Principles of Communications Week 5 to Nov 3rd/L9

This week were covering Mobile Networking(*rant*) (briefly) and then telephony networks(^motives for innovation!) - architecture, but in particular, random routing as it is a neat illustration of the use of non-deterministic approaches to distributed systems which yields efficient results See also Valiant Load Balancing (for example this work at Microsoft, also used in Operating Systems (NGINX, for example. 

 For a general discussion of random choices and their application, see this study (very much deep background reading only!) 

 *rant* The TCP checksum is designed to protect end-to-end packets against corruption - it is fairly low cost to. compute (ones-compliment sum of all the 16 bit fields in the TCP header and application data) compared to more complex integrity checks used in lower level protocols (link level checks are usually designed to be implemented in hardware). 

However, it features one incredibly poor additional design choice, which is that the check also includes adding in fields from the "pseudo header" which includes parts of the IP header (source & destination addresses, protocol field and IP segment length). (see for details) This has two problems: firstly it messes with the fate sharing design of the Internet so that end system comms is now entangled with network layer. Secondly, it stops a TCP connection surviving IP address re-assignment, which is essentially what happens if you move network (e.g. switch from using your wifi to using your cellular data link). 

The argument made by the TCP designers was that this prevents some security threat from someone messing with your IP address. It is not clear that that threat actually really represents a real problem since nowadays, TCP implements multipath with multiple IP addresses and switches or combines them safely. Also other protocols (e.g. TLS, above TCP, or else QUIC, as a replacement for TCP+TLS) make use of a secure association between end systems, which is far more secure and resilient to mobility anyhow. So another mistake in the Internet design.

^Motivation for Innovation! The first automatic exchange was designed by an undertaker, Almon Strwoger who believed human operators were diverting business calls to his competitors - real story is a bit less interesting, but still important - precursor to the cross-bar switch!

Started on Flow Control - for closed loop/feedback control systems, this paper by James CLerk Maxwell, back in 1868 covered some nice maths behind how to desgin controllers!