GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc4362

Network Working Group L-E. Jonsson Request for Comments: 4362 G. Pelletier Obsoletes: 3242 K. Sandlund Category: Standards Track Ericsson

                                                          January 2006
                 RObust Header Compression (ROHC):
            A Link-Layer Assisted Profile for IP/UDP/RTP

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

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 related to the usage of the header-free packet format.
 This document is a replacement for RFC 3242, which it obsoletes.

Jonsson, et al. Standards Track [Page 1] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

Table of Contents

 1. Introduction ....................................................2
    1.1. Differences from RFC 3242 ..................................5
 2. Terminology .....................................................5
 3. Overview of the Link-Layer Assisted Profile .....................6
    3.1. Providing Packet Type Identification .......................7
    3.2. Replacing the Sequence Number ..............................7
    3.3. CRC Replacement ............................................8
    3.4. Applicability of This Profile ..............................8
 4. Additions and Exceptions Compared to ROHC RTP ...................9
    4.1. Additional Packet Types ....................................9
         4.1.1. No-Header Packet (NHP) ..............................9
         4.1.2. Context Synchronization Packet (CSP) ................9
         4.1.3. Context Check Packet (CCP) .........................11
    4.2. Interfaces Towards the Assisting Layer ....................12
         4.2.1. Interface, Compressor to Assisting Layer ...........13
         4.2.2. Interface, Assisting Layer to Decompressor .........13
    4.3. Optimistic Approach Agreement .............................14
    4.4. Fast Context Initialization, IR Redefinition ..............15
    4.5. Feedback Option, CV-REQUEST ...............................16
    4.6. Periodic Context Verification .............................16
    4.7. Use of Context Identifier .................................16
 5. Implementation Issues ..........................................17
    5.1. Implementation Parameters and Signals .....................17
         5.1.1. Implementation Parameters at the Compressor ........17
         5.1.2. Implementation Parameters at the Decompressor ......19
    5.2. Implementation over Various Link Technologies .............19
 6. IANA Considerations ............................................20
 7. Security Considerations ........................................20
 8. Acknowledgements ...............................................20
 9. References .....................................................20
    9.1. Normative References ......................................20
    9.2. Informative References ....................................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

Jonsson, et al. Standards Track [Page 2] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 improve compression efficiency further for real-time traffic using
 the Real-Time Transport Protocol [RTP].
 The schemes mentioned above have all been designed by 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 when 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 also been designed to 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

Jonsson, et al. Standards Track [Page 3] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 next higher fixed packet size supported by the link, something that
 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 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 describes a link-layer-assisted ROHC RTP profile,
 originally defined by [LLA], extending ROHC RTP (profile 0x0001)
 [ROHC], and compliant with the ROHC 0-byte requirements [0B-REQ].
 The purpose of this 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.
 The profile defined by this document was originally specified by RFC
 3242 [LLA], but to address one technical flaw and clarify one
 implementation issue, this document has been issued to replace RFC
 3242, which becomes obsolete.

Jonsson, et al. Standards Track [Page 4] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

1.1. Differences from RFC 3242

 This section briefly summarizes the differences of this document from
 RFC 3242.  Acronyms and terminology can be found in Section 2.
 The format of the CSP packet, as defined in [LLA], was identified as
 non-interoperable when carrying a RHP header with a 3-bit or 7-bit
 CRC.  This problem occurs because the payload has been dropped by the
 compressor, and the decompressor is supposed to use the payload
 length to infer certain fields in the uncompressed header.  These
 fields are the IPv4 total length, the IPv6 payload length, the UDP
 length, and the IPv4 header checksum field (all INFERRED fields in
 [ROHC]).  To correct this flaw, the CSP packet must carry information
 about the payload length of the RHP packet.  Therefore, the length of
 the RTP payload has been included in the CSP packet.
 This document also clarifies an unclear referencing in RFC 3242,
 where Section 4.1.3 of [LLA] states that upon CRC failure, the
 actions of [ROHC], Section 5.3.2.2.3 MUST be taken.  That section
 specifies that detection of SN wraparound and local repair must be
 performed, but neither of these steps apply when the failing packet
 is a CCP.  Therefore, upon CRC failure, actions to be taken are the
 ones specified in Section 5.3.2.2.3, but steps a-d only.

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 [RFC2119].
 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.

Jonsson, et al. Standards Track [Page 5] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 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" refers to the IP/UDP/RTP profile as defined in [ROHC].

3. Overview of the Link-Layer Assisted Profile

 The ROHC IP/UDP/RTP profile defined in [LLA] and updated by this
 document, profile 0x0005 (hex), is designed to be used over channels
 that have been optimized for specific payload sizes and that
 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, this profile replaces 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.

Jonsson, et al. Standards Track [Page 6] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

   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.  It will be possible to distinguish ROHC RTP packets
 with compressed headers 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, the compressing side, if
 needed, 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 that which 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 link losses and bit errors in
 the compressed sequence number.

Jonsson, et al. Standards Track [Page 7] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

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
 an 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.
 Thus, whether LLA or ROHC RTP should be implemented depends on the
 characteristics of the link itself.  For most RTP packet streams, LLA
 will work exactly as ROHC RTP, and it will have a higher compression
 efficiency for packet streams with certain characteristics.  LLA will
 never have a lower compression efficiency 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.

Jonsson, et al. Standards Track [Page 8] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 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) is a packet that consists only of the
 payload of the original packet.  The NHP MAY be used when only the
 sequence information needs to be conveyed to the decompressor.  In
 other words, the NHP can be used 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
 sections 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 discarded data, which may result in decompressor context
 invalidation.  It might therefore be beneficial to send a packet with
 only the header information and to discard the payload.  This would
 be helpful to maintain synchronization of the decompressor context
 while efficiently using the available bandwidth.

Jonsson, et al. Standards Track [Page 9] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 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
 +===+===+===+===+===+===+===+===+
 /       RTP Payload Length      / 2 octets
 +---+---+---+---+---+---+---+---+
 :  ROHC header without padding  :
 :    see [ROHC, Section 5.7]    :
 +---+---+---+---+---+---+---+---+
   RTP Payload Length: This field is the length of the payload carried
                       inside the RTP header, stored in network byte
                       order.  That is, this field will be set by the
                       compressor to (UDP length - size of the UDP
                       header - size of the RTP header including CSRC
                       identifiers).
 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.
 In the CSP packet, the payload has been dropped by the compressor.
 However, the decompressor is supposed to use the payload length to
 infer certain fields in the uncompressed header (the IPv4 total
 length, the IPv6 payload length, the UDP length, and the IPv4 header
 checksum field).  When dropping the payload, the CSP packet needs to
 contain information about the payload length carried in the RHP
 packet.  Therefore, the length of the RTP payload is carried in the
 CSP packet.  When the decompressor receives a CSP packet, it can use
 the RTP payload length field to calculate the value of fields
 classified as INFERRED in [ROHC] when attempting to verify a 3- or
 7-bit CRC carried in the RHP header enclosed in 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.

Jonsson, et al. Standards Track [Page 10] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

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.  If C=1, the CRC field MUST be set to the 7-bit 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 to 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

Jonsson, et al. Standards Track [Page 11] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 [ROHC, Section 5.3.2.2.3, steps a-d only].  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.
 When using the 7-bit CRC in the CCP packet to verify the context, the
 decompressor needs to have access to the entire uncompressed header
 of the latest packet decompressed.  Some implementations of [ROHC]
 might not save the values of INFERRED fields.  An implementation of
 ROHC LLA MUST save these fields in the decompressor context to be
 able to successfully verify CCP packets.
 The use of CCP by an assisting layer is optional and depends on the
 characteristics of the actual link.  Whether it is used 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

Jonsson, et al. Standards Track [Page 12] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 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 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

Jonsson, et al. Standards Track [Page 13] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 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:
  1. an indication for each packet loss over the link between the

compressing and decompressing sides for CID=0.

  1. the received packet together with an indication of whether the

packet received is an NHP.

 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 if three or more consecutive physical packets are lost.
 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 to between the compressing side and the
 decompressing side.  It could be handled with a fixed principle, with

Jonsson, et al. Standards Track [Page 14] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 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 that the payload has been discarded.
 After an IR packet with D=0 has been processed by the decompressor,
 the packet MUST be discarded.

Jonsson, et al. Standards Track [Page 15] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

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 NHPs 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 that context updating packets be sent periodically,
 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.

Jonsson, et al. Standards Track [Page 16] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

5. Implementation Issues

 This document specifies mechanisms for the protocol and leaves
 details on the use of these mechanisms to the implementers.  The
 present section 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 ROHC padding
    of one octet, minimum.
    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

Jonsson, et al. Standards Track [Page 17] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

    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.

Jonsson, et al. Standards Track [Page 18] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

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].

Jonsson, et al. Standards Track [Page 19] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

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
 using random CRC values onto the link, 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

9.1. Normative References

 [ROHC]    Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
           Hannu, H., Jonsson, L-E., 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", STD 64, RFC 3550, July 2003.

Jonsson, et al. Standards Track [Page 20] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

 [LLA]     Jonsson, L-E. and G. Pelletier, "RObust Header Compression
           (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP", RFC
           3242, April 2002.
 [TCP]     Postel, J., "Transmission Control Protocol", STD 7, RFC
           793, September 1981.
 [RTP-REQ] Degermark, M., "Requirements for robust 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.

Jonsson, et al. Standards Track [Page 21] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

Authors' Addresses

 Lars-Erik Jonsson
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 29 61
 Fax:   +46 920 996 21
 EMail: lars-erik.jonsson@ericsson.com
 Ghyslain Pelletier
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 29 43
 Fax:   +46 920 996 21
 EMail: ghyslain.pelletier@ericsson.com
 Kristofer Sandlund
 Ericsson AB
 Box 920
 SE-971 28 Lulea, Sweden
 Phone: +46 8 404 41 58
 Fax:   +46 920 996 21
 EMail: kristofer.sandlund@ericsson.com

Jonsson, et al. Standards Track [Page 22] RFC 4362 A Link-Layer Assisted ROHC RTP January 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Jonsson, et al. Standards Track [Page 23]

/data/webs/external/dokuwiki/data/pages/rfc/rfc4362.txt · Last modified: 2006/01/18 23:40 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki