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

TCP over OBS

Joint Work with F. Baccelli, D. Hong, G. Petit, F. Poppe

Julien Reynier

Article available at http://www.eleves.ens.fr/home/jreynier/

http://www.eleves.ens.fr/home/jreynier/





Capacity requirements of common services — 2 All rights reserved © 2002, Alcatel

Overview

> Introduction

> Optical Burst Switching (OBS) : description

> FDL

> Optimization problem

> The formula

> Conclusion


Goal

> Understand the interaction between TCP and physical layers for data transmission over OBS

> OBS : Optical Burst Switching

TCP layer

IP layer

OBS layer

optical electricalelectrical


Overview

> Introduction


> FDL


> The formula

> Conclusion


SituationOptical Burst Switching : Optimize burst size

Burstification of IP packets into Data Bursts

(DBs)IP router

OBSedge

routerOBS core

router

DWDM link

Electronic processing of Burst Header Packets (BHPs)

IP packets

BHP

BHPBHP

Channel Group (CG)

control (CCG)

data (DCG)


SituationOptical Burst Switching

IP router

OBS Ingress

edge routerOBS core

router

DBs are routed in the optical domain

IP packets

DB

DBDB

Bursts disassemb

ly


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


Overview

> Introduction


> FDL


> The formula

> Conclusion


FDL and a classical queue :

> Without FDL :

> With FDL :

> The Void size is :

),0max(1 nnnn WW

),0max(1 nnnnn VWW

DV nnn mod


Voids

DV nnn mod Tn Tn+1

Tn+kD Tn+1+k’D

input :

output :

n Vn


Load factor for an FDL with infinite capacity


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%


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.


Overview

> Introduction


> FDL


> The formula

> Conclusion


Notation

N sourcesTCP-

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).


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


Synchronization - AIMD

> TCP is AIMD : Additive Increase, Multiplicative decrease

> The proportion of sources that lose packets at that time is the synchronization

X(t)

t

X(t) = throughput seen by a user

Ti = congestion epochs in the network

TiTi-2T2 TET1TS

Congestion avoidance: additive increase, multiplicative

decrease

T3 Ti-1Ti-3

Slowstart

Additive Additive IncreaseIncrease

Multiplicative Multiplicative DecreaseDecrease


Overview

> Introduction


> FDL


> The formula

> Conclusion


AIMD framework

Gb

bDbp

N

C

'4

),(4

The goodput is :

The synchronization p(b,D) is : ),(/1),( DbLNCReDbp

The AIMD framework models the impact of losses on the TCP-bandwidth.• AIMD : “The AIMD model” : Infocom ‘02 by F. Baccelli et

D. Hong


Formula

> The study of FDL allows one to deduct the ratio of lost busts from the bandwidth.

> This leads to a simple formula :

CR

NBD

G

bopt

22

2

YD

bXDbL ),(

For X=B/2 and Y=1/2


Overview

> Introduction


> FDL


> The formula

> Conclusion


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).


Further research

> A better representation of the increase of RTT due to burstification

> Model Void Filling

> Core routers with wavelength converters

www.alcatel.com


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Documents

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