Internet Engineering Task Force Dave Thaler INTERNET-DRAFT Merit Expires July 1998 13 January 1997 Multipath Issues in Unicast and Multicast Status of this Memo This document is an Internet Draft. 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 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 a "work in progress". 1. Introduction Various routing protocols, including OSPF [1] and ISIS, allow ''Equal- Cost Multipath'' routing. At least two vendors also allow equal-cost multipath usage with RIP [2]. Using equal-cost multipath means that if multiple equal-cost routes to the same destination exist, they can all be discovered and used to provide load balancing among redundant paths. The effects of multipath routing on a forwarder is that the forwarder potentially has several next-hops for any given destination and must use some method to choose which next-hop should be used for a given data packet. In this memo, we describe current practice, problems with this, and potential solutions. Expires July 1998 [Page 1] Draft Multipath January 1997 2. Concerns At least two deployed router implementations allow multipath forwarding. This is typically done via round-robin, where each packet matching a given destination route is forwarded using the subsequent next-hop in a round-robin fashion. This approach does provide a form of load balancing, but there are several problems with this approach: Variable Path MTU Since each of the redundant paths may have a different MTU, this means that the overall path MTU can change on a packet-by-packet basis, negating the usefulness of path MTU discovery. Variable Bandwidth Since each of the redundant paths may have a different bandwidth available, this may also change on a packet-by-packet basis. Rate-adaptive protocols such as TCP are designed to optimize their performance to adapt to the available bandwidth. Varying this on a packet-by-packet basis causes problems with TCP's congestion control mechanisms, resulting in much lower throughputs. Variable Latencies Since each of the redundant paths may have a different latency involved, having packets take separate paths can cause packets to always arrive out of order, increasing delivery latency and buffering requirements. Debugging Common debugging utilities such as ping and traceroute are much less reliable in the presence of multiple paths and may even present completely wrong results. In multicast routing, the problem with multiple paths is that loops and duplicates are prevented by constructing a single tree to all receivers of the same group address. Multicast routing protocols available today construct use shortest-path trees rooted at some point (either the source address, or the address of another router known as a Core or Rendezvous Point) [2]. Hence, the way they insure that duplicates will not arise is that a given tree must use only a single next-hop towards the root of the tree. Expires July 1998 [Page 2] Draft Multipath January 1997 3. Requirements All of the problems outlined in the previous section arise when packets in the same (unicast or multicast) session are split among multiple paths. The natural solution is therefore to insure that packets for the same session (or flow) always use the same path. Two additional features are desirable: Minimal disruption When multipath is used, meaning that multiple routes contribute valid next-hops, the chances are higher of routes being added and deleted from consideration than when only the "best" route is used (in which case metric changes in alternate routes have no effect on traffic paths). Hence, it is desirable to minimize the number of sessions affected by the addition or deletion of another path. Fast implementation The amount of additional computation required to forward a packet must be as small as possible. For example, when doing round-robin, this computation might consist of incrementing (modulo the number of next-hops) a next-hop index. 4. Solutions We now provide two possible methods for improving the performance of multipath and then discuss their applicability to unicast and multicast forwarding. Modulo-N Hash To select a next-hop from the list of N next-hops, the router performs a modulo-N hash over the IP header fields that identify a session. This has the advantage of being fast, at the expense of (N-1)/N of all sessions changing paths whenever a next-hop is added or removed. Highest Random Weight (HRW) The router uses a simple pseudo-random number function seeded with the IP header fields that identify a session, as well as a next-hop identifier (address or index) to assign a weight to each of the N Expires July 1998 [Page 3] Draft Multipath January 1997 next hops. An analysis of various deterministic weight functions can be found in [3]. The next-hop receiving the highest weight is chosen as the next hop. This has the advantage of minimizing the number of sessions affected by a next-hop addition or deletion, but is approximately N times as expensive as a modulo-N hash. The applicability of these two alternatives depends on (at least) two factors: whether the forwarder maintains per-flow state (or any finer classification than "all unicast traffic" and "all multicast traffic"), and how precious CPU is to a multipath forwarder. If per-flow state is maintained in a multipath forwarder, then computation of the next-hop can be done by the router at state creation time. This entails no additional computations at packet forwarding time, since the next-hop is precomputed. In this case, any method can be used, including round-robin, random, modulo-N, or HRW. Hash functions such as modulo-N and HRW are better if the forwarder state may be deleted for any reason during the lifetime of a session since subsequent next-hop computations by the router will always select the same path. This also improves the usefulness of debugging utilities such as traceroute. Finally, to maximize the stability of paths (and hence the usefulness of traceroute, etc.), we specifically recommend the use of HRW. If no state finer than "all packets" is maintained in the forwarder, then using multiple next-hops requires that the next-hop be calculated at packet arrival time. When CPU is more precious than stability of session paths, a simple modulo-N hash may be used. 4.1. Unicast Forwarding Depending on the implementation, unicast forwarding may or may keep per-flow state. We recommend that where forwarder implementations keep flow state, routers should use HRW at state creation time (and next-hop deletion time) to select next-hop, and that forwarders without per-flow state use a modulo-N hash over the source and destination addresses. 4.2. Multicast Forwarding Multicast forwarding uses a cache of forwarding entries indexed by group (or group prefix) and source (or source prefix). This means that, logically, a multicast forwarder always keeps per-"session" state, although a "session" may be fairly coarse (e.g., traffic from all Expires July 1998 [Page 4] Draft Multipath January 1997 sources to the same destination), depending on the multicast routing protocol in use. Since per-session state is kept by the forwarder, it is recommended that the router always use HRW to select the next-hop. Routers using explicit-joining protocols such as PIM-SM [4] should thus use the multipath information when determining to which neighbor a join message should be sent. For example, when multiple next-hops exist for a given Rendezvous Point (RP) toward which a (*,G) Join should be sent, it is recommended that HRW be used to select the next-hop to use for each group. 5. References [1] Moy, J., "OSPF Version 2", RFC 2178, July 1997. [2] Semeria, C., and T. Maufer, "Introduction to IP Multicast Routing", draft-ietf-mboned-intro-multicast-03.txt, October 1997. [3] Thaler, D., and C.V. Ravishankar, "Using Name-Based Mappings to Increase Hit Rates", IEEE/ACM Transactions on Networking, February 1998. [4] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S., Handley, M., Jacobson, V., Liu, C., Sharma, P., and L. Wei, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification", RFC 2117, June 1997. 6. Security Considerations Security issues are not discussed in this memo. 7. Author's Address Dave Thaler Merit Network, Inc 4251 Plymouth Rd., Suite C Ann Arbor, MI 48105-2785 Phone: +1 313 647 4813 EMail: thalerd@merit.net Expires July 1998 [Page 5]