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: September 17, 2013 Adara Networks
March 16, 2013
SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
draft-ietf-tsvwg-sctp-sack-immediately-02.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.
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This Internet-Draft will expire on September 17, 2013.
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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 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.
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] 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 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]. It should be noted that 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
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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, 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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