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RFC 3241

Robust Header Compression (ROHC) over PPP

Pages: 12
Proposed Standard
Updates:  1332
Updated by:  4815

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Network Working Group                                         C. Bormann
Request for Comments: 3241                                TZI/Uni Bremen
Updates: 1332                                                 April 2002
Category: Standards Track


               Robust Header Compression (ROHC) over PPP

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.

Abstract

This document describes an option for negotiating the use of robust header compression (ROHC) on IP datagrams transmitted over the Point-to-Point Protocol (PPP). It defines extensions to the PPP Control Protocols for IPv4 and IPv6.

1. Introduction

Robust Header Compression (ROHC) as defined in [RFC3095] may be used for compression of both IPv4 and IPv6 datagrams or packets encapsulated with multiple IP headers. The initial version of ROHC focuses on compression of the packet headers in RTP streams, while supporting compression of other UDP flows; however, it also defines a framework into which further header compression mechanisms can be plugged as new profiles. Planned additions to the set of profiles supported by ROHC will be capable of compressing TCP transport protocol headers as well. In order to establish compression of IP datagrams sent over a PPP link each end of the link must agree on a set of configuration parameters for the compression. The process of negotiating link parameters for network layer protocols is handled in PPP by a family of network control protocols (NCPs). Since there are separate NCPs for IPv4 and IPv6, this document defines configuration options to be used in both NCPs to negotiate parameters for the compression scheme.
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   ROHC does not require that the link layer be able to indicate the
   types of datagrams carried in the link layer frames.  However, there
   are two basic types of ROHC headers defined in the ROHC framework:
   small-CID headers (zero or one bytes are used to identify the
   compression context) and large-CID headers (one or two bytes are used
   for this purpose).  To keep the PPP packets self-describing, in this
   document two new types for the PPP Data Link Layer Protocol Field are
   defined, one for small-CID ROHC packets and one for large-CID ROHC
   packets.  (This also avoids a problem that would occur if PPP were to
   negotiate which of the formats to use in each of IPCP and IPV6CP and
   the two negotiation processes were to arrive at different results.)
   A PPP ROHC sender may send packets in either small-CID or large-CID
   format at any time, i.e., the LARGE_CIDS parameter from [RFC3095] is
   not used.  Any PPP ROHC receiver MUST be able to process both small-
   CID and large-CID ROHC packets, therefore no negotiation of this
   function is required.

   ROHC assumes that the link layer delivers packets in sequence.  PPP
   normally does not reorder packets.  When using reordering mechanisms
   such as multiclass multilink PPP [RFC2686], care must be taken so
   that packets that share the same compression context are not
   reordered.  (Note that in certain cases, reordering may be acceptable
   to ROHC, such as within a sequence of packets that all do not change
   the decompression context.)

   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.

2. Configuration Option

This document specifies a new compression protocol value for the IPCP IP-Compression-Protocol option as specified in [RFC1332]. The new value and the associated option format are described in section 2.1. The option format is structured to allow future extensions to the ROHC scheme. It may be worth repeating [RFC1332], section 4: "The IP-Compression- Protocol Configuration Option is used to indicate the ability to receive compressed packets. Each end of the link must separately request this option if bi-directional compression is desired." I.e., the option describes the capabilities of the decompressor (receiving side) of the peer that sends the Configure-Request.
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      NOTE: The specification of link and network layer parameter
      negotiation for PPP [RFC1661], [RFC1331], [RFC1332] does not
      prohibit multiple instances of one configuration option but states
      that the specification of a configuration option must explicitly
      allow multiple instances.  From the current specification of the
      IPCP IP-Compression-Protocol configuration option [RFC1332] one
      can infer that it can only be used to select a single compression
      protocol at any time.

      This was appropriate at a time when only one header compression
      scheme existed.  With the advent of IP header compression
      [RFC2507, RFC2509], this did not really change, as RFC 2507
      essentially superseded RFC 1144.  However, with ROHC, it may now
      very well be desirable to use RFC 2507 TCP compression in
      conjunction with RFC 3095 RTP/UDP compression.

   The present document now updates RFC 1332 by explicitly allowing the
   sending of multiple instances of the IP-Compression-Protocol
   configuration option, each with a different value for IP-
   Compression-Protocol.  Each type of compression protocol may
   independently establish its own parameters.

   This change is believed to not cause significant harm in existing PPP
   implementations, as they would most likely Configure-Nak or
   Configure-Reject the duplicate option, or simply happen to accept the
   one option they understand.  To aid interoperability, the peer
   implementing the present specification SHOULD react to a Configure-
   Nak or Configure-Reject by reducing the number of options offered to
   one.

2.1. Configuration Option Format

Both the network control protocol for IPv4, IPCP [RFC1332] and the IPv6 NCP, IPV6CP [RFC2472] may be used to negotiate IP Header Compression parameters for their respective protocols. The format of the configuration option is the same for both IPCP and IPV6CP. Description This NCP configuration option is used to negotiate parameters for Robust Header Compression. The option format is summarized below. The fields are transmitted from left to right.
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           Figure 1: Robust Header Compression (ROHC) Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    IP-Compression-Protocol    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            MAX_CID            |             MRRU              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           MAX_HEADER          |          suboptions...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      2

   Length
      >= 10

      The length may be increased if the presence of additional
      parameters is indicated by additional suboptions.

   IP-Compression-Protocol
      0003 (hex)

   MAX_CID
      The MAX_CID field is two octets and indicates the maximum value of
      a context identifier.

         Suggested value: 15

      MAX_CID must be at least 0 and at most 16383 (The value 0 implies
      having one context).

   MRRU
      The MRRU field is two octets and indicates the maximum
      reconstructed reception unit (see [RFC3095], section 5.1.1).

         Suggested value: 0

   MAX_HEADER
      The largest header size in octets that may be compressed.

         Suggested value: 168 octets
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      The value of MAX_HEADER should be large enough so that at least
      the outer network layer header can be compressed.  To increase
      compression efficiency MAX_HEADER should be set to a value large
      enough to cover common combinations of network and transport layer
      headers.

      NOTE: The four ROHC profiles defined in RFC 3095 do not provide
      for a MAX_HEADER parameter.  The parameter MAX_HEADER defined by
      this document is therefore without consequence in these profiles.
      Other profiles (e.g., ones based on RFC 2507) can make use of the
      parameter by explicitly referencing it.

   suboptions
      The suboptions field consists of zero or more suboptions.  Each
      suboption consists of a type field, a length field and zero or
      more parameter octets, as defined by the suboption type.  The
      value of the length field indicates the length of the suboption in
      its entirety, including the lengths of the type and length fields.

                          Figure 2: Suboption

             0                   1                   2
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |     Type      |    Length     |  Parameters...
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2. PROFILES Suboption

The set of profiles to be enabled is subject to negotiation. Most initial implementations of ROHC implement profiles 0x0000 to 0x0003. This option MUST be supplied. Description Define the set of profiles supported by the decompressor. Figure 3: PROFILES suboption 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Profiles... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 1
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      Length
         2n+2

      Value
         n octet-pairs in ascending order, each octet-pair specifying a
         ROHC profile supported.

3. Multiple Network Control Protocols

The ROHC protocol is able to compress both IPv6 and IPv4 datagrams. Both IPCP and IPV6CP are able to negotiate option parameter values for ROHC. The ROHC capability negotiated as a whole applies to the compression of packets where the outer header is an IPv4 header and an IPv6 header, respectively; e.g., an outer IPv6 header MUST NOT be sent if the ROHC IP-Compression-Protocol option was not negotiated for IPV6CP. Offering a specific ROHC capability in a Configure-Request in either IPCP or IPV6CP indicates that the capability is provided for the entire ROHC channel formed by the PPP link. When the option has been negotiated with different values in IPCP and IPV6CP, the result is that the set of parameter values for the entire ROHC channel is the logical union of the two values, i.e., the maximum for MAX_CID, MRRU or MAX_HEADER, and the logical union of the suboptions. For the PROFILES suboption, the logical union is the union of the two sets of profiles. The unified values are kept as valid parameter values for the ROHC channel even when either of the NCPs is taken down. Note that each new suboption for this option must define the meaning of "logical union", if the concept applies.

3.1. Sharing Context Identifier Space

For the compression and decompression of IPv4 and IPv6 datagram headers, the context identifier space is shared. While the parameter values are independently negotiated, sharing the context identifier spaces becomes more complex when the parameter values differ. Since the compressed packets share context identifier space, the compression engine must allocate context identifiers out of a common pool; for compressed packets, the decompressor has to examine the context state to determine what parameters to use for decompression. In particular, the context identifier space is shared between ROHC small-CID packets and ROHC large-CID packets. From the point of view of the ROHC framework, the PPP NCP instances for IPCP and IPV6CP together constitute exactly one ROHC channel; its feedback is destined for the ROHC channel defined by the NCP instances for IPCP and IPV6CP in the reverse direction on the same PPP link.
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   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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   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: cabo@tzi.org
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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 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 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 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. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.