Network Working Group Ch. Schmidt
Internet-Draft M. Tuexen
Expires: February 18, 2003 Siemens AG
August 20, 2002
Requirements for RoHC IP/SCTP Robust Header Compression
draft-ietf-rohc-sctp-requirements-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document contains requirements for the IP/SCTP header
compression scheme (profile) to be developed by the ROHC WG. The
structure of this document is inherited from the document defining
IP/TCP requirements for ROHC.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Header compression requirements . . . . . . . . . . . . . . . 4
2.1 Impact on Internet infrastructure . . . . . . . . . . . . . . 4
2.2 Supported headers . . . . . . . . . . . . . . . . . . . . . . 4
2.3 SCTP specific requirements . . . . . . . . . . . . . . . . . . 5
2.4 Performance issues . . . . . . . . . . . . . . . . . . . . . . 6
2.5 Capability requirements related to link layer
characteristics . . . . . . . . . . . . . . . . . . . . . . . 7
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The goal of the ROHC WG is to develop header compression schemes that
perform well over links with high error rates and long link round
trip times. The schemes must perform well for cellular links, using
technologies such as WCDMA, EDGE, and CDMA-2000. However, the
schemes should also be applicable to other future link technologies
with high loss and long round trip times.
The main objective for ROHC has been robust compression of IP/UDP/
RTP. Next step was IP/TCP compression.
SCTP is the new reliable transport protocol from the IETF. It offers
a number of features not available in other reliable transport
protocols such as TCP, including multi-streaming, multi-homing and
resistance to flooding and masquerade attacks. SCTP is designed to
transport PSTN signaling messages over IP networks but its rich
feature set makes it capable of many broader applications. Main
known application today is the transport of SIP signaling messages.
One of the most important innovations of SCTP is the multi-streaming
function. This feature allows data to be partitioned into multiple
streams where each stream is delivered independently, so in-sequence
delivery can be guaranteed for data sent within a single stream. The
advantage of this technique is that when a packet is lost, only
certain streams are affected.
From the header compression point of view the multi-streaming
function raises a number of new issues to solve. Most importantly a
SCTP packet consists of a common header followed by a sequence of
chunks. User payload is transported in DATA chunks which contain
stream specific information. All other chunks do not contain stream
specific information. To obtain maximum compression efficiency it is
important to maintain a separate context for the stream-specific
fields from each stream, but to use a shared context for all general
fields.
The remaining requirements will be similar to IP / TCP compression
[5].
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2. Header compression requirements
The following requirements have, more or less arbitrarily, been
divided into five groups.
The first group deals with requirements concerning the impact of a
header compression scheme on the rest of the Internet infrastructure.
The second group defines what kind of headers that must be compressed
efficiently. The third group defines SCTP specific requirements,
while the forth and fifth groups concern performance requirements and
capability requirements from the properties of the anticipated link
technologies.
2.1 Impact on Internet infrastructure
Transparency:
When a header is compressed and then decompressed, the resulting
header must be semantically identical to the original header. If
this cannot be achieved, the packet containing the erroneous
header must be discarded.
Justification: The header compression process must not produce
headers that might cause problems for any current or future part
of the Internet infrastructure.
Note: The ROHC WG has not found a case where "semantically
identical" is not the same as "bitwise identical".
Ubiquity:
Must not require modifications to existing IP (v4 or v6) or SCTP
implementations.
Justification: Ease of deployment.
2.2 Supported headers
IPv4 and IPv6:
Must support both IPv4 and IPv6. This means that all possible
changes in the IP header fields must be handled by the compression
scheme and commonly changing fields should be compressed
efficiently.
Justification: IPv4 and IPv6 will both be around during the
foreseeable future.
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Mobile IP:
The kinds of headers used by Mobile IP{v4,v6} must be supported
and should be compressed efficiently. For IPv4 these include
headers of tunneled packets. For IPv6 these include headers
containing the Routing Header, the Binding Update Destination
Option, and the Home Address option.
Justification: It is very likely that Mobile IP will be used by
cellular devices.
IPSEC:
The scheme should be able to compress headers containing IPSEC
sub-headers.
Justification: IPSEC is expected to be used to provide necessary
end-to-end security.
Note: It is of course not possible to compress the encrypted part
of an ESP header, nor the cryptographic data in an AH header.
2.3 SCTP specific requirements
Generality:
Must support efficient compression of the SCTP information in a
SCTP packet. This means that the scheme must be able to work with
the protocol structure of the SCTP protocol (SCTP common header,
chunk-1 header, chunk-1 body, chunk-2 header, chunk-2 body...) in
a proper way.
Justification: There must be a generic scheme which reflects the
structure of SCTP packets.
Streams:
Multi-streaming function of SCTP has to be kept in most of the
cases.
Justification: The independent transport of multiple streams is a
big advantage of SCTP. In case of a packet loss at the compressed
link, two cases have to be differentiated:
Case 1: The verification of the decompression via CRC compression
checksum went well. In this case, uncompressed SCTP packets
will be forwarded and the SCTP endpoints will take care about
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multi-streaming functionality.
Case 2: The verification of the decompression via CRC compression
checksum fails. In this case, the release of the related SCTP
packet could influence unrelated streams as well. The only way
to avoid this would be the generation of a new SCTP packet by
the decompressor (without the data chunks from the involved
stream) - in violation to the transparency transport
requirement.
The compression stream must support the multiple streams feature
in a way that head of line blocking is introduced by RoHC only in
very rare cases. Context update should be restricted to a
minimum.
Extensions:
SCTP extensions as described in ADDIP [2] and PRSCTP [3] should be
compressed efficiently.
Justification: SCTP extensions will be a normal part of the
protocol. To reach good efficiency for SCTP, these extension have
to be handled in an appropriate way.
Extendibility:
Generic extendibility describes the handling of yet not defined
chunks, the compression scheme must be able to handle this chunks.
Justification: The compression scheme must support full SCTP
functionality.
2.4 Performance issues
Performance/Spectral Efficiency:
Must provide low relative overhead under expected operating
conditions.
Justification: Spectrum efficiency is the primary goal here.
Error propagation:
For SCTP traffic, link layer retransmissions should be applied to
make use of the bandwidth in the most efficient way. Lost or
damaged headers should thus not occur and therefore it is not a
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primary goal to have mechanisms for error propagation avoidance in
case of such events.
Justification: To provide robustness against loss or damage
introduced by the link, efficiency must be sacrificed. Since loss
or damage is not expected for SCTP traffic, efficiency should
instead be prioritized. This does not mean that some robustness
should not be provided, if efficiency can still be optimized.
Note: In general, error propagation due to header compression
should be kept at an absolute minimum. Error propagation is
defined as the loss or damage of headers subsequent to headers
lost or damaged by the link, even if those subsequent headers are
not lost or damaged.
Note: There are at least two kinds of error propagation; loss
propagation, where a lost header causes subsequent headers to be
lost or damaged, and damage propagation, where a damaged header
causes subsequent headers to be lost or damaged.
Moderate Packet Reordering:
The scheme should efficiently handle moderate reordering (2-3
packets) in the packet stream reaching the compressor.
Justification: This kind of reordering is common.
Packet Reordering:
The scheme should be able to compress when there are reordered
packets in the packet stream reaching the compressor.
Justification: Reordering happens regularly in the Internet.
However, since the Internet is engineered to run SCTP reasonably
well, excessive reordering will not be common and need not be
handled with optimum efficiency.
Processing delay:
The scheme must not contribute significantly to system delay
budget.
2.5 Capability requirements related to link layer characteristics
Unidirectional links:
Must be possible to implement (possibly with less efficiency)
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without explicit feedback messages from decompressor to
compressor.
Justification: There are links that do not provide a feedback
channel or feedback is not desirable for other reasons.
Link delay:
Must operate under all expected link delay conditions.
Header compression coexistence:
The scheme must fit into the ROHC framework together with other
ROHC profiles.
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3. IANA Considerations
A protocol which meets these requirements will require the IANA to
assign various numbers. This document by itself, however, does not
require any IANA involvement.
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4. Security Considerations
A protocol specified to meet these requirements must be able to
compress packets containing IPSEC headers according to the IPSEC
requirement, 2.2.4. The efficiency of the compression may be
influenced by encrypted protocol header elements. This document by
itself, however, does not add any security risks.
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References
[1] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
"Stream Control Transmission Protocol", RFC 2960, October 2000.
[2] Stewart, R., "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", draft-ietf-tsvwg-addip-sctp-
05 (work in progress), May 2002.
[3] Ramalho, M. and R. Stewart, "SCTP Partial Reliability
Extension", draft-stewart-tsvwg-prsctp-01 (work in progress),
July 2002.
[4] Degermark, M., "Requirements for robust IP/UDP/RTP header
compression", RFC 3096, July 2001.
[5] Jonsson, L., "Requirements for ROHC IP/TCP Header Compression",
draft-ietf-rohc-tcp-requirements-04 (work in progress), May
2002.
Authors' Addresses
Christian Schmidt
Siemens AG
Hofmannstr. 51
81359 Munich
Germany
Phone: +49 89 722 27822
EMail: Christian.Schmidt@icn.siemens.de
Michael Tuexen
Siemens AG
Hofmannstr. 51
81359 Munich
Germany
Phone: +49 89 722 47210
EMail: michael.tuexen@siemens.com
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