Network Working Group L-E. Jonsson
Request for Comments: 3242 G. Pelletier
Category: Standards Track Ericsson
April 2002
RObust Header Compression (ROHC):
A Link-Layer Assisted Profile for IP/UDP/RTP
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a ROHC (Robust Header Compression) profile for
compression of IP/UDP/RTP (Internet Protocol/User Datagram
Protocol/Real-Time Transport Protocol) packets, utilizing
functionality provided by the lower layers to increase compression
efficiency by completely eliminating the header for most packets
during optimal operation. The profile is built as an extension to
the ROHC RTP profile. It defines additional mechanisms needed in
ROHC, states requirements on the assisting layer to guarantee
transparency, and specifies general logic for compression and
decompression making use of this header-free packet.
Table of Contents
1. Introduction....................................................2
2. Terminology.....................................................4
3. Overview of the Link-Layer Assisted Profile.....................5
3.1. Providing Packet Type Identification.....................6
3.2. Replacing the Sequence Number............................6
3.3. CRC Replacement..........................................7
3.4. Applicability of This Profile............................7
4. Additions and Exceptions Compared to ROHC RTP...................8
4.1. Additional Packet Types..................................8
4.1.1. No-Header Packet (NHP)..........................8
4.1.2. Context Synchronization Packet (CSP)............8
4.1.3. Context Check Packet (CCP)......................9
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4.2. Interfaces Towards the Assisting Layer..................11
4.2.1. Interface, Compressor to Assisting Layer.......11
4.2.2. Interface, Assisting Layer to Decompressor.....12
4.3. Optimistic Approach Agreement...........................13
4.4. Fast Context Initialization, IR Redefinition............13
4.5. Feedback Option, CV-REQUEST.............................14
4.6. Periodic Context Verification...........................15
4.7. Use of Context Identifier...............................15
5. Implementation Issues..........................................15
5.1. Implementation Parameters and Signals...................15
5.1.1. Implementation Parameters at the Compressor....16
5.1.2. Implementation Parameters at the Decompressor..17
5.2. Implementation over Various Link Technologies...........18
6. IANA Considerations............................................18
7. Security Considerations........................................18
8. Acknowledgements...............................................18
9. References.....................................................19
10. Authors' Addresses.............................................20
11. Full Copyright Statement.......................................21
1. Introduction
Header compression is a technique used to compress and transparently
decompress the header information of a packet on a per-hop basis,
utilizing redundancy within individual packets and between
consecutive packets within a packet stream. Over the years, several
protocols [VJHC, IPHC] have been developed to compress the network
and transport protocol headers [IPv4, IPv6, UDP, TCP], and these
schemes have been successful in improving efficiency over many wired
bottleneck links, such as modem connections over telephone networks.
In addition to IP, UDP, and TCP compression, an additional
compression scheme called Compressed RTP [CRTP] has been developed to
further improve compression efficiency for the case of real-time
traffic using the Real-Time Transport Protocol [RTP].
The schemes mentioned above have all been designed taking into
account normal assumptions about link characteristics, which
traditionally have been based on wired links only. However, with an
increasing number of wireless links in the Internet paths, these
assumptions are no longer generally valid. In wireless environments,
especially wide coverage cellular environments, relatively high error
rates are tolerated in order to allow efficient usage of the radio
resources. For real-time traffic, which is more sensitive to delays
than to errors, such operating conditions will be norm over, for
example, 3rd generation cellular links, and header compression must
therefore tolerate packet loss. However, with the previously
mentioned schemes, especially for real-time traffic compressed by
CRTP, high error rates have been shown to significantly degrade
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header compression performance [CRTPC]. This problem was the driving
force behind the creation of the RObust Header Compression (ROHC) WG
in the IETF.
The ROHC WG has developed a header compression framework on top of
which profiles can be defined for different protocol sets, or for
different compression strategies. Due to the limited packet loss
robustness of CRTP, and the demands of the cellular industry for an
efficient way of transporting voice over IP over wireless, the main
focus of ROHC has so far been on compression of IP/UDP/RTP headers,
which are generous in size, especially compared to the payloads often
carried by packets with such headers.
ROHC RTP has become a very efficient, robust and capable compression
scheme, able to compress the headers down to a total size of one
octet only. Also, transparency is guaranteed to an extremely great
extent even when residual bit errors are present in compressed
headers delivered to the decompressor. The requirements for RTP
compression [RTP-REQ], defined by the WG before and during the
development process, have thus been fulfilled.
As mentioned above, the 3rd generation cellular systems, where IP
will be used end-to-end, have been one of the driving forces behind
ROHC RTP, and the scheme has been designed to also suit new cellular
air interfaces, such as WCDMA, making it possible to run even speech
services with spectrum efficiency insignificantly lower than for
existing one-service circuit switched solutions [VTC2000]. However,
other air interfaces such as those based on GSM and IS-95 will also
be used in all-IP networks, with further implications for the header
compression issue. These older air interfaces are less flexible,
with radio bearers optimized for specific payload sizes. This means
that not even a single octet of header can be added without using the
next higher fixed packet size supported by the link, something which
is obviously very costly. For the already deployed speech vocoders,
the spectrum efficiency over these links will thus be low compared to
existing circuit switched solutions. To achieve high spectrum
efficiency overall with any application, more flexible air interfaces
must be deployed, and then the ROHC RTP scheme will perform
excellently, as shown for WCDMA [MOMUC01]. However, for deployment
reasons, it is however important to also provide a suitable header
compression strategy for already existing vocoders and air
interfaces, such as for GERAN and for CDMA2000, with minimal effects
on spectral efficiency.
This document defines a new link-layer assisted ROHC RTP profile
extending ROHC RTP (profile 0x0001) [ROHC], compliant with the ROHC
0-byte requirements [0B-REQ]. The purpose of this new profile is to
provide a header-free packet format that, for a certain application
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behavior, can replace a majority of the 1-octet header ROHC RTP
packets during normal U/O-mode operation, while still being fully
transparent and complying with all the requirements of ROHC RTP
[RTP-REQ]. For other applications, compression will be carried out
as with normal ROHC RTP.
To completely eliminate the compressed header, all functionality
normally provided by the 1-octet header has to be provided by other
means, typically by utilizing functionality provided by the lower
layers and sacrificing efficiency for less frequently occurring
larger compressed headers. The latter is not a contradiction since
the argument for eliminating the last octet for most packets is not
overall efficiency in general. It is important to remember that the
purpose of this profile is to provide efficient matching of existing
applications to existing link technologies, not efficiency in
general. The additional complexity introduced by this profile,
although minimized by a tight integration with already existing ROHC
functionality, implies that it should therefore only be used to
optimize performance of specific applications over specific links.
When implementing this profile over various link technologies, care
must be taken to guarantee that all the functionality needed is
provided by ROHC and the lower layers together. Therefore,
additional documents should specify how to incorporate this profile
on top of various link technologies.
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.
CCP Context Check Packet
CRC Cyclic Redundancy Check
CSP Context Synchronization Packet
LLA Link Layer Assisted ROHC RTP profile
NHP No Header Packet
ROHC RObust Header Compression
RHP ROHC Header Packet (a non-NHP packet, i.e., RRP, CSP or CCP)
RRP ROHC RTP Packet as defined in [ROHC, profile 0x0001]
Assisting layer
"Assisting layer" refers to any entity implementing the interface
to ROHC (section 4.2). It may, for example, refer to a sub-layer
used to adapt the ROHC implementation and the physical link layer.
This layer is assumed to have knowledge of the physical layer
synchronization.
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Compressing side
"Compressing side" refers to the combination of the header
compressor, operating with the LLA profile, and its associated
assisting layer.
Lower layers
"Lower layers" in this document refers to entities located below
ROHC in the protocol stack, including the assisting layer.
ROHC RTP
"ROHC RTP" in this document refers to the IP/UDP/RTP profile
(profile 0x0001) as defined in [ROHC].
3. Overview of the Link-Layer Assisted Profile
The ROHC IP/UDP/RTP profile defined in this document, profile 0x0005
(hex), is designed to be used over channels that have been optimized
for specific payload sizes and therefore cannot efficiently
accommodate header information when transmitted together with
payloads corresponding to these optimal sizes.
The LLA profile extends, and thus also inherits all functionality
from, the ROCH RTP profile by defining some additional functionality
and an interface from the ROHC component towards an assisting lower
layer.
+---------------------------------------+
| |
The LLA | ROHC RTP, |
profile | Profile #1 +-----------------+
| | LLA Additions |
+---------------------+-----------------+
By imposing additional requirements on the lower layers compared to
[ROHC], it is possible to infer the information needed to maintain
robust and transparent header compression even though the headers are
completely eliminated during most of the operation time.
Basically, what this profile does is to replace the smallest and most
frequent ROHC U/O-mode headers with a no-header format, for which the
header functionality must be provided by other means.
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Smallest header in Smallest header in
ROHC RTP (profile #1) LLA (profile #5)
+--+--+--+--+--+--+--+--+ ++
| 1 octet | -----> || No Header
+--+--+--+--+--+--+--+--+ ++
|
| Header field functionality
+-------------------> provided by other means
The fields present in the ROHC RTP headers for U/O-mode PT0 are the
packet type identifier, the sequence number and the CRC. The
subsequent sections elaborate more on how the functionality of these
fields is replaced for NHP.
3.1. Providing Packet Type Identification
All ROHC headers carry a packet type identifier, indicating to the
decompressor how the header should be interpreted. This is a
function that must be provided by some means in 0-byte header
compression. ROHC RTP packets with compressed headers will be
possible to distinguish thanks to the packet type identifier, but a
mechanism is needed to separate packets with a header from packets
without a header. This function MUST therefore be provided by the
assisting layer in one way or another.
3.2. Replacing the Sequence Number
From the sending application, the RTP sequence number is increased by
one for each packet sent. The purpose of the sequence number is to
cope with packet reordering and packet loss. If reordering or loss
has occurred before the transmission point, if needed the compressing
side can easily avoid problems by not allowing the use of a header-
free packet.
However, at the transmission point, loss or reordering that may occur
over the link can not be anticipated and covered for. Therefore, for
NHP the assisting layer MUST guarantee in-order delivery over the
link (already assumed by [ROHC]) and at the receiving side it MUST
provide an indication for each packet loss over the link. This is
basically the same principle as the VJ header compression [VJHC]
relies on.
Note that guaranteeing in-order delivery and packet loss indication
over the link not only makes it possible to infer the sequence number
information, but also supersedes the main function of the CRC, which
normally takes care of errors due to long link losses and bit errors
in the compressed sequence number.
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3.3. CRC Replacement
All context updating RRP packets carry a CRC calculated over the
uncompressed header. The CRC is used by the decompressor to verify
that the updated context is correct. This verification serves three
purposes in U/O-mode:
1) Detection of longer losses than can be covered by the sequence
number LSBs
2) Protection against failures caused by residual bit errors in
compressed headers
3) Protection against faulty implementations and other causes of
error
Since this profile defines an NHP packet without this CRC, care must
be taken to fulfill these purposes by other means, when an NHP is
used as a replacement for a context updating packet. Detection of
long losses (1) is already covered since the assisting layer MUST
provide indication of all packet losses. Furthermore, the NHP packet
has one important advantage over RHP packets in that residual bit
errors (2) cannot damage a header that is not even sent.
It is thus reasonable to assume that compression and decompression
transparency can be assured with high confidence even without a CRC
in header-free packets. However, to provide additional protection
against damage propagation due to undetected residual bit errors in
context updating packets (2) or other unexpected errors (3), periodic
context verifications SHOULD be performed (see section 4.6).
3.4. Applicability of This Profile
The LLA profile can be used with any link technology capable of
providing the required functionality described in previous sections.
Whether LLA or ROHC RTP should be implemented thus depends on the
characteristics of the link itself. For most RTP packet streams, LLA
will work exactly as ROHC RTP, while it will be more efficient for
packet streams with certain characteristics. LLA will never be less
efficient than ROHC RTP.
Note as well that LLA, like all other ROHC profiles, is fully
transparent to any packet stream reaching the compressor. LLA does
not make any assumptions about the packet stream but will perform
optimally for packet streams with certain characteristics, e.g.,
synchronized streams exactly timed with the assisting link over which
the LLA profile is implemented.
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The LLA profile is obviously not applicable if the UDP checksum (2
bytes) is enabled, which is always the case for IPv6/UDP. For
IPv4/UDP, the sender may choose to disable the UDP checksum.
4. Additions and Exceptions Compared to ROHC RTP
4.1. Additional Packet Types
The LLA profile defines three new packet types to be used in addition
to the RRP packet types defined by [ROHC]. The following sections
describe these packet types and their purpose in detail.
4.1.1. No-Header Packet (NHP)
A No-Header Packet (NHP), i.e., a packet consisting only of a
payload, is defined and MAY be used when only sequencing must be
conveyed, i.e., when all header fields are either unchanged or follow
the currently established change pattern. In addition, there are
some considerations for the use of the NHP (see 4.3, 4.5 and 4.6).
An LLA compressor is not allowed to deliver NHP packets when
operating in R-mode.
The assisting layer MAY send the NHP for RTP SN = X only if an NHP
was delivered by the LLA compressor AND the assisting layer can
guarantee that the decompressor will infer the proper sequencing for
this NHP. This guarantee is based on the confidence that the
decompressor
a) has the means to infer proper sequencing for the packet
corresponding to SN = X-1, AND
b) has either received a loss indication or the packet itself for the
packet corresponding to SN = X-1.
Updating properties: NHP packets update context (RTP Sequence
Number).
4.1.2. Context Synchronization Packet (CSP)
The case where the packet stream overruns the channel bandwidth may
lead to data being discarded, which may result in decompressor
context invalidation. It might therefore be beneficial to send a
packet with only the header information and discard the payload.
This would be helpful to maintain synchronization of the decompressor
context, while efficiently using the available bandwidth.
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This case can be handled with the Context Synchronization Packet
(CSP), which has the following format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 1 0 | Packet type identifier
+===+===+===+===+===+===+===+===+
: ROHC header without padding :
: see [ROHC, section 5.7] :
+---+---+---+---+---+---+---+---+
Updating properties: CSP maintains the updating properties of the
ROHC header it carries.
The CSP is defined by one of the unused packet type identifiers from
ROHC RTP, carried in the one-octet base header. As for any ROHC
packet, except the NHP, the packet may begin with ROHC padding and/or
feedback. It may also carry context identification after the packet
type identifier. It is possible to have two CID fields present, one
after the packet type ID and one within the encapsulated ROHC header.
If a decompressor receives a CSP with two non-equal CID values
included, the packet MUST be discarded. ROHC segmentation may also
be applied to the CSP.
Note that when the decompressor has received and processed a CSP, the
packet (including any possible data following the CSP encapsulated
compressed header) MUST be discarded.
4.1.3. Context Check Packet (CCP)
A Context Check Packet (CCP), which does not carry any payload but
only an optional CRC value in addition to the packet type identifier,
is defined.
The purpose of the CCP is to provide a useful packet that MAY be sent
by a synchronized physical link layer in the case where data must be
sent at fixed intervals, even if no compressed packet is available.
Whether the CCP is sent over the link and delivered to the
decompressor is decided by the assisting layer. The CCP has the
following format:
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 1 1 | Packet type identifier
+===+===+===+===+===+===+===+===+
| C | CRC |
+---+---+---+---+---+---+---+---+
C: C = 0 indicates that the CRC field is not used;
C = 1 indicates that a valid CRC is present.
Updating properties: CCP packets do not update context.
The CCP is defined by one of the unused packet type identifiers from
ROHC RTP, carried in the first octet of the base header. The first
bit of the second octet, the C bit, indicates whether the CRC field
is used or not. If C=1, the CRC field MUST be set to the 7-bits CRC
calculated over the original uncompressed header defined in [ROHC
section 5.9.2]. As for any ROHC packet, except NHP, the packet MAY
begin with ROHC padding and/or carry context identification.
The use of the CRC field to perform decompressor context verification
is optional and is therefore a compressor implementation issue.
However, a CCP MUST always be made available to the assisting layer.
If the assisting layer receives CCPs with the C-bit set (C=1) from
the compressor, it MUST use the last CCP received if a CCP is to be
sent, i.e., the CCP corresponding to the last non-CCP packet sent
(NHP, RRP or CSP). An assisting layer MAY use the CCP for other
purposes, such as signaling a packet loss before the link.
The decompressor is REQUIRED to handle a CCP received with the C bit
set (C=1), indicating a valid CRC field, and perform context
verification. The received CRC MUST then be applied to the last
decompressed packet, unless a packet loss indication was previously
received. Upon CRC failure, actions MUST be taken as specified in
[ROHC, section 5.3.2.2.3]. A CCP received with C=0 MUST be ignored
by the decompressor. The decompressor is not allowed to make any
further interpretation of the CCP.
The use of CCP by an assisting layer is optional and depends on the
characteristics of the actual link. Whether it is used or not MUST
therefore be specified in link layer implementation specifications
for this profile.
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4.2. Interfaces Towards the Assisting Layer
This profile relies on the lower layers to provide the necessary
functionality to allow NHP packets to be sent. This interaction
between LLA and the assisting layer is defined as interfaces between
the LLA compressor/decompressor and the LLA applicable link
technology.
| |
+ +
+-------------------------+ +-------------------------+
| ROHC RTP HC | | ROHC RTP HD |
+-------------------------+ +-------------------------+
| LLA profile | | LLA profile |
+=========================+ +=========================+
| Interface | | Interface |
| ROHC to assisting layer | | Assisting layer to ROHC |
+=========================+ +=========================+
| Applicable | | Applicable |
| link technology | | link technology |
+=========================+ +=========================+
| |
+------>---- CHANNEL ---->-----+
The figure above shows the various levels, as defined in [ROHC] and
this document, constituting a complete implementation of the LLA
profile. The figure also underlines the need for additional
documents to specify how to implement these interfaces for a link
technology for which this profile is relevant.
This section defines the information to be exchanged between the LLA
compressor and the assisting layer for this profile to operate
properly. While it does define semantics, it does not specify how
these interfaces are to be implemented.
4.2.1. Interface, Compressor to Assisting Layer
This section defines the interface semantics between the compressor
and the assisting layer, providing rules for packet delivery from the
compressor.
The interface defines the following parameters: RRP, RRP segmentation
flag, CSP, CSP segmentation flag, NHP, and RTP Sequence Number. All
parameters, except the NHP, MUST always be delivered to the assisting
layer. This leads to two possible delivery scenarios:
a. RRP, CSP, CCP, NHP and RTP Sequence Number are delivered, along
with the corresponding segmentation flags set accordingly.
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This corresponds to the case when the compressor allows sending of
an NHP packet, with or without segmentation being applied to the
corresponding RRP/CSP packets.
Recall that delivery of an NHP packet occurs when the ROHC RTP
compressor would have used a ROHC UO-0.
b. RRP, CSP, CCP and RTP Sequence Number are delivered, along with
the corresponding segmentation flags set accordingly.
This corresponds to the case when the compressor does not allow
sending of an NHP packet. Segmentation might be applied to the
corresponding RRP and CSP packets.
Segmentation may be applied independently to an RRP or a CSP packet
if its size exceeds the largest value provided in the PREFERRED
PACKET_SIZES list and if the LARGE_PACKET_ALLOWED parameter is set to
false. The segmentation flags are explicitly stated in the interface
definition to emphasize that the RRP and the CSP may be delivered by
the compressor as segmented packets.
The RTP SN MUST be delivered for each packet by the compressor to
allow the assisting layer to maintain the necessary sequencing
information.
4.2.2. Interface, Assisting Layer to Decompressor
Here the interface semantics between the assisting layer and the
decompressor are defined, providing simple rules for the delivery of
received packets to the decompressor. The decompressor needs a way
to distinguish NHP packets from RHP packets. Also, when receiving
packets without a header, the decompressor needs a way to infer the
sequencing information to keep synchronization between the received
payload and the sequence information of the decompressed headers. To
achieve this, the decompressor MUST receive the following from the
assisting layer:
- an indication for each packet loss over the link between the
compressing and decompressing sides for CID=0
- the received packet together with an indication whether the packet
received is an NHP or not
Note that the context is updated from a packet loss indication.
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4.3. Optimistic Approach Agreement
ROHC defines an optimistic approach for updates to reduce the header
overhead. This approach is fully exploited in the Optimistic and
Unidirectional modes of operation. Due to the presence of a CRC in
all compressed headers, the optimistic approach is defined as a
compressor issue only because the decompressor will always be able to
detect an invalid context through the CRC verification.
However, no CRC is present in the NHP packet defined by the LLA
profile. Therefore the loss of an RHP packet updating the context
may not always be detected. To avoid this problem, the compressing
and decompressing sides must agree on the principles for the
optimistic approach, and the agreed principles MUST be enforced not
only by the compressor but also by the transmitting assisting layer.
If, for example, three consecutive updates are sent to convey a
header field change, the decompressor must know this and invalidate
the context in case of three or more consecutive physical packet
losses. Note that the mechanism used to enforce the optimistic
approach must be reinitialized if a new field change needs to be
conveyed while the compressing side is already sending packets to
convey non-linear context updates.
An LLA decompressor MUST use the optimistic approach knowledge to
detect possible context loss events. If context loss is suspected it
MUST invalidate the context and not forward any packets before the
context has been synchronized.
It is REQUIRED that all documents, describing how the LLA profile is
implemented over a certain link technology, define how the optimistic
approach is agreed between the compressing side and the decompressing
side. It could be handled with a fixed principle, negotiation at
startup, or by other means, but the method must be unambiguously
defined.
4.4. Fast Context Initialization, IR Redefinition
As initial IR packets might overrun the channel bandwidth and
significantly delay decompressor context establishment, it might be
beneficial to initially discard the payload. This allows state
transitions and higher compression efficiency to be achieved with
minimal delay.
To serve this purpose, the D-bit from the basic structure of the ROHC
RTP IR packet [ROHC section 5.7.7.1] is redefined for the LLA
profile. For D=0 (no dynamic chain), the meaning of the D-bit is
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extended to indicate that the payload has been discarded when
assembling the IR packet. All other fields keep their meanings as
defined for ROHC RTP.
The resulting structure, using small CIDs and CID=0, becomes:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 | 1 | 1 | 1 | 1 | 0 | D |
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| Static | variable length
| chain |
- - - - - - - - - - - - - - - -
| Dynamic | not present if D = 0
| chain | present if D = 1, variable length
- - - - - - - - - - - - - - - -
| Payload | not present if D = 0
| | present if D = 1, variable length
- - - - - - - - - - - - - - - -
D: D = 0 indicates that the dynamic chain is not present
and the payload has been discarded.
After an IR packet with D=0 has been processed by the decompressor,
the packet MUST be discarded.
4.5. Feedback Option, CV-REQUEST
The CV-REQUEST option MAY be used by the decompressor to request an
RRP or CSP for context verification. This option should be used if
only NHP have been received for a long time and the context therefore
has not been verified recently.
+---+---+---+---+---+---+---+---+
| Opt Type = 8 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
If the compressor receives a feedback packet with this option, the
next packet compressed SHOULD NOT be delivered to the assisting layer
as an NHP.
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4.6. Periodic Context Verification
As described in section 3.3, transparency is expected to be
guaranteed by the functionality provided by the lower layers. This
ROHC profile would therefore be at least as reliable as the older
header compression schemes [VJHC, IPHC, CRTP], which do not make use
of a header compression CRC. However, since ROHC RTP normally is
extremely safe to use from a transparency point of view, it would be
desirable to be able to achieve this with LLA also.
To provide an additional guarantee for transparency and also catch
unexpected errors, such as errors due to faulty implementations, it
is RECOMMENDED to periodically send context updating packets, even
when the compressor logic allows NHP packets to be used.
4.7. Use of Context Identifier
Since an NHP cannot carry a context identifier (CID), there is a
restriction on how this profile may be used, related to context
identification. Independent of which CID size has been negotiated,
NHP packets can only be used for CID=0. If the decompressor receives
an NHP packet, it can only belong to CID=0.
Note that if multiple packet streams are handled by a compressor
operating using LLA, the assisting layer must in case of physical
packet loss be able to tell for which CID the loss occurred, or at
least it MUST be able to tell if packets with CID=0 (packet stream
with NHPs) have been lost.
5. Implementation Issues
This document specifies mechanisms for the protocol and leaves
details on the use of these mechanisms to the implementers. The
present chapter aims to provide guidelines, ideas and suggestions for
implementation of LLA.
5.1. Implementation Parameters and Signals
As described in [ROHC, section 6.3], implementations use parameters
to set up configuration information and to stipulate how a ROHC
implementation is to operate. The following parameters are
additions, useful to LLA, to the parameter set defined for ROHC RTP
implementations. Note that if the PREFERRED_PACKET_SIZES parameters
defined here are used, they obsolete all PACKET_SIZE and PAYLOAD_SIZE
parameters of ROHC RTP.
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5.1.1. Implementation Parameters at the Compressor
ALWAYS_PAD -- value: boolean
This parameter may be set by an external entity to specify to the
compressor that every RHP packet MUST be padded with a minimum of
one octet ROHC padding.
The assisting layer MUST provide a packet type identification. If
no field is available for this purpose from the protocol at the
link layer, then a leading sequence may be used to distinguish RHP
packets from NHP packets. Although the use of a leading sequence
is obviously not efficient, since it sacrifices efficiency for RHP
packets, the efficiency loss should be insignificant because the
leading sequence applies only to packets with headers in order to
favor the use of packets without headers. If a leading sequence
is desired for RHP identification, the lower layer MAY use ROHC
padding for the leading sequence by setting the ALWAYS_PAD
parameter. Note that in such cases, possible collisions of the
padding with the NHP payload must be avoided.
By default, this parameter is set to FALSE.
PREFERRED_PACKET_SIZES -- list of:
SIZE -- value: integer (octets)
RESTRICTED_TYPE -- values: [NHP_ONLY, RHP_ONLY, NO_RESTRICTION]
This parameter set governs which packet sizes are preferred by the
assisting layer. If this parameter set is used, all RHP packets
MUST be padded to fit the smallest possible preferred size. If
the size of the unpadded packet (or, in the case of ALWAYS_PAD
being set, the packet with minimal one octet padding) is larger
than the maximal preferred packet size, the compressor has two
options. Either, it may deliver this larger packet with an
arbitrary size, or it may split the packet into several segments
using ROHC segmentation and pad each segment to one of the
preferred sizes. Which method to use depends on the value of the
LARGE_PACKETS_ALLOWED parameter below.
NHP packets can be delivered to the lower layer only if the
payload size is part of the preferred packet size set.
Furthermore, if RESTRICTED_TYPE is set to one of NHP_ONLY or
RHP_ONLY for any of the preferred packet sizes, that size is
allowed only for packets of the specified type.
By default, no preferred packet sizes are specified. When sizes
are specified, the default value for RESTRICTED_TYPE is
NO_RESTRICTION.
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LARGE_PACKETS_ALLOWED -- value: boolean
This parameter may be set by an external entity to specify how to
handle packets that do not fit any of the preferred packet sizes
specified. If it is set to TRUE, the compressor MUST deliver the
larger packet as-is and MUST NOT use segmentation. If it is set
to FALSE, the ROHC segmentation scheme MUST be used to split the
packet into two or more segments, and each segment MUST further be
padded to fit one of the preferred packet sizes.
By default, this parameter is set to TRUE, which means that
segmentation is disabled.
VERIFICATION_PERIOD -- value: integer
This parameter may be set by an external entity to specify to the
compressor the minimum frequency with which a packet validating
the context must be sent. This tells the compressor that a packet
containing a CRC field MUST be sent at least once every N packets,
where N=VERIFICATION_PERIOD (see section 4.6).
By default, this parameter is set to 0, which indicates that
periodical verifications are disabled.
5.1.2. Implementation Parameters at the Decompressor
NHP_PACKET -- value: boolean
This parameter informs the decompressor that the packet being
delivered is an NHP packet. The decompressor MUST accept this
packet type indicator from the lower layer. An assisting layer
MUST set this indicator to true for every NHP packet delivered,
and to false for any other packet.
PHYSICAL_PACKET_LOSS -- signal
This signal indicates to the decompressor that a packet has been
lost on the link between the compressing and the decompressing
sides, due to a physical link error. The signal is given once for
each packet that was lost, and a decompressor must increase the
sequence number accordingly when this signal is received.
PRE_LINK_PACKET_LOSS -- signal
This signal tells the decompressor to increase the sequence number
due to a gap in the sequencing, not related to a physical link
error. A receiving assisting layer may for example use this
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RFC 3242 A Link-Layer Assisted ROHC RTP April 2002
signal to indicate to the decompressor that a packet was lost
before the compressor, or that a packet was discarded by the
transmitting assisting layer.
5.2. Implementation over Various Link Technologies
This document provides the semantics and requirements of the
interface needed from the ROHC compressor and decompressor towards
the assisting layer to perform link-layer assisted header
compression.
However, this document does not provide any link layer specific
operational information, except for some implementation suggestions.
Further details about how this profile is to be implemented over
various link technologies must be described in other documents, where
specific characteristics of each link layer can be taken into account
to provide optimal usage of this profile.
These specifications MAY use a packet type bit pattern unused by this
profile to implement signaling on the lower layer. The pattern
available to lower layer implementations is [11111001].
6. IANA Considerations
ROHC profile identifier 0x0005 has been reserved by the IANA for the
IP/UDP/RTP profile defined in this document.
7. Security Considerations
The security considerations of ROHC RTP [ROHC section 7] apply also
to this document with one addition: in the case of a denial-of-
service attack scenario where an intruder injects bogus CCP packets
onto the link using random CRC values, the CRC check will fail for
incorrect reasons at the decompressor side. This would obviously
greatly reduce the advantages of ROHC and any extra efficiency
provided by this profile due to unnecessary context invalidation,
feedback messages and refresh packets. However, the same remarks
related to the presence of such an intruder apply.
8. Acknowledgements
The authors would like to thank Lila Madour, Ulises Olvera-Hernandez
and Francis Lupien for input regarding the typical links in which LLA
can be applied. Thanks also to Mikael Degermark for fruitful
discussions that led to improvements of this profile, and to Zhigang
Liu for many valuable comments.
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9. References
[ROHC] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
H., Hannu, H., Jonsson, L., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP,
UDP, ESP, and uncompressed", RFC 3095, July 2001.
[IPv4] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[UDP] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RTP] Schulzrinne, H., Casner S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[TCP] Postel, P., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RTP-REQ] Degermark, M., "Requirements for IP/UDP/RTP Header
Compression", RFC 3096, July 2001.
[0B-REQ] Jonsson, L-E., "RObust Header Compression (ROHC):
Requirements and Assumptions for 0-byte IP/UDP/RTP
Compression", RFC 3243, April 2002.
[VJHC] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[IPHC] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[CRTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[CRTPC] Degermark, M., Hannu, H., Jonsson, L-E. and K. Svanbro,
"Evaluation of CRTP Performance over Cellular Radio
Networks", IEEE Personal Communications Magazine, Volume
7, number 4, pp. 20-25, August 2000.
Jonsson, et. al Standards Track [Page 19]
RFC 3242 A Link-Layer Assisted ROHC RTP April 2002
[VTC2000] Svanbro, K., Hannu, H., Jonsson, L-E. and M. Degermark,
"Wireless real time IP-services enabled by header
compression", proceedings of IEEE VTC2000, May 2000.
[MOMUC01] Liu, G., et al., "Experimental field trials results of
Voice-over IP over WCDMA links", MoMuC'01 - The
International Workshop on Mobile Multimedia
Communications, Conference proceedings, February 2001.
10. Authors' Addresses
Lars-Erik Jonsson
Ericsson AB
Box 920
SE-971 28 Lulea
Sweden
Phone: +46 920 20 21 07
Fax: +46 920 20 20 99
EMail: lars-erik.jonsson@ericsson.com
Ghyslain Pelletier
Ericsson AB
Box 920
SE-971 28 Lulea
Sweden
Phone: +46 920 20 24 32
Fax: +46 920 20 20 99
EMail: ghyslain.pelletier@epl.ericsson.se
Jonsson, et. al Standards Track [Page 20]
RFC 3242 A Link-Layer Assisted ROHC RTP April 2002
11. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Jonsson, et. al Standards Track [Page 21]