Chapter 5 Introducing SDN Control in MPLS Networks - High ...

205In Fig. 5.14 we show two tunnels that were configured by the network-operator,routed by the CSPF module, and established in the data-path using OpenFlow, to enterflow-entries in the OVS flow-tables. The tunnels originate in SFO (head-end) andterminate in Houston and NYC (tail-ends).The panels show information for each tunnel. It is worth noting here that Fig. 5.14shows the tunnels from a conceptual standpoint (similar to Fig. 5.7a). But the tunnels areactually routed over the IP links. For example, the SFONYC tunnel is routed along theSFODENKANNYC links, reserving 123 Mbps of bandwidth on each link in theeastward direction. Similarly, the SFOHOU tunnel is routed viaSFODENKANHOU links reserving 700Mbps of bandwidth along the path. Butanother way to think about this is that this is also the annotated-topology-view presentedto the packet-flow routing applications in Fig. 5.9 or Fig. 5.12. The tunnels are, from apacket-flow routing standpoint, uni-directional links which form the shortest (1-hop) pathbetween routers. And so the routing-modules perform Auto-Route, as described in theprevious section, by re-routing packet-flows into the tunnels.For example, flows from SFONYC which previously traveled over the IP linksnow get re-routed (or Auto-Routed) via the 1-hop tunnel. It so happens that the tunnel isalso routed along those same links. But this does not matter – the packets get imposedwith labels in SFO and get label-switched in the DEN and KAN routers, which no longerhave visibility into the IP packet. The flow-entries in DEN and KAN which previouslymatched on the IP-information in the packet-header, subsequently idle timeout, as they nolonger match on the labeled-packets. We also use penultimate hop popping (PHP) of thelabel at the KAN LSR so that the NYC LSR receives the unlabeled IP packet; and so itonly has to do a single lookup to forward the packet.Another aspect of our design worth pointing out is that for the packet-routingapplications to treat the tunnels as unidirectional links, they need to have the concept of avirtual-port. For example, the SFONYC tunnel is represented as a unidirectional linkconnecting a virtual-port of the SFO router to a virtual-port on the NYC router. But the