In message <199607231657.LAA14164@uh.msc.edu>, Tim Salo writes:
On a more theoretical note, switches, being circuit-switched, make the complicated decisions when the connection is established, (or configured for PVCs), and need to make relatively simple decisions to switch each cell. This probably scales very well is terms of speed, although at some point might have some difficulty in scaling to a very large number of simultaneous connections.
Routers, on the other hand, have to make a bit more complicated decisions per packet. This has some limitations in terms of speed and number of simultaneous "connections."
-tjs
Tim et al, Delete now if you have anything better to do. :-) Just addressing you theoretical note here for the moment. Routers scale well to "number of connections if there are a large number of sources and destinations per routing prefix (ie: good aggregation). Routers scale O(logN) wrt the number of prefixes used for forwarding if a radix tree is used or required O(N) storage for hashed lookup methods (failure of the host based cache - proven avout 2 years ago - dead horse dept). Switches scale O(N) with the number of connections established and torn down. Routers are not affected by setup/tear-down. So the (theoretic) question is whether the O(logN) swamps the forwarding lookup in the router model before the O(N) connection setup overhead kills the switch. Back to real world considerations. Which scales better depends on things like average packet size for the router limits and average connection duration for the switch limits. Good ol' HTTP is a nightmare for either one. As a result, hybrid approaches start to look attractive, using routers on the periphery and building fat pipes through the switches. What you end up with there is all sorts of traffic on the same fat pipe (or forget about setup scaling) and so out the window goes "the advantages of ATM QoS". The state of the art of ISP needs (where the barage of tinygrams and very short flows is felt full force) is PVC pipes between routers to offload the routers a little bit. In practice the advantage is very little (but enough to have MCI doing it). For all the investment made in custom silicon for ATM it may turn out that a general purpose processor and a good router design (such as the DEC Alpha used in the BBN router) will take us to OC12. Hope its been amusing. I'd say the jury is still out on this one. :-) Curtis