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rfc:rfc2509

Network Working Group Request for Comments: 2509 M. Engan Category: Standards Track Effnet

                                                             S. Casner
                                                         Cisco Systems
                                                            C. Bormann
                                               Universitaet Bremen TZI
                                                         February 1999
                   IP Header Compression 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 (1999).  All Rights Reserved.

Abstract

 This document describes an option for negotiating the use of header
 compression on IP datagrams transmitted over the Point-to-Point
 Protocol [RFC1661]. It defines extensions to the PPP Control
 Protocols for IPv4 and IPv6 [RFC1332, RFC2023]. Header compression
 may be applied to IPv4 and IPv6 datagrams in combination with TCP,
 UDP and RTP transport protocols as specified in [IPHC] and [CRTP].

1. Introduction

 The IP Header Compression (IPHC) defined in [IPHC] may be used for
 compression of both IPv4 and IPv6 datagrams or packets encapsulated
 with multiple IP headers. IPHC is also capable of compressing both
 TCP and UDP transport protocol headers.  The IP/UDP/RTP header
 compression defined in [CRTP] fits within the framework defined by
 IPHC so that it may also be applied to both IPv4 and IPv6 packets.
 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

Engan, et. al. Standards Track [Page 1] RFC 2509 IP Header Compression over PPP February 1999

 used in both NCPs to negotiate parameters for the compression scheme.
 IPHC relies on the link layer's ability to indicate the types of
 datagrams carried in the link layer frames. In this document nine new
 types for the PPP Data Link Layer Protocol Field are defined along
 with their meaning.
 In general, header compression schemes that use delta encoding of
 compressed packets require that the lower layer does not reorder
 packets between compressor and decompressor. IPHC uses delta encoding
 of compressed packets for TCP and RTP.  The IPHC specification [IPHC]
 includes methods that allow link layers that may reorder packets to
 be used with IPHC.  Since PPP does not reorder packets these
 mechanisms are disabled by default.  When using reordering mechanisms
 such as multiclass multilink PPP [MCML], care must be taken so that
 packets that share the same compression context are not reordered.

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
 IPHC scheme.
    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, p 6]
    it follows that it can only be used to select a single compression
    protocol at any time.
    NOTE: [RFC1332] is not explicit about whether the option
    negotiates the capabilities of the receiver or of the sender.  In
    keeping with current practice, we assume that the option describes
    the capabilities of the decompressor (receiving side) of the peer
    that sends the Config-Req.

Engan, et. al. Standards Track [Page 2] RFC 2509 IP Header Compression over PPP February 1999

2.1. Configuration Option Format

 Both the network control protocol for IPv4, IPCP [RFC1332] and the
 IPv6 NCP, IPV6CP [RFC2023] 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
    IP Header Compression.  The option format is summarized below.
    The fields are transmitted from left to right.
     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    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           TCP_SPACE           |         NON_TCP_SPACE         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         F_MAX_PERIOD          |          F_MAX_TIME           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           MAX_HEADER          |          suboptions...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    2
 Length
    >= 14
    The length may be increased if the presence of additional
    parameters is indicated by additional suboptions.
 IP-Compression-Protocol
    0061 (hex)
 TCP_SPACE
    The TCP_SPACE field is two octets and indicates the maximum value
    of a context identifier in the space of context identifiers
    allocated for TCP.
       Suggested value: 15
    TCP_SPACE must be at least 0 and at most 255 (The value 0 implies
    having one context).

Engan, et. al. Standards Track [Page 3] RFC 2509 IP Header Compression over PPP February 1999

 NON_TCP_SPACE
    The NON_TCP_SPACE field is two octets and indicates the maximum
    value of a context identifier in the space of context identifiers
    allocated for non-TCP. These context identifiers are carried in
    COMPRESSED_NON_TCP, COMPRESSED_UDP and COMPRESSED_RTP packet
    headers.
       Suggested value: 15
    NON_TCP_SPACE must be at least 0 and at most 65535 (The value 0
    implies having one context).
 F_MAX_PERIOD
    Maximum interval between full headers.  No more than F_MAX_PERIOD
    COMPRESSED_NON_TCP headers may be sent between FULL_HEADER
    headers.
       Suggested value: 256
    A value of zero implies infinity, i.e. there is no limit to the
    number of consecutive COMPRESSED_NON_TCP headers.
 F_MAX_TIME
    Maximum time interval between full headers.  COMPRESSED_NON_TCP
    headers may not be sent more than F_MAX_TIME seconds after sending
    the last FULL_HEADER header.
       Suggested value: 5 seconds
    A value of zero implies infinity.
 MAX_HEADER
    The largest header size in octets that may be compressed.
       Suggested value: 168 octets
    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.
 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.

Engan, et. al. Standards Track [Page 4] RFC 2509 IP Header Compression over PPP February 1999

           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 RTP-Compression Suboption

 The RTP-Compression suboption is included in the NCP
 IP-Compression-Protocol option for IPHC if IP/UDP/RTP compression is
 to be enabled.
 After successful negotiation of parameters for IP Header Compression
 the use of Protocol Identifiers FULL_HEADER, COMPRESSED_TCP,
 COMPRESSED_TCP_NODELTA and COMPRESSED_NON_TCP is enabled, regardless
 of the prescence of an RTP-Compression suboption.
 Description
    Enable use of Protocol Identifiers COMPRESSED_RTP, COMPRESSED_UDP and
    CONTEXT_STATE as specified in [CRTP].
           0                   1
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |     Type      |    Length     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Type
          1
       Length
          2

3. Multiple Network Control Protocols

 The IPHC protocol is able to compress both IPv6 and IPv4 datagrams.
 Both IPCP and IPV6CP are able to negotiate option parameter values
 for IPHC.  These values apply to the compression of packets where the
 outer header is an IPv4 header and an IPv6 header, respectively.

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

Engan, et. al. Standards Track [Page 5] RFC 2509 IP Header Compression over PPP February 1999

 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.
 Context identifier spaces are not shared between TCP and non-
 TCP/UDP/RTP.  Doing so would require additional mechanisms to ensure
 that no error can occur when switching from using a context
 identifier for TCP to non-TCP.

4. Demultiplexing of Datagrams

 The IPHC specification [IPHC] defines four header formats for
 different types of compressed headers.  They are compressed TCP,
 compressed TCP with no delta encoding, compressed non-TCP with 8 bit
 CID and compressed non-TCP with 16 bit CID. The two non-TCP formats
 may be distinguished by their contents so both may use the same
 link-level identifier.  A fifth header format, the full header is
 distinct from a regular header in that it carries additional
 information to establish shared context between the compressor and
 decompressor.
 The specification of IP/UDP/RTP Header Compression [CRTP] defines
 four additional formats of compressed headers.  They are for
 compressed UDP and compressed RTP (on top of UDP), both with either
 8- or 16-bit CIDs.  In addition, there is an explicit error message
 from the decompressor to the compressor.
 The link layer must be able to indicate these header formats with
 distinct values.  Nine PPP Data Link Layer Protocol Field values are
 specified below.
    FULL_HEADER
       The frame contains a full header as specified in [CRTP] Section
       3.3.1.  This is the same as the FULL_HEADER specified in [IPHC]
       Section 5.3.
          Value: 0061 (hex)
    COMPRESSED_TCP
       The frame contains a datagram with a compressed header with the
       format as specified in [IPHC] Section 6a.
          Value: 0063 (hex)
    COMPRESSED_TCP_NODELTA
       The frame contains a datagram with a compressed header with the
       format as specified in [IPHC] Section 6b.
          Value: 2063 (hex)

Engan, et. al. Standards Track [Page 6] RFC 2509 IP Header Compression over PPP February 1999

    COMPRESSED_NON_TCP
       The frame contains a datagram with a compressed header with the
       format as specified in either Section 6c or Section 6d of
       [IPHC].
          Value: 0065 (hex)
    COMPRESSED_RTP_8
       The frame contains a datagram with a compressed header with the
       format as specified in [CRTP] Section 3.3.2, using 8-bit CIDs.
          Value: 0069 (hex)
    COMPRESSED_RTP_16
       The frame contains a datagram with a compressed header with the
       format as specified in [CRTP] Section 3.3.2, using 16-bit CIDs.
          Value: 2069 (hex)
    COMPRESSED_UDP_8
       The frame contains a datagram with a compressed header with the
       format as specified in [CRTP] Section 3.3.3, using 8-bit CIDs.
          Value: 0067 (hex)
    COMPRESSED_UDP_16
       The frame contains a datagram with a compressed header with the
       format as specified in [CRTP] Section 3.3.3, using 16-bit CIDs.
          Value: 2067 (hex)
    CONTEXT_STATE
       The frame is a link-level message sent from the decompressor to
       the compressor as specified in [CRTP] Section 3.3.5.
          Value: 2065 (hex)

5. References

 [CRTP]     Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
            Headers for Low-Speed Serial Links", RFC 2508, February
            1999.
 [IPHC]     Degermark, M., Nordgren, B. and S. Pink, "Header
            Compression for IP", RFC 2507, February 1999.
 [RFC2023]  Haskin, E. and E. Allan, "IP Version 6 over PPP", RFC
            2023, October 1996.
 [RFC1144]  Jacobson, V., "Compressing TCP/IP Headers for Low- Speed
            Serial Links", RFC 1144, February 1990.
 [RFC1332]  McGregor, G., "The PPP Internet Protocol Control Protocol
            (IPCP)", RFC 1332, May 1992.

Engan, et. al. Standards Track [Page 7] RFC 2509 IP Header Compression over PPP February 1999

 [RFC1889]  Schulzrinne, H., Casner, S., Frederick, R. and V.
            Jacobson, "RTP: A Transport Protocol for real-time
            applications", RFC 1889, January 1996.
 [RFC1661]  Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", STD
            51, RFC 1661, July 1994.
 [MCML]     Bormann, C., "The Multi-Class Extension to Multi-Link
            PPP", Work in Progress.

6. Security Considerations

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

Engan, et. al. Standards Track [Page 8] RFC 2509 IP Header Compression over PPP February 1999

7. Authors' Addresses

 Mathias Engan
 Effnet
 Aurorum 2
 SE-977 75 Lulea, Sweden
 Phone: +46 920 75600
 Mobile: +46 70 833 8932
 Fax: +46 920 75610
 EMail: engan@effnet.com
 Stephen L. Casner
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134-1706
 United States
 EMail: casner@cisco.com
 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

Engan, et. al. Standards Track [Page 9] RFC 2509 IP Header Compression over PPP February 1999

8. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  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.

Engan, et. al. Standards Track [Page 10]

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