From: miw@cc.uq.oz.au (Mark I. Williams)
Subject: Re: Future of IP routers
Organization: Prentice Centre, University of Queensland
Date: Thu, 3 Sep 1992 01:13:21 GMT

jtw@lcs.mit.edu (John Wroclawski) writes:

>This distinction seems rather artificial. IP routing and ATM switching
>require exactly the same fast-path operations: classify packet/cell
>(IP, based on dest address; ATM based on VCI), look up next hop
>information, send packet/cell to correct output port. Store-and-
>forward vs passthrough doesn't affect this. Implementation of even a
>minimal queue management scheme requires adding decision-making power
>to the ATM switch; in a pure ATM scheme queueing decisions are likely
>made per-cell, which takes more processing power than making the
>equivalent decision in an IP router because ATM cells are smaller. 
>In both cases the actual route calculation is done out-of-band.

While the rest of your comments were correct, this isn't quite true. In
an ATM switch, routing decisions are made on a per-connection basis.
There is no lookup of next-hop information, since the VCI usually *is*
the route. If a particular output port dies, then so do all virtual
circuits that were carried over that line, and connections need to be
remade.

In an IP switch, the address has no connection with any output port
except in a lookup table, and the router has to look up next-hop
information for each datagram. If a particular output port dies, this is
merely reflected in a change to the routing table.

Not that this discussion is really relevant, anyway. IP routers and ATM
switches are as different in function as IP routers and ethernet
bridges. They just don't occupy the same level in whatever reference
model you happen to be using.

cheers,


>[From a different article]

>   Because routers are store and forward, I would have thought that time
>   critical traffic such as video/audio traffic would be very difficult
>   to pass across a router at high speed. Since the telcos are billing
>   ATM as ideal to carry multimedia traffic, this implies that the idea
>   of using ATM routing at the network level is more suitable for high
>   speed multi media traffic than IP.

>Several issues interact here. First, one has to compare the latency
>allowable in a system against the latency introduced by various
>components. On a 1GB link, a 1000 byte IP packet occupies roughly 8
>microseconds, meaning that an optimal store-and-forward introduces
>about 16 microseconds of delay over optimal pass-through. Queueing
>delay in the presence of loaded links is likely to be a more
>significant issue. Here ATM has an advantage because the minimum delay
>is smaller (since cells are shorter), but IP may have an advantage
>because the larger packet size gives you more time to run a management
>algorithm. The question of what sort of quality-of-service (queue
>management) to provide in a switch is open, but there seems to be
>general agreement that -something- is required.

>				-john

>John Wroclawski
>jtw@lcs.mit.edu
--
Mark Williams         The University of Queensland       miw@cc.uq.edu.au
+61 7 36 54012   (w)  Prentice Centre
+61 7 36 54477  (fax) Qld 4072  Australia
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