RoHC Working Group Ch. Schmidt INTERNET DRAFT M. Tuexen Siemens Expires March 14, 2002 September 14, 2001 Requirements for RoHC IP/SCTP Robust 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 to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be acessed at http://www.ietf.org/shadow.html. Abstract This document contains requirements for the IP/SCTP header compression scheme (profile) to be developed by the ROHC WG. The structure of this document is inherited from the document defining IP/TCP requirements for ROHC. 1. Document history September 14, 2001 - draft-schmidt-rohc-sctp-requirements-00.txt. Initial version of this document to initiate discussion on requirements for SCTP compression in ROHC. Schmidt, Tuexen [Page 1] Internet Draft SCTP Header Compression March 2002 2. 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 round trip 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 round trip times. The main objective for ROHC has been robust compression of IP/UDP/RTP. Next step was IP/TCP compression. SCTP is the new reliable transport protocol from the IETF. It offers a number of features not available in other reliable transport protocols such as TCP, including multi-streaming, multi-homing and resistance to flooding and masquerade attacks. SCTP is designed to transport PSTN signaling messages over IP networks but its rich feature set makes it capable of many broader applications. Main known application today is the transport of SIP signaling messages. One of the most important innovations of SCTP is the multi-streaming function. This feature allows data to be partitioned into multiple streams where each stream is delivered independently, so in-sequence delivery can be guaranteed only for packets within a single stream. The advantage of this technique is that when a packet is lost, only the data from the corresponding stream is delayed whilst the packet is retransmitted. Packets from other streams are unaffected by the loss. From the header compression point of view the multi-streaming function raises a number of new issues to solve. Most importantly a SCTP packet consists of a common header followed by a sequence of chunks. User payload is transported in DATA chunks which contain stream specific information. All other chunks do not contain stream specific information. To obtain maximum compression efficiency it is important to maintain a separate context for the stream-specific fields from each stream, but to use a shared context for all general fields. The remaining requirements will be similar to IP / TCP compression. 3. 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 Schmidt, Tuexen [Page 2] Internet Draft SCTP Header Compression March 2002 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 SCTP compression. 3.1. 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 SCTP implementations. Justification: Ease of deployment. Note: The ROHC WG may recommend changes that would increase the compression efficiency for the SCTP streams emitted by implementations. However, ROHC cannot rely on such recommendations being followed. 3.2. Supported headers (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. Justification: IPv4 and IPv6 will both be around during the foreseeable future. (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. Schmidt, Tuexen [Page 3] Internet Draft SCTP Header Compression March 2002 Justification: It is very likely that Mobile IP will be used by cellular devices. (3) 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. 3.3. SCTP specific requirements (1) Generality: Must support efficient compression of the SCTP information in a SCTP packet. This means that the scheme must be able to work with the protocol structure of the SCTP protocol (chunk header, chunk body, chunk header, chunk body ...) in a proper way. Justification: There must be a generic scheme which reflects the structure of SCTP packets. (2) Context synchronization failure, concerning certain stream, must not affect the handling of other streams. Justification: The compression stream must support the multiple streams feature in a way that head of line blocking is not introduced again by RoHC. Context update should be restricted to a minimum. (3) SCTP extensions as described in [USCTP] and [ADDIP] should be compressed efficiently. Justification: SCTP extensions will be a normal part of the protocol. To reach good efficiency for SCTP, these extension have to be handled in an appropriate way. (4) Generic extendibility describes the handling of yet not defined chunks, the compression scheme must be able to handle this chunks. Justification: The compression scheme must support full SCTP functionality. Schmidt, Tuexen [Page 4] Internet Draft SCTP Header Compression March 2002 3.4. Performance issues (1) Performance/Spectral Efficiency: Must provide low relative overhead under expected operating conditions. Justification: Spectrum efficiency is the primary goal here. (2) Error propagation: For SCTP traffic, link layer retransmissions should be applied to make use of the bandwidth in the most efficient way. Lost or damaged headers should thus not occur and therefore it is not a primary goal to have mechanisms for error propagation avoidance in case of such events. Justification: To provide robustness against loss or damage introduced by the link, efficiency must be sacrificed. Since loss or damage is not expected for SCTP traffic, efficiency should instead be prioritized. This does not mean that some robustness should not be provided, if efficiency can still be optimized. Note: In general, error propagation due to header compression should be kept at an absolute minimum. Error propagation is defined as the loss or damage of headers subsequent to headers lost or damaged by the link, even if those subsequent headers are not lost or damaged. Note: There are at least two kinds of error propagation; loss propagation, where a lost header causes subsequent headers to be lost or damaged, and damage propagation, where a damaged header causes subsequent headers to be lost or damaged. (3) Short lived SCTP transfers: The scheme should provide mechanisms for efficient compression of short-lived SCTP transfers, minimizing the size of context initiation headers. Justification: Many SCTP 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. (4a) 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. Schmidt, Tuexen [Page 5] Internet Draft SCTP Header Compression March 2002 (4b) Packet Reordering: The scheme should be able to compress when there are reordered packets in the SCTP stream reaching the compressor. Justification: Reordering happens regularly in the Internet. However, since the Internet is engineered to run SCTP reasonably well, excessive reordering will not be common and need not be handled with optimum efficiency. (5) Processing delay: The scheme must not contribute significantly to system delay budget. 3.5. 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 3.6. Open issues - For further discussions (1) What is the level of complexity, are we willing to pay for the most efficient way to compress multi-streaming SCTP? 4. IANA Considerations A protocol which meets these requirements, e.g., [ROHC], 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, e.g., [ROHC], must be able to compress packets containing IPSEC headers according to the IPSEC Schmidt, Tuexen [Page 6] Internet Draft SCTP Header Compression March 2002 requirement, 2.2.4. The efficiency of the compression may be influenced by encrypted protocol header elements. This document by itself, however, does not add any security risks. 6. References [RFC-2960] R. R. Stewart et al., "Stream Control Transmission Protocol", November 2000. [ADDIP] R. R. Stewart et al., "SCTP Extensions for Dynamic Reconfiguration of IP Addresses and Enforcement of Flow and Message Limits", draft-ietf-tsvwg-addip-sctp-02.txt, June 2001. [USCTP] Q. Xie et al., "SCTP Unreliable Data Mode Extension", draft- ietf-tsvwg-usctp-00.txt, April 2001. [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. [TCPREQ] Lars-Erik Jonsson, "Requirements for ROHC IP/TCP Compression", draft-ietf-rohc-tcp-requirements-01.txt, June 2001. 7. Authors' Addresses Christian Schmidt Tel.: +49 89 722 27822 Siemens AG e-mail: Christian.Schmidt@icn.siemens.de Werner von Siemens Ring 20 D-85630 Grasbrunn Germany Schmidt, Tuexen [Page 7] Internet Draft SCTP Header Compression March 2002 Michael Tuexen Tel.: +49 89 722 47210 Siemens AG e-mail: Michael.Tuexen@icn.siemens.de Hofmannstrasse 51 D-81359 Munich Germany This Internet Draft expires March 14, 2002. Schmidt, Tuexen [Page 8]