Network Working Group                                         R. Stewart
Internet-Draft                                            Adara Networks
Intended status: Standards Track                               M. Tuexen
Expires: April 23, 2014                 Muenster Univ. of Appl. Sciences
                                                               S. Loreto
                                                           R. Seggelmann
                                            T-Systems International GmbH
                                                        October 20, 2013

       A New Data Chunk for Stream Control Transmission Protocol


   The Stream Control Transmission Protocol (SCTP) is a message oriented
   transport protocol supporting arbitrary large user messages.
   However, the sender can not interleave different user messages which
   which causes head of line blocking at the sender side.  To overcome
   this limitation, this document adds a new data chunk to SCTP.

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

   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 April 23, 2014.

Copyright Notice

   Copyright (c) 2013 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
   ( in effect on the date of
   publication of this document.  Please review these documents

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  N-DATA Chunk  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Socket API Considerations . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

1.1.  Overview

   When SCTP [RFC4960] was initially designed it was mainly envisioned
   for 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 MTU sized message would be too small.
   Unfortunately this design decision, though valid at the time, did not
   account for other applications which might send very large messages
   over SCTP.  When such large messages are now sent over SCTP a form of
   sender side head of line blocking becomes created within the
   protocol.  This head of line blocking is caused by the use of the
   Transmission Sequence Number (TSN) for two 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

   The protocol requires all fragments of a user message to have
   consecutive TSNs.  Therefore the sender can not interleave different

   This document describes a new Data chunk called N-DATA.  This chunk
   incorporates all the flags and properties of the current SCTP Data
   chunk but also adds a new field in its chunk header, the Fragment
   Sequence Number (FSN).  Then the FSN is only used for reassembly and

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   the TSN only for the reliability.  Therefore, the head of line
   blocking caused by the original design is avoided.

1.2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  N-DATA Chunk

   The following Figure 1 shows the new data chunk N-DATA.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |   Type = 17   |  Res  |I|U|B|E|           Length              |
   |                              TSN                              |
   |        Stream Identifier      |     Stream Sequence Number    |
   |                  Payload Protocol Identifier                  |
   |                      Message Identifier                       |
   |                  Fragment Sequence Number                     |
   \                                                               \
   /                           User Data                           /
   \                                                               \

                       Figure 1: N-DATA chunk format

   The only differences between the N-DATA chunk in Figure 1 and the
   DATA chunk defined in [RFC4960] and
   [I-D.ietf-tsvwg-sctp-sack-immediately] is the addition of the new
   Message Identifier (MID) and Fragment Sequence Number (FSN).

   Message Identifier (MID): 32 bits (unsigned integer)
      The Message Identifier .  Please note that the MID is in "network
      byte order", a.k.a. Big Endian.

   Fragment Sequence Number (FSN): 32 bits (unsigned integer)
      Identifies the fragment number of this piece of a message.  FSN's
      are unsigned number, the first fragment MUST start at 0 and MUST
      have the 'B' bit set.  The last fragment of a message MUST have

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      the 'E' bit set.  Note that the FSN may wrap completely multiple
      times allowing arbitrary large messages.  Please note that the FSN
      is in "network byte order", a.k.a. Big Endian.

3.  Procedures

3.1.  Sender Side Considerations

   A sender MUST NOT send a N-DATA chunk unless the peer has indicated
   its support of the N-DATA chunk type within the Supported Extensions
   Parameter as defined in [RFC5061].

   A sender MUST NOT use the N-DATA chunk unless the user has requested
   that use via the socket API (see Section 4).  This constraint is made
   since usage of this chunk requires that the application be willing to
   interleave messages upon reception within an association.  This is
   not the default choice within the socket API (see [RFC6458]) thus the
   user MUST indicate support to the protocol of the reception of
   completely interleaved messages.  Note that for stacks that do not
   implement [RFC6458] they may use other methods to indicate
   interleaved message support and thus enable the usage of the N-DATA
   chunk, the key is that the the stack MUST know the application has
   indicated its choice in wanting to use the extension.

   Sender side usage of the N-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.  If the 'I' bit is set the 'E' bit MUST also be set, i.e. the
   'I' bit may only be set on the last fragment of a message.  All other
   properties of the existing SCTP DATA chunk also apply to the N-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 should also imply late binding of
   the actual TSN to any chunk being sent.  This way other messages from
   other streams may be interleaved with the fragmented message.

   The sender MUST NOT have more than one ordered fragmented message
   being produced in any one stream.  The sender MUST NOT have more than
   one un-ordered fragmented message being produced in any one stream.
   The sender MAY have one ordered and one unordered fragmented message
   being produced within a single stream.  At any time multiple streams
   MAY be producing an ordered or unordered fragmented message.

3.2.  Receiver Side Considerations

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   Upon reception of an SCTP packet containing a N-DATA chunk if the
   message needs to be reassembled, then the receiver MUST use the FSN
   for reassembly of the message and not the TSN.  Note that a non-
   fragmented messages is indicated by the fact that both the 'E' and
   'B' bits are set.  An ordered or unordered fragmented message is thus
   identified with any message not having both bits set.

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

   Please note that this section is informational only.

4.1.  Socket Options

       | option name       | data type                | get | set |
       | SCTP_NDATA_ENABLE | int                      |  X  |  X  |
       | SCTP_PLUGGABLE_SS | struct sctp_assoc_value  |  X  |  X  |
       | SCTP_SS_VALUE     | struct sctp_stream_value |  X  |  X  |

4.1.1.  Enable or Disable the Interleaving Capability

   A new socket option to turn on/off the usage of the N-DATA chunk.
   Turning this option on only effect future associations, and MUST be
   turned on for the protocol stack to indicate support of the N-DATA
   chunk to the peer during association setup.  Turning this option off,
   will prevent the N-DATA chunk from being indicated supported in
   future associations, and will also prevent current associations from
   producing N-DATA chunks for future large fragmented messages.  Note
   that this does not stop the peer from sending N-DATA chunks.

   An N-DATA chunk aware application should also set the fragment
   interleave level to 2.  This allows the reception from multiple
   streams simultaneously.  Failure to set this option can possibly lead
   to application deadlock.

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4.1.2.  Get or Set the Stream Scheduler (SCTP_PLUGGABLE_SS)

   A stream scheduler can be selected with the SCTP_PLUGGABLE_SS 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

   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_ROUND_ROBIN or

      SCTP_SS_ROUND_ROBIN:  This scheduler provides a fair scheduling
            based on the number of user messages by cycling around non-
            empty stream queues.

      SCTP_SS_ROUND_ROBIN_PACKET:  This is a round-robin scheduler but
            only bundles user messages of the same stream in one packet.
            This minimizes head-of-line blocking when a packet is lost
            because only a single stream is affected.

      SCTP_SS_PRIORITY:  Scheduling with different priorities is used.
            Streams having a higher priority will be scheduled first and
            when multiple streams have the same priority, the default
            scheduling should be used for them.  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_FAIR_BANDWITH:  A fair bandwidth distribution between the
            streams can be activated using this value.  This scheduler

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            considers the lengths of the messages of each stream and
            schedules them in a certain way to maintain an equal
            bandwidth for all streams.

      SCTP_SS_FIRST_COME:  The simple first-come, first-serve algorithm
            is selected by using this value.  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

4.1.3.  Get or Set the Stream Scheduler Parameter (SCTP_SS_VALUE)

   Some schedulers require additional information to be set for single
   streams as shown in the following table:

                | name                 | per stream info |
                | SCTP_SS_DEFAULT      |        no       |
                | SCTP_SS_RR           |        no       |
                | SCTP_SS_RR_INTER     |        no       |
                | SCTP_SS_RR_PKT       |        no       |
                | SCTP_SS_RR_PKT_INTER |        no       |
                | SCTP_SS_PRIO         |       yes       |
                | SCTP_SS_PRIO_INTER   |       yes       |
                | SCTP_SS_FB           |        no       |
                | SCTP_SS_FB_INTER     |        no       |
                | SCTP_SS_FCFS         |        no       |

   This is achieved with the SCTP_SS_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.

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   stream_id:  Holds the stream id for 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.

5.  IANA Considerations

   [NOTE to RFC-Editor:

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


   [NOTE to RFC-Editor:

      The suggested values for the chunk type 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.

   A new chunk type has to be assigned by IANA.  IANA should assign this
   value from the pool of chunks with the upper two bits set to '00'.
   This requires an additional line in the "Chunk Types" registry for

            | ID Value | Chunk Type              | Reference |
            | 17       | New DATA chunk (N-DATA) | [RFCXXXX] |

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

            | Chunk Flag Value | Chunk Flag Name | Reference |
            | 0x01             | E bit           | [RFCXXXX] |
            | 0x02             | B bit           | [RFCXXXX] |
            | 0x04             | U bit           | [RFCXXXX] |
            | 0x08             | I bit           | [RFCXXXX] |

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            | 0x10             | Unassigned      |           |
            | 0x20             | Unassigned      |           |
            | 0x40             | Unassigned      |           |
            | 0x80             | Unassigned      |           |

6.  Security Considerations

   This document does not add any additional security considerations in
   addition to the ones given in [RFC4960] and [RFC6458].

7.  Acknowledgments

   The authors wish to thank Lixia Zhang for her invaluable comments.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061, September

   [RFC6096]  Tuexen, M. and R. Stewart, "Stream Control Transmission
              Protocol (SCTP) Chunk Flags Registration", RFC 6096,
              January 2011.

              Tuexen, M., Ruengeler, I., and R. Stewart, "SACK-
              IMMEDIATELY Extension for the Stream Control Transmission
              Protocol", draft-ietf-tsvwg-sctp-sack-immediately-04 (work
              in progress), August 2013.

8.2.  Informative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

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   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458, December 2011.

Authors' Addresses

   Randall R. Stewart
   Adara Networks
   Chapin, SC  29036


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt


   Salvatore Loreto
   Hirsalantie 11
   Jorvas  02420


   Robin Seggelmann
   T-Systems International GmbH
   Fasanenweg 5
   70771 Leinfelden-Echterdingen


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