Loop-Free Forwarding during Routing Transitions via Cost-Carrying Packets

We present the design and evaluation of an IP fast restoration system in which routers can restore forwarding connectivity without forming temporary loops (micro-loops) when a failure is detected and routes are still in transition. Our design is based on cost-carrying packets, in which routers stamp a short label in a packet to indicate the remaining path cost to reach a destination. Downstream routers use path costs carried by packets and their locally computed path costs to detect inconsistent forwarding paths and repair the inconsistencies using pre-computed paths. Our evaluation shows that this design can shorten service disruption times to be commensurate with the failure detection time for single topology update event and suppress micro-loops in multiple concurrent topology update events, outperforming other known solutions actively being discussed at IETF. Our design is suitable for implementation in high-speed routers: it requires at most two table lookups at packet forwarding times, increases the packet header length by a small constant, and does not alter routing protocols or require additional control messages.