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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.


 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.

Bormann Standards Track [Page 1] RFC 3241 ROHC over PPP April 2002

 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",
 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.

Bormann Standards Track [Page 2] RFC 3241 ROHC over PPP April 2002

    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

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.
    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.

Bormann Standards Track [Page 3] RFC 3241 ROHC over PPP April 2002

         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...
    >= 10
    The length may be increased if the presence of additional
    parameters is indicated by additional suboptions.
    0003 (hex)
    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).
    The MRRU field is two octets and indicates the maximum
    reconstructed reception unit (see [RFC3095], section 5.1.1).
       Suggested value: 0
    The largest header size in octets that may be compressed.
       Suggested value: 168 octets

Bormann Standards Track [Page 4] RFC 3241 ROHC over PPP April 2002

    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
    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.
    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.
    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...

Bormann Standards Track [Page 5] RFC 3241 ROHC over PPP April 2002

       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.

Bormann Standards Track [Page 6] 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
    Value: 0003 (hex)
 ROHC large-CIDs
    The frame contains a ROHC packet with large CIDs as defined in
    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.

Bormann Standards Track [Page 7] RFC 3241 ROHC over PPP April 2002

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

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 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.

Bormann Standards Track [Page 8] 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
 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

Bormann Standards Track [Page 9] RFC 3241 ROHC over PPP April 2002

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.

Bormann Standards Track [Page 10] RFC 3241 ROHC over PPP April 2002

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

Bormann Standards Track [Page 11] RFC 3241 ROHC over PPP April 2002

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
 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.

Bormann Standards Track [Page 12]

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