Network Working Group Lars-Erik Jonsson
INTERNET-DRAFT Ghyslain Pelletier
Expires: February 2002 Ericsson
August 15, 2001
A Link-Layer Assisted ROHC Profile for IP/UDP/RTP
<draft-ietf-rohc-rtp-lla-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 defines a ROHC profile for compression of IP/UDP/RTP
packets, utilizing functionality provided by the lower layers to
increase compression efficiency by completely eliminating the header
for most packets during normal operation. The profile is built as an
extension to the ROHC RTP profile [ROHC]. 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.
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Table of contents
1. Introduction....................................................3
2. Terminology.....................................................5
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
4.2. Interfaces towards the assisting layer..................10
4.2.1. Compressor to assisting layer interface........10
4.2.2. Assisting layer to decompressor interface......11
4.3. Optimistic approach agreement (U/O-mode)................12
4.4. NHP and the secure reference principle (R-mode).........12
4.5. Fast context initialization, IR redefinition............13
4.6. Feedback option, RHP-REQUEST............................14
4.7. Periodic context verification...........................14
4.8. Use of context identifier...............................14
5. Implementation issues..........................................15
5.1. Implementation parameters and signals...................15
5.1.1. Implementation parameters at the compressor....15
5.1.2. Implementation parameters at the decompressor..16
5.2. Implementation structures...............................17
5.2.1. Compressor context.............................17
5.2.2. Decompressor context...........................17
5.3. Implementation over various link technologies...........17
6. IANA considerations............................................18
7. Security considerations........................................18
8. Acknowledgements...............................................18
9. References.....................................................18
10. Author's addresses.............................................19
11. Full copyright statement.......................................20
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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 at 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 not valid as general anymore. In wireless
environments, especially wide coverage cellular environments, the
error rates are relatively high to provide efficient usage of the
radio resources. For real-time traffic, which is more sensitive to
delays than to errors, this will be normal operating conditions over
links such as the 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
reduce header compression performance [CRTPC]. This problem was the
driving force for 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 various profiles can be defined for different protocol sets, or
for different compression strategies. Due to the packet loss
robustness problems 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 the packets with these 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 high
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, has thus been fulfilled.
As mentioned above, the 3rd generation cellular systems, where IP
will be used end-to-end, has been one of the driving forces for ROHC
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RTP and the scheme has been designed to also suit new cellular air
interfaces, such as WCDMA, making even speech services possible with
an insignificantly lower spectrum efficiency than with existing one-
service circuit switched solutions [VTC2000]. However, other existing
air interfaces such as GSM and IS-95 will be used in all-IP networks,
adding new implications to the header compression issue. These 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
of the link and that is obviously very costly. For the already
deployed speech vocoders, the spectrum efficiency over these links
will thus be low compared to the 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 be excellent, as shown for WCDMA [MOMUC01]. For
deployment reasons, it is however important to also achieve
efficiency with already existing vocoders and air interfaces, such as
GSM and IS-95, with minimal effects on spectral efficiency.
This document defines a new link-layer assisted ROHC RTP profile
extending ROHC RTP (profile #1) [ROHC], compliant to 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 behavior,
can replace a majority of the 1-octet header ROHC RTP packets during
normal operation, while still being fully transparent and comply 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.
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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 1]
Assisting layer
Assisting layer refers to any entity implementing the
interface to ROHC as defined in section 4.2. It may, as an
example, refer to a sub-layer used to adapt the ROHC implementation
and the physical link 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 #1) as defined in [ROHC].
3. Overview of the link-layer assisted profile
This ROHC IP/UDP/RTP profile 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 ROHC | ROHC RTP, |
profile | Profile #1 +-----------------+
| | LLA Additions |
+---------------------+-----------------+
The LLA profile extends, thus also inherits all functionality from,
the ROCH RTP profile by defining some additional functionality and an
interface between the ROHC component towards the assisting layer.
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By putting 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 ROHC
header of one octet with a no-header format by providing the header
functionality by other means.
Smallest header in Smallest header in
ROHC RTP (profile #1) LLA ROHC RTP profile
+--+--+--+--+--+--+--+--+ ++
| 1 octet | -----> || No Header
+--+--+--+--+--+--+--+--+ ++
|
| Header field functionality
+-------------------> provided by other means
The fields present in the one octet ROHC RTP header for PT0 are the
packet type identifier, the sequence number and the CRC (U/O-mode).
The subsequent sections elaborate more on the replacement of the
functionality of these fields.
3.1. Providing packet type identification
All ROHC headers carry a packet type identifier, indicating to the
decompressor how it should be interpreted. This is a functionality
that must be provided by some means. ROHC RTP packets with compressed
header will still be possible to distinguish between since they have
this identifier, but a mechanism to separate those packets with
header from the packets without header is needed. This functionality
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 thus
to cope with packet reordering and packet loss. If reordering or loss
has occurred before the compression point, the compressor can easily
avoid problems by not allowing usage of a header-free packet.
However, the compressor can not in beforehand anticipate loss or
reordering that may occur between compressor and decompressor.
Therefore, the assisting layer MUST guarantee in-order delivery
(already assumed by [ROHC]) and it MUST provide an indication for
each packet loss over the link. This is basically the same principle
as VJ header compression [VJHC] relies on.
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Note that guarantees for in-order delivery and packet loss indication
not only makes it possible to infer the sequence number information,
it also supersedes the main functionality of the CRC, which normally
takes care of errors due to long losses and bit errors in the
compressed sequence number.
3.3. CRC replacement
Most RRP packets carry a CRC calculated over the uncompressed header.
This CRC is used by the decompressor to verify that the decompressed
header is correct. This verification serves three purposes:
- Detection of longer losses than can be covered by the sequence
number LSBs (this applies to U/O-mode only)
- Protection against failures caused by residual bit errors in the
compressed header
- Protection against faulty implementations or 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. Detection of long
losses is already covered since the assisting layer MUST provide
indication of all packet losses. Furthermore, the NHP packet has one
important advantage compared to RHP packets because residual bit
errors can not damage a header that is not even sent.
It is thus reasonable to assume that compression and decompression
transparency can be guaranteed even without a CRC in header-free
packets. However, to detect also unexpected errors and thereby
provide transparency in the ROHC class, periodical context
verifications SHOULD be performed (see section 4.7).
3.4. Applicability of this profile
The LLA profile can be used on any link technology capable of
providing the necessary required functionality described in previous
sections. Whether LLA ROHC RTP 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 produce
optimal performance for packet streams with certain characteristics,
e.g. synchronized streams exactly matching the timing of the
assisting link over which the LLA profile is implemented.
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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), thus a packet consisting only of a payload,
is defined and MAY be used instead of ROHC RTP packet type 0 (PT0).
Note that the requirement for using PT0 in the first place is
basically that all header fields must be unchanged or follow the
currently established change pattern. In addition, there are some
considerations for the use of the NHP (see 4.3, 4.4, 4.6 and 4.7).
The NHP packet, if delivered by the LLA compressor, can only be sent
by the assisting layer if the corresponding RTP Sequence Number for
this packet has incremented by one from the value for the packet
previously sent. Otherwise, the RHP or CSP MUST be sent.
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 only the payload.
This would allow for both context verification and context
synchronization at the decompressor, while still saving bandwidth.
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 0 1 | Packet type identifier
+---+---+---+---+---+---+---+---+
| 1 | CRC |
+---+---+---+---+---+---+---+---+
: ROHC header without padding :
: or context identification :
+---+---+---+---+---+---+---+---+
This packet is defined by one of the unused packet type identifiers
from ROHC RTP, carried in the first octet of the base header, and the
first bit of the following octet being set to 1. As for any ROHC
packet, except NHP, the packet MAY begin with ROHC padding and/or
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carry context identification. ROHC segmentation might also be applied
to CSP packets. The CRC used in the CSP packet is the 7-bits CRC over
the original uncompressed header defined in [ROHC section 5.9.2].
Note that when the decompressor has received a CSP packet and updated
the context accordingly, 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 a CRC value in addition to the packet type identifier, is
defined. The CCP packet MAY be created by the assisting layer from
the CSP packet provided by the compressor. Whether the packet is sent
over the link and delivered to the decompressor is therefore decided
by the assisting layer.
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.
The CCP has the following format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 1 | Packet type identifier
+---+---+---+---+---+---+---+---+
| 0 | CRC |
+---+---+---+---+---+---+---+---+
The CPP packet is identified using the same packet type identifier as
the CSP packet with the first bit of the second octet being set to 0,
and it uses the same CRC. The two octets of the CPP packet are thus
identical to the first two octets of the CSP packet, except for the
first bit of the second octet being set to 0 instead of 1.
The difference of the CPP from the CSP is the absence of a compressed
ROHC header. This packet can therefore only be used by the
decompressor to perform context verification. An assisting layer MAY
use the CCP packet to signal a packet loss before the link.
If the assisting layer implementation makes use of the CCP, the last
CCP packet received from the compressor MUST always be used, i.e. the
CCP corresponding to the last non-CCP packet sent (NHP, RRP or CSP).
Accordingly, if a CCP packet is received by the decompressor and used
for context verification, it MUST be for the last packet decompressed
unless a packet loss indication was previously received. The 7-bit
CRC MUST always be calculated for all decompressed packets and saved
in the decompressor context in order to use the CCP. Upon CRC
failure, actions MUST be taken as specified in [ROHC, section
5.3.2.2.3].
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Note that the usage of CCP 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.
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 ROHC LLA compressor/decompressor and the LLA applicable link
technology. The figure below shows the various levels, as defined in
[ROHC] and this document, making up for a complete implementation of
the LLA profile.
| |
+ +
+-------------------------+ +-------------------------+
| ROHC RTP HC | | ROHC RTP HD |
+-------------------------+ +-------------------------+
| LLA profile | | LLA profile |
+=========================+ +=========================+
| ROHC to assisting layer | | Assisting layer to ROHC |
| interface | | interface |
+=========================+ +=========================+
| Applicable | | Applicable |
| link technology | | link technology |
+=========================+ +=========================+
| |
+------>---- CHANNEL ---->-----+
The figure also underline the need for additional documents to
specify how to fulfill these interfaces for a link technology for
which this profile is relevant.
4.2.1. Compressor to assisting layer interface
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, NHP and RTP Sequence Number are delivered, along with
the corresponding segmentation flags accordingly set.
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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 PT0.
b. RRP, CSP and RTP Sequence Number are delivered, along with the
corresponding segmentation flags accordingly set.
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.
Note that the CCP can always be inferred from the CSP, which in turn
must always be delivered by the compressor. The CCP is not required
to be provided by the compressor and therefore does not appear in the
list of required interface parameters.
4.2.2. Assisting layer to decompressor interface
The interface semantics between the assisting layer and the
decompressor are defined here, and provide simple rules for the
delivery of received packets to the decompressor. The decompressor
needs a way to identify NHP packets from RHP packets. Also, when
receiving packets without header, the decompressor needs a way to
infer the sequencing information to keep synchronization between
received payload and the sequence information of the decompressed
headers. To achieve this, the assisting layer MUST provide the
following to the decompressor:
- an indication for each packet loss for CID=0
- the received packet together with a indication whether the packet
received is an NHP or not
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4.3. Optimistic approach agreement (U/O-mode)
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 check.
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 compressor and
decompressor must agree on the principles for the optimistic
approach. If, for example, the compressor sends three consecutive
updates to convey a header field change, the decompressor must know
this and invalidate the context in case of three or more consecutive
packet losses.
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 a
context synchronization event has happened.
It is REQUIRED that all documents describing how the LLA profile is
implemented over a certain link technology MUST define how the
optimistic approach is agreed between compressor and decompressor. It
could be with a fixed principle, negotiation at startup or by other
means but it must be unambiguously defined.
4.4. NHP and the secure reference principle (R-mode)
Similar to the optimistic approach used for U/O-mode, R-mode relies
on the secure reference principle. In parallel, the NHP packet
introduces some additional considerations also in relation to the
secure reference principle.
At the decompression side, the secure reference principle [ROHC,
section 5.5] states that only packets carrying a 7- or 8-bit CRC can
update the context and be used for decompression of subsequent
packets. This rule is not in any way modified by LLA.
However, to be able to decompress subsequent NHP packets, the
decompressor must keep a counter for the number of received non-
updating packets following the established linear pattern (NHP or R-
0). The application of this counter to the context defines a pseudo-
context. This pseudo-context is used to decompress NHPs, while
subsequent updating packets are decompressed based on the context, as
usual.
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Since the NHP packets do not update the decompressor context, the
compressor sliding windows can obviously not be updated with values
corresponding to these packets. The compressor must therefore also
keep a counter for the number of non-updating packets sent following
the established linear pattern. NHP transmission is thus allowed if
PT0 would normally be allowed, not only based on the context but also
based on the pseudo-context.
4.5. 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. The meaning of the D bit for D=0 (no dynamic chain) is
extended to indicate that the payload has been discarded when
assembling the IR packet. All other fields keep their meaning 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.
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4.6. Feedback option, RHP-REQUEST
The RHP-REQUEST option MAY be used by the decompressor to request an
RHP 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. If the compressor receives a feedback packet
with this option, the sending of an RRP with CRC SHOULD be triggered
immediately.
+---+---+---+---+---+---+---+---+
| Opt Type = 8 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
4.7. 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 if that also could be achieved with this profile.
To provide an additional guarantee for transparency and also catch
non expected errors, such as errors due to faulty implementations, it
is RECOMMENDED that RRP packets (with the CRC present also for R-
mode PT0 packets) be sent periodically (possibly with an increasing
period), even when the compressor logic allows for NHP packets to be
used.
4.8. Use of context identifier
Since a NHP can not 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
a NHP packet, it can only belong to CID=0.
Note that if multiple packet streams are handled by a compressor
running LLA, the assisting layer MUST in case of packet loss be able
to tell for which CID the loss occurred, at least it must be able to
tell if packets with CID=0 (packet stream with NHPs) have been lost.
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5. Implementation Issues
This document specifies mechanisms for the protocol and leaves
details on the use of these mechanisms to the implementers. This
chapter aims to provide guidelines, ideas and suggestions for
implementation of this profile.
5.1. Implementation parameters and signals
As described in [ROHC, section 6.3], implementations uses parameters
to set up configuration information and to stipulate how a ROHC
implementation is to operate. The following are additions to the ones
used by ROHC RTP implementations, needed by this profile. Note that
if the PREFERRED_PACKET_SIZES parameters defined here are used, they
obsolete all PACKET_SIZE and PAYLOAD_SIZE parameters of ROHC RTP.
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 using the 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 identify RHP
packets from NHP packets. Although the use of a leading sequence
is obviously not efficient since it sacrifices efficiency for RHP
packets, this 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 this by setting the ALWAYS_PAD
parameter.
By default, this parameter is set to FALSE.
PREFERRED PACKET SIZES -- list of: SIZE -- value: integer (octets)
ONLY_NHP -- value: boolean
This parameter set governs which packet sizes that 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. It may either 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
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preferred sizes. Which method to use depends on the value of the
LARGE_PACKETS_ALLOWED parameter below.
NHP packets can only be delivered to the lower layer if the
payload size is part of the preferred packet size set.
Furthermore, if ONLY_NHP is set to TRUE for any of the preferred
packet sizes, that size is only allowed to be used for NHP
packets.
By default, no preferred packet sizes are specified and when used
the default value of ONLY_NHP is FALSE for the specified sizes.
LARGE_PACKETS_ALLOWED -- value: boolean
This parameter may be set by an external entity to specify how to
handle packets that can not fit in any of the preferred packet
sizes specified. If set to TRUE, the compressor MUST deliver the
larger packet as it is and not use segmentation. If 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 into any of the preferred packet sizes.
By default, this parameter is set to TRUE, which means that
segmentation is disabled.
VERIFICATION_PERIOD -- value: integer (octets)
This parameter may be set by an external entity to specify to the
compressor the minimum frequency for which a packet that
validates the context must be sent. This tells the compressor
that a packet containing a CRC field MUST be sent at least every
number of packets equals to this value (see section 4.7). A value
of 0 indicates that no periodical verification are needed.
By default, this parameter is set to the value 1, which indicates
that packets with the CRC field MUST always be sent.
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.
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PACKET_LOST -- signal
This parameter indicates to the decompressor that a packet has
been lost on the link between the compressor and the
decompressor, for each packet that was lost.
5.2. Implementation structures
This section provides some explanatory material on data structures
specific to this profile that a ROHC implementation will have to
maintain in one form or another. What is listed here are additions to
ROHC RTP [ROHC section 6.5], imposed by this profile.
5.2.1. Compressor context
For the compressor context, this profile does not require any
additions to the data structures needed for ROHC RTP.
5.2.2. Decompressor context
For all modes, an additional field MUST be kept in memory to store a
CRC value. The decompressor MUST calculate the CRC for every header
decompressed and always keep in one form or another the value
calculated for the last packet received.
CRC_PREVIOUS: CRC calculated from the decompressed header of the last
packet received
5.3. Implementation over various link technologies
This document provides the interface semantics and requirements
needed from the ROHC compressor and decompressor towards the
assisting layer to perform link-layer assisted header compression.
However, the 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 layers implementations is [1111101].
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6. IANA considerations
A ROHC profile identifier must be reserved by the IANA for the
IP/UDP/RTP profile defined in this document. Since this additional
profile will be used concurrent to the ROHC IP/UDP/RTP profile in
[ROHC] and is part of the IETF standards track, an ordinary
identifier in the range from 4 to 127 should be reserved.
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 inject 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 applies.
8. Acknowledgements
The authors would like to thank Ulises Olvera-Hernandez and Francis
Lupien for inputs about the typical links that LLA can be applied to.
Thanks also to Mikael Degermark for fruitful discussions that led to
improvements of this profile, and to Zhigang Liu for valuable inputs
especially regarding R-mode operation.
9. References
[ROHC] Bormann, C., et. al., "Robust Header Compression
(ROHC)", 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", RFC 793,
September 1981.
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[RTP-REQ] Degermark, M., "Requirements for IP/UDP/RTP Header
Compression", RFC 3096, July 2001.
[0B-REQ] Jonsson, L-E., "Requirements and Assumptions for ROHC 0-
byte Header Compression", Internet Draft, work in
progress, August 2001.
<draft-ietf-rohc-rtp-0-byte-requirements-01.txt
[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.
[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. Author's addresses
Lars-Erik Jonsson Tel: +46 920 20 21 07
Ericsson Erisoft AB Fax: +46 920 20 20 99
Box 920
SE-971 28 Lulea
Sweden EMail: lars-erik.jonsson@ericsson.com
Ghyslain Pelletier Tel: +46 920 20 24 32
Ericsson Erisoft AB Fax: +46 920 20 20 99
Box 920
SE-971 28 Lulea
Sweden EMail: ghyslain.pelletier@epl.ericsson.se
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11. Full copyright statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
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
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
This Internet-Draft expires February 15, 2002.
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