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Network Working Group                                       L-E. JonssonRequest for Comments: 4362                                  G. PelletierObsoletes:3242                                              K. SandlundCategory: Standards Track                                       Ericsson                                                            January 2006RObust Header Compression (ROHC):A Link-Layer Assisted Profile for IP/UDP/RTPStatus 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 forRFC 3242, which it obsoletes.Jonsson, et al.             Standards Track                     [Page 1]

RFC 4362             A Link-Layer Assisted ROHC RTP         January 2006Table of Contents1. Introduction ....................................................21.1. Differences fromRFC 3242 ..................................52. Terminology .....................................................53. Overview of the Link-Layer Assisted Profile .....................63.1. Providing Packet Type Identification .......................73.2. Replacing the Sequence Number ..............................73.3. CRC Replacement ............................................83.4. Applicability of This Profile ..............................84. Additions and Exceptions Compared to ROHC RTP ...................94.1. Additional Packet Types ....................................94.1.1. No-Header Packet (NHP) ..............................94.1.2. Context Synchronization Packet (CSP) ................94.1.3. Context Check Packet (CCP) .........................114.2. Interfaces Towards the Assisting Layer ....................124.2.1. Interface, Compressor to Assisting Layer ...........134.2.2. Interface, Assisting Layer to Decompressor .........134.3. Optimistic Approach Agreement .............................144.4. Fast Context Initialization, IR Redefinition ..............154.5. Feedback Option, CV-REQUEST ...............................164.6. Periodic Context Verification .............................164.7. Use of Context Identifier .................................165. Implementation Issues ..........................................175.1. Implementation Parameters and Signals .....................175.1.1. Implementation Parameters at the Compressor ........175.1.2. Implementation Parameters at the Decompressor ......195.2. Implementation over Various Link Technologies .............196. IANA Considerations ............................................207. Security Considerations ........................................208. Acknowledgements ...............................................209. References .....................................................209.1. Normative References ......................................209.2. Informative References ....................................211.  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 toJonsson, 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 theJonsson, 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 byRFC3242 [LLA], but to address one technical flaw and clarify one   implementation issue, this document has been issued to replaceRFC3242, which becomes obsolete.Jonsson, et al.             Standards Track                     [Page 4]

RFC 4362             A Link-Layer Assisted ROHC RTP         January 20061.1.  Differences fromRFC 3242   This section briefly summarizes the differences of this document fromRFC 3242.  Acronyms and terminology can be found inSection 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 inRFC 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 inSection 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 inRFC 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 20063.3.  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 (seeSection 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 RTP4.1.  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   sections4.3,4.5, and4.6).  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 20064.1.3.  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 inJonsson, 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 operateJonsson, 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 wayJonsson, 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:      -  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 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, withJonsson, 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 20064.5.  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 inSection 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 20065.  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_PADJonsson, 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 (seeSection 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 20065.1.2.  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 20066.  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.  References9.1.  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",RFC3242, April 2002.   [TCP]     Postel, J., "Transmission Control Protocol", STD 7,RFC793, 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 2006Authors' 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.comJonsson, et al.             Standards Track                    [Page 22]

RFC 4362             A Link-Layer Assisted ROHC RTP         January 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 inBCP 78 andBCP 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 athttp://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]