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Network Working Group                                      R. R. Stewart
Internet-Draft                                            Adara Networks
Intended status: Standards Track                               M. Tuexen
Expires: October 19, 2013               Muenster Univ. of Appl. Sciences
                                                                 X. Dong
                                                          April 17, 2013

          ECN for Stream Control Transmission Protocol (SCTP)


   This document describes the addition of the ECN to the Stream Control
   Transmission Protocol (SCTP).

Status of This Memo

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

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   This Internet-Draft will expire on October 19, 2013.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Chunk and Parameter Formats . . . . . . . . . . . . . . . . .   3
     4.1.  ECN Support Parameter (32768) . . . . . . . . . . . . . .   3
     4.2.  ECN Echo (12) . . . . . . . . . . . . . . . . . . . . . .   3
     4.3.  CWR Chunk(13) . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  SCTP Initialization . . . . . . . . . . . . . . . . . . .   5
     5.2.  The SCTP Sender . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  The SCTP Receiver . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Congestion on the SACK path . . . . . . . . . . . . . . .   9
     5.5.  Retransmitted SCTP Packets  . . . . . . . . . . . . . . .   9
     5.6.  SCTP Window Probes  . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative references  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informational References  . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   At the time SCTP was initially defined in [RFC2960] ECN - [RFC2481]
   was still an experimental document.  This left the authors of SCTP in
   a position where they could not directly refer to ECN without
   creating a normative reference in a standards track document to an
   experimental RFC.  To work around this problem the authors of SCTP
   decided to add two reserved chunk types for ECN (CWR and ECNE) but
   did not fully specify how they were to be used except in a vague way
   within an appendix of the document.  This worked around the document
   reference problem, but left ECN and its implementation for SCTP
   unspecified.  This document is intended to fill in the details of ECN
   processing in SCTP in a standards track document.

   This document assumes that the reader is familiar with ECN [RFC3168].
   Readers unfamiliar with ECN are strongly encouraged to first read
   [RFC3168] since this document will not repeat any of the details on
   how the various IP level bits are set.  This document will use the
   same terminology has [RFC3168].  For example the term ECT is used to
   indicate that the IP level packet is marked indicating the transport
   (SCTP) supports ECN.

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2.  Conventions

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

3.  Terminology

   All integer fields defined in this document included in an SCTP
   packet MUST be transmitted in network byte order, unless otherwise

   ECT     -  The term used to indicate that the IP level packet is
      marked indicating the transport is willing to support ECN for this

   not-ECT -  The term used to indicate that the IP level packet is
      marked indicating the transport is NOT willing to support ECN for
      this packet.

   CE      -   The term used to indicate that the IP level packet is
      marked indicating that a router in the network has marked the
      packet as having experienced congestion

4.  Chunk and Parameter Formats

4.1.  ECN Support Parameter (32768)

       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
      |   Parameter Type = 32768      |     Parameter Length = 4      |

   This parameter is used to indicate the support for ECN.  If this
   parameter is present, the sender of the chunk is indicating that it
   supports ECN and wishes to use ECN for the newly forming association.

   Valid Chunk Appearance

   The ECN Supported Parameter may appear in the INIT, or the INIT-ACK
   chunk type.

4.2.  ECN Echo (12)

          0                   1                   2                   3

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          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
         | Chunk Type=12 | Flags=00000000|    Chunk Length = 12          |
         |                      Lowest TSN Number                        |
         |            Number CE Marked Packets Seen since CWR            |

   Chunk Flags: 8 bits

      Set to all zeros on transmit and ignored on receipt.

   Lowest TSN Number: 32 bits (unsigned integer)

      This parameter contains the lowest TSN number contained in the
      last packet received that was marked by the network with a CE

   Number CE Marked Packets: 32 bits (unsigned integer)

      This parameter contains the total number of CE marked packets that
      has been seen since the first CE mark received while waiting for a
      CWR chunk.  Note that the CE counter will overflow from 0xffffffff
      to 0 if a CWR chunk is not recieved.

   Note that the appendix of [RFC4960] did not have the field Number CE
   Marked Packets.  Implementations SHOULD accept an 8 byte form of this
   chunk that does not include this field.  In such a case the
   implementation SHOULD treat the missing field as indicating one CE
   marked packet for any purpose for which the implementation is using
   this field.

4.3.  CWR Chunk(13)

          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
         | Chunk Type=13 | Flags=0000000R|    Chunk Length = 8           |
         |                      TSN Number                               |

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   Chunk Flags: 8 bits

   The R Bit indicates if the CWR is a retransmission of an earlier CWR
   that may have been lost.  If this bit is set, then the TSN number
   included is the latest TSN that a CWR has been responded to.  If the
   o bit is clear, than the TSN indicated is the latest TSN for that

      Set to all zeros on transmit and ignored on receipt.

   TSN Number: 32 bits (unsigned integer)

      This parameter contains the TSN number to which the sender has
      reduced his congestion window to.

5.  Procedures

5.1.  SCTP Initialization

   In the SCTP association setup phase, the source and destination SCTP
   endpoints exchange information about their willingness to use ECN.
   After the completion of this negotiation, an SCTP sender sets an ECT
   codepoint in the IP header of data packets to indicate to the network
   that the transport is capable and willing to participate in ECN for
   this packet.  This indicates to the routers that they may mark this
   packet with the CE codepoint.

   If the SCTP association does not wish to use ECN notification for a
   particular packet, the sending SCTP sets the ECN codepoint to not-
   ECT, and the SCTP receiver ignores the CE codepoint in the received

   For this discussion we will call the endpoint initiating the SCTP
   association as EP-A and the listening SCTP endpoint as EP-Z.

   Before an SCTP association can use ECN, EP-A sends an INIT chunk
   which includes the ECN Support parameter.  By including the ECN
   Support parameter the sending endpoint (EP-A) will participate in ECN
   as both a sender and a receiver.  Specifically, as a receiver, it
   will respond to incoming data packets that have the CE codepoint set
   in the IP header by sending an ECN Echo chunk bundled with the next
   outgoing SACK Chunk.  As a sender, it will respond to incoming
   packets that include an ECN Echo chunk by reducing the congestion
   window and sending a CWR chunk when appropriate.

   Including an ECN Support parameter in an INIT or INIT-ACK does not
   commit the SCTP sender to setting the ECT codepoint in any or all of

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   the packets it may transmit.  However, the commitment to respond
   appropriately to incoming packets with the CE codepoint set remains.

   When EP-Z sends INIT-ACK chunk, it also includes an ECN Support
   parameter.  Including the ECN Support parameter indicates that the
   SCTP transmitting the INIT-ACK chunk is ECN-Capable.

   The following rules apply to the use of ECN for an SCTP association.

   *  If the SCTP Endpoint supports ECN a sender of either an INIT or
      INIT-ACK chunk MUST ALWAYS include the ECN Supported Parameter.

   *  After the exchange of the INIT and INIT-ACK if both endpoints have
      NOT indicated support of ECN by including an ECN Supported
      Parameter, then ECT MUST NOT be set on any IP packets sent by any
      endpoint which is ECN capable.  Furthermore upon receiving IP
      packets with a CE codepoint set, the ECN capable endpoint SHOULD
      ignore the CE codepoint.

   *  If both endpoints have included an ECN Supported Parameter in the
      INIT and INIT-ACK exchange, then both endpoints MUST follow the
      ECN procedures defined in the rest of this document.

   *  A sending endpoint SHOULD set the ECT code points on IP packets
      that carry Data chunk.  This includes IP packets that have other
      control chunks bundled with the Data.

5.2.  The SCTP Sender

   For an SCTP association using ECN, new data packets are transmitted
   with an ECT codepoint set in the IP header.  When only one ECT
   codepoint is needed by a sender for all packets sent on an SCTP
   association ECT(0) SHOULD be used.  If the sender receives an ECN-
   Echo chunk packet, then the sender knows that congestion was
   encountered in the network on the path from the sender to the
   receiver.  The indication of congestion should be treated just as a
   congestion loss in non-ECN-Capable SCTP.  That is, the SCTP source
   halves the congestion window "cwnd" for the destination address that
   the sender transmitted the data to and reduces the slow start
   threshold "ssthresh".  A packet containing an ECN-Echo chunk
   shouldn't trigger new data to be sent.  SCTP follows the normal
   procedures for increasing the congestion window when it receives a
   packet with a SACK chunk without the ECN Echo chunk.

   SCTP should not react to congestion indications more than once every
   round-trip time.  That is, the SCTP sender's congestion window should
   be reduced only once in response to a series of dropped and/or CE
   packets from a single window of data.  In addition, the SCTP source

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   should not decrease the slow-start threshold, ssthresh, if it has
   been decreased within the last round trip time.

   One method to accomplish this is as following:

   1) During association setup, create a new state variable ECN_ECHO_TSN
      and ECN_ECHO_LAST for each destination.  The initial value of
      these variables are set to the initial TSN that will be assigned
      minus 1.

   2) When an ECN Echo chunk arrives, use the TSN in the ECN Echo to
      establish which destination the packet was sent to.  We will call
      this destination the selected destination.If the chunk cannot be
      found note that an override is occuring From the selected
      destination (if found) select its ECN Echo TSN.

   3) Compare the ECN Echo TSN with the ECN_ECHO_TSN for the selected
      destination.  If an override is not noted and the value of the
      ECN_ECHO_TSN is greater than the ECN Echo TSN proceed to step 4;
      else proceed to step 6b.

   4) Reduce the cwnd and ssthresh for the selected destination the same
      as if a loss was detected during a fast retransmit.  For details,
      see [RFC4960] Section 7.2.3 and Section 7.2.4.

   5) Record in the ECN_ECHO_TSN value, the last TSN that was sent and
      recorded in ECN_ECHO_LAST the TSN number from the ECN Echo Chunk.

   6a)  If the implementation is tracking the number of marked packets,
      record the value found in the 'Number CE Marked Packets Seen since
      CWR' field and also add this number to the running loss count.  If
      such a count is not being maintained, then proceed to step 7.

   6b)  If the implementation is tracking the number of marked packets,
      compare the number in the ECN Echo Chunk TSN to the ECN_ECHO_LAST.
      If it is greater than ECN_ECHO_LAST, update ECN_ECHO_LAST with
      this value.  Take the difference between the stored 'Number CE
      Marked Packets' field and the value from the newly arriving
      'Number CE Marked Packets' and add this difference to the total
      loss count.  Then update the stored 'Number CE Marked Packets'
      with the ECN Echo Chunk TSN.

   7) Create a CWR chunk with the value found in the ECN_ECHO_LAST for
      the selected destination.If an override was noted, set the 'O' bit
      within the CWR flags.  Queue this chunk for transmission to the
      peer destination.  Note if there is already such a chunk in queue
      to be sent, remove that chunk and replace it with the new chunk.

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   After the sending SCTP reduces its congestion window in response to a
   ECN Echo, incoming SACKs that continue to arrive can "clock out"
   outgoing packets as allowed by the reduced congestion window.  Note
   that continued arrival of ECN Echo chunks should still be processed
   as described above, possibly reducing the cwnd, but always sending a
   CWR to the receiving SCTP.  This assures that the ECN Echo and CWR
   are robust with regard to loss in either direction and that the
   implementation, if it desires, can maintain an accurate loss count
   per destination.

   Note, originally in the appendix of [RFC4960] a definition was
   supplied for the ECN Echo chunk.  This definition did NOT include the
   'Number CE Marked Packets' field.  An implementation SHOULD accept
   such a chunk, delineating it from the standards track version by the
   fact that the length field will be 8 bytes instead of 12.  When
   processing this older style chunk, the 'Number CE Marked Packets'
   should be treated as if it contains the number 1.  This may cause
   incorrect loss counts but will NOT cause any issues with SCTP's ECN

5.3.  The SCTP Receiver

   When an SCTP endpoint first receives a CE data packet at the
   destination end-system, the SCTP data receiver creates an ECN Echo
   chunk and records the lowest TSN number found in the data packet.  It
   also sets the 'Number CE Marked Packets' to 1 and queues this chunk
   for transmission at the next opportunity.  If there is any ACK
   withholding implemented, as in current "delayed-SACK" SCTP
   implementations where the SCTP receiver can send an SACK for two
   arriving data packets, then the ECN Echo chunk will not be sent until
   the SACK is sent.  If the next arriving data packet also has the CE
   codepoint set, then the receiver updates the queued ECN Echo chunk to
   have a higher TSN value (the lowest one in the newly arriving data
   packet) and increments the 'Number CE Marked Packets' field in the
   queued chunk.

   Multi-homing requires one added restriction upon the ECN Echo chunk,
   such a chunk MUST be bundled with a SACK, and the SACK MUST follow
   the ECN Echo Chunk.  This ordering is necessary so that the receiver
   of the ECN Echo chunk will at least one time find the proper
   destination to which the chunk was originally sent.  Without this
   restriction it is possible a SACK could arrive ahead of the ECN Echo
   Chunk, no matter what the sending order, causing the sender to free
   the DATA chunk and thus loose the association with what destination
   it was sent to.  For the same reason we also require the ECN Echo
   Chunk be earlier in the packet ahead of the SACK so that the SACK is
   not processed before the ECN Echo Chunk.

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   After transmission of the ECN Echo chunk, usually bundled with the
   SACK, the receiver does NOT discard the ECN Echo chunk.  Instead it
   keeps the chunk in its queue and continues to send this chunk bundled
   with at least a SACK chunk on each outgoing packet, updating it as
   described above if other CE codepoint data packets arrive.  The ECN
   Echo chunk should only be discarded when a CWR Chunk arrives holding
   a TSN value that is greater than or equal to the value inside the ECN
   Echo Chunk.

   This provides robustness against the possibility of a dropped SACK
   packet carrying an ECN Echo chunk.  The SCTP receiver continues to
   transmit the ECN Echo chunk in subsequent SACK packets until the
   correct CWR is received.

   After the receipt of the CWR chunk, acknowledgments for subsequent
   non-CE data packets will not have an ECN Echo chunk bundled with
   them.  If another CE packet is received by the data receiver, the
   receiver would once again send SACK packets bundled with a newly
   created ECN Echo chunk.  The receipt of a CWR packet guarantees that
   the data sender has received the ECN Echo chunk for the TSN
   specified, and reduced its congestion window at some point *after* it
   sent the data packet for which the CE codepoint was set.

   When processing a CWR, it is important that the receiver of the CWR
   validate the source address from which the CWR came from.  It SHOULD
   match the destination the ECN Echo was sent to unless the override
   bit is set in the CWR Chunk.

5.4.  Congestion on the SACK path

   For the current generation of SCTP congestion control algorithms,
   pure acknowledgement packets (e.g., packets that do not contain any
   accompanying data) MUST be sent with the not-ECT codepoint.  Current
   SCTP receivers have no mechanisms for reducing traffic on the SACK-
   path in response to congestion notification.  Mechanisms for
   responding to congestion on the SACK-path are areas for current and
   future research.  For current SCTP implementations, a single dropped
   SACK generally has only a very small effect on SCTP's sending rate.

5.5.  Retransmitted SCTP Packets

   This document specifies ECN-capable SCTP implementations MUST NOT set
   either ECT codepoint (ECT(0) or ECT(1)) in the IP header for
   retransmitted data packets, and that the SCTP data receiver SHOULD
   ignore the ECN field on arriving data packets that are outside of the
   receiver's current window.  The reasons for this can be found in
   [RFC3168] Section 6.1.5.

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5.6.  SCTP Window Probes

   When the SCTP data receiver advertises a zero window, the SCTP data
   sender sends window probes to determine if the receiver's window has
   increased.  Window probe packets for SCTP do contain user data (one
   chunk).  If a window probe packet is dropped in the network, this
   loss can be detected by the receiver.  Therefore, the SCTP data
   sender MAY set an ECT codepoint on the initial send of the window
   probe, but the SCTP sender MUST NOT set the ECT codepoint on
   retransmissions of that TSN.

6.  Security Considerations

   [RFC3168] defines the security considerations for ECN.  These same
   consideration that are described for TCP are applicable to SCTP.

7.  IANA Considerations


8.  Acknowledgements

   Thanks to Richard Scheffenegger for his helpful comments and review.

9.  References

9.1.  Normative references

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP", RFC
              3168, September 2001.

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

9.2.  Informational References

   [RFC2481]  Ramakrishnan, K.K. and S. Floyd, "A Proposal to add
              Explicit Congestion Notification (ECN) to IP", RFC 2481,
              January 1999.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

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Authors' Addresses

   Randall R. Stewart
   Adara Networks
   Chapin, SC  29036

   Email: randall@lakerest.net

   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt

   Email: tuexen@fh-muenster.de

   Xuesong Dong
   Pleasanton, CA  94566

   Email: stevedong@huawei.com

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