gigabit router (was Re: Getting a "portable" /19 or /20)

Wed Apr 11 20:47:28 UTC 2001

On Wed, Apr 11, 2001 at 12:26:54AM -0700, David Schwartz wrote:

> > > 	Why do you think central fowarding is superior to distributed
> > > 	forwarding?
> >
> > Because you will have consistency problem. You are nearly 100% guaranteed
> > to have them.
> >
> > Alex
> 
> 	Ahh, so that's what you're thinking.
> 
> 	If you have forwarding table F(X) at time X and forwarding table F(X+1)
> 	at time X+1, a packet that arrives between times X and X+2 can
> 	reasonably be forwarded by any of the tables. There is no special
> 	sequencing present or required between the packets that involve routing
> 	protocols and the data packets.

I think Alex was referring to internal consistency within the router (between
linecards), not external consistency.  For example, if linecard X believes that
a packet should be forwarded to linecard Y, but linecard Y's forwarding table
is older than X's, Y could misforward the packet, causing a forwarding loop or
a dropped packet.  Thus, it can be the case that neither the old path nor the
new path is taken.

Yes, there are ways to approach this problem, but it is a problem that
central-forwarding systems will not have.

> 	We misroute packets between routers because routing table updates don't
> 	happen fast enough. It's not a problem -- IP is designed to tolerate
> 	packet losses and has never guaranteed sequencing.

It is true that IP does not make guarantees about delivery, but packet loss has
a detrimental effect on performance nonetheless.

> 	The added occasional misroutes due to inconsistency will be
> 	proportional to the ratio of the average network transport time for a
> 	routing protocol packet to the average delay in propogating forwarding
> 	table changes to a linecard. You do the math.

I think a more useful model is this:

S(X) = (% of time that a router X spends in a consistent state) *
          (packets/sec through router X)

For the percentage of packets which will be successfully routed.  The total
end-to-end loss is 1 - S(X)^N for N identical routers.  N >= 20 is not uncommon
these days, and packets/sec gets higher all the time.

-- 
 - mdz