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rfc:rfc7053

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.

Tuexen, et al. Standards Track [Page 1] RFC 7053 SACK-IMMEDIATELY November 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
 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 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.

Tuexen, et al. Standards Track [Page 2] RFC 7053 SACK-IMMEDIATELY November 2013

 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.

Tuexen, et al. Standards Track [Page 3] RFC 7053 SACK-IMMEDIATELY November 2013

 [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.

Tuexen, et al. Standards Track [Page 4] RFC 7053 SACK-IMMEDIATELY November 2013

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.

Tuexen, et al. Standards Track [Page 5] RFC 7053 SACK-IMMEDIATELY November 2013

 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      |           |
          +------------------+-----------------+-----------+

Tuexen, et al. Standards Track [Page 6] RFC 7053 SACK-IMMEDIATELY November 2013

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.

Tuexen, et al. Standards Track [Page 7] RFC 7053 SACK-IMMEDIATELY November 2013

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

Tuexen, et al. Standards Track [Page 8]

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