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Segment Routing

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WAN SDN meet Segment Routing
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Segment Routing

  1. 1. Segment Routing A Control Plane Simplification Cengiz Alaettinoglu
  2. 2. Motivation Forwarding traffic over non-shortest paths is necessary for: • Traffic engineering • Protection against failures (link/node/srlg) • BGP exit selection • SLA conforming service paths (e.g. low delay paths) • … Network operators have two choices: • RSVP-TE • Segment routing Copyright © 2017 Packet Design. All rights reserved. 2
  3. 3. Segment Routing Provides Uncompromised Functionality With Simplicity Segment routing simplifies the IP/MPLS control plane • No need to run LDP or RSVP-TE • No signaling overhead • Small overhead in IGP and BGP in comparison to RSVP-TE overhead in IGP Functionality is not compromised • Traffic engineering • Shortest and non-shortest paths • ECMP-aware TE • Protection against link/node/SRLG failures • Primary/secondary paths • SLA conforming service paths (e.g. L2/L3 VPNs) • Egress BGP exit selection (mainly for content providers/data centers) Copyright © 2017 Packet Design. All rights reserved. 3
  4. 4. Why Simplify? It is a challenge to operate LDP and RSVP-TE at large scale • N^2 RSVP-TE tunnels • N^2 LDP sessions over these tunnels • RSVP-TE signaling overhead on the routers and race conditions • Excessive IGP churn due to available link bandwidth changes • IGP/LDP synchronization issues As a result, many SPs are reluctant to run RSVP-TE Copyright © 2017 Packet Design. All rights reserved. 4
  5. 5. State of RSVP-TE in the Network More than 6% of the tunnels are down due to race conditions Traffic is forced to IGP paths or stuck on FRR paths • Creates congestion Copyright © 2017 Packet Design. All rights reserved. 5
  6. 6. Segment Routing (by SPRING WG) Each packet contains its path as a list of path segments • Similar to “source” routing • Any router along the path can insert segments • Typically ingress router or protecting router 6 A B Z DC V W YX 3 Segments • Segment to C • Segment to X • Segment to Z All link costs are 1 Copyright © 2017 Packet Design. All rights reserved.
  7. 7. IGP Distributes SIDs and SRGBs Z announces SID 9 to reach Z C announces SID 3 to reach C X announces SID 7 to reach X Each router announces its Segment Routing Global Block (SRGB) • MPLS label = SID + offset • Let’s assume that offset is 0 • MPLS label = SID All routers set up MPLS forwarding state for these labels • ECMP-aware shortest paths to each Z, C and X 7 A B Z DC V W YX 3 Segments • Go to C on shortest path • Go to X on shortest path • Go to Z on shortest path All link costs are 1 Copyright © 2017 Packet Design. All rights reserved.
  8. 8. A Wants Path A B C X Y Z Copyright © 2017 Packet Design. All rights reserved. 8 A B Z DC V W YX 3 Segments: 3 -> 7-> 9 • Go to C on shortest path (SID 3) • Go to X on shortest path (SID 7) • Go to Z on shortest path (SID 9) OSPF says SIDs for • C is 3 • X is 7 • Z is 9 7 9 Payload 9 Payload 9 Payload Payload 3 7 9 Payload MPLS Label Stack
  9. 9. Any Path Can Be Encoded Copyright © 2017 Packet Design. All rights reserved. 9 A B Z DC V W YX 1 Segment (shortest IGP path) • Go to Z on shortest path (node segment) A B Z DC V W YX 5 Segments • Go to B on shortest path • Go to W on shortest path • Go to Y on shortest path • Go to D on shortest path • Go to Z on shortest path A B Z DC V W YX 3 Segments • Go to C on shortest path • Go to X on link 3 (adjacency segment) • Go to Z on shortest path
  10. 10. How Does SPRING Eliminate LDP? LDP distributes MPLS labels for prefixes • Usually these are BGP next hops IGP distributes prefix-SID for BGP next hops (I am conveniently ignoring PWE signaling) 10Copyright © 2017 Packet Design. All rights reserved.
  11. 11. SDN TE Use Case Using on-device RSVP-TE for optimization is problematic • Not network-wide optimization • Race conditions lead to failed tunnels or stuck FRRs • See my Apricot talk yesterday SDN application centralizes bandwidth allocation • No need to signal intermediate routers these bandwidth reservations • No need to carry available bandwidth in IGP and interfere with IGP convergence • No need for a reservation protocol • No need for refreshes Still need to encode shortest and non-shortest paths • Segment routing can do this without any of this overhead • Segment routing IGP overhead is small and not changing with bw reservations; hence small Copyright © 2017 Packet Design. All rights reserved. 11
  12. 12. TI-LFA Topology-independent loop-free alternative Protects against link/node/SRLG failures Protects any IP/MPLS traffic • Segment routed or not Same protection path as post-convergence path 100% coverage For most networks, very few segments necessary • Usually just one more • Up to 3 in some networks Copyright © 2017 Packet Design. All rights reserved. 12
  13. 13. Operational Challenges Each router needs to have a large enough SRGB for the network • Network is growing Max SID depth (very large label stack) is a problem for old routers • For most networks, TE paths can be encoded by less than 5 labels • TI-LFA needs even less labels than this • Unfortunately, some old hardware can handle only 4 or 5 labels • They can swap but can not push these many labels • Recent hardware can handle more than 15 labels • Location of entropy label is also problematic with deep stack • Binding SID can remedy these problems Copyright © 2017 Packet Design. All rights reserved. 13

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