INTERNET-DRAFT Fred L. Templin Mark G. Lewis SRI International Expires 22 August 2002 22 February 2002 Issues for Redirects in Mobile Ad-hoc Networks (MANETs) draft-templin-manet-redirect-00.txt 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 accessed at http://www.ietf.org/shadow.html. Abstract This document discusses issues for the use of network layer redirects in Mobile Ad-hoc Networks (MANETs). The document observes that the use of redirects requires a transitive property for network layer connectivity that does not hold for MANETs, thus the use of redirects in MANETs can cause communication problems. Additionally, the document proposes several possible solutions for the problem. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. 1. Introduction Network layer redirects are used by the Internet protocols Templin, Lewis Expires 22 August 2002 [Page 1] INTERNET-DRAFT Redirects in MANETs 22 February 2002 [IPV4][IPV6] to eliminate redundant network layer hops whenever pos- sible. Their use is governed by host and router specifications that MUST be followed in compliant implementations. But, network layer redirects can defeat the use of multiple network layer hops on media that require such. An example is the Mobile Ad-hoc Network [MANET] paradigm, in which multiple network layer hops are often required to enable communications between correspondent nodes. The following document presents issues and proposed solutions for the use of redirects in MANETs. 2. Applicability Statement - This memo applies to Mobile Ad-hoc Networks and may also apply to other types of multi-hop media 3. Terminology The terminology of [IPV4], [IPV6], [MANET] and [SHARE] apply. The additional term is also defined: multi-hop media: a shared network media such that any two systems MAY NOT be capable of communicating directly at the link layer (and thus MAY require multiple network layer hops) at a given point in time 4. Problem Statement As for shared media [SHARE,3], multi-hop media MAY comprise multiple LISs (Logical IP Subnets) [DISCUSS]. [HREQ,3.2.2.2][RREQ,5.2.7.2] specify host and router requirements for an [IPV4] redirect mechanism to eliminate redundant network layer hops within a single LIS. [SHARE,4.1] proposes a hop-by-hop redirection extension using "foreign redirects" to eliminate redundant network layer hops between nodes belonging to different LISs. Finally, [DISC,8.2-8.3] specifies host and router requirements for an analogous redirect function for [IPv6]. The above-mentioned redirect mechanisms all assume a transitive pro- perty for connectivity at the network layer, i.e., if bi-directional links A<->B and B<->C exist on the same physical interface, then B can assume the link A<->C also exists. Under this assumption, B (when acting as a router) can send a redirect to A if it forwards a message from A using C as the next-hop (respectively to C if it forwards a message from C using A as the next-hop). Templin, Lewis Expires 22 August 2002 [Page 2] INTERNET-DRAFT Redirects in MANETs 22 February 2002 But, by definition, the transitive assumption for network-layer con- nectivity does not apply to all circumstances in multi-hop media; particularly for highly mobile environments such as MANETs. The use of redirects in multi-hop media could therefore disrupt communica- tions and/or lead to poor performance. Thus, as currently specified, the redirect mechanisms mentioned above MUST NOT be used on multi-hop media. 5. Proposed Solutions 5.1. Extensions to the Existing Standards A possible extension to the current router specifications in [RREQ,5.2.7.2] and [DISC,8.2] would require B to have explicit knowledge of the link A<->C before sending redirects. But, the link A<->C could be temporary or of marginal quality such that redirection from B could present problems for future communications between A and C. Thus, this extension used alone seems dangerously inadequate. Whether suitable extensions can be added to the existing specifica- tions to allow some useful form of redirects in multi-hop is a matter for further study. 5.2. Redirect Avoidance Using Restrictive Netmasks The router specification in [RREQ,5.2.7.2] states: "Routers MUST NOT generate a Redirect Message unless all the following conditions are met: o The packet is being forwarded out the same physical interface that it was received from, o The IP source address in the packet is on the same Logical IP (sub)network as the next-hop IP address, and o The packet does not contain an IP source route option." In an effort to avoid redirects, [IPv4] implementations MAY configure a totally-restrictive netmask (i.e. 32-bit) on an IP interface such that no address other than the one assigned to the interface appears to be "on-link" when netmasking is applied [DISCUSS]. While such a configuration may be useful for other purposes, one implication is that the second validity check in the above router specification should never be true. That, is there should never be a case in which "the IP source address in the packet is on the same logical IP Templin, Lewis Expires 22 August 2002 [Page 3] INTERNET-DRAFT Redirects in MANETs 22 February 2002 (sub)network as the next-hop IP address". However, comparing IP addresses based on the netmask assigned to an IP interfaces is arguably only one possible interpretation of what is meant by: "on the same Logical IP (sub)network" and this interpreta- tion may not be shared among all host and router implementations. Additionally, when hop-by-hop redirection is enabled as specified in [SHARE,4.1] the second validity check in the above specification is excused, therefore the restrictive netmask scheme does not avoid redirects. Finally, it is not clear whether an analogous solution exists for [IPV6]. 5.3. Redirect Avoidance Using Explicit Configuration Options A final method that clearly avoids the issue is disabling redirects for multi-hop media through explicit configuration options. The host and router specifications mentioned above refer to configurable options to enable/disable the use of redirects (default is typically "enabled"). However, a survey of representative implementations shows that some expose the appropriate configuration options to the user while others do not. In particular, all host implementations surveyed provide a configura- tion option to disable receipt of redirects but some router implemen- tations did not provide a configuration option to disable sending redirects. This fact seems to be slightly at odds with the host and router specifications and dangerous for multi-hop media operation. Therefore we recommend that all implementations provide configurable options to disable sending/receiving redirects to support efficient operation on multi-hop media, as directed by the standards. 6. IANA considerations This document introduces no new IANA considerations 7. Security considerations This document introduces no new security considerations Acknowledgements The authors acknowledge the members of the community who have contri- buted to discussions relating to addressing issues for MANET. Normative References Templin, Lewis Expires 22 August 2002 [Page 4] INTERNET-DRAFT Redirects in MANETs 22 February 2002 [ARP] Plummer, D., "Address Resolution Protocol", STD 37, RFC 826, November 1982. [DISC] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [HREQ] Braden, R., Editor, "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, October 1989. [RREQ] Baker, F., Editor, "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [SHARE] Braden, B., Postel, J., and Y. Rekhter, "Internet Architecture Extensions for Shared Media", RFC 1620, May 1994. Informative References [DISCUSS] discussions on the MANET WG mailing list dating 03/01 - 04/01, ftp://manet.itd.nrl.navy.mil/pub/manet. [IPV4] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [IPV6] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [MANET] Corson, S. and J. Macker, "Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations", RFC 2501. Authors Addresses Fred L. Templin SRI International 333 Ravenswood Ave. Menlo Park, CA 94025, USA Phone: (650)-859-3144 Email: templin@erg.sri.com Mark G. Lewis SRI International 333 Ravenswood Ave. Menlo Park, CA 94025, USA Phone: (650)-859-3144 Email: lewis@erg.sri.com Templin, Lewis Expires 22 August 2002 [Page 5] INTERNET-DRAFT Redirects in MANETs 22 February 2002 Intellectual Property Placeholder for IPR statement. Templin, Lewis Expires 22 August 2002 [Page 6]