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Network Working Group                                           D. ThalerRequest for Comments: 2991                                      MicrosoftCategory: Informational                                          C. Hopps                                                     NextHop Technologies                                                            November 2000Multipath Issues in Unicast and Multicast Next-Hop SelectionStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.Abstract   Various routing protocols, including Open Shortest Path First (OSPF)   and Intermediate System to Intermediate System (ISIS), explicitly   allow "Equal-Cost Multipath" (ECMP) routing.  Some router   implementations also allow equal-cost multipath usage with RIP and   other routing protocols.  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.1.  Introduction   Various routing protocols, including OSPF 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.Thaler & Hopps               Informational                      [Page 1]

RFC 2991                    Multipath Issues               November 20002.  Concerns   Several router implementations allow multipath forwarding.  This is   sometimes done naively via round-robin, where each packet matching a   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 approaches such as   round-robin or random:   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 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.         Packet reordering causes TCP to believe that loss has taken         place when packets with higher sequence numbers arrive before         an earlier one.  When three or more packets are received before         a "late" packet, TCP enters a mode called "fast-retransmit" [6]         which consumes extra bandwidth (which could potentially cause         more loss, decreasing throughput) as it attempts to         unnecessarily retransmit the delayed packet(s).  Hence,         reordering can be detrimental to network performance.   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) [2] construct shortest-path trees rooted at either the   source, or another router known as a Core or Rendezvous Point.   Hence, the way they ensure that duplicates will not arise is that a   given tree must use only a single next-hop towards the root of the   tree.Thaler & Hopps               Informational                      [Page 2]

RFC 2991                    Multipath Issues               November 20003.  Requirements   In the remainder of this document, we will use the term "flow" to   represent the granularity at which the router keeps state (if at all)   for classes of traffic.  The exact definition of a flow may depend on   the actual implementation.  For example, a flow might be identified   solely by destination address, or it might be identified by (source   address, destination address, protocol id) triplet.  Hence "flow" is   not necessarily synonymous with the term "microflow" as used inRFC2474 [7], which also includes port numbers.  Indeed, including   transport-layer information in the next-hop selection process can   actually be problematic.  For example, if packets are fragmented, the   transport-layer information may not be available in every packet.   Furthermore, having the choice of path depend on transport-layer   fields may negate the benefit of caching information such as MTU for   use in subsequent connections between the same endpoints.   All of the problems outlined in the previous section arise when   packets in the same unicast or multicast "flow" are split among   multiple paths.  The natural solution is therefore to ensure 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).  Since a higher number of routes         may actually be used for forwarding when multipath is in use,         the potential for packet reordering and packet loss due to         route flaps can be much greater than when not using multipath.         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 should be small.  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 three possible methods for improving the performance   of multipath and then discuss their applicability to unicast and   multicast forwarding.Thaler & Hopps               Informational                      [Page 3]

RFC 2991                    Multipath Issues               November 2000   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.   Hash-Threshold         The router first selects a key by performing a hash over the         packet header fields that identify the flow.  The N next-hops         have been assigned unique regions in the hash function's output         space.  By comparing the hash value against region boundaries         the router can determine which region the hash value belongs to         and thus which next-hop to use.  This method has the advantage         of only affecting flows near the region boundaries (or         thresholds) when next-hops are added or removed.  For ECMP         hash-threshold's lookup can be done with a simple division         (hash_value / fixed_region_size).  When a next-hop is added or         removed, between 1/4 and 1/2 of all flows change paths.  An         analysis of this method can be found in [3].   Highest Random Weight (HRW)         The router computes a key for EACH next-hop by performing a         hash over the packet header fields that identify the flow, as         well as over the address of the next-hop.  The router then         chooses the next-hop with the highest resulting key value [4].         This has the advantage of minimizing the number of flows         affected by a next-hop addition or deletion (only 1/N of them),         but is approximately N times as expensive as a modulo-N hash.   The applicability of these three alternatives depends on (at least)   two factors: whether the forwarder maintains per-flow state, and how   precious CPU is to a multipath forwarder.   Some routers may maintain per-flow state for reasons other than for   supporting multipath.  For example, routers typically keep per-flow   state for multicast flows so that they can maintain the list of   interfaces to which packets in the flow should be copied.   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 compared with normal forwarding to a single next-hop,   since the next-hop is precomputed.  In this case, any method can be   used, including round-robin, random, modulo-N, hash-threshold or HRW.   Hash functions such as modulo-N, hash-threshold 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 willThaler & Hopps               Informational                      [Page 4]

RFC 2991                    Multipath Issues               November 2000   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 over the other methods mentioned   herein.   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, hash-threshold is recommended over the other methods mentioned   herein.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 hash-threshold.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, 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 [5] 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.  Applicability   The algorithms discussed above (except round-robin) all rely on some   form of hash function.  Equal flow distribution is achieved when the   hash function is uniformly distributed.  Since the commonly used hash   functions only become uniformly distributed when the number of inputs   is relatively large, these algorithms are more applicable to routers   used to route many flows, than in, for example, a small business   setting.Thaler & Hopps               Informational                      [Page 5]

RFC 2991                    Multipath Issues               November 20006.  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.7.  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.   One issue that is worth mentioning, however, is that when next-hop   selection is predictable, an attacker can synthesize traffic that   will all hash the same, making it possible to launch a denial-of-   service attack that overloads a particular path.  Since a special   case of this is when the same (single) next-hop is always selected,   such an attack is easiest when multipath is not being used.   Introducing multipath routing can make such an attack more difficult;   the more unpredictable the hash is, the harder it becomes to conduct   a denial-of-service attack against any single link.Thaler & Hopps               Informational                      [Page 6]

RFC 2991                    Multipath Issues               November 20008.  References   [1]  Moy, J., "OSPF Version 2", STD 54,RFC 2328, April 1998.   [2]  Maufer, T., "Deploying IP Multicast in the Enterprise",        Prentice-Hall, 1998.   [3]  Hopps, C., "Analysis of an Equal-Cost Multi-Path Algorithm",RFC2992, November 2000.   [4]  Thaler, D., and C.V. Ravishankar, "Using Name-Based Mappings to        Increase Hit Rates", IEEE/ACM Transactions on Networking,        February 1998.   [5]  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.   [6]  Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control",RFC 2581, April 1999.   [7]  Nichols, K., Blake, S., Baker, F. and D. Black., "Definition of        the Differentiated Services Field (DS Field) in the IPv4 and        IPv6 Headers",RFC 2474, December 1998.Thaler & Hopps               Informational                      [Page 7]

RFC 2991                    Multipath Issues               November 20009.  Authors' Addresses   Dave Thaler   Microsoft   One Microsoft Way   Redmond, WA  98052   Phone: +1 425 703 8835   EMail: dthaler@dthaler.microsoft.com   Christian E. Hopps   NextHop Technologies, Inc.   517 W. William Street   Ann Arbor, MI 48103-4943   U.S.A   Phone: +1 734 936 0291   EMail: chopps@nexthop.comThaler & Hopps               Informational                      [Page 8]

RFC 2991                    Multipath Issues               November 200010.  Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Thaler & Hopps               Informational                      [Page 9]