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