Improving TCP Performanceover Ad-hoc Network

11/28/2000

Xuanming Dong, Duke Lee, and Jin Wang

Course Project for EE228A --- Fall 2000(Professor Jean Walrand)

Outline

TCP basics and congestion control Ad-hoc network basics Impact of ad-hoc network on TCP

performance Explicit Route Notification Simulation and Results Summary

Basic function of TCP

1. Basic Data Transfer: use IP services;2. Reliability: sequence number; positive

ACK; retransmission and time out; checksum;

3. Flow Control: 4. Congestion Control: 5. Multiplexing: addresses or ports within

each host;6. Connections: 7. Precedence and Security:

TCP: Congestion Control

Variables: Congestion Window (W) & Slow Start

threshold size (ssthresh) Indication of congestion:

Timeout and DupACK Congestion Control :

Slow Start + Congestion Avoidance Fast Recovery (Fast retransmission +

Congestion Avoidance)

Ad-hoc Networks

Autonomous mobile nodes are free to move around arbitrarily

network topology may change randomly and rapidly at unpredictable times

Without (necessarily) using a pre-existing infrastructure

Routes between nodes may potentially contain multiple hops

Infrastructure-oriented Wireless Network Have fixed and wired gateways The bridges for these networks are

known as base stations A mobile node within these networks

connects to the nearest base station As the mobile node travels out of

range of one base station and into range of another, a “handoff” occurs

Usually only the last link is wireless

How does mobility affect TCP

High bit error rate cause packets(TCP data segment or ACKs)

corrupted and then congestion control invoked

Route recomputation Discovering a new route may take

significantly longer than RTO and then cause retransmission and invoke congestion control

Network may be temporarily partitioned Cause packets dropped and invoke

congestion control

Route Re-computation

Figure 1: Route Re-Computation

Temporary Network Partition

Figure 2: Temporary Network Partition

mobility causeslink breakage,resulting in routefailure

TCP data and acksen route discarded

TCP sendertimes out.Backs off timer.

Route isrepaired

TCP sendertimes out.Resumessending

Larger route repair delaysespecially harmful

No throughput

No throughput

despite route repair

Why Does Throughput Degrade?

Explicit Route Notification(ERN)

Router-based feedback Need to modify TCP Source & Router

to receive/send feedback Need mechanisms for information

exchange between layers Inform TCP of route failure by

explicit message Let TCP know when route is

repaired By Explicit notification Reduces repeated TCP timeouts

and backoff

TCP FSM

Normal Frozen

ERFN

Frozen Timeout

ERRN

Figure 4: Extended TCP Finite State Machine

Explicit Route Notification(ERN)

Router-based feedback Need to modify TCP Source & Router

to receive/send feedback Need mechanisms for information

exchange between layers Inform TCP of route failure by

explicit message Let TCP know when route is

repaired By Explicit notification Reduces repeated TCP timeouts

and backoff

Support from Router

1. Maintain a packet cache for every routing entry in the routing table. Once the router finds that the routing entry for a packet becomes invalid recently, it caches the packet and creates an ERFN (Explicit Route Failure Notification) packet.

2. Associate a timer with every packet in the cache. If the timer expires, then drop the packet from the cache.

3. If the invalid routing entry is removed from the routing table, drop all packets in its cache and cancel the associated timers.

4. If the invalid routing entry becomes valid again, forward all the cached packets and create an ERRN (Explicit Route Recovery Notification) packet for each cached packet.

Support from Router

Destination Next Hop Metric SeqNo A A 0 3 B B 3 2 C 2 D 6

Cache - C

Cache - D

Figure 3: Routing table in DSDV Routers

Support from TCP

Context Switch The Frozen State

1. Stop sending further packets. 2. Stop invoking congestion control. 3. All TCP timers are frozen, except the new frozen

timeout timer.4. But TCP source may accept ACK packets and advance

the acknowledgment window.

Support from TCP

Normal Frozen

ERFN

Frozen Timeout

ERRN

Figure 4: Extended TCP Finite State Machine

Simulation Results

Source Packet Trace Source Congestion Window trace TCP Throughput

Simulation Results

Figure 6: TCP Source Packet Seq No vs Time (Reno)

-400

100

600

1100

1600

24 26 28 30 32 34 36

Time(Second)

Se

qu

en

ce N

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Simulation Results

Figure 7: TCP Source Congestion Window Size vs Time (Reno)

0

5

10

15

20

24 26 28 30 32 34 36

Time(Second)

Co

ng

est

ion

Win

do

w

Siz

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Simulation Results

Figure 8: TCP Source Packet Seq No vs Time (ERN)

0

200

400

600

800

1000

1200

1400

1600

24 26 28 30 32 34 36

Time(Second)

Seq

uen

ce N

o

Simulation Results

Figure 9: TCP Source Congestion Window Size vs Time (ERN)

0

5

10

15

24 26 28 30 32 34 36

Time(Second)

Con

gest

ion

Win

dow

S

ize

Summary

By Xuanming Dong