Network Working Group L-E. Jonsson
INTERNET-DRAFT Ericsson
Expires: March 2005 September 14, 2004
Requirements on ROHC TCP/IP Header Compression
<draft-ietf-rohc-tcp-requirements-08.txt>
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Abstract
This document contains requirements on the TCP/IP header compression
scheme (profile) to be developed by the ROHC WG. The document
discusses the scope of TCP compression, performance considerations,
assumptions on the surrounding environment, as well as IPR concerns.
The structure of this document is inherited from the document
defining RTP/UDP/IP requirements [5] for ROHC.
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Table of Contents
1. Introduction.....................................................2
2. Header Compression Requirements..................................2
2.1. Impact on Internet Infrastructure...........................3
2.2. Supported Headers and Kinds of TCP Streams..................3
2.3. Performance Issues..........................................4
2.4. Requirements Related to Link Layer Characteristics..........6
2.5. Intellectual Property Rights (IPR)..........................7
3. Security Consideration...........................................7
4. IANA Considerations..............................................7
5. Acknowledgments..................................................7
6. Authors' Address.................................................7
7. Informative References...........................................8
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 roundtrip
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 link technologies with high loss
and long roundtrip times.
The main objective for ROHC has been robust compression of
IP/UDP/RTP, but the WG is also chartered to develop new header
compression solutions for IP/TCP [1], [2]. Since TCP traffic, in
contrast to RTP, has usually been sent over reliable links, existing
schemes for TCP, [3] and [4], have not experienced the same
robustness problems as RTP compression. However, there are still many
scenarios where TCP header compression will be implemented over less
reliable links [11], [12], making robustness an important objective
also for the new TCP compression scheme. Other, equally important,
objectives for ROHC TCP compression are: improved compression
efficiency, enhanced capabilities for compression of header fields
including TCP options, and finally incorporation of TCP compression
into the ROHC framework [6].
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 must be compressed efficiently, while the third and fourth
groups concern performance requirements and capability requirements
that stem from the properties of link technologies where ROHC TCP is
expected to be used. Finally, the fifth section discusses
Intellectual Property Rights related to ROHC TCP compression.
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2.1. Impact on Internet Infrastructure
1. 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".
2. Ubiquity: Must not require modifications to existing IP (v4 or
v6) or TCP implementations.
Justification: Ease of deployment.
Note: The ROHC WG may recommend changes that would increase the
compression efficiency for the TCP streams emitted by
implementations. However, ROHC cannot rely on such recommendations
being followed.
Note: Several TCP variants are currently in use on the Internet.
This requirement implies that the header compression scheme must
work efficiently and correctly for all expected TCP variants.
2.2. Supported Headers and Kinds of TCP Streams
1. IPv4 and IPv6: Must support both IPv4 and IPv6. This means that
all expected changes in the IP header fields must be handled by
the compression scheme, and commonly changing fields should be
compressed efficiently. Compression must still be possible when
IPv6 Extensions are present in the header. When designing the
compression scheme, the usage of Explicit Congestion Notification
(ECN) [10] should be considered as a common behavior. Therefore,
the scheme must compress efficiently also in the case when the ECN
bits are used.
Justification: IPv4 and IPv6 will both be around for the
foreseeable future, and Options/Extensions are expected to be more
commonly used. ECN is expected to have a breakthrough and be
widely deployed, especially in combination with TCP.
2. 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 they include headers
containing the Routing Header, and the Home Address Option.
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Justification: It is very likely that Mobile IP will be used by
cellular devices.
3. Generality: Must handle all headers from arbitrary TCP streams.
Justification: There must be a generic scheme which can compress
reasonably well for any TCP traffic pattern. This does not
preclude optimizations for certain traffic patterns.
4. IPSEC: The scheme should be able to compress headers containing
IPSEC sub-headers where the NULL encryption algorithm is used.
Justification: IPSEC is expected to be used to provide necessary
end-to-end security.
Note: It is not possible to compress the encrypted part of an ESP
header, nor the cryptographic data in an AH header.
5. TCP: All fields supported by [4] should be handled with efficient
compression, and so also the cases when the SYN, FIN or TCP ECN
[10] bits are set.
Justification: These bits are expected to be commonly used.
6. TCP options: The scheme must support compression of packets with
any TCP option present, even if the option itself is not
compressed. Further, for some commonly used options the scheme
should provide compression mechanisms also for the options.
Justification: Since various TCP options are commonly used,
applicability of the compression scheme would be significantly
reduced if packets with options could not be compressed.
Note: Options that should be compressed are:
- Selective Acknowledgement (SACK), [8], [9]
- Timestamp, [7]
2.3. Performance Issues
1. Performance/Spectral Efficiency: The scheme must provide low
relative overhead under expected operating conditions; compression
efficiency should be better than for RFC 2507 [4] under equivalent
operating conditions.
Justification: Spectrum efficiency is a primary goal.
Note: The relative overhead is the average header overhead
relative to the payload. Any auxiliary (e.g., control or feedback)
channels used by the scheme should be taken into account when
calculating the header overhead.
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2. Losses between compressor and decompressor: The scheme should make
sure losses between compressor and decompressor do not result in
losses of subsequent packets, or cause damage to the context that
result in incorrect decompression of subsequent packet headers.
Justification: Even though link layer retransmission in most cases
is expected to almost eliminate losses between compressor and
decompressor, there are still many scenarios where TCP header
compression will be implemented over less reliable links [11],
[12]. In such cases, loss propagation due to header compression
could affect certain TCP mechanisms that are capable of handling
some losses, and have a negative impact on the performance of TCP
loss recovery.
3. Residual errors in compressed headers: Residual errors in
compressed headers may result in delivery of incorrectly
decompressed headers not only for the damaged packet itself, but
also for subsequent packets, since errors may be saved in the
context state. For TCP, the compression scheme is not required to
implement explicit mechanisms for residual error detection, but
the compression scheme must not affect TCP's end-to-end mechanisms
for error detection.
Justification: For links carrying TCP traffic, the residual error
rate is expected to be insignificant. However, residual errors may
still occur, especially in the end-to-end path, and therefore it
is crucial that TCP is not prevented from handling these.
Note: This requirement implies that the TCP checksum must be
carried unmodified in all compressed headers.
Note: The error detection mechanism in TCP may be able to detect
residual bit errors, but the mechanism is not designed for this
purpose, and might actually provide a rather weak protection.
Therefore, although it is not a requirement on the compression
scheme, it should be possible for the decompressor to detect
residual errors and discard such packets.
4. Short-lived TCP transfers: The scheme should provide mechanisms
for efficient compression of short-lived TCP transfers, minimizing
the size of context initiation headers.
Justification: Many TCP transfers are short-lived. This may lead
to a low gain for header compression schemes that, for each new
packet stream, require full headers to be sent initially and allow
small compressed headers only after the initialization phase.
Note: This requirement implies that mechanisms for building new
contexts based on information from previous contexts or for
concurrent packet streams to share context information should be
considered.
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5a. Moderate Packet Misordering: The scheme should efficiently handle
moderate misordering (2-3 packets) in the packet stream reaching
the compressor.
Justification: This kind of misordering is common.
5b. Packet Misordering: The scheme must be able to correctly handle
and preferably compress also when there are misordered packets in
the TCP stream reaching the compressor.
Justification: Misordering happens regularly in the Internet.
However, since the Internet is engineered to run TCP reasonably
well, excessive misordering will not be common and need not be
handled with optimum efficiency.
6. Processing delay: The scheme should not contribute significantly
to the system delay budget.
2.4. Requirements Related to Link Layer Characteristics
1. Unidirectional links: Must be possible to implement (possibly with
less efficiency) without explicit feedback messages from
decompressor to compressor.
Justification: There are links that do not provide a feedback
channel or where feedback is not desirable for other reasons.
2. Link delay: Must operate under all expected link delay conditions.
3. Header compression coexistence: The scheme must fit into the ROHC
framework together with other ROHC profiles (e.g. [6]).
4. Note on misordering between compressor and decompressor:
When compression is applied over tunnels, misordering often cannot
be completely avoided. The header compression scheme should not
prohibit misordering between compressor and decompressor, as it
would therefore not be applicable in many tunneling scenarios.
However, in the case of tunneling, it is usually possible to get
misordering indications. Therefore, the compression scheme does
not have to support detection of misordering, but can assume that
such information is available from lower layers when misordering
can occur.
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2.5. Intellectual Property Rights (IPR)
The ROHC WG must spend effort to achieve a high degree of confidence
that there are no known IPR that covers a final compression solution
for TCP.
Justification: Currently there is no TCP header compression scheme
available that can efficiently compress the packet headers of modern
TCP, e.g. with SACK, ECN, etc. ROHC is expected to fill this gap by
providing a ROHC TCP scheme that is applicable in the wide area
Internet, not only over error-prone radio links. It must thus attempt
to be as future-proof as possible, and only unencumbered solutions
will be acceptable to the Internet at large.
3. Security Consideration
A protocol specified to meet these requirements must be able to
compress packets containing IPSEC headers according to the IPSEC
requirement, 2.2.4. There may be other security aspects to consider
in such protocols. This document by itself, however, does not add
any security risks.
4. IANA Considerations
A protocol that meets these requirements will require the IANA to
assign various numbers. This document by itself, however, does not
require any IANA involvement.
5. Acknowledgments
This document has evolved through fruitful discussions with and input
from Gorry Fairhurst, Carsten Bormann, Mikael Degermark, Mark West,
Jan Kullander, Qian Zhang, Richard Price, and Aaron Falk. The
document quality was significantly improved thanks to Peter Eriksson,
who made a thorough linguistic review.
Last, but not least, Ghyslain Pelletier and Kristofer Sandlund served
as committed working group document reviewers.
6. Authors' Address
Lars-Erik Jonsson Phone: +46 8 404 29 61
Ericsson AB Fax: +46 920 996 21
Box 920
SE-971 28 Lulea
Sweden EMail: lars-erik.jonsson@ericsson.com
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7. Informative References
[1] Jon Postel, Internet Protocol, RFC 791, September 1981.
[2] Jon Postel, Transport Control Protocol, RFC 793, September 1981.
[3] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed Serial
Links", RFC 1144, February 1990.
[4] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP Header
Compression", RFC 2507, February 1999.
[5] Mikael Degermark, "Requirements for IP/UDP/RTP header
compression", RFC 3096, July 2001.
[6] Carsten Bormann, et. al., "Robust Header Compression (ROHC)",
RFC 3095, July 2001.
[7] Van Jacobson, Bob Braden, Dave Borman, "TCP Extensions for High
Performance", RFC 1323, May 1992.
[8] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow, "TCP
Selective Acknowledgement Option", RFC 2018, October 1996.
[9] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matthew Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option for
TCP", RFC 2883, July 2000.
[10] K. K. Ramakrishnan, Sally Floyd, David L. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[11] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent
Magret, "End-to-end Performance Implications of Slow Links", RFC
3150, July 2001.
[12] Gorry Fairhurst, Lloyd Wood, "Advice to link designers on link
Automatic Repeat reQuest (ARQ)", RFC 3366, August 2002.
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This Internet-Draft expires March 14, 2005.
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