Internet Engineering Task Force Dave Thaler INTERNET-DRAFT Microsoft Expires May 1999 15 November 1998 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, explicitly allow "Equal-Cost Multipath" routing. Some router implementations also allow equal-cost multipath usage with RIP and other routing protocols. Using equal-cost multipath means that if multiple equal-cost routes to the same destination exist, they can be discovered and used to provide load balancing among redundant paths. The effect 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. This memo summarizes current practices, problems, and solutions. 2. Concerns Several router implementations allow multipath forwarding. This is sometimes done naively via round-robin, where each packet matching a Expires May 1999 [Page 1] Draft Multipath Issues November 1998 given destination route is forwarded using the subsequent next-hop, in a round-robin fashion. This does provide a form of load balancing, but there are several problems with a round-robin 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 amount of bandwidth available, bandwidth 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 the bandwidth 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 multicast routing protocols prevent loops and duplicates by constructing a single tree to all receivers of the same group address. Multicast routing protocols deployed today (DVMRP, PIM-DM, PIM-SM) construct shortest-path trees rooted at either the source, or another router known as a Core or Rendezvous Point. 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 May 1999 [Page 2] Draft Multipath Issues November 1998 3. Requirements All of the problems outlined in the previous section arise when packets in the same unicast or multicast "flow" (or session) are split among multiple paths. The natural solution is therefore to insure that packets for the same 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 active flows affected by the addition or deletion of another next- hop. 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 packet header fields that identify a flow. This has the advantage of being fast, at the expense of (N-1)/N of all flows 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 packet header fields that identify a flow, as well as a next- Expires May 1999 [Page 3] Draft Multipath Issues November 1998 hop identifier (address or index), to assign a weight to each of the N next hops. The next-hop receiving the highest weight is chosen as the next hop. This has the advantage of minimizing the number of flows affected by a next-hop addition or deletion, but is approximately N times as expensive as a modulo-N hash. An analysis of various deterministic weight functions can be found in [3]. The applicability of these two alternatives depends on (at least) two factors: whether the forwarder maintains per-flow state, 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 flow 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.), the use of HRW is recommended. If per-flow state is not maintained by 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 flow paths, a simple modulo-N hash over all packet header fields identifying a flow is recommended. 4.1. Unicast Forwarding Depending on the implementation, unicast forwarding may or may not 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 the next-hop, and that forwarders without per- flow state use a modulo-N hash over the source and destination addresses. 4.2. Multicast Forwarding Today's multicast forwarding engines use a cache of forwarding entries indexed by group (or group prefix) and source (or source prefix). This means that today's multicast forwarder's always keep per-flow state, Expires May 1999 [Page 4] Draft Multipath Issues November 1998 although for some multicast routing protocols, the "flow" may be fairly coarse (e.g., traffic from all sources to the same destination). Since per-flow 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. Redundant Parallel Links A related problem occurs when multiple parallel links are used between the same pair of routers. A common solution is to bundle the two links together into a "super"-link when is then used for routing. For multicast forwarding, this results in the two links being reduced to a single next-hop (over the combined link) which can be used to prevent duplicates. When a unicast or multicast packet is queued to the combined link, some method, such as those discussed earlier, is still required to determine the physical link on which to transmit the packet. If the parallel links are identical, then most of the concerns discussed in this document are avoided with the combined link. The exception is packet reordering, which can still occur with round-robin, adversely affecting TCP. 6. Security Considerations This document discusses issues with various methods of choosing a next- hop from among multiple valid next-hops. As such, it does not directly impact the security of the Internet infrastructure or its applications. 7. 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 Expires May 1999 [Page 5] Draft Multipath Issues November 1998 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 2362, June 1998. 8. Author's Address Dave Thaler Microsoft One Microsoft Way Redmond, WA 98052 Phone: +1 425 703 8835 EMail: dthaler@microsoft.com Expires May 1999 [Page 6]