Network Working Group Krister Svanbro, Ericsson
INTERNET-DRAFT Sweden
Expires: April 2001 October 10, 2000
Lower Layer Guidelines for Robust RTP/UDP/IP Header Compression
<draft-ietf-rohc-rtp-lower-layer-guidelines-00.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 its mailing list, rohc@cdt.luth.se.
Abstract
This document describes lower layer guidelines for robust header
compression (ROHC) and the requirements ROHC put on lower layers. The
purpose of this document is to support the incorporation of robust
header compression algorithms, as specified in the ROHC working
group, into different systems such as those specified by 3GPP, 3GPP2,
ETSI, etc. The document covers only lower layer guidelines for
compression of RTP/UDP/IP and UDP/IP headers as specified in [ROHC].
Both general guidelines and guidelines specific for cellular systems
are treated.
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TABLE OF CONTENTS
1. Introduction..................................................3
2. Terminology...................................................3
3. General guidelines............................................4
3.1 Error detection.......................................4
3.2 Inferred header field information.....................4
3.3 Handling of header size variation.....................5
3.4 Negotiation of header compression parameters..........6
3.5 Demultiplexing of flows onto logically
separated channels..................................6
3.6 Packet type identification............................6
3.7 Packet duplication....................................6
3.8 Feedback packets......................................7
4. Cellular system specific guidelines...........................7
4.1 Handover procedures...................................7
4.2 Unequal error detection...............................8
4.3 Unequal error protection..............................9
5. Author's addresses............................................9
6. References...................................................10
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1. Introduction
Almost all header compression algorithms [RFC1144, RFC2507, RFC2508]
rely on some functionality from underlying link layer. Headers
(compressed or not) are expected to be delivered without any residual
bit errors, IP length fields are inferred from link layer length
fields and packet type identification may be separated from the
header compression scheme and instead done at underlying link layer.
[RFC2509], for example, elaborates on how to incorporate IP header
compression [RFC2507] in PPP [RFC1661].
It is important to be aware of such assumptions on required
functionality from underlying layers when incorporating a header
compression scheme into a system. The functionality required by a
specific header compression scheme from lower layers may also be
needed if incorporation of a header compression scheme is to be
prepared without knowing the exact details of the final scheme.
This document describes lower layer guidelines for robust RTP/UDP/IP
header compression [ROHC] as being specified by the ROHC working
group. [ROHC] will from this point be referenced to as ROHC. These
guidelines should simplify incorporation of the robust header
compression algorithms into cellular system like those standardized
by 3GPP, 3GPP2, ETSI, etc, and also into specific link layer
protocols such as PPP. The document should also enable preparation of
this incorporation without requiring detailed knowledge about the
final header compression scheme. Relevant standardization groups
standardizing link layers should, aided by this document, include
required functionality in "their" link layers to support robust
header compression.
Hence, this document clarify the requirements [ROHC] put on lower
layers, while the requirements on ROHC may be found in [REQ].
2. Terminology
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 RFC 2119.
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3. General guidelines
3.1 Error detection
All current header compression schemes [RFC1144, RFC2507, RFC2508]
rely on lower layers to detect errors in (compressed) headers. This
is usually done with link layer checksums covering at least the
compressed header. However, all error detecting mechanisms may fail
to detect some bit errors and cause residual bit errors (undetected
bit errors).
Lower layers MUST provide error detection for at least ROHC headers.
ROHC has been designed not to increase residual bit error rate (for
reasonable residual error rates) compared to the case when no header
compression is used. Headers passed up to the header decompressor
should, however, have a residual bit error close to zero.
It is RECOMMENDED that erroneous headers are passed up to the
decompressor instead of being discarded before the decompressor, but
in that case an indication that the header has errors MUST be
included to the decompressor together with the erroneous header.
3.2 Inferred header field information
Some fields of the RTP/UDP/IP headers may be classified as inferred,
that is their values are to be inferred from other values or from an
underlying link layer. An underlying link layer MUST therefore
provide the header decompressor with the following information:
Packet Length (IPv4)
Information about the received packet (with the compressed header)
length MUST be provided by the link layer.
Payload Length (IPv6)
Information about the received packet (with the compressed header)
length MUST be provided by the link layer.
Length (UDP)
This field is redundant with the Packet Length (IPv4) or the
Payload Length (IPv6) field. Hence, the value of this field MUST in
some way be provided to the decompressor.
In summary, all of the fields above relate to the length of the
packet the compressed header is included in. All three fields may
thus be inferred by the decompressor if one packet length value is
signaled from the link layer to the decompressor on a per packet
basis. This packet length value should be the length of the received
packet including the (compressed) header.
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3.3 Handling of header size variations
It is desirable for many cellular link layer technologies that bit
rate variations and thus packet size variations are minimized.
However, there will always be some variation in compressed header
sizes since there is a trade-off between header size variations and
compression efficiency, and also due to events in the header flow and
on the channel. Variations in header sizes cause variations in packet
sizes depending on variations of payload size. The following will
only treat header size variations caused by ROHC and not packet size
variations due to variations of payload size.
The link layer MUST in some manner support varying header sizes from
40 bytes (full RTP/UDP/IPv4 header) or 60 bytes (full RTP/UDP/IPv6)
down to 1 byte for the minimal compressed header. It is likely that
the small compressed headers dominate the flow of headers, and that
the largest headers are sent rarely, e.g. only a few times in the
initialization phase of the header compression scheme.
Header size variations and thus packet size variations depend on
numerous factors. Un-predictable changes in the RTP, UDP or IP
headers may cause compressed headers to momentarily increase in size
as well as header sizes may depend on packet loss rate at lower
layers. Header size distributions depend also on the mode ROHC
operate in. However, for e.g. a voice application, carried by
RTP/UDP/IPv4, with a constant speech frame size and silence
suppression, the following basic header size changes may be
considered as typical:
In the very beginning of the speech session, the ROHC scheme is
initialized by sending full headers called IR/DYN. These are the
largest headers and with sizes depending on basically IP-version. For
IPv4 the size is approximately 40 bytes, and for IPv6 approximately
60 bytes. The IR/DYN headers are used typically during one round trip
time, possible interleaved with compressed headers. After that,
usually only compressed headers are sent. Compressed headers may vary
in size from 1 byte up to several bytes. The smallest compressed
headers is used when there is no unpredictable changes in header
fields, typically during a talk spurt. In the beginning of a talk
spurt, compressed header sizes may increase with one or a few bytes
momentarily. Apart from increases due to new talk spurts, compressed
headers may increase in size momentarily due to unpredictable changes
in header fields.
ROHC provides some means to limit the amount of produced header
sizes. In some cases a larger header than needed may be used to limit
the number of header sizes used. Padding octets may also be used to
fill up to a desired size. Chapter 6.3 Implementation parameters in
[ROHC] provide optional implementation parameters that make it
possible to mandate how a ROHC implementation should operate, for
instance to mandate how many header sizes that may be used.
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3.4 Negotiation of header compression parameters
ROHC have some parameters that needs to be set at an initial setup
phase. Which header compression profile to use may have to be
determined and also what kind of context identification (CID)
mechanism to use.
The lower layers supporting ROHC MUST include mechanisms for
negotiating header compression parameters such as, CID usage and/or
header compression profiles. It is RECOMMENDED that the lower layer
have mechanisms that support re-negotiations of these parameters.
3.5 Demultiplexing of flows onto logically separated channels
In some cellular technologies there are a demultiplexing of flows
onto radio bearers suitable to the particular flows, i.e., onto
logically separated channels. For instance, real-time flows such as
voice and video may be carried on logically separated bearers. It is
RECOMMENDED that this kind of demultiplexing is done in the lower
layers supporting robust header compression. By doing so, the need
for context identification in the header compression scheme is
reduced. If there is a one to one mapping between flow and logical
channel, there is no need at all for context identification at the
header compression level.
3.6 Packet type identification
Header compression schemes like [RFC2507, RFC2508] have relied on the
underlying link layer to identify different kinds of headers by means
of packet type identifiers on link layers. This kind of mechanism is
not necessary for ROHC. The packet type identification is
incorporated in the ROHC scheme and there is thus, no need for such a
mechanism in the link layer. If ROHC is used together with header
compression schemes requiring packet type identification at the link
layer, e.g. [RFC2507, RFC2508], or if ROHC is used on top of link
layers where packet type identifiers already are present, it is
RECOMMENDED that one (1) ROHC packet type identifier is supported on
lower layers. Thus, only one ROHC packet type is needed to mix ROHC
and e.g. RFC2507 flows or to support ROHC on links where packet type
identifiers already are present.
3.7 Packet duplication
Exact duplications of one and the same packet may waste transmission
resources and is in contradiction to compression. Even so, packet
duplication may occur for various reasons. Packet duplication may
also occur in different places along the path for a packet.
ROHC can handle packet duplication before the compressor but it is
RECOMMENDED that such packet duplications are avoided. Lower layers
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MUST NOT duplicate packets on the path between ROHC compressor and
decompressor.
3.8 Feedback packets
ROHC consist of three modes; Unidirectional mode (U-mode),
bidirectional optimistic mode (O-mode) and bidirectional reliable
mode (R-mode). A brief description of the modes can be found in
chapter 4.4 in [ROHC].
In U-mode it is not necessary to send any feedback from the
decompressor to the compressor. O-mode and R-mode requires however
that feedback messages from the decompressor to the compressor may be
sent. Feedback message consist of small ROHC internal packets without
any application payload. It is possible in ROHC to piggy-back
feedback packets onto regular packets with ROHC compressed headers
and payload, if there is ROHC type of compression in both the forward
and reverse direction. However, this piggy-backing may not be desired
or possible in some cases.
Lower layer MUST support transport of feedback packets from
decompressor to compressor if ROHC is to be used in O-mode or R-mode.
Lower layers MUST support transport of small stand-alone feedback
packets if piggybacking of feedback packets is not used. The feedback
packets from the decompressor SHOULD be delivered as soon as possible
to the compressor.
4. Cellular system specific guidelines
An important group of link layer technologies where robust header
compression will be needed are future cellular systems, which may
have a very large number of users in some years. The need for header
compression is large in these kinds of systems to achieve spectrum
efficiency. Hence, it is important that future cellular systems can
efficiently incorporate the robust header compression scheme.
4.1 Handover procedures
One cellular specific property that may affect header compression is
mobility and thus, handover (i.e., change of serving base station or
radio network controller).
The main characteristics of handovers relevant for robust header
compression are: the length of the longest packet loss event due to
handover (i.e. the number of consecutive packet losses); relocation
of header compression context when necessary.
Depending on the location of the header compressor/decompressor in
the radio access network and the type of handover, handover may or
may not cause disruptions or packet loss events in the (compressed)
header flow relevant for the header compression scheme. For instance,
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if soft handover is used and if the header compressor/decompressor
reside above the combining point for soft handover, there will be no
extra packet losses visible to the decompressor due to handover. In
hard handovers, where packet loss events due to handover is
introduced, the length of the longest consecutive packet loss is most
relevant and should thus, be minimized.
Handover SHOULD NOT cause significant long events of consecutive
packet loss. Significant in this context relates to the kind of loss
tolerable for the carried real-time application.
If hard handovers are performed, which may cause significant long
events of consecutive packet loss, it is RECOMMENDED that the radio
access network notifies the compressor when such a handover has
started and completed.
The cellular system supporting robust header compression may also
have internal mechanisms for transferring the header compression
context between nodes where contexts may reside, at or before
handover.
If the cellular system does not have any internal mechanism for
transferring header compression context between nodes, the context
relocation may be done by the header compression scheme by means of a
context refresh. The header compression scheme may perform a new
header compression initialization, e.g. by sending full headers. This
will, however, introduce an increase in the average header size
dependent on how often a transfer of context is needed.
In this latter case, the lower layers MUST indicate to the header
compressor that such a handover has occurred, so that it knows when
to refresh the context. Chapter 6.3 Implementation parameters in
[ROHC] provide optional implementation parameters that make it
possible to trigger e.g. a complete context refresh.
4.2 Unequal error detection
Chapter 3.1 states that ROHC requires error detection from lower
layers for at least the compressed header. However, some cellular
technologies may differentiate the amount of error detection for
different parts of a packet. For instance, it could be possible to
have a stronger error detection for the header part of a packet, if
the application payload part of the packet is less sensitive to
errors, e.g. some cellular types of speech codecs.
ROHC does not require UED from lower layers, ROHC requires only a
error detection mechanism that detects errors in at least the header
part of the packet. There is thus no requirement on lower layers to
provide separate error detection for the header and payload part of a
packet. However, overall performance may be increased if UED is used.
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For example, if equal error detection is used in the form of one link
layer checksum covering the entire packet including both header and
payload part, any bit error will cause the packet to be discarded at
the ROHC decompressor. It is not possible to distinguish between
errors in the header and the payload part of the packet with this
error detection mechanism and the ROHC decompressor must assume that
the header is damaged, even if the bit error hit the payload part of
the packet. If the header is assumed to be damaged, it is not
possible to ensure correct decompression and that packet will thus be
discarded. If the application is such that it tolerates some errors
in the payload, it could have been better to deliver that packet to
the application and let the application judge whether the payload was
usable or not. Hence, with a unequal error detection scheme where it
is possible to separate detection of errors in the header and payload
part of a packet, more packets may be delivered to applications in
some cases for the same lower layer error rates. The final benefit
depend of course on the cost of UED for the radio interface and
related protocols.
4.3 Unequal error protection
Some cellular technologies can provide different error probabilities
for different parts of a packet, unequal error protection (UEP). For
instance, the lower layers may provide a stronger error protection
for the header part of a packet compared to the payload part of the
packet.
ROHC does not require UEP. UEP may be beneficial in some cases to
reduce the error rate in ROHC headers, but only if it is possible to
distinguish between errors in header and payload parts of a packets.
I.e. only if unequal error detection (UED) is used. The benefit of
UEP depend of course on the cost of UEP for the radio interface and
related protocols.
5. Author's Addresses
Krister Svanbro Tel: +46 920 20 20 77
Box 920 Fax: +46 920 20 20 99
Ericsson Erisoft AB Email: krister.svanbro@ericsson.com
SE-971 28 Lulea, Sweden
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6. References
[ROHC] Carsten Borman et al, "Robust Header Compression (ROHC)",
Internet-Draft (work in progress), October 2000.
<draft-ietf-rohc-rtp-03.txt>
[REQ] Mikael Degermark, "Requirements for robust IP/UDP/RTP
header compression", Internet Draft (work in progress),
June 2000.
<draft-ietf-rohc-rtp-requirements-01.txt>
[RFC1144] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[RFC2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP
Header Compression", RFC 2507, February 1999.
[RFC2508] Steven Casner, Van Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[RFC2509] Mattias Engan, Steven Cassner, "IP Header Compression
over PPP", RFC2509, February 1999
[RFC1661] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
This Internet-Draft expires in April 2001.
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