DSN 2003

A Study of Packet Delivery Performance during Routing Convergence

Dan Pei, Lan Wang, Lixia Zhang, UCLA

Dan Massey, USC/ISI

S. Felix Wu, UC Davis

06/24/2003 2/14DSN 2003

Packet Delivery during Routing Convergence

Failures do occur in the Internet 20% of intra-ISP links have a MTTF < 1 day [Diot:IMW02] 40% of Inter-ISP routes have a MTT-Change < 1 day [Labovitz:FTCS-29]

Routing convergence after failure takes time IS-IS(Intra-ISP protocol): 5+ seconds [Diot:IMW02] BGP(Inter-ISP protocol): 3+ minutes [Labovitz:Sigcomm00]

Packets can be delivered during convergence

A B C

E F

D

G

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Goal of this paper

How to maximize packet delivery during routing convergence?

Topological connectivity’s impact?

Studying: RIP, Distributed Bellman-Ford(DBF), BGP

Previous work focused on: preventing loops, minimizing convergence time and routing overhead

This problem becomes more important with

Larger Internet topology [Huston01] --> higher freq. of component failuresRicher connectivity[Huston01] --> potentially helps with more alternate pathsHigher bandwidth --> more packets sent during convergence

06/24/2003 4/14DSN 2003

Outline for the rest of the talk

Introduction of RIP, DBF and BGP

Simulation results and lessons learned

Conclusion

06/24/2003 5/14DSN 2003

Protocols Examined (I):RIP and DBF

RIP

• Keep shortest path only

Distributed Bellman-Ford(DBF)• Keep distance info from all neighbors

A

B

C

E F

D

D:1

D:3

D:2

D:2

D:3

•B’s route to D: Nexthop=A, Dist=4

•B’s route to D: Nexthop=A, dist=4Alternate Nexthop=C, Dist=4

D: infin

ity

• 30sec refreshing interval •Damping timer to space out two triggered updates: 1~5 seconds

•Poison reverse: B sends infinity distance to A

Both RIP and DBF:

•Exchange distance info.

06/24/2003 6/14DSN 2003

BGP is similar to DBF, but route includes entire path

Route via A = <A E F>Route via C = <C E F>

B’s route to D:A

B

C

E F

D

D:<F>D:<A E F>

D:<E F>

D:<E F>

D:<C E F>

BGP: damping timer: 25 ~ 35 seconds

BGP’: damping timer: 1~5 seconds

†

Protocols Examined (II): BGP

D: <B A E F>

06/24/2003 7/14DSN 2003

Outline for the rest of the talk

Introduction of RIP, DBF and BGP

Simulation results and lessons learned

Conclusion

06/24/2003 8/14DSN 2003

Simulation conducted 7 by 7 mesh topologies similar

those in [Baran64]

20 pkts/second

Measure Packet loss, loops, path convergence time, throughput, and e2e delay.

Simulated node degree range [3 ~ 16]

06/24/2003 9/14DSN 2003

Packet Losses (I) : Observation

RIP

DBF, BGP’ and BGP

Packet losses of DBF, BGP’ and BGP decrease to zero at degree 6.

Richer connectivity helps RIP little.

Node Degree

Pac

ket L

oss

06/24/2003 10/14DSN 2003

Packet Loss(II): Lessons Learned

Keeping alternate paths

F

DA

B

C

E

F

DA

B

C

E

Connectivity Mattersno immediate available alternative due to poor connectivity and poison reverse

RIP:

DBF, BGP:

alternative is more likely with richer connectivity

06/24/2003 11/14DSN 2003

Packet Loss(III): Is an alternate path valid?

Valid Alternate Paths: not using the failed link

Poison reverse and BGP’s path information are not enough! [Pei:Infocom2002]

F

DA

B

C

E

U

X

VW

Richer connectivity --> reduces one single link’s impact better availability of valid(but may be suboptimal) path

C2

D: < >

D: < >

D: < >

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Transient Loops(I): Observation

DBF BGP’

BGP•BGP has the most loops!

•RIP has no loops

•Richer connectivity reduces the chance of looping.

Node Degree

Los

ses

due

to lo

ops

06/24/2003 13/14DSN 2003

F

D

A

B

C

E

Transient Loops(II): Msg PropagationDamping timer slows the msg propagation, causing looping

UX

V W

Y

D:<C A E F>

D: <B A E F>

D: <B C A E F>

D:<C B A E F>

Richer connectivity can reduce the chance of loopingMore details in:

“A Study of Transient Loops in BGP”

30 seconds!

D: < >

D: < >

D: < >D: <

>

D: <

>

06/24/2003 14/14DSN 2003

Conclusion Network’s Ultimate goal is to deliver happy packets, so Routing Protocols should

Maximize packet delivery during convergence

Achieve a good balance between packet delivery AND loop prevention, routing conv. time and routing overhead

Utilize the connectivity redundancy

Future work Apply insights to BGP; study link state protocols, e2e TCP performance; Larger topologies, multiple pairs of S/D,

multiple failures

DSN 2003

Questions?

06/24/2003 16/14DSN 2003

Instantaneous Throughput

RIP

DBF

BGP’

BGP

RIP

Time

Thr

ough

put(

pkts

/sec

ond

06/24/2003 17/14DSN 2003

Packet Delay During Convergence

06/24/2003 18/14DSN 2003

Forwarding Path Convergence timeBGP: no loss at degree 6 or higher

Shall we still tune MRAI timer to minimize convergence time(with the risk of increasing overhead)?Node Degree

BGP:70

BGP’:10

Time till there is no routing msg.

BGP:13

BGP’:2

Time till the forwarding path from S to D stabilizes.

06/24/2003 19/14DSN 2003