Subject: [RADIR] Section 4.2. Multihoming

From: Thomas Narten <[email-masked]>
Date: Thu, 31 May 2007 15:11:51 -0400

Multihoming refers generically to the case in which a site is
   served by more than one ISP [RFC4116]. Multihoming is used to
   provide backup paths (i.e., to remove single points of failure), to
   achieve load-sharing, and to achieve policy or performance
   objectives. Multihoming may also be a requirement due to contract
   or law.

   Multihoming can be accomplished using either PI or PA address
   space.  A multihomed site advertises its site prefix into the
   routing system of each of its providers. For PI space, the site's
   PI space is used, and the prefix is propagated throughout the
   DFZ. For PA space, the PA site prefix may (or may not) be
   propagated throughout the DFZ, with the details depending on what
   type of multihoming is sought.

   If the site uses PA space, the PA site prefix allocated from one of
   its provider's (whom we'll call the Primary Provider) is used. The
   PA site prefix will be aggregatable by the Primary Provider but not
   the others.  To achieve the same level of multihoming as described
   in the case with PI address above, the PA site prefix will need to
   be injected into the routing system of all of its ISPs, and
   throughout the DFZ.  In addition, because of the longest-match
   forwarding rule, the Primary Provider must also advertise and
   propagate the individual PA site prefix; otherwise, the path via
   the primary provider (as advertised via the aggregate) will never
   be selected due to the longest match rule. For the type of
   multihoming described here, where the PA site prefix is propagated
   throughout the DFZ, the use of PI vs. PA space has no impact on the
   load placed on the routing system. The increased load is due
   entirely to the need to propage the site's individual prefix that
   is relevant.

   The demand for multihoming is increasing [XXX do we have data to
   cite?]. The increase in multihoming demand is due to the increased
   reliance on the Internet for mission and business-critical
   applications (where businesses require 7x24 availability for their
   services) and the general decrease in cost of Internet
   connectivity.

Thomas

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From: Erik Nordmark <[email-masked]>
Date: Thu, 31 May 2007 14:07:55 -0700

>    objectives. Multihoming may also be a requirement due to contract

Would it be useful to talk more about the 'performance' motivation?
I don't know how common it is, but my understanding is that large 
content providers multihome so that they can get closer to more of their 
users; lower latency and better performance.
*If* this is believed to be a noticable driver of more multihoming it 
would be good to add a few words about it.

>    type of multihoming is sought.

I wonder if somebody that looks at the world through IPv6-tinted glasses 
will be concerned that the above omits the IPv6 suggestion to use 
multiple PA prefixes. Given that such an approach doesn't really work 
(no failover etc etc) I think what you've written is fine.

>    entirely to the need to propage the site's individual prefix that

The last part of the last sentence reads odd. It reads better if you 
drop "that is relevant" but perhaps you indented something different.

>    services) and the general decrease in cost of Internet

Geoff has said it is increasing, but I don't know the details of the 
data on which he bases that conclusion.

    Erik