RFC 2991 (rfc2991) - Page 2 of 9

Multipath Issues in Unicast and Multicast Next-Hop Selection

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