U-turn Alternates for IP/LDP Fast-Reroute draft-atlas-ip-local-protect-uturn-01.txt Alia Atlas Gagan Choudhury Christian Martin Brent Imhoff Don Fedyk Raveendra Torvi

Slide 2 draft-atlas-ip-local-protect-loopfree-00.txt IETF-60 Routing Area WG U-Turn Alternates: Cooperatively Breaking the Loop S can locally determine to use N as a U-turn alternate if and only if: S is the primary neighbor of N for any shortest paths from N to D that go through S (N is a U-turn neighbor of S). N has signaled that it is capable of breaking U-turns on that interface (U-turn traffic received from S destined to D will go to N’s alternate and not back to S). N has a loop-free node-protecting alternate (R) to reach the destination (D). R S A N Time D E N breaks the loop and sends traffic to alternate interface Time 3

Slide 3 draft-atlas-ip-local-protect-loopfree-00.txt IETF-60 Routing Area WG Original Implicit Packet Identification Method Router signals type of U-turn packet identification supported (implicit or explicitly marked) Implicit Method Packet is U-turn Packet if received from primary neighbor No need to mark packets Allows breaking of upstream micro-forwarding loops; this is best-effort because alternate computed for local failure, not a remote failure.

Slide 4 draft-atlas-ip-local-protect-loopfree-00.txt IETF-60 Routing Area WG Added Explicitly Marked Packet Identification Method Packet is a U-turn Packet if Explicitly Marked (well-known label suggested) Identifies packet as a potential U-turn packet – doesn’t imply switching. Received from primary neighbor This is required to handle transition cases where network isn’t converged. Advantages Easier to implement – marking identifies candidate U-turn packets. Handles more broadcast link scenarios – can have neighbors in different areas (or just hosts) on same link. Properly forwards packets from a TE LSP with Penultimate Hop Popping. Easier to predict – Traffic gets the behavior requested. Disadvantages Requires an explicit marking – must be below Layer 3, b/c no bits available in IP header Doesn’t mitigate micro-forwarding loops

Slide 5 draft-atlas-ip-local-protect-loopfree-00.txt IETF-60 Routing Area WG Other Changes Added example algorithm to look for U-turn alternates Worst-case for node or link protection is 1 additional SPF per neighbor. Connection to alternate selection clarified. Section on SRLGs More details needed Clarified local hold-down considerations Cleaned up text & terminology

Slide 6 draft-atlas-ip-local-protect-loopfree-00.txt IETF-60 Routing Area WG What’s Next? Simplification of Alternate Selection Same behavior regardless of number of Potential Primary Neighbors More Detailed Explanation Considering Link Protection Focus on broadcast links Link to use to access the U-turn neighbor or loop-free neighbor Other suggestions? What is working group interest? Please send comments to the list.