Re: DNS based Destination Selection

[Jay Ford <jay-ford@uiowa.edu>]

On Thu, 19 Jun 2003, David Gethings wrote:
> >    o  DNS changes are difficult->impossible to get deployed globally
> >    o  for some cases, relying on remotely-maintained DNS information is not
> >       sufficient or desirable
> I agree that they are not easy, but this method is used for MX records.
> I agree that by adding the priority value to the AAAA & A6 records
> alters the purpose. But I do not see this as being a major sticking
> point.

I think changing AAAA and/or A6 RRs at this point would be tough, but it
might be possible.

The second of my two above points was that the remote end (which owns the
DNS) would be specifying the preferences of the sender of a packet.  I'd
rather decide the preferences at my end for traffic I send.  Most net
interactions are bidirectional, so this is admittedly not a single-ended
problem, & the issue of who (sender or receiver) gets to decide path
selection for a packet borders on religious, so getting consensus on this
won't be easy.  (By the way, we have the same issues with IPv4, but based on
AS-path info... instead of addresses.)

> >    o  you cover dstaddr selection; what about srcaddr selection?
> One thought I have just had (so I have no idea how good or bad this idea
> is) is if a source is also multihomed it can perform PMTU discovery for
> each of the IPv6 addresses it has in that scope. The source address to
> use is determined by which ever returns the best PMTU and RTT.
>
> This is a slight waste of bandwidth. But if the "best" possible path is
> your ultimate goal there have to be trade offs.

The definition of "best" is tricky.  It isn't always just bandwidth & RTT.

________________________________________________________________________
Jay Ford, Network Engineering Group, Information Technology Services
University of Iowa, Iowa City, IA 52242
email: jay-ford@uiowa.edu, phone: 319-335-5555, fax: 319-335-2951