Modeling Inter-Domain Routing

Protocol Dynamics

ISMA 2000December 6, 2000

In collaboration with Abha, Ahuja, Roger Wattenhofer, Srinivasan Venkatachary, Madan Musuvathi

Craig LabovitzMerit Network/Microsoft Research

labovit@merit.edu

2

Routing Dynamics

Goal: Develop a model of Internet inter-domain routing protocol dynamics. Easy, right?

Subgoals– Model impact of failures and topological changes on end-to-end

paths– Predict/measure reliability of inter-AS links, routers, etc.– Compare steady-state topology compare to topologies under

failure– Figure out where all of those darn BGP updates come from

3

Stuff

• Old stuff– Measurements of BGP updates and convergence

– Model BGP convergence (upper and lower bounds)

• New Stuff– Protocol timer trade-offs

– Improvements to BGP (BGP-CT)

4

Data Sets & Tools

• Default-free BGP peering sessions– (routeviews.merit.edu, 2 Equinix probes, 1 Mae-West,

several iBGP probes, Merit RSNG route servers)– Daily tables and all BGP updates/events sent to RS

over last five years– Daily default-free dumps (and all updates/events) for

20-30 peers for last two years• Fault injection probes (OSPF/BGP)• Analysis/Tools

– MRT/Perl (playing with SSFNet)– RouteTracker (whois.routetracker.net)

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Internet BGP Update Volume

• Withdraws in millions until 2/1998 due to withdraw looping/Cisco bug. Dramatic drop after IOS release

Announcements growing after 6/98 due to MED policy and convergence?

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MTTF of Backbone Networks

• Informally: How long before a network is unreachable?

• Majority of Internet routes unreachable within 30 days

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Mean Time to Fail-Over

• How long before traffic is re-routed?

• Majority of Internet routes which possess backup paths fail-over every 3 days

8

Internet Route Repair

• How long before a network is reachable again?

• Long-tailed distribution with plateau at 30 minutes. Why this plateau?

9

BGP Convergence

• If complete graph, N! upper theoretic bound and 30*(N-3) lower bound

• In practice, Internet has hierarchy and customer/provider/sibling relationships. Bounded by length longest possible path

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BGP Convergence ExampleR

AS0 AS1

AS2AS3

*B R via 3 B R via 03 B R via 23

*B R via 3 B R via 03 B R via 13

*B R via 3 B R via 13 B R via 23

AS0 AS1 AS2

** **B R via 203

*B R via 013 B R via 103

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Observed Fault Injection Topologies

• In steady-state, topologies between ISP1, ISP2, ISP3 similar – all direct BGP peers of ISP4.

• Repeatedly withdrew single-homed route (R1, R2, R3)

Steady State

ISP 1R1

Withdraw

ISP 4

ISP 2R2

Withdraw

Steady State

ISP 3R3

Withdraw

Steady State

MAE-WEST

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Comparing ISP Convergence Latencies

• CDF of faults injected into three Mae-West providers and observed at ISP router in Japan• Significant variations between providers

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ISP1-ISP4 Paths During Failure

• Only one back up path (length 3)

Steady State

ISP 1

ISP 5

P2

P2ISP 4

R1

FAULT

96% Average: 92 (min/max 63/140) seconds

Announce AS4 AS5 AS1 (44 seconds)

Withdraw (92 seconds)

4% Average: 32 (min/max 27/38) seconds

Withdraw (32 seconds)

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ISP2-ISP4 Paths During Failure

Ste

ady

Sta

te

ISP 2

ISP 4

P2

P2

ISP 5

P3

P3

P3

ISP 6

R2

FAULT

Vagabond

P4

P4ISP 10

ISP 11

ISP 12

ISP 13

P4

P4

P4

63% Average: 79 (min/max 44/208) seconds

AS4 AS5 AS2 (35 seconds)

Withdraw (79 seconds)

7% Average: 88 (min/max 80/94) seconds

Announce AS4 AS5 AS2 (33 seconds)

Announce AS4 AS6 AS5 AS2 (61 seconds)

Withdraw (88 seconds)

7% Average: 54 (min/max 29/9) seconds

Withdraw (54 seconds)

23% Other

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ISP3-ISP4 Paths During Failure

ISP 3

Ste

ady

St a

t e

ISP 4

R3

P2

P2

ISP 5

FAULT

ISP 1

P3

P3

P6

P7

P7P4

P4

P5

P5

P5P5

P5

P6

P6

P6

ISP 7

ISP 9

ISP 8

P7

P7

P4

36% Average: 110 (min/max 78/135) seconds

Announce AS4 AS5 AS (52 seconds)

Withdraw (110 seconds)

35% Average: 107 (min/max 91/133) seconds

Announce AS4 AS1 AS3 (39 seconds)

Announce AS4 AS5 AS3 (68 seconds)

Withdraw (107 seconds)

2% Average:140.00 (min/max 120/142)

Announce AS4 AS5 AS8 AS7 AS3 (27) Announce AS4 AS5AS9 AS8 AS7 AS3 (86)

Withdraw (140 seconds)

27% Other

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Race Conditions and Paths

• T(shortest path) <= Tdown <= T(longest path)

B

A

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Relationship Between Backup Paths and Convergence

• Convergence related to length of longest possible backup ASPath between two nodes

Longest Observed ASPath Between AS Pair

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Towards Fast BGP Convergence

Four possible solutions

• No transit/One-hop topology (peer and filter everyone)

• Turn off/Change MinRouteAdver timer

• “Tag” BGP updates and provide hint so nodes can detect bogus state information

• Entirely new protocol

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255 AS Topology0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350 400 450

Seconds

Cumulative Percentage

MRA, CT

MRA, No CT

No MRA, No CT

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255 AS Topology0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 100012001400 16001800

Number of Messages

Cumulative Percentage

MRA, CT

MRA, No CT

No MRA, No CT

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BGP-CT

• Incremental addition to BGP4– Capability negotiation– Tags carried in as multi-protocol NRLI extension– Invalidate alternative paths if match tag (and other necessary

conditions met)

• Details– New state machine additions (temporary invalidation)– Works with iBGP– Implemented MRT and deployed on CAIRN– Improves BGP convergence by an order of magnitude in most

cases (in a few cases, behavior is worse)