U-Turn Alternates for IP/LDP Local Protection draft-atlas-ip-local-protect-uturn-00.txt Alia Atlas Gagan Choudhury Christian Martin Brent Imhoff Don Fedyk Raveendra Torvi

Slide 2 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Outline of Talk Overview of Solution Control-Plane Modifications Data-Plane Modifications What Needs to Be Standardized Repair Coverage Complexity Analysis Comparison with Other Methods

Slide 3 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Loop-Free Alternates: Limited Coverage The coverage in a network provided by loop-free alternates is limited. U-turn alternates expand the coverage on real networks. Analysis on networks shows improvement on average from 79.5% to 98.4% coverage of source-destination pairs. Sufficient to become a network engineering problem and not a technology problem (with its associated technical complexity). R5 R2 R6 R1 Time R4 R No loop-free alternate path from R2 to R4

Slide 4 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG U-Turn Alternates: Cooperatively Breaking the Loop R2 can locally determine to use R1 as a U-turn alternate if and only if: R2 is the primary neighbor of R1 for any shortest paths from R1 to R4 that go through R2 (R1 is a U-turn neighbor of R2). R1 has signaled that it is capable of breaking U-turns on that interface (traffic received from R2 destined to R4 will go to R1’s alternate and not back to R2). R1 has a loop-free node-protecting alternate (R5) to reach the destination (R4). R5 R2 R6 R1 Time R4 R R1 breaks the loop and sends traffic to alternate port Time 3

Slide 5 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Control-Plane : Node-Local Computation A router S must compute: For each destination D (via an enhanced SPF) If a neighbor N has indicated that it can break U- turns for traffic coming in an interface, Does that neighbor N have a loop-free node- protecting alternate to reach the destination D? Does that alternate path also avoid the router S’s primary neighbor P? If a loop-free node-protecting alternate is available, select it for use. If not, pick among loop-free link-protecting alternates and u-turn alternates as desired (router-local decision).

Slide 6 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Control-Plane: Routing Protocol Changes A router must know If a neighbor can redirect U-turning traffic on a particular interface Interface-wide capability - not tied to particular traffic prefixes And the policy configuration that neighbor has for using its interfaces as alternates. Assumes operator has administrative control to disallow using an interface as an alternate. Signal this information via a new Link Capabilities sub-TLV in IGP: 1 bit : U-turn capable recipient 1 bit : Eligible Alternate No additional signaling required based on topology changes (i.e. at time of failure or after).

Slide 7 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Data-Plane Mechanisms: Rerouting Forwarding Outcome is based on incoming interface, which is similar to VRFs, RPF checks, ACLs, policy-based forwarding, etc. DestinationIncoming Interface Out Interface R4R4 L1L2 L3 L2

Slide 8 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Data-Plane Modifications: Encapsulation Traffic redirected to U-turn alternates does not require any type of encapsulation.

Slide 9 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG What Needs to Be Standardized U-Turn Alternates Require A Link Capabilities sub-TLV with 2 bits used. These Signaling Protocol extensions would be for ISIS and OSPF. A common selection method for deciding To use a loop-free node-protecting alternate, if any is available How to break ties among those

Slide 10 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Repair Coverage U-Turn Alternates improve coverage on real networks. Improvement is topology-dependent. Minor changes to network can lead to further improved coverage. Analysis based on source/destination pairs, not % of traffic covered or % of link or node failures fully covered. I’d be happy to analyze any network with the automated tool.

Slide 11 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Complexity Analysis: Control Plane Computational Complexity is O(neighbors) Neighbors which aren’t available for use as alternates don’t count. (They decrease the complexity to O(alternate-capable neighbors)) Feel free to discuss the details after meeting…

Slide 12 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Complexity Analysis: Data Plane For interface-specific FIBs, no changes required - only different information in FIB. For non-interface-specific FIBS, need to look at the results of forwarding decision and decide based on the primary out-going interface and incoming interface whether to send traffic to primary or alternate. Requires additional comparison for determination Has potential requirement to read a second forwarding result. No more look-up complexity than uRPF, VRFs, Policy-based forwarding, etc.

Slide 13 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Comparison with Other Methods Commonalities Assumes a common framework of alternate pre-computation and traffic redirection on failure. Assumes a base of loop-free alternates. Provides a mechanism to break the loop needed to go through an upstream node that can provide an alternate path. Looking for operationally simpler method than TE Fast-Reroute Computation of alternates may be similar.

Slide 14 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Comparison with Other Methods: Differences U-turn Alternates: Computational complexity is less ( O(neighbors) ) No encapsulation is needed. No set-up, monitoring or maintenance of explicit tunnels is required. Particularly important because tunnels may need to change when the topology does. This can leave protection gaps while the new tunnels are created. No added complexity or support to learn mechanism for directed forwarding. No new adjacencies (such as for LDP) need to be considered. I.e. a targeted LDP session isn’t necessary to learn the labels understood by a neighbor’s neighbor. Simply works for LDP. Lowest impact on security Goal is to simplify operations and provide local protection – not to make so complex that RSVP-TE Fast-Reroute is preferable

Slide 15 draft-atlas-ip-local-protect-uturn-00.txt IETF-60 Routing Area WG Conclusion U-Turn Alternates offer improved coverage with Similar computational complexity to loop-free alternates No new encapsulation or explicit tunnels Simple notification of capability is the only signaling extension Simple to manage and deploy Orthogonal to MPLS Comments?