Internet Engineering Task Force (IETF) M. Kohno
Request for Comments: 6164 Juniper Networks, Keio University
Category: Standards Track B. Nitzan
ISSN: 2070-1721 Juniper Networks
R. BushY. Matsuzaki
Internet Initiative Japan
April 2011 Using 127-Bit IPv6 Prefixes on Inter-Router Links
On inter-router point-to-point links, it is useful, for security and
other reasons, to use 127-bit IPv6 prefixes. Such a practice
parallels the use of 31-bit prefixes in IPv4. This document
specifies the motivation for, and usages of, 127-bit IPv6 prefix
lengths on inter-router point-to-point links.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................22. Scope of This Memo ..............................................33. Conventions Used in This Document ...............................34. Problems Identified with 127-Bit Prefix Lengths in the Past .....35. Reasons for Using Longer Prefixes ...............................45.1. Ping-Pong Issue ............................................45.2. Neighbor Cache Exhaustion Issue ............................45.3. Other Reasons ..............................................56. Recommendations .................................................57. Security Considerations .........................................68. Contributors ....................................................69. Acknowledgments .................................................610. References .....................................................610.1. Normative References ......................................610.2. Informative References ....................................71. Introduction
[RFC4291] specifies that interface IDs for all unicast addresses,
except those that start with the binary value 000, are required to be
64 bits long and to be constructed in Modified EUI-64 format. In
addition, it defines the Subnet-Router anycast address, which is
intended to be used for applications where a node needs to
communicate with any one of the set of routers on a link.
Some operators have been using 127-bit prefixes, but this has been
discouraged due to conflicts with Subnet-Router anycast [RFC3627].
However, using 64-bit prefixes creates security issues that are
particularly problematic on inter-router links, and there are other
valid reasons to use prefixes longer than 64 bits, in particular /127
(see Section 5).
This document provides a rationale for using 127-bit prefix lengths,
reevaluates the reasons why doing so was considered harmful, and
specifies how /127 prefixes can be used on inter-router links
configured for use as point-to-point links.
2. Scope of This Memo
This document is applicable to cases where operators assign specific
addresses on inter-router point-to-point links and do not rely on
link-local addresses. Many operators assign specific addresses for
the purposes of network monitoring, reverse DNS resolution for
traceroute and other management tools, External Border Gateway
Protocol (EBGP) [RFC4271] peering sessions, and so on.
For the purposes of this document, an inter-router point-to-point
link is a link to which only two routers and no hosts are attached.
This may include Ethernet links that are configured to be point-to-
point. Links between a router and a host, or links to which both
routers and hosts are attached, are out of scope of this document.
The recommendations in this document do not apply to the link-local
3. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
4. Problems Identified with 127-Bit Prefix Lengths in the Past
[RFC3627] discourages the use of 127-bit prefix lengths due to
conflicts with the Subnet-Router anycast addresses, while stating
that the utility of Subnet-Router anycast for point-to-point links is
[RFC5375] also says the usage of 127-bit prefix lengths is not valid
and should be strongly discouraged, but the stated reason for doing
this is to be in compliance with [RFC3627].
Though the analyses in the RFCs are correct, operational experience
with IPv6 has shown that /127 prefixes can be used successfully.
5. Reasons for Using Longer Prefixes
There are reasons network operators use IPv6 prefix lengths greater
than 64, particularly 127, for inter-router point-to-point links.
5.1. Ping-Pong Issue
A forwarding loop may occur on a point-to-point link with a prefix
length shorter than 127. This does not affect interfaces that
perform Neighbor Discovery, but some point-to-point links, which use
a medium such as the Synchronous Optical Network (SONET), do not use
Neighbor Discovery. As a consequence, configuring any prefix length
shorter than 127 bits on these links can create an attack vector in
The ping-pong issue happens in the case of IPv4 as well. But due to
the scarcity of IPv4 address space, the current practice is to assign
long prefix lengths such as /30 or /31 [RFC3021] on point-to-point
links; thus, the problem did not come to the fore.
The latest ICMPv6 specification [RFC4443] mitigates this problem by
specifying that a router receiving a packet on a point-to-point link,
where the packet is destined to an address within a subnet assigned
to that same link (other than one of the receiving router's own
addresses), MUST NOT forward the packet back on that link. Instead,
it SHOULD generate an ICMPv6 Destination Unreachable message (code 3)
in response. This check is on the forwarding processing path, so it
may have performance impact.
5.2. Neighbor Cache Exhaustion Issue
As described in Section 4.3.2 of [RFC3756], the use of a 64-bit
prefix length on an inter-router link that uses Neighbor Discovery
(e.g., Ethernet) potentially allows for denial-of-service attacks on
the routers on the link.
Consider an Ethernet link between two routers, A and B, to which a
/64 subnet has been assigned. A packet sent to any address on the
/64 (except the addresses of A and B) will cause the router
attempting to forward it to create a new cache entry in INCOMPLETE
state, send a Neighbor Solicitation message on the link, start a
retransmit timer, and so on [RFC4861].
By sending a continuous stream of packets to a large number of the
2^64 - 3 unassigned addresses on the link (one for each router and
one for Subnet-Router anycast), an attacker can create a large number
of neighbor cache entries and cause one of the routers to send a
large number of Neighbor Solicitation packets that will never receive
replies, thereby consuming large amounts of memory and processing
resources. Sending the packets to one of the 2^24 addresses on the
link that has the same Solicited-Node multicast address as one of the
routers also causes the victim to spend large amounts of processing
time discarding useless Neighbor Solicitation messages.
Careful implementation and rate-limiting can limit the impact of such
an attack, but are unlikely to neutralize it completely. Rate-
limiting Neighbor Solicitation messages will reduce CPU usage, and
following the garbage-collection recommendations in [RFC4861] will
maintain reachability, but if the link is down and neighbor cache
entries have expired while the attack is ongoing, legitimate traffic
(for example, BGP sessions) over the link might never be
re-established, because the routers cannot resolve each others' IPv6
addresses to link-layer addresses.
This attack is not specific to point-to-point links, but is
particularly harmful in the case of point-to-point backbone links,
which may carry large amounts of traffic to many destinations over
While there are a number of ways to mitigate this kind of issue,
assigning /127 subnets eliminates it completely.
5.3. Other Reasons
Though address space conservation considerations are less important
for IPv6 than they are in IPv4, some operators prefer not to assign
/64s to individual point-to-point links. Instead, they may be able
to number all of their point-to-point links out of a single /64 or a
small number of /64s.
Routers MUST support the assignment of /127 prefixes on point-to-
point inter-router links. Routers MUST disable Subnet-Router anycast
for the prefix when /127 prefixes are used.
When assigning and using any /127 prefixes, the following
considerations apply. Some addresses have special meanings, in
particular addresses corresponding to reserved anycast addresses.
When assigning prefixes (and addresses) to links, care should be
taken to ensure that addresses reserved for such purposes aren't
inadvertently assigned and used as unicast addresses. Otherwise,
nodes may receive packets that they are not intended to receive.
Specifically, assuming that a number of point-to-point links will be
numbered out of a single /64 prefix:
(a) Addresses with all zeros in the rightmost 64 bits SHOULD NOT be
assigned as unicast addresses, to avoid colliding with the
Subnet-Router anycast address [RFC4291].
(b) Addresses in which the rightmost 64 bits are assigned the
highest 128 values (i.e., ffff:ffff:ffff:ff7f to ffff:ffff:ffff:
ffff) SHOULD NOT be used as unicast addresses, to avoid
colliding with reserved subnet anycast addresses [RFC2526].
7. Security Considerations
This document does not have inherent security considerations. It
does discuss security-related issues and proposes a solution to them.
Chris Morrow, email@example.com
Pekka Savola, firstname.lastname@example.org
Remi Despres, email@example.com
Seiichi Kawamura, firstname.lastname@example.org
The authors would like to thank Ron Bonica, Pramod Srinivasan,
Olivier Vautrin, Tomoya Yoshida, Warren Kumari, and Tatsuya Jinmei
for their helpful inputs.
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
10.2. Informative References
[RFC2526] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson,
"Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
RFC 3021, December 2000.
[RFC3627] Savola, P., "Use of /127 Prefix Length Between Routers
Considered Harmful", RFC 3627, September 2003.
[RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats",
RFC 3756, May 2004.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
[RFC5375] Van de Velde, G., Popoviciu, C., Chown, T., Bonness, O.,
and C. Hahn, "IPv6 Unicast Address Assignment
Considerations", RFC 5375, December 2008.
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