"Selfish Routing"

Mike Lloyd drmike at routescience.com
Sat Feb 15 21:11:45 UTC 2003




Stephen Sprunk wrote:
> The problem is eliminating the possibility of a packet taking a "near
> optimal" path from A to B, and then taking another "near optimal" path from
> B back to A
> 
> I suspect this is impossible to fix while retaining hop-by-hop routing.

Looping does indeed present a problem.  If you want all nodes to play 
the "near optimality" game, you have to move very slowly - if I choose 
you, then we need a sensible way to guarantee that you won't choose me 
at the same instant.  A mechanism to do so will slow all decisions down, 
potentially beyond the point of usefulness.  However, I don't quite get 
from this problem to a need to abandon hop-by-hop routing.

Paul Vixie wrote:
 > i dunno, i don't think igrp would scale to the size of the internet.
 > wasn't there a 1/(n^2) relationship between metadata size and
 > network capacity as a function of total delay*bandwidth product
 > in the whole system?

Combining the scale problem and the looping problem suggests 
optimization would fit most sensibly in an environment where the number 
of choices under consideration is small, and some loop-free properties 
already exist.

Such conditions happen to occur where autonomous systems meet, and 
especially for a stub AS.  Careful inter-AS BGP optimization makes a 
good deal more practical sense than, say, hysterically 
self-reconfiguring OSPF.

I could observe that the edge of a BGP AS is also a place where money 
sometimes changes hands, but I wouldn't want to infect a nice academic 
discussion with anything too commercial :-)

Mike




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