gigabit router (was Re: Getting a "portable" /19 or /20)
Kavi, Prabhu
prabhu_kavi at tenornetworks.com
Wed Apr 11 21:16:15 UTC 2001
Vendors have known how to solve this problem for many years.
Failure to do so is a poor implementation and has nothing to do
with centralized forwarding being better/worse than distributed
forwarding.
Prabhu
----------------------------------------------------------------------
Prabhu Kavi Phone: 1-978-264-4900 x125
Director, Adv. Prod. Planning Fax: 1-978-264-0671
Tenor Networks Email: prabhu_kavi at tenornetworks.com
100 Nagog Park WWW: www.tenornetworks.com
Acton, MA 01720
> -----Original Message-----
> From: Matt Zimmerman [mailto:mdz at csh.rit.edu]
> Sent: Wednesday, April 11, 2001 4:47 PM
> To: nanog at merit.edu
> Subject: Re: gigabit router (was Re: Getting a "portable" /19 or /20)
>
>
>
> 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
>
>
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