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Versions: 00 01 RFC 3178

Internet Engineering Task Force                        Jun-ichiro Hagino
INTERNET-DRAFT                                  Research Laboratory, IIJ
Expires: October 12, 2001                                 April 12, 2001

             IPv6 multihoming support at site exit routers

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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or to cite them other than as ``work in progress.''

The list of current Internet-Drafts can be accessed at

To view the list Internet-Draft Shadow Directories, see

Distribution of this memo is unlimited.

The internet-draft will expire in 6 months.  The date of expiration will
be October 12, 2001.


The document describes a mechanism for basic IPv6 multihoming support,
and its operational requirements.  The mechanism can be combined with
more sophisticated (or complex) multihoming support mechanisms, and can
be used as a foundation for other mechanisms.  The document is largely
based on RFC2260 [Bates, 1998] by Tony Bates.

1.  Problem

IPv6 specifications try to decrease the number of backbone routes, to
cope with possible memory overflow problem in the backbone routers.  To
achieve this, the IPv6 addressing architecture [Hinden, 1998] only
allows the use of aggregatable addresses.  Also, IPv6 network
administration rules [Durand, 1999] do not allow non-aggregatable
routing announcements to the backbone.

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In IPv4, a multihomed site uses either of the following technique to
achieve better reachability:

o  Obtain a portable IPv4 address prefix, and announce it from multiple
   upstream providers.

o  Obtain single IPv4 address prefix from ISP A, and announce it from
   multiple upstream providers the site is connected to.

The above two methodologies are not available in IPv6, but on the other
hand IPv6 sites and hosts may obtain multiple simultaneous address
prefixes to achieve the same result.

The document provides a way to configure site exit routers and ISP
routers, so that the site can achieve better reachability from
multihomed connectivity, without violating IPv6 rules.  Since the
technique uses already-defined routing protocol (BGP or RIPng) and
tunnelling of IPv6 packets, the document introduces no new protocol

The document is largely based on RFC2260 [Bates, 1998] by Tony Bates.

2.  Goals and non-goals

The goal of this document is to achieve better packet delivery from a
site to the outside, or from the outside to the site, even when some of
the site exit links are down.

Non goals are:

o  Choose the "best" exit link as possible.  Note that there can be no
   common definition of the "best" exit link.

o  Achieve load-balancing between multiple exit links.

3.  Basic mechanisms

We use technique described in RFC2260 section 5.2 onto our
configuration.  To summarize, for IPv4-only networks, RFC2260 says that:

o  We assume that our site is connected to 2 ISPs, ISP-A and ISP-B.

o  We are assigned IP address prefix, Pref-A and Pref-B, from ISP-A and
   ISP-B respectively.  Hosts near ISP-A will get an address from Pref-
   A, and vice versa.

o  In the site, we locally exchanage routes for Pref-A and Pref-B, so
   that hosts in the site can communicate with each other without using
   external link.

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o  ISP-A and our site is connected by ``primary link'' between ISP
   router ISP-BR-A and our router E-BR-A.  ISP B and our site is
   connected by primary link between ISP router ISP-BR-B and our router

        (ISP A)                         (ISP B)

        ISP-BR-A                       ISP-BR-B
            |                             |
            |Primary link                 |
            |                             |
            |                             |
        | E-BR-A                      E-BR-B |
        |                                    |
        | Pref-A     <---------->     Pref-B |

o  Establish a secondary link, between ISP-BR-A and E-BR-B, and ISP-BR-B
   and E-BR-A, respectively.  Secondary link usually is IP-over-IP
   tunnel.  It is important to have secondary link on top of different
   medium than primary link, so that one of them survives link failure.
   For example, secondary link between ISP-BR-A and E-BR-B should go
   through different medium than primary link between ISP-BR-A and E-BR-
   A.  If secondary link is an IPv4-over-IPv4 tunnel, tunnel endpoint at
   E-BR-A needs to be an address in Pref-A, not in Pref-B (tunnelled
   packet needs to travel from ISP-BR-B to E-BR-A, over the primary link
   between ISP-BR-A and E-BR-A).

        (ISP A)                         (ISP B)

        ISP-BR-A                       ISP-BR-B
            | |                         | |
            | \-----------------------+ | |
            |     Secondary link      | | |
            |  +----------------------|-/ |
            |  |                      |   |
            |  |                      |   |
            |  |                      |   |
            |  |                      |   |
        | E-BR-A                      E-BR-B |
        |                                    |
        |                                    |

o  For inbound packets, E-BR-A will advertise (1) Pref-A toward ISP-BR-A
   with strong preference over primary link, and (2) Pref-B toward ISP-
   BR-B with weak preference over secondary link.  Similarly, E-BR-B
   will advertise (1) Pref-B toward ISP-BR-B with strong preference over

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   primary link, and (2) Pref-A toward ISP-BR-A with weak preference
   over secondary link.
   Note that we always announce Pref-A to ISP-BR-A, and Pref-B to ISP-

o  For outbound packets, ISP-BR-A will advertise (1) default route (or
   specific routes) toward E-BR-A with strong preference over primary
   link, and (2) default route (or specific routes) toward E-BR-B with
   weak preference over secondary link.  Similarly, ISP-BR-B will
   advertise (1) default route (or specific routes) toward E-BR-B with
   strong preference over primary link, and (2) default route (or
   specific routes) toward E-BR-A with weak preference over secondary

Under this configuration, both inbound and outbound packet can survive
link failure on either side.  Routing information with weak preference
will be available as backup, for both inbound and outbound cases.

4.  Extensions for IPv6

RFC2260 is written for IPv4 and BGP.  With IPv6 and BGP4+, or IPv6 and
RIPng, similar result can be achieved, without violating IPv6
addressing/routing rules.

4.1.  IPv6 rule conformance

In RFC2260, we announce Pref-A toward ISP-BR-A only, and Pref-B toward
ISP-BR-B only.  Therefore, there will be no extra routing announcement
to the outside of the site.  This conforms to the aggregation
requirement in IPv6 documents.  Also, RFC2260 does not require portable

4.2.  Address assignment to the nodes

In IPv4, it is usually assumed that a node will be assigned single IPv4
address.  Therefore, RFC2260 assumed that addresses from Pref-A will be
assigned to nodes near E-BR-A, and vice versa (second bullet in the
previous section).

With IPv6, multiple IPv6 addresses can be assigned to a node.  So we can
assign (1) one address from Pref-A, (2) one address from Pref-B, or (3)
two addresses from both address prefixes, to a single node in the site.

This will allow more flexibility in node configuration.  However, this
may make source address selection on a node more complex.  Source
address selection itself is out of scope of the document.

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4.3.  Configuration of links

With IPv6, primary link can be IPv6 native connectivity, RFC1933
[Gilligan, 1996] IPv6-over-IPv4 configured tunnel, 6to4 [Carpenter,
2000] IPv6-over-IPv4 encapsulation, or some others.

If tunnel-based connectivity is used in some of primary links,
administrators may want to avoid IPv6-over-IPv6 tunnels for secondary
links.  For example, if:

o  primary links to ISP-A and ISP-B are RFC1933 IPv6-over-IPv4 tunnels,

o  ISP-A, ISP-B and the site have IPv4 connectivity with each other,

it makes no sense to configure a secondary link by IPv6-over-IPv6
tunnel, since it will actually be IPv6-over-IPv6-over-IPv4 tunnel.  In
this case, IPv6-over-IPv4 tunnel should be used for secondary link.
IPv6-over-IPv4 configuration has a big advantage against IPv6-over-
IPv6-over-IPv4 configuration, as secondary link will be able to have the
same path MTU than the primary link.

4.4.  Using RFC2260 with IPv6 and BGP4+

RFC2260 approach on top of IPv6 will work fine as documented in RFC2260.
There will be no extra twists necessary.

4.5.  Using RFC2260 with IPv6 and RIPng

It is possible to run RFC2260-like configuration with RIPng [Malkin,
1997] , with careful control of metric.  Routers in the figure needs to
increase RIPng metric on secondary link, to make primary link a
preferred path.

If we denote the RIPng metric for route announcement, from router R1
toward router R2, as metric(R1, R2), the invariants that must hold are:

o  metric(E-BR-A, ISP-BR-A) < metric(E-BR-B, ISP-BR-A)

o  metric(E-BR-B, ISP-BR-B) < metric(E-BR-A, ISP-BR-B)

o  metric(ISP-BR-A, E-BR-A) < metric(ISP-BR-A, E-BR-B)

o  metric(ISP-BR-B, E-BR-B) < metric(ISP-BR-B, E-BR-A)

Note that smaller metric means stronger route in RIPng.

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5.  Issues with ingress filters in ISP

If the upstream ISP imposes ingress filters [Ferguson, 1998] to outbound
traffic, story becomes much more complex.  A packet with source address
taken from Pref-A must go out from ISP-BR-A.  Similarly, a packet with
source address taken from Pref-B must go out from ISP-BR-B.  Since none
of the routers in the site network will route packets based on source
address, packets can easily be routed to incorrect border router.

One possible way is to negotiate with both ISPs, to allow both Pref-B
and Pref-A to be used as source address.  This approach does not work if
upstream ISP of ISP-A imposes ingress filtering.  Since there will be
multiple levels of ISP on top of ISP-A, it will be hard to understand
which upstream ISP imposes the filter.  In reality, this problem will be
very rare, as ingress filter is not suitable for use in large ISPs where
smaller ISPs are connected beneath.

Another possibility is to use source-based routing at E-BR-A and E-BR-B.
Here we assume that IPv6-over-IPv6 tunnel is used for secondary links.
When an outbound packet arrives to E-BR-A with source address in Pref-B,
E-BR-A will forward it to secondary link (tunnel to ISP-BR-B) based on
source-based routing decision.  The packet will look like this:

o  Outer IPv6 header: source = address of E-BR-A in Pref-A, dest = ISP-

o  Inner IPv6 header: source = address in Pref-B, dest = final dest

Tunneled packet will travel across ISP-BR-A toward ISP-BR-B.  The packet
can go through ingress filter at ISP-BR-A, since it has outer IPv6
source address in Pref-A.  Packet will reach ISP-BR-B and decapsulated
before ingress filter is applied.  Decapsulated packet can go through
ingress filter at ISP-BR-B, since it now has source address in Pref-B
(from inner IPv6 header).  Notice the following facts when configuring

o  Not every router implements source-based routing.

o  The interaction between normal routing and source-based routing at E-
   BR-A (and/or E-BR-B) varies by router implementations.

o  At ISP-BR-B (and/or ISP-BR-A), the interaction between tunnel egress
   processing and filtering rules varies by router implementations and
   filter configurations.

6.  Observations

The document discussed the cases where a site has two upstream ISPs.
The document can easily be extended to the cases where there are 3 or
more upstream ISPs.

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If you have many upstream providers, you would not make all ISPs backup
each other, as it requires O(N^2) tunnels for N ISPs.  Rather, it is
better to make N/2 pairs of ISPs, and let each pair of ISP backup each
other.  It is important to pick pairs which are unlikely to be down
simultaneously.  In this way, number of tunnels will be O(N).

Suppose that the site is very large and it has ISP links in very distant
locations, such as in the United States and in Japan.  In such case, it
is wiser to use this technique only among ISP links in the US, and only
among ISP links in Japan.  If you use this technique between ISP link A
in the US and ISP link B in Japan, the secondary link makes packets
travel very long path, for example, from host in the site in the US, to
E-BR-B in Japan, to ISP-BR-B (again in Japan), and then to the final
destination in the US.  This may not make sense for actual use, due to
excessive delay.

Similarly, in a large site, addresses must be assigned to end nodes with
great care, to minimize delays due to extra path packets may travel.  It
may be wiser to avoid assigning an address in a prefix assigned from
Japanese ISP, to an end node in the US.

If one of primary link is down for a long time, administrators may want
to control source address selection on end hosts so that secondary link
is less likely to be used.  This can be achieved by marking unwanted
prefix as deprecated.  Suppose the primary link toward ISP-A has been
down.  You will issue router advertisement [Thomson, 1998; Narten, 1998]
packets from routers, with preferred lifetime set to 0 in prefix
information option for Pref-A.  End hosts will consider addresses in
Pref-A as deprecated, and will not use any of them as source address for
future connections.  If an end host in the site makes new connection to
outside, the host will use an address in Pref-B as source address, and
reply packet to the end host will travel primary link from ISP-BR-B
toward E-BR-B.

Some of non-goals (such as "best" exit link selection) can be achieved
by combining technique described in this document, with some other
techniques.  One example of the technique would be the
source/destination address selection heuristics on the end nodes.

7.  Security considerations

The configuration described in the document introduces no new security

If primary links toward ISP-A and ISP-B have different security
characteristics (like encrypted link and non-encrypted link),
administrators needs to be careful setting up secondary links tunneled
on them.  Packets may travel unwanted path, if secondary links are
configured without care.

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Bates, 1998.
T. Bates and Y. Rekhter, "Scalable Support for Multi-homed Multi-
provider Connectivity" in RFC2260 (January 1998). ftp://ftp.isi.edu/in-

Hinden, 1998.
R. Hinden and S. Deering, "IP Version 6 Addressing Architecture" in
RFC2373 (July 1998). ftp://ftp.isi.edu/in-notes/rfc2373.txt.

Durand, 1999.
A. Durand and B. Buclin, "6Bone Routing Practice" in RFC2546 (March
1999). ftp://ftp.isi.edu/in-notes/rfc2546.txt.

Gilligan, 1996.
R. Gilligan and E. Nordmark, "Transition Mechanisms for IPv6 Hosts and
Routers" in RFC1933 (April 1996). ftp://ftp.isi.edu/in-

Carpenter, 2000.
Brian Carpenter and Keith Moore, "Connection of IPv6 Domains via IPv4
Clouds without Explicit Tunnels" in draft-ietf-ngtrans-6to4-06.txt (June
2000). work in progress.

Malkin, 1997.
G. Malkin and R. Minnear, "RIPng for IPv6" in RFC2080 (January 1997).

Ferguson, 1998.
P. Ferguson and D. Senie, "Network Ingress Filtering: Defeating Denial
of Service Attacks which employ IP Source Address Spoofing" in RFC2267
(January 1998). ftp://ftp.isi.edu/in-notes/rfc2267.txt.

Thomson, 1998.
S. Thomson and T. Narten, "IPv6 Stateless Address Autoconfiguration" in
RFC2462 (December 1998). ftp://ftp.isi.edu/in-notes/rfc2462.txt.

Narten, 1998.
T. Narten, E. Nordmark, and W. Simpson, "Neighbor Discovery for IP
Version 6 (IPv6)" in RFC2461 (December 1998). ftp://ftp.isi.edu/in-


The document was made possible by cooperation from people in ipngwg
multihoming design team, people in KAME project and George Tsirtsis.

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Author's address

     Jun-ichiro Hagino
     Research Laboratory, Internet Initiative Japan Inc.
     Takebashi Yasuda Bldg.,
     3-13 Kanda Nishiki-cho,
     Chiyoda-ku,Tokyo 101-0054, JAPAN
     Tel: +81-3-5259-6350
     Fax: +81-3-5259-6351
     email: itojun@iijlab.net

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