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rfc:rfc8260

Internet Engineering Task Force (IETF) R. Stewart Request for Comments: 8260 Netflix, Inc. Category: Standards Track M. Tuexen ISSN: 2070-1721 Muenster Univ. of Appl. Sciences

                                                             S. Loreto
                                                              Ericsson
                                                         R. Seggelmann
                                   Metafinanz Informationssysteme GmbH
                                                         November 2017
          Stream Schedulers and User Message Interleaving
            for the Stream Control Transmission Protocol

Abstract

 The Stream Control Transmission Protocol (SCTP) is a message-oriented
 transport protocol supporting arbitrarily large user messages.  This
 document adds a new chunk to SCTP for carrying payload data.  This
 allows a sender to interleave different user messages that would
 otherwise result in head-of-line blocking at the sender.  The
 interleaving of user messages is required for WebRTC data channels.
 Whenever an SCTP sender is allowed to send user data, it may choose
 from multiple outgoing SCTP streams.  Multiple ways for performing
 this selection, called stream schedulers, are defined in this
 document.  A stream scheduler can choose to either implement, or not
 implement, user message interleaving.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8260.

Stewart, et al. Standards Track [Page 1] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Stewart, et al. Standards Track [Page 2] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.2.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   6
 2.  User Message Interleaving . . . . . . . . . . . . . . . . . .   6
   2.1.  The I-DATA Chunk Supporting User Message Interleaving . .   7
   2.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .   9
     2.2.1.  Negotiation . . . . . . . . . . . . . . . . . . . . .  10
     2.2.2.  Sender-Side Considerations  . . . . . . . . . . . . .  10
     2.2.3.  Receiver-Side Considerations  . . . . . . . . . . . .  11
   2.3.  Interaction with Other SCTP Extensions  . . . . . . . . .  11
     2.3.1.  SCTP Partial Reliability Extension  . . . . . . . . .  11
     2.3.2.  SCTP Stream Reconfiguration Extension . . . . . . . .  13
 3.  Stream Schedulers . . . . . . . . . . . . . . . . . . . . . .  14
   3.1.  First-Come, First-Served Scheduler (SCTP_SS_FCFS) . . . .  14
   3.2.  Round-Robin Scheduler (SCTP_SS_RR)  . . . . . . . . . . .  14
   3.3.  Round-Robin Scheduler per Packet (SCTP_SS_RR_PKT) . . . .  14
   3.4.  Priority-Based Scheduler (SCTP_SS_PRIO) . . . . . . . . .  14
   3.5.  Fair Capacity Scheduler (SCTP_SS_FC)  . . . . . . . . . .  15
   3.6.  Weighted Fair Queueing Scheduler (SCTP_SS_WFQ)  . . . . .  15
 4.  Socket API Considerations . . . . . . . . . . . . . . . . . .  15
   4.1.  Exposure of the Stream Sequence Number (SSN)  . . . . . .  15
   4.2.  SCTP_ASSOC_CHANGE Notification  . . . . . . . . . . . . .  16
   4.3.  Socket Options  . . . . . . . . . . . . . . . . . . . . .  16
     4.3.1.  Enable or Disable the Support of User Message
             Interleaving (SCTP_INTERLEAVING_SUPPORTED)  . . . . .  16
     4.3.2.  Get or Set the Stream Scheduler
             (SCTP_STREAM_SCHEDULER) . . . . . . . . . . . . . . .  17
     4.3.3.  Get or Set the Stream Scheduler Parameter
             (SCTP_STREAM_SCHEDULER_VALUE) . . . . . . . . . . . .  18
   4.4.  Explicit EOR Marking  . . . . . . . . . . . . . . . . . .  19
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   5.1.  I-DATA Chunk  . . . . . . . . . . . . . . . . . . . . . .  19
   5.2.  I-FORWARD-TSN Chunk . . . . . . . . . . . . . . . . . . .  20
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
   7.2.  Informative References  . . . . . . . . . . . . . . . . .  22
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

Stewart, et al. Standards Track [Page 3] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

1. Introduction

1.1. Overview

 When SCTP [RFC4960] was initially designed, it was mainly envisioned
 for the transport of small signaling messages.  Late in the design
 stage, it was decided to add support for fragmentation and reassembly
 of larger messages with the thought that someday signaling messages
 in the style of Session Initiation Protocol (SIP) [RFC3261] may also
 need to use SCTP, and a message that is a single Maximum Transmission
 Unit (MTU) would be too small.  Unfortunately this design decision,
 though valid at the time, did not account for other applications that
 might send large messages over SCTP.  The sending of such large
 messages over SCTP, as specified in [RFC4960], can result in a form
 of sender-side head-of-line blocking (e.g., when the transmission of
 a message is blocked from transmission because the sender has started
 the transmission of another, possibly large, message).  This head-of-
 line blocking is caused by the use of the Transmission Sequence
 Number (TSN) for three different purposes:
 1.  As an identifier for DATA chunks to provide a reliable transfer.
 2.  As an identifier for the sequence of fragments to allow
     reassembly.
 3.  As a sequence number allowing up to 2**16 - 1 Stream Sequence
     Numbers (SSNs) outstanding.
 The protocol requires all fragments of a user message to have
 consecutive TSNs.  This document allows an SCTP sender to interleave
 different user messages.
 This document also defines several stream schedulers for general SCTP
 associations allowing different relative stream treatments.  The
 stream schedulers may behave differently depending on whether or not
 user message interleaving has been negotiated for the association.
 Figure 1 illustrates the behavior of a round-robin stream scheduler
 using DATA chunks when three streams with the Stream Identifiers
 (SIDs) 0, 1, and 2 are used.  Each queue for SID 0 and SID 2 contains
 a single user message requiring three chunks.  The queue for SID 1
 contains three user messages each requiring a single chunk.  It is
 shown how these user messages are encapsulated in chunks using TSN 0
 to TSN 8.  Please note that the use of such a scheduler implies late

Stewart, et al. Standards Track [Page 4] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 TSN assignment, but it can be used with an implementation that is
 compliant with [RFC4960] and that does not support user message
 interleaving.  Late TSN assignment means that the sender generates
 chunks from user messages and assigns the TSN as late as possible in
 the process of sending the user messages.
 +---+---+---+
 |    0/0    |-+
 +---+---+---+ |
               |  +---+---+---+---+---+---+---+---+---+
 +---+---+---+ +->|1/2|1/1|2/0|2/0|2/0|1/0|0/0|0/0|0/0|
 |1/2|1/1|1/0|--->|---|---|---|---|---|---|---|---|---|
 +---+---+---+ +->| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
               |  +---+---+---+---+---+---+---+---+---+
 +---+---+---+ |
 |    2/0    |-+
 +---+---+---+
                                +-------+
   +-------+                    |SID/SSN|
   |SID/SSN|                    |-------|
   +-------+                    |  TSN  |
                                +-------+
   Figure 1: Round-Robin Scheduler without User Message Interleaving
 This document describes a new chunk carrying payload data called
 I-DATA.  This chunk incorporates the properties of the current SCTP
 DATA chunk, all the flags and fields except the Stream Sequence
 Number (SSN), and also adds two new fields in its chunk header -- the
 Fragment Sequence Number (FSN) and the Message Identifier (MID).  The
 FSN is only used for reassembling all fragments that have the same
 MID and the same ordering property.  The TSN is only used for the
 reliable transfer in combination with Selective Acknowledgment (SACK)
 chunks.
 In addition, the MID is also used for ensuring ordered delivery
 instead of using the stream sequence number (the I-DATA chunk omits
 an SSN).
 Figure 2 illustrates the behavior of an interleaving round-robin
 stream scheduler using I-DATA chunks.

Stewart, et al. Standards Track [Page 5] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

+—+—+—+

0/0

+—+—+—+ |

            |  +-----+-----+-----+-----+-----+-----+-----+-----+-----+

+—+—+—+ +→|2/0/2|1/2/0|0/0/2|2/0/1|1/1/0|0/0/1|2/0/0|1/0/0|0/0/0|

1/21/11/0—>—–—–—–—–—–—–—–—–—–

+—+—+—+ +→| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |

            |  +-----+-----+-----+-----+-----+-----+-----+-----+-----+

+—+—+—+ |

2/0

+—+—+—+

                                   +-----------+
+-------+                          |SID/MID/FSN|
|SID/MID|                          |-----------|
+-------+                          |    TSN    |
                                   +-----------+
    Figure 2: Round-Robin Scheduler with User Message Interleaving
 The support of the I-DATA chunk is negotiated during the association
 setup using the Supported Extensions Parameter, as defined in
 [RFC5061].  If I-DATA support has been negotiated for an association,
 I-DATA chunks are used for all user messages.  DATA chunks are not
 permitted when I-DATA support has been negotiated.  It should be
 noted that an SCTP implementation supporting I-DATA chunks needs to
 allow the coexistence of associations using DATA chunks and
 associations using I-DATA chunks.
 In Section 2, this document specifies the user message interleaving
 by defining the I-DATA chunk, the procedures to use it, and its
 interactions with other SCTP extensions.  Section 3 defines multiple
 stream schedulers, and Section 4 describes an extension to the socket
 API for using the mechanism specified in this document.

1.2. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

2. User Message Interleaving

 The protocol mechanisms described in this document allow the
 interleaving of user messages sent on different streams.  They do not
 support the interleaving of multiple messages (ordered or unordered)
 sent on the same stream.

Stewart, et al. Standards Track [Page 6] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 The interleaving of user messages is required for WebRTC data
 channels, as specified in [DATA-CHAN].
 An SCTP implementation supporting user message interleaving is
 REQUIRED to support the coexistence of associations using DATA chunks
 and associations using I-DATA chunks.  If an SCTP implementation
 supports user message interleaving and the Partial Reliability
 extension described in [RFC3758] or the Stream Reconfiguration
 Extension described in [RFC6525], it is REQUIRED to implement the
 corresponding changes specified in Section 2.3.

2.1. The I-DATA Chunk Supporting User Message Interleaving

 The following Figure 3 shows the new I-DATA chunk allowing user
 message interleaving.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Type = 64   |  Res  |I|U|B|E|       Length = Variable       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                              TSN                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Stream Identifier      |           Reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Message Identifier                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Payload Protocol Identifier / Fragment Sequence Number     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                           User Data                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 3: I-DATA Chunk Format
 The only differences between the I-DATA chunk in Figure 3 and the
 DATA chunk defined in [RFC4960] and [RFC7053] are the addition of the
 new Message Identifier (MID) and the new Fragment Sequence Number
 (FSN) and the removal of the Stream Sequence Number (SSN).  The
 Payload Protocol Identifier (PPID), which is already defined for DATA
 chunks in [RFC4960], and the new FSN are stored at the same location
 of the packet using the B bit to determine which value is stored at
 the location.  The length of the I-DATA chunk header is 20 bytes,
 which is 4 bytes more than the length of the DATA chunk header
 defined in [RFC4960] and [RFC7053].

Stewart, et al. Standards Track [Page 7] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 The old fields are:
 Res: 4 bits
    These bits are reserved.  They MUST be set to 0 by the sender and
    MUST be ignored by the receiver.
 I bit: 1 bit
    The (I)mmediate Bit, if set, indicates that the receiver SHOULD
    NOT delay the sending of the corresponding SACK chunk.  Same as
    the I bit for DATA chunks, as specified in [RFC7053].
 U bit: 1 bit
    The (U)nordered bit, if set, indicates the user message is
    unordered.  Same as the U bit for DATA chunks, as specified in
    [RFC4960].
 B bit: 1 bit
    The (B)eginning fragment bit, if set, indicates the first fragment
    of a user message.  Same as the B bit for DATA chunks, as
    specified in [RFC4960].
 E bit: 1 bit
    The (E)nding fragment bit, if set, indicates the last fragment of
    a user message.  Same as the E bit for DATA chunks, as specified
    in [RFC4960].
 Length: 16 bits (unsigned integer)
    This field indicates the length in bytes of the DATA chunk from
    the beginning of the Type field to the end of the User Data field,
    excluding any padding.  Similar to the Length for DATA chunks, as
    specified in [RFC4960].
 TSN: 32 bits (unsigned integer)
    This value represents the TSN for this I-DATA chunk.  Same as the
    TSN for DATA chunks, as specified in [RFC4960].
 Stream Identifier: 16 bits (unsigned integer)
    Identifies the stream to which the user data belongs.  Same as the
    Stream Identifier for DATA chunks, as specified in [RFC4960].
 The new fields are:
 Reserved: 16 bits (unsigned integer)
    This field is reserved.  It MUST be set to 0 by the sender and
    MUST be ignored by the receiver.

Stewart, et al. Standards Track [Page 8] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 Message Identifier (MID): 32 bits (unsigned integer)
    The MID is the same for all fragments of a user message; it is
    used to determine which fragments (enumerated by the FSN) belong
    to the same user message.  For ordered user messages, the MID is
    also used by the SCTP receiver to deliver the user messages in the
    correct order to the upper layer (similar to the SSN of the DATA
    chunk defined in [RFC4960]).  The sender uses two counters for
    each outgoing stream: one for ordered messages and one for
    unordered messages.  All of these counters are independent and
    initially 0.  They are incremented by 1 for each user message.
    Please note that the serial number arithmetic defined in [RFC1982]
    using SERIAL_BITS = 32 applies.  Therefore, the sender MUST NOT
    have more than 2**31 - 1 ordered messages for each outgoing stream
    in flight and MUST NOT have more than 2**31 - 1 unordered messages
    for each outgoing stream in flight.  A message is considered in
    flight if at least one of its I-DATA chunks is not acknowledged in
    a way that cannot be reneged (i.e., not acknowledged by the
    cumulative TSN Ack).  Please note that the MID is in "network byte
    order", a.k.a.  Big Endian.
 Payload Protocol Identifier (PPID) / Fragment Sequence Number (FSN):
    32 bits (unsigned integer)
    If the B bit is set, this field contains the PPID of the user
    message.  Note that in this case, this field is not touched by an
    SCTP implementation; therefore, its byte order is not necessarily
    in network byte order.  The upper layer is responsible for any
    byte order conversions to this field, similar to the PPID of DATA
    chunks.  In this case, the FSN is implicitly considered to be 0.
    If the B bit is not set, this field contains the FSN.  The FSN is
    used to enumerate all fragments of a single user message, starting
    from 0 and incremented by 1.  The last fragment of a message MUST
    have the E bit set.  Note that the FSN MAY wrap completely
    multiple times, thus allowing arbitrarily large user messages.
    For the FSN, the serial number arithmetic defined in [RFC1982]
    applies with SERIAL_BITS = 32.  Therefore, a sender MUST NOT have
    more than 2**31 - 1 fragments of a single user message in flight.
    A fragment is considered in flight if it is not acknowledged in a
    way that cannot be reneged.  Please note that the FSN is in
    "network byte order", a.k.a.  Big Endian.

2.2. Procedures

 This subsection describes how the support of the I-DATA chunk is
 negotiated and how the I-DATA chunk is used by the sender and
 receiver.
 The handling of the I bit for the I-DATA chunk corresponds to the
 handling of the I bit for the DATA chunk described in [RFC7053].

Stewart, et al. Standards Track [Page 9] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

2.2.1. Negotiation

 An SCTP endpoint indicates user message interleaving support by
 listing the I-DATA chunk within the Supported Extensions Parameter,
 as defined in [RFC5061].  User message interleaving has been
 negotiated for an association if both endpoints have indicated I-DATA
 support.
 If user message interleaving support has been negotiated for an
 association, I-DATA chunks MUST be used for all user messages and
 DATA chunks MUST NOT be used.  If user message interleaving support
 has not been negotiated for an association, DATA chunks MUST be used
 for all user messages and I-DATA chunks MUST NOT be used.
 An endpoint implementing the socket API specified in [RFC6458] MUST
 NOT indicate user message interleaving support unless the user has
 requested its use (e.g., via the socket API; see Section 4.3).  This
 constraint is made since the usage of this chunk requires that the
 application is capable of handling interleaved messages upon
 reception within an association.  This is not the default choice
 within the socket API (see the SCTP_FRAGMENT_INTERLEAVE socket option
 in Section 8.1.20 of [RFC6458]); thus, the user MUST indicate to the
 SCTP implementation its support for receiving completely interleaved
 messages.
 Note that stacks that do not implement [RFC6458] may use other
 methods to indicate interleaved message support and thus indicate the
 support of user message interleaving.  The crucial point is that the
 SCTP stack MUST know that the application can handle interleaved
 messages before indicating the I-DATA support.

2.2.2. Sender-Side Considerations

 The sender-side usage of the I-DATA chunk is quite simple.  Instead
 of using the TSN for fragmentation purposes, the sender uses the new
 FSN field to indicate which fragment number is being sent.  The first
 fragment MUST have the B bit set.  The last fragment MUST have the E
 bit set.  All other fragments MUST NOT have the B or E bit set.  All
 other properties of the existing SCTP DATA chunk also apply to the
 I-DATA chunk, i.e., congestion control as well as receiver window
 conditions MUST be observed, as defined in [RFC4960].
 Note that the usage of this chunk implies the late assignment of the
 actual TSN to any chunk being sent.  Each I-DATA chunk uses a single
 TSN.  This way messages from other streams may be interleaved with
 the fragmented message.  Please note that this is the only form of
 interleaving support.  For example, it is not possible to interleave
 multiple ordered or unordered user messages from the same stream.

Stewart, et al. Standards Track [Page 10] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 The sender MUST NOT process (move user data into I-DATA chunks and
 assign a TSN to it) more than one user message in any given stream at
 any time.  At any time, a sender MAY process multiple user messages,
 each of them on different streams.
 The sender MUST assign TSNs to I-DATA chunks in a way that the
 receiver can make progress.  One way to achieve this is to assign a
 higher TSN to the later fragments of a user message and send out the
 I-DATA chunks such that the TSNs are in sequence.

2.2.3. Receiver-Side Considerations

 Upon reception of an SCTP packet containing an I-DATA chunk whose
 user message needs to be reassembled, the receiver MUST first use the
 SID to identify the stream, consider the U bit to determine if it is
 part of an ordered or unordered message, find the user message
 identified by the MID, and use the FSN for reassembly of the message
 and not the TSN.  The receiver MUST NOT make any assumption about the
 TSN assignments of the sender.  Note that a non-fragmented message is
 indicated by the fact that both the E and B bits are set.  A message
 (either ordered or unordered) whose E and B bits are not both set may
 be identified as being fragmented.
 If I-DATA support has been negotiated for an association, the
 reception of a DATA chunk is a violation of the above rules and
 therefore the receiver of the DATA chunk MUST abort the association
 by sending an ABORT chunk.  The ABORT chunk MAY include the 'Protocol
 Violation' error cause.  The same applies if I-DATA support has not
 been negotiated for an association and an I-DATA chunk is received.

2.3. Interaction with Other SCTP Extensions

 The usage of the I-DATA chunk might interfere with other SCTP
 extensions.  Future SCTP extensions MUST describe if and how they
 interfere with the usage of I-DATA chunks.  For the SCTP extensions
 already defined when this document was published, the details are
 given in the following subsections.

2.3.1. SCTP Partial Reliability Extension

 When the SCTP extension defined in [RFC3758] is used in combination
 with the user message interleaving extension, the new I-FORWARD-TSN
 chunk MUST be used instead of the FORWARD-TSN chunk.  The difference
 between the FORWARD-TSN and the I-FORWARD-TSN chunk is that the
 16-bit Stream Sequence Number (SSN) has been replaced by the 32-bit
 Message Identifier (MID), and the largest skipped MID can also be

Stewart, et al. Standards Track [Page 11] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 provided for unordered messages.  Therefore, the principle applied to
 ordered messages when using FORWARD-TSN chunks is applied to ordered
 and unordered messages when using I-FORWARD-TSN chunks.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Type = 194  | Flags = 0x00  |      Length = Variable        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       New Cumulative TSN                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Stream Identifier       |          Reserved           |U|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Message Identifier                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Stream Identifier       |          Reserved           |U|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Message Identifier                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 4: I-FORWARD-TSN Chunk Format
 The old fields are:
 Flags: 8 bits (unsigned integer)
    These bits are reserved.  They MUST be set to 0 by the sender and
    MUST be ignored by the receiver.  Same as the Flags for FORWARD
    TSN chunks, as specified in [RFC3758].
 Length: 16 bits (unsigned integer)
    This field holds the length of the chunk.  Similar to the Length
    for FORWARD TSN chunks, as specified in [RFC3758].
 New Cumulative TSN: 32 bits (unsigned integer)
    This indicates the New Cumulative TSN to the data receiver.  Same
    as the New Cumulative TSN for FORWARD TSN chunks, as specified in
    [RFC3758].
 The new fields are:
 Stream Identifier (SID): 16 bits (unsigned integer)
    This field holds the stream number this entry refers to.

Stewart, et al. Standards Track [Page 12] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 Reserved: 15 bits
    This field is reserved.  It MUST be set to 0 by the sender and
    MUST be ignored by the receiver.
 U bit: 1 bit
    The U bit specifies if the Message Identifier of this entry refers
    to unordered messages (U bit is set) or ordered messages (U bit is
    not set).
 Message Identifier (MID): 32 bits (unsigned integer)
    This field holds the largest Message Identifier for ordered or
    unordered messages indicated by the U bit that was skipped for the
    stream specified by the Stream Identifier.  For ordered messages,
    this is similar to the FORWARD-TSN chunk, just replacing the
    16-bit SSN by the 32-bit MID.
 Support for the I-FORWARD-TSN chunk is negotiated during the SCTP
 association setup via the Supported Extensions Parameter, as defined
 in [RFC5061].  The partial reliability extension is negotiated and
 can be used in combination with user message interleaving only if
 both endpoints indicated their support of user message interleaving
 and the I-FORWARD-TSN chunk.
 The FORWARD-TSN chunk MUST be used in combination with the DATA chunk
 and MUST NOT be used in combination with the I-DATA chunk.  The
 I-FORWARD-TSN chunk MUST be used in combination with the I-DATA chunk
 and MUST NOT be used in combination with the DATA chunk.
 If I-FORWARD-TSN support has been negotiated for an association, the
 reception of a FORWARD-TSN chunk is a violation of the above rules
 and therefore the receiver of the FORWARD-TSN chunk MUST abort the
 association by sending an ABORT chunk.  The ABORT chunk MAY include
 the 'Protocol Violation' error cause.  The same applies if
 I-FORWARD-TSN support has not been negotiated for an association and
 a FORWARD-TSN chunk is received.

2.3.2. SCTP Stream Reconfiguration Extension

 When an association resets the SSN using the SCTP extension defined
 in [RFC6525], the two counters (one for the ordered messages, one for
 the unordered messages) used for the MIDs MUST be reset to 0.
 Since most schedulers, especially all schedulers supporting user
 message interleaving, require late TSN assignment, it should be noted
 that the implementation of [RFC6525] needs to handle this.

Stewart, et al. Standards Track [Page 13] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

3. Stream Schedulers

 This section defines several stream schedulers.  The stream
 schedulers may behave differently depending on whether or not user
 message interleaving has been negotiated for the association.  An
 implementation MAY implement any subset of them.  If the
 implementation is used for WebRTC data channels, as specified in
 [DATA-CHAN], it MUST implement the Weighted Fair Queueing Scheduler
 defined in Section 3.6.
 The selection of the stream scheduler is done at the sender side.
 There is no mechanism provided for signaling the stream scheduler
 being used to the receiver side or even for letting the receiver side
 influence the selection of the stream scheduler used at the sender
 side.

3.1. First-Come, First-Served Scheduler (SCTP_SS_FCFS)

 The simple first-come, first-served scheduler of user messages is
 used.  It just passes through the messages in the order in which they
 have been delivered by the application.  No modification of the order
 is done at all.  The usage of user message interleaving does not
 affect the sending of the chunks, except that I-DATA chunks are used
 instead of DATA chunks.

3.2. Round-Robin Scheduler (SCTP_SS_RR)

 When not interleaving user messages, this scheduler provides a fair
 scheduling based on the number of user messages by cycling around
 non-empty stream queues.  When interleaving user messages, this
 scheduler provides a fair scheduling based on the number of I-DATA
 chunks by cycling around non-empty stream queues.

3.3. Round-Robin Scheduler per Packet (SCTP_SS_RR_PKT)

 This is a round-robin scheduler, which only switches streams when
 starting to fill a new packet.  It bundles only DATA or I-DATA chunks
 referring to the same stream in a packet.  This scheduler minimizes
 head-of-line blocking when a packet is lost because only a single
 stream is affected.

3.4. Priority-Based Scheduler (SCTP_SS_PRIO)

 Scheduling of user messages with strict priorities is used.  The
 priority is configurable per outgoing SCTP stream.  Streams having a
 higher priority will be scheduled first and when multiple streams
 have the same priority, the scheduling between them is implementation

Stewart, et al. Standards Track [Page 14] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 dependent.  When the scheduler interleaves user messages, the sending
 of large, lower-priority user messages will not delay the sending of
 higher-priority user messages.

3.5. Fair Capacity Scheduler (SCTP_SS_FC)

 A fair capacity distribution between the streams is used.  This
 scheduler considers the lengths of the messages of each stream and
 schedules them in a specific way to maintain an equal capacity for
 all streams.  The details are implementation dependent.  interleaving
 user messages allows for a better realization of the fair capacity
 usage.

3.6. Weighted Fair Queueing Scheduler (SCTP_SS_WFQ)

 A Weighted Fair Queueing scheduler between the streams is used.  The
 weight is configurable per outgoing SCTP stream.  This scheduler
 considers the lengths of the messages of each stream and schedules
 them in a specific way to use the capacity according to the given
 weights.  If the weight of stream S1 is n times the weight of stream
 S2, the scheduler should assign to stream S1 n times the capacity it
 assigns to stream S2.  The details are implementation dependent.
 Interleaving user messages allows for a better realization of the
 capacity usage according to the given weights.
 This scheduler, in combination with user message interleaving, is
 used for WebRTC data channels, as specified in [DATA-CHAN].

4. Socket API Considerations

 This section describes how the socket API defined in [RFC6458] is
 extended to allow applications to use the extension described in this
 document.
 Please note that this section is informational only.

4.1. Exposure of the Stream Sequence Number (SSN)

 The socket API defined in [RFC6458] defines several structures in
 which the SSN of a received user message is exposed to the
 application.  The list of these structures includes:
 struct sctp_sndrcvinfo
    Specified in Section 5.3.2 of [RFC6458] and marked as deprecated.
 struct sctp_extrcvinfo
    Specified in Section 5.3.3 of [RFC6458] and marked as deprecated.

Stewart, et al. Standards Track [Page 15] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 struct sctp_rcvinfo
    Specified in Section 5.3.5 of [RFC6458].
 If user message interleaving is used, the lower-order 16 bits of the
 MID are used as the SSN when filling out these structures.

4.2. SCTP_ASSOC_CHANGE Notification

 When an SCTP_ASSOC_CHANGE notification (specified in Section 6.1.1 of
 [RFC6458]) is delivered indicating a sac_state of SCTP_COMM_UP or
 SCTP_RESTART for an SCTP association where both peers support the
 I-DATA chunk, SCTP_ASSOC_SUPPORTS_INTERLEAVING should be listed in
 the sac_info field.

4.3. Socket Options

 +-----------------------------+-------------------------+-----+-----+
 | Option Name                 | Data Type               | Get | Set |
 +-----------------------------+-------------------------+-----+-----+
 | SCTP_INTERLEAVING_SUPPORTED | struct sctp_assoc_value |  X  |  X  |
 | SCTP_STREAM_SCHEDULER       | struct sctp_assoc_value |  X  |  X  |
 | SCTP_STREAM_SCHEDULER_VALUE | struct                  |  X  |  X  |
 |                             | sctp_stream_value       |     |     |
 +-----------------------------+-------------------------+-----+-----+

4.3.1. Enable or Disable the Support of User Message Interleaving

      (SCTP_INTERLEAVING_SUPPORTED)
 This socket option allows the enabling or disabling of the
 negotiation of user message interleaving support for future
 associations.  For existing associations, it allows for querying
 whether or not user message interleaving support was negotiated on a
 particular association.
 This socket option uses IPPROTO_SCTP as its level and
 SCTP_INTERLEAVING_SUPPORTED as its name.  It can be used with
 getsockopt() and setsockopt().  The socket option value uses the
 following structure defined in [RFC6458]:
 struct sctp_assoc_value {
   sctp_assoc_t assoc_id;
   uint32_t assoc_value;
 };

Stewart, et al. Standards Track [Page 16] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 assoc_id:  This parameter is ignored for one-to-one style sockets.
    For one-to-many style sockets, this parameter indicates upon which
    association the user is performing an action.  The special
    sctp_assoc_t SCTP_FUTURE_ASSOC can also be used; it is an error to
    use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
 assoc_value:  A non-zero value encodes the enabling of user message
    interleaving, whereas a value of zero encodes the disabling of
    user message interleaving.
 sctp_opt_info() needs to be extended to support
 SCTP_INTERLEAVING_SUPPORTED.
 An application using user message interleaving should also set the
 fragment interleave level to 2 by using the SCTP_FRAGMENT_INTERLEAVE
 socket option specified in Section 8.1.20 of [RFC6458].  This allows
 the interleaving of user messages from different streams.  Please
 note that it does not allow the interleaving of user messages
 (ordered or unordered) on the same stream.  Failure to set this
 option can possibly lead to application deadlock.  Some
 implementations might therefore put some restrictions on setting
 combinations of these values.  Setting the interleaving level to at
 least 2 before enabling the negotiation of user message interleaving
 should work on all platforms.  Since the default fragment interleave
 level is not 2, user message interleaving is disabled per default.

4.3.2. Get or Set the Stream Scheduler (SCTP_STREAM_SCHEDULER)

 A stream scheduler can be selected with the SCTP_STREAM_SCHEDULER
 option for setsockopt().  The struct sctp_assoc_value is used to
 specify the association for which the scheduler should be changed and
 the value of the desired algorithm.
 The definition of struct sctp_assoc_value is the same as in
 [RFC6458]:
 struct sctp_assoc_value {
   sctp_assoc_t assoc_id;
   uint32_t assoc_value;
 };
 assoc_id:  Holds the identifier of the association for which the
    scheduler should be changed.  The special
    SCTP_{FUTURE|CURRENT|ALL}_ASSOC can also be used.  This parameter
    is ignored for one-to-one style sockets.

Stewart, et al. Standards Track [Page 17] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 assoc_value:  This specifies which scheduler is used.  The following
    constants can be used:
    SCTP_SS_DEFAULT:  The default scheduler used by the SCTP
       implementation.  Typical values are SCTP_SS_FCFS or SCTP_SS_RR.
    SCTP_SS_FCFS:  Use the scheduler specified in Section 3.1.
    SCTP_SS_RR:  Use the scheduler specified in Section 3.2.
    SCTP_SS_RR_PKT:  Use the scheduler specified in Section 3.3.
    SCTP_SS_PRIO:  Use the scheduler specified in Section 3.4.  The
       priority can be assigned with the sctp_stream_value struct.
       The higher the assigned value, the lower the priority.  That
       is, the default value 0 is the highest priority, and therefore
       the default scheduling will be used if no priorities have been
       assigned.
    SCTP_SS_FB:  Use the scheduler specified in Section 3.5.
    SCTP_SS_WFQ:  Use the scheduler specified in Section 3.6.  The
       weight can be assigned with the sctp_stream_value struct.
 sctp_opt_info() needs to be extended to support
 SCTP_STREAM_SCHEDULER.

4.3.3. Get or Set the Stream Scheduler Parameter

      (SCTP_STREAM_SCHEDULER_VALUE)
 Some schedulers require additional information to be set for
 individual streams as shown in the following table:
                 +-----------------+-----------------+
                 | Name            | Per-Stream Info |
                 +-----------------+-----------------+
                 | SCTP_SS_DEFAULT |       n/a       |
                 | SCTP_SS_FCFS    |        no       |
                 | SCTP_SS_RR      |        no       |
                 | SCTP_SS_RR_PKT  |        no       |
                 | SCTP_SS_PRIO    |       yes       |
                 | SCTP_SS_FB      |        no       |
                 | SCTP_SS_WFQ     |       yes       |
                 +-----------------+-----------------+

Stewart, et al. Standards Track [Page 18] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 This is achieved with the SCTP_STREAM_SCHEDULER_VALUE option and the
 corresponding struct sctp_stream_value.  The definition of struct
 sctp_stream_value is as follows:
 struct sctp_stream_value {
   sctp_assoc_t assoc_id;
   uint16_t stream_id;
   uint16_t stream_value;
 };
 assoc_id:  Holds the identifier of the association for which the
    scheduler should be changed.  The special
    SCTP_{FUTURE|CURRENT|ALL}_ASSOC can also be used.  This parameter
    is ignored for one-to-one style sockets.
 stream_id:  Holds the identifier of the stream for which additional
    information has to be provided.
 stream_value:  The meaning of this field depends on the scheduler
    specified.  It is ignored when the scheduler does not need
    additional information.
 sctp_opt_info() needs to be extended to support
 SCTP_STREAM_SCHEDULER_VALUE.

4.4. Explicit EOR Marking

 Using explicit End of Record (EOR) marking for an SCTP association
 supporting user message interleaving allows the user to interleave
 the sending of user messages on different streams.

5. IANA Considerations

 Two new chunk types have been assigned by IANA.

5.1. I-DATA Chunk

 IANA has assigned the chunk type for this chunk from the pool of
 chunks with the upper two bits set to '01'.  This appears in the
 "Chunk Types" registry for SCTP as follows:
 +----------+--------------------------------------------+-----------+
 | ID Value | Chunk Type                                 | Reference |
 +----------+--------------------------------------------+-----------+
 | 64       | Payload Data supporting Interleaving       | RFC 8260  |
 |          | (I-DATA)                                   |           |
 +----------+--------------------------------------------+-----------+

Stewart, et al. Standards Track [Page 19] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

 The registration table (as defined in [RFC6096]) for the chunk flags
 of this chunk type is initially as follows:
          +------------------+-----------------+-----------+
          | Chunk Flag Value | Chunk Flag Name | Reference |
          +------------------+-----------------+-----------+
          | 0x01             | E bit           | RFC 8260  |
          | 0x02             | B bit           | RFC 8260  |
          | 0x04             | U bit           | RFC 8260  |
          | 0x08             | I bit           | RFC 8260  |
          | 0x10             | Unassigned      |           |
          | 0x20             | Unassigned      |           |
          | 0x40             | Unassigned      |           |
          | 0x80             | Unassigned      |           |
          +------------------+-----------------+-----------+

5.2. I-FORWARD-TSN Chunk

 IANA has assigned the chunk type for this chunk from the pool of
 chunks with the upper two bits set to '11'.  This appears in the
 "Chunk Types" registry for SCTP as follows:
               +----------+---------------+-----------+
               | ID Value | Chunk Type    | Reference |
               +----------+---------------+-----------+
               | 194      | I-FORWARD-TSN | RFC 8260  |
               +----------+---------------+-----------+
 The registration table (as defined in [RFC6096]) for the chunk flags
 of this chunk type is initially empty.

6. Security Considerations

 This document does not add any additional security considerations in
 addition to the ones given in [RFC4960] and [RFC6458].
 It should be noted that the application has to consent that it is
 willing to do the more complex reassembly support required for user
 message interleaving.  When doing so, an application has to provide a
 reassembly buffer for each incoming stream.  It has to protect itself
 against these buffers taking too many resources.  If user message
 interleaving is not used, only a single reassembly buffer needs to be
 provided for each association.  But the application has to protect
 itself for excessive resource usages there too.

Stewart, et al. Standards Track [Page 20] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

7. References

7.1. Normative References

 [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
            DOI 10.17487/RFC1982, August 1996,
            <https://www.rfc-editor.org/info/rfc1982>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
            Conrad, "Stream Control Transmission Protocol (SCTP)
            Partial Reliability Extension", RFC 3758,
            DOI 10.17487/RFC3758, May 2004,
            <https://www.rfc-editor.org/info/rfc3758>.
 [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
            RFC 4960, DOI 10.17487/RFC4960, September 2007,
            <https://www.rfc-editor.org/info/rfc4960>.
 [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
            Kozuka, "Stream Control Transmission Protocol (SCTP)
            Dynamic Address Reconfiguration", RFC 5061,
            DOI 10.17487/RFC5061, September 2007,
            <https://www.rfc-editor.org/info/rfc5061>.
 [RFC6096]  Tuexen, M. and R. Stewart, "Stream Control Transmission
            Protocol (SCTP) Chunk Flags Registration", RFC 6096,
            DOI 10.17487/RFC6096, January 2011,
            <https://www.rfc-editor.org/info/rfc6096>.
 [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
            Transmission Protocol (SCTP) Stream Reconfiguration",
            RFC 6525, DOI 10.17487/RFC6525, February 2012,
            <https://www.rfc-editor.org/info/rfc6525>.
 [RFC7053]  Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
            IMMEDIATELY Extension for the Stream Control Transmission
            Protocol", RFC 7053, DOI 10.17487/RFC7053, November 2013,
            <https://www.rfc-editor.org/info/rfc7053>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Stewart, et al. Standards Track [Page 21] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

7.2. Informative References

 [DATA-CHAN]
            Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
            Channels", Work in Progress,
            draft-ietf-rtcweb-data-channel-13, January 2015.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <https://www.rfc-editor.org/info/rfc3261>.
 [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
            Yasevich, "Sockets API Extensions for the Stream Control
            Transmission Protocol (SCTP)", RFC 6458,
            DOI 10.17487/RFC6458, December 2011,
            <https://www.rfc-editor.org/info/rfc6458>.

Acknowledgments

 The authors wish to thank Benoit Claise, Julian Cordes, Spencer
 Dawkins, Gorry Fairhurst, Lennart Grahl, Christer Holmberg, Mirja
 Kuehlewind, Marcelo Ricardo Leitner, Karen E. Egede Nielsen, Maksim
 Proshin, Eric Rescorla, Irene Ruengeler, Felix Weinrank, Michael
 Welzl, Magnus Westerlund, and Lixia Zhang for their invaluable
 comments.
 This work has received funding from the European Union's Horizon 2020
 research and innovation program under grant agreement No. 644334
 (NEAT).  The views expressed are solely those of the authors.

Stewart, et al. Standards Track [Page 22] RFC 8260 Stream Schedulers and the I-DATA Chunk November 2017

Authors' Addresses

 Randall R. Stewart
 Netflix, Inc.
 Chapin, SC  29036
 United States of America
 Email: randall@lakerest.net
 Michael Tuexen
 Muenster University of Applied Sciences
 Stegerwaldstrasse 39
 48565 Steinfurt
 Germany
 Email: tuexen@fh-muenster.de
 Salvatore Loreto
 Ericsson
 Torshamnsgatan 21
 164 80 Stockholm
 Sweden
 Email: Salvatore.Loreto@ericsson.com
 Robin Seggelmann
 Metafinanz Informationssysteme GmbH
 Leopoldstrasse 146
 80804 Muenchen
 Germany
 Email: rfc@robin-seggelmann.com

Stewart, et al. Standards Track [Page 23]

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