rfc3241.txt

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RFC 3241                     ROHC over PPP                    April 2002


   In particular, this means that taking down either of the NCPs while
   the other is still open means that the contexts of the channel stay
   active.  To avoid race conditions, the same is true if both NCPs are
   taken down and then one or more is reopened.  Taking down LCP
   destroys the channel, however; reopening LCP and then one or more of
   IPCP and IPV6CP restarts ROHC with all contexts in no-context state.

4.  Demultiplexing of Datagrams

   The ROHC specification [RFC3095] defines a single header format for
   all different types of compressed headers, with a variant for small
   CIDs and a variant for large CIDs.  Two PPP Data Link Layer Protocol
   Field values are specified below.

   ROHC small-CIDs

      The frame contains a ROHC packet with small CIDs as defined in
      [RFC3095].

      Value: 0003 (hex)

   ROHC large-CIDs

      The frame contains a ROHC packet with large CIDs as defined in
      [RFC3095].

      Value: 0005 (hex)

   Note that this implies that all CIDs within one ROHC packet MUST be
   of the same size as indicated by the Data Link Layer Protocol field,
   either small or large.  In particular, embedded feedback MUST have a
   CID of the same size as indicated by the Protocol field value.  For
   piggybacking feedback, a compressor must be able to control the
   feedback CID size used by the associated decompressor, ensure that
   all CIDs are of the same size, and indicate this size with the
   appropriate Protocol Field value.

   To make CID interpretation unambiguous when ROHC segmentation is
   used, all packets that contribute to a segment MUST be sent with the
   same Data Link Layer Protocol Field value, either 0003 or 0005, which
   then also applies to the CID size in the reconstructed unit.  A unit
   reconstructed out of packets with Protocol field values that differ
   MUST be discarded.








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5.  ROHC Usage Considerations

   Certain considerations are required for any ROHC-over-X protocol.
   This section describes how some of these are handled for ROHC over
   PPP.

5.1.  Uncompressed profile

   There is no need for the ROHC uncompressed profile in ROHC over PPP,
   as uncompressed packets can always be sent using the PPP protocol
   demultiplexing method.  Therefore, no consideration was given to
   locking down one of the context numbers for the uncompressed profile
   (see [RFC3095] section 5.1.2).  Note, however, that according to the
   ROHC specification, profile 0x0000 must not be rejected [RFC3095], so
   it MUST be implemented by all receivers.

5.2.  Parameter selection

   For each of the ROHC channel parameters MAX_CID and MRRU, the value
   is the maximum of the respective values negotiated for the IPCP and
   IPv6CP instances, if any.  The ROHC channel parameter FEEDBACK_FOR is
   set implicitly to the reverse direction on the same PPP link (see
   "Sharing Context Identifier Space" above).  The ROHC channel
   parameter LARGE_CIDS is not used, instead the PPP protocol ID on the
   packet is used (see "Demultiplexing of Datagrams" above).

   A number of parameters for ROHC must be set correctly for good
   compression on a specific link.  E.g., the parameters k_1, n_1, k_2,
   n_2 in section 5.3.2.2.3 of [RFC3095] need to be set based on the
   error characteristics of the underlying links.  As PPP links are
   usually run with a strong error detection scheme [RFC1662], k_1 = n_1
   = k_2 = n_2 = 1 is usually a good set of values.  (Note that in any
   case k values need to be set low enough relative to n values to allow
   for the limited ability of the CRC to detect errors, i.e., the CRC
   will succeed for about 1/8 of the packets even in case of context
   damage, so k/n should be significantly less than 7/8.)

6.  Security Considerations

   Negotiation of the option defined here imposes no additional security
   considerations beyond those that otherwise apply to PPP [RFC1661].

   The security considerations of ROHC [RFC3095] apply.

   The use of header compression can, in rare cases, cause the
   misdelivery of packets.  If necessary, confidentiality of packet
   contents should be assured by encryption.




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RFC 3241                     ROHC over PPP                    April 2002


   Encryption applied at the IP layer (e.g., using IPSEC mechanisms)
   precludes header compression of the encrypted headers, though
   compression of the outer IP header and authentication/security
   headers is still possible as described in [RFC3095].  For RTP
   packets, full header compression is possible if the RTP payload is
   encrypted by itself without encrypting the UDP or RTP headers, as
   described in [RFC1889].  This method is appropriate when the UDP and
   RTP header information need not be kept confidential.

7.  IANA considerations

   The ROHC suboption identifier is a non-negative integer.  Following
   the policies outlined in [RFC2434], the IANA policy for assigning new
   values for the suboption identifier shall be Specification Required:
   values and their meanings must be documented in an RFC or in some
   other permanent and readily available reference, in sufficient detail
   that interoperability between independent implementations is
   possible.  The range 0 to 127 is reserved for IETF standard-track
   specifications; the range 128 to 254 is available for other
   specifications that meet this requirement (such as Informational
   RFCs).  The value 255 is reserved for future extensibility of the
   present specification.

   The following suboption identifiers are already allocated:

   Suboption   Document       Usage
   identifier

   1           RFC3241        Profiles

   The RFC 3006 compressibility hint [RFC3006] for ROHC is 0x0003pppp,
   where 0xpppp is the profile assumed.

   (Note that the PPP protocol identifier values 0003 and 0005 were
   taken from a previously reserved space that exhibits inefficient
   transparency in the presence of asynchronous control character
   escaping, as it is considered rather unlikely that ROHC will be used
   over links with highly populated ACCMs.)

8.  Acknowledgments

   The present document borrows heavily from [RFC2509].

   The author would like to thank Pete McCann and James Carlson for
   clarifying the multiple option instance issue, Craig Fox for helping
   with some PPP arcana, and Lars-Erik Jonsson for supplying some final
   clarifications.




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9.  References

9.1.  Normative References


   [RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
             (IPCP)", RFC 1332, May 1992.

   [RFC1661] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", STD
             51, RFC 1661, July 1994.

   [RFC2472] Haskin, E. and E. Allan, "IP Version 6 over PPP", RFC 2472,
             December 1998.

   [RFC3006] Davie, B., Casner, S., Iturralde, C., Oran, D. and J.
             Wroclawski, "Integrated Services in the Presence of
             Compressible Flows", RFC 3006, November 2000.

   [RFC3095] 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.

9.2.  Informative References

   [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
             Serial Links", RFC 1144, February 1990.

   [RFC1889] Schulzrinne, H., Casner, S., Frederick, R. and V.
             Jacobson, "RTP: A Transport Protocol for real-time
             applications", RFC 1889, January 1996.

   [RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

   [RFC2507] Degermark, M., Nordgren, B. and S. Pink, "IP Header
             Compression", RFC 2507, February 1999.

   [RFC2509] Engan, M., Casner, S. and C. Bormann, "IP Header
             Compression over PPP", RFC 2509, February 1999.

   [RFC2686] Bormann, C., "The Multi-Class Extension to Multi-Link PPP",
             RFC 2686, September 1999.





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10.  Author's Address

   Carsten Bormann
   Universitaet Bremen FB3 TZI
   Postfach 330440
   D-28334 Bremen, GERMANY

   Phone: +49.421.218-7024
   Fax:   +49.421.218-7000
   EMail: [e-mail]









































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11.  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
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   followed, or as required to translate it into languages other than
   English.

   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
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

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



















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