Principles of Communications Week 7 18-20/11/2025

 This week we'll cover

• qjump[qj] - data center networking - a big special corner case of traffic scheduling.

note qj puts the token bucket pacer in a hypversions so it is a regulator and a policer in one. it could also be offloaded to a smart NIC If you had one, or just put in the guest OS if you trust it.

• optimisation - routes for traffic, and traffic for routes

The start of traffic engineering by steering traffic by weights - note the hill descent is a classic old school AI technique.  And the origin of why tcp does AIMD is also a type of optimisation, of joint source and network utility!

Optimisation is a very large topic in itself,  and underpins many of the ideas in machine learning when it comes to training - ideas like stochastic gradient descent (SGD) are seen in how assign traffic flows to routes, here. In contrast, the decentralised, implicit optimisation that a collection of TCP or "TCP friendly" flows use is more akin to federated learning, which is another whole topic in itself.

Why a log function? maybe see bernouilli on risk

Why proportional fairness? from social choice theory!

Are people prepared to pay more for more bandwidth? One famous Index Experiment says yes.

0. See Computer Systems Modeling for why the delay grows quickly as load approaches capacity.

1. see IB Distributed Systems for clock synch

2. see prev year's Cloud Computing (II) module for a bit more about data centers&platforms.

[qj] The qj paper gives lots more details of setup, for anyone interested (the figures in the paper are clickable and take you to software and switch/network configurations and data).