" = 129) are used, the "payload_len" and
 "payload_offset" fields contain actual length and offset values for
 the encapsulated application data segment for those NORM_DATA
 messages containing source data symbols.  In NORM_DATA messages that
 contain parity information, these fields are not actual length or

Adamson, et al. Experimental [Page 17] RFC 3940 NORM Protocol November 2004

 offset values, but instead are values computed from FEC encoding the
 "payload_len" and "payload_offset" fields of the _source_ data
 symbols of the corresponding applicable coding block.
 The "version", "type", "hdr_len", "sequence", and "source_id" fields
 form the NORM Common Message Header as described in Section 4.1.  The
 value of the NORM_DATA "type" field is 2.  The NORM_DATA _base_
 "hdr_len" value is 4 (32-bit words) plus the size of the
 "fec_payload_id" field.  The "fec_payload_id" field size depends upon
 the FEC encoding used for the referenced NormObject.  The "fec_id"
 field is used to indicate the FEC coding type.  For example, when
 small block, systematic codes are used, a "fec_id" value of 129 is
 indicated and the size of the "fec_payload_id" is two 32-bit words.
 In this case the NORM_DATA base "hdr_len" value is 6.  The cumulative
 size of any header extensions applied is added into the "hdr_len"
 field.
 The "instance_id" field contains a value generated by the sender to
 uniquely identify its current instance of participation in the
 NormSession.  This allows receivers to detect when senders have
 perhaps left and rejoined a session in progress.  When a sender
 (identified by its "source_id") is detected to have a new
 "instance_id", the NORM receivers SHOULD drop their previous state on
 the sender and begin reception anew.
 The "grtt" field contains a non-linear quantized representation of
 the sender's current estimate of group round-trip time (GRTT) (this
 is also referred to as R_max in [19]).  This value is used to control
 timing of the NACK repair process and other aspects of protocol
 operation as described in this document.  The algorithm for encoding
 and decoding this field is described in the RMT NORM Building Block
 document [4].
 The "backoff" field value is used by receivers to determine the
 maximum backoff timer value used in the timer-based NORM NACK
 feedback suppression.  This 4-bit field supports values from 0-15
 which is multiplied by the sender GRTT to determine the maximum
 backoff timeout.  The "backoff" field informs the receiver set of the
 sender's backoff factor parameter "Ksender".  Recommended values and
 their use are described in the NORM receiver NACK procedure
 description in Section 5.3.  The "gsize" field contains a
 representation of the sender's current estimate of group size.  This
 4-bit field can roughly represent values from ten to 500 million
 where the most significant bit value of 0 or 1 represents a mantissa
 of 1 or 5, respectively and the three least significant bits
 incremented by one represent a base 10 exponent (order of magnitude).
 For examples, a field value of "0x0" represents 1.0e+01 (10), a value
 of "0x8" represents 5.0e+01 (50), a value of "0x1" represents 1.0e+02

Adamson, et al. Experimental [Page 18] RFC 3940 NORM Protocol November 2004

 (100), and a value of "0xf" represents 5.0e+08.  For NORM feedback
 suppression purposes, the group size does not need to be represented
 with a high degree of precision.  The group size may even be
 estimated somewhat conservatively (i.e., overestimated) to maintain
 low levels of feedback traffic.  A default group size estimate of
 10,000 ("gsize" = 0x4) is recommended for general purpose reliable
 multicast applications using the NORM protocol.
 The "flags" field contains a number of different binary flags
 providing information and hints regarding how the receiver should
 handle the identified object.  Defined flags in this field include:

+——————–+——-+—————————————–+

Flag Value Purpose

+——————–+——-+—————————————–+

NORM_FLAG_REPAIR 0x01 Indicates message is a repair
transmission

+——————–+——-+—————————————–+

NORM_FLAG_EXPLICIT 0x02 Indicates a repair segment intended to
meet a specific receiver erasure, as
compared to parity segments provided by
the sender for general purpose (with
respect to an FEC coding block) erasure
filling.

+——————–+——-+—————————————–+

NORM_FLAG_INFO 0x04 Indicates availability of NORM_INFO for
object.

+——————–+——-+—————————————–+

NORM_FLAG_UNRELIABLE 0x08 Indicates that repair transmissions for
the specified object will be unavailable
(One-shot, best effort transmission).

+——————–+——-+—————————————–+

NORM_FLAG_FILE 0x10 Indicates object is "file-based" data
(hint to use disk storage for
reception).

+——————–+——-+—————————————–+

NORM_FLAG_STREAM 0x20 Indicates object is of type
NORM_OBJECT_STREAM.

+——————–+——-+—————————————–+

NORM_FLAG_MSG_START 0x40 Marks the first segment of application
messages embedded in
NORM_OBJECT_STREAMs.

+——————–+——-+—————————————–+

 NORM_FLAG_REPAIR is set when the associated message is a repair
 transmission.  This information can be used by receivers to help
 observe a join policy where it is desired that newly joining
 receivers only begin participating in the NACK process upon receipt

Adamson, et al. Experimental [Page 19] RFC 3940 NORM Protocol November 2004

 of new (non-repair) data content.  NORM_FLAG_EXPLICIT is used to mark
 repair messages sent when the data sender has exhausted its ability
 to provide "fresh" (previously untransmitted) parity segments as
 repair.  This flag could possibly be used by intermediate systems
 implementing functionality to control sub-casting of repair content
 to different legs of a reliable multicast topology with disparate
 repair needs.  NORM_FLAG_INFO is set only when optional NORM_INFO
 content is actually available for the associated object.  Thus,
 receivers will NACK for retransmission of NORM_INFO only when it is
 available for a given object.  NORM_FLAG_UNRELIABLE is set when the
 sender wishes to transmit an object with only "best effort" delivery
 and will not supply repair transmissions for the object.  NORM
 receivers SHOULD NOT execute repair requests for objects marked with
 the NORM_FLAG_UNRELIABLE flag.  Note that receivers may inadvertently
 request repair of such objects when all segments (or info content)
 for those objects are not received (i.e., a gap in the
 "object_transport_id" sequence is noted).  In this case, the sender
 should invoke the NORM_CMD(SQUELCH) process as described in Section
 4.2.3.  NORM_FLAG_FILE can be set as a "hint" from the sender that
 the associated object should be stored in non-volatile storage.
 NORM_FLAG_STREAM is set when the identified object is of type
 NORM_OBJECT_STREAM.  When NORM_FLAG_STREAM is set, the
 NORM_FLAG_MSG_START can be optionally used to mark the first data
 segments of application-layer messages transported within the NORM
 stream.  This allows NORM receiver applications to "synchronize" with
 NORM senders and to be able to properly interpret application layer
 data when joining a NORM session already in progress.  In practice,
 the NORM implementation MAY set this flag for the segment transmitted
 following an explicit "flush" of the stream by the application.
 The "fec_id" field corresponds to the FEC Encoding Identifier
 described in the FEC Building Block document [5].  The "fec_id" value
 implies the format of the "fec_payload_id" field and, coupled with
 FEC Object Transmission Information, the procedures to decode FEC
 encoded content.  Small block, systematic codes ("fec_id" = 129) are
 expected to be used for most NORM purposes and the NORM_OBJECT_STREAM
 requires systematic FEC codes for most efficient performance.
 The "object_transport_id" field is a monotonically and incrementally
 increasing value assigned by the sender to NormObjects being
 transmitted.  Transmissions and repair requests related to that
 object use the same "object_transport_id" value.  For sessions of
 very long or indefinite duration, the "object_transport_id" field may
 be repeated, but it is presumed that the 16-bit field size provides
 an adequate enough sequence space to avoid object confusion amongst
 receivers and sources (i.e., receivers SHOULD re-synchronize with a
 server when receiving object sequence identifiers sufficiently out-
 of-range with the current state kept for a given source).  During the

Adamson, et al. Experimental [Page 20] RFC 3940 NORM Protocol November 2004

 course of its transmission within a NORM session, an object is
 uniquely identified by the concatenation of the sender "source_id"
 and the given "object_transport_id".  Note that NORM_INFO messages
 associated with the identified object carry the same
 "object_transport_id" value.
 The "fec_payload_id" identifies the attached NORM_DATA "payload"
 content.  The size and format of the "fec_payload_id" field depends
 upon the FEC type indicated by the "fec_id" field.  These formats are
 given in the FEC Building Block document [5] and any subsequent
 extensions of that document.  As an example, the format of the
 "fec_payload_id" format small block, systematic codes ("fec_id" =
 129) given here:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       source_block_number                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        source_block_len       |      encoding_symbol_id       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Small Block, Systematic Code ("fec_id" = 129) "fec_payload_id" Format
 The FEC payload identifier "source_block_number", "source_block_len",
 and "encoding_symbol_id" fields correspond to the "Source Block
 Number", "Source Block Length, and "Encoding Symbol ID" fields of the
 FEC Payload ID format given by the IETF FEC Building Block document
 [5].  The "source_block_number" identifies the coding block's
 relative position with a NormObject.  Note that, for NormObjects of
 type NORM_OBJECT_STREAM, the "source_block_number" may wrap for very
 long lived sessions.  The "source_block_len" indicates the number of
 user data segments in the identified coding block.  Given the
 "source_block_len" information of how many symbols of application
 data are contained in the block, the receiver can determine whether
 the attached segment is data or parity content and treat it
 appropriately.  The "encoding_symbol_id" identifies which specific
 symbol (segment) within the coding block the attached payload
 conveys.  Depending upon the value of the "encoding_symbol_id" and
 the associated "source_block_len" parameters for the block, the
 symbol (segment) referenced may be a user data or an FEC parity
 segment.  For systematic codes, encoding symbols numbered less than
 the source_block_len contain original application data while segments
 greater than or equal to source_block_len contain parity symbols
 calculated for the block.  The concatenation of

Adamson, et al. Experimental [Page 21] RFC 3940 NORM Protocol November 2004

 object_transport_id::fec_payload_id can be viewed as a unique
 transport protocol data unit identifier for the attached segment with
 respect to the NORM sender's instance within a session.
 Additional FEC Object Transmission Information (as described in the
 FEC Building Block document [5]) is required to properly receive and
 decode NORM transport objects.  This information MAY be provided as
 out-of-band session information.  However, in some cases, it may be
 useful for the sender to include this information "in band" to
 facilitate receiver operation with minimal preconfiguration.  For
 this purpose, the NORM FEC Object Transmission Information Header
 Extension (EXT_FTI) is defined.  This header extension MAY be applied
 to NORM_DATA and NORM_INFO messages to provide this necessary
 information.  The exact format of the extension depends upon the FEC
 code in use, but in general it SHOULD contain any required details on
 the FEC code in use (e.g., FEC Instance ID, etc.) and the byte size
 of the associated NormObject (For the NORM_OBJECT_STREAM type, this
 size corresponds to the stream buffer size maintained by the NORM
 sender).  As an example, the format of the EXT_FTI for small block
 systematic codes ("fec_id" = 129) is given here:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    het = 64   |    hel = 4    |      object_length (msb)      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      object_length (lsb)                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       fec_instance_id         |          segment_size         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       fec_max_block_len       |         fec_num_parity        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 FEC Object Transmission Information Header Extension (EXT_FTI) for
 Small Block Systematic Codes ("fec_id" = 129)
 The header extension type "het" field value for this header extension
 is 64.  The header extension length "hel" depends upon the format of
 the FTI for FEC code type identified by the "fec_id" field.  In this
 example (for "fec_id" = 129), the "hel" field value is 4.
 The 48-bit "object_length" field indicates the total size of the
 object (in bytes) for the static object types of NORM_OBJECT_FILE and
 NORM_OBJECT_DATA.  This information is used by receivers to determine
 storage requirements and/or allocate storage for the received object.
 Receivers with insufficient storage capability may wish to forego
 reliable reception (i.e., not NACK for) of the indicated object.  In
 the case of objects of type NORM_OBJECT_STREAM, the "object_length"

Adamson, et al. Experimental [Page 22] RFC 3940 NORM Protocol November 2004

 field is used by the sender to indicate the size of its stream buffer
 to the receiver group.  In turn, the receivers SHOULD use this
 information to allocate a stream buffer for reception of
 corresponding size.
 The "fec_instance_id" corresponds to the "FEC Instance ID" described
 in the FEC Building Block document [5].  In this case, the
 "fec_instance_id" SHALL be a value corresponding to the particular
 type of Small Block Systematic Code being used (e.g., Reed-Solomon
 GF(2^8), Reed-Solomon GF(2^16), etc).  The standardized assignment of
 FEC Instance ID values is described in [5].  The "segment_size" field
 indicates the sender's current setting for maximum message payload
 content (in bytes).  This allows receivers to allocate appropriate
 buffering resources and to determine other information in order to
 properly process received data messaging.
 The "fec_max_block_len" indicates the current maximum number of user
 data segments per FEC coding block to be used by the sender during
 the session.  This allows receivers to allocate appropriate buffer
 space for buffering blocks transmitted by the sender.
 The "fec_num_parity" corresponds to the "maximum number of encoding
 symbols that can be generated for any source block" as described in
 for FEC Object Transmission Information for Small Block Systematic
 Codes in the FEC Building Block document [5].  For example, Reed-
 Solomon codes may be arbitrarily shortened to create different code
 variations for a given block length.  In the case of Reed-Solomon
 (GF(2^8) and GF(2^16)) codes, this value indicates the maximum number
 of parity segments available from the sender for the coding blocks.
 This field MAY be interpreted differently for other systematic codes
 as they are defined.
 The payload portion of NORM_DATA messages includes source data or FEC
 encoded application content.
 The "payload_reserved", "payload_len" and "payload_offset" fields are
 present ONLY for transport objects of type NORM_OBJECT_STREAM.  These
 fields indicate the size and relative position (within the stream) of
 the application content represented by the message payload.  For
 senders employing systematic FEC encoding, these fields contain
 _actual_ length and offset values (in bytes) for the payload of
 messages which contain original data source symbols.  For NORM_DATA
 messages containing calculated parity content, these fields will
 actually contain values computed by FEC encoding of the "payload_len"
 and "payload_offset" values of the NORM_DATA data segments of the
 corresponding FEC coding block.  Thus, the "payload_len" and
 "payload_offset" values of missing data content can be determined
 upon decoding a FEC coding block.  Note that these fields do NOT

Adamson, et al. Experimental [Page 23] RFC 3940 NORM Protocol November 2004

 contribute to the value of the NORM_DATA "hdr_len" field.  These
 fields are NOT present when the "flags" portion of the NORM_DATA
 message indicate the transport object if of type NORM_OBJECT_FILE or
 NORM_OBJECT_DATA.  In this case, the length and offset information
 can be calculated from the "fec_payload_id" using the methodology
 described in Section 5.1.1.  Note that for long-lived streams, the
 "payload_offset" field can wrap.
 The "payload_data" field contains the original application source  or
 parity content for the symbol identified by the "fec_payload_id".
 The length of this field SHALL be limited to a maximum of the
 sender's NormSegmentSize bytes as given in the FTI for the object.
 Note the length of this field for messages containing parity content
 will always be of length NormSegmentSize.  When encoding data
 segments of varying sizes, the FEC encoder SHALL assume ZERO value
 padding for data segments with length less than the NormSegmentSize.
 It is RECOMMENDED that a sender's NormSegmentSize generally be
 constant for the duration of a given sender's term of participation
 in the session, but may possibly vary on a per-object basis.  The
 NormSegmentSize is expected to be configurable by the sender
 application prior to session participation as needed for network
 topology maximum transmission unit (MTU) considerations.  For IPv6,
 MTU discovery may be possibly leveraged at session startup to perform
 this configuration.  The "payload_data" content may be delivered
 directly to the application for source symbols (when systematic FEC
 encoding is used) or upon decoding of the FEC block.  For
 NORM_OBJECT_FILE and NORM_OBJECT_STREAM objects, the data segment
 length and offset can be calculated using the algorithm described in
 Section 5.1.1.  For NORM_OBJECT_STREAM objects, the length and offset
 is obtained from the segment's corresponding "payload_len" and
 "payload_offset" fields.

4.2.2. NORM_INFO Message

 The NORM_INFO message is used to convey OPTIONAL, application-
 defined, "out-of-band" context information for transmitted
 NormObjects.  An example NORM_INFO use for bulk file transfer is to
 place MIME type information for the associated file, data, or stream
 object into the NORM_INFO payload.  Receivers may use the NORM_INFO
 content to make a decision as whether to participate in reliable
 reception of the associated object.  Each NormObject can have an
 independent unit of NORM_INFO associated with it.  NORM_DATA messages
 contain a flag to indicate the availability of NORM_INFO for a given
 NormObject.  NORM receivers may NACK for retransmission of NORM_INFO
 when they have not received it for a given NormObject.  The size of
 the NORM_INFO content is limited to that of a single NormSegmentSize

Adamson, et al. Experimental [Page 24] RFC 3940 NORM Protocol November 2004

 for the given sender.  This atomic nature allows the NORM_INFO to be
 rapidly and efficiently repaired within the NORM reliable
 transmission process.
 When NORM_INFO content is available for a NormObject, the
 NORM_FLAG_INFO flag SHALL be set in NORM_DATA messages for the
 corresponding "object_transport_id" and the NORM_INFO message shall
 be transmitted as the first message for the NormObject.
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=1|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     flags     |     fec_id    |     object_transport_id       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                header_extensions (if applicable)              |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         payload_data                          |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      NORM_INFO Message Format
 The "version", "type", "hdr_len", "sequence", and "source_id" fields
 form the NORM Common Message Header as described in Section 4.1.  The
 value of "hdr_len" field when no header extensions are present is 4.
 The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
 "object_transport_id" fields carry the same information and serve the
 same purpose as with NORM_DATA messages.  These values allow the
 receiver to prepare appropriate buffering, etc, for further
 transmissions from the sender when NORM_INFO is the first message
 received.
 As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
 may be optionally applied to NORM_INFO messages.  To conserve
 protocol overhead, some NORM implementations may wish to apply the
 EXT_FTI when used to NORM_INFO messages only and not to NORM_DATA
 messages.

Adamson, et al. Experimental [Page 25] RFC 3940 NORM Protocol November 2004

 The NORM_INFO "payload_data" field contains sender application-
 defined content which can be used by receiver applications for
 various purposes as described above.

4.2.3. NORM_CMD Messages

 NORM_CMD messages are transmitted by senders to perform a number of
 different protocol functions.  This includes functions such as
 round-trip timing collection, congestion control functions,
 synchronization of sender/receiver repair "windows", and notification
 of sender status.  A core set of NORM_CMD messages is enumerated.
 Additionally, a range of command types remain available for potential
 application-specific use.  Some NORM_CMD types may have dynamic
 content attached.  Any attached content will be limited to maximum
 length of the sender NormSegmentSize to retain the atomic nature of
 commands.  All NORM_CMD messages begin with a common set of fields,
 after the usual NORM message common header.  The standard NORM_CMD
 fields are:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     flavor    |                                               |
 +-+-+-+-+-+-+-+-+        NORM_CMD Content                       +
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      NORM_CMD Standard Fields
 The "version", "type", "hdr_len", "sequence", and "source_id" fields
 form the NORM Common Message Header as described in Section 4.1.  The
 value of the "hdr_len" field for NORM_CMD messages without header
 extensions present depends upon the "flavor" field.
 The "instance_id", "grtt", "backoff", and "gsize" fields provide the
 same information and serve the same purpose as with NORM_DATA and
 NORM_INFO messages.  The "flavor" field indicates the type of command
 to follow.  The remainder of the NORM_CMD message is dependent upon
 the command type ("flavor").  NORM command flavors include:

Adamson, et al. Experimental [Page 26] RFC 3940 NORM Protocol November 2004

+———————-+————-+———————————+

Command Flavor Value Purpose

+———————-+————-+———————————+

NORM_CMD(FLUSH) 1 Used to indicate sender
temporary end-of-transmission.
(Assists in robustly initiating
outstanding repair requests from
receivers). May also be
optionally used to collect
positive acknowledgment of
reliable reception from subset
of receivers.

+———————-+————-+———————————+

NORM_CMD(EOT) 2 Used to indicate sender
permanent end-of-transmission.

+———————-+————-+———————————+

NORM_CMD(SQUELCH) 3 Used to advertise sender's
current repair window in
response to out-of-range NACKs
from receivers.

+———————-+————-+———————————+

NORM_CMD(CC) 4 Used for GRTT measurement and
collection of congestion control
feedback.

+———————-+————-+———————————+

NORM_CMD(REPAIR_ADV) 5 Used to advertise sender's
aggregated repair/feedback state
for suppression of unicast
feedback from receivers.

+———————-+————-+———————————+

NORM_CMD(ACK_REQ) 6 Used to request application-
defined positive acknowledgment
from a list of receivers
(OPTIONAL).

+———————-+————-+———————————+

NORM_CMD(APPLICATION) 7 Used for application-defined
purposes which may need to
temporarily preempt data
transmission (OPTIONAL).

+———————-+————-+———————————+

4.2.3.1. NORM_CMD(FLUSH) Message

 The NORM_CMD(FLUSH) command is sent when the sender reaches the end
 of all data content and pending repairs it has queued for
 transmission.  This may indicate a temporary or permanent end of data
 transmission, but the sender is still willing to respond to repair
 requests.  This command is repeated once per 2*GRTT to excite the

Adamson, et al. Experimental [Page 27] RFC 3940 NORM Protocol November 2004

 receiver set for any outstanding repair requests up to and including
 the transmission point indicated within the NORM_CMD(FLUSH) message.
 The number of repeats is equal to NORM_ROBUST_FACTOR unless a list of
 receivers from which explicit positive acknowledgment is expected
 ("acking_node_list") is given.  In that case, the "acking_node_list"
 is updated as acknowledgments are received and the NORM_CMD(FLUSH) is
 repeated according to the mechanism described in Section 5.5.3.  The
 greater the NORM_ROBUST_FACTOR, the greater the probability that all
 applicable receivers will be excited for acknowledgment or repair
 requests (NACKs) _and_ that the corresponding NACKs are delivered to
 the sender.  If a NORM_NACK message interrupts the flush process, the
 sender will re-initiate the flush process after any resulting repair
 transmissions are completed.
 Note that receivers also employ a timeout mechanism to self-initiate
 NACKing (if there are outstanding repair needs) when no messages of
 any type are received from a sender.  This inactivity timeout is
 related to 2*GRTT*NORM_ROBUST_FACTOR and will be discussed more
 later.  With a sufficient NORM_ROBUST_FACTOR value, data content is
 delivered with a high assurance of reliability.  The penalty of a
 large NORM_ROBUST_FACTOR value is potentially excess sender
 NORM_CMD(FLUSH) transmissions and a longer timeout for receivers to
 self-initiate the terminal NACK process.
 For finite-size transport objects such as NORM_OBJECT_DATA and
 NORM_OBJECT_FILE, the flush process (if there are no further pending
 objects) occurs at the end of these objects.  Thus, FEC repair
 information is always available for repairs in response to repair
 requests elicited by the flush command.  However, for
 NORM_OBJECT_STREAM, the flush may occur at any time, including in the
 middle of an FEC coding block if systematic FEC codes are employed.
 In this case, the sender will not yet be able to provide FEC parity
 content as repair for the concurrent coding block and will be limited
 to explicitly repairing stream data content for that block.
 Applications that anticipate frequent flushing of stream content
 SHOULD be judicious in the selection of the FEC coding block size
 (i.e., do not use a very large coding block size if frequent flushing
 occurs).  For example, a reliable multicast application transmitting
 an on-going series of intermittent, relatively small messaging
 content will need to trade-off using the NORM_OBJECT_DATA paradigm
 versus the NORM_OBJECT_STREAM paradigm with an appropriate FEC coding
 block size.  This is analogous to application trade-offs for other
 transport protocols such as the selection of different TCP modes of
 operation such as "no delay", etc.

Adamson, et al. Experimental [Page 28] RFC 3940 NORM Protocol November 2004

    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   flavor = 1  |    fec_id     |      object_transport_id      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         fec_payload_id                        |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                acking_node_list (if applicable)               |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   NORM_CMD(FLUSH) Message Format
 In addition to the NORM common message header and standard NORM_CMD
 fields, the NORM_CMD(FLUSH) message contains fields to identify the
 current status and logical transmit position of the sender.
 The "fec_id" field indicates the FEC type used for the flushing
 "object_transport_id" and implies the size and format of the
 "fec_payload_id" field.  Note the "hdr_len" value for the
 NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
 field when no header extensions are present.
 The "object_transport_id" and "fec_payload_id" fields indicate the
 sender's current logical "transmit position".  These fields are
 interpreted in the same manner as in the NORM_DATA message type.
 Upon receipt of the NORM_CMD(FLUSH), receivers are expected to check
 their completion state _through_ (including) this transmission
 position.  If receivers have outstanding repair needs in this range,
 they SHALL initiate the NORM NACK Repair Process as described in
 Section 5.3.  If receivers have no outstanding repair needs, no
 response to the NORM_CMD(FLUSH) is generated.
 For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
 MUST request "explicit-only" repair of the identified
 "source_block_number" if the given "encoding_symbol_id" is less than
 the "source_block_len".  This condition indicates the sender has not
 yet completed encoding the corresponding FEC block and parity content
 is not yet available.  An "explicit-only" repair request consists of
 NACK content for the applicable "source_block_number" which does not
 include any requests for parity-based repair.  This allows NORM

Adamson, et al. Experimental [Page 29] RFC 3940 NORM Protocol November 2004

 sender applications to "flush" an ongoing stream of transmission when
 needed, even if in the middle of an FEC block.  Once the sender
 resumes stream transmission and passes the end of the pending coding
 block, subsequent NACKs from receivers SHALL request parity-based
 repair as usual.  Note that the use of a systematic FEC code is
 assumed here.  Normal receiver NACK initiation and construction is
 discussed in detail in Section 5.3.  The OPTIONAL "acking_node_list"
 field contains a list of NormNodeIds for receivers from which the
 sender is requesting explicit positive acknowledgment of reception up
 through the transmission point identified by the
 "object_transport_id" and "fec_payload_id" fields.  The length of the
 list can be inferred from the length of the received NORM_CMD(FLUSH)
 message.  When the "acking_node_list" is present, the lightweight
 positive acknowledgment process described in Section 5.5.3 SHALL be
 observed.

4.2.3.2. NORM_CMD(EOT) Message

 The NORM_CMD(EOT) command is sent when the sender reaches permanent
 end-of-transmission with respect to the NormSession and will not
 respond to further repair requests.  This allows receivers to
 gracefully reach closure of operation with this sender (without
 requiring any timeout) and free any resources that are no longer
 needed.  The NORM_CMD(EOT) command SHOULD be sent with the same
 robust mechanism as used for NORM_CMD(FLUSH) commands to provide a
 high assurance of reception by the receiver set.
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   flavor = 2  |                    reserved                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    NORM_CMD(EOT) Message Format
 The value of the "hdr_len" field for NORM_CMD(EOT) messages without
 header extensions present is 4.  The "reserved" field is reserved for
 future use and MUST be set to an all ZERO value.  Receivers MUST
 ignore the "reserved" field.

Adamson, et al. Experimental [Page 30] RFC 3940 NORM Protocol November 2004

4.2.3.3. NORM_CMD(SQUELCH) Message

 The NORM_CMD(SQUELCH) command is transmitted in response to outdated
 or invalid NORM_NACK content received by the sender.  Invalid
 NORM_NACK content consists of repair requests for NormObjects for
 which the sender is unable or unwilling to provide repair.  This
 includes repair requests for outdated objects, aborted objects, or
 those objects which the sender previously transmitted marked with the
 NORM_FLAG_UNRELIABLE flag.  This command indicates to receivers what
 content is available for repair, thus serving as a description of the
 sender's current "repair window".  Receivers SHALL not generate
 repair requests for content identified as invalid by a
 NORM_CMD(SQUELCH).
 The NORM_CMD(SQUELCH) command is sent once per 2*GRTT at the most.
 The NORM_CMD(SQUELCH) advertises the current "repair window" of the
 sender by identifying the earliest (lowest) transmission point for
 which it will provide repair, along with an encoded list of objects
 from that point forward that are no longer valid for repair.  This
 mechanism allows the sender application to cancel or abort
 transmission and/or repair of specific previously enqueued objects.
 The list also contains the identifiers for any objects within the
 repair window that were sent with the NORM_FLAG_UNRELIABLE flag set.
 In normal conditions, it is expected the NORM_CMD(SQUELCH) will be
 needed infrequently, and generally only to provide a reference repair
 window for receivers who have fallen "out-of-sync" with the sender
 due to extremely poor network conditions.
 The starting point of the invalid NormObject list begins with the
 lowest invalid NormTransportId greater than the current "repair
 window" start from the invalid NACK(s) that prompted the generation
 of the squelch.  The length of the list is limited by the sender's
 NormSegmentSize.  This allows the receivers to learn the status of
 the sender's applicable object repair window with minimal
 transmission of NORM_CMD(SQUELCH) commands.  The format of the
 NORM_CMD(SQUELCH) message is:

Adamson, et al. Experimental [Page 31] RFC 3940 NORM Protocol November 2004

    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    version    |   type = 3    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  flavor = 3   |     fec_id    |      object_transport_id      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         fec_payload_id                        |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        invalid_object_list                    |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  NORM_CMD(SQUELCH) Message Format
 In addition to the NORM common message header and standard NORM_CMD
 fields, the NORM_CMD(SQUELCH) message contains fields to identify the
 earliest logical transmit position of the sender's current repair
 window and an "invalid object list" beginning with the index of the
 logically earliest invalid repair request from the offending NACK
 message which initiated the squelch transmission.
 The "object_transport_id" and "fec_payload_id" fields are
 concatenated to indicate the beginning of the sender's current repair
 window (i.e., the logically earliest point in its transmission
 history for which the sender can provide repair).  The "fec_id" field
 implies the size and format of the "fec_payload_id" field.  This
 serves as an advertisement of a "synchronization point" for receivers
 to request repair.  Note, that while an "encoding_symbol_id" may be
 included in the "fec_payload_id" field, the sender's repair window
 SHOULD be aligned on FEC coding block boundaries and thus the
 "encoding_symbol_id" SHOULD be zero.
 The "invalid_object_list" is a list of 16-bit NormTransportIds that,
 although they are within the range of the sender's current repair
 window, are no longer available for repair from the sender.  For
 example, a sender application may dequeue an out-of-date object even
 though it is still within the repair window.  The total size of the
 "invalid_object_list" content is can be determined from the packet's
 payload length and is limited to a maximum of the NormSegmentSize of
 the sender.  Thus, for very large repair windows, it is possible that
 a single NORM_CMD(SQUELCH) message may not be capable of listing the
 entire set of invalid objects in the repair window.  In this case,

Adamson, et al. Experimental [Page 32] RFC 3940 NORM Protocol November 2004

 the sender SHALL ensure that the list begins with a NormObjectId that
 is greater than or equal to the lowest ordinal invalid NormObjectId
 from the NACK message(s) that prompted the NORM_CMD(SQUELCH)
 generation.  The NormObjectIds in the "invalid_object_list" MUST be
 greater than the "object_transport_id" marking the beginning of the
 sender's repair window.  This insures convergence of the squelch
 process, even if multiple invalid NACK/ squelch iterations are
 required.  This explicit description of invalid content within the
 sender's current window allows the sender application (most notably
 for discrete "object" based transport) to arbitrarily invalidate
 (i.e., dequeue) portions of enqueued content (e.g., certain objects)
 for which it no longer wishes to provide reliable transport.

4.2.3.4. NORM_CMD(CC) Message

 The NORM_CMD(CC) messages contains fields to enable sender-to-
 receiver group greatest round-trip time (GRTT) measurement and to
 excite the group for congestion control feedback.  A baseline NORM
 congestion control scheme (NORM-CC), based on the TCP-Friendly
 Multicast Congestion Control (TFMCC) scheme of [19] is described in
 Section 5.5.2 of this document.  The NORM_CMD(CC) message is usually
 transmitted as part of NORM-CC congestion control operation.  A NORM
 header extension is defined below to be used with the NORM_CMD(CC)
 message to support NORM-CC operation.  Different header extensions
 may be defined for the NORM_CMD(CC) (and/or other NORM messages as
 needed) to support alternative congestion control schemes in the
 future.  If NORM is operated in a private network with congestion
 control operation disabled, the NORM_CMD(CC) message is then used for
 GRTT measurement only and may optionally be sent less frequently than
 with congestion control operation.

Adamson, et al. Experimental [Page 33] RFC 3940 NORM Protocol November 2004

    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |            sequence           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   flavor = 4  |    reserved   |          cc_sequence          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         send_time_sec                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        send_time_usec                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               header extensions (if applicable)               |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  cc_node_list (if applicable)                 |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    NORM_CMD(CC) Message Format
 The NORM common message header and standard NORM_CMD fields serve
 their usual purposes.
 The "reserved" field is for potential future use and should be set to
 ZERO in this version of the NORM protocol.
 The "cc_sequence" field is a sequence number applied by the sender.
 For NORM-CC operation, it is used to provide functionality equivalent
 to the "feedback round number" (fb_nr)described in [19].  The most
 recently received "cc_sequence" value is recorded by receivers and
 can be fed back to the sender in congestion control feedback
 generated by the receivers for that sender.  The "cc_sequence" number
 can also be used in NORM implementations to assess how recently a
 receiver has received NORM_CMD(CC) probes from the sender.  This can
 be useful instrumentation for complex or experimental multicast
 routing environments.
 The "send_time" field is a timestamp indicating the time that the
 NORM_CMD(CC) message was transmitted.  This consists of a 64-bit
 field containing 32-bits with the time in seconds ("send_time_sec")
 and 32-bits with the time in microseconds ("send_time_usec") since
 some reference time the source maintains (usually 00:00:00, 1 January
 1970).  The byte ordering of the fields is "Big Endian" network
 order.  Receivers use this timestamp adjusted by the amount of delay

Adamson, et al. Experimental [Page 34] RFC 3940 NORM Protocol November 2004

 from the time they received the NORM_CMD(CC) message to the time of
 their response as the "grtt_response" portion of NORM_ACK and
 NORM_NACK messages generated.  This allows the sender to evaluate
 round-trip times to different receivers for congestion control and
 other (e.g., GRTT determination) purposes.
 To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
 a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
 group of the sender's current transmission rate.  This is used along
 with the loss detection "sequence" field of all NORM sender messages
 and the NORM_CMD(CC) GRTT collection process to support NORM-CC
 congestion control operation.  The format of this header extension is
 as follows:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    het = 128  |    reserved   |           send_rate           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          NORM-CC Rate Header Extension Format (EXT_RATE)
 The "send_rate" field indicates the sender's current transmission
 rate in bytes per second.  The 16-bit "send_rate" field consists of
 12 bits of mantissa in the most significant portion and 4 bits of
 base 10 exponent (order of magnitude) information in the least
 significant portion.  The 12-bit mantissa portion of the field is
 scaled such that a floating point value of 0.0 corresponds to 0 and a
 floating point value of 10.0 corresponds to 4096.  Thus:
 send_rate = (((int)(Value_mantissa * 4096.0 / 10.0 + 0.5)) << 4) |
 Value_exponent;
 For example, to represent a transmission rate of 256kbps (3.2e+04
 bytes per second), the lower 4 bits of the 16-bit field contain a
 value of 0x04 to represent the exponent while the upper 12 bits
 contain a value of 0x51f as determined from the equation given above:

send_rate = 1) « 4) | 4;

        = (0x51f << 4) | 0x4
        = 0x51f4

To decode the "send_rate" field, the following equation can be used:

value = (send_rate » 4) * 10.0 / 4096.0 *

      power(10.0, (send_rate & x000f))

Adamson, et al. Experimental [Page 35] RFC 3940 NORM Protocol November 2004

 Note the maximum transmission rate that can be represented by this
 scheme is approximately 9.99e+15 bytes per second.
 When this extension is present, a "cc_node_list" may be attached as
 the payload of the NORM_CMD(CC) message.  The presence of this header
 extension also implies that NORM receivers should respond according
 to the procedures described in Section 5.5.2.  The "cc_node_list"
 consists of a list of NormNodeIds and their associated congestion
 control status.  This includes the current limiting receiver (CLR)
 node, any potential limiting receiver (PLR) nodes that have been
 identified, and some number of receivers for which congestion control
 status is being provided, most notably including the receivers'
 current RTT measurement.  The maximum length of the "cc_node_list"
 provides for at least the CLR and one other receiver, but may be
 configurable for more timely feedback to the group.  The list length
 can be inferred from the length of the NORM_CMD(CC) message.
 Each item in the "cc_node_list" is in the following format:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          cc_node_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    cc_flags   |     cc_rtt    |            cc_rate            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Congestion Control Node List Item Format
 The "cc_node_id" is the NormNodeId of the receiver which the item
 represents.
 The "cc_flags" field contains flags indicating the congestion control
 status of the indicated receiver.  The following flags are defined:

Adamson, et al. Experimental [Page 36] RFC 3940 NORM Protocol November 2004

+——————+——-+——————————————+

Flag Value Purpose

+——————+——-+——————————————+

NORM_FLAG_CC_CLR 0x01 Receiver is the current limiting
receiver (CLR).

+——————+——-+——————————————+

NORM_FLAG_CC_PLR 0x02 Receiver is a potential limiting
receiver (PLR).

+——————+——-+——————————————+

NORM_FLAG_CC_RTT 0x04 Receiver has measured RTT with respect
to sender.

+——————+——-+——————————————+

NORM_FLAG_CC_START 0x08 Sender/receiver is in "slow start" phase
of congestion control operation (i.e.,
The receiver has not yet detected any
packet loss and the "cc_rate" field is
the receiver's actual measured receive
rate).

+——————+——-+——————————————+

NORM_FLAG_CC_LEAVE 0x10 Receiver is imminently leaving the
session and its feedback should not be
considered in congestion control
operation.

+——————+——-+——————————————+

 The "cc_rtt" contains a quantized representation of the RTT as
 measured by the sender with respect to the indicated receiver.  This
 field is valid only if the NORM_FLAG_CC_RTT flag is set in the
 "cc_flags" field.  This one byte field is a quantized representation
 of the RTT using the algorithm described in the NORM Building Block
 document [4].  The "cc_rate" field contains a representation of the
 receiver's current calculated (during steady-state congestion control
 operation) or twice its measured (during the "slow start" phase)
 congestion control rate.  This field is encoded and decoded using the
 same technique as described for the NORM_CMD(CC) "send_rate" field.

4.2.3.5. NORM_CMD(REPAIR_ADV) Message

 The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
 its aggregated repair state from NORM_NACK messages accumulated
 during a repair cycle and/or congestion control feedback received.
 This message is sent only when the sender has received NORM_NACK
 and/or NORM_ACK(CC) (when congestion control is enabled) messages via
 unicast transmission instead of multicast.  By "echoing" this
 information to the receiver set, suppression of feedback can be
 achieved even when receivers are unicasting that feedback instead of
 multicasting it among the group [13].

Adamson, et al. Experimental [Page 37] RFC 3940 NORM Protocol November 2004

    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  flavor = 5   |     flags     |            reserved           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               header extensions (if applicable)               |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       repair_adv_payload                      |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                NORM_CMD(REPAIR_ADV) Message Format
 The "instance_id", "grtt", "backoff", "gsize", and "flavor" fields
 serve the same purpose as in other NORM_CMD messages.  The value of
 the "hdr_len" field when no extensions are present is 4.
 The "flags" field provide information on the NORM_CMD(REPAIR_ADV)
 content.  There is currently one NORM_CMD(REPAIR_ADV) flag defined:
                   NORM_REPAIR_ADV_FLAG_LIMIT = 0x01
 This flag is set by the sender when it is unable to fit its full
 current repair state into a single NormSegmentSize.  If this flag is
 set, receivers should limit their NACK response to generating NACK
 content only up through the maximum ordinal transmission position
 (objectId::fecPayloadId) included in the "repair_adv_content".
 When congestion control operation is enabled, a header extension may
 be applied to the NORM_CMD(REPAIR_ADV) representing the most limiting
 (in terms of congestion control feedback suppression) congestion
 control response.  This allows the NORM_CMD(REPAIR_ADV) message to
 suppress receiver congestion control responses as well as NACK
 feedback messages.  The field is defined as a header extension so
 that alternative congestion control schemes may be used with NORM
 without revision to this document.  A NORM-CC Feedback Header
 Extension (EXT_CC) is defined to encapsulate congestion control
 feedback within NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV)
 messages.  If another congestion control technique (e.g., Pragmatic
 General Multicast Congestion Control (PGMCC) [20]) is used within a

Adamson, et al. Experimental [Page 38] RFC 3940 NORM Protocol November 2004

 NORM implementation, an additional header extension MAY need to be
 defined to encapsulate any required feedback content.  The NORM-CC
 Feedback Header Extension format is:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     het = 3   |    hel = 3    |          cc_sequence          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    cc_flags   |     cc_rtt    |            cc_loss            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            cc_rate            |          cc_reserved          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         NORM-CC Feedback Header Extension (EXT_CC) Format
 The "cc_sequence" field contains the current greatest "cc_sequence"
 value receivers have  received in NORM_CMD(CC) messages from the
 sender.  This information assists the sender in congestion control
 operation by providing an indicator of how current ("fresh") the
 receiver's round-trip measurement reference time is and whether the
 receiver has been successfully receiving recent congestion control
 probes.  For example, if it is apparent the receiver has not been
 receiving recent congestion control probes (and thus possibly other
 messages from the sender), the sender may choose to take congestion
 avoidance measures.  For NORM_CMD(REPAIR_ADV) messages, the sender
 SHALL set the "cc_sequence" field value to the value set in the last
 NORM_CMD(CC) message sent.
 The "cc_flags" field contains bits representing the receiver's state
 with respect to congestion control operation.  The possible values
 for the "cc_flags" field are those specified for the NORM_CMD(CC)
 message node list item flags.  These fields are used by receivers in
 controlling (suppressing as necessary) their congestion control
 feedback.  For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
 should be set only when all feedback messages received by the sender
 have the flag set.  Similarly, the NORM_FLAG_CC_CLR or
 NORM_FLAG_CC_PLR should be set only when no feedback has been
 received from non-CLR or non-PLR receivers.  And the
 NORM_FLAG_CC_LEAVE should be set only when all feedback messages the
 sender has received have this flag set.  These heuristics for setting
 the flags in NORM_CMD(REPAIR_ADV) ensure the most effective
 suppression of receivers providing unicast feedback messages.
 The "cc_rtt" field SHALL be set to a default maximum value and the
 NORM_FLAG_CC_RTT flag SHALL be cleared when no receiver has yet
 received RTT measurement information.  When a receiver has received
 RTT measurement information, it shall set the "cc_rtt" value
 accordingly and set the NORM_FLAG_CC_RTT flag in the "cc_flags"
 field.

Adamson, et al. Experimental [Page 39] RFC 3940 NORM Protocol November 2004

 For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_rtt"
 field value to the largest non-CLR/non-PLR RTT it has measured from
 receivers for the current feedback round.
 The "cc_loss" field represents the receiver's current packet loss
 fraction estimate for the indicated source.  The loss fraction is a
 value from 0.0 to 1.0 corresponding to a range of zero to 100 percent
 packet loss.  The 16-bit "cc_loss" value is calculated by the
 following formula:
              "cc_loss" = decimal_loss_fraction * 65535.0
 For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
 field value to the largest non-CLR/non-PLR loss estimate it has
 received from receivers for the current feedback round.
 The "cc_rate" field represents the receivers current local congestion
 control rate.  During "slow start", when the receiver has detected no
 loss, this value is set to twice the actual rate it has measured from
 the corresponding sender and the NORM_FLAG_CC_START is set in the
 "cc_flags' field.  Otherwise, the receiver calculates a congestion
 control rate based on its loss measurement and RTT measurement
 information (even if default) for the "cc_rate" field.  For
 NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
 field value to the lowest non-CLR/non-PLR "cc_rate" report it has
 received from receivers for the current feedback round.
 The "cc_reserved" field is reserved for future NORM protocol use.
 Currently, senders SHALL set this field to ZERO, and receivers SHALL
 ignore the content of this field.
 The "repair_adv_payload" is in exactly the same form as the
 "nack_content" of NORM_NACK messages and can be processed by
 receivers for suppression purposes in the same manner, with the
 exception of the condition when the NORM_REPAIR_ADV_FLAG_LIMIT is
 set.

4.2.3.6. NORM_CMD(ACK_REQ) Message

 The NORM_CMD(ACK_REQ) message is used by the sender to request
 acknowledgment from a specified list of receivers.  This message is
 used in providing a lightweight positive acknowledgment mechanism
 that is OPTIONAL for use by the reliable multicast application.  A
 range of acknowledgment request types is provided for use at the
 application's discretion.  Provision for application-defined,
 positively-acknowledged commands allows the application to
 automatically take advantage of transmission and round-trip timing
 information available to the NORM protocol.  The details of the NORM

Adamson, et al. Experimental [Page 40] RFC 3940 NORM Protocol November 2004

 positive acknowledgment process including transmission of the
 NORM_CMD(ACK_REQ) messages and the receiver response (NORM_ACK) are
 described in Section 5.5.3.  The format of the NORM_CMD(ACK_REQ)
 message is:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  flavor = 6   |    reserved   |    ack_type   |    ack_id     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       acking_node_list                        |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  NORM_CMD(ACK_REQ) Message Format
 The NORM common message header and standard NORM_CMD fields serve
 their usual purposes.  The value of the "hdr_len" field for
 NORM_CMD(ACK_REQ) messages with no header extension present is 4.
 The "ack_type" field indicates the type of acknowledgment being
 requested and thus implies rules for how the receiver will treat this
 request.  The following "ack_type" values are defined and are also
 used in NORM_ACK messages described later:

+———————+——–+———————————+

ACK Type Value Purpose

+———————+——–+———————————+

NORM_ACK_CC 1 Used to identify NORM_ACK
messages sent in response to
NORM_CMD(CC) messages.

+———————+——–+———————————+

NORM_ACK_FLUSH 2 Used to identify NORM_ACK
messages sent in response to
NORM_CMD(FLUSH) messages.

+———————+——–+———————————+

NORM_ACK_RESERVED 3-15 Reserved for possible future
NORM protocol use.

+———————+——–+———————————+

NORM_ACK_APPLICATION 16-255 Used at application's
discretion.

+———————+——–+———————————+

Adamson, et al. Experimental [Page 41] RFC 3940 NORM Protocol November 2004

 The NORM_ACK_CC value is provided for use only in NORM_ACKs generated
 in response to the NORM_CMD(CC) messages used in congestion control
 operation.  Similarly, the NORM_ACK_FLUSH is provided for use only in
 NORM_ACKs generated in response to applicable NORM_CMD(FLUSH)
 messages.  NORM_CMD(ACK_REQ) messages with "ack_type" of NORM_ACK_CC
 or NORM_ACK_FLUSH SHALL NOT be generated by the sender.
 The NORM_ACK_RESERVED range of "ack_type" values is provided for
 possible future NORM protocol use.
 The NORM_ACK_APPLICATION range of "ack_type" values is provided so
 that NORM applications may implement application-defined,
 positively-acknowledged commands that are able to leverage internal
 transmission and round-trip timing information available to the NORM
 protocol implementation.
 The "ack_id" provides a sequenced identifier for the given
 NORM_CMD(ACK_REQ) message.  This "ack_id" is returned in NORM_ACK
 messages generated by the receivers so that the sender may associate
 the response with its corresponding request.
 The "reserved" field is reserved for possible future protocol use and
 SHALL be set to ZERO by senders and ignored by receivers.
 The "acking_node_list" field contains the NormNodeIds of the current
 NORM receivers that are desired to provide positive acknowledge
 (NORM_ACK) to this request.  The packet payload length implies the
 length of the "acking_node_list" and its length is limited to the
 sender NormSegmentSize.  The individual NormNodeId items are listed
 in network (Big Endian) byte order.  If a receiver's NormNodeId is
 included in the "acking_node_list", it SHALL schedule transmission of
 a NORM_ACK message as described in Section 5.5.3.

4.2.3.7. NORM_CMD(APPLICATION) Message

 This command allows the NORM application to robustly transmit
 application-defined commands.  The command message preempts any
 ongoing data transmission and is repeated up to NORM_ROBUST_FACTOR
 times at a rate of once per 2*GRTT.  This rate of repetition allows
 the application to observe any response (if that is the application's
 purpose for the command) before it is repeated.  Possible responses
 may include initiation of data transmission, other
 NORM_CMD(APPLICATION) messages, or even application-defined,
 positively-acknowledge commands from other NormSession participants.
 The transmission of these commands will preempt data transmission
 when they are scheduled and may be multiplexed with ongoing data
 transmission.  This type of robustly transmitted command allows NORM
 applications to define a complete set of session control mechanisms

Adamson, et al. Experimental [Page 42] RFC 3940 NORM Protocol November 2004

 with less state than the transfer of FEC encoded reliable content
 requires while taking advantage of NORM transmission and round-trip
 timing information.
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=3|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          instance_id          |     grtt      |backoff| gsize |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  flavor = 7   |                    reserved                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Application-Defined Content                 |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                NORM_CMD(APPLICATION) Message Format
 The NORM common message header and NORM_CMD fields are interpreted as
 previously described.  The value of the NORM_CMD(APPLICATION)
 "hdr_len" field when no header extensions are present is 4.
 The "Application-Defined Content" area contains information in a
 format at the discretion of the application.  The size of this
 payload SHALL be limited to a maximum of the sender's NormSegmentSize
 setting.

4.3. Receiver Messages

 The NORM message types generated by participating receivers consist
 of NORM_NACK and NORM_ACK message types.  NORM_NACK messages are sent
 to request repair of missing data content from sender transmission
 and NORM_ACK messages are generated in response to certain sender
 commands including NORM_CMD(CC) and NORM_CMD(ACK_REQ).

4.3.1. NORM_NACK Message

 The principal purpose of NORM_NACK messages is for receivers to
 request repair of sender content via selective, negative
 acknowledgment upon detection of incomplete data.  NORM_NACK messages
 will be transmitted according to the rules of NORM_NACK generation
 and suppression described in Section 5.3.  NORM_NACK messages also
 contain additional fields to provide feedback to the sender(s) for
 purposes of round-trip timing collection and congestion control.

Adamson, et al. Experimental [Page 43] RFC 3940 NORM Protocol November 2004

 The payload of NORM_NACK messages contains one or more repair
 requests for different objects or portions of those objects.  The
 NORM_NACK message format is as follows:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=4|    hdr_len    |            sequence           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           server_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           instance_id         |            reserved           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       grtt_response_sec                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       grtt_response_usec                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               header extensions (if applicable)               |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          nack_payload                         |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      NORM_NACK Message Format
 The NORM common message header fields serve their usual purposes.
 The value of the "hdr_len" field for NORM_NACK messages without
 header extensions present is 6.
 The "server_id" field identifies the NORM sender to which the
 NORM_NACK message is destined.
 The "instance_id" field contains the current session identifier given
 by the sender identified by the "server_id" field in its sender
 messages.  The sender SHOULD ignore feedback messages which contain
 an invalid "instance_id" value.
 The "grtt_response" fields contain an adjusted version of the
 timestamp from the most recently received NORM_CMD(CC) message for
 the indicated NORM sender.  The format of the "grtt_response" is the
 same as the "send_time" field of the NORM_CMD(CC).  The
 "grtt_response" value is _relative_ to the "send_time" the source
 provided with a corresponding NORM_CMD(CC) command.  The receiver
 adjusts the source's NORM_CMD(CC) "send_time" timestamp by adding the
 time differential from  when the receiver received the NORM_CMD(CC)

Adamson, et al. Experimental [Page 44] RFC 3940 NORM Protocol November 2004

 to when the NORM_NACK is transmitted to calculate the value in the
 "grtt_response" field.  This is the
 "receive_to_response_differential" value used in the following
 formula:
 "grtt_response" = NORM_CMD(CC) "send_time" +
 receive_to_response_differential
 The receiver SHALL set the "grtt_response" to a ZERO value, to
 indicate that it has not yet received a NORM_CMD(CC) message from the
 indicated sender and that the sender should ignore the
 "grtt_response" in this message.
 For NORM-CC operation, the NORM-CC Feedback Header Extension, as
 described in the NORM_CMD(REPAIR_ADV} message description, is added
 to NORM_NACK messages to provide feedback on the receivers current
 state with respect to congestion control operation.  Note that
 alternative header extensions for congestion control feedback may be
 defined for alternative congestion control schemes for NORM use in
 the future.
 The "reserved" field is for potential future NORM use and SHALL be
 set to ZERO for this version of the protocol.
 The "nack_content" of the NORM_NACK message specifies the repair
 needs of the receiver with respect to the NORM sender indicated by
 the "server_id" field.  The receiver constructs repair requests based
 on the NORM_DATA and/or NORM_INFO segments it requires from the
 sender in order to complete reliable reception up to the sender's
 transmission position at the moment the receiver initiates the NACK
 Procedure as described in Section 5.3.  A single NORM Repair Request
 consists of a list of items, ranges, and/or FEC coding block erasure
 counts for needed NORM_DATA and/or NORM_INFO content.  Multiple
 repair requests may be concatenated within the "nack_payload" field
 of a NORM_NACK message.  Note that a single NORM Repair Request can
 possibly include multiple "items", "ranges", or "erasure_counts".  In
 turn, the "nack_payload" field may contain multiple repair requests.
 A single NORM Repair Request has the following format:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      form     |     flags     |             length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      repair_request_items                     |
 |                             ...                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Adamson, et al. Experimental [Page 45] RFC 3940 NORM Protocol November 2004

                     NORM Repair Request Format
 The "form" field indicates the type of repair request items given in
 the "repair_request_items" list.  Possible values for the "form"
 field include:
              Form          Value
       NORM_NACK_ITEMS        1
       NORM_NACK_RANGES       2
       NORM_NACK_ERASURES     3
 A "form" value of NORM_NACK_ITEMS indicates each repair request item
 in the "repair_request_items" list is to be treated as an individual
 request.  A value of NORM_NACK_RANGES indicates that the
 "repair_request_items" list consists of pairs of repair request items
 that correspond to inclusive ranges of repair needs.  And the
 NORM_NACK_ERASURES "form" indicates that the repair request items are
 to be treated individually and that the "encoding_symbol_id" portion
 of the "fec_payload_id" field of the repair request item (see below)
 is to be interpreted as an "erasure count" for the FEC coding block
 identified by the repair request item's "source_block_number".
 The "flags" field is currently used to indicate the level of data
 content for which the repair request items apply (i.e., an individual
 segment, entire FEC coding block, or entire transport object).
 Possible flag values include:

+——————+——-+—————————————–+

Flag Value Purpose

+——————+——-+—————————————–+

NORM_NACK_SEGMENT 0x01 Indicates the listed segment(s) or range
of segments are required as repair.

+——————+——-+—————————————–+

NORM_NACK_BLOCK 0x02 Indicates the listed block(s) or range
of blocks in entirety are required as
repair.

+——————+——-+—————————————–+

NORM_NACK_INFO 0x04 Indicates that NORM_INFO is required as
repair for the listed object(s).

+——————+——-+—————————————–+

NORM_NACK_OBJECT 0x08 Indicates the listed object(s) or range
of objects in entirety are required as
repair.

+——————+——-+—————————————–+

 When the NORM_NACK_SEGMENT flag is set, the "object_transport_id" and
 "fec_payload_id" fields are used to determine which sets or ranges of
 individual NORM_DATA segments are needed to repair content at the

Adamson, et al. Experimental [Page 46] RFC 3940 NORM Protocol November 2004

 receiver.  When the NORM_NACK_BLOCK flag is set, this indicates the
 receiver is completely missing the indicated coding block(s) and
 requires transmissions sufficient to repair the indicated block(s) in
 their entirety.  When the NORM_NACK_INFO flag is set, this indicates
 the receiver is missing the NORM_INFO segment for the indicated
 "object_transport_id".  Note the NORM_NACK_INFO may be set in
 combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags, or
 may be set alone.  When the NORM_NACK_OBJECT flag is set, this
 indicates the receiver is missing the entire NormTransportObject
 referenced by the "object_transport_id".  This also implicitly
 requests any available NORM_INFO for the NormObject, if applicable.
 The "fec_payload_id" field is ignored when the flag NORM_NACK_OBJECT
 is set.
 The "length" field value is the length in bytes of the
 "repair_request_items" field.
 The "repair_request_items" field consists of a list of individual or
 range pairs of transport data unit identifiers in the following
 format.
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     fec_id    |   reserved    |      object_transport_id      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        fec_payload_id                         |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  NORM Repair Request Item Format
 The "fec_id" indicates the FEC type and can be used to determine the
 format of the "fec_payload_id" field.  The "reserved" field is kept
 for possible future use and SHALL be set to a ZERO value and ignored
 by NORM nodes processing NACK content.
 The "object_transport_id" corresponds to the NormObject for which
 repair is being requested and the "fec_payload_id" identifies the
 specific FEC coding block and/or segment being requested.  When the
 NORM_NACK_OBJECT flag is set, the value of the "fec_payload_id" field
 is ignored.  When the NORM_NACK_BLOCK flag is set, only the FEC code
 block identifier portion of the "fec_payload_id" is to be
 interpreted.
 The format of the "fec_payload_id" field depends upon the "fec_id"
 field value.

Adamson, et al. Experimental [Page 47] RFC 3940 NORM Protocol November 2004

 When the receiver's repair needs dictate that different forms (mixed
 ranges and/or individual items) or types (mixed specific segments
 and/or blocks or objects in entirety) are required to complete
 reliable transmission, multiple NORM Repair Requests with different
 "form" and or "flags" values can be concatenated within a single
 NORM_NACK message.  Additionally, NORM receivers SHALL construct
 NORM_NACK messages with their repair requests in ordinal order with
 respect to "object_transport_id" and "fec_payload_id" values.  The
 "nack_payload" size SHALL NOT exceed the NormSegmentSize for the
 sender to which the NORM_NACK is destined.
 NORM_NACK Content Examples:
 In these examples, a small block, systematic FEC code ("fec_id" =
 129) is assumed with a user data block length of 32 segments.  In
 Example 1, a list of individual NORM_NACK_ITEMS repair requests is
 given.  In Example 2, a list of NORM_NACK_RANGES requests _and_ a
 single NORM_NACK_ITEMS request are concatenated to illustrate the
 possible content of a NORM_NACK message.  Note that FEC coding block
 erasure counts could also be provided in each case.  However, the
 erasure counts are not really necessary since the sender can easily
 determine the erasure count while processing the NACK content.
 However, the erasure count option may be useful for operation with
 other FEC codes or for intermediate system purposes.

Adamson, et al. Experimental [Page 48] RFC 3940 NORM Protocol November 2004

 Example 1:  NORM_NACK "nack_payload" for: Object 12, Coding Block 3,
 Segments 2,5,8
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   form = 1    | flags = 0x01  |       length  = 36            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    source_block_number = 3                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    source_block_length = 32   |    encoding_symbol_id = 2     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    source_block_number = 3                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    source_block_length = 32   |    encoding_symbol_id = 5     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    source_block_number = 3                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    source_block_length = 32   |    encoding_symbol_id = 8     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Adamson, et al. Experimental [Page 49] RFC 3940 NORM Protocol November 2004

 Example 2:  NORM_NACK "nack_payload" for: Object 18 Coding Block 6,
 Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block
 1, segment 3
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   form = 2    | flags = 0x01  |       length  = 24            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    source_block_number = 6                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    source_block_length = 32   |    encoding_symbol_id = 5     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    source_block_number = 6                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    source_block_length = 32   |    encoding_symbol_id = 10    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   form = 1    | flags = 0x05  |       length  = 12            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  fec_id = 129 |   reserved    |    object_transport_id = 19   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    source_block_number = 1                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    source_block_length = 32   |    encoding_symbol_id = 3     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3.2. NORM_ACK Message

 The NORM_ACK message is intended to be used primarily as part of NORM
 congestion control operation and round-trip timing measurement.  As
 mentioned in the NORM_CMD(ACK_REQ) message description, the
 acknowledgment type NORM_ACK_CC is provided for this purpose.  The
 generation of NORM_ACK(CC) messages for round-trip timing estimation
 and congestion-control operation is described in Sections 5.5.1 and
 5.5.2, respectively.  However, some multicast applications may
 benefit from some limited form of positive acknowledgment for certain
 functions.  A simple, scalable positive acknowledgment scheme is
 defined in Section 5.5.3 that can be leveraged by protocol
 implementations when appropriate.  The NORM_CMD(FLUSH) may be used
 for OPTIONAL collection of positive acknowledgment of reliable
 reception to a certain "watermark" transmission point from specific
 receivers using this mechanism.  The NORM_ACK type NORM_ACK_FLUSH is
 provided for this purpose and the format of the "nack_payload" for
 this acknowledgment type is given below.  Beyond that, a range of

Adamson, et al. Experimental [Page 50] RFC 3940 NORM Protocol November 2004

 application-defined "ack_type" values is provided for use at the NORM
 application's discretion.  Implementations making use of
 application-defined positive acknowledgments may also make use the
 "nack_payload" as needed, observing the constraint that the
 "nack_payload" field size be limited to a maximum of the
 NormSegmentSize for the sender to which the NORM_ACK is destined.
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |version| type=5|    hdr_len    |          sequence             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           source_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           server_id                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           instance_id         |    ack_type  |     ack_id     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       grtt_response_sec                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       grtt_response_usec                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               header extensions (if applicable)               |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   ack_payload (if applicable)                 |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      NORM_ACK Message Format
 The NORM common message header fields serve their usual purposes.
 The "server_id", "instance_id",  and "grtt_response" fields serve the
 same purpose as the corresponding fields in NORM_NACK messages.  And
 header extensions may be applied to support congestion control
 feedback or other functions in the same manner.
 The "ack_type" field indicates the nature of the NORM_ACK message.
 This directly corresponds to the "ack_type" field of the
 NORM_CMD(ACK_REQ) message to which this acknowledgment applies.
 The "ack_id" field serves as a sequence number so that the sender can
 verify that a NORM_ACK message received actually applies to a current
 acknowledgment request.  The "ack_id" field is not used in the case
 of the NORM_ACK_CC and NORM_ACK_FLUSH acknowledgment types.

Adamson, et al. Experimental [Page 51] RFC 3940 NORM Protocol November 2004

 The "ack_payload" format is a function of the "ack_type".  The
 NORM_ACK_CC message has no attached content.  Only the NORM_ACK
 header applies.  In the case of NORM_ACK_FLUSH, a specific
 "ack_payload" format is defined:
    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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     fec_id    |   reserved    |      object_transport_id      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        fec_payload_id                         |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                NORM_ACK_FLUSH "ack_payload" Format
 The "object_transport_id" and "fec_payload_id" are used by the
 receiver to acknowledge applicable NORM_CMD(FLUSH) messages
 transmitted by the sender identified by the "server_id" field.
 The "ack_payload" of NORM_ACK messages for application-defined
 "ack_type" values is specific to the application but is limited in
 size to a maximum the NormSegmentSize of the sender referenced by the
 "server_id".

4.4. General Purpose Messages

 Some additional message formats are defined for general purpose in
 NORM multicast sessions whether the participant is acting as a sender
 and/or receiver within the group.

4.4.1. NORM_REPORT Message

 This is an optional message generated by NORM participants.  This
 message could be used for periodic performance reports from receivers
 in experimental NORM implementations.  The format of this message is
 currently undefined.  Experimental NORM implementations may define
 NORM_REPORT formats as needed for test purposes.  These report
 messages SHOULD be disabled for interoperability testing between
 different NORM implementations.

5. Detailed Protocol Operation

 This section describes the detailed interactions of senders and
 receivers participating in a NORM session.  A simple synopsis of
 protocol operation is given here:

Adamson, et al. Experimental [Page 52] RFC 3940 NORM Protocol November 2004

 1) The sender periodically transmits NORM_CMD(CC) messages as needed
    to initialize and collect roundtrip timing and congestion control
    feedback from the receiver set.
 2) The sender transmits an ordinal set of NormObjects segmented in
    the form of NORM_DATA messages labeled with NormTransportIds and
    logically identified with FEC encoding block numbers and symbol
    identifiers.  NORM_INFO messages may optionally precede the
    transmission of data content for NORM transport objects.
 3) As receivers detect missing content from the sender, they initiate
    repair requests with NORM_NACK messages.  Note the receivers track
    the sender's most recent objectId::fecPayloadId transmit position
    and NACK _only_ for content ordinally prior to that transmit
    position.  The receivers schedule random backoff timeouts before
    generating NORM_NACK messages and wait an appropriate amount of
    time before repeating the NORM_NACK if their repair request is not
    satisfied.
 4) The sender aggregates repair requests from the receivers and
    logically "rewinds" its transmit position to send appropriate
    repair messages.  The sender sends repairs for the earliest
    ordinal transmit position first and maintains this ordinal repair
    transmission sequence.  Previously untransmitted FEC parity
    content for the applicable FEC coding block is used for repair
    transmissions to the greatest extent possible.  If the sender
    exhausts its available FEC parity content on multiple repair
    cycles for the same coding block, it resorts to an explicit repair
    strategy (possibly using parity content) to complete repairs.
    (The use of explicit repair is expected to be an exception in
    general protocol operation, but the possibility does exist for
    extreme conditions).  The sender immediately assumes transmission
    of new content once it has sent pending repairs.
 5) The sender transmits NORM_CMD(FLUSH) messages when it reaches the
    end of enqueued transmit content and pending repairs.  Receivers
    respond to the NORM_CMD(FLUSH) messages with NORM_NACK
    transmissions (following the same suppression backoff timeout
    strategy as for data) if they require further repair.
 6) The sender transmissions are subject to rate control limits
    determined by congestion control mechanisms.  In the baseline
    NORM-CC operation, each sender in a NormSession maintains its own
    independent congestion control state.  Receivers provide
    congestion control feedback in NORM_NACK and NORM_ACK messages.
    NORM_ACK feedback for congestion control purposes is governed
    using a suppression mechanism similar to that for NORM_NACK
    messages.

Adamson, et al. Experimental [Page 53] RFC 3940 NORM Protocol November 2004

 While this overall concept is relatively simple, there are details to
 each of these aspects that need to be addressed for successful,
 efficient, robust, and scalable NORM protocol operation.

5.1. Sender Initialization and Transmission

 Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
 messages to collect round trip timing and other information from the
 potential group.  If NORM-CC congestion control operation is enabled,
 the NORM-CC Rate header extension MUST be included in these messages.
 Congestion control operation SHALL be observed at all times when
 operating in the general Internet.  Even if congestion control
 operation is disabled at the sender, it may be desirable to use the
 NORM_CMD(CC) messaging to collect feedback from the group using the
 baseline NORM-CC feedback mechanisms.  This proactive feedback
 collection can be used to establish a GRTT estimate prior to data
 transmission and potential NACK operation.
 In some cases, applications may wish for the sender to also proceed
 with data transmission immediately.  In other cases, the sender may
 wish to defer data transmission until it has received some feedback
 or request from the receiver set indicating that receivers are indeed
 present.  Note, in some applications (e.g., web push), this
 indication may come out-of-band with respect to the multicast session
 via other means.  As noted, the periodic transmission of NORM_CMD(CC)
 messages may precede actual data transmission in order to have an
 initial GRTT estimate.
 With inclusion of the OPTIONAL NORM FEC Object Transmission
 Information Header Extension, the NORM protocol sender message
 headers can contain all information necessary to prepare receivers
 for subsequent reliable reception.  This includes FEC coding
 parameters, the sender NormSegmentSize, and other information.  If
 this header extension is not used, it is presumed that receivers have
 received the FEC Object Transmission Information via other means.
 Additionally, applications may leverage the use of NORM_INFO messages
 associated with the session data objects in the session to provide
 application-specific context information for the session and data
 being transmitted.  These mechanisms allow for operation with minimal
 pre-coordination among the senders and receivers.
 The NORM sender begins segmenting application-enqueued data into
 NORM_DATA segments and transmitting it to the group.  The
 segmentation algorithm is described in Section 5.1.1.  The rate of
 transmission is controlled via congestion control mechanisms or is a
 fixed rate if desired for closed network operations.  The receivers
 participating in the multicast group provide feedback to the sender
 as needed.  When the sender reaches the end of data it has enqueued

Adamson, et al. Experimental [Page 54] RFC 3940 NORM Protocol November 2004

 for transmission or any pending repairs, it transmits a series of
 NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT.  Receivers may
 respond to these NORM_CMD(FLUSH) messages with additional repair
 requests.  A protocol parameter "NORM_ROBUST_FACTOR" determines the
 number of flush messages sent.  If receivers request repair, the
 repair is provided and flushing occurs again at the end of repair
 transmission.  The sender may attach an OPTIONAL "acking_node_list"
 to NORM_CMD(FLUSH) containing the NormNodeIds for receivers from
 which it expects explicit positive acknowledgment of reception.  The
 NORM_CMD(FLUSH) message may be also used for this optional function
 any time prior to the end of data enqueued for transmission with the
 NORM_CMD(FLUSH) messages multiplexed with ongoing data transmissions.
 The OPTIONAL NORM positive acknowledgment procedure is described in
 Section 5.5.3.

5.1.1. Object Segmentation Algorithm

 NORM senders and receivers must use a common algorithm for logically
 segmenting transport data into FEC encoding blocks and symbols so
 that appropriate NACKs can be constructed to request repair of
 missing data.  NORM FEC coding blocks are comprised of multi-byte
 symbols which are transmitted in the payload of NORM_DATA messages.
 Each NORM_DATA message contains one source or encoding symbol and the
 NormSegmentSize sender parameter defines the maximum symbol size in
 bytes.  The FEC encoding type and associated parameters govern the
 source block size (number of source symbols per coding block).  NORM
 senders and receivers use these FEC parameters, along with the
 NormSegmentSize and transport object size to compute the source block
 structure for transport objects.  These parameters are provided in
 the FEC Transmission Information for each object.  The algorithm
 given below is used to compute a source block structure such that all
 source blocks are as close to being equal length as possible.  This
 helps avoid the performance disadvantages of "short" FEC blocks.
 Note this algorithm applies only to the statically-sized
 NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
 the object size is fixed and predetermined.  For NORM_OBJECT_STREAM
 objects, the object is segmented according to the maximum source
 block length  given in the FEC Transmission Information, unless the
 FEC Payload ID indicates an alternative size for a given block.
 The NORM block segmentation algorithm is defined as follows.  For a
 transport object of a given length (L_obj) in bytes, a first number
 of FEC source blocks (N_large) is delineated of a larger block size
 (B_large), and a second number of source blocks (N_small) is
 delineated of a smaller block size (B_small).  Given the maximum FEC
 source block size (B_max) and the sender's NormSegmentSize, the block
 segmentation for a given NORM transport object is determined as
 follows:

Adamson, et al. Experimental [Page 55] RFC 3940 NORM Protocol November 2004

 Inputs:
 B_max = Maximum source block length (i.e., maximum number of source
         symbols per source block)
 L_sym = Encoding symbol length in bytes (i.e., NormSegmentSize)
 L_obj = Object length in bytes
 Outputs:
 N_total = The total number of source blocks into which the transport
           object is partitioned.
 N_large = Number of larger source blocks (first set of blocks)
 B_large = Size (in encoding symbols) of the larger source blocks
 N_small = Number of smaller source blocks (second set of blocks)
 B_small = Size (in encoding symbols) of the smaller source blocks
 L_final = Length (in bytes) of the last source symbol of the last
           source block (All other symbols are of length L_sym).
 Algorithm:
 1) The total number of source symbols in the transport object is
    computed as:  S_total = L_obj/L_sym [rounded up to the nearest
    integer]
 2) The transport object is partitioned into N_total source blocks,
    where:  N_total = S_total/B_max [rounded up to the nearest
    integer]
 3) The average length of a source block is computed as:  B_ave =
    S_total/N_total (this may be non-integer)
 4) The size of the first set of larger blocks is computed as:
    B_large = B_ave [rounded up to the nearest integer] (Note it will
    always be the case that B_large <= B_max)
 5) The size of the second set of smaller blocks is computed as:
    B_small = B_ave [rounded down to the nearest integer] (Note if
    B_ave is an integer B_small = B_large; otherwise B_small = B_large
    - 1)

Adamson, et al. Experimental [Page 56] RFC 3940 NORM Protocol November 2004

 6) The fractional part of B_ave is computed as:  B_fraction = B_ave -
    B_small
 7) The number of larger source blocks is computed as:  N_large =
    B_fraction * N_total (Note N_large is an integer in the range 0
    through N_total - 1)
 8) The number of smaller source blocks is computed as:  N_small =
    N_total - N_large
 9) Each of the first N_large source blocks consists of B_large source
    symbols.  Each of the remaining N_small source blocks consists of
    B_small source symbols.  All symbols are L_sym bytes in length
    except for the final source symbol of the final source block which
    is of length (in bytes):
    L_final = L_obj - (N_large*B_large + N_small*B_small - 1) * L_sym

5.2. Receiver Initialization and Reception

 The NORM protocol is designed such that receivers may join and leave
 the group at will.  However, some applications may be constrained
 such that receivers need to be members of the group prior to start of
 data transmission.  NORM applications may use different policies to
 constrain the impact of new receivers joining the group in the middle
 of a session.  For example, a useful implementation policy is for new
 receivers joining the group to limit or avoid repair requests for
 transport objects already in progress.  The NORM sender
 implementation may wish to impose additional constraints to limit the
 ability of receivers to disrupt reliable multicast performance by
 joining, leaving, and rejoining the group often.  Different receiver
 "join policies" may be appropriate for different applications and/or
 scenarios.  For general purpose operation, default policy where
 receivers are allowed to request repair only for coding blocks with a
 NormTransportId and FEC coding block number greater than or equal to
 the first non-repair NORM_DATA or NORM_INFO message received upon
 joining the group is RECOMMENDED.  For objects of type
 NORM_OBJECT_STREAM it is RECOMMENDED that the join policy constrain
 receivers to start reliable reception at the current FEC coding block
 for which non-repair content is received.

5.3. Receiver NACK Procedure

 When the receiver detects it is missing data from a sender's NORM
 transmissions, it initiates its NACKing procedure.  The NACKing
 procedure SHALL be initiated _only_ at FEC coding block boundaries,
 NormObject boundaries, and upon receipt of a NORM_CMD(FLUSH) message.

Adamson, et al. Experimental [Page 57] RFC 3940 NORM Protocol November 2004

 The NACKing procedure begins with a random backoff timeout.  The
 duration of the backoff timeout is chosen using the "RandomBackoff"
 algorithm described in the NORM Building Block document [4] using
 (Ksender*GRTTsender) for the "maxTime" parameter and the sender
 advertised group size (GSIZEsender) as the "groupSize" parameter.
 NORM senders provide values for GRTTsender, Ksender and GSIZEsender
 via the "grtt", "backoff", and "gsize" fields of transmitted
 messages.  The GRTTsender value is determined by the sender based on
 feedback it has received from the group while the Ksender and
 GSIZEsender values may determined by application requirements and
 expectations or ancillary information.  The backoff factor "Ksender"
 MUST be greater than one to provide for effective feedback
 suppression.  A value of K = 4 is RECOMMENDED for the Any Source
 Multicast (ASM) model while a value of K = 6 is RECOMMENDED for
 Single Source Multicast (SSM) operation.
 Thus:
      T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)
 To avoid the possibility of NACK implosion in the case of sender or
 network failure during SSM operation, the receiver SHALL
 automatically suppress its NACK and immediately enter the "holdoff"
 period described below when T_backoff is greater than (Ksender-
 1)*GRTTsender.  Otherwise, the backoff period is entered and the
 receiver MUST accumulate external pending repair state from NORM_NACK
 messages and NORM_CMD(REPAIR_ADV) messages received.  At the end of
 the backoff time, the receiver SHALL generate a NORM_NACK message
 only if the following conditions are met:
 1) The sender's current transmit position (in terms of
    objectId::fecPayloadId) exceeds the earliest repair position of
    the receiver.
 2) The repair state accumulated from NORM_NACK and
    NORM_CMD(REPAIR_ADV) messages do not equal or supersede the
    receiver's repair needs up to the sender transmission position at
    the time the NACK procedure (backoff timeout) was initiated.
 If these conditions are met, the receiver immediately generates a
 NORM_NACK message when the backoff timeout expires.  Otherwise, the
 receiver's NACK is considered to be "suppressed" and the message is
 not sent.  At this time, the receiver begins a "holdoff" period
 during which it constrains itself to not reinitiate the NACKing
 process.  The purpose of this timeout is to allow the sender worst-
 case time to respond to the repair needs before the receiver requests
 repair again.  The value of this "holdoff" timeout  (T_rcvrHoldoff)
 as described in [4] is:

Adamson, et al. Experimental [Page 58] RFC 3940 NORM Protocol November 2004

                 T_rcvrHoldoff =(Ksender+2)*GRTTsender
 The NORM_NACK message contains repair request content beginning with
 lowest ordinal repair position of the receiver up through the coding
 block prior to the most recently heard ordinal transmission position
 for the sender.  If the size of the NORM_NACK content exceeds the
 sender's NormSegmentSize, the NACK content is truncated so that the
 receiver only generates a single NORM_NACK message per NACK cycle for
 a given sender.  In summary, a single NACK message is generated
 containing the receiver's lowest ordinal repair needs.
 For each partially-received FEC coding block requiring repair, the
 receiver SHALL, on its _first_ repair attempt for the block, request
 the parity portion of the FEC coding block beginning with the lowest
 ordinal _parity_ "encoding_symbol_id" (i.e., "encoding_symbol_id" =
 "source_block_len") and request the number of FEC symbols
 corresponding to its data segment erasure count for the block.  On
 _subsequent_ repair cycles for the same coding block, the receiver
 SHALL request only those repair symbols from the first set it has not
 yet received up to the remaining erasure count for that applicable
 coding block.  Note that the sender may have provided other
 different, additional parity segments for other receivers that could
 also be used to satisfy the local receiver's erasure-filling needs.
 In the case where the erasure count for a partially-received FEC
 coding block exceeds the maximum number of parity symbols available
 from the sender for the block (as indicated by the NORM_DATA
 "fec_num_parity" field), the receiver SHALL request all available
 parity segments plus the ordinally highest missing data segments
 required to satisfy its total erasure needs for the block.  The goal
 of this strategy is for the overall receiver set to request a lowest
 common denominator set of repair symbols for a given FEC coding
 block.  This allows the sender to construct the most efficient repair
 transmission segment set and enables effective NACK suppression among
 the receivers even with uncorrelated packet loss.  This approach also
 requires no synchronization among the receiver set in their repair
 requests for the sender.
 For FEC coding blocks or NormObjects missed in their entirety, the
 NORM receiver constructs repair requests with NORM_NACK_BLOCK or
 NORM_NACK_OBJECT flags set as appropriate.  The request for
 retransmission of NORM_INFO is accomplished by setting the
 NORM_NACK_INFO flag in a corresponding repair request.

5.4. Sender NACK Processing and Response

 The principle goal of the sender is to make forward progress in the
 transmission of data its application has enqueued.  However, the
 sender must occasionally "rewind" its logical transmission point to

Adamson, et al. Experimental [Page 59] RFC 3940 NORM Protocol November 2004

 satisfy the repair needs of receivers who have NACKed.  Aggregation
 of multiple NACKs is used to determine an optimal repair strategy
 when a NACK event occurs.  Since receivers initiate the NACK process
 on coding block or object boundaries, there is some loose degree of
 synchronization of the repair process even when receivers experience
 uncorrelated data loss.

5.4.1. Sender Repair State Aggregation

 When a sender is in its normal state of transmitting new data and
 receives a NACK, it begins a procedure to accumulate NACK repair
 state from NORM_NACK messages before beginning repair transmissions.
 Note that this period of aggregating repair state does _not_
 interfere with its ongoing transmission of new data.
 As described in [4], the period of time during which the sender
 aggregates NORM_NACK messages is equal to:
                  T_sndrAggregate = (Ksender+1)*GRTT
 where "Ksender" is the same backoff scaling value used by the
 receivers, and "GRTT" is the sender's current estimate of the group's
 greatest round-trip time.
 When this period ends, the sender "rewinds" by incorporating the
 accumulated repair state into its pending transmission state and
 begins transmitting repair messages.  After pending repair
 transmissions are completed, the sender continues with new
 transmissions of any enqueued data.  Also, at this point in time, the
 sender begins a "holdoff" timeout during which time the sender
 constrains itself from initiating a new repair aggregation cycle,
 even if NORM_NACK messages arrive.  As described in [4], the value of
 this sender "holdoff" period is:
                       T_sndrHoldoff = (1*GRTT)
 If additional NORM_NACK messages are received during this sender
 "holdoff" period, the sender will immediately incorporate these "late
 messages" into its pending transmission state ONLY if the NACK
 content is ordinally greater than the sender's current transmission
 position.  This "holdoff" time allows worst case time for the sender
 to propagate its current transmission sequence position to the group,
 thus avoiding redundant repair transmissions.  After the holdoff
 timeout expires, a new NACK accumulation period can be begun (upon
 arrival of a NACK) in concert with the pending repair and new data
 transmission.  Recall that receivers are not to initiate the NACK
 repair process until the sender's logical transmission position
 exceeds the lowest ordinal position of their repair needs.  With the

Adamson, et al. Experimental [Page 60] RFC 3940 NORM Protocol November 2004

 new NACK aggregation period, the sender repeats the same process of
 incorporating accumulated repair state into its transmission plan and
 subsequently "rewinding" to transmit the lowest ordinal repair data
 when the aggregation period expires.  Again, this is conducted in
 concert with ongoing new data and/or pending repair transmissions.

5.4.2. Sender FEC Repair Transmission Strategy

 The NORM sender should leverage transmission of FEC parity content
 for repair to the greatest extent possible.  Recall that the
 receivers use a strategy to request a lowest common denominator of
 explicit repair (including parity content) in the formation of their
 NORM_NACK messages.  Before falling back to explicitly satisfying
 different receivers' repair needs, the sender can make use of the
 general erasure-filling capability of FEC-generated parity segments.
 The sender can determine the maximum erasure filling needs for
 individual FEC coding blocks from the NORM_NACK messages received
 during the repair aggregation period.  Then, if the sender has a
 sufficient number (less than or equal to the maximum erasure count)
 of previously unsent parity segments available for the applicable
 coding blocks, the sender can transmit these in lieu of the specific
 packets the receiver set has requested.  Only after exhausting its
 supply of "fresh" (unsent) parity segments for a given coding block
 should the sender resort to explicit transmission of the receiver
 set's repair needs.  In general, if a sufficiently powerful FEC code
 is used, the need for explicit repair will be an exception, and the
 fulfillment of reliable multicast can be accomplished quite
 efficiently.  However, the ability to resort to explicit repair
 allows the protocol to be reliable under even very extreme
 circumstances.
 NORM_DATA messages sent as repair transmissions SHALL be flagged with
 the NORM_FLAG_REPAIR flag.  This allows receivers to obey any
 policies that limit new receivers from joining the reliable
 transmission when only repair transmissions have been received.
 Additionally, the sender SHOULD additionally flag NORM_DATA
 transmissions sent as explicit repair with the NORM_FLAG_EXPLICIT
 flag.
 Although NORM end system receivers do not make use of the
 NORM_FLAG_EXPLICIT flag, this message transmission status could be
 leveraged by intermediate systems wishing to "assist" NORM protocol
 performance.  If such systems are properly positioned with respect to
 reciprocal reverse-path multicast routing, they need to sub-cast only
 a sufficient count of non-explicit parity repairs to satisfy a
 multicast routing sub-tree's erasure filling needs for a given FEC
 coding block.  When the sender has resorted to explicit repair, then
 the intermediate systems should sub-cast all of the explicit repair

Adamson, et al. Experimental [Page 61] RFC 3940 NORM Protocol November 2004

 packets to those portions of the routing tree still requiring repair
 for a given coding block.  Note the intermediate systems will be
 required to conduct repair state accumulation for sub-routes in a
 manner similar to the sender's repair state accumulation in order to
 have sufficient information to perform the sub-casting.
 Additionally, the intermediate systems could perform additional
 NORM_NACK suppression/aggregation as it conducts this repair state
 accumulation for NORM repair cycles.  The detail of this type of
 operation are beyond the scope of this document, but this information
 is provided for possible future consideration.

5.4.3. Sender NORM_CMD(SQUELCH) Generation

 If the sender receives a NORM_NACK message for repair of data it is
 no longer supporting, the sender generates a NORM_CMD(SQUELCH)
 message to advertise its repair window and squelch any receivers from
 additional NACKing of invalid data.  The transmission rate of
 NORM_CMD(SQUELCH) messages is limited to once per 2*GRTT.  The
 "invalid_object_list" (if applicable) of the NORM_CMD(SQUELCH)
 message SHALL begin with the lowest "object_transport_id" from the
 invalid NORM_NACK messages received since the last NORM_CMD(SQUELCH)
 transmission.  Lower ordinal invalid "object_transport_ids" should be
 included only while the NORM_CMD(SQUELCH) payload is less than the
 sender's NormSegmentSize parameter.

5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation

 When a NORM sender receives NORM_NACK messages from receivers via
 unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
 advertise its accumulated repair state to the receiver set since the
 receiver set is not directly sharing their repair needs via multicast
 communication.  The NORM_CMD(REPAIR_ADV) message is multicast to the
 receiver set by the sender.  The payload portion of this message has
 content in the same format as the NORM_NACK receiver message payload.
 Receivers are then able to perform feedback suppression in the same
 manner as with NORM_NACK messages directly received from other
 receivers.  Note the sender does not merely retransmit NACK content
 it receives, but instead transmits a representation of its aggregated
 repair state.  The transmission of NORM_CMD(REPAIR_ADV) messages are
 subject to the sender transmit rate limit and NormSegmentSize
 limitation.  When the NORM_CMD(REPAIR_ADV) message is of maximum
 size, receivers SHALL consider the maximum ordinal transmission
 position value embedded in the message as the senders "current"
 transmission position and implicitly suppress requests for ordinally
 higher repair.  For congestion control operation, the sender may also
 need to provide information so that dynamic congestion control
 feedback can be suppressed as needed among receivers.  This document
 specifies the NORM-CC Feedback Header Extension that is applied for

Adamson, et al. Experimental [Page 62] RFC 3940 NORM Protocol November 2004

 baseline NORM-CC operation.  If other congestion control mechanisms
 are used within a NORM implementation, other header extensions may be
 defined.  Whatever content format is used for this purpose should
 ensure that maximum possible suppression state is conveyed to the
 receiver set.

5.5. Additional Protocol Mechanisms

 In addition to the principal function of data content transmission
 and repair, there are some other protocol mechanisms that help NORM
 to adapt to network conditions and play fairly with other coexistent
 protocols.

5.5.1. Greatest Round-trip Time Collection

 For NORM receivers to appropriately scale backoff timeouts and the
 senders to use proper corresponding timeouts, the participants must
 agree on a common timeout basis.  Each NORM sender monitors the
 round-trip time of active receivers and determines the group greatest
 round-trip time (GRTT).  The sender advertises this GRTT estimate in
 every message it transmits so that receivers have this value
 available for scaling their timers.  To measure the current GRTT, the
 sender periodically sends NORM_CMD(CC) messages that contain a
 locally generated timestamp.  Receivers are expected to record this
 timestamp along with the time the NORM_CMD(CC) message is received.
 Then, when the receivers generate feedback messages to the sender, an
 adjusted version of the sender timestamp is embedded in the feedback
 message (NORM_NACK or NORM_ACK).  The adjustment adds the amount of
 time the receiver held the timestamp before generating its response.
 Upon receipt of this adjusted timestamp, the sender is able to
 calculate the round-trip time to that receiver.
 The round-trip time for each receiver is fed into an algorithm that
 weights and smoothes the values for a conservative estimate of the
 GRTT.  The algorithm and methodology are described in the NORM
 Building Block document [4] in the section entitled "One-to-Many
 Sender GRTT Measurement".  A conservative estimate helps feedback
 suppression at a small cost in overall protocol repair delay.  The
 sender's current estimate of GRTT is advertised in the "grtt" field
 found in all NORM sender messages.  The advertised GRTT is also
 limited to a minimum of the nominal inter-packet transmission time
 given the sender's current transmission rate and system clock
 granularity.  The reason for this additional limit is to keep the
 receiver somewhat "event driven" by making sure the sender has had
 adequate time to generate any response to repair requests from
 receivers given transmit rate limitations due to congestion control
 or configuration.

Adamson, et al. Experimental [Page 63] RFC 3940 NORM Protocol November 2004

 When the NORM-CC Rate header extension is present in NORM_CMD(CC)
 messages, the receivers respond to NORM_CMD(CC) messages as described
 in Section 5.5.2, "NORM Congestion Control Operation".  The
 NORM_CMD(CC) messages are periodically generated by the sender as
 described for congestion control operation.  This provides for
 proactive, but controlled, feedback from the group in the form of
 NORM_ACK messages.  This provides for GRTT feedback even if no
 NORM_NACK messages are being sent.  If operating without congestion
 control in a closed network, the NORM_CMD(CC) messages may be sent
 periodically without the NORM-CC Rate header extension.  In this
 case, receivers will only provide GRTT measurement feedback when
 NORM_NACK messages are generated since no NORM_ACK messages are
 generated.  In this case, the NORM_CMD(CC) messages may be sent less
 frequently, perhaps as little as once per minute, to conserve network
 capacity.  Note that the NORM-CC Rate header extension may also be
 used proactively solicit RTT feedback from the receiver group per
 congestion control operation even though the sender may not be
 conducting congestion control rate adjustment.  NORM operation
 without congestion control should be considered only in closed
 networks.

5.5.2. NORM Congestion Control Operation

 This section describes baseline congestion control operation for the
 NORM protocol (NORM-CC).  The supporting NORM message formats and
 approach described here are an adaptation of the equation-based TCP-
 Friendly Multicast Congestion Control (TFMCC) approach described in
 [19].  This congestion control scheme is REQUIRED for operation
 within the general Internet unless the NORM implementation is adapted
 to use another IETF-sanctioned reliable multicast congestion control
 mechanism (e.g., PGMCC [20]).  With this TFMCC-based approach, the
 transmissions of NORM senders are controlled in a rate-based manner
 as opposed to window-based congestion control algorithms as in TCP.
 However, it is possible that the NORM protocol message set may
 alternatively be used to support a window-based multicast congestion
 control scheme such as PGMCC.  The details of that alternative may be
 described separately or in a future revision of this document.  In
 either case (rate-based TFMCC or window-based PGMCC), successful
 control of sender transmission depends upon collection of sender-to-
 receiver packet loss estimates and RTTs to identify the congestion
 control bottleneck path(s) within the multicast topology and adjust
 the sender rate accordingly.  The receiver with loss and RTT
 estimates that correspond to the lowest result transmission rate is
 identified as the "current limiting receiver" (CLR).

Adamson, et al. Experimental [Page 64] RFC 3940 NORM Protocol November 2004

 As described in [21], a steady-state sender transmission rate, to be
 "friendly" with competing TCP flows can be calculated as:
                                     S

Rsender = ————————————————————–

        tRTT * (sqrt((2/3)*p) + 12 * sqrt((3/8)*p) * p *
        (1 + 32*(p^2)))

where

 S = Nominal transmitted packet size. (In NORM, the "nominal"
     packet size can be determined by the sender as an
     exponentially weighted moving average (EWMA) of transmitted
     packet sizes to account for variable message sizes).

tRTT = The RTT estimate of the current "current limiting receiver"

     (CLR).
 p = The loss event fraction of the CLR.
 To support congestion control feedback collection and operation, the
 NORM sender periodically transmits NORM_CMD(CC) command messages.
 NORM_CMD(CC) messages are multiplexed with NORM data and repair
 transmissions and serve several purposes:
 1) Stimulate explicit feedback from the general receiver set to
    collect congestion control information.
 2) Communicate state to the receiver set on the sender's current
    congestion control status including details of the CLR.
 3) Initiate rapid (immediate) feedback from the CLR in order to
    closely track the dynamics of congestion control for that current
    "worst path" in the group multicast topology.
 The format of the NORM_CMD(CC) message is describe in Section 4.2.3
 of this document.  The NORM_CMD(CC) message contains information to
 allow measurement of RTTs, to inform the group of the congestion
 control CLR, and to provide feedback of individual RTT measurements
 to the receivers in the group.  The NORM_CMD(CC) also provides for
 exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
 nodes that may be determined administratively or possibly
 algorithmically based on congestion control feedback.  PLR nodes are
 receivers that have been identified to have potential for (perhaps
 soon) becoming the CLR and thus immediate, up-to-date feedback is
 beneficial for congestion control performance.  The details of PLR
 selection are not discussed in this document.

Adamson, et al. Experimental [Page 65] RFC 3940 NORM Protocol November 2004

5.5.2.1. NORM_CMD(CC) Transmission

 The NORM_CMD(CC) message is transmitted periodically by the sender
 along with its normal data transmission.  Note that the repeated
 transmission of NORM_CMD(CC) messages may be initiated some time
 before transmission of user data content at session startup.  This
 may be done to collect some estimation of the current state of the
 multicast topology with respect to group and individual RTT and
 congestion control state.
 A NORM_CMD(CC) message is immediately transmitted at sender startup.
 The interval of subsequent NORM_CMD(CC) message transmission is
 determined as follows:
 1) By default, the interval is set according to the current sender
    GRTT estimate.  A startup GRTT of 0.5 seconds is recommended when
    no feedback has yet been received from the group.
 2) If a CLR has been identified (based on previous receiver
    feedback), the interval is the RTT between the sender and CLR.
 3) Additionally, if the interval of nominal data message transmission
    is greater than the GRTT or RTT_clr interval, the NORM_CMD(CC)
    interval is set to this greater value.  This ensures that the
    transmission of this control message is not done to the exclusion
    of user data transmission.
 The NORM_CMD(CC) "cc_sequence" field is incremented with each
 transmission of a NORM_CMD(CC) command.  The greatest "cc_sequence"
 recently received by receivers is included in their feedback to the
 sender.  This allows the sender to determine the "age" of feedback to
 assist in congestion avoidance.
 The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
 message and the sender advertises its current transmission rate in
 the "send_rate" field.  The rate information is used by receivers to
 initialize loss estimation during congestion control startup or
 restart.
 The "cc_node_list" contains a list of entries identifying receivers
 and their current congestion control state (status "flags", "rtt" and
 "loss" estimates).  The list may be empty if the sender has not yet
 received any feedback from the group.  If the sender has received
 feedback, the list will minimally contain an entry identifying the
 CLR.  A NORM_FLAG_CC_CLR flag value is provided for the "cc_flags"
 field to identify the CLR entry.  It is RECOMMENDED that the CLR
 entry be the first in the list for implementation efficiency.
 Additional entries in the list are used to provide sender-measured

Adamson, et al. Experimental [Page 66] RFC 3940 NORM Protocol November 2004

 individual RTT estimates to receivers in the group.  The number of
 additional entries in this list is dependent upon the percentage of
 control traffic the sender application is willing to send with
 respect to user data message transmissions.  More entries in the list
 may allow the sender to be more responsive to congestion control
 dynamics.  The length of the list may be dynamically determined
 according to the current transmission rate and scheduling of
 NORM_CMD(CC) messages.  The maximum length of the list corresponds to
 the sender's NormSegmentSize parameter for the session.  The
 inclusion of additional entries in the list based on receiver
 feedback are prioritized with following rules:
 1) Receivers that have not yet been provided RTT feedback get first
    priority.  Of these, those with the greatest loss fraction receive
    precedence for list inclusion.
 2) Secondly, receivers that have previously been provided RTT are
    included with receivers yielding the lowest calculated congestion
    rate getting precedence.
 There are "cc_flag" values in addition to NORM_FLAG_CC_CLR that are
 used for other congestion control functions.  The NORM_FLAG_CC_PLR
 flag value is used to mark additional receivers from that the sender
 would like to have immediate, non-suppressed feedback.  These may be
 receivers that the sender algorithmically identified as potential
 future CLRs or that have been pre-configured as potential congestion
 control points in the network.  The NORM_FLAG_CC_RTT indicates the
 validity of the "cc_rtt" field for the associated receiver node.
 Normally, this flag will be set since the receivers in the list will
 typically be receivers from which the sender has received feedback.
 However, in the case that the NORM sender has been pre-configured
 with a set of PLR nodes, feedback from those receivers may not yet
 have been collected and thus the "cc_rtt" and "cc_rate" fields do not
 contain valid values when this flag is not set.

5.5.2.2. NORM_CMD(CC) Feedback Response

 Receivers explicitly respond to NORM_CMD(CC) messages in the form of
 a NORM_ACK(RTT) message.  The goal of the congestion control feedback
 is to determine the receivers with the lowest congestion control
 rates.  Receivers that are marked as CLR or PLR nodes in the
 NORM_CMD(CC) "cc_node_list" immediately provide feedback in the form
 of a NORM_ACK to this message.  When a NORM_CMD(CC) is received,
 non-CLR or non-PLR nodes initiate random feedback backoff timeouts
 similar to that used when the receiver initiates a repair cycle (see
 Section 5.3) in response to detection of data loss.  The backoff
 timeout for the congestion control response is generated as follows:

Adamson, et al. Experimental [Page 67] RFC 3940 NORM Protocol November 2004

         T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)
 The "RandomBackoff()" algorithm provides a truncated exponentially
 distributed random number and is described in the NORM Building Block
 document [4].  The same backoff factor K = Ksender MAY be used as
 with NORM_NACK suppression.  However, in cases where the application
 purposefully specifies a very small Ksender backoff factor to
 minimize the NACK repair process latency (trading off group size
 scalability), it may still be desirable to maintain a larger backoff
 factor for congestion control feedback, since there may often be a
 larger volume of congestion control feedback than NACKs in many cases
 and congestion control feedback latency may be tolerable where
 reliable delivery latency is not.  As previously noted, a backoff
 factor value of K = 4 is generally recommended for ASM operation and
 K = 6 for SSM operation.  A receiver SHALL cancel the backoff timeout
 and thus its pending transmission of a NORM_ACK(RTT) message under
 the following conditions:
 1) The receiver generates another feedback message (NORM_NACK or
    other NORM_ACK) before the congestion control feedback timeout
    expires,
 2) A NORM_CMD(CC) or other receiver feedback with an ordinally
    greater "cc_sequence" field value is received before the
    congestion control feedback timeout expires (this is similar to
    the TFMCC feedback round number),
 3) When the T_backoff is greater than 1*GRTT.  This prevents NACK
    implosion in the event of sender or network failure,
 4) "Suppressing" congestion control feedback is heard from another
    receiver (in a NORM_ACK or NORM_NACK) or via a
    NORM_CMD(REPAIR_ADV) message from the sender.  The local
    receiver's feedback is "suppressed" if the rate of the competing
    feedback (Rfb) is sufficiently close to or less than the local
    receiver's calculated rate (Rcalc).  The local receiver's feedback
    is canceled when:
                           Rcalc > (0.9 * Rfb)
    Also note receivers that have not yet received an RTT measurement
    from the sender are suppressed only by other receivers that have
    not yet measured RTT.  Additionally, receivers whose RTT estimate
    has "aged" considerably (i.e., they haven't been included in the
    NORM_CMD(CC) "cc_node_list" in a long time) may wish to compete as
    a receiver with no prior RTT measurement after some expiration
    period.

Adamson, et al. Experimental [Page 68] RFC 3940 NORM Protocol November 2004

 When the backoff timer expires, the receiver SHALL generate a
 NORM_ACK(RTT) message to provide feedback to the sender and group.
 This message may be multicast to the group for most effective
 suppression in ASM topologies or unicast to the sender depending upon
 how the NORM protocol is deployed and configured.
 Whenever any feedback is generated (including this NORM_ACK(RTT)
 message), receivers include an adjusted version of the sender
 timestamp from the most recently received NORM_CMD(CC) message and
 the "cc_sequence" value from that command in the applicable NORM_ACK
 or NORM_NACK message fields.  For NORM-CC operation, any generated
 feedback message SHALL also contain the NORM-CC Feedback header
 extension.  The receiver provides its current "cc_rate" estimate,
 "cc_loss" estimate, "cc_rtt" if known, and any applicable "cc_flags"
 via this header extension.
 During slow start (when the receiver has not yet detected loss from
 the sender), the receiver uses a value equal to two times its
 measured rate from the sender in the "cc_rate" field.  For steady-
 state congestion control operation, the receiver "cc_rate" value is
 from the equation-based value using its current loss event estimate
 and sender<->receiver RTT information.  (The GRTT is used when the
 receiver has not yet measured its individual RTT).
 The "cc_loss" field value reflects the receiver's current loss event
 estimate with respect to the sender in question.
 When the receiver has a valid individual RTT measurement, it SHALL
 include this value in the "cc_rtt" field.  The NORM_FLAG_CC_RTT MUST
 be set when the "cc_rtt" field is valid.
 After a congestion control feedback message is generated or when the
 feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
 period during which it will restrain itself from providing congestion
 control feedback, even if NORM_CMD(CC) messages are received from the
 sender (unless the receive becomes marked as a CLR or PLR node).  The
 value of this holdoff timeout (T_ccHoldoff) period is:
                        T_ccHoldoff = (K*GRTT)
 Thus, non-CLR receivers are constrained to providing explicit
 congestion control feedback once per K*GRTT intervals.  Note,
 however, that as the session progresses, different receivers will be
 responding to different NORM_CMD(CC) messages and there will be
 relatively continuous feedback of congestion control information
 while the sender is active.

Adamson, et al. Experimental [Page 69] RFC 3940 NORM Protocol November 2004

5.5.2.3. Congestion Control Rate Adjustment

 During steady-state operation, the sender will directly adjust its
 transmission rate to the rate indicated by the feedback from its
 currently selected CLR.  As noted in [19], the estimation of
 parameters (loss and RTT) for the CLR will generally constrain the
 rate changes possible within acceptable bounds.  For rate increases,
 the sender SHALL observe a maximum rate of increase of one packet per
 RTT at all times during steady-state operation.
 The sender processes congestion control feedback from the receivers
 and selects the CLR based on the lowest rate receiver.  Receiver
 rates are either determined directly from the slow start "cc_rate"
 provided by the receiver in the NORM-CC Feedback header extension or
 by performing the equation-based calculation using individual RTT and
 loss estimates ("cc_loss") as feedback is received.
 The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
 using the "grtt_response" timestamp included in feedback messages.
 When the "cc_rtt" value in a response is not valid, the sender simply
 uses this RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
 For non-CLR and non-PLR receivers, the sender can use the "cc_rtt"
 value provided in the NORM-CC Feedback header extension as the
 receiver's previous RTT measurement (RTT_rcvrPrev) to smooth
 according to:
           RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew
 For CLR receivers where feedback is received more regularly, the
 sender SHOULD maintain a more smoothed RTT estimate upon new feedback
 from the CLR where:
              RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
 "RTT_clrNew" is the new RTT calculated from the timestamp in the
 feedback message received from the CLR.  The RTT_clr is initialized
 to RTT_clrNew on the first feedback message received.  Note that the
 same procedure is observed by the sender for PLR receivers and that
 if a PLR is "promoted" to CLR status, the smoothed estimate can be
 continued.
 There are some additional periods besides steady-state operation that
 need to be considered in NORM-CC operation.  These periods are:
 1) during session startup,
 2) when no feedback is received from the CLR, and

Adamson, et al. Experimental [Page 70] RFC 3940 NORM Protocol November 2004

 3) when the sender has a break in data transmission.
 During session startup, the congestion control operation SHALL
 observe a "slow start" procedure to quickly approach its fair
 bandwidth share.  An initial sender startup rate is assumed where:
 Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.
 The rate is increased only when feedback is received from the
 receiver set.  The "slow start" phase proceeds until any receiver
 provides feedback indicating that loss has occurred.  Rate increase
 during slow start is applied as:
                           Rnew = Rrecv_min
 where "Rrecv_min" is the minimum reported receiver rate in the
 "cc_rate" field of congestion control feedback messages received from
 the group.  Note that during "slow start", receivers use two times
 their measured rate from the sender in the "cc_rate" field of their
 feedback.  Rate increase adjustment is limited to once per GRTT
 during slow start.
 If the CLR or any receiver intends to leave the group, it will set
 the NORM_FLAG_CC_LEAVE in its congestion control feedback message as
 an indication that the sender should not select it as the CLR.  When
 the CLR changes to a lower rate receiver, the sender should
 immediately adjust to the new lower rate.  The sender is limited to
 increasing its rate at one additional packet per RTT towards any new,
 higher CLR rate.
 The sender should also track the "age" of the feedback it has
 received from the CLR by comparing its current "cc_sequence" value
 (Seq_sender) to the last "cc_sequence" value received from the CLR
 (Seq_clr).  As the "age" of the CLR feedback increases with no new
 feedback, the sender SHALL begin reducing its rate once per RTT_clr
 as a congestion avoidance measure.
 The following algorithm is used to determine the decrease in sender
 rate (Rsender bytes/sec) as the CLR feedback, unexpectedly,
 excessively ages:
 Age = Seq_sender - Seq_clr;
 if (Age > 4) Rsender = Rsender * 0.5;
 This rate reduction is limited to the lower bound on NORM
 transmission rate.  After NORM_ROBUST_FACTOR consecutive NORM_CMD(CC)
 rounds without any feedback from the CLR, the sender SHOULD assume
 the CLR has left the group and pick the receiver with the next lowest

Adamson, et al. Experimental [Page 71] RFC 3940 NORM Protocol November 2004

 rate as the new CLR.  Note this assumes that the sender does not have
 explicit knowledge that the CLR intentionally left the group.  If no
 receiver feedback is received, the sender MAY wish to withhold
 further transmissions of NORM_DATA segments and maintain NORM_CMD(CC)
 transmissions only until feedback is detected.  After such a CLR
 timeout, the sender will be transmitting with a minimal rate and
 should return to slow start as described here for a break in data
 transmission.
 When the sender has a break in its data transmission, it can continue
 to probe the group with NORM_CMD(CC) messages to maintain RTT
 collection from the group.  This will enable the sender to quickly
 determine an appropriate CLR upon data transmission restart.
 However, the sender should exponentially reduce its target rate to be
 used for transmission restart as time since the break elapses.  The
 target rate SHOULD be recalculated once per RTT_clr as:
                       Rsender = Rsender * 0.5;
 If the minimum NORM rate is reached, the sender should set the
 NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and
 the group should observer "slow start" congestion control procedures
 until any receiver experiences a new loss event.

5.5.3. NORM Positive Acknowledgment Procedure

 NORM provides options for the source application to request positive
 acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ)
 messages from members of the group.  There are some specific
 acknowledgment requests defined for the NORM protocol and a range of
 acknowledgment request types that are left to be defined by the
 application.  One predefined acknowledgment type is the
 NORM_ACK_FLUSH type.  This acknowledgment is used to determine if
 receivers have achieved completion of reliable reception up through a
 specific logical transmission point with respect to the sender's
 sequence of transmission.  The NORM_ACK_FLUSH acknowledgment may be
 used to assist in application flow control when the sender has
 information on a portion of the receiver set.  Another predefined
 acknowledgment type is NORM_ACK(CC), which is used to explicitly
 provide congestion control feedback in response to NORM_CMD(CC)
 messages transmitted by the sender for NORM-CC operation.  Note the
 NORM_ACK(CC) response does NOT follow the positive acknowledgment
 procedure described here.  The NORM_CMD(ACK_REQ) and NORM_ACK
 messages contain an "ack_type" field to identify the type of
 acknowledgment requested and provided.  A range of "ack_type" values
 is provided for application-defined use.  While the application is
 responsible for initiating the acknowledgment request and interprets
 application-defined "ack_type" values, the acknowledgment procedure

Adamson, et al. Experimental [Page 72] RFC 3940 NORM Protocol November 2004

 SHOULD be conducted within the protocol implementation to take
 advantage of timing and transmission scheduling information available
 to the NORM transport.
 The NORM positive acknowledgment procedure uses polling by the sender
 to query the receiver group for response.  Note this polling
 procedure is not intended to scale to very large receiver groups, but
 could be used in large group setting to query a critical subset of
 the group.  Either the NORM_CMD(ACK_REQ), or when applicable, the
 NORM_CMD(FLUSH) message is used for polling and contains a list of
 NormNodeIds for receivers that should respond to the command.  The
 list of receivers providing acknowledgment is determined by the
 source application with "a priori" knowledge of participating nodes
 or via some other application-level mechanism.
 The ACK process is initiated by the sender that generates
 NORM_CMD(FLUSH) or NORM_CMD(ACK_REQ) messages in periodic "rounds".
 For NORM_ACK_FLUSH requests, the NORM_CMD(FLUSH) contain a
 "object_transport_id" and "fec_payload_id" denoting the watermark
 transmission point for which acknowledgment is requested.  This
 watermark transmission point is "echoed" in the corresponding fields
 of the NORM_ACK(FLUSH) message sent by the receiver in response.
 NORM_CMD(ACK_REQ) messages contain an "ack_id" field which is
 similarly "echoed" in response so that the sender may match the
 response to the appropriate request.
 In response to the NORM_CMD(ACK_REQ), the listed receivers randomly
 spread NORM_ACK messages uniformly in time over a window of (1*GRTT).
 These NORM_ACK messages are typically unicast to the sender.  (Note
 that NORM_ACK(CC) messages SHALL be multicast or unicast in the same
 manner as NORM_NACK messages).
 The ACK process is self-limiting and avoids ACK implosion in that:
 1) Only a single NORM_CMD(ACK_REQ) message is generated once per
    (2*GRTT), and,
 2) The size of the "acking_node_list" of NormNodeIds from which
    acknowledgment is requested is limited to a maximum of the sender
    NormSegmentSize setting per round of the positive acknowledgment
    process.
 Because the size of the included list is limited to the sender's
 NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds may be
 required to achieve responses from all receivers specified.  The
 content of the attached NormNodeId list will be dynamically updated
 as this process progresses and NORM_ACK responses are received from
 the specified receiver set.  As the sender receives valid responses

Adamson, et al. Experimental [Page 73] RFC 3940 NORM Protocol November 2004

 (i.e., matching watermark point or "ack_id") from receivers, it SHALL
 eliminate those receivers from the subsequent NORM_CMD(ACK_REQ)
 message "acking_node_list" and add in any pending receiver
 NormNodeIds while keeping within the NormSegmentSize limitation of
 the list size.  Each receiver is  queried a maximum number of times
 (NORM_ROBUST_FACTOR, by default).  Receivers not responding within
 this number of repeated requests are removed from the payload list to
 make room for other potential receivers pending acknowledgment.  The
 transmission of the NORM_CMD(ACK_REQ) is repeated until no further
 responses are required or until the repeat threshold is exceeded for
 all pending receivers.  The transmission of NORM_CMD(ACK_REQ) or
 NORM_CMD(FLUSH) messages to conduct the positive acknowledgment
 process is multiplexed with ongoing sender data transmissions.
 However, the NORM_CMD(FLUSH) positive acknowledgment process may be
 interrupted in response to negative acknowledgment repair requests
 (NACKs) received from receivers during the acknowledgment period.
 The NORM_CMD(FLUSH) positive acknowledgment process is restarted for
 receivers pending acknowledgment once any the repairs have been
 transmitted.
 In the case of NORM_CMD(FLUSH) commands with an attached
 "acking_node_list", receivers will not ACK until they have received
 complete transmission of all data up to and including the given
 watermark transmission point.  All receivers SHALL interpret the
 watermark point provided in the request NACK for repairs if needed as
 for NORM_CMD(FLUSH) commands with no attached "acking_node_list".

5.5.4. Group Size Estimate

 NORM sender messages contain a "gsize" field that is a representation
 of the group size and is used in scaling random backoff timer ranges.
 The use of the group size estimate within the NORM protocol does not
 require a precise estimation and works reasonably well if the
 estimate is within an order of magnitude of the actual group size.
 By default, the NORM sender group size estimate may be
 administratively configured.  Also, given the expected scalability of
 the NORM protocol for general use, a default value of 10,000 is
 recommended for use as the group size estimate.
 It is possible that group size may be algorithmically approximated
 from the volume of congestion control feedback messages which follow
 the exponentially weighted random backoff.  However, the
 specification of such an algorithm is currently beyond the scope of
 this document.

Adamson, et al. Experimental [Page 74] RFC 3940 NORM Protocol November 2004

6. Security Considerations

 The same security considerations that apply to the NORM, and FEC
 Building Blocks also apply to the NORM protocol.  In addition to
 vulnerabilities that any IP and IP multicast protocol implementation
 may be generally subject to, the NACK-based feedback of NORM may be
 exploited by replay attacks which force the NORM sender to
 unnecessarily transmit repair information.  This MAY be addressed by
 network layer IP security implementations that guard against this
 potential security exploitation.  It is RECOMMENDED that such IP
 security mechanisms be used when available.  Another possible
 approach is for NORM senders to use the "sequence" field from the
 NORM Common Message Header to detect replay attacks.  This can be
 accomplished if the NORM packets are cryptographically protected and
 the sender is willing to maintain state on receivers which are
 NACKing.  A cache of receiver state may provide some protection
 against replay attacks.  Note that the "sequence" field of NORM
 messages should be incremented with independent values for different
 destinations (e.g., group-addressed versus unicast-addressed messages
 versus "receiver" messages).  Thus, the congestion control loss
 estimation function of the "sequence" field can be preserved for
 sender messages when receiver messages are unicast to the sender.
 The NORM protocol is compatible with the use of the IP security
 (IPsec) architecture described in [22].  It is important to note that
 while NORM does leverage FEC-based repair for scalability, this does
 not alone guarantee integrity of received data.  Application-level
 integrity-checking of data content is highly RECOMMENDED.

7. IANA Considerations

 No information in this specification is currently subject to IANA
 registration.  However, several Header Extensions are defined within
 this document.  If/when additional Header Extensions are developed,
 the first RFC MUST establish an IANA registry for them, with a
 "Specification Required" policy [6] and all Header Extensions,
 including those in the present document, MUST be registered
 thereafter.  Additionally, building blocks components used by NORM
 may introduce additional IANA considerations.  In particular, the FEC
 Building Block used by NORM does require IANA registration of the FEC
 codecs used.  The registration instructions for FEC codecs are
 provided in [5].

8. Suggested Use

 The present NORM protocol is seen as useful tool for the  reliable
 data transfer over generic IP multicast  services.  It is not the
 intention of the authors to suggest it is suitable for  supporting
 all envisioned multicast reliability requirements.  NORM provides a

Adamson, et al. Experimental [Page 75] RFC 3940 NORM Protocol November 2004

 simple and flexible framework for multicast applications with a
 degree of concern for network traffic implosion and protocol overhead
 efficiency.  NORM-like protocols have been successfully demonstrated
 within the MBone for bulk data dissemination applications, including
 weather satellite compressed imagery updates servicing a large group
 of receivers and a generic web content reliable "push" application.
 In addition, this framework approach has some design features making
 it attractive for bulk transfer in asymmetric and wireless
 internetwork applications.  NORM is capable of successfully operating
 independent of network structure and in environments with high packet
 loss, delay, and misordering.  Hybrid proactive/reactive FEC-based
 repairing improve protocol performance in some multicast scenarios.
 A sender-only repair approach often makes additional engineering
 sense in asymmetric networks.  NORM's unicast feedback capability may
 be suitable for use in asymmetric networks or in networks where only
 unidirectional multicast routing/delivery service exists.  Asymmetric
 architectures supporting multicast delivery are likely to make up an
 important portion of the future Internet structure (e.g.,
 DBS/cable/PSTN hybrids) and efficient, reliable bulk data transfer
 will be an important capability for servicing large groups of
 subscribed receivers.

9. Acknowledgments (and these are not Negative)

 The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
 Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
 and comments on this document.  The authors would also like to thank
 the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
 their support in development of this specification, and Sally Floyd
 for her early input into this document.

10. References

10.1. Normative References

 [1]  Kermode, R. and L. Vicisano, "Author Guidelines for Reliable
      Multicast Transport (RMT) Building Blocks and Protocol
      Instantiation documents", RFC 3269, April 2002.
 [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [3]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
      1112, August 1989.

Adamson, et al. Experimental [Page 76] RFC 3940 NORM Protocol November 2004

 [4]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
      "Negative-Acknowledgment (NACK)-Oriented Reliable Multicast
      (NORM) Building Blocks", RFC 3941, November 2004.
 [5]  Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
      J. Crowcroft, "Forward Error Correction (FEC) Building Block",
      RFC 3452, December 2002.
 [6]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

10.2. Informative References

 [7]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [8]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
      Protocol", RFC 2974, October 2000.
 [9]  S. Pingali, D. Towsley, J. Kurose, "A Comparison of Sender-
      Initiated and Receiver-Initiated Reliable Multicast Protocols",
      In Proc. INFOCOM, San Francisco CA, October 1993.
 [10] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
      J. Crowcroft, "The Use of Forward Error Correction (FEC) in
      Reliable Multicast", RFC 3453, December 2002.
 [11] Macker, J. and B. Adamson, "The Multicast Dissemination Protocol
      (MDP) Toolkit", Proc. IEEE MILCOM 99, October 1999.
 [12] Nonnenmacher, J. and E. Biersack, "Optimal Multicast Feedback",
      Proc. IEEE INFOCOMM, p. 964, March/April 1998.
 [13] J. Macker, B. Adamson, "Quantitative Prediction of Nack Oriented
      Reliable Multicast (NORM) Feedback", Proc. IEEE MILCOM 2002,
      October 2002.
 [14] H.W. Holbrook, "A Channel Model for Multicast", Ph.D.
      Dissertation, Stanford University, Department of Computer
      Science, Stanford, California, August 2001.
 [15] D. Gossink, J. Macker, "Reliable Multicast and Integrated Parity
      Retransmission with Channel Estimation", IEEE GLOBECOMM 98',
      September 1998.
 [16] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S.,
      and M. Luby, "Reliable Multicast Transport Building Blocks for
      One-to-Many Bulk-Data Transfer", RFC 3048, January 2001.

Adamson, et al. Experimental [Page 77] RFC 3940 NORM Protocol November 2004

 [17] Mankin, A., Romanow, A., Bradner, S., and V. Paxson, "IETF
      Criteria for Evaluating Reliable Multicast Transport and
      Application Protocols", RFC 2357, June 1998.
 [18] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
      "RTP:  A Transport Protocol for Real-Time Applications", STD 64,
      RFC 3550, July 2003.
 [19] J. Widmer and M. Handley, "Extending Equation-Based Congestion
      Control to Multicast Applications", Proc ACM SIGCOMM 2001, San
      Diego, August 2001.
 [20] L. Rizzo, "pgmcc: A TCP-Friendly Single-Rate Multicast
      Congestion Control Scheme", Proc ACM SIGCOMM 2000, Stockholm,
      August 2000.
 [21] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, "Modeling TCP
      Throughput: A Simple Model and its Empirical Validation", Proc
      ACM SIGCOMM 1998.
 [22] Kent, S. and R. Atkinson, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.

Adamson, et al. Experimental [Page 78] RFC 3940 NORM Protocol November 2004

11. Authors' Addresses

 Brian Adamson
 Naval Research Laboratory
 Washington, DC, USA, 20375
 EMail: adamson@itd.nrl.navy.mil
 Carsten Bormann
 Universitaet Bremen TZI
 Postfach 330440
 D-28334 Bremen, Germany
 EMail: cabo@tzi.org
 Mark Handley
 Department of Computer Science
 University College London
 Gower Street
 London
 WC1E 6BT
 UK
 EMail: M.Handley@cs.ucl.ac.uk
 Joe Macker
 Naval Research Laboratory
 Washington, DC, USA, 20375
 EMail: macker@itd.nrl.navy.mil

Adamson, et al. Experimental [Page 79] RFC 3940 NORM Protocol November 2004

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Adamson, et al. Experimental [Page 80]

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