Robust Header Compression C. Bormann Internet-Draft Universitaet Bremen TZI Expires: April 17, 2005 October 17, 2004 Robust Header Compression (ROHC) over 802 networks draft-bormann-rohc-over-802-00.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. 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 accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 17, 2005. Copyright Notice Copyright (C) The Internet Society (2004). Abstract Various proposals have been submitted to the ROHC working group for enabling the use of ROHC [RFC 3095] header compression over Ethernet, 802.11 and other 802-based links. Previous proposals generally suffered from a lack of systems perspective on 802 networks. The present document attempts to supply some systems perspective and provides a rough outline for a solution. Bormann Expires April 17, 2005 [Page 1] Internet-Draft ROHC over 802 October 2004 This is a submission to the IETF ROHC WG. Please direct discussion to its mailing list, rohc@ietf.org Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Overall Requirements . . . . . . . . . . . . . . . . . . . 3 2.2 Elements of a Solution . . . . . . . . . . . . . . . . . . 4 2.3 Who Should Standardize? . . . . . . . . . . . . . . . . . 4 2.4 Why not just use PPPoE? . . . . . . . . . . . . . . . . . 4 3. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1 Ethernet Minimum Frame Size . . . . . . . . . . . . . . . 5 3.2 Negotiation and the existing IP-over-802 model . . . . . . 6 4. Non-Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1 Reordering . . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 Padding a non-issue? . . . . . . . . . . . . . . . . . . . 6 5. A Proposal for the Encapsulation . . . . . . . . . . . . . . . 7 6. A Way Forward for the Negotiation . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . 10 Bormann Expires April 17, 2005 [Page 2] Internet-Draft ROHC over 802 October 2004 1. Introduction RFC 3095 [3] defines four ROHC profiles for the header compression of IP, UDP, RTP, ESP, and related protocol headers, as well as a framework that has been used to define a number of related profiles (LLA [5], R-mode LLA [6], IP ROHC [7], and UDP-lite based RTP ROHC [8]). To enable robust header compression over a specific link layer, the ROHC profile specifications have to be complemented by a link-layer specific specification, typically called "ROHC-over-X". One such specification has been defined in the IETF, ROHC over PPP [4]. Other ROHC-over-X specifications have been defined by the organizations defining specific link layers, such as 3GPP. No specification currently exists for applying robust header compression to IEEE 802 networks such as Ethernet, 802.11, or 802.16. A number of proposals have been made to the IETF ROHC WG [[list these]], but it became obvious quickly that the solutions that seem to suggest themselves do not work at the desirable level of efficiency. This document first discusses some issues about ROHC-over-802, then lists some potential non-issues, makes a rough proposal for an encapsulation format for ROHC-over-802, and finally discusses a way forward towards an appropriate negotiation mechanism. 2. Discussion 2.1 Overall Requirements There is little need for robust header compression in a classical Ethernet (802.3) environment, which is both relatively high-speed and (at least at the segment level) virtually error-free. However, WLAN (802.11) and WPAN (802.15) links are often bandwidth limited; the same will hold for WMAN (802.16) links. They also have typical loss and delay patterns that would motivate the use of ROHC in such scenarios. Since voice over IP is and will be commonly used in these networks, header compression will continue to be useful. In the ROHC framework, header compression is performed at the boundary between Layer 3 (IP) and Layer 2 (802, in the case of ROHC-over-802). 802 networks are often bridged, i.e. multiple 802 technologies may contribute to a Layer 2 path that constitutes what is considered to be "the link" from a ROHC framework point of view. In practical implementation, nodes such as routers (often one end of a ROHC channel) in many cases don't connect directly to 802.11 links, but send packets on 802.3 (Ethernet) links that then are bridged by Bormann Expires April 17, 2005 [Page 3] Internet-Draft ROHC over 802 October 2004 "Access Points" to 802.11 links. System architectures for other 802 technologies also often make use of bridging. One can conclude: It is not sufficient to just look at the wireless links -- ROHC-over-802 also needs to work on 802.3 networks. In effect, a single solution for applying ROHC to all 802 environments needs to be defined independent of the physical layer technology. A nice side effect is that this will simplify both standardization and implementation. 2.2 Elements of a Solution A ROHC-over-X specification needs to define two elements: o An encapsulation for ROHC framework packets, and o a negotiation mechanism for agreeing on the use of ROHC and on the parameters of the ROHC channel. (While a negotiation mechanism is not strictly needed for every ROHC-over-X document, it is clearly too late for the alternative, i.e. making ROHC mandatory and defining fixed channel parameter values for any use of IP over 802.) 2.3 Who Should Standardize? In previous discussions, the question was raised which body should standardize ROHC-over-802. As mentioned in the introduction, one ROHC-over-X protocol has been defined in the IETF, other ones have been defined in the standards bodies defining the link layers under consideration. In the view of the author, a good test would be to see who has defined the IP-over-X specification. The ROHC-over-PPP specification clearly fits in the IETF as both the IP-over-PPP specification and the PPP specification itself are IETF specifications. For 802 networks, the IETF also has specified the link layer mapping of IP, including a number of ancillary protocols (ARP and ND) necessary for these mappings. If these protocols need to be extended, it would be more appropriate for the IETF to do so. The system issues of complex 802 networks do have a bearing on ROHC-over-802 and are in the domain of the IEEE; on the other hand, no good arguments exist currently that would call for an extension to the 802 protocols for ROHC. In summary, the author believes that IETF is the right body to work on ROHC-over-802. 2.4 Why not just use PPPoE? An informational RFC specifies a widely deployed specification for PPP over Ethernet (PPPoE [9]), and, as mentioned there is a Bormann Expires April 17, 2005 [Page 4] Internet-Draft ROHC over 802 October 2004 specification for ROHC over PPP [4]. For a number of reasons, just combining these as a ROHC-over-802 solution would be suboptimal: 1. PPPoE's encapsulation together with the PPP encapsulation has a fixed overhead of eight bytes per packet, negating some of the savings provided by header compression. 2. PPPoE does not solve the minimum-size padding problem (see below). 3. PPPoE has a different model than the usual IP-over-802 model, with discovery and session stages, and possibly multiple PPP sessions. This complexity is often not required. On the other hand, if PPPoE is in active use on an 802 link, adding ROHC-over-PPP may be a simple way to add robust header compression. 3. Issues 3.1 Ethernet Minimum Frame Size Due to its roots in the CSMA/CD protocol, Ethernet (IEEE 802.3) defines a minimum frame size of 64 bytes. Of these, 14 bytes are used for the MAC header and 4 are used for the MAC trailer (frame check sequence), which means that the minimum payload of an Ethernet packet is 46 bytes. The existing IPv4-over-802 [10] specification uses the "total size" field in the IP packet to indicate how much of the 802 packet payload is actually an IP packet; this indirectly indicates the presence of padding, if any. ROHC compresses away the "total size" field. Instead, it relies on the link layer (or the ROHC-over-X protocol) to provide a packet size. A ROHC-over-802 encapsulation could use a number of ways to provide this packet size: 1. It still could rely on the link layer size and use ROHC padding schemes to always inflate the size to at least 46 bytes. 2. It could add a length field. 3. It could make use of the length-field variant of the 802 MAC header format; this requires a different way of demultiplexing ROHC packets from other LLC packets. Note that solutions 1 and 2 mean that ROHC-compressed packets shorter than 46 bytes will be padded out to this length if they ever go over an 802.3 link. Worse, there will be no way for an 802.3-to-802.x bridge to identify this padding and remove it, so the padding will remain on any wireless segments of the link layer path. Given that many voice over IP packets will have payloads of 10 to 20 bytes and Bormann Expires April 17, 2005 [Page 5] Internet-Draft ROHC over 802 October 2004 headers often can be compressed down to 3 bytes or less, this entails a significant overhead. So, apart from the issue of properly indicating padding, a more interesting property of a ROHC-over-802 encapsulation is whether it allows 802.3-to-802.x bridges to remove any padding inserted on the 802.3 segments. 3.2 Negotiation and the existing IP-over-802 model In the existing IP-over-802 model (as exemplified by IPv4-over-802 [10]) assumes that once the MAC (link layer) address of a node is known, packets can be sent to it. No channel setup/teardown is provided for. In particular, a node can lose its state (be rebooted) and packets can still be sent to it based on the knowledge of the MAC address. The ROHC channel model [11] assumes that channel state is maintained explicitly, at least if the more advanced O- and R-modes are to be used. In addition, this channel setup is used to negotiate parameters of the channel (such as variants of the encapsulation format or the maximum number of compression contexts supported). Also, while there is a ROHC channel for each direction, each ROHC channel itself is bidirectional in the sense that (at least if not just U-mode is to be used) there needs to be a way to return feedback. Finally, only the receiving end of a packet flow may be aware that there is a benefit in using header compression (for illustration, consider a VoWLAN phone that is receiving packets from a router that is different than the router it chose as its default router and thus for the reverse packet flow). Therefore, there should be a way to initiate the setup of a ROHC channel from the receiving end. 4. Non-Issues 4.1 Reordering Fortunately, 802 links are sequence-preserving, so there is no need to re-sequence packets to avoid reordering (as would be required by the current ROHC framework). 4.2 Padding a non-issue? One argument could be that the padding issue outlined in Section 3.1 can be ignored for most 802 networks, either because the payloads will be larger than for the most heavily compressing voice codecs or Bormann Expires April 17, 2005 [Page 6] Internet-Draft ROHC over 802 October 2004 because the header overhead is already rather high (e.g., for 802.11b, the link-layer header overhead in typical configurations is about as large as that of three-digit numbers of bytes in the payload). The author takes the viewpoint that a solution that is intended to be used universally through the 802 space does need to address padding. 5. A Proposal for the Encapsulation Based on the considerations above, this document proposes to use LLC encapsulation of ROHC packets. Several approaches are possible: 1. An SAP value is allocated to ROHC. The per-packet overhead is reduced to three bytes. Note that this means the negotiation protocol needs to fix the small-CID vs. large-CID choice (alternatively, ROHC-over-802 could simply always use large CIDs, or even a pair of SAP values could be allocated). 2. SAP 0xAA is used. By setting the first byte of the OUI to a value illegal for an OUI (multicast/private), the rest of the frame can be used for the ROHC packet, reducing the overhead to four bytes. The first (illegal OUI) byte can be used to demultiplex variants, e.g. small-CID and large-CID ROHC packets as well as possible negotiation protocol packets (see below). What would be the second and third OUI bytes are already used for the ROHC packet. 3. A full SNAP header is used. (From an overhead perspective, this is not better than the PPPoE case, but, like the previous proposals, it does allow the removal of padding by bridges.) The author proposes to obtain additional input from encapsulation graybeards about this. 6. A Way Forward for the Negotiation Negotiation of ROHC channels can either be piggy-backed on the existing address resolution/neighbor discovery protocols or a completely separate negotiation protocol can be used. For IPv4, extending ARP sounds rather difficult at this point in the evolution of this protocol. For IPv6, while ND is probably a more extensible protocol, it is not clear that it is the right place for negotiating link-layer characteristics. Instead, a simple negotiation protocol should be defined that is based on regularly probing the peer node for ROHC capability and offering a capability set. A magic number scheme can be used both to ensure liveness of the peer state assumed and as a basic security Bormann Expires April 17, 2005 [Page 7] Internet-Draft ROHC over 802 October 2004 measure. The negotiation protocol should preferably share its encapsulation with the ROHC encapsulation itself to ensure probes only arrive if there is no obstacle to LLC-style frames. 7. Security Considerations Making a node believe its peer node is ROHC capable when it isn't is one way to stage a denial of service attack, as is maliciously tearing down ROHC state. The ROHC negotiation protocol probably needs to have security that is commensurate to that of the address resolution/neighbor discovery protocol in use. The ROHC protocol itself is quite susceptible to attacks. To quote RFC 3095 [3]: Denial-of-service attacks are possible if an intruder can introduce (for example) bogus STATIC, DYNAMIC or FEEDBACK packets onto the link and thereby cause compression efficiency to be reduced. However, an intruder having the ability to inject arbitrary packets at the link layer in this manner raises additional security issues that dwarf those related to the use of header compression. 8 References [1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [4] Bormann, C., "Robust Header Compression (ROHC) over PPP", RFC 3241, April 2002. [5] Jonsson, L-E. and G. Pelletier, "RObust Header Compression (ROHC): A Link-Layer Assisted Profile for IP/UDP/RTP", RFC 3242, April 2002. [6] Liu, Z. and K. Le, "Zero-byte Support for Bidirectional Bormann Expires April 17, 2005 [Page 8] Internet-Draft ROHC over 802 October 2004 Reliable Mode (R-mode) in Extended Link-Layer Assisted RObust Header Compression (ROHC) Profile", RFC 3408, December 2002. [7] Jonsson, L-E. and G. Pelletier, "RObust Header Compression (ROHC): A Compression Profile for IP", RFC 3843, June 2004. [8] Pelletier, G., "RObust Header Compression (ROHC):Profiles for UDP-Lite", draft-ietf-rohc-udp-lite-04 (work in progress), June 2004. [9] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D. and R. Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC 2516, February 1999. [10] Postel, J. and J. Reynolds, "Standard for the transmission of IP datagrams over IEEE 802 networks", STD 43, RFC 1042, February 1988. [11] Jonsson, L-E., "RObust Header Compression (ROHC): Terminology and Channel Mapping Examples", RFC 3759, April 2004. Author's Address Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28334 Germany Phone: +49 421 218 7024 Fax: +49 421 218 7000 EMail: cabo@tzi.org Appendix A. Acknowledgements The issue of enabling robust header compression over 802 networks has been brought up repeatedly, e.g. by Kris Fleming in a contribution called ROHCoWEM (draft-fleming-rohc-wireless-ethernet-med). The participant comments at the Atlanta IETF ROHC WG meeting (November 2002) provided the basis for the analytical part of this document. Bormann Expires April 17, 2005 [Page 9] Internet-Draft ROHC over 802 October 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Bormann Expires April 17, 2005 [Page 10]