Network Working Group Lars-Erik Jonsson (Editor), Ericsson
INTERNET-DRAFT Sweden
Expires: December 2001 June 20, 2001
Requirements for ROHC IP/TCP Header Compression
<draft-ietf-rohc-tcp-requirements-01.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This document is a submission of the IETF ROHC WG. Comments should be
directed to the authors or the ROHC WG mailing list,
rohc@cdt.luth.se.
Abstract
This document contains requirements for the IP/TCP header compression
scheme (profile) to be developed by the ROHC WG. The structure of
this document is inherited from the document defining IP/UDP/RTP
requirements for ROHC.
Jonsson (Ed) [Page 1]
INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
0. Document history
February 23, 2001 - draft-ietf-rohc-tcp-requirements-00.txt
Initial version of this document to initiate discussion on
requirements for TCP compression in ROHC.
June 20, 2001 - draft-ietf-rohc-tcp-requirements-01.txt
ECN requirements, note to "Short lived TCP transfers", a section
discussing the IPR issue, and a new open issue about reordering
have been added. The packet size restriction requirement has been
removed. Several open issues still not solved.
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 future 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 [RFC-791, RFC-793]. However,
existing schemes for TCP [RFC-1144, RFC-2507] do not have the same
robustness problems as RTP compression has had since overall
efficiency for TCP traffic is usually achieved by using reliable
links where losses do not occur. Therefore the objectives for a new
TCP compression are instead ; improved compression efficiency,
enhanced capabilities for compression of header fields including TCP
options and finally incorporation of TCP compression into the ROHC
framework [ROHC].
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 while the third and forth
groups concern performance requirements and capability requirements
which stem from the properties of the anticipated link technologies.
Finally, the fifth section discusses Intellectual Property Rights
related to ROHC TCP compression.
>> In chapter 2.5, some issues are listed that MUST be subject to
>> further discussions within the WG.
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INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
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.
2.2. Supported headers and kinds of TCP streams
1. 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. The compression scheme must consider as
normal operation the scenario where Early Congestion Notification
[ECN] is used and support efficient compression also in the case
when the ECN bits are used.
Justification: IPv4 and IPv6 will both be around during the
foreseeable future. ECN is expected to get 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 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.
Jonsson (Ed) [Page 3]
INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
3. Generality: Must support compression of headers of 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.
Note: This applies to the TCP stream before as well as after it
has passed through an internet.
4. 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.
5. TCP: All fields supported by [RFC-2507] must be handled with
efficient compression, and so also the cases when the SYN, FIN or
ECN 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:
- Window scale, [RFC-1323]
- Selective Acknowledgement (SACK), [RFC-2018, RFC-2883]
- Timestamp, [RFC-1323]
2.3. Performance issues
1. Performance/Spectral Efficiency: Must provide low relative
overhead under expected operating conditions; compression
efficiency should be better than for RFC2507 under equivalent
operating conditions.
Justification: Spectrum efficiency is the primary goal here since
the requirements for robustness [see 2. below] is less stringent
for TCP traffic than for UDP(/RTP) [RTP-REQ].
Jonsson (Ed) [Page 4]
INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
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.
2. Error propagation: For TCP 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 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 TCP 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.
3. 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 means
that the gain of header compression could be low since normally
header compression sends full headers initially and small
compressed headers first after the initiation phase.
Note: This requirement implies that mechanisms for "context
sharing" or "context re-use" should be considered.
4a. 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.
4b. Packet Reordering: The scheme should be able to compress when
there are reordered packets in the TCP stream reaching the
compressor.
Jonsson (Ed) [Page 5]
INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
Justification: Reordering happens regularly in the Internet.
However, since the Internet is engineered to run TCP reasonably
well, excessive reordering will not be common and need not be
handled with optimum efficiency.
5. Processing delay: The scheme must not contribute significantly to
system delay budget.
2.4. Capability 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 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
2.5. Intellectual property rights (IPR)
The ROHC WG must spend effort to achieve a high degree of
confidence that there is no IPR covering 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 can be 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, in
particular, only unencumbered solutions will be acceptable to the
Internet at large.
2.6. Open issues - For further discussions
1. The draft is still written to assume that TCP traffic is sent
over links where link-layer retransmissions are applied. This
issue was somewhat discussed at the last IETF meeting in
Minneapolis, but without conclusions in any direction.
Alternative approaches could be to require a scheme with some
robustness capabilities, an adaptable scheme or TWO different
solutions, one for links with retransmissions and one for links
where errors and loss may occur.
Jonsson (Ed) [Page 6]
INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
2. What should be the approach for residual bit errors? Should we
split the error propagation section into two sections, one for
loss propagation and one for damage propagation?
3. Should we require the scheme to be capable of handling some
reordering between compressor and decompressor? Should we have a
separate requirement for reordering of acks?
3. IANA Considerations
A protocol which meets these requirements, e.g., [ROHC], will require
the IANA to assign various numbers. This document by itself, however,
does not require any IANA involvement.
4. Security Considerations
A protocol specified to meet these requirements, e.g., [ROHC], 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.
5. References
[RFC-791] Jon Postel, Internet Protocol, RFC 791, September 1981.
[RFC-793] Jon Postel, Transport Control Protocol, RFC 793,
September 1981.
[RFC-1144] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[RFC-2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP
Header Compression", RFC 2507, February 1999.
[RTP-REQ] Mikael Degermark, "Requirements for IP/UDP/RTP header
compression", Internet draft (work in progress),
February 2001.
<draft-ietf-rohc-rtp-requirements-05.txt>
[ROHC] C. Bormann, "Robust Header Compression (ROHC)", Internet
draft (work in progress), February 2001.
<draft-ietf-rohc-rtp-08.txt>
[RFC-1323] Van Jacobson, Bob Braden, Dave Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
Jonsson (Ed) [Page 7]
INTERNET-DRAFT Requirements for IP/TCP ROHC June 20, 2001
[RFC-2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn
Romanow, "TCP Selective Acknowledgement Option", RFC
2018, October 1996.
[RFC-2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matthew
Podolsky, "An Extension to the Selective Acknowledgement
(SACK) Option for TCP", RFC 2883, July 2000.
[ECN] K. K. Ramakrishnan, Sally Floyd, David L. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", Internet Draft (work in progress), March 2001.
<draft-ietf-tsvwg-ecn-03.txt>
6. Editor's address
Lars-Erik Jonsson Tel: +46 920 20 21 07
Ericsson Erisoft AB Fax: +46 920 20 20 99
Box 920 Mobile: +46 70 554 82 71
SE-971 28 Lulea
Sweden EMail: lars-erik.jonsson@ericsson.com
This Internet-Draft expires December 20, 2001.
Jonsson (Ed) [Page 8]