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Versions: (draft-stewart-tsvwg-sctp-ndata) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 8260

Network Working Group                                         R. Stewart
Internet-Draft                                             Netflix, Inc.
Intended status: Standards Track                               M. Tuexen
Expires: December 23, 2017              Muenster Univ. of Appl. Sciences
                                                               S. Loreto
                                                                Ericsson
                                                           R. Seggelmann
                                     Metafinanz Informationssysteme GmbH
                                                           June 21, 2017


 Stream Schedulers and User Message Interleaving for the Stream Control
                         Transmission Protocol
                   draft-ietf-tsvwg-sctp-ndata-11.txt

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.

   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.  A stream
   scheduler can choose to either implement, or not implement, user
   message interleaving.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 23, 2017.






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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
   (http://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  User Message Interleaving . . . . . . . . . . . . . . . . . .   5
     2.1.  The I-DATA Chunk Supporting User Message Interleaving . .   6
     2.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.  Negotiation . . . . . . . . . . . . . . . . . . . . .   8
       2.2.2.  Sender Side Considerations  . . . . . . . . . . . . .   8
       2.2.3.  Receiver Side Considerations  . . . . . . . . . . . .   9
     2.3.  Interaction with other SCTP Extensions  . . . . . . . . .   9
       2.3.1.  SCTP Partial Reliability Extension  . . . . . . . . .   9
       2.3.2.  SCTP Stream Reconfiguration Extension . . . . . . . .  11
   3.  Stream Schedulers . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  First Come First Served Scheduler (SCTP_SS_FCFS)  . . . .  11
     3.2.  Round Robin Scheduler (SCTP_SS_RR)  . . . . . . . . . . .  12
     3.3.  Round Robin Scheduler per Packet (SCTP_SS_RR_PKT) . . . .  12
     3.4.  Priority Based Scheduler (SCTP_SS_PRIO) . . . . . . . . .  12
     3.5.  Fair Capacity Scheduler (SCTP_SS_FC)  . . . . . . . . . .  12
     3.6.  Weighted Fair Queueing Scheduler (SCTP_SS_WFQ)  . . . . .  12
   4.  Socket API Considerations . . . . . . . . . . . . . . . . . .  13
     4.1.  Exposure of the Stream Sequence Number (SSN)  . . . . . .  13
     4.2.  SCTP_ASSOC_CHANGE Notification  . . . . . . . . . . . . .  13
     4.3.  Socket Options  . . . . . . . . . . . . . . . . . . . . .  13
       4.3.1.  Enable or Disable the Support of User Message
               Interleaving (SCTP_INTERLEAVING_SUPPORTED)  . . . . .  14
       4.3.2.  Get or Set the Stream Scheduler
               (SCTP_STREAM_SCHEDULER) . . . . . . . . . . . . . . .  15
       4.3.3.  Get or Set the Stream Scheduler Parameter
               (SCTP_STREAM_SCHEDULER_VALUE) . . . . . . . . . . . .  16
     4.4.  Explicit EOR Marking  . . . . . . . . . . . . . . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17



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     5.1.  I-DATA Chunk  . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.  I-FORWARD-TSN Chunk . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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 Session Initiation
   Protocol (SIP) [RFC3261] style signaling messages may also need to
   use SCTP and a single Maximum Transmission Unit (MTU) sized message
   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 an urgent
   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.  They can be used with and without user message
   interleaving being negotiated and possibly behave differently.

   Figure 1 illustrates the behaviour 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



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   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 chunk using TSN 0
   to TSN 8.  Please note that the use of such a scheduler implies late
   TSN assignment but it can be used with an [RFC4960] compliant
   implementation that does not support user message interleaving.

   +---+---+---+
   |    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), but 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 having the same MID
   and 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 a
   SSN.).

   Figure 2 illustrates the behaviour of an interleaving round robin
   stream scheduler using I-DATA chunks.










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+---+---+---+
|    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|
|1/2|1/1|1/0|--->|-----|-----|-----|-----|-----|-----|-----|-----|-----|
+---+---+---+ +->|  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.  Multiple stream schedulers
   are defined in Section 3 followed in Section 4 by describing an
   extension to the socket API for using what is specified in this
   document.

1.2.  Conventions

   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 [RFC2119].

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.




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   The interleaving of user messages is required for WebRTC Datachannels
   as specified in [I-D.ietf-rtcweb-data-channel].

   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 extension described in
   [RFC3758] or [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              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              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) and the 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].

   The new fields are:

   Reserved: 16 bits (unsigned integer)



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      This field is reserved.  It MUST be set to 0 by the sender and
      MUST be ignored by the receiver.

   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, one for unordered
      messages.  All 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
      on of its I-DATA chunks is not acknowledged in a non-renegable
      way.  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 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
      non-renegable way.  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.




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2.2.1.  Negotiation

   An SCTP end point 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 end points 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 end point 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.




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   The sender MUST NOT be fragmenting more than one user message in any
   given stream at any time.  At any time, a sender MAY fragment
   multiple user messages, each of them on different streams.

   The sender MUST assign TSNs 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 TSNs 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 finally 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) may be identified as
   being fragmented whose 'E' and 'B' bits are not set both.

   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
   be 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
   provided for unordered messages.  Therefore, the principle applied to
   ordered message when using FORWARD-TSN chunks is applied to ordered
   and unordered messages when using I-FORWARD-TSN chunks.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 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 relevant new fields are:

   Stream Identifier (SID): 16-bits (unsigned integer)
      This field holds the stream number this entry refers to.

   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].  Only if both end points indicated their support of
   user message interleaving and the I-FORWARD-TSN chunk, the partial
   reliability extension is negotiated and can be used in combination
   with user message interleaving.



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   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 be 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.

3.  Stream Schedulers

   This section defines several stream schedulers.  The stream
   schedulers may behave differently depending on whether user message
   interleaving has been negotiated for the association or not.  An
   implementation MAY implement any subset of them.

   The selection of the stream scheduler is done at the sender side.
   There is no mechanism provided for signalling the stream scheduler
   being used to the receiver side or even let 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.







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3.2.  Round Robin Scheduler (SCTP_SS_RR)

   When not using user message interleaving, this scheduler provides a
   fair scheduling based on the number of user messages by cycling
   around non-empty stream queues.  When using user message
   interleaving, 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
   dependent.  When using user message interleaving, the sending of
   lower priority user messages will not block 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.  Using user
   message interleaving 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.
   Using user message interleaving allows for a better realization of
   the capacity usage according to the given weights.





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   This scheduler in combination with user message interleaving is used
   for WebRTC Datachannels as specified in
   [I-D.ietf-rtcweb-data-channel].

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 SCTP Header Information Structure
      (SCTP_SNDRCV) of [RFC6458] and marked as deprecated.

   struct sctp_extrcvinfo
      Specified in Section 5.3.3 Extended SCTP Header Information
      Structure (SCTP_EXTRCV)of [RFC6458] and marked as deprecated.

   struct sctp_rcvinfo
      Specified in Section 5.3.5 SCTP Receive Information Structure
      (SCTP_RCVINFO) 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









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   +-----------------------------+-------------------------+-----+-----+
   | 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 to query whether
   user message interleaving support was negotiated or not 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;
   };

   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 0 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



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   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 for the association of 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.

   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.





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

   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 for the association of 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 stream id 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.



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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

   [NOTE to RFC-Editor:

      "RFCXXXX" is to be replaced by the RFC number you assign this
      document.

   ]

   [NOTE to RFC-Editor:

      The suggested values for the chunk types and the chunk flags are
      tentative and to be confirmed by IANA.

   ]

   This document (RFCXXXX) is the reference for all registrations
   described in this section.

   Two new chunk types have to be assigned by IANA.

5.1.  I-DATA Chunk

   IANA should assign the chunk type for this chunk from the pool of
   chunks with the upper two bits set to '01'.  This requires an
   additional line in the "Chunk Types" registry for SCTP:

   +----------+--------------------------------------------+-----------+
   | ID Value | Chunk Type                                 | Reference |
   +----------+--------------------------------------------+-----------+
   | 64       | Payload Data supporting Interleaving       | [RFCXXXX] |
   |          | (I-DATA)                                   |           |
   +----------+--------------------------------------------+-----------+

   The registration table as defined in [RFC6096] for the chunk flags of
   this chunk type is initially given by the following table:









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            +------------------+-----------------+-----------+
            | Chunk Flag Value | Chunk Flag Name | Reference |
            +------------------+-----------------+-----------+
            | 0x01             | E bit           | [RFCXXXX] |
            | 0x02             | B bit           | [RFCXXXX] |
            | 0x04             | U bit           | [RFCXXXX] |
            | 0x08             | I bit           | [RFCXXXX] |
            | 0x10             | Unassigned      |           |
            | 0x20             | Unassigned      |           |
            | 0x40             | Unassigned      |           |
            | 0x80             | Unassigned      |           |
            +------------------+-----------------+-----------+

5.2.  I-FORWARD-TSN Chunk

   IANA should assign the chunk type for this chunk from the pool of
   chunks with the upper two bits set to '11'.  This requires an
   additional line in the "Chunk Types" registry for SCTP:

                 +----------+---------------+-----------+
                 | ID Value | Chunk Type    | Reference |
                 +----------+---------------+-----------+
                 | 194      | I-FORWARD-TSN | [RFCXXXX] |
                 +----------+---------------+-----------+

   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
   up to two reassembly buffers (one for ordered messages, one for
   unordered messages) 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.

7.  Acknowledgments

   The authors wish to thank Julian Cordes, Gorry Fairhurst, Lennart
   Grahl, Christer Holmberg, Marcelo Ricardo Leitner, Karen E.  Egede
   Nielsen, Maksim Proshin, Irene Ruengeler, Felix Weinrank, Michael



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   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 programme under grant agreement No. 644334
   (NEAT).  The views expressed are solely those of the author(s).

8.  References

8.1.  Normative References

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3758>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6525>.






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   [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,
              <http://www.rfc-editor.org/info/rfc7053>.

8.2.  Informative References

   [I-D.ietf-rtcweb-data-channel]
              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
              Channels", draft-ietf-rtcweb-data-channel-13 (work in
              progress), 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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6458>.

Authors' Addresses

   Randall R. Stewart
   Netflix, Inc.
   Chapin, SC  29036
   United States

   Email: randall@lakerest.net


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   Germany

   Email: tuexen@fh-muenster.de










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   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



































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