Principles of Communications Week 4 L7@LT2 11am, 28th Oct 2021 - Multicast&Mobile...

 Seen problems for scaling caused by Multihoming. Now lets look at Multicast, and briefly, at Mobile Routing.

We've seen a very simple form of multicast already - flooding, as used in link-state advertisements for intra-domain routing is a way to deliver updates to all routers (but not everyone else) - i.e. selectively get a packet (or a stream or flow of packets) from one of more sources to more than just one destination, without the source having to repeatedly transmit separate times for each recipient...

An interesting question area for supervisions is: what possible security problems could arise because of IP multicast, if widely available to end users?

The best analysis I know of MOSPF that NASA used is by John Moy That was very successful in practice due to a well managed network, and very good optimisations based on real world usage.

For a view of what you do "above" IP multicast to acieve reliable delivery, one good example is Pragmatic General Multicast, which was deployed in share trading networks. Why not TCP is an interesting excercise!.

Principles of Communications Week 4 L6@LT2 11am, 26th Oct 2021 BGP stable paths - theory & practice

convergence or not (no stable state reached - deadlocked or oscillatory)  - the fact that we end up with seemingly arbitrary ordering of AS path choices really is another way of saying that we don't have a metric (e.g. path vectors are not distance vectors, unless everyone was just using shortest AS path and everything was in one single customer-provider hierarchy.

pace of convergence (and why should we need rate limits or damping?) - again, the world has operational rules which are not tidy (in this case, over time, rather than in space)...

it would certainly be nice if the clustering of network nodes/edges could be reflected in the routing and addressing, so that hierarchy and summarisation (at the IP prefix and at the AS level) would result in much smaller tables and also in less traffic (and mostly, much less dynamic traffic). But life is just not that simple in the policy world.

the damping mechanism in BGP is a simple form of a controller - for a very old article about this idea see james clerk maxwell on governors

as always xkcd has some relevant remarks

Principles of Communications Week 3 L5@LT2 11am, 21 Oct 2021 - BGP TE&Stable Paths...

 For L4, go check through slides 106,113,123,124,126, in the interdomain routing section, then in this lecture, I cover traffic engineering tricks with BGP and start on the stable paths problem (what does BGP really "mean"?). L5 covers use of BGP attributes for maintaining customer provider and peering routes, then looks at tricks for traffic engineering, and finally, starts to take a look at an abstraction for thinking about what BGP is trying to do and some interesting ways it may go wrong. 

Crucially, BGP is one of the first systems that lets people program at planetary scale - nowadays, many cloud services do this, but inter-domain routing was one of the first, and suffers as a result. 

Global (as in plant wide, or wider) means you have failures due to the real world (power,  earthquakes, floods, solar storms, etc) and thes aren't always correlated with your topology in a neat way. You also have latency - the speed of light is quite quick, but actually, so are computers, so the amount of computation you can get done while waiting for a message/update from a device only a few hundred kilometers away, is significant.

p.s. this lecture recorded ok, I believe:-)

Principles of Communications Week 3 L4@LT2 11am, 19-21 Oct 2021 - BGP

 apologies, failed to record tuesday lecture (have recordings from last year if slides/notes/chapter not clear enough.

Principles of Communications Week 2, L2, 12.10.2021

 Recommend revisiting 1B networking lecture on Link State Routing and Longest Prefix Matching - see pp 25 onwards, and pp 141 onwards - relates both to fibbing (IGP is typically link state) and to MPLS (trying to avoid longest prefix matching by using fixed length labels instead).

Note: MPLS requries a setup, either management (controller/SDN like) or implicit from routing, or a signaling protocol (e.g. RSVP - see later lectures) - clearly, only management or signaling could add the details for FEC->PHB mapping for performance. willentail other algorithms (e.g. some sort of account of topology & capacity of links) to compute whether performance can be met (see later) - similar for protection, which might need multiple runs (e.g. k-shortest paths) of dijkstra...

Principles of Communications Week 1, L1, 7.10.2021

 Course outline, and refresher on routing. video up 

https://www.cl.cam.ac.uk/teaching/2122/PrincComm/video/

also

 links to slides/notes/explanatons/back stories.