draft-ietf-rohc-rtp-lower-layer-guidelines-01.txt   draft-ietf-rohc-rtp-lower-layer-guidelines-02.txt 
Network Working Group Krister Svanbro, Ericsson Network Working Group Krister Svanbro, Ericsson
INTERNET-DRAFT Sweden INTERNET-DRAFT Sweden
Expires: August 2001 February 7, 2001 Expires: May 2002 November 19, 2001
Lower Layer Guidelines for Robust RTP/UDP/IP Header Compression Lower Layer Guidelines for Robust RTP/UDP/IP Header Compression
<draft-ietf-rohc-rtp-lower-layer-guidelines-01.txt> <draft-ietf-rohc-rtp-lower-layer-guidelines-02.txt>
Status of this memo Status of this memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts. groups may also distribute working documents as Internet-Drafts.
skipping to change at page 1, line 41 skipping to change at page 1, line 41
directed to its mailing list, rohc@cdt.luth.se. directed to its mailing list, rohc@cdt.luth.se.
Abstract Abstract
This document describes lower layer guidelines for robust header This document describes lower layer guidelines for robust header
compression (ROHC) and the requirements ROHC put on lower layers. The compression (ROHC) and the requirements ROHC put on lower layers. The
purpose of this document is to support the incorporation of robust purpose of this document is to support the incorporation of robust
header compression algorithms, as specified in the ROHC working header compression algorithms, as specified in the ROHC working
group, into different systems such as those specified by 3GPP, 3GPP2, group, into different systems such as those specified by 3GPP, 3GPP2,
ETSI, etc. The document covers only lower layer guidelines for ETSI, etc. The document covers only lower layer guidelines for
compression of RTP/UDP/IP and UDP/IP headers as specified in [ROHC]. compression of RTP/UDP/IP and UDP/IP headers as specified in
Both general guidelines and guidelines specific for cellular systems [RFC3095]. Both general guidelines and guidelines specific for
are treated. cellular systems are treated.
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 Packet reordering.....................................7
3.9 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. IANA Considerations...........................................9
6. Security considerations......................................10
7. Author's addresses...........................................10 Table of Contents
8. References...................................................10 1. Introduction..................................................2
2. General guidelines............................................3
2.1. Error detection.......................................3
2.2. Inferred header field information.....................3
2.3. Handling of header size variation.....................4
2.4. Negotiation of header compression parameters..........5
2.5. Demultiplexing of flows onto logical channels.........5
2.6. Packet type identification............................5
2.7. Packet duplication....................................6
2.8. Packet reordering.....................................6
2.9. Feedback packets......................................6
3. Cellular system specific guidelines...........................6
3.1. Handover procedures...................................7
3.2. Unequal error detection...............................8
3.3. Unequal error protection..............................8
4. IANA considerations...........................................9
5. Security considerations.......................................9
6. Author's addresses............................................9
7. References....................................................9
1. Introduction 1. Introduction
Almost all header compression algorithms [RFC1144, RFC2507, RFC2508] Almost all header compression algorithms [RFC1144, RFC2507, RFC2508]
rely on some functionality from underlying link layer. Headers rely on some functionality from underlying link layer. Headers
(compressed or not) are expected to be delivered without any residual (compressed or not) are expected to be delivered without any residual
bit errors, IP length fields are inferred from link layer length bit errors, IP length fields are inferred from link layer length
fields and packet type identification may be separated from the fields and packet type identification may be separated from the
header compression scheme and instead done at underlying link layer. header compression scheme and instead done at underlying link layer.
[RFC2509], for example, elaborates on how to incorporate IP header [RFC2509], for example, elaborates on how to incorporate IP header
compression [RFC2507] in PPP [RFC1661]. compression [RFC2507] in PPP [RFC1661].
It is important to be aware of such assumptions on required It is important to be aware of such assumptions on required
functionality from underlying layers when incorporating a header functionality from underlying layers when incorporating a header
compression scheme into a system. The functionality required by a compression scheme into a system. The functionality required by a
specific header compression scheme from lower layers may also be specific header compression scheme from lower layers may also be
needed if incorporation of a header compression scheme is to be needed if incorporation of a header compression scheme is to be
prepared without knowing the exact details of the final scheme. prepared without knowing the exact details of the final scheme.
This document describes lower layer guidelines for robust RTP/UDP/IP This document describes lower layer guidelines for robust RTP/UDP/IP
header compression [ROHC] as being specified by the ROHC working header compression [RFC3095] as specified by the ROHC working group.
group. [ROHC] will from this point be referenced to as ROHC. These [RFC3095] will from this point be referenced to as ROHC. These
guidelines should simplify incorporation of the robust header guidelines should simplify incorporation of the robust header
compression algorithms into cellular system like those standardized compression algorithms into cellular system like those standardized
by 3GPP, 3GPP2, ETSI, etc, and also into specific link layer by 3GPP, 3GPP2, ETSI, etc, and also into specific link layer
protocols such as PPP. The document should also enable preparation of protocols such as PPP. The document should also enable preparation of
this incorporation without requiring detailed knowledge about the this incorporation without requiring detailed knowledge about the
final header compression scheme. Relevant standardization groups final header compression scheme. Relevant standardization groups
standardizing link layers should, aided by this document, include standardizing link layers should, aided by this document, include
required functionality in "their" link layers to support robust required functionality in "their" link layers to support robust
header compression. header compression.
Hence, this document clarifies the requirements [ROHC] put on lower Hence, this document clarify the requirements ROHC put on lower
layers, while the requirements on ROHC may be found in [REQ]. layers, while the requirements on ROHC may be found in [RFC3096].
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.
3. General guidelines 2. General guidelines
3.1 Error detection 2.1. Error detection
All current header compression schemes [RFC1144, RFC2507, RFC2508] All current header compression schemes [RFC1144, RFC2507, RFC2508]
rely on lower layers to detect errors in (compressed) headers. This rely on lower layers to detect errors in (compressed) headers. This
is usually done with link layer checksums covering at least the is usually done with link layer checksums covering at least the
compressed header. However, all error detecting mechanisms may fail compressed header. However, any error detecting mechanism may fail to
to detect some bit errors and cause residual bit errors (undetected detect some bit errors, which are usually called residual bit errors.
bit errors).
Lower layers MUST provide error detection for at least ROHC headers. As for non-compressed IP packets, lower layers must provide similar
ROHC has been designed not to increase residual bit error rate (for error detection, at least for ROHC headers. ROHC has been designed
reasonable residual error rates) compared to the case when no header not to increase the residual bit error rate (for reasonable residual
compression is used. Headers passed up to the header decompressor error rates) compared to the case when no header compression is used.
should, however, have a residual bit error close to zero. Headers passed up to the header decompressor should, however, have a
residual bit error probability close to zero.
It is RECOMMENDED that erroneous headers are passed up to the A ROHC decompressor might make use of packets with erroneous headers,
decompressor instead of being discarded before the decompressor, but even if they must be discarded. It is therefore recommended that also
in that case an indication that the header has errors MUST be such invalid packets are passed up to the decompressor instead of
included to the decompressor together with the erroneous header. being discarded by lower layers, but the packet must then be
accompanied with an error indication.
3.2 Inferred header field information 2.2. Inferred header field information
Some fields of the RTP/UDP/IP headers may be classified as inferred, 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 that is their values are to be inferred from other values or from an
underlying link layer. An underlying link layer MUST therefore underlying link layer. A ROHC decompressor require that at least the
provide the header decompressor with the following information: following information from any underlying link layer:
Packet Length (IPv4)
Information about the received packet (with the compressed header)
length MUST be provided by the link layer.
Payload Length (IPv6) Packet Length (IPv4) / Payload Length (IPv6)
Information about the received packet (with the compressed header) The received packet (with compressed header) length.
length MUST be provided by the link layer.
Length (UDP) Length (UDP)
This field is redundant with the Packet Length (IPv4) or the This field is redundant with the Packet Length (IPv4) or the
Payload Length (IPv6) field. Hence, the value of this field MUST in Payload Length (IPv6) field.
some way be provided to the decompressor.
In summary, all of the fields above relate to the length of the In summary, all these fields relate to the length of the packet the
packet the compressed header is included in. All three fields may compressed header is included in. These fields may thus be inferred
thus be inferred by the decompressor if one packet length value is by the decompressor if one packet length value is signaled from the
signaled from the link layer to the decompressor on a per packet link layer to the decompressor on a per packet basis. This packet
basis. This packet length value should be the length of the received length value should be the length of the received packet including
packet including the (compressed) header. the (compressed) header.
3.3 Handling of header size variations 2.3. Handling of header size variations
It is desirable for many cellular link layer technologies that bit It is desirable for many cellular link layer technologies that bit
rate variations and thus packet size variations are minimized. rate variations and thus packet size variations are minimized.
However, there will always be some variation in compressed header However, there will always be some variation in compressed header
sizes since there is a trade-off between header size variations and sizes since there is a trade-off between header size variations and
compression efficiency, and also due to events in the header flow and compression efficiency, and also due to events in the header flow and
on the channel. Variations in header sizes cause variations in packet on the channel. Variations in header sizes cause variations in packet
sizes depending on variations of payload size. The following will sizes depending on variations of payload size. The following will
only treat header size variations caused by ROHC and not packet size only treat header size variations caused by ROHC and not packet size
variations due to variations of payload size. variations due to variations of payload size.
The link layer MUST in some manner support varying header sizes from 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) 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 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 small compressed headers dominate the flow of headers, and that
the largest headers are sent rarely, e.g. only a few times in the the largest headers are sent rarely, e.g. only a few times in the
initialization phase of the header compression scheme. initialization phase of the header compression scheme.
Header size variations and thus packet size variations depend on Header size variations and thus packet size variations depend on
numerous factors. Un-predictable changes in the RTP, UDP or IP numerous factors. Unpredictable changes in the RTP, UDP or IP headers
headers may cause compressed headers to momentarily increase in size may cause compressed headers to momentarily increase in size as well
as well as header sizes may depend on packet loss rate at lower as header sizes may depend on packet loss rate at lower layers.
layers. Header size distributions depend also on the mode ROHC Header size distributions depend also on the mode ROHC operate in.
operates in. However, for e.g. a voice application, carried by However, for e.g. a voice application, carried by RTP/UDP/IPv4, with
RTP/UDP/IPv4, with a constant speech frame size and silence a constant speech frame size and silence suppression, the following
suppression, the following basic header size changes may be basic header size changes may be considered as typical:
considered as typical:
In the very beginning of the speech session, the ROHC scheme is In the very beginning of the speech session, the ROHC scheme is
initialized by sending full headers called IR/DYN. These are the initialized by sending full headers called IR/DYN. These are the
largest headers and with sizes depending on basically IP-version. For largest headers and with sizes depending on basically IP-version. For
IPv4 the size is approximately 40 bytes, and for IPv6 approximately IPv4 the size is approximately 40 bytes, and for IPv6 approximately
60 bytes. The IR/DYN headers are used typically during one round trip 60 bytes. The IR/DYN headers are used typically during one round trip
time, possible interleaved with compressed headers. After that, time, possible interleaved with compressed headers. After that,
usually only compressed headers are sent. Compressed headers may vary usually only compressed headers are sent. Compressed headers may vary
in size from 1 byte up to several bytes. The smallest compressed in size from 1 byte up to several bytes. The smallest compressed
headers is used when there is no unpredictable changes in header headers is used when there is no unpredictable changes in header
fields, typically during a talk spurt. In the beginning of a talk fields, typically during a talk spurt. In the beginning of a talk
spurt, compressed header sizes may increase with one or a few bytes spurt, compressed header sizes may increase with one or a few bytes
momentarily. Apart from increases due to new talk spurts, compressed momentarily. Apart from increases due to new talk spurts, compressed
headers may increase in size momentarily due to unpredictable changes headers may increase in size momentarily due to unpredictable changes
in header fields. in header fields.
ROHC provides some means to limit the amount of produced header 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 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 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 fill up to a desired size. Chapter 6.3 Implementation parameters in
[ROHC] provide optional implementation parameters that make it [RFC3095] provide optional implementation parameters that make it
possible to mandate how a ROHC implementation should operate, for possible to mandate how a ROHC implementation should operate, for
instance to mandate how many header sizes that may be used. instance to mandate how many header sizes that may be used.
3.4 Negotiation of header compression parameters 2.4. Negotiation of header compression parameters
ROHC has some parameters that needs to be set at an initial setup ROHC have some parameters that needs to be configured in an initial
phase. Which header compression profile to use may have to be setup phase. Which header compression profiles that are allowed may
determined and also what kind of context identification (CID) have to be determined and also what kind of context identification
mechanism to use. (CID) mechanism to use.
The lower layers supporting ROHC MUST include mechanisms for The lower layers supporting ROHC should thus include mechanisms for
negotiating header compression parameters such as, CID usage and/or negotiation of header compression parameters such as, CID usage and
header compression profiles. It is RECOMMENDED that the lower layer header compression profile support. In certain environments, it might
have mechanisms that support re-negotiations of these parameters. also be desirable to have mechanisms for re-negotiation of these
parameters.
For unidirectional links, this negotiation may be performed out-of- For unidirectional links, this configuration might have to be
band or even a priori. performed out-of-band or a priori, and similar methods could of
course also be used for bi-directional links if direct negotiation is
not possible.
3.5 Demultiplexing of flows onto logically separated channels 2.5. Demultiplexing of flows onto logical channels
In some cellular technologies there are a demultiplexing of flows In some cellular technologies flows are demultiplexed onto radio
onto radio bearers suitable to the particular flows, i.e., onto bearers suitable to the particular flows, i.e., onto logically
logically separated channels. For instance, real-time flows such as separated channels. For instance, real-time flows such as voice and
voice and video may be carried on logically separated bearers. It is video may be carried on logically separated bearers. It is
RECOMMENDED that this kind of demultiplexing is done in the lower recommended that this kind of demultiplexing is done in the lower
layers supporting robust header compression. By doing so, the need layers supporting robust header compression. By doing so, the need
for context identification in the header compression scheme is for context identification in the header compression scheme is
reduced. If there is a one to one mapping between flow and logical 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 channel, there is no need at all for context identification at the
header compression level. header compression level.
3.6 Packet type identification 2.6. Packet type identification
Header compression schemes like [RFC2507, RFC2508] have relied on the Header compression schemes like [RFC2507, RFC2508] have relied on the
underlying link layer to identify different kinds of headers by means underlying link layer to identify different kinds of headers by means
of packet type identifiers on link layers. This kind of mechanism is of packet type identifiers on link layers. This kind of mechanism is
not necessary for ROHC. The packet type identification is not necessarily needed for ROHC since a ROHC packet type identifier
incorporated in the ROHC scheme and there is thus, no need for such a is included in all compressed ROHC headers. Only if ROHC packets are
mechanism in the link layer. If ROHC is used together with header to be mixed with other packets, such as packets compressed by other
compression schemes requiring packet type identification at the link header compression schemes, the link layer must provide a packet type
layer, e.g. [RFC2507, RFC2508], or if ROHC is used on top of link identifier. In such cases, or if ROHC is used on top of link layers
layers where packet type identifiers already are present, it is already providing packet type identification, one (1) packet type
RECOMMENDED that one (1) ROHC packet type identifier is supported on identifier must be reserved for identification of ROHC packets. Thus,
lower layers. Thus, only one ROHC packet type is needed to mix ROHC only one ROHC packet type is needed to mix ROHC and e.g. RFC2507
and e.g. RFC2507 flows or to support ROHC on links where packet type flows, or to support ROHC on links where packet type identifiers are
identifiers already are present. already present.
3.7 Packet duplication 2.7. Packet duplication
Exact duplications of one and the same packet may waste transmission Exact duplications of one and the same packet may waste transmission
resources and is in contradiction to compression. Even so, packet resources and is in contradiction to compression. Even so, packet
duplication may occur for various reasons. Packet duplication may duplication may occur for various reasons. Packet duplication may
also occur in different places along the path for a packet. also occur in different places along the path for a packet.
ROHC can handle packet duplication before the compressor but it is ROHC can handle packet duplication before the compressor but such
RECOMMENDED that such packet duplications are avoided. Lower layers packet duplications should be avoided for optimal compression
MUST NOT duplicate packets on the path between ROHC compressor and efficiency. For correct ROHC operation, lower layers are not allowed
decompressor. to duplicate packets on the ROHC compressor-decompressor path.
3.8 Packet reordering 2.8. Packet reordering
Lower layers between compressor and decompressor is not assumed to Lower layers between compressor and decompressor is assumed not to
reorder packets, i.e., the decompressor receives packets in the same reorder packets, i.e., the decompressor must receive packets in the
order as the compressor sends them. ROHC handles, however, reordering same order as the compressor sends them. ROHC handles, however,
before the compression point. That is, there is no assumption that reordering before the compression point. That is, there is no
the compressor will only receive packets in sequence. assumption that the compressor will only receive packets in sequence.
3.9 Feedback packets 2.9. Feedback packets
ROHC consist of three modes; Unidirectional mode (U-mode), ROHC may operate in three different modes; Unidirectional mode (U-
bidirectional optimistic mode (O-mode) and bidirectional reliable mode), bidirectional optimistic mode (O-mode) and bidirectional
mode (R-mode). A brief description of the modes can be found in reliable mode (R-mode). A brief description of the modes can be found
chapter 4.4 in [ROHC]. in chapter 4.4 of [RFC3095].
In U-mode it is not necessary to send any feedback from the In U-mode it is not necessary to send any feedback from the
decompressor to the compressor. O-mode and R-mode requires however decompressor to the compressor. O-mode and R-mode requires however
that feedback messages from the decompressor to the compressor may be that feedback messages from the decompressor to the compressor may be
sent. Feedback message consist of small ROHC internal packets without sent. Feedback message consist of small ROHC internal packets without
any application payload. It is possible in ROHC to piggy-back any application payload. It is possible in ROHC to piggy-back
feedback packets onto regular packets with ROHC compressed headers feedback packets onto regular packets with ROHC compressed headers
and payload, if there is ROHC type of compression in both the forward and payload, if there is ROHC type of compression in both the forward
and reverse direction. However, this piggy-backing may not be desired and reverse direction. However, this piggy-backing may not be desired
or possible in some cases. or possible in some cases.
Lower layer MUST support transport of feedback packets from To support ROHC O-mode or R-mode operation, lower layers must provide
decompressor to compressor if ROHC is to be used in O-mode or R-mode. transport of feedback packets from decompressor to compressor. If
Lower layers MUST support transport of small stand-alone feedback piggybacking of feedback packets is not used, lower layers must be
packets if piggybacking of feedback packets is not used. The feedback able to handle feedback as small stand-alone packets. For optimal
packets from the decompressor SHOULD be delivered as soon as possible compression efficiency, feedback packets from the decompressor should
to the compressor. be delivered as soon as possible to the compressor.
4. Cellular system specific guidelines 3. Cellular system specific guidelines
An important group of link layer technologies where robust header An important group of link layer technologies where robust header
compression will be needed are future cellular systems, which may compression will be needed are future cellular systems, which may
have a very large number of users in some years. The need for header 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 compression is large in these kinds of systems to achieve spectrum
efficiency. Hence, it is important that future cellular systems can efficiency. Hence, it is important that future cellular systems can
efficiently incorporate the robust header compression scheme. efficiently incorporate the robust header compression scheme.
4.1 Handover procedures 3.1. Handover procedures
One cellular specific property that may affect header compression is One cellular specific property that may affect header compression is
mobility and thus, handover (i.e., change of serving base station or mobility and thus, handover (i.e., change of serving base station or
radio network controller). radio network controller).
The main characteristics of handovers relevant for robust header The main characteristics of handovers relevant for robust header
compression are: the length of the longest packet loss event due to compression are: the length of the longest packet loss event due to
handover (i.e. the number of consecutive packet losses); relocation handover (i.e. the number of consecutive packet losses); relocation
of header compression context when necessary. of header compression context when necessary.
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the radio access network and the type of handover, handover may or the radio access network and the type of handover, handover may or
may not cause disruptions or packet loss events in the (compressed) may not cause disruptions or packet loss events in the (compressed)
header flow relevant for the header compression scheme. For instance, header flow relevant for the header compression scheme. For instance,
if soft handover is used and if the header compressor/decompressor if soft handover is used and if the header compressor/decompressor
reside above the combining point for soft handover, there will be no reside above the combining point for soft handover, there will be no
extra packet losses visible to the decompressor due to handover. In extra packet losses visible to the decompressor due to handover. In
hard handovers, where packet loss events due to handover is hard handovers, where packet loss events due to handover is
introduced, the length of the longest consecutive packet loss is most introduced, the length of the longest consecutive packet loss is most
relevant and should thus, be minimized. relevant and should thus, be minimized.
Handover SHOULD NOT cause significant long events of consecutive To maintain efficient ROHC operation, it should be ensured that
handover events do not cause significant long events of consecutive
packet loss. Significant in this context relates to the kind of loss packet loss. Significant in this context relates to the kind of loss
tolerable for the carried real-time application. tolerable for the carried real-time application.
If hard handovers are performed, which may cause significant long If hard handovers are performed, which may cause significant long
events of consecutive packet loss, it is RECOMMENDED that the radio events of consecutive packet loss, the radio access network should
access network notifies the compressor when such a handover has notify the compressor when such a handover has started and completed.
started and completed. The compressor could then be implemented to take proper actions and
prevent consequences from such long loss events.
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 Cellular systems supporting robust header compression may have
compressor that such a handover has occurred, so that it knows when internal mechanisms for transferring the header compression context
to refresh the context. Chapter 6.3 Implementation parameters and between nodes where contexts may reside, at or before handover. If no
signals in [ROHC] provide optional implementation parameters that such mechanism for transferring header compression context between
make it possible to trigger e.g. a complete context refresh. nodes is available, the contexts may be resynchronized by the header
compression scheme itself by means of a context refresh. The header
compressor will than 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. To reinitialize the context in such cases, the
lower layers must indicate to the header compressor when a handover
has occurred, so that it knows when to refresh the context. Chapter
6.3 "Implementation parameters" in [RFC3095] provide optional
implementation parameters that make it possible to trigger e.g. a
complete context refresh.
4.2 Unequal error detection 3.2. Unequal error detection
Chapter 3.1 states that ROHC requires error detection from lower Chapter 3.1 states that ROHC requires error detection from lower
layers for at least the compressed header. However, some cellular layers for at least the compressed header. However, some cellular
technologies may differentiate the amount of error detection for technologies may differentiate the amount of error detection for
different parts of a packet. For instance, it could be possible to 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 have a stronger error detection for the header part of a packet, if
the application payload part of the packet is less sensitive to the application payload part of the packet is less sensitive to
errors, e.g. some cellular types of speech codecs. errors, e.g. some cellular types of speech codecs.
ROHC does not require UED from lower layers, ROHC requires only a ROHC does not require UED from lower layers, ROHC requires only a
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payload part, any bit error will cause the packet to be discarded at payload part, any bit error will cause the packet to be discarded at
the ROHC decompressor. It is not possible to distinguish between the ROHC decompressor. It is not possible to distinguish between
errors in the header and the payload part of the packet with this errors in the header and the payload part of the packet with this
error detection mechanism and the ROHC decompressor must assume that 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 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 the packet. If the header is assumed to be damaged, it is not
possible to ensure correct decompression and that packet will thus be possible to ensure correct decompression and that packet will thus be
discarded. If the application is such that it tolerates some errors discarded. If the application is such that it tolerates some errors
in the payload, it could have been better to deliver that packet to in the payload, it could have been better to deliver that packet to
the application and let the application judge whether the payload was the application and let the application judge whether the payload was
usable or not. Hence, with an unequal error detection scheme where it usable or not. Hence, with a unequal error detection scheme where it
is possible to separate detection of errors in the header and payload is possible to separate detection of errors in the header and payload
part of a packet, more packets may be delivered to applications in part of a packet, more packets may be delivered to applications in
some cases for the same lower layer error rates. The final benefit some cases for the same lower layer error rates. The final benefit
depends of course on the cost of UED for the radio interface and depend of course on the cost of UED for the radio interface and
related protocols. related protocols.
4.3 Unequal error protection 3.3. Unequal error protection
Some cellular technologies can provide different error probabilities Some cellular technologies can provide different error probabilities
for different parts of a packet, unequal error protection (UEP). For for different parts of a packet, unequal error protection (UEP). For
instance, the lower layers may provide a stronger error protection instance, the lower layers may provide a stronger error protection
for the header part of a packet compared to the payload part of the for the header part of a packet compared to the payload part of the
packet. packet.
ROHC does not require UEP. UEP may be beneficial in some cases to 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 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. distinguish between errors in header and payload parts of a packets.
I.e. only if unequal error detection (UED) is used. The benefit of 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 UEP depend of course on the cost of UEP for the radio interface and
related protocols. related protocols.
5. IANA Considerations 4. IANA Considerations
A protocol which follows these guidelines, e.g., [ROHC], will require A protocol which follows these guidelines, e.g., [RFC3095], will
the IANA to assign various numbers. This document by itself, however, require the IANA to assign various numbers. This document by itself,
does not require IANA involvment. however, does not require IANA involvement.
6. Security Considerations 5. Security Considerations
A protocol which follows these guidelines, e.g., [ROHC], must be able A protocol which follows these guidelines, e.g., [RFC3095], must be
to compress packets containing IPSEC headers according to [REQ]. able to compress packets containing IPSEC headers according to
There may be other sequrity aspects to consider in such protocols. [RFC3096]. There may be other sequrity aspects to consider in such
This document by itself, however, does not add security risks. protocols. This document by itself, however, does not add security
risks.
7. Author's Addresses 6. Author's Addresses
Krister Svanbro Tel: +46 920 20 20 77 Krister Svanbro Tel: +46 920 20 20 77
Box 920 Fax: +46 920 20 20 99 Box 920 Fax: +46 920 20 20 99
Ericsson Erisoft AB Email: krister.svanbro@ericsson.com Ericsson Erisoft AB Email: krister.svanbro@ericsson.com
SE-971 28 Lulea, Sweden SE-971 28 Lulea, Sweden
8. References 7. References
[ROHC] Carsten Borman et al, "Robust Header Compression (ROHC)", [RFC3095] Carsten Borman et al, "Robust Header Compression (ROHC)",
Internet-Draft (work in progress), February 2001. RFC 3095, July 2001.
<draft-ietf-rohc-rtp-08.txt>
[REQ] Mikael Degermark, "Requirements for robust IP/UDP/RTP [RFC3096] Mikael Degermark, "Requirements for robust IP/UDP/RTP
header compression", Internet Draft (work in progress), header compression", RFC 3096, July 2001.
February 2001.
<draft-ietf-rohc-rtp-requirements-05.txt>
[RFC1144] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed [RFC1144] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990. Serial Links", RFC 1144, February 1990.
[RFC2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP [RFC2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP
Header Compression", RFC 2507, February 1999. Header Compression", RFC 2507, February 1999.
[RFC2508] Steven Casner, Van Jacobson, "Compressing IP/UDP/RTP [RFC2508] Steven Casner, Van Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February Headers for Low-Speed Serial Links", RFC 2508,
1999. February 1999.
[RFC2509] Mattias Engan, Steven Cassner, "IP Header Compression [RFC2509] Mattias Engan, Steven Casner, Carsten Bormann, "IP Header
over PPP", RFC2509, February 1999 Compression over PPP", RFC2509, February 1999
[RFC1661] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", [RFC1661] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994. STD 51, RFC 1661, July 1994.
This Internet-Draft expires in April 2001. Full copyright statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
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followed, or as required to translate it into languages other than
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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This Internet-Draft expires May 19, 2002.
 End of changes. 

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