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

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Abstract

This document defines a method for a sender of a DATA chunk to indicate that the corresponding SACK chunk should be sent back immediately.

Table of Contents

1.  Introduction
2.  Conventions
3.  The I-bit in the DATA Chunk Header
4.  Procedures
    4.1.  Sender Side Considerations
    4.2.  Receiver Side Considerations
5.  Interoperability Considerations
6.  IANA Considerations
7.  Security Considerations
8.  Normative References
§  Authors' Addresses

1.  Introduction

[RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) states that an SCTP implementation should use delayed SACKs. In combination with the Nagle algorithm, reduced congestion windows after timeouts, the handling of the SHUTDOWN-SENDING state, or other situations this might result in reduced performance of the protocol.

This document describes a simple extension of the SCTP DATA chunk by defining a new flag, the I-bit. The sender indicates by setting this bit that the corresponding SACK chunk should be sent back without delaying it.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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                           /
\                                                               \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.  Procedures

4.1.  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.

Reasons for setting the I-bit include

• The sender has not enough queued user data to send the remaining DATA chunks due to the Nagle algorithm.
• The sending of a DATA chunk fills the congestion or receiver window.
• The sender is in the SHUTDOWN-PENDING state.
• The sender has reduced its RTO.Min such that a retransmission timeout will occur if the receiver would delay its SACK.

4.2.  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 SHOULD send it back immediately.

5.  Interoperability Considerations

According to [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) a 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 feature described in this document.

6.  IANA Considerations

There are no actions required from IANA.

7.  Security Considerations

This document does not add any additional security considerations in addition to the ones given in [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.).

8. Normative References

Authors' Addresses