Multipath Issues in Unicast and Multicast Next-Hop Selection
RFC 2991

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Network Working Group                                           D. Thaler
Request for Comments: 2991                                      Microsoft
Category: Informational                                          C. Hopps
                                                     NextHop Technologies
                                                            November 2000

      Multipath Issues in Unicast and Multicast Next-Hop Selection

Status 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 2000

2.  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 2000

3.  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 in RFC