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Network Working Group Z. Liu Request for Comments: 3408 K. Le Category: Standards Track Nokia

                                                         December 2002
     Zero-byte Support for Bidirectional Reliable Mode (R-mode)
     in Extended Link-Layer Assisted RObust Header Compression
                           (ROHC) Profile

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 (2002).  All Rights Reserved.


 This document defines an additional mode of the link-layer assisted
 RObust Header Compression (ROHC) profile, also known as the zero-byte
 profile, beyond the two defined in RFC 3242.  Zero-byte header
 compression exists in order to prevent the single-octet ROHC header
 from pushing a packet voice stream into the next higher fixed packet
 size for the radio.  It is usable in certain widely deployed older
 air interfaces.  This document adds the zero-byte operation for ROHC
 Bidirectional Reliable mode (R-mode) to the ones specified for
 Unidirectional (U-mode) and Bidirectional Optimistic (O-mode) modes
 of header compression in RFC 3242.

1. Introduction

 [RFC3242] defines a zero-byte solution for compression of IP/UDP/RTP
 packets only for Unidirectional (U-) and Bidirectional Optimistic
 (O-) modes [RFC3095].  The present specification extends the profile
 defined in [RFC3242] to provide zero-byte support for Bidirectional
 Reliable (R-) mode.  This specification and [RFC3242] allow a
 header-free packet format to be used in all modes to replace the
 majority of the 1-octet headers of ROHC RTP packets sent during
 normal operation.  Specifically, the compressor operating in R-mode
 is allowed to deliver a No-Header Packet (NHP) when [RFC3242] would
 have required it to deliver a ROHC Reliable Packet Type Zero (R-0)
 packet [RFC3095].

Liu & Le Standards Track [Page 1] RFC 3408 0-byte Support for R-mode December 2002

 For simplification, this profile is defined in the form of the
 additions and exceptions to [RFC3242] that are required to extend the
 RFC 3242 profile with zero-byte support for R-mode.  All terminology
 used in this document is the same as in [RFC3242].
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in BCP 14, RFC 2119

2. Extensions to the assisting layer (AL) interface

 This section describes additions (some are optional) to the assisting
 layer interface as defined in [RFC3242, section 4.2].

2.1. Additional parameters to the compressor to AL interface

  1. Mode, indicating the mode in which the compressor is operating.

The AL has slightly different logic depending on the mode value.

  1. SN_ACKed, indicating the latest RTP SN that has been acknowledged.

It is used only when Mode value = R-mode.

 Note that these two parameters MUST always be attached to every
 packet delivered to the AL.

2.2. Additional interface, assisting layer to compressor

 To improve the compression efficiency of this profile in some
 specific cases, e.g., when the AL operates in such a way that it
 often becomes unsafe to send NHPs, it is RECOMMENDED to implement
 this additional interface.  Here, the word "unsafe" means that the
 compressor allows the AL to send NHP but the AL cannot guarantee that
 the RTP SN of the NHP will be correctly decompressed at the receiving
 side.  The interface is used to carry update_request as described in
 section 3.  Note that this interface is not required in the sense
 that the impossibility of implementing such an interface should not
 be an obstacle to implement this profile over a specific link.

3. R-mode operation

 For the R-mode, this profile extends ROHC RTP by performing a mapping
 of the R-0 packet to the NHP packet.  Note that R-0 is the only type
 of packets in R-mode that can be replaced with NHP.
 On the receiving side, the RTP SN of an NHP is determined by the
 decompressor as = SN_Ref_D + Offset_D, where SN_Ref_D is the RTP SN
 of the last update packet received by the decompressor, and Offset_D

Liu & Le Standards Track [Page 2] RFC 3408 0-byte Support for R-mode December 2002

 the sequence number offset between the NHP and the last update
 packet.  How to derive Offset_D depends on the implementation of this
 profile over a specific link technology and must be specified in the
 implementation document(s).  For example, it can be calculated by
 counting the total number of non-context-updating packets (including
 NHPs) and packet loss indications received since the last successful
 context update.  Alternatively, it can be derived using the link
 timing in the case where the linear mapping between RTP SN and link
 timing is maintained.
 On the transmitting side, the AL follows the same rule defined in
 section 4.1.1 of [RFC3242] to determine whether it can send NHP or
 not, with one modification.  That is, when the AL determines that it
 has become unsafe (see section 2.2) to send NHPs, the AL records the
 corresponding RTP SN as SN_break.  Then it waits until the rule is
 satisfied again and SN_ACKed > SN_break before it resumes sending
 NHPs.  The latter condition is essentially the counterpart of
 optimistic approach agreement [RFC3242, section 4.3] of U/O-mode
 which states that when the AL in U/O-mode determines it is unsafe to
 send NHP, it must send headers in the subsequent X packets, where X
 is some agreed number.  There are two reasons for the difference: a)
 R-mode relies on acknowledgements to synchronize contexts, instead of
 optimistic approach principle as in U/O-mode; and b) R-0 packets do
 not update decompressor context while UO-0 packets do.  To meet the
 condition SN_ACKed > SN_break, the AL can either wait passively for
 the compressor to send a context update packet (e.g., R-0-CRC
 triggered by 6-bit SN wrap-around), or send an update_request via the
 interface from AL to the compressor (section 2.2) to request the
 compressor to send a context updating packet.  The update_request
 carries the last SN_break.  Upon receiving an update_request, the
 compressor SHOULD use a context updating packet (e.g. R-0-CRC) when
 sending the next packet.  Context updating packets are handled as in
 Note: the passive waiting as described above might take a long time
 in the worst case, during which NHPs cannot be sent.  Therefore,
 sending update_request via the optional AL to compressor interface is
 RECOMMENDED to improve the worst case performance.
 Note: the update_request may be lost if the AL and compressor are at
 different locations and the channel between them is unreliable, but
 such a loss only delays the AL from resuming sending NHP.  Therefore,
 how frequent the AL sends update_request is an implementation issue.
 For example, the AL may send one update_request for each packet it
 receives from the compressor until the conditions to send NHP are

Liu & Le Standards Track [Page 3] RFC 3408 0-byte Support for R-mode December 2002

 Note: as no CRC field is present in R-0 packets, only the function
 related to RTP SN and packet type identifier needs to be replaced.
 In addition, NHP packets and packet loss indications in R-mode do not
 update either the compressor or the decompressor context (as opposed
 to U/O-mode).  Consequently, the secure reference principle [RFC3095,
 section 5.5] is not affected in any way and there is no loss of
 robustness in this profile compared to ROHC RTP.

4. Differences between R-mode and U/O-mode

 This section clarifies some differences between R-mode and U/O-mode
 in this profile.
 a) CRC replacement
    Unlike U/O-mode, CRC replacement [RFC3242, section 3.3] is not an
    issue for R-mode since R-0 packets do not carry CRC field.
 b) Periodic context verification
    For U/O-mode, periodic context verification [RFC3242, section 4.6]
    is RECOMMENDED to provide additional protection against damage
    propagation after CRC is replaced.  For R-mode, since there is no
    CRC replacement (see above), no change to ROHC RTP is needed in
    this regard.  In particular, R-mode has this feature naturally
    built-in, since the sending of R-0-CRC when 6-bit SN wraps around
    implicitly provides periodic context verification for R-mode.
 c) CV-REQUEST option
    For the same reasons as above, the decompressor operating in R-
    mode SHOULD NOT send CV-REQUEST [RFC3242, section 4.5] to
    compressor.  This is to avoid unnecessary overhead on the feedback
 d) Context Check Packet (CCP)
    When CCP [RFC3242, section 4.1.3] is used, compressor operating in
    R-mode SHOULD set C-bit to 0 (zero) and not generate 7-bit CRC if
    computation cost at compressor and decompressor causes concern.
    The use of the CRC field in CCP to perform decompressor context
    verification is not critical in R-mode (see last note of section 3
    and item b) above).
 e) Handling of Acknowledgements (ACKs)
    Special care in the realization of ACKs should be taken for R-mode
    implementations.  It is RECOMMENDED to avoid the use of
    interspersed feedback packets [RFC3095, section 5.2.1] to carry
    ACK information.  The reason is that interspersed feedback packets
    will interrupt the RTP SN sequencing and thus temporarily disable
    the sending of NHPs.

Liu & Le Standards Track [Page 4] RFC 3408 0-byte Support for R-mode December 2002

5. IANA Considerations

 A ROHC profile identifier has been reserved by the IANA for the
 profile defined in this document (0x0105), where 0x0005 is the
 profile identifier assigned for LLA [RFC3242].

6. Security Considerations

 The security considerations of ROHC RTP [RFC3095, section 7] apply
 also to this document with one addition: in the case of a denial-of-
 service attack scenario where an intruder injects bogus CCP packets
 onto the link using random CRC values, the CRC check will fail for
 incorrect reasons at the decompressor side.  This would obviously
 greatly reduce the advantages of ROHC and any extra efficiency
 provided by this profile due to unnecessary context invalidation,
 feedback messages and refresh packets.  However, the same remarks
 related to the presence of such an intruder apply.

7. Acknowledgements

 The authors would like to thank Lars-Erik Jonsson and Ghyslain
 Pelletier for intriguing discussions on LLA that helped to nail down
 the R-mode operation.  The authors also appreciate valuable input
 from Carsten Bormann, Christopher Clanton, Mark Cheng, and Thinh

8. References

 [RFC3242]   Jonsson, L. and G. Pelletier, "RObust Header Compression
             (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP",
             RFC 3242, April 2002.
 [RFC3095]   Bormann, C., Burmeister, C., Degermark, M., Fukushima,
             H., Hannu, H., Jonsson, L., Hakenberg, R., Koren, T., Le,
             K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
             Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header
             Compression (ROHC): Framework and four profiles: RTP,
             UDP, ESP, and uncompressed", RFC 3095, July 2001.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to indicate
             requirement levels", BCP 14, RFC 2119, March 1997.

Liu & Le Standards Track [Page 5] RFC 3408 0-byte Support for R-mode December 2002

9. Authors' Addresses

 Zhigang Liu
 Nokia Research Center
 6000 Connection Drive
 Irving, TX 75039
 Phone: +1 972 894-5935
 Fax:   +1 972 894-4589
 Khiem Le
 Nokia Research Center
 6000 Connection Drive
 Irving, TX 75039
 Phone: +1 972 894-4882
 Fax:   +1 972 894-4589

Liu & Le Standards Track [Page 6] RFC 3408 0-byte Support for R-mode December 2002

10. Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Liu & Le Standards Track [Page 7]

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