1 / 24

Jennifer Rexford Fall 2010 (TTh 1:30-2:50 in COS 302) COS 561: Advanced Computer Networks

Routing Convergence. Jennifer Rexford Fall 2010 (TTh 1:30-2:50 in COS 302) COS 561: Advanced Computer Networks http://www.cs.princeton.edu/courses/archive/fall10/cos561/. Intradomain Routing. Convergence. Getting consistent routing information to all nodes

feleti
Télécharger la présentation

Jennifer Rexford Fall 2010 (TTh 1:30-2:50 in COS 302) COS 561: Advanced Computer Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Routing Convergence Jennifer Rexford Fall 2010 (TTh 1:30-2:50 in COS 302) COS 561: Advanced Computer Networks http://www.cs.princeton.edu/courses/archive/fall10/cos561/

  2. Intradomain Routing

  3. Convergence • Getting consistent routing information to all nodes • E.g., all nodes having the same link-state database • Consistent forwarding after convergence • All nodes have the same link-state database • All nodes forward packets on shortest paths • The next router on the path forwards to the next hop 2 1 3 1 4 2 1 5 4 3

  4. Transient Disruptions • Detection delay • A node does not detect a failed link immediately • … and forwards data packets into a “blackhole” • Depends on timeout for detecting lost hellos • Or link media that can detect “loss of light” 2 1 3 1 4 2 1 5 4 3

  5. Transient Disruptions • Inconsistent link-state database • Some routers know about failure before others • The shortest paths are no longer consistent • Can cause transient forwarding loops 2 2 1 1 3 3 1 1 4 4 2 2 1 1 5 4 4 3 3

  6. Convergence Delay • Sources of convergence delay • Detection latency • Flooding of link-state information • Shortest-path computation • Creating the forwarding table • Performance during convergence period • Lost packets due to blackholes and TTL expiry • Looping packets consuming resources • Out-of-order packets reaching the destination • Very bad for VoIP, online gaming, and video

  7. Reducing Convergence Delay • Faster detection • Smaller hello timers • Link-layer technologies that can detect failures • Faster flooding • Flooding immediately • Sending link-state packets with high-priority • Faster computation • Faster processors on the routers • Incremental Dijkstra’s algorithm • Faster forwarding-table update • Data structures supporting incremental updates

  8. Reducing Convergence Delay • Weight tuning for planned maintenance • Gradually increase link weight • Before taking down the link • MPLS fast-reroute • Backup paths to use when the primary path fails • Local protection to circumvent a failed link 2 1 3 1 4 2 1 5 4 3

  9. Interdomain Routing

  10. Causes of BGP Routing Changes • Topology changes • Equipment going up or down • Deployment of new routers or sessions • BGP session failures • Due to equipment failures, maintenance, etc. • Or, due to congestion on the physical path • Changes in routing policy • Changes in preferences in the routes • Changes in whether the route is exported • Persistent protocol oscillation • Conflicts between policies in different ASes

  11. BGP Session Failure • BGP runs over TCP • BGP only sends updates when changes occur • TCP doesn’t detect lost connectivity on its own • Detecting a failure • Keep-alive: 60 seconds • Hold timer: 180 seconds • Reacting to a failure • Discard all routes learned from the neighbor • Send new updates for any routes that change AS1 AS2

  12. Routing Change: Before and After 0 0 (2,0) (2,0) (1,0) (1,2,0) 1 1 2 2 (3,2,0) (3,1,0) 3 3

  13. Routing Change: Path Exploration • AS 1 • Delete the route (1,0) • Switch to next route (1,2,0) • Send route (1,2,0) to AS 3 • AS 3 • Sees (1,2,0) replace (1,0) • Compares to route (2,0) • Switches to using AS 2 0 (2,0) (1,2,0) 1 2 (3,2,0) 3

  14. 0 Routing Change: Path Exploration (2,0) (2,1,0) (2,3,0) (2,1,3,0) (1,0) (1,2,0) (1,3,0) • Initial situation • Destination 0 is alive • All ASes use direct path • When destination dies • All ASes lose direct path • All switch to longer paths • Eventually withdrawn • E.g., AS 2 • (2,0)  (2,1,0) • (2,1,0)  (2,3,0) • (2,3,0)  (2,1,3,0) • (2,1,3,0)  null 1 2 3 (3,0) (3,1,0) (3,2,0)

  15. BGP Converges Slowly • Path vector avoids count-to-infinity • But, ASes still must explore many alternate paths • … to find the highest-ranked path that is still available • Fortunately, in practice • Most popular destinations have very stable BGP routes • And most instability lies in a few unpopular destinations • Still, lower BGP convergence delay is a goal • Can be tens of seconds to a few minutes • High for important interactive applications • … or even conventional application, like Web browsing

  16. Beyond Faster Convergence

  17. Research Ideas • Modeling of routing convergence • Bounds for BGP as function of topology and policy • Impact on timer configurations on convergence • Much smaller timers for faster convergence • Understand trade-off between convergence time and protocol overhead • Distributed coordination of routing changes • Avoid loops and blackholes during convergence • Failure carrying packets • Faster detection by piggybacking on data packets

  18. Research Ideas • Temporary backup routes in BGP • Have an alternate path through another AS • Multipath routing to survive failures • Multiple paths, possibly computed in advance • Load balancing over the currently-working paths • Error-correction codes on multiple paths • Spreading redundant traffic over multiple paths • Reconstructing the traffic at the receiver

  19. BGP Instability

  20. 5 2 1 0 5 0 3 4 2 1 Stable Paths Problem (SPP) Instance • Node • BGP-speaking router • Node 0 is destination • Edge • BGP adjacency • Permitted paths • Set of routes to 0 at each node • Ranking of the paths 2 1 0 2 0 2 4 2 0 4 3 0 3 0 1 3 0 1 0 1 most preferred … least preferred

  21. 5 0 3 4 2 1 A Solution to a Stable Paths Problem 2 • Solution • Path assignment per node • Can be the “null” path • If node u has path uwP • {u,w} is an edge in the graph • Node w is assigned path wP • Each node is assigned • The highest ranked path consistent with assignment of its neighbors 2 1 0 2 0 5 2 1 0 4 2 0 4 3 0 3 0 1 3 0 1 0 1 A solution need not represent a shortest path tree, or a spanning tree.

  22. 1 1 1 0 0 0 2 2 2 An SPP May Have Multiple Solutions 1 2 0 1 0 1 2 0 1 0 1 2 0 1 0 2 1 0 2 0 2 1 0 2 0 2 1 0 2 0 First solution Second solution

  23. 1 An SPP May Have No Solution 2 1 0 2 0 2 4 0 3 2 0 3 0 1 3 0 1 0 3 3

  24. Avoiding BGP Instability • Detecting conflicting policies • Computationally expensive • Requires too much cooperation • Detecting oscillations • Observing the repetitive BGP routing messages • Restricted routing policies and topologies • Policies based on business relationships • Prefer paths through customers • Don’t provide transit service to peers and providers • No cycles of provider-customer relationship • Getting rid of BGP 

More Related