Network Working Group Mikael Degermark (editor) /University of Arizona
INTERNET-DRAFT USA
Expires: December 7, 2000 June 7, 2000
Requirements for robust IP/UDP/RTP header compression
<draft-ietf-rohc-rtp-requirements-02.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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This document is a submission of the IETF ROHC WG. Comments should be
directed to its mailing list, rohc@cdt.luth.se.
Abstract
This document contains requirements for robust IP/UDP/RTP header
compression to be developed by the ROHC WG. It is based on the ROHC
charter, discussion in the WG, the 3GPP document "3GPP TR 23.922",
version 1.0.0 of october 1999 [TR], as well as contributions from
3G.IP.
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INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC June 7, 2000
0. History and Change Log
June 7, 2000- draft-ietf-rohc-rtp-requirements-02.txt
Transparency: Added note saying that the ROHC WG has not found an
instance where semantically identical is not the same as bitwise
identical.
Ubiquity: Added note saying that ROHC may recommend changes that
will improve compression efficiency.
2.2.4 IPSEC: new requirement
Performance/Spectral Efficiency: changed "error conditions" to
"operating conditions".
Split Cellular Handover requirement in two: one dealing with loss
events caused by handover, and another dealing with context
recreation after switching compressor or decompressor to a new
node.
Added note to Genericity requirement stating that it applies to an
RTP stream before as well as after it has passed through IP-
networks.
Added note to Error Propagation requirement that explains the
terms "loss propagation" and "damage propagation".
2.3.11: Residual bit-error rate: New requirement
May 20, 2000 - draft-ietf-rohc-rtp-requirements-01.txt
added "robust" to the title of the document.
2.1.2 Mobile-IP: Added requirement for compression of Home Address
option.
2.3.3 Cellular handover: Added note to requirement stating that
handover procedure will not be prescribed as the procedure will be
highly system-dependent. However, it must be possible to run ROHC
such that handover is an efficient operation.
2.3.7 Packet misordering: Added note to explain why only moderate
misordering needs to be handled efficiently.
2.3.10 Delay. New requirement.
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INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC June 7, 2000
March 29, 2000 - draft-ietf-rohc-rtp-requirements-00.txt
Initial version of this document. Distributed over the ROHC WG
mailing list prior to 47th IETF in Adelaide.
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INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC June 7, 2000
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 build 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 following requirements have, more or less arbitrarily, been
divided into three groups. The first group deals with requirements
concerning the impact of an header compression scheme on the rest of
the Internet infrastructure. The second group concerns what kind of
headers that must be compressed efficiently. The final group concerns
efficiency requirements and requirements which stem from the
properties of the anticipated link technologies.
2. Header compression requirements
Several current standardization efforts in the cellular arena aim at
supporting voice over IP and other real-time services over IP, e.g.,
GERAN (specified by the ETSI SMG2 standards group), and UTRAN
(specified by the 3GPP standards organization). It is critical for
these standardization efforts that a suitable header compression
scheme is developed before completion of the Release 2000 standards.
Therefore, it is imperative that the ROHC WG keeps its schedule.
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), UDP, or RTP implementations.
Justification: Ease of deployment.
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Note: The ROHC WG may recommend changes that would increase the
compression efficiency for the RTP streams emitted by
implementations. However, ROHC cannot rely on such recommendations
being followed.
2.2 Supported headers and kinds of RTP streams
1. Ipv4 and Ipv6: Must support both IPv4 and IPv6.
Justification: IPv4 and IPv6 will both be around during the
foreseeable future.
2. Mobile IP: The kinds of headers used by Mobile IP{v4,v6} 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.
3. Genericity: Must support compression of headers of arbitrary RTP
streams.
Justification: There must be a generic scheme which can compress
reasonably well for any payload type and traffic pattern. This does
not preclude optimizations for certain media types where the traffic
pattern is known, e.g., for low-bandwidth voice and low-bandwidth
video.
Note: This applies to the RTP stream before as well as after it has
passed through an internet.
4. IPSEC: ROHC should be able to compress headers containing IPSEC
subheaders.
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 Efficiency
1. Performance/Spectral Efficiency: Must provide low relative
overhead under expected operating conditions; compression efficiency
should be better than for RFC2508 under equivalent operating
conditions. The error rate should only marginally increase the
overhead under expected operating conditions.
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Justification: Spectrum efficiency is a primary goal. RFC2508 does
not perform well enough. Notes: 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: Error propagation due to header compression
should be kept at an absolute minimum. Error propagation is defined
as the loss or damage of packets subsequent to packets lost or
damaged by the link, even if those subsequent packets are not lost or
damaged.
Justification: Error propagation reduces spectral efficiency and
reduces quality. CRTP suffers severely from error propagation.
Note: There are at least two kinds of error propagation; loss
propagation, where a lost packet causes subsequent packets to be
lost, and damage propagation, where a damaged packet causes
subsequent packets to be damaged.
3a. Handover loss events: Loss events of lenght 150 ms should be
handled efficiently and without additional packet loss being
introduced by ROHC.
Justification: Such loss events can be introduced by handover
operations in the (radio) network between compressor and
decompressor. Handover operations can be frequent in cellular
systems. Failure to handle handover well can adversely impact
spectrum efficiency and quality.
3b. Handover context recreation: There must be a way to run ROHC
that deals well with events where the route of an RTP conversation
changes such that either the compressor or the decompressor of the
conversation is relocated to another node, where the context needs to
be recreated. ROHC must not create erroneous packets during such
events.
Justification: Such events can be frequent in cellular systems where
the compressor/decompressor on the network side is close to the radio
base stations.
Note: In order to aid developers of context recreation schemes where
context is transfered to the new node, the specification shall
outline what parts of the context is to be transfered, as well as
conditions for its use. Procedures for context recreation (and
discard) without such context transfer will also be provided.
4. Link delay: Must operate under all expected link delay conditions.
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5. Processing delay: The scheme must not contribute significantly to
system delay budget.
6. Multiple links: The scheme must perform well when there are two or
more cellular links in the end-to-end path.
Justification: Such paths will occur.
Note: loss on previous links will cause irregularities in the RTP
stream reaching the compressor. Such irregularities should only
marginally affect performance.
7a. Packet Misordering: The scheme should be able to compress when
there are misordered packets in the RTP stream reaching the
compressor. No misordering is expected on the link between
compressor and decompressor.
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 efficiently.
7b. Moderate Packet Misordering: The scheme should efficiently handle
moderate misordering (2-3 packets) in the packet stream reaching the
compressor.
Note: This kind of reordering is common.
8. Unidirectional links/multicast: Must operate (possibly with less
efficiency) over links where there is no feedback channel or where
there are several receivers.
9. Configurable frame size fluctuation: It should be possible to
restrict the number of different frame sizes used by the scheme.
Justification: Some radio technologies support only a limited number
of frame sizes efficiently.
Note: Somewhat degraded performance is to be expected when such
restrictions are applied.
Note: This implies that a list of "good" frame sizes must be made
available and that ROHC may pick a suitable header format to utilize
available space as well as possible.
10. Delay: ROHC should not noticeably add to the end-to-end delay.
Justification: RTP packets carrying data for interactive voice or
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video have a limited end-to-end delay budget.
Note: This requirement is intended to prevent schemes that achieve
robustness at the expense of delay, for example schemes that require
that packet i+x, x>0, is received before packet i can be
decompressed.
Note: Together with 2.3.5, this requirement implies that ROHC will
not noticeably add to the jitter of the RTP stream, other than what
is caused by variations in header size.
Note: This requirement does not prevent a queue from forming upstream
a bottleneck link. Nor does it prevent a compressor from utilizing
information from more than one header in such a queue.
11. Residual errors: For a residual bit-error rate of at most 1e-5,
the ROHC scheme must not increase the error rate.
Justification: Some target links have a residual error rate, i.e,
rate of undetected errors, of this magnitude.
Note: In the presence of residual bit-errors, ROHC will need error
detection mechanisms to prevent damage propagation. These mechanisms
will catch some residual errors, but those not caught might cause
damage propagation. This requirement states that the reduction of
the damage rate due to the error detection mechanisms must not be
less than the increase in error rate due to damage propagation.
3. Editor's address
Mikael Degermark Tel (May-July): +46 70 833-8933
Dept. of Computer Science Tel: +1 520 621-3489
University of Arizona Fax: +1 520 621-4246
P.O. Box 210077
Tucson, AZ 85721-0077
USA EMail: micke@cs.arizona.edu
4. References
[TR] 3GPP TR 23.922 version 1.0.0, 3rd Generation partnership
Project; Technical Specification Group Services and
Systems Aspects; Architecture for an All IP network.
[TS] TS 22.101 version 3.6.0: Service Principles
[RFC-768] J. Postel, User Datagram Protocol, RFC 761, August 1980.
[RFC-791] J. Postel, Internet Protocol, RFC 791, September 1981.
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[RFC-1144] V. Jacobson, Compressing TCP/IP Headers for Low-Speed
Serial Links, RFC 1144, February 1990.
[CRTP] S. Casner, V. Jacobson, Compressing IP/UDP/RTP Headers
for Low-Speed Serial Links, RFC 2508.
This draft expires December 7, 2000.
Degermark (Ed) [Page 9]
