SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
RFC 7053

Internet Engineering Task Force (IETF)                         M. Tuexen
Request for Comments: 7053                                  I. Ruengeler
Updates: 4960                           Muenster Univ. of Appl. Sciences
Category: Standards Track                                     R. Stewart
ISSN: 2070-1721                                           Adara Networks
                                                           November 2013

                     SACK-IMMEDIATELY Extension for
                the Stream Control Transmission Protocol

Abstract

   This document updates RFC 4960 by defining a method for the sender of
   a DATA chunk to indicate that the corresponding Selective
   Acknowledgment (SACK) chunk should be sent back immediately and
   should not be delayed.  It is done by specifying a bit in the DATA
   chunk header, called the (I)mmediate bit, which can get set by either
   the Stream Control Transmission Protocol (SCTP) implementation or 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7053.

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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
   carefully, as they describe your rights and restrictions with respect
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  The (I)mmediate Bit in the DATA Chunk Header  . . . . . . . . . 3
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Triggering at the Application Level . . . . . . . . . . . . 4
     4.2.  Triggering at the SCTP Level  . . . . . . . . . . . . . . . 4
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.1.  Sender-Side Considerations  . . . . . . . . . . . . . . . . 5
     5.2.  Receiver Side Considerations  . . . . . . . . . . . . . . . 5
   6.  Interoperability Considerations . . . . . . . . . . . . . . . . 5
   7.  Socket API Considerations . . . . . . . . . . . . . . . . . . . 5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     11.1. Normative References  . . . . . . . . . . . . . . . . . . . 7
     11.2. Informative References  . . . . . . . . . . . . . . . . . . 7

1.  Introduction

   According to [RFC4960], the receiver of a DATA chunk should use
   delayed SACKs.  This delay 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:

   1.  If such a situation can be detected by the receiver, the
       corresponding SACK can be sent immediately.  For example,
       [RFC4960] recommends immediately sending the SACK if the receiver
       has detected message loss or message duplication.

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   2.  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.  Examples of these situations include ones
       that require interaction with the application (e.g., applications
       using the SCTP_SENDER_DRY_EVENT, see Section 4.1) and ones that
       can be detected by the SCTP stack itself (e.g., closing the
       association, hitting window limits, or resetting streams, see
       Section 4.2).

   To overcome the limitation described in the second case, this
   document describes a simple extension of the SCTP DATA chunk by
   defining a new flag, the "I bit".  By setting this bit, the sender of
   a DATA chunk indicates that the corresponding SACK chunk should not
   be delayed.

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].

3.  The (I)mmediate Bit in the DATA Chunk Header

   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.

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   [RFC4960] defines the Reserved field and specifies that these bits
   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 of how the I
   bit may be set.

4.1.  Triggering at the Application Level

   One example of a situation in which it may be desirable for an
   application to trigger the setting of the I bit involves the
   SCTP_SENDER_DRY_EVENT in the SCTP socket API [RFC6458].  Upper layers
   of SCTP that use the socket API as defined in [RFC6458] may subscribe
   to the SCTP_SENDER_DRY_EVENT to be notified as soon as no user data
   is outstanding.  To avoid an unnecessary delay, the application can
   request that the I bit be set 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; this 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-lived 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.

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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 why the
   sender has set the I bit is irrelevant to the receiver.

   Reasons for setting the I bit include, but are not limited to (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

   Upon receipt 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.

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   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

   Following the chunk flag registration procedure defined in [RFC6096],
   IANA has registered a new bit, the I bit, for the DATA chunk.

   The "Chunk Flags" registry for SCTP has been updated as described in
   the following table.

                             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           | [RFC7053] |
            | 0x10             | Unassigned      |           |
            | 0x20             | Unassigned      |           |
            | 0x40             | Unassigned      |           |
            | 0x80             | Unassigned      |           |
            +------------------+-----------------+-----------+

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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 received packet containing DATA chunks.
   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 simplest SACK chunk (no gap reports, no
   duplicate TSNs), since in cases of packet drops or reordering in the
   network a SACK chunk would be sent immediately anyway.  Therefore,
   this does not introduce a significant additional processing cost on
   the receiver side.  This also does not result in more traffic in the
   network, because a receiver sending a SACK for every packet is
   already permitted.

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.

   [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.

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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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