[36612] in North American Network Operators' Group
Re: gigabit router (was Re: Getting a "portable" /19 or /20)
daemon@ATHENA.MIT.EDU (Matt Zimmerman)
Wed Apr 11 16:57:06 2001
Date: Wed, 11 Apr 2001 16:47:28 -0400
From: Matt Zimmerman <mdz@csh.rit.edu>
To: nanog@merit.edu
Message-ID: <20010411164727.B647@alcor.net>
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In-Reply-To: <NCBBLIEPOCNJOAEKBEAKAEKJOEAA.davids@webmaster.com>; from davids@webmaster.com on Wed, Apr 11, 2001 at 12:26:54AM -0700
Errors-To: owner-nanog-outgoing@merit.edu
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
