Network Working Group Lars-Erik Jonsson INTERNET-DRAFT Ericsson Expires: February 2002 August 30, 2001 Requirements and Assumptions for ROHC 0-byte IP/UDP/RTP 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 its mailing list, rohc@cdt.luth.se. Abstract This document contains requirements for the 0-byte IP/UDP/RTP header compression scheme to be developed by the ROHC WG. It also includes the basic assumptions for the typical link layers over which 0-byte compression may be implemented, and assumptions about its usage in general. Jonsson [Page 1] INTERNET-DRAFT Requirements and assumptions August 30, 2001 for 0-byte ROHC RTP 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. ROHC RTP has become a very efficient, robust and capable compression scheme, able to compress the IP/UDP/RTP headers down to a total size of one octet only. This makes ROHC RTP an excellent solution for future cellular environments with new air interfaces, such as WCDMA, making even speech services possible over IP with an insignificantly lower spectrum efficiency compared to existing circuit switched solutions. However, all-IP cellular networks will be built also with already existing air interfaces such as GSM and IS-95, which are less flexible using radio bearers optimized for specific frame sizes matching the speech codecs used. This means that not a single octet of header can be added without switching to the next higher fixed packet size supported by the link, something which is obviously very costly. In the long term, this drawback should of course be eliminated with new, more flexible air interfaces, but in the short term it would be desirable if an efficiency comparable to the circuit switched case could be achieved also for already deployed speech codecs when used over the existing air interfaces. To achieve that, it must be possible to completely eliminate the headers for a majority of the packets during normal operation, and this is the purpose of 0-byte header compression. All functionality normally provided by the 1-octet header must then be provided by some other means, typically by utilizing functionality from the lower layer. It is important to remember that the purpose of 0-byte header compression is to provide optimal efficiency for applications matching the link layer characteristics, not efficiency in general. As a starting point for these requirements, the well-established requirements base developed in the ROHC WG has been used. From that, the requirements have evolved through inputs from the 3GPP2 community and from discussions within the WG. 2. Assumptions for the applicability of 0-byte RTP header compression The purpose of 0-byte header compression is to provide optimal usage of certain links when the traffic pattern of a packet stream completely matches the characteristics of that link. There are no assumptions that only packet streams complying with that pattern will occur, but optimal efficiency cannot of course be provided when this is not the case. Jonsson [Page 2] INTERNET-DRAFT Requirements and assumptions August 30, 2001 for 0-byte ROHC RTP To make 0-byte header compression feasible, it is assumed that lower layers can provide the necessary functionality needed to replace the 1-octet headers and fulfil the requirements defined in section 3. An example is the synchronized nature of most cellular links, which can provide sequencing and timing information and make packet loss detection possible. 3. Requirements on 0-byte RTP header compression Since 0-byte header compression for ROHC IP/UDP/RTP is a variant of regular ROHC RTP compression [ROHC], these requirements are described as deltas to those defined in the regular RTP requirements [RTP-REQ]. For simplicity, this section is also separated into the same three subsections as the requirements in [RTP-REQ], where the first deals with the impact of header compression on the rest of the Internet infrastructure, the second concerns the headers to be compressed, and the third covers efficiency and link technology related issues. 3.1. Impact on Internet infrastructure The meaning of header compression is in no way changed by the introduction of 0-byte header compression. No additional impact on the Internet infrastructure is thus allowed. The "Transparency" and "Ubiquity" requirements of [RTP-REQ, section 2.1] therefore also apply to 0-byte RTP compression without any modifications. 3.2. Supported headers and kinds of RTP streams The 0-byte RTP compression scheme in general imposes the same requirements on supported headers and RTP streams as regular ROHC RTP [RTP-REQ, section 2.2]. However, there are some aspects regarding the "Genericity" and IPSEC requirements that should be noted. The "Genericity" requirement of [RTP-REQ] states that compression of headers of arbitrary RTP streams must be supported, and this is also true for the 0-byte compression scheme to the extent that it is not allowed to assume certain RTP behavior. However, as also stated in [RTP-REQ], this does not preclude optimizations for certain media types where the traffic pattern is known. For 0-byte RTP, this means that the scheme must be able to handle arbitrary RTP streams in order to fulfil the requirements of section 3.1. However, due to the typical characteristics of 0-byte compression, by requiring a traffic pattern that suits the link over which it is implemented to be able to compress down to 0-byte headers, it becomes optimized for applications with link-suited traffic patterns. For traffic that does not comply with the link properties, the scheme must automatically Jonsson [Page 3] INTERNET-DRAFT Requirements and assumptions August 30, 2001 for 0-byte ROHC RTP and immediately fall back to non-0-byte RTP compression and must not have any impact on the packet stream. Regarding IPSEC, it should be noted that 0-byte compression cannot be achieved if parts of the original headers are encrypted or carry randomly changing fields. IPSEC and 0-byte RTP header compression therefore do not go well together. If IPSEC is used and prevents 0- byte compression, the scheme must fall back to a less efficient compression that can handle all present header fields. Of course, this applies not only to IPSEC but to all cases where headers cannot be compressed down to 0-byte. 3.3. Performance issues All the performance requirements of [RTP-REQ] also apply to 0-byte RTP header compression, with the following additions and exceptions: - Performance/Spectral Efficiency: For packet streams with traffic patterns that match the characteristics of the link over which 0- byte header compression is implemented, the performance should be such that 0-byte header packets are generated most of the time during normal operation. 0-byte headers would then replace most of the 1-octet headers used by regular ROHC RTP [ROHC]. Justification: Spectrum efficiency is a primary goal. Studies have shown that for certain applications and link technologies, even a single octet of header may result in a significant decrease in spectrum efficiency compared to existing circuit switched solutions. - Header Compression Coexistence: The scheme must fit into the ROHC framework together with other ROHC profiles. Justification: Implementation simplicity is an important issue and the 0-byte RTP compression scheme should therefore have as much as possible in common with the regular IP/UDP/RTP profile. - Unidirectional links: It is of less importance that the 0-byte header compression scheme be able to work also over unidirectional links. Justification. 0-byte header compression targets links that typically are bi-directional. Jonsson [Page 4] INTERNET-DRAFT Requirements and assumptions August 30, 2001 for 0-byte ROHC RTP 4. IANA Considerations A protocol which meets these requirements, e.g., [LLA], 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., [LLA], may have a number of security aspects that need to be considered. This document by itself, however, does not add any security risks. 6. References [RTP-REQ] Degermark, M., "Requirements for robust IP/UDP/RTP header compression", RFC 3096, July 2001. [ROHC] Bormann, C., et. al., "Robust Header Compression (ROHC)", RFC 3095, July 2001. [LLA] Jonsson, L-E. and G. Pelletier, "A Link-Layer Assisted ROHC Profile for IP/UDP/RTP", 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 SE-971 28 Lulea Sweden E-mail: lars-erik.jonsson@ericsson.com Jonsson [Page 5] INTERNET-DRAFT Requirements and assumptions August 30, 2001 for 0-byte ROHC RTP Full copyright statement Copyright (C) The Internet Society (2001). All Rights Reserved. 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