Subject: Re: [RADIR] Section 4.2. Multihoming

From: Thomas Narten <[email-masked]>
Date: Fri, 01 Jun 2007 12:07:15 -0400

> 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.

Another thing I thought of (which may be obvious). There  is a fine
line between multihoming and TE. If you have a large site that has
upstream connections  in US and Japan, presumably longer prefixes are
advertised in each region in order to prevent all traffic from flowing
through (say) the US exit point.

> >    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.

> 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.

Yeah, someone will probably raise the issue, but I'd rather not go
there.

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

Stray words, I think. :-)

> >    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.

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

OK. We'll need to chase that down to a reference.

Thomas

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