Why doesn't BGP... -Reply

Wed Nov 20 17:14:33 UTC 1996

In message <199611171205.MAA02263 at diamond.xara.net>, "Alex.Bligh" writes:
> 
> > >SS7 (Signalling System 7) is a connectionless packet switched technology
> > >used to control the setup and teardown of circuit switched calls.
> > >Originally is was used as a database query technology to make 800
> > >numbers portable across carriers.  If this did not make sense I can descib
> e
> > >it in a little mnore detail offline.
> > 
> 
> Vadim makes my point better than I did - SS7 etc. make their routing
> decision OTO once per call (on call setup). IP makes it once per packet.
> Therefore you can put a lot more effort into finding the correct route
> if you only have to perform the calculation once. This was an algorthmic
> point not a network/transport layer point.
> 
> Alex Bligh
> Xara Networks

The route computation is made by the routing protocol and is only made
when a route changes.  Maybe you know of a router implementation
where the forwarding code checks the BGP and OSPF paths for every
packet, but I don't know of any that work that way.

If routers were stable, routers would make all of their routing
decisions within minutes of power up and never change those decisions.
Forwarding is then based on the table builtbased on those decisions.

Call setup happens every time a connection is setup.  Even as much as
routes flap in the Internet, the load on the routers (ones that
actually work, mythical) only affects mostly stable routes by slowing
their convergence when they make their very rare bounce.  That's why
IP routing scales as well as it does and switches using comparable
processors have already exceeded call setup rates in limited
environments.

IP routing has an scaling advantage over call setup.  Call setup can
take advantage of the lowest loaded path at the time of setup,
providing a better chance to load balance (ala PNNI).  The VC table
has a speed forwarding advantage over the IP radix tree.  The radix
tree may have a space advantage due to aggregation.  That in a
nutshell is the IP vs ATM flamewar (did I miss anything - ommisions
private mail unless it was major).  The interest in IP switching
(CSRs, tag switching, IP switching proposal de jour) comes from a
desire to combine the best of the two.

Curtis