Principles of Communications Week 4 28-30/10/2025

 This week we wrap up BGP - looking at abstractions of the algorithm (The Stable Paths Problem in Interdomain Routing) and concrete realisations of problems in implementations.

While you may find the stable paths model helpful in removing noise from BGP complexity, I am not so sure its a great abstraction for thinking about how actually to resolve the problem(s) (non convergence etc). A nicer approach (by same lead person, Tim Griffin) is meta routing, which is very powerful and general, but would need an entire other course to discuss and I just put here for background in case anyone is interested !

Then we'll next make a start on Multicast Routing.

Principles of Communications Week 3 21-23/10/2025

 Interdomin routing-  BGP - key 4 slides - 124 126 131 132

Moving from intra-domain (within one autonomous system/routing domain/internet service provider) to intradomain, the key change is from policy within a domain (as used for steering traffic in centralised routing or in mpls or segment routing) to policy between multiple autonomous (i.e. independent) domains who may have conflicts and often require some level of information hiding (protecting knowledge of their customers' needs from competitors). So while connectivity is the minimum requirement, there's often no shared goal in terms of what is "optimal" (i.e. what routing metric to use) - we'll see that in default cases, for traffic engineering, and various tie breaking reasons, metrics implicitly creep in as an implici part of BGP routing, but not in any way consistently.

The last thing on BGP is going to follow from Traffic ENgineering and asks two questions: what really is the model BGP implements, to better understand how it works (and goes wrong)? and What engineering tweaks can we do to make it actually operate better in practice. We'll finish up on this on Oct 28th

Principles of Communications Week 2 14-16/10/2025

1 Centralised (!) Routing - Fibbing (see fibbing paper for more details - esp. figure 9/10 on failure/recovery modes) - in particular, fail-open and fail-close is used in the paper to refer to the persistence of a path made up by fibbing in the event of a controller failure where in some cases this is needed, and in others it needs to be removed! [1,2]

2 Stateful Routing - MPLS. Multi-protocol label switching has a long history (probably started in Cambridge!). It simplifes switch/router design in terms of forwarding, at the cost of increasing complexity in the control plane  -- possibly in routing, but more crucially, signaling. Signaling protocols have a very long history (from railways over 200 years ago). One interesting computer science dimension that arose from signaling is the concept of mutual exclusion. The first algorithms for avoiding contention for a limited resource are direct descendents of the P and V flags used to prevent two trains entering the same section of track....

In a sense, these two ideas (central and stateful) can be reconciled via "soft state" protocols (see last lecture).

Note also: MLPS involves a "shim" layer between IP and lower levels. Segment routing may use that, or may just use IP6 routing options directly. Recall layering from IB networking course. It is often not a pure picture - IP tunnels are another example of extra layers  between this and that. MPLS can also simplify switch & router port processing (and possibly, if switch is "cell switched" scheduling forwarding packets across the switch fabric - again, recall router architecure from IB networking course). Segment Routing is a re-think of MPLS, which can use IPv6 routing options as labels, and then use IP routing updates to distribute the label information to ( amongst others, upstream) neighbours. There's a nice slidepack SR explainer from CERN which shows the interaction with routing... Note segment routing with IPv6 dispenses with the potential hardware speedup of having 20 bit MLPS labels for forwarding, so one assumes router NICs and Processors may have ASIC support for v6 header processing!

Optional background reading...

2. Another dimension of signaling is that it requires a level of accesss control authentication and authorisation not typically present in pure datagram networks like traditional IP. For a measure of how bad it can get, look no further than the old digital telephone network signaling system number 7 (SS7) which is more complex than the whole TCP/IP regular data stack (see report on vulnerabilities in SS7). RSVP (serves similar function for signaling for MPLS if you don't just rely on routing!) is about as bad.

1. Further work was done based on the fibbing idea:

https://www.michaelschapira.com/_files/ugd/3b1e1e_dc51c8d43c7c41cc9ffbfa84923cd864.pdf

Basically, it leverages FIBbing-like mechanisms to optimize (oblivious) TE in IP networks.

https://www.michaelschapira.com/_files/ugd/3b1e1e_b928d21ed137475bb89f51c9073ed8de.pdf

This paper is about the hardness of configuring ECMP for TE. This is actually based on a very cute hardness amplification proof technique.

Principles of Communications Week 1 9/10/2025

 I'll be noting progress and also adding occasional related reading/ and corrections on this blog.

If you want to revise anything to warm up for the course, I suggest last year's Computer Networks course should be a quick re-read (e..g on routing )! A fun review of 40+ years of the Internet

This week we'll just make a start on routing.

For fun, you might find this discussion on why LLMs aren't much use for networking, mostly interesting - video recording of panel session

Reference requested for Glossary of Terms: from ISOC

Some acronyms come from the 7 layer model of the communications stack including terms like PHY (short for physical, so not really an acronym).