RFC 2991 (rfc2991) - Page 2 of 9
Multipath Issues in Unicast and Multicast Next-Hop Selection
Alternative Format: Original Text Document
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
