Outbound Route Optimization

Mon Jan 26 10:35:38 UTC 2004

>BGP is relatively good at determining the best path when you a major
>carrier with connectivity to "everyone" (i.e. when traffic flows
>"naturally"), in many locations, and you engineer your network so that 
you
>have sufficient capacity to support the traffic flows.

In other words, BGP really only works well when most networks
are overbuilt so that there is a single uncongested best
path through each network from every ingress to every egress
and the paths within any given network's core are roughly
similar in capacity.

Nowadays there is a lot more variability both within networks
and between different networks. How can a simple protocol
provide optimal behavior between an MPLS network, an IP over
ATM network, a network that is half GRE tunnels, and a network
that has core links ranging from DS3 to OC48? I think BGP is 
another example where something that is "good enough" has risen
to prominence in spite of the fact that it is not optimal.

And another thing. How do we know this problem can ever be
solved when we continue to use routing protocols which choose
the *BEST* path. The best path is always a single path and,
by definition, this is a single point of failure. How can we
ever have a diverse and reliable network when its core routing
paradigm is a single point of failure?

Note that people have built IP networks that provide two
diverse paths at all times using multicast
http://www.simc-inc.org/archive9899/Jun01-1999/bach2/Default.htm
and such things may also be possible with MPLS. But are any of
the researchers seriously looking at how to provide a network
in which all packets flow through two diverse paths to provide
better reliability?

--Michael Dillon