IPv4 Multihoming Practices and Limitations
RFC 4116

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Network Working Group                                           J. Abley
Request for Comments: 4116                                           ISC
Category: Informational                                     K. Lindqvist
                                                Netnod Internet Exchange
                                                               E. Davies
                                                  Independent Researcher
                                                                B. Black
                                                         Layer8 Networks
                                                                 V. Gill
                                                                     AOL
                                                               July 2005

               IPv4 Multihoming Practices and Limitations

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 (2005).

Abstract

   Multihoming is an essential component of service for many Internet
   sites.  This document describes some implementation strategies for
   multihoming with IPv4 and enumerates features for comparison with
   other multihoming proposals (particularly those related to IPv6).

Abley, et al.                Informational                      [Page 1]
RFC 4116                    IPv4 Multihoming                   July 2005

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. IPv4 Multihoming Practices ......................................4
      3.1. Multihoming with BGP .......................................4
           3.1.1. Addressing Considerations ...........................4
           3.1.2. AS Number Considerations ............................6
      3.2. Multiple Attachments to a Single Transit Provider ..........6
      3.3. NAT- or RFC2260-based Multihoming ..........................7
   4. Features of IPv4 Multihoming ....................................7
      4.1. Redundancy .................................................7
      4.2. Load Sharing ...............................................8
      4.3. Performance ................................................8
      4.4. Policy .....................................................8
      4.5. Simplicity .................................................9
      4.6. Transport-Layer Survivability ..............................9
      4.7. Impact on DNS ..............................................9
      4.8. Packet Filtering ...........................................9
      4.9. Scalability ................................................9
      4.10. Impact on Routers ........................................10
      4.11. Impact on Hosts ..........................................10
      4.12. Interactions between Hosts and the Routing System ........10
      4.13. Operations and Management ................................10
      4.14. Cooperation between Transit Providers ....................10
   5. Security Considerations ........................................10
   6. Acknowledgements ...............................................10
   7. Informative References .........................................11

Abley, et al.                Informational                      [Page 2]
RFC 4116                    IPv4 Multihoming                   July 2005

1.  Introduction

   Multihoming is an important component of service for many Internet
   sites.  Current IPv4 multihoming practices have been added on to the
   Classless Inter Domain Routing (CIDR) architecture [RFC1519], which
   assumes that routing table entries can be aggregated based upon a
   hierarchy of customers and service providers.

   Multihoming is a mechanism by which sites can satisfy a number of
   high-level requirements.  It is widely used in the IPv4 Internet.
   There are some practical limitations, however, including concerns as
   to how it would scale with future Internet growth.  This document
   aims to document common IPv4 multihoming practices and enumerate
   their features for comparison with other multihoming approaches.

   There are a number of different ways to route and manage traffic in
   and out of a multihomed site: the majority rely on the routing policy
   capabilities of the inter-domain routing protocol, the Border Gateway
   Protocol, version 4 (BGP) [RFC1771].  This document also discusses a
   multi-homing strategy which does not rely on the capabilities of BGP.

2.  Terminology

   A "site" is an entity autonomously operating a network using IP, and
   in particular, determining the addressing plan and routing policy for
   that network.  This definition is intended to be equivalent to