Re: [grobj] Referral definition and its purpose?

Keith Moore <moore@network-heretics.com> Thu, 27 May 2010 10:48 UTC

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Date: Thu, 27 May 2010 06:48:30 -0400
From: Keith Moore <moore@network-heretics.com>
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To: bo zhou <zhouboyj@gmail.com>
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Subject: Re: [grobj] Referral definition and its purpose?
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On 5/27/10 3:29 AM, bo zhou wrote:
>
>
>     here's an example: over path P1, A sends to B a referral that
>     specifies three paths by which A can be reached: P1, P2, and P3. 
>     when B tries to reconnect to A, its circumstances are such that P1
>     appears to be the best of the three paths.  I don't know why B
>     shouldn't use P1.
>
>  
> [bo] I am not sure why B will use P1 to connect to A, because it is
> hard for B to understand which path is the best. A send B a referral
> over P1 is bacause only P1 is known at the beginning of
> communication. A can tell B there are three paths, but cannot tell B
> which one is the best.
It is indeed difficult for B to know which path is best.  And in many
cases "best" depends on the specific needs of the application.   One
application might need maximum bandwidth, another low delay, another
might prefer a stable address.  An IPv4 only host would want to use an
IPv4 address for its peer.  etc.

Last I knew there was a current IETF WG looking at a similar problem -
though its name escapes me at the moment and I haven't been following
their work.

But just for the sake of discussion, suppose that B tries each path and
picks the one that seems to work best.  Or suppose that it tries each
path in turn, in a random order, and sticks with one that seems to work
adequately.  There are other strategies that make sense in certain
situations; e.g. a file transfer program might use all paths and adjust
the amount of data transferred over each according to the measured
available bandwidth.
> And in the real world, could you give me an example why A will give B
> three path. For my knowledge, A always tells B only one path. Or you
> want to say the communication between A and B support MIF scenario?
If A has three IP addresses, it would presumably supply multiple
addresses in a referral.  There are many reasons why A might have
multiple IP addresses:

    * multiple network interfaces;
    * mobile IP;
    * A supports both IPv4 and IPv6;
    * there are multiple IPv6 prefixes advertised on one or more of the
      networks to which A attaches;
    * A is behind a NAT and thus have both an "external" and an
      "internal" address (and A won't know which will work better for
      its peers);
    * A has arranged to have "remote" IPv4 or IPv6 access via a tunnel
      to another network (which makes a very flexible transition strategy)

Note also that an _application_ can have multiple addresses because it
exists on multiple hosts.  So we don't necessarily want to think of
referrals as being tied to hosts, but rather, to applications or services.

Keith