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Network Working GroupRequest for Comments: 2509 M. EnganCategory: Standards Track Effnet S. Casner Cisco Systems C. Bormann Universitaet Bremen TZI February 1999IP Header Compression over PPPStatus 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 beEngan, 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 insection 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 19992.1. 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            23. 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.  SinceEngan, 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]Section3.3.1. 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 eitherSection 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",RFC2023, 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 19997. 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.orgEngan, et. al.              Standards Track                     [Page 9]

RFC 2509 IP Header Compression over PPP February 19998. 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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