Principles of Communications Week 2, to Oct 17th, L3

This week we started on more fundamentals of routing, which can be

centralised - we gave an example, using SDN controllers to add FIBs into a routing domain - in "fibbing" this is done in a hybrid distributed/centralised way, so that the default is that a distributed routing algorithm (typically link state routing) runs, but is then enhanced by central control. At one extreme, everything could be done through central control and dirct control of routers via OpenFlow, and robustness achieved by replicated the controllers.

decentralised - one decentralise approach is source routing, where routers are completely dumn with regards paths, but just forward packets based in fields in the packet. One example  of this is the use of loose source routes in iPv6 in Segment Routing.

distributed - classic Link State Routing is distributed, and does not distinguish any special node.  Of course, link metrics might be configured centrally to influence path selection (see traffic enginering)

federated. - the internet is a network of networks, where each autonomous system runs its own routing protocol (any of the paradigms here work), but to connect networks together and find paths,  BGP is a classic example of fedeated routing. 

We'll start on BGP details next week.

MPLS is not a routing protocol, but rather addresses on the forwarding problem, and interacts with other protocols to install labels - this can be done via label distribution protocols (LDP) or signaling protocols like RSVP (which amount to source routing in some senses). In Segment Routing (SR), which is an IP layer analog of MPLS, we can also choose to distibute segment labels via additional link state update fields, or we can use lists of intermediate router addresses in IPv6 to achieve source routing. 

Note MPLS operates "below" IP in the protocol stack, and hence can support multiple network layers for forwarding (hence its name), while SR operates "within" the IP layer.

The main way we illustrate that neither MPLS nor SR are routnig protocols themselves, is that they ave ad hoc methods to deal with switch/router outages and recover (or a priori protection).