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

["Kavi, Prabhu" <prabhu_kavi () tenornetworks com>]

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 () tenornetworks com
100 Nagog Park                  WWW:    www.tenornetworks.com
Acton, MA 01720


> -----Original Message-----
> From: Matt Zimmerman [mailto:mdz () csh rit edu]
> Sent: Wednesday, April 11, 2001 4:47 PM
> To: nanog () 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