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Re: 16-bit ASN kludge

  • From: Iljitsch van Beijnum
  • Date: Sat Dec 04 06:20:28 2004

On 4-dec-04, at 9:47, Owen DeLong wrote:

I think the general idea of dividing ASNs into LEAF and TRANSIT categories
is a good one. A method of determining which ASs need to know about
a given LEAF AS is needed, and, I think a lot of optimizations may well
be possible.
So now people have to renumber their AS when they start selling transit? Not such a great idea...

Now watch out, since you completely unnecessirily quoted Eddy's message in full, I'm going to reply to that here rather than use a separate message for this.

OD> I think all the meaningful parties have already pretty much agreed on
OD> 32bit ASNs in BGP4. I think that will be coded in the routers well
before OD> any attribute-based thing for 32bit ASNs is. As such, I don't
see much OD> point to kludging this instead of just going for it assuming
a 32bit world.

Then belay my 16-bit ramblings. I'm probably a bit naive in thinking a
new attribute would be passed along by enough transits to be useful; an
"adopt this incompatible protocol or become an island" approach may well
be needed.
This is not what the 32 bit AS draft proposes. (From memory, so I might get some of the small details wrong.) The idea is that the new 32 bit AS path is a new transitive attribute, which should be carried by existing BGP implementations. However, the 16 bit AS path is still there as well, with all the 16 bit incompatible ASes replaced by a "special" AS.

So all of this should work with existing implementations except that they don't see the full picture so AS path filtering on 32 bit ASes won't work. Basic operation shouldn't be a problem, though.

Note that I suggested starting to give out 32 bit AS numbers to new 32 bit compatible leaf sites while giving out 16 bit AS numbers to transit ASes as a way to ease in to all of this with the least amount of operational trouble. But at some point we'll run out of 16 bit AS numbers and 32 bit leaf networks will become transit networks, so people should upgrade at some point or live with the reduced filtering capabilities. And new ASes can't get around 32 bit support if their AS number isn't 16 bit safe, of course.

I still have to wonder if some leaf optimizations are possible. Perhaps
an incompatible protocol would leave more implementation wiggle room.
What would you like to optimize for?