Network Working Group Lars-Erik Jonsson, Ericsson INTERNET-DRAFT Sweden Expires: May 2002 November 21, 2001 Requirements for ROHC IP/TCP Header Compression Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is a submission of the IETF ROHC WG. Comments should be directed to the ROHC WG mailing list, rohc@cdt.luth.se. Abstract This document contains requirements for the IP/TCP header compression scheme (profile) to be developed by the ROHC WG. The structure of this document is inherited from the document defining IP/UDP/RTP requirements for ROHC. Jonsson [Page 1] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 0. Document history February 23, 2001 - draft-ietf-rohc-tcp-requirements-00.txt Initial version to initiate discussion of TCP aspects in ROHC. June 20, 2001 - draft-ietf-rohc-tcp-requirements-01.txt ECN requirements, note to "Short lived TCP transfers", and a section discussing the IPR issue have been added. November 21, 2001 - draft-ietf-rohc-tcp-requirements-02.txt The error propagation requirement has been split in two parts and updated. Further have references to PILC documents and a note for "Ubiquity" about TCP versions been added. 1. Introduction The goal of the ROHC WG is to develop header compression schemes that perform well over links with high error rates and long link roundtrip times. The schemes must perform well for cellular links, using technologies such as WCDMA, EDGE, and CDMA-2000. However, the schemes should also be applicable to other future link technologies with high loss and long roundtrip times. The main objective for ROHC has been robust compression of IP/UDP/RTP, but the WG is also chartered to develop new header compression solutions for IP/TCP [RFC-791, RFC-793]. Since TCP traffic, in contrast to RTP, has usually been sent over reliable links, existing schemes for TCP [RFC-1144, RFC-2507] have not experienced the same robustness problems as RTP compression. However, there are still many scenarios where TCP header compression will be implemented over less reliable links [RFC-3150, PILC-ARQ], making robustness an important objective also for the new TCP compression scheme. Other, as important, objectives for ROHC TCP compression are; improved compression efficiency, enhanced capabilities for compression of header fields including TCP options, and finally incorporation of TCP compression into the ROHC framework [RFC-3095]. 2. Header compression requirements The following requirements have, more or less arbitrarily, been divided into five groups. The first group deals with requirements concerning the impact of a header compression scheme on the rest of the Internet infrastructure. The second group defines what kind of headers that must be compressed efficiently while the third and forth groups concern performance requirements and capability requirements which stem from the properties of the anticipated link technologies. Finally, the fifth section discusses Intellectual Property Rights related to ROHC TCP compression. Jonsson [Page 2] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 2.1. Impact on Internet infrastructure 1. Transparency: When a header is compressed and then decompressed, the resulting header must be semantically identical to the original header. If this cannot be achieved, the packet containing the erroneous header must be discarded. Justification: The header compression process must not produce headers that might cause problems for any current or future part of the Internet infrastructure. Note: The ROHC WG has not found a case where "semantically identical" is not the same as "bitwise identical". 2. Ubiquity: Must not require modifications to existing IP (v4 or v6) or TCP implementations. Justification: Ease of deployment. Note: The ROHC WG may recommend changes that would increase the compression efficiency for the TCP streams emitted by implementations. However, ROHC cannot rely on such recommendations being followed. Note: Several TCP versions are currently in use on the Internet. This requirement implies that the header compression scheme must work efficiently and correct for all expected TCP versions. 2.2. Supported headers and kinds of TCP streams 1. IPv4 and IPv6: Must support both IPv4 and IPv6. This means that all possible changes in the IP header fields must be handled by the compression scheme and commonly changing fields should be compressed efficiently. The compression scheme must consider as normal operation the scenario where Early Congestion Notification [RFC-3168] is used and support efficient compression also in the case when the ECN bits are used. Justification: IPv4 and IPv6 will both be around during the foreseeable future. ECN is expected to get a breakthrough and be widely deployed, especially in combination with TCP. 2. Mobile IP: The kinds of headers used by Mobile IP{v4,v6} must be supported and should be compressed efficiently. For IPv4 these include headers of tunneled packets. For IPv6 these include headers containing the Routing Header, the Binding Update Destination Option, and the Home Address option. Justification: It is very likely that Mobile IP will be used by cellular devices. Jonsson [Page 3] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 3. Generality: Must compress headers from arbitrary TCP streams. Justification: There must be a generic scheme which can compress reasonably well for any TCP traffic pattern. This does not preclude optimizations for certain traffic patterns. Note: This applies to the TCP stream before as well as after it has passed through an internet. 4. IPSEC: The scheme should be able to compress headers containing IPSEC sub-headers. Justification: IPSEC is expected to be used to provide necessary end-to-end security. Note: It is of course not possible to compress the encrypted part of an ESP header, nor the cryptographic data in an AH header. 5. TCP: All fields supported by [RFC-2507] must be handled with efficient compression, and so also the cases when the SYN, FIN or ECN bits are set. Justification: These bits are expected to be commonly used. 6. TCP options: The scheme must support compression of packets with any TCP option present, even if the option itself is not compressed. Further, for some commonly used options the scheme should provide compression mechanisms also for the options. Justification: Since various TCP options are commonly used, applicability of the compression scheme would be significantly reduced if packets with options could not be compressed. Note: Options that should be compressed are: - Window scale, [RFC-1323] - Selective Acknowledgement (SACK), [RFC-2018, RFC-2883] - Timestamp, [RFC-1323] Jonsson [Page 4] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 2.3. Performance issues 1. Performance/Spectral Efficiency: Must provide low relative overhead under expected operating conditions; compression efficiency should be better than for RFC2507 under equivalent operating conditions. Justification: Spectrum efficiency is a primary goal. Note: the relative overhead is the average header overhead relative to the payload. Any auxiliary (e.g., control or feedback) channels used by the scheme should be taken into account when calculating the header overhead. 2. Loss propagation: Loss propagation due to header compression should be avoided. Loss propagation is defined as the loss of headers subsequent to headers lost by the link, even if those subsequent headers are not lost by the link. Justification: Even though link layer retransmission in most cases is expected to make loss propagation an irrelevant issue, there are still many scenarios where TCP header compression will be implemented over less reliable links [RFC-3150, PILC-ARQ]. In such cases, loss propagation due to header compression could affect certain TCP mechanisms that are capable of handling some losses, and thereby cause significant performance degradation. 3. Damage propagation and handling of residual errors: Damage propagation is defined as the damage of headers subsequent to headers damaged by the link, even if those subsequent headers are not damaged by the link. For TCP, the compression scheme is not required to explicitly implement mechanisms for damage propagation avoidance, but the compression scheme must not affect TCP's end-to-end mechanisms for handling of residual errors. Justification: For links carrying TCP traffic, the residual error rate is expected to be insignificant. However, residual errors may still occur, especially in the end-to-end path, and therefore it is crucial that TCP is not prevented from handling these. 4. Short lived TCP transfers: The scheme should provide mechanisms for efficient compression of short-lived TCP transfers, minimizing the size of context initiation headers. Justification: Many TCP transfers are short-lived. This means that the gain of header compression could be low since normally header compression sends full headers initially and small compressed headers first after the initiation phase. Note: This requirement implies that mechanisms for "context sharing" or "context re-use" should be considered. Jonsson [Page 5] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 5a. Moderate Packet Reordering: The scheme should efficiently handle moderate reordering (2-3 packets) in the packet stream reaching the compressor. Justification: This kind of reordering is common. 5b. Packet Reordering: The scheme should be able to compress when there are reordered packets in the TCP stream reaching the compressor. Justification: Reordering happens regularly in the Internet. However, since the Internet is engineered to run TCP reasonably well, excessive reordering will not be common and need not be handled with optimum efficiency. 6. Processing delay: The scheme must not contribute significantly to system delay budget. 2.4. Capability requirements related to link layer characteristics 1. Unidirectional links: Must be possible to implement (possibly with less efficiency) without explicit feedback messages from decompressor to compressor. Justification: There are links that do not provide a feedback channel or feedback is not desirable for other reasons. 2. Link delay: Must operate under all expected link delay conditions. 3. Header compression coexistence: The scheme must fit into the ROHC framework together with other ROHC profiles 2.5. Intellectual property rights (IPR) The ROHC WG must spend effort to achieve a high degree of confidence that there is no IPR covering a final compression solution for TCP. Justification: Currently there is no TCP header compression scheme available that can efficiently compress the packet headers of modern TCP, e.g. with SACK, ECN, etc. ROHC is expected to fill this gap by providing a ROHC TCP scheme that can be applicable in the wide area Internet, not only over error-prone radio links. It must thus attempt to be as future-proof as possible, and, in particular, only unencumbered solutions will be acceptable to the Internet at large. Jonsson [Page 6] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 3. Open questions - For further discussion Q: Should we require the scheme to be capable of handling some reordering between compressor and decompressor? Q: Should we have a separate requirement for reordering of acks? 4. IANA Considerations A protocol which meets these requirements, will require the IANA to assign various numbers. This document by itself, however, does not require any IANA involvement. 5. Security Considerations A protocol specified to meet these requirements must be able to compress packets containing IPSEC headers according to the IPSEC requirement, 2.2.4. There may be other security aspects to consider in such protocols. This document by itself, however, does not add any security risks. 6. References [RFC-791] Jon Postel, Internet Protocol, RFC 791, September 1981. [RFC-793] Jon Postel, Transport Control Protocol, RFC 793, September 1981. [RFC-1144] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC-2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP Header Compression", RFC 2507, February 1999. [RFC-3096] Mikael Degermark, "Requirements for IP/UDP/RTP header compression", RFC 3096, July 2001. [RFC-3095] Carsten Bormann, et. al., "Robust Header Compression (ROHC)", RFC 3095, July 2001. [RFC-1323] Van Jacobson, Bob Braden, Dave Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [RFC-2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow, "TCP Selective Acknowledgement Option", RFC 2018, October 1996. [RFC-2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matthew Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000. Jonsson [Page 7] INTERNET-DRAFT Requirements for IP/TCP ROHC November 21, 2001 [RFC-3168] K. K. Ramakrishnan, Sally Floyd, David L. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. [RFC-3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent Magret, "End-to-end Performance Implications of Slow Links", RFC 3150, July 2001. [PILC-ARQ] Gorry Fairhurst, Lloyd Wood, "Advice to link designers on link Automatic Repeat reQuest (ARQ)", Internet Draft (work in progress), August 2001. 7. Author's address Lars-Erik Jonsson Tel: +46 920 20 21 07 Ericsson Erisoft AB Fax: +46 920 20 20 99 Box 920 Mobile: +46 70 513 56 21 SE-971 28 Lulea Sweden EMail: lars-erik.jonsson@ericsson.com This Internet-Draft expires May 21, 2002. Jonsson [Page 8]