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Versions: (draft-tuexen-tsvwg-sctp-sack-immediately) 03 04 RFC 7053

Network Working Group                                          M. Tuexen
Internet-Draft                                              I. Ruengeler
Updates: 4960 (if approved)             Muenster Univ. of Appl. Sciences
Intended status: Standards Track                           R. R. Stewart
Expires: October 10, 2013                                 Adara Networks
                                                          April 08, 2013


SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
             draft-ietf-tsvwg-sctp-sack-immediately-03.txt

Abstract

   This document updates RFC 4960 by defining 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.  Since unknown flags in chunk headers are ignored by SCTP
   implementations, this extension does not introduce any
   interoperability problems.

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 October 10, 2013.

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
   (http://trustee.ietf.org/license-info) 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.  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  . . . . . . . . . . . . . .   4
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Sender Side Considerations  . . . . . . . . . . . . . . .   4
     5.2.  Receiver Side Considerations  . . . . . . . . . . . . . .   5
   6.  Interoperability Considerations . . . . . . . . . . . . . . .   5
   7.  Socket API Considerations . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     11.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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 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 a delay in sending the SACK.

   This document 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.

   This bit was Reserved in [RFC4960].  [RFC4960] specified that this
   bit should be set to 0 by the sender and ignored by the receiver.

4.  Use Cases

   The setting of the I-bit can either be triggered by the application
   using SCTP or by the SCTP stack itself.  The following two
   subsections provide a non-exhaustive list of examples.

4.1.  Triggering at the Application Level

   One example of a situation in which it may be desirable for an
   application to trigger setting of the I-bit involves the
   SCTP_SENDER_DRY_EVENT in the SCTP socket API [RFC6458].  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 can request the



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   setting of the I-Bit when sending the last user message before
   waiting for the event.  This results in setting the I-bit of the last
   DATA chunk corresponding to the user message and is possible using
   the extension of the socket API described in Section 7.

4.2.  Triggering at the SCTP Level

   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, setting the
   I-bit reduces the number of simultaneous associations for 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
   that require that there be 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].



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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, i.e., the receiver SHOULD immediately respond with the
   corresponding SACK chunk.

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] needs to be extended
   to allow the application to set the I-bit of the last DATA chunk when
   sending each 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 flag 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 and the reference should be RFCXXXX.

   This requires an update of the "DATA Chunk Flags" registry for SCTP:



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                             DATA Chunk Flags

            +------------------+-----------------+-----------+
            | Chunk Flag Value | Chunk Flag Name | Reference |
            +------------------+-----------------+-----------+
            | 0x01             | E bit           | [RFC4960] |
            | 0x02             | B bit           | [RFC4960] |
            | 0x04             | U bit           | [RFC4960] |
            | 0x08             | I Bit           | [RFCXXXX] |
            | 0x10             | Unassigned      |           |
            | 0x20             | Unassigned      |           |
            | 0x40             | Unassigned      |           |
            | 0x80             | Unassigned      |           |
            +------------------+-----------------+-----------+


9.  Security Considerations

   See [RFC4960] for general security considerations for SCTP.  In
   addition, a malicious sender can force its peer to send packets
   containing a SACK chunk for each received packet containing DATA
   chunks instead of every other.  This could impact the network,
   resulting in more packets sent on the network, or the peer because
   the generating and sending of the packets has some processing cost.
   However, the additional packets can only contain the most simplest
   SACK chunk (no gap reports, no duplicate TSNs), since in case of
   packet drop or reordering in the network a SACK chunk would be sent
   immediately anyway.  Therefore this does neither introduce a
   significant additional processing cost on the receiver side nor does
   it cause congestion on the network.

10.  Acknowledgments

   The authors wish to thank Mark Allmann, Brian Bidulock, David Black,
   Anna Brunstrom, Gorry Fairhurst, Janardhan Iyengar, Kacheong Poon,
   and Michael Welzl 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.





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   [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


   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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