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Network Working Group                                       L-E. JonssonRequest for Comments: 3843                                  G. PelletierCategory: Standards Track                                       Ericsson                                                               June 2004RObust Header Compression (ROHC): A Compression Profile for IPStatus 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 (2004).Abstract   The original RObust Header Compression (ROHC) RFC (RFC 3095) defines   a framework for header compression, along with compression protocols   (profiles) for IP/UDP/RTP, IP/ESP (Encapsulating Security Payload),   IP/UDP, and also a profile for uncompressed packet streams.  However,   no profile was defined for compression of IP only, which has been   identified as a missing piece inRFC 3095.  This document defines a   ROHC compression profile for IP, similar to the IP/UDP profile   defined byRFC 3095, but simplified to exclude UDP, and enhanced to   compress IP header chains of arbitrary length.   Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . . .23.  ROHC IP Compression (Profile 0x0004) . . . . . . . . . . . . .33.1.  Static Chain Termination . . . . . . . . . . . . . . . .33.2.  Handling Multiple Levels of IP Headers . . . . . . . . .33.3.  Constant IP-ID . . . . . . . . . . . . . . . . . . . . .43.4.  Additional Mode Transition Logic . . . . . . . . . . . .63.5.  Initialization . . . . . . . . . . . . . . . . . . . . .83.6.  Packet Types . . . . . . . . . . . . . . . . . . . . . .83.7.  The CONTEXT_MEMORY Feedback Option . . . . . . . . . . .104.  Security Considerations. . . . . . . . . . . . . . . . . . . .105.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .106.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .107.  Normative References . . . . . . . . . . . . . . . . . . . . .11Jonsson & Pelletier         Standards Track                     [Page 1]

RFC 3843                 A ROHC Profile for IP                 June 2004Appendix A.  Detailed Procedures for Canceling Mode Transitions. .12A.1.  Transition from Optimistic to Reliable Mode. . . . . . .12A.2.  Transition from Unidirectional to Reliable Mode. . . . .13A.3.  Transition from Reliable to Optimistic Mode. . . . . . .13A.4.  Transition Back to Unidirectional Mode . . . . . . . . .14   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .15   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . .161.  Introduction   The original RObust Header Compression (ROHC) RFC [RFC-3095] defines   a framework for header compression, along with compression protocols   (profiles) for IP/UDP/RTP, IP/ESP (Encapsulating Security Payload),   IP/UDP, and also a profile for uncompressed packet streams.  The   profile for uncompressed data was defined to provide a means to   encapsulate all traffic over a link within ROHC packets.  Through   this profile, the lower layers do not have to provide multiplexing   for different packet types, but instead ROHC can handle any packet   stream, even if compression profiles for all kinds of packet streams   have not yet been defined or implemented over the link.   Although the profile without compression is simple and can tunnel   arbitrary packets, it has of course a major weakness in that it does   not compress the headers at all.  When considering that normally all   packets are expected to be IP [RFC-791,RFC-2460] packets, and that   the IP header often represents a major part of the total header, a   useful alternative to no compression would for most packets be   compression of the IP header only.  Unfortunately, such a profile was   not defined in [RFC-3095], and this has thus been identified as an   important missing piece in the ROHC toolbox.   This document addresses this missing compression support and defines   a ROHC compression profile for IP [RFC-791,RFC-2460] only, similar   to the IP/UDP profile defined by [RFC-3095], but simplified to   exclude UDP.  Due to the similarities with the IP/UDP profile, the IP   compression profile is described based on the IP/UDP profile, mainly   covering differences.  The most important differences are a different   way of terminating the static header chain, and the capability of   compressing IP header chains of arbitrary length.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].Jonsson & Pelletier         Standards Track                     [Page 2]

RFC 3843                 A ROHC Profile for IP                 June 2004   ROHC UDP      "ROHC UDP" in this document refers to the IP/UDP profile (Profile      0x0002) as defined in [RFC-3095].3.  ROHC IP Compression (Profile 0x0004)   In general, there are no major differences between the ROHC UDP   profile and the IP profile (ROHC IP) defined in this document, since   the removal of UDP has no impact on the compression mechanisms in   principle.  As for ROHC UDP, the compressor generates a 16-bit   sequence number which increases by one for each packet compressed in   the packet stream, simply called SN below.  The most important   difference between this profile and ROHC UDP is about static chain   termination and the handling of multiple IP headers.  Unless stated   explicitly below, mechanisms and formats are the same as for ROHC   UDP.3.1.  Static Chain Termination   One difference for IP-only compression, compared to IP/UDP   compression, is related to the termination of the static chain in IR   headers.  For the UDP profile, the chain always ends with a UDP   header part, which per definition provides the boundaries for the   chain.  The UDP header is also the last header in the uncompressed   packet (except for a potential application header).  For the IP-only   profile, there is no single last header that per profile definition   terminates the chain.  Instead, the static chain is terminated if the   "Next Header / Protocol" field of a static IP header part indicates   anything but IP (IPinIP or IPv6).  Alternatively, the compressor can   choose to end the static chain at any IP header, and indicate this by   setting the MSB of the IP version field to 1 (0xC for IPv4 or 0xE for   IPv6).  The decompressor must store this indication in the context   for correct decompression of subsequent headers.  Note that the IP   version field in decompressed headers must be restored to its   original value.3.2.  Handling Multiple Levels of IP Headers   The ROHC IR and IR-DYN packets defined in [RFC-3095] are used to   communicate static and/or dynamic parts of a context.  For each of   the compression profiles defined in [RFC-3095], there is a single   last header in the header chain that clearly marks the termination of   the static chain.  The length of the dynamic chain is then inferred   from the static chain in the IR header itself, or from the static   chain in the context for the IR-DYN header.  The length of both   static and dynamic chains may thus be of arbitrary length and may, in   theory, initialize a context with an arbitrary number of IP levels.Jonsson & Pelletier         Standards Track                     [Page 3]

RFC 3843                 A ROHC Profile for IP                 June 2004   However, the general compressed header formats defined in [RFC-3095,section 5.7.] specifies that at most two levels of IP headers (the   'Inner' and the 'Outer' level of IP headers) may be included in a   compressed header.  Specifically, the format defined for Extension 3   [RFC-3095,section 5.7.5.] can only carry one single 'Outer' IP   header.  In addition, while list compression may be used to compress   other types of headers, it cannot be used to compress additional IP   headers, as IP headers may not be part of an extension header chain   in compressed headers [RFC-3095,section 5.8.].   For the compression profiles defined in [RFC-3095], the consequence   is that at most two levels of IP headers can be compressed.  In other   words, the presence of additional IP headers at best partially   disables header compression, as the compressor will only be allowed   to send IR and IR-DYN packets in such cases.   For the compression of IP headers only, the additional IP headers   would however not have to cause header compression to be disabled   because there is no single packet type that ends the compressed   chain.  The excess IP headers could simply be left uncompressed by   implicitly terminating the static and dynamic chains after at most   two levels of IP headers.   The IP-only profile defined in this document goes one step further   and supports compression of an arbitrary number of IP levels.  This   is achieved by adding a dynamic chain to the general format of   compressed headers, to include the header part of each IP level in   excess of the first two.   As explained above, the static chain within IR packets can be of   arbitrary length, and the chain is terminated by the presence of a   non-IP header (not IPinIP nor IPv6).  Alternatively, the chain may be   explicitly terminated with a special code value in the IP version   field, as described insection 3.1.  The dynamic chain is structured   analogously.   For compressed headers, the information related to the initial two IP   headers is carried as for the IP/UDP profile, and a chain of dynamic   header information is added to the end of the compressed header for   each and every additional IP header.  Thus, this additional data   structure is exactly the same as the one used in IR and IR-DYN   packets.  The length of the chain is inferred from the chain of   static parameters in the context.  While a dynamic chain carries   dynamically changing parameters using an uncompressed representation,   this ensures that flows with arbitrary levels of IP headers will not   impair compression efficiency.Jonsson & Pelletier         Standards Track                     [Page 4]

RFC 3843                 A ROHC Profile for IP                 June 20043.3.  Constant IP-ID   Most IPv4 stacks assign an IP-ID according to the value of a counter,   increasing by one for each outgoing packet.  ROHC UDP compresses the   IP-ID field using offset IP-ID encoding based on the UDP SN [RFC-   3095].  For stacks generating IP-ID values using a pseudo-random   number generator, the field is not compressed and is sent as-is in   its entirety as additional octets after the compressed header.   Cases have also been found where an IPv4 stack uses a constant value   for the IP Identifier.  When the IP-ID field is constant, it cannot   be compressed using offset IP-ID encoding and the field must be sent   in its entirety.  This overhead can be avoided with the addition of a   flag within the dynamic part of the chain used to initialize the IPv4   header, as follow:   Dynamic part:      +---+---+---+---+---+---+---+---+      |        Type of Service        |      +---+---+---+---+---+---+---+---+      |         Time to Live          |      +---+---+---+---+---+---+---+---+      /        Identification         /   2 octets      +---+---+---+---+---+---+---+---+      | DF|RND|NBO|SID|       0       |      +---+---+---+---+---+---+---+---+      / Generic extension header list /  variable length      +---+---+---+---+---+---+---+---+   SID: Static IP Identifier.      For IR and IR-DYN packets, the logic is the same as for ROHC UDP      with the addition that field(SID) must be kept in the context.      For compressed headers other than IR and IR-DYN:         If value(RND) = 0 and context(SID) = 0, hdr(IP-ID) is         compressed using Offset IP-ID encoding (see [RFC-3095section4.5.5]) using p = 0 and default-slope(IP-ID offset) = 0.         If value(RND) = 0 and context(SID) = 1, hdr(IP-ID) is constant         and compressed away; hdr(IP-ID) is the value of context(IP-ID).         If value(RND) = 1, IP-ID is the uncompressed hdr(IP-ID).  IP-ID         is then passed as additional octets at the end of the         compressed header, after any extensions.Jonsson & Pelletier         Standards Track                     [Page 5]

RFC 3843                 A ROHC Profile for IP                 June 2004   Note: Only IR and IR-DYN packets can update context(SID).   Note: All other fields are the same as for ROHC UDP [RFC-3095].3.4.  Additional Mode Transition Logic   The profiles defined in [RFC-3095] operate using different modes of   compression.  A mode transition can be requested once a packet has   reached the decompressor by sending feedback indicating the desired   mode.  As per the specifications found in [RFC-3095], the compressor   is compelled to honor such requests.   For the IP profile defined in this document, the Mode parameter for   the value mode = 0 (packet types UOR-2, IR and IR-DYN) is redefined   to allow the compressor to decline a mode transition requested by the   decompressor:      Mode: Compression mode.  0 = (C)ancel Mode Transition   Upon receiving the Mode parameter set to '0', the decompressor MUST   stay in its current mode of operation and SHOULD refrain from sending   further mode transition requests for the declined mode for a certain   amount of time.   More specifically, with reference to the parameters C_TRANS, C_MODE,   D_TRANS, and D_MODE defined in [RFC-3095,section 5.6.1.], the   following modifications apply when the compressor cancels a mode   transition:      Parameters for the compressor side:      -  C_MODE:         This value must not be changed when sending mode information         within packets if the mode parameter is set to '0' (as a         response to a mode transition request from the decompressor).      -  C_TRANS:         C_TRANS is (P)ending when receiving a mode transition request         from the decompressor.  C_TRANS is set to (D)one when the         compressor receives an ACK for a UOR-2, IR-DYN, or IR packet         sent with the mode parameter set to the mode in use at the time         the mode transition request was initiated.Jonsson & Pelletier         Standards Track                     [Page 6]

RFC 3843                 A ROHC Profile for IP                 June 2004      Parameters for the decompressor side:      -  D_MODE:         D_MODE MUST remain unchanged when receiving a UOR-2, an IR-DYN,         or an IR packet sent with the mode parameter set to '0'.      -  D_TRANS:         D_TRANS is (P)ending when a UOR-2, IR-DYN, or IR packet sent         with the mode parameter set to '0' is received.  It is set to         (D)one when a packet of type 1 or 0 corresponding to the         unchanged mode is received.   The resulting mode transition procedure is described below:              Compressor                     Decompressor             ----------------------------------------------   C_MODE = X      |                               |  D_MODE = X                   |       Mode Request(Y) +-<-<-<-|  D_TRANS = I                   |       +-<-<-<-<-<-<-<-+       |   C_TRANS = P     |-<-<-<-+                       |   C_MODE = X      |                               |                   |->->->-+ IR/IR-DYN/UOR-2(SN,C) |                   |       +->->->->->->->-+       |                   |->-..                  +->->->-|  D_TRANS = P                   |->-..                          |  D_MODE = X                   |           ACK(SN,X)   +-<-<-<-|                   |       +-<-<-<-<-<-<-<-+       |   C_TRANS = D     |-<-<-<-+                       |                   |                               |                   |->->->-+   X-0, X-1*           |                   |       +->->->->->->->-+       |                   |                       +->->->-|  D_TRANS = D                   |                               |           where X: mode in use before the mode transition was initiated                 Y: mode requested by the decompressor                 C: (C)ancel mode transitionJonsson & Pelletier         Standards Track                     [Page 7]

RFC 3843                 A ROHC Profile for IP                 June 20043.5.  Initialization   The static context for ROHC IP compression can be initialized in   either of two ways:   1) By using an IR packet as in ROHC UDP, where the profile is 0x0004,      and the static chain ends with the static part of an IP header,      where the Next Header/Protocol field has any value but IPinIP (4)      or IPv6 (41) [PROTOCOL], or where the IP version field indicates      termination (seesection 3.1).  At the compressor, SN is      initialized to a random value when the first IR packet is sent.   2) By reusing an existing context.  This is done with an IR-DYN      packet, identifying profile 0x0004, where the dynamic chain      corresponds to the prefix of the existing static chain, ending      with an IP header where the Next Header/Protocol field has any      value but IPinIP (4) or IPv6 (41) [PROTOCOL], or where the IP      version field indicates termination (seesection 3.1).  At the      compressor, SN is initialized to a random value when the first      IR-DYN packet is sent.   For ROHC IP, the dynamic part of an IR or IR-DYN packet is similar to   the one for ROHC UDP, with a two-octet field containing the SN   present at the end of the dynamic chain in IR and IR-DYN packets.  It   should be noted that the static and dynamic chains have an arbitrary   length, and the SN is added only once, at the end of the dynamic   chain in IR and IR-DYN packets.3.6.  Packet Types   Except for one new feedback option (seesection 3.7), the only packet   format that differs from ROHC UDP is the general format for   compressed packets, which has no UDP checksum in the end.  Instead,   it ends with a list of dynamic header portions, one for each IP   header above the initial two (if any, as indicated by the presence of   corresponding header portions in the static chain).Jonsson & Pelletier         Standards Track                     [Page 8]

RFC 3843                 A ROHC Profile for IP                 June 2004   The general format for a compressed header is thus as follows:     0   1   2   3   4   5   6   7    --- --- --- --- --- --- --- ---   :         Add-CID octet         :                    |   +---+---+---+---+---+---+---+---+                    |   |   first octet of base header  |                    |   +---+---+---+---+---+---+---+---+                    |   :                               :                    |   /   0, 1, or 2 octets of CID    /                    |   :                               :                    |   +---+---+---+---+---+---+---+---+                    |   /   remainder of base header    /                    |   +---+---+---+---+---+---+---+---+                    |   :                               :                    |   /           Extension           /                    |   :                               :                    |    --- --- --- --- --- --- --- ---                     |   :                               :                    |   +   IP-ID of outer IPv4 header  +   :                               :     (seesection 5.7 of [RFC-3095])    --- --- --- --- --- --- --- ---   /    AH data for outer list     /                    |    --- --- --- --- --- --- --- ---                     |   :                               :                    |   +         GRE checksum          +                    |   :                               :                    |    --- --- --- --- --- --- --- ---                     |   :                               :                    |   +   IP-ID of inner IPv4 header  +                    |   :                               :                    |    --- --- --- --- --- --- --- ---                     |   /    AH data for inner list     /                    |    --- --- --- --- --- --- --- ---                     |   :                               :                    |   +         GRE checksum          +                    |   :                               :                    |    --- --- --- --- --- --- --- ---   :            List of            :   /        Dynamic chains         /    variable, given by static chain   :   for additional IP headers   :           (includes no SN)    --- --- --- --- --- --- --- ---   Note that the list of dynamic chains for the additional IP headers in   compressed packets do not have a sequence number at the end of the   chain, as SN is present within compressed base headers.Jonsson & Pelletier         Standards Track                     [Page 9]

RFC 3843                 A ROHC Profile for IP                 June 20043.7.  The CONTEXT_MEMORY Feedback Option   The CONTEXT_MEMORY option informs the compressor that the   decompressor does not have sufficient memory resources to handle the   context of the packet stream, as the stream is currently compressed.     0   1   2   3   4   5   6   7   +---+---+---+---+---+---+---+---+   |  Opt Type = 9 |  Opt Len = 0  |   +---+---+---+---+---+---+---+---+   When receiving a CONTEXT_MEMORY option, the compressor SHOULD take   actions to compress the packet stream in a way that requires less   decompressor memory resources, or stop compressing the packet stream.4.  Security Considerations   The security considerations of [RFC-3095] apply equally to this   document, without exceptions or additions.5.  IANA Considerations   ROHC profile identifier 0x0004 has been reserved by the IANA for the   profile defined in this document.6.  Acknowledgements   The authors would like to thank Carsten Bormann, Fredrik Lindstrom,   Tommy Lundemo, and especially the committed document reviewers   Kristofer Sandlund and Mark West, for valuable input and review.Jonsson & Pelletier         Standards Track                    [Page 10]

RFC 3843                 A ROHC Profile for IP                 June 20047.  Normative References   [RFC-791]   Postel, J., "Internet Protocol",RFC 791, September 1981.   [RFC-2119]  Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC-2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6               (IPv6) Specification",RFC 2460, December 1998.   [RFC-3095]  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)",RFC 3095, July 2001.   [PROTOCOL]  "Assigned Internet Protocol Numbers", IANA registry at:http://www.iana.org/assignments/protocol-numbersJonsson & Pelletier         Standards Track                    [Page 11]

RFC 3843                 A ROHC Profile for IP                 June 2004Appendix A.  Detailed Procedures for Canceling Mode Transitions   The profiles defined in [RFC-3095] operate using different modes of   compression: Unidirectional (U-Mode), Bi-directional Optimistic   (O-Mode), and Bi-directional Reliable (R-Mode).  Compression always   starts in the U-Mode, and mode transitions can only be initiated by   the decompressor [RFC-3095,section 5.6.].  A mode transition can be   requested once a packet has reached the decompressor by sending   feedback indicating the desired mode.   With reference to the parameters C_TRANS, C_MODE, D_TRANS, and D_MODE   defined in [RFC-3095,section 5.6.1.], the following sub-sections   describe the resulting procedures when a compressor declines a mode   transition request from the decompressor as described insection 3.4.A.1.  Transition from Optimistic to Reliable Mode   When the decompressor initiates a mode transition from Optimistic to   Reliable mode, the cancellation of the transition procedure is as   follows:             Compressor                     Decompressor             ----------------------------------------------                   |                               |                   |        ACK(R)/NACK(R) +-<-<-<-|  D_TRANS = I                   |       +-<-<-<-<-<-<-<-+       |   C_TRANS = P     |-<-<-<-+                       |   C_MODE = O      |                               |                   |->->->-+ IR/IR-DYN/UOR-2(SN,C) |                   |       +->->->->->->->-+       |                   |->-..                  +->->->-|  D_TRANS = P                   |->-..                          |  D_MODE = O                   |           ACK(SN,O)   +-<-<-<-|                   |       +-<-<-<-<-<-<-<-+       |   C_TRANS = D     |-<-<-<-+                       |                   |                               |                   |->->->-+  UO-0, UO-1*          |                   |       +->->->->->->->-+       |                   |                       +->->->-|  D_TRANS = D   The compressor must not send packet types 1 or 0 when C_TRANS is P,   i.e., not until it has received an ACK for a UOR-2, IR-DYN, or IR   packet sent with the mode transition parameter set to C.  When the   decompressor receives a UOR-2, IR-DYN, or IR packet sent with the   mode transition parameter set to C, it must keep the value D_MODE as   O and set D_TRANS to P.  When the decompressor receives packet types   0 or 1, after having ACKed a UOR-2, IR-DYN, or IR packet, it sets   D_TRANS to D.Jonsson & Pelletier         Standards Track                    [Page 12]

RFC 3843                 A ROHC Profile for IP                 June 2004A.2.  Transition from Unidirectional to Reliable Mode   The cancellation of a transition from Unidirectional to Reliable mode   follows the same procedure as defined insection 4.2 above.A.3.  Transition from Reliable to Optimistic Mode   When the decompressor initiates a mode transition from Reliable to   Optimistic mode, the cancellation of the transition procedure is   described as follows:               Compressor                     Decompressor             ----------------------------------------------                   |                               |                   |        ACK(O)/NACK(O) +-<-<-<-|  D_TRANS = I                   |       +-<-<-<-<-<-<-<-+       |   C_TRANS = P     |-<-<-<-+                       |   C_MODE = R      |                               |                   |->->->-+ IR/IR-DYN/UOR-2(SN,C) |                   |       +->->->->->->->-+       |                   |->-..                  +->->->-|  D_MODE = R                   |->-..                          |                   |           ACK(SN,R)   +-<-<-<-|                   |       +-<-<-<-<-<-<-<-+       |   C_TRANS = D     |-<-<-<-+                       |                   |                               |                   |->->->-+   R-0, R-1*           |                   |       +->->->->->->->-+       |                   |                       +->->->-|  D_TRANS = D                   |                               |   The compressor must not send packet types 1 or 0 when C_TRANS is P,   i.e.,  not until it has received an ACK for a UOR-2, IR-DYN, or IR   packet sent with the mode transition parameter set to C.  When the   decompressor receives a UOR-2, IR-DYN, or IR packet sent with the   mode transition parameter set to C, it must keep the value D_MODE as   R.  When the decompressor receives packet types 0 or 1, after having   ACKed a UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D.Jonsson & Pelletier         Standards Track                    [Page 13]

RFC 3843                 A ROHC Profile for IP                 June 2004A.4.  Transition Back to Unidirectional Mode   When the decompressor initiates a mode transition from Reliable or   Optimistic mode back to Unidirectional mode, the cancellation of the   transition procedure is as follows:              Compressor                     Decompressor             ----------------------------------------------               |                               |               |        ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I               |       +-<-<-<-<-<-<-<-+       |   C_TRANS = P |-<-<-<-+                       |   C_MODE = O/R|                               |               |->->->-+ IR/IR-DYN/UOR-2(SN,C) |               |       +->->->->->->->-+       |               |->-..                  +->->->-|               |->-..                          |               |          ACK(SN,O/R)  +-<-<-<-|               |       +-<-<-<-<-<-<-<-+       |   C_TRANS = D |-<-<-<-+                       |               |          R-0, R-1* or         |               |->->->-+  UO-0, UO-1*          |               |       +->->->->->->->-+       |               |                       +->->->-| D_TRANS = D                                                 D_MODE = O/R   When the decompressor receives a UOR-2, IR-DYN, or IR packet sent   with the mode transition parameter set to C, it must keep the value   D_MODE to the bi-directional mode already in use (either O- or R-   mode).  After ACKing the first UOR-2(C), IR-DYN(C), or IR(C), the   decompressor MUST continue to send feedback with the Mode parameter   set to the bi-directional mode in use (either O- or R-mode) until it   receives packet types 0 or 1.  When the decompressor receives packet   types 0 or 1, after having ACKed a UOR-2, IR-DYN, or IR packet, it   sets D_TRANS to D.Jonsson & Pelletier         Standards Track                    [Page 14]

RFC 3843                 A ROHC Profile for IP                 June 2004Authors' 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.comJonsson & Pelletier         Standards Track                    [Page 15]

RFC 3843                 A ROHC Profile for IP                 June 2004Full Copyright Statement   Copyright (C) The Internet Society (2004).  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 currently provided by the   Internet Society.Jonsson & Pelletier         Standards Track                    [Page 16]