IPv6 Neighbor Discovery On-Link Assumption Considered Harmful
RFC 4943

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Network Working Group                                             S. Roy
Request for Comments: 4943                        Sun Microsystems, Inc.
Category: Informational                                        A. Durand
                                                                 Comcast
                                                                J. Paugh
                                                           Nominum, Inc.
                                                          September 2007

     IPv6 Neighbor Discovery On-Link Assumption Considered Harmful

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.

Abstract

   This document describes the historical and background information
   behind the removal of the "on-link assumption" from the conceptual
   host sending algorithm defined in Neighbor Discovery for IP Version 6
   (IPv6).  According to the algorithm as originally described, when a
   host's default router list is empty, the host assumes that all
   destinations are on-link.  This is particularly problematic with
   IPv6-capable nodes that do not have off-link IPv6 connectivity (e.g.,
   no default router).  This document describes how making this
   assumption causes problems and how these problems outweigh the
   benefits of this part of the conceptual sending algorithm.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Background on the On-link Assumption  . . . . . . . . . . . . . 2
   3.  Problems  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  First Rule of Destination Address Selection . . . . . . . . 3
     3.2.  Delays Associated with Address Resolution . . . . . . . . . 3
     3.3.  Multi-interface Ambiguity . . . . . . . . . . . . . . . . . 4
     3.4.  Security-Related Issues . . . . . . . . . . . . . . . . . . 4
   4.  Changes to RFC 2461 . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 7

Roy, et al.                  Informational                      [Page 1]
RFC 4943               On-Link Assumption Harmful         September 2007

1.  Introduction

   Neighbor Discovery for IPv6 [RFC4861] defines a conceptual sending
   algorithm for hosts.  The version of the algorithm described in
   [RFC2461] states that if a host's default router list is empty, then
   the host assumes that all destinations are on-link.  This memo
   documents the removal of this assumption in the updated Neighbor
   Discovery specification [RFC4861] and describes the reasons why this
   assumption was removed.

   This assumption is problematic with IPv6-capable nodes that do not
   have off-link IPv6 connectivity.  This is typical when systems that
   have IPv6 enabled on their network interfaces (either on by default
   or administratively configured that way) are attached to networks
   that have no IPv6 services such as off-link routing.  Such systems
   will resolve DNS names to AAAA and A records, and they may attempt to
   connect to unreachable IPv6 off-link nodes.

   The on-link assumption creates problems for destination address
   selection as defined in [RFC3484], and it adds connection delays
   associated with unnecessary address resolution and neighbor
   unreachability detection.  The behavior associated with the
   assumption is undefined on multi-interface nodes and has some subtle
   security implications.  All of these issues are discussed in this
   document.

2.  Background on the On-link Assumption

   This part of Neighbor Discovery's [RFC2461] conceptual sending
   algorithm was created to facilitate communication on a single link
   between systems configured with different global prefixes in the
   absence of an IPv6 router.  For example, consider the case where two
   systems on separate links are manually configured with global
   addresses and are then plugged in back-to-back.  They can still
   communicate with each other via their global addresses because
   they'll correctly assume that each is on-link.

   Without the on-link assumption, the above scenario wouldn't work, and
   the systems would need to be configured to share a common prefix such
   as the link-local prefix.  On the other hand, the on-link assumption
   introduces several problems to various parts of the networking stack
   described in Section 3.  As such, this document points out that the
   problems introduced by the on-link assumption outweigh the benefit
   that the assumption lends to this scenario.  It is more beneficial to
   the end user to remove the on-link assumption from Neighbor Discovery
   and declare this scenario illegitimate (or a misconfiguration).