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Multipath Issues in Unicast and Multicast Next-Hop Selection (RFC2991)

IP.com Disclosure Number: IPCOM000005185D
Original Publication Date: 2000-Nov-01
Included in the Prior Art Database: 2001-Aug-17
Document File: 10 page(s) / 18K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

D. Thaler: AUTHOR [+2]

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.

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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, decr...