TCP over OBS Joint Work with F. Baccelli, D. Hong, G. Petit, F. Poppe Julien Reynier Article available at

Capacity requirements of common services — 2 All rights reserved © 2002, Alcatel Overview > Introduction > Optical Burst Switching (OBS) : description > FDL > Optimization problem > The formula > Conclusion

Capacity requirements of common services — 3 All rights reserved © 2002, Alcatel Goal > Understand the interaction between TCP and physical layers for data transmission over OBS > OBS : Optical Burst Switching TCP layer IP layer OBS layer opticalelectrical

Capacity requirements of common services — 4 All rights reserved © 2002, Alcatel Overview > Introduction > Optical Burst Switching (OBS) : description > FDL > Optimization problem > The formula > Conclusion

Capacity requirements of common services — 5 All rights reserved © 2002, Alcatel Situation Optical Burst Switching : Optimize burst size Burstification of IP packets into Data Bursts (DBs) IP router OBS edge router OBS core router DWDM link Electronic processing of Burst Header Packets (BHPs) IP packets BHP Channel Group (CG) control (CCG) data (DCG)

Capacity requirements of common services — 6 All rights reserved © 2002, Alcatel Situation Optical Burst Switching IP router OBS Ingress edge router OBS core router DBs are routed in the optical domain IP packets DB Bursts disassembly

Capacity requirements of common services — 7 All rights reserved © 2002, Alcatel Specificity of OBS > At the ingress edge router, IP packets are assembled in lrager processing units, namely the Data Bursts (DBs). The loss of a single DB is the loss of many TCP-IP packets at the same time. > In the core routers, bursts wait in a queue implemented with optical fibers : the Fiber Delay Lines (FDLs). Bursts are not released as soon as the link becomes ready. – “Classical” queuing models do not work. Certain methods as Void Filling mitigate this effect. – The drawback is a complex architecture and a possible reordering of bursts, therefore of IP packets. Classical queue FDL length D

Capacity requirements of common services — 8 All rights reserved © 2002, Alcatel Overview > Introduction > Optical Burst Switching (OBS) : description > FDL > Optimization problem > The formula > Conclusion

Capacity requirements of common services — 9 All rights reserved © 2002, Alcatel FDL and a classical queue : > Without FDL : > With FDL : > The Void size is :

Capacity requirements of common services — 10 All rights reserved © 2002, Alcatel Voids TnTn T n+1 T n +kDT n+1 +k’D input : output : nn VnVn

Capacity requirements of common services — 11 All rights reserved © 2002, Alcatel Load factor for an FDL with infinite capacity

Capacity requirements of common services — 12 All rights reserved © 2002, Alcatel Example of an FDL workload Workload distribution of an FDL queue and for an M/M/1 queue with the same equivalent load D=0.1 hpkt  =7.8 1/hpkt =1 hpkt/s 1 hpkt=100 IP- packets  ’=95%

Capacity requirements of common services — 13 All rights reserved © 2002, Alcatel Loss rate due to buffer overflow with FDL b=1/  B=8D =1/b’ B:buffer D:delay line b:burst size b’:burst size + void size The loss rate at a “congestion epoch” is affine for a large range of values.

Capacity requirements of common services — 14 All rights reserved © 2002, Alcatel Overview > Introduction > Optical Burst Switching (OBS) : description > FDL > Optimization problem > The formula > Conclusion

Capacity requirements of common services — 15 All rights reserved © 2002, Alcatel Notation N sources TCP- controlled Link capacity C Queue size B RTT R Bottleneck router > N users share a bottleneck router with capacity C (Mb/s) with queue size B (byte), their RTT being R (ms).

Capacity requirements of common services — 16 All rights reserved © 2002, Alcatel Small bursts or large bursts ? > For large bursts Lower bandwidth waste due to guard bands – OBS needs time to reconfigure its switching array Lower processing time inside nodes > For small bursts Lower synchronization between sources due to TCP Lower burst formation time

Capacity requirements of common services — 17 All rights reserved © 2002, Alcatel Synchronization - AIMD > TCP is AIMD : Additive Increase, Multiplicative decrease > The proportion of sources that lose packets at that time is the synchronization X(t)X(t) t X ( t ) = throughput seen by a user T i = congestion epochs in the network TiTi T i-2 T2T2 TETE T1T1 TSTS Congestion avoidance: additive increase, multiplicative decrease T3T3 T i-1 T i-3 Slow startAdditiveIncreaseMultiplicativeDecrease

Capacity requirements of common services — 18 All rights reserved © 2002, Alcatel Overview > Introduction > Optical Burst Switching (OBS) : description > FDL > Optimization problem > The formula > Conclusion

Capacity requirements of common services — 19 All rights reserved © 2002, Alcatel AIMD framework The goodput is : The synchronization p(b,D) is : The AIMD framework models the impact of losses on the TCP- bandwidth. AIMD : “The AIMD model” : Infocom ‘02 by F. Baccelli et D. Hong

Capacity requirements of common services — 20 All rights reserved © 2002, Alcatel Formula  The study of FDL allows one to deduct the ratio of lost busts from the bandwidth.  This leads to a simple formula : For X=B/2 and Y=1/2

Capacity requirements of common services — 21 All rights reserved © 2002, Alcatel Overview > Introduction > Optical Burst Switching (OBS) : description > FDL > Optimization problem > The formula > Conclusion

Capacity requirements of common services — 22 All rights reserved © 2002, Alcatel Our results > Model FDL > Determine impact of burst size on aggregate TCP throughput the impact of FDL on synchronization of TCP sources the AIMD model allows one to find the bandwidth. > Determine the optimal burst size maximize the bandwidth. minimize the bandwidth waste due to OBS (Burst Header Packets, voids, guard bands).

Capacity requirements of common services — 23 All rights reserved © 2002, Alcatel Further research > A better representation of the increase of RTT due to burstification > Model Void Filling > Core routers with wavelength converters

Capacity requirements of common services — 25 All rights reserved © 2002, Alcatel Backup slides