Computer Networks Winter 2002 / 2003

Computer Networks Winter 2002 / 2003
Chapter 4 NETWORK LAYER Dynamic PowerPoint Slides Only Distributed Computing Group Computer Networks Winter 2002 / 2003

Remember: Count to Infinity Problem
a b c c: 3 c: 4 c: 5 c: 6 c: 7 c: 8 Distributed Computing Group Computer Networks R. Wattenhofer

BGP does not count to infinity
a b c d e Zurich Destination Dir Dst Path Zurich c 4 cdeZ Destination Dir Dst Path Zurich b 5 bcdeZ Distributed Computing Group Computer Networks R. Wattenhofer

BGP does not count to infinity
“withdraw Zurich” a b c d e Zurich Destination Dir Dst Path Zurich c 4 cdeZ Destination Dir Dst Path Zurich b 5 bcdeZ Distributed Computing Group Computer Networks R. Wattenhofer

Distributed Computing Group Computer Networks R. Wattenhofer
BGP Basics Continued “announce bcdeZ” a b c d e Zurich Destination Dir Dst Path Zurich c 4 cdeZ Destination Dir Dst Path Zurich b 5 bcdeZ Distributed Computing Group Computer Networks R. Wattenhofer

BGP Basics Continued “announce bfeZ” f 30s a b c d e Zurich Destination Dir Dst Path Zurich c 4 cdeZ f 3 feZ backup active Destination Dir Dst Path Zurich b 4 bfeZ Distributed Computing Group Computer Networks R. Wattenhofer

BGP Basics Continued “announce bcdeZ” f a b c d e Zurich Destination Dir Dst Path Zurich c 4 cdeZ f 3 feZ backup active Destination Dir Dst Path Zurich b 4 bfeZ Distributed Computing Group Computer Networks R. Wattenhofer

BGP (Border Gateway Protocol)
BGP is the Internet de-facto standard Path Vector protocol Receive BGP update (announce or withdrawal) from a neighbor. Update routing table. Does update affect active route? (Loop detection, policy, etc.) If yes, send update to all neighbors that are allowed by policy. MinRouteAdver: At most 1 announce per neighbor per 30+jitter seconds. Store the active routes of the neighbors. Distributed Computing Group Computer Networks R. Wattenhofer

Internet Architecture
BGP Destination Dir Dst Path Zurich c 4 cdeZ /19 R1 iBGP Route flap dampening Multipath Soft configuration … Distributed Computing Group Computer Networks R. Wattenhofer

Internet inter-AS routing: BGP
BGP messages exchanged using TCP. BGP messages OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection Policy Even if two BGP routers are connected they may not announce all their routes or use all the routes of the other Example: if AS A does not want to route traffic of AS B, then A should simply not announce anything to B. Distributed Computing Group Computer Networks R. Wattenhofer

Robustness of BGP a d b c a startet mit acd, b dann bd (weil ueber a haben wir bacd was 3 hops sind, zu lange…), c mit cbd, doch dann muss a sich wieder umentscheiden und geht auf direkt was wiederum b aendern laesst… etc. We are interested in routes to destination d. Nodes a,b,c all have the policy to prefer a 2-hop route through their clockwise neighbor over a direct 1-hop route to destination d. Distributed Computing Group Computer Networks R. Wattenhofer

BGP Update Traffic (Mae-East)
Cisco bug “withdraw loop” is fixed with IOS release. Distributed Computing Group Computer Networks R. Wattenhofer

Internet Evolution: NSFNet (1995)
NSFNet Backbone Hello/EGP Hello/EGP Regional Regional Regional Campus Campus Campus Campus Distributed Computing Group Computer Networks R. Wattenhofer

Internet Evolution: Today
AS1 AS2 AS5 BGP AS6 AS4 AS3 AS8 AS7 Distributed Computing Group Computer Networks R. Wattenhofer

Experimental Setup Analyzed secondary paths of 20x20 AS pairs: Inject and monitor BGP faults. Survey providers on policies. Distributed Computing Group Computer Networks R. Wattenhofer

BGP Convergence Times 180 Distributed Computing Group Computer Networks R. Wattenhofer

BGP Convergence Results
If a link comes up, the convergence time is in the order of time to forward a message on the shortest path. If a link goes down, the convergence time is in the order of time to forward a message on the longest path. Distributed Computing Group Computer Networks R. Wattenhofer

Intuition for Slow Convergence
p a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

p Os a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

p a p:p W W W W W b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

a W c d e a:p c:ap Distributed Computing Group Computer Networks R. Wattenhofer

a dcap c d e a:p c:ap Distributed Computing Group Computer Networks R. Wattenhofer

p O.1s a p:p W cbap dcap edap b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

p O.2s a - W cbap dcap edap b c d e f - b:ap c:ap d:ap e:ap d:cap e:dap c:bap Distributed Computing Group Computer Networks R. Wattenhofer

p 30s!!! a - W dcbap edcap b c d e f - - c:bap d:cap e:dap Distributed Computing Group Computer Networks R. Wattenhofer

p 60s a - W edcbap b c d e f - - - d:cbap e:dcap Distributed Computing Group Computer Networks R. Wattenhofer

p 90s a - W b c d e f - - - - e:dcbap Distributed Computing Group Computer Networks R. Wattenhofer

p a b c d e f Convergence in the time to forward a message on the longest path. Distributed Computing Group Computer Networks R. Wattenhofer

The longest path… p a g e b h f d c j i Distributed Computing Group Computer Networks R. Wattenhofer

… is NP-complete (APX) p a g e b h f d c j i Distributed Computing Group Computer Networks R. Wattenhofer

Example of BGP Convergence
Time BGP Message/Event 10:40: withdraws p 10:41: announces p 10:41: announces p 10:41: announces p 10:42: announces p 10:43: announces p 10:43: announces p 10:43: withdraws p Distributed Computing Group Computer Networks R. Wattenhofer

Remember the Example p a W b c d e f edap edcap edcbap Distributed Computing Group Computer Networks R. Wattenhofer W

What might help? Idea: Attach a “cause tag” to the withdrawal message identifying the failed link/node (for a given prefix). It can be shown that a cause tag reduces the convergence time to the shortest path Problems Since BGP is widely deployed, it cannot be changed easily ISP’s (AS’s) don’t like the world to know that it is their link that is not stable, and cause tags do exactly that. Race conditions make the cause tags protocol intricate Distributed Computing Group Computer Networks R. Wattenhofer

Example with BGP-CT (Cause Tags)
p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

Example with BGP-CT p Os a p:p b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

Example with BGP-CT p O.1s a p:p W(ap) W(ap) W(ap) W(ap) W(ap) b c d e f a:p a:p b:ap a:p c:ap a:p d:ap a:p e:ap Distributed Computing Group Computer Networks R. Wattenhofer

Convergence Time using Cause Tags
b c p x e f Distributed Computing Group Computer Networks R. Wattenhofer

Convergence Time using Cause Tags
b c p x e f Convergence in the time to forward a message on the new shortest path (instead of the longest). Distributed Computing Group Computer Networks R. Wattenhofer

Computer Networks Winter 2002 / 2003

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