SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
draft-ietf-tsvwg-sctp-sack-immediately-01

Network Working Group                                          M. Tuexen
Internet-Draft                                              I. Ruengeler
Intended status: Standards Track        Muenster Univ. of Appl. Sciences
Expires: September 15, 2013                                R. R. Stewart
                                                          Adara Networks
                                                          March 14, 2013

SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
             draft-ietf-tsvwg-sctp-sack-immediately-01.txt

Abstract

   This document defines a method for the sender of a DATA chunk to
   indicate that the corresponding SACK chunk should be sent back
   immediately and not be delayed.  It is done by specifying a bit in
   the DATA chunk header, called the I-bit, which can get set either by
   the SCTP implementation or by the application using an SCTP stack.

Status of This Memo

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   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  The I-bit in the DATA Chunk Header  . . . . . . . . . . . . .   3
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Triggering at the Application Level . . . . . . . . . . .   3
     4.2.  Triggering at the SCTP Level  . . . . . . . . . . . . . .   3
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Sender Side Considerations  . . . . . . . . . . . . . . .   4
     5.2.  Receiver Side Considerations  . . . . . . . . . . . . . .   4
   6.  Interoperability Considerations . . . . . . . . . . . . . . .   5
   7.  Socket API Considerations . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     11.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   According to [RFC4960] the receiver of a DATA chunk should use
   delayed SACKs.  This delaying is completely controlled by the
   receiver of the DATA chunk and remains the to be the default
   behavior.

   In specific situations the delaying of SACKs results in reduced
   performance of the protocol.  If such a situation can be detected by
   the receiver, the corresponding SACK can be sent immediately.  For
   example, [RFC4960] recommends the immediate sending if the receiver
   has detected message loss or message duplication.  However, if the
   situation can only be detected by the sender of the DATA chunk,
   [RFC4960] provides no method of avoiding the delaying of the SACK.
   Thus the protocol performance might be reduced.

   This document overcomes this limitation and describes a simple
   extension of the SCTP DATA chunk by defining a new flag, the I-bit.
   The sender of a DATA chunk indicates by setting this bit that the
   corresponding SACK chunk should not be delayed.  Use-cases are
   described in Section 4.

2.  Conventions

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   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].

3.  The I-bit in the DATA Chunk Header

   The following Figure 1 shows the extended DATA chunk.

    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 = 0    |  Res  |I|U|B|E|           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              TSN                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Stream Identifier      |     Stream Sequence Number    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Payload Protocol Identifier                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                           User Data                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: Extended DATA chunk format

   The only difference between the DATA chunk in Figure 1 and the DATA
   chunk defined in [RFC4960] is the addition of the I-bit in the flags
   field of the DATA chunk header.

4.  Use Cases

   The setting of the I-bit can either be triggered by the application
   using SCTP or by the SCTP stack itself.

4.1.  Triggering at the Application Level

   Upper layers of SCTP using the socket API as defined in [RFC6458] may
   subscribe to the SCTP_SENDER_DRY_EVENT for getting a notification as
   soon as no user data is outstanding anymore.  To avoid an unnecessary
   delay while waiting for such an event, the application might set the
   I-Bit on the last DATA chunk sent before waiting for the event.  This
   enabling is possible using the extension of the socket API described
   in Section 7.

4.2.  Triggering at the SCTP Level

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   There are also situations in which the SCTP implementation can set
   the I-bit without interacting with the upper layer.

   If the association is in the SHUTDOWN-PENDING state, the I-bit should
   be set.  This reduces the number of simultaneous associations in case
   of a busy server handling short living associations.

   Another case is where the sending of a DATA chunk fills the
   congestion or receiver window.  Setting the I-bit in these cases
   improves the throughput of the transfer.

   If an SCTP association supports the SCTP Stream Reconfiguration
   extension defined in [RFC6525], the performance can be improved by
   setting the I-bit when there are pending reconfiguration requests
   requiring no outstanding DATA chunks.

5.  Procedures

5.1.  Sender Side Considerations

   Whenever the sender of a DATA chunk can benefit from the
   corresponding SACK chunk being sent back without delay, the sender
   MAY set the I-bit in the DATA chunk header.  Please note that it is
   irrelevant to the receiver why the sender has set the I-bit.

   Reasons for setting the I-bit include, but are not limited to, the
   following (see Section 4 for the benefits):

   o  The application requests to set the I-bit of the last DATA chunk
      of a user message when providing the user message to the SCTP
      implementation (see Section 7).

   o  The sender is in the SHUTDOWN-PENDING state.

   o  The sending of a DATA chunk fills the congestion or receiver
      window.

   o  The sending of an Outgoing SSN Reset Request Parameter or an SSN/
      TSN Reset Request Parameter is pending, if the association
      supports the Stream Reconfiguration extension defined in
      [RFC6525].

5.2.  Receiver Side Considerations

   On reception of an SCTP packet containing a DATA chunk with the I-bit
   set, the receiver SHOULD NOT delay the sending of the corresponding
   SACK chunk and send it back immediately.

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6.  Interoperability Considerations

   According to [RFC4960] the receiver of a DATA chunk with the I-bit
   set should ignore this bit when it does not support the extension
   described in this document.  Since the sender of the DATA chunk is
   able to handle this case, there is no requirement for negotiating the
   support of the feature described in this document.

7.  Socket API Considerations

   This section describes how the socket API defined in [RFC6458] is
   extended to provide a way for the application to set the I-bit.

   Please note that this section is informational only.

   A socket API implementation based on [RFC6458] is extended to allow
   the application to set the I-bit of the last DATA chunk for each
   provided user message.

   This can be done by setting a flag called SCTP_SACK_IMMEDIATELY in
   the snd_flags field of the struct sctp_sndinfo structure when using
   sctp_sendv() or sendmsg().  If the deprecated struct sctp_sndrcvinfo
   structure is used instead when calling sctp_send(), sctp_sendx(), or
   sendmsg(), the SCTP_SACK_IMMEDIATELY flags can be set in the
   sinfo_flags field.  When using the deprecated function sctp_sendmsg()
   the SCTP_SACK_IMMEDIATELY flag can be in the flags parameter.

8.  IANA Considerations

   [NOTE to RFC-Editor:

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

   ]

   Following the chunk flag registration procedure defined in [RFC6096]
   IANA should register a new bit, the I-bit, for the DATA chunk.  The
   suggested value is 0x08.  The reference for the new chunk flag in the
   chunk flags table for the DATA chunk should be RFCXXXX.

9.  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 an malicious sender can force its peer to send packets
   containing SACK chunks for each received packet containing DATA
   chunks instead of every other.  However, every receiver has to be

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   able to do this anyway.  It might be configured to do so or has to do
   this because of packet loss or reordering in the network.

10.  Acknowledgments

   The authors wish to thank Mark Allmann, Brian Bidulock, Gorry
   Fairhurst, Janardhan Iyengar, and Kacheong Poon for their invaluable
   comments.

11.  References

11.1.  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.

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

11.2.  Informative References

   [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.

   [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration", RFC
              6525, February 2012.

Authors' Addresses

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

   Email: tuexen@fh-muenster.de

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   Irene Ruengeler
   Muenster University of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt
   DE

   Email: i.ruengeler@fh-muenster.de

   Randall R. Stewart
   Adara Networks
   Chapin, SC  29036
   US

   Email: randall@lakerest.net

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