Network Working Group
Request for Comments: 3242
Category: Standards Track

L-E. Jonsson
G. Pelletier
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

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.

   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
   
        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 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 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.

   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.

     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.

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.

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.

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:

     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.

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.

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.

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 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.

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.

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.

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 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.

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.
   
   [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

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