This week, we've got two random[ref] examples
1. random telephone routing
c.f. triangles
can we greedily find the tandem? (only) if you want to check the reasoning behind that part of DAR!
2. random drop congestion
c.f.tcp arena
how many TCPs are there, really? again, only here for the keen background reader!
ref: there's a very nice study of general applicability of random choices in this harvard paper if you want some more background reading...
separately, in a discussion with a supervisee/supervisor, i realise some people may be interested in looking at real code to reinforce their understanding - for IB material, I strongly recommend Rich Stevens' TCP/IP Illustrated Volumes 1 and 2, but for this course, there's not really any such a nice single text - an alternative might be this:-
There's a neat network simulator called NS3 which has real code in it - it is used both for routing experiments and for transport - the package also has a nice visualisation system, so when you configure a simulation with a topology of routers and hosts and links, you can record the traces of events (packets) and later 'play it back' with a nice layout - see
for where to get it & documentation including examples...
Simulations are interesting as they often (in this case) include fragments of real code from real systems, so they are also test harnesses. however, more fundamentally, packet-based event driven simulators are implented with much the same design pattern as packet switching systems (i.e. routers) - effectively the simulator is an aggregsation of all the event schedules for all the entities in the system confituation being simulators- there are basically two types of events
- timers - both for when to send periodic route updates/keepalives, and for tcp/quic retransmit type actions
- packets arriving to go to the next stage (whether next layer or next hop...)
Associated with events is some state (e.g. forwarding information base searched by destination address in IP header, or label switched path indexed by packet label in MPLS shim header) - or in transport protocols, the
whole bunch of information about sequence numbers, windows and congestion, etc
Event driven code tends to look a bit different from classic application layer code
[that said, proxies/caches/servers can look similar).
From the router perspective, there's the event/timer driven distributed routing control protocol,
and separately theres the actual packet forwarding processes/threads ...the former (as i mentioned before) is often in the pattern of a unix daemon/service, whereas the latter is much lower level (and often has to interface to custom hardware for supportng fast lookup and even switch fabric interfaces) - there are open source routers other than just linux based (home broadband) ones - (e.g.
www.nongnu.org/quagga ) but i think this might be a lot to read with few extra lessons..
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