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New Router Manages Flows, Not Packets
An anonymous reader writes "A new router, designed by one of the creators of ARPANET, manages flows of packets instead of only managing individual packets. The router recognizes packets that are following the first and sends them along faster than if it had to route them as individuals. When overloaded, the router can make better choices of which packets to drop. 'Indeed, during most of my career as a network engineer, I never guessed that the queuing and discarding of packets in routers would create serious problems. More recently, though, as my Anagran colleagues and I scrutinized routers during peak workloads, we spotted two serious problems. First, routers discard packets somewhat randomly, causing some transmissions to stall. Second, the packets that are queued because of momentary overloads experience substantial and nonuniform delays, significantly reducing throughput (TCP throughput is inversely proportional to delay). These two effects hinder traffic for all applications, and some transmissions can take 10 times as long as others to complete.'"
All older cisco equipment worked this way. This was nice, and worked very well for the first router(s) closest to the end customer. However for routers meant to route for large numbers of users this turned out to be a disaster.
Just to give you an idea, this was EOS (end of support) before I turned 10 (look for "netflow routing")
There are a number of very problematic properties :
-> trivial to ddos (just generate too many flows to fit in memory, or generally increase the per-packet lookup time)
-> not p2p compatible (p2p will cause flow based routers to perform at a snail's pace, because they open so much connections)
-> possible triple penalty for every new flow (first a failed flow lookup, followed by a failed route lookup, going to default route)
-> very hard to have a good qos policy this way. A pipe has a fixed bandwidth, and you almost always oversubscribe. Therefore useful policies are very hard to formulate per-flow.
-> if you divide bandwidth per-flow over tcp then a large overload will "synchronize" everything. So let's explain what happens if 3 users are happily surfing about and another user starts bittorrent. Bandwidth gets divided over all the flows, and *every* connection closes, due to timeouts.
There are a number of advantages
-> easy, very extensive QOS is trivial to implement
-> stateful firewalling is almost laughably easy to implement, and very advanced firewalling can be done (e.g. easy to block ssh but not https, just filter on the string "openssh" anywhere in the connection. Added bonus : hilarity ensues if you email someone the text "openssh", and his pop3 connection keeps getting closed)
Here's the deal : a router has to lookup in a table of about 300.000 entries in per-packet switching (excepting MPLS P routers). My PC is, at this moment, opening 331 flows to various destinations, each sending an average of 5 packets (probably a lot of DNS requests are dragging this number down), but you have to keep in mind that a flow-based router has to look up first in the "flow table" AND in the route table (which still has 300.000 entries).
As soon as a flow-based router services more than 1000 machines (in either direction, ie. 100 clients communicating with 900 internet hosts = 1000 machines serviced), it's performance will fail to keep up with a packet-based router. That's not a lot. If a single client torrents or p2p's you will hit this limit easily, resulting in slower performance. 2000 machines and packet-based switching is double as efficient.
So : flow-based routing ... for your wireless access point ... perhaps. For anything more serious than that ? No way in hell.