wdiff draft-jonsson-rohc-lla-rtp-00.txt draft-jonsson-rohc-lla-rtp-01.txt

Network Working Group Lars-Erik Jonsson, Ericsson
INTERNET-DRAFT Ghyslain Pelletier, Ericsson
Expires: August 23, 2001 January 2002 Sweden
February 23,
July 20, 2001

A Link-Layer Assisted ROHC Profile for IP/UDP/RTP
<draft-jonsson-rohc-lla-rtp-00.txt>
<draft-jonsson-rohc-lla-rtp-01.txt>

Status of this memo

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all provisions of Section 10 of RFC2026.

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This document is an individual submission to the IETF. Comments
should be directed to the authors.

Abstract

This document defines a ROHC profile for compression of IP/UDP/RTP
packets, utilizing functionality provided by the lower layer 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 lower layer to
guarantee transparency, and specifies general logic for compression
and decompression making use of this header-free packet.

Table of contents

1. Introduction....................................................3

2. Terminology.....................................................5

3. Overview of the link-layer assisted profile.....................6
3.1. Providing packet type identification.....................6
3.2. Replacing the sequence number............................6 number............................7
3.3. CRC replacement..........................................7
3.4. Applicability of this profile............................8

4. Additions and exceptions compared to ROHC RTP...................7 RTP...................8
4.1. Additional packet types..................................8
4.1.1. A no-header packet (NHP).................................7
4.2. (NHP)........................8
4.1.2. A context check packet (CCP).............................8
4.3. (CCP)....................8
4.1.3. A context synchronization packet (CSP)..........9
4.2. Interfaces towards the assisting lower layers...........10
4.2.1. Interface between compressor and lower layer.............9
4.4. layer...11
4.2.2. Interface between lower layer and decompressor...........9 decompressor.12
4.3. Agreement on optimistic approach........................12
4.4. Feedback option RHP-REQUEST.............................13
4.5. Periodic context verification...........................10 verification...........................13
4.6. Feedback option RHP-REQUEST.............................10
4.7. Use of context identifiers..............................10 identifiers..............................13

5. Implementation issues..........................................10 issues..........................................14
5.1. Implementation parameters and signals...................11 signals...................14
5.1.1. Implementation parameters at compressor........11 compressor........14
5.1.2. Implementation parameters at decompressor......12 decompressor......15
5.2. Implementation structures...............................13 structures...............................16
5.2.1. Compressor context.............................13 context.............................16
5.2.2. Decompressor context...........................13 context...........................16
5.3. Implementation over various link technologies...........13 technologies...........16

6. IANA considerations............................................13 considerations............................................17

7. Security considerations........................................14 considerations........................................17

8. Acknowledgements...............................................14 Acknowledgements...............................................17

9. References.....................................................14 References.....................................................17

10. Authors addresses..............................................15 addresses..............................................18

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 [IP, 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. Because of 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
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. 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 this 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. 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 the ROHC RTP profile in [ROHC] (profile number 1). 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 regular 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 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 layer and sacrificing
efficiency for less frequently occurring larger header packets. 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 layer together. Therefore, additional
standards-track 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
LL Link Layer
LLA Link Layer Assisted ROHC RTP Profile
MSB Most Significant Bit
NHP No Header Packet
ROHC Robust Header Compression
RHP ROHC Header Packet (either a CCP (a non-NHP packet, i.e. RRP, CSP or a RRP packet) CCP)
RRP ROHC RTP Packet as defined in [ROHC, profile 1]

Link layer

Link layer in this document refers to the physical link layer.

Lower layer

Lower layer in this document refers to any entity implementing the
interface to ROHC as defined in section 4.3 and 4.4. 4.2. It may, as an
example, refer to a sub-layer between the ROHC implementation and
the physical link layer used to adapt both implementations.

ROHC RTP

ROHC RTP in this document refers to the IP/UDP/RTP profile 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 to be transmitted
together with payloads corresponding to these optimal sizes.

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 lower layer.

By putting additional requirements on the lower layer compared to [RTP-
LLG],
[RTP-LLG], it is possible for ROHC 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 (optional in
PT0 for Reliable 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 which context the compressed packet belongs to and how
it should be interpreted. This is a functionality that must be
provided by some means. ROHC RTP packets with header will still be
possible to distinguish between since they have this identifier, but
what must be provided is a mechanism to separate those packets with
header from the packets without header. This functionality MUST
therefore be provided by the lower 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 lower layer MUST guarantee in-order delivery and
provide an indication of packet loss over the link. This is basically
the same principle as VJ header compression [VJHC] relies on.

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

All RRP packets carry a CRC calculated over the uncompressed header
(optional in PT0 for Reliable mode). 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 Unidirectional and Optimistic mode)
- 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 a 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 lower 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 checks SHOULD be performed.

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 profile #1 should be
implemented thus depends on the characteristics of the link itself.
For most RTP packet streams, LLA will work exactly as profile #1,
while it will be more efficient for packet streams with certain
characteristics. LLA will never be less efficient than ROHC RTP
profile #1.

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.

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. A 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)
when the sequence number has incremented with one from the previous
packet. 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
cases
restrictions on when NHP should not be used (see section 4.4 and 4.5).

Note that since the lower layer is responsible of separating NHP
packets from RHP packets, an indicated indication from the compressor to the
lower layer MUST be provided upon delivery of an NHP packet.

4.2.

4.1.2. A context check packet (CCP)

A Context Check Packet (CCP) is defined, (CCP), which does not carry any payload but, but
only a CRC value in addition to the packet type identifier, only a CRC
value. A is
defined. The CCP packet MAY be created by the compressor in addition
to the compressed packet and provided to the lower layer. Whether the
packet is sent over the link and delivered to the decompressor is not
decided by the compressor, but by the lower layer.

The purpose of
this packet the CCP is to provide a useful packet that MAY be sent
by a synchronized physical link layer in the case when it where data must send
something on a regular basis, be
sent at fixed intervals, even if no compressed packet is available. This is the format of the
The CCP packet: has the following format:

0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 1 | Packet type identifier
+---+---+---+---+---+---+---+---+
| R 0 | CRC |
+---+---+---+---+---+---+---+---+

This packet is defined by one of the unused packet type identifiers
from ROHC RTP, carried in the first octet of the packet. packet, and the
first bit of the following octet being set to 0. As for any ROHC
packet, except NHP, the packet MAY begin with ROHC padding and/or
carry context identification. The (R)eserved bit MUST be set
to 0 in all CCP packets. The CRC is the 7-bits CRC over the
original uncompressed header as described in [ROHC section 5.9.2].

If the lower layer implementation makes use of the CCP feature, the
last CCP packet received from the compressor MUST always be used,
i.e. the CCP corresponding to the last data packet sent (NHP or RRP).
Accordingly, if a CCP packet is received by the decompressor, it MUST
be used as a context verification for the last packet decompressed
unless a packet loss indication was previously received. To
facilitate this verification, the 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].

Note that the usage of CCP is optional, depending optional and depends on the link layer
this profile is implemented over.
characteristics of the actual link. Whether it is used or not MUST
therefore be specified in the link layer implementation
specifications for this profile.

4.3.

4.1.3. A context synchronization packet (CSP)

The CCP packet can be used to ensure correctness of the decompressor
context, but when this verification fails a request must be sent to
get an update. All packets must then be discarded until the proper
context is re-established.

However, it would in many cases be beneficial to send not only the
header verifying CRC of the previous packet but also the header
itself without its associated payload. This would allow not only to
perform a context verification but also to provide a context repair
mechanism.

Another situation when it could be useful to send a packet with only
the header information is when the packet stream overruns the channel
bandwidth and data must be discarded. Instead of discarding the whole
packet at the compressor side, which would mean that the decompressor
context might be invalidated, only the payload could be discarded and
the header sent to keep the decompressor context synchronized while
still saving bandwidth.

Both cases above 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 :
+---+---+---+---+---+---+---+---+

The CSP packet is identified using the same packet type identifier as
the CCP packet with the first bit of the second octet being set to 1,
and it uses the same CRC. The first two octets of the CSP packet are
thus identical to the CCP packet, except for the first bit of the
second octet being set to 1 instead of 0. As for any ROHC packet,
except NHP, the packet MAY begin with ROHC padding and/or carry
context identification.

The difference of the CSP from the CCP packet is the presence of a
compressed ROHC header that can be used to synchronize the
decompressor context. 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.

4.2. Interfaces towards the assisting lower layers

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 lower layers is defined as interfaces between the
ROHC LLA compressor/decompressor elements 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.

The figure also underline the need for additional standards-track
documents to specify how to fulfill these interfaces for a link
technology for which this profile is relevant.

4.2.1. Interface between compressor and the lower layer

This section defines the interface semantics between the compressor
and the lower layer, providing rules for packet delivery from the
compressor.

+-----------+-----+-----+-----+-----+-----+-----+-----+

+------------------+-----+-----+-----+-----+-----+-----+-----+
| \ Interface | RRP | Seg | NHP | CCP | Seq | Seg | NHP |
| Case \ Parameter | | RRP | | | Num | Flg | Flg |
+-----------+-----+-----+-----+-----+-----+-----+-----+
+------------------+-----+-----+-----+-----+-----+-----+-----+
| Case 1 - NHP, No Seg. | x | | x | x | x | | x |
+-----------+-----+-----+-----+-----+-----+-----+-----+
+------------------+-----+-----+-----+-----+-----+-----+-----+
| Case 2 - NHP, Seg. | | x | x | x | x | x | x |
+-----------+-----+-----+-----+-----+-----+-----+-----+
+------------------+-----+-----+-----+-----+-----+-----+-----+
| Case 3 - No Seg. | x | | | x | x | | |
+-----------+-----+-----+-----+-----+-----+-----+-----+
+------------------+-----+-----+-----+-----+-----+-----+-----+
| Case 4 - Seg. | | x | | x | x | x | |
+-----------+-----+-----+-----+-----+-----+-----+-----+
+------------------+-----+-----+-----+-----+-----+-----+-----+
Table 1: Data delivery from the compressor

As seen in table 1, four different delivery scenarios are possible.
Case 1 represent what needs to be delivered from compressor to lower
layers when the compressor allows sending of an NHP packet, without
any segmentation applied to the corresponding RRP packet. Case 2
differs from case 1 in that a segmented RRP is being delivered. Case
3 and case 4 shows are the case cases where a packet without header cannot be
delivered.

If the compressor delivers a NHP packet to the lower layer, it MUST
also provide the RRP packet, a CCP packet, the Sequence Number and
the indication that a NHP packet MAY be used. Otherwise the RRP
packet MUST be delivered together with a CCP packet.

Furthermore, for the case where the RRP packet is delivered to the
lower layer as a segmented ROHC packet according to the rules in
chapter 5.5.1, an indication MUST be provided by the compressor.

4.4.

4.2.2. Interface between lower layer and decompressor

The interface semantics between the lower 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 headers, 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 lower layer MUST provide the following to the decompressor:

- an indication of packet loss
- the received packets together with a indication whether the packet
received is an NHP or not

4.3. Agreement on optimistic approach

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, but it may also be partially used
in the Reliable mode. 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, with the LLA profile the CRC is not present in the NHP
packet and 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.

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.

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.

4.4. Feedback option, RHP-REQUEST

The RHP-REQUEST option could 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, at least one RRP with CRC SHOULD be sent
immediately.

+---+---+---+---+---+---+---+---+
| Opt Type = 8 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+

4.5. Periodic context verification

As described in chapter 3.3, transparency is expected to be
guaranteed by the functionality provided by the lower layer. 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 Reliable
mode PT0 packets) be sent periodically, periodically (possibly with an increasing
period), even when the normal logic allows for NHP packets to be
used.

Since a CCP packet serves the same purpose as a regular periodic
verification with RRP, indication of CCP transmission is beneficial
for
to the compressor, which can ignore some periodic RRP verifications.

4.6. Feedback option, RHP-REQUEST

+---+---+---+---+---+---+---+---+
| Opt Type = 8 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+

4.7. 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 lower
layers 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
(NHP packet stream) 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. 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 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 lower layer MUST provide a packet type identification. If no
field is available for this purpose from the protocol at the link
layer, then it is suggested to use a leading sequence 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 lower layer. If this parameter set is used, all RHP
packets MUST be padded to fit the smallest possible of the
preferred sizes. If the unpadded packet size, 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
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 packet 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. 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 decompressor

NHP_PACKET -- value: boolean

This parameter informs the decompressor that the packet being
delivered is a NHP packet. The decompressor MUST accept this
packet type indicator from the lower layer. A lower layer MUST
set this indicator for every NHP packet delivered to true, and to
false for any other packet.

PACKET_LOST -- signal

This parameter indicates to the decompressor that a packet has
been lost on the link between the compressor and the
decompressor.

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
compared 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

An additional field MUST for all modes be kept in memory to store a
CRC value. The decompressor MUST calculate the CRC for every packet
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 link
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 should be implemented over
various link technologies must be described in additional standards
track 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 a lower layer implementations is [1111101].

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 valuable inputs. inputs about the typical links that LLA can be
applied to. Thanks also to Mikael Degermark and Zhigang Liu for
fruitful discussions that led to improvements of this profile.

9. References

[ROHC] C. Bormann, "Robust Header Compression (ROHC)", Internet
draft (work in progress), February
RFC 3095, July 2001.
<draft-ietf-rohc-rtp-08.txt>

[VJHC] V. Jacobson, "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.

[IPHC] M. Degermark, B. Nordgren, S. Pink, "IP Header
Compression", RFC 2507, February 1999.

[CRTP] S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers
for Low-Speed Serial Links", RFC 2508, February 1999.

[CRTPC] M. Degermark, H. Hannu, L.-E. Jonsson, K. Svanbro,
"Evaluation of CRTP Performance over Cellular Radio
Networks", IEEE Personal Communications Magazine, Volume
7, number 4, pp. 20-25, August 2000.

[RTP-REQ] M. Degermark, "Requirements for IP/UDP/RTP Header
Compression", Internet draft (work in progress),
February RFC 3096, July 2001.
<draft-ietf-rohc-rtp-requirements-05.txt>

[RTP-LLG] K. Svanbro, "Lower Layer Guidelines for Robust
RTP/UDP/IP Header Compression", Internet draft (work in
progress), February 2001.
<draft-ietf-rohc-rtp-lower-layer-guidelines-01.txt>

[IP] J. Postel, "Internet Protocol", RFC 791, September 1981.

[IPv6] S. Deering, R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[UDP] J. Postel, "User Datagram Protocol", RFC 768, August
1980.

[TCP] J. Postel, "Transmission Control Protocol", RFC 793,
September 1981.

[RTP] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 1889, January 1996.

[VTC2000] K. Svanbro, H. Hannu, L.-E. Jonsson, M. Degermark,
"Wireless real time IP-services enabled by header
compression", proceedings of IEEE VTC2000, May 2000.

[MOMUC01] G. Liu, 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 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

This Internet-Draft expires August 23, 2001. January 20, 2002.