Network Working Group J. Hagino
Request for Comments: 3178 Research Laboratory, IIJ
Category: Informational H. Snyder
Vail Systems
October 2001
IPv6 Multihoming Support at Site Exit Routers
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 (2001). All Rights Reserved.
Abstract
The document describes a mechanism for basic IPv6 multihoming
support, and its operational requirements. Unlike currently-
practiced IPv4 multihoming, the technique does not impact the
worldwide routing table size, nor IGP (Interior Gateway Protocol)
routing table size in upstream ISPs. 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 by Tony Bates.
1. Problem
Routing table size has been a major issue for both IPv4 and IPv6. As
IPv6 addresses are 4 times larger in bit width than IPv4, the routing
table size issue would have more serious negative effects on router
memory usage, as well as routing table lookup performance. To cope
with this problem, the IPv6 addressing architecture [Hinden, 1998] is
designed to take advantage of aggregated routing announcements to
redUCe the number of routes in default-free zone. Also, 6bone
operation guideline [Rockell, 2000] (which is the currently-practiced
guideline for IPv6 network operation) suggests that ASes not announce
non-aggregatable announcements to the default-free zone, if there is
no special agreement with the peer.
In IPv4, a multihomed site uses either of the following techniques to
achieve better reachability:
o OBTain a portable IPv4 address prefix, and announce it from
multiple upstream providers.
o Obtain a single IPv4 address prefix from ISP A, and announce it
from multiple upstream providers the site is connected to.
Since the above two methodologies effectively inject additional
routes to the worldwide routing table, they have negative impact on
the worldwide routing table size issue. They also are not compatible
with current IPv6 operational practice.
This document provides a way to configure site exit routers and ISP
routers, so that the site can achieve better reachability from
multihomed connectivity, without impacting worldwide routing table
size issues. The technique uses multiple distinct IPv6 address
prefixes, assigned from multiple upstream ISPs. The technique uses
an already-defined routing protocol (BGP or RIPng) and tunneling of
IPv6 packets; therefore, this document introduces no new protocol
standard (the document describes how to operate the configuration).
This 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.
o Cope with breakage of any of the upstream ISPs.
3. Basic mechanisms
We use the technique described in RFC2260 section 5.2 in 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 prefixes, 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 exchange routes for Pref-A and Pref-B, so
that hosts in the site can communicate with each other without
using external link.
o ISP-A and our site are connected by a "primary link" between ISP
router ISP-BR-A and our router E-BR-A. ISP B and our site are
connected by a primary link between ISP router ISP-BR-B and our
router E-BR-B.
(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. The secondary link usually is an
IP-over-IP tunnel. It is important to have the secondary link on
top of a different medium than the primary link, so that one of
them survives link failure. For example, the secondary link
between ISP-BR-A and E-BR-B should go through a different medium
than the primary link between ISP-BR-A and E-BR-A. If the
secondary link is an IPv4-over-IPv4 tunnel, the tunnel endpoint at
E-BR-A needs to be an address in Pref-A, not in Pref-B (tunneled
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 the over primary link, and (2) Pref-B
toward ISP-BR-B with weak preference over the secondary link.
Similarly, E-BR-B will advertise (1) Pref-B toward ISP-BR-B with
strong preference over the primary link, and (2) Pref-A toward
ISP-BR-A with weak preference over the secondary link.
Note that we always announce Pref-A to ISP-BR-A, and Pref-B to
ISP-BR-B.
o For outbound packets, ISP-BR-A will advertise (1) default route
(or specific routes) toward E-BR-A with strong preference over the
primary link, and (2) default route (or specific routes) toward
E-BR-B with weak preference over the secondary link. Similarly,
ISP-BR-B will advertise (1) default route (or specific routes)
toward E-BR-B with strong preference over the primary link, and
(2) default route (or specific routes) toward E-BR-A with weak
preference over the secondary link.
Under this configuration, both inbound and outbound packets 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 results can be achieved, without impacting
worldwide IPv6 routing table size.
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 meets the suggestions
in 6bone aggregation guidelines [Rockell, 2000]. Also, RFC2260 does
not require portable addresses.
4.2. Address assignment to the nodes
In IPv4, it is usually assumed that a node will be assigned a 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) addresses from both prefixes, to a single node in the site.
This will allow more flexibility in node configuration.
When multiple IPv6 global addresses are assigned to an IPv6 node,
source address selection must take place on packet transmissions.
Source address selection itself is out of scope of the document.
Refer to a separate draft [Draves, 2001] for more discussions.
One simplifying approach is to place the site's Internet hosts on
separate subnets, one with addresses in Pref-A and connected to E-
BR-A, the other having addresses in Pref-B and connected to E-BR-B.
This approach generalizes to having E-BR-A and E-BR-B at different
sites, where site A and site B have links to the Internet and to each
other.
4.3. Configuration of links
With IPv6, the primary link can be IPv6 native connectivity, RFC2893
[Gilligan, 2000] 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 RFC2893 IPv6-over-IPv4
tunnels, and
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.
In the figure, ISP-BR-A and E-BR-A are both single points of failure
for inbound traffic to Pref-A. This could be remedied by using
different routers for primary vs. backup links.
4.4. Using RFC2260 with IPv6 and BGP4+
The RFC2260 approach on top of IPv6 will work fine as documented in
RFC2260. There will be no extra twists necessary. Since the
multihomed site is not doing transit, variations are possible that do
not require it to have a public AS number.
4.5. Using RFC2260 with IPv6 and RIPng
It is possible to run an RFC2260-like configuration with RIPng
[Malkin, 1997] , with careful control of metric. Routers in the
figure need to increase RIPng metric on the secondary link, to make
the 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.
5. Issues with ingress filters in ISP
If the upstream ISP imposes ingress filters [Ferguson, 1998] to
outbound traffic, the 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 the 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-BR-B
o Inner IPv6 header: source = address in Pref-B, dest = final dest
A 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. The packet will reach ISP-BR-B and be
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 this:
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.
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 ISPs
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 a 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 a very long path, for example,
from a 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 the primary links 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 the 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 a new connection to outside, the host will use an
address in Pref-B as source address, and the reply packet to the end
host will travel the primary link from ISP-BR-B toward E-BR-B. A
great care must be taken when you try to automate this by using
router renumbering protocols [Crawford, 2000] , as the approach could
lead your site into very unstable state if any of the links flap.
The author does not recommend to automate it.
Some of non-goals (such as "best" exit link selection) can be
achieved by combining the technique described in this document, with
some other techniques. One example of the technique would be the
source/destination address selection [Draves, 2001] on the end nodes.
7. Operational eXPeriences
Hal Snyder has been running the technique, with two upstream ISPs
(lava.net and iijlab), using 2 RFC2893 IPv6-over-IPv4 tunnels to
each of them (in total 4 tunnels), and BGP4+ peering over them.
As expected, when the primary links goes down the routing switches to
the secondary link within BGP hold time, i.e., we see approximately
the relations:
o (hold time - keepalive time) < failover time
o failover time < hold time
o failback time < keepalive time
This has been tested with keepalive and hold times from as low as 3
and 10 seconds respectively, up to 60 and 180 seconds respectively.
The routing change will affect ISP-BR-A (or B) only. Because route
instability is not propagated beyond one ISP, it should be feasible
to use lower hold and keepalive times than in a conventional IPv4
setting. If primary and backup links terminate on the same router at
the ISP, then failover from primary to backup link need not affect
reachability information upstream of that router.
Many of the existing IPv6 networks (connected to worldwide 6bone) are
assigned multiple IPv6 prefixes from multiple upstreams. In many
cases people assign global IPv6 addresses generated from multiple
address prefixes. There has been almost no problems raised about
complication due to source address selection.
8. Security Considerations
The configuration described in the document introduces no new
security problem.
If primary links toward ISP-A and ISP-B have different security
characteristics (like encrypted link and non-encrypted link),
administrators need to be careful setting up secondary links tunneled
on them. Packets may travel an unwanted path, if secondary links are
configured without care.
References
[Bates, 1998] Bates, T. and Y. Rekhter, "Scalable Support for
Multi-homed Multi-provider Connectivity", RFC2260,
January 1998.
[Hinden, 1998] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC2373, July 1998.
[Rockell, 2000] Rockell, R. and B. Fink, "6Bone Backbone Routing
Guidelines", RFC2772, February 2000.
[Draves, 2001] Draves, R., "Default Address Selection for IPv6",
Work in Progress.
[Gilligan, 2000] Gilligan, R. and E. Nordmark, "Transition
Mechanisms for IPv6 Hosts and Routers", RFC2893,
August 2000.
[Carpenter, 2000] Carpenter, B. and K. Moore, "Connection of IPv6
Domains via IPv4 Clouds", RFC3056, February 2001.
[Malkin, 1997] Malkin, G. and R. Minnear, "RIPng for IPv6", RFC
2080, January 1997.
[Ferguson, 1998] Ferguson, P. and D. Senie, "Network Ingress
Filtering: Defeating Denial of Service Attacks
which employ IP Source Address Spoofing", RFC2267,
January 1998.
[Thomson, 1998] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC2462, December 1998.
[Narten, 1998] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC2461,
December 1998.
[Crawford, 2000] Crawford, M., "Router Renumbering for IPv6", RFC
2894, August 2000.
Acknowledgements
The document was made possible by cooperation from people
participated in JEPG-IP IPv6 multihoming study meeting (1999), people
in ipngwg multihoming design team, people in WIDE/KAME project and
George Tsirtsis.
Authors' Addresses
Jun-ichiro itojun Hagino
Research Laboratory, Internet Initiative Japan Inc.
Takebashi Yasuda Bldg.,
3-13 Kanda Nishiki-cho,
Chiyoda-ku, Tokyo 101-0054, JAPAN
Phone: +81-3-5259-6350
Fax: +81-3-5259-6351
EMail: itojun@iijlab.net
Hal Snyder
Vail Systems, Inc.
570 Lake Cook Rd, Ste 408
Deerfield, IL 60015, US
Phone: +1-312-360-8245
EMail: hal@vailsys.com
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