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