performance of tcp protocol running over wireless lan network … · 2001. 4. 23. · n tcp is a...
TRANSCRIPT
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ENSC 833 Network Protocols And Performance
Final Project Presentations - Spring 2001
Performance of TCP Protocol Running over Wireless LAN
Network using the Snoop Protocol
Chi-ho Ng and Jack [email protected]
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The Roadmapn Introduction (Problem and Scope of
Project)n TCP retransmission and window sizen Wireless LAN using TCPn The Snoop Protocoln Opnet Implementation
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The Problemn TCP is a reliable protocol with packet
retransmission and congestion control (transmission window adjustment)
n Loss packets seen as network congestion; packets are re-sent with a smaller window size
n Scheme works well in wired networkn However, in wireless networks, with the high bit
error rate, TCP reduces window size excessively and under-utilizes the bandwidth available
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Our projectn Investigate the TCP congestion control
policy and its problem on wireless LAN networks
n Research for possible enhancing algorithmsn Investigate the Snoop Protocoln Implement Snoop on Opnetn Compare results with and without Snoop
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TCP retransmission policyn For every packet
received, the recipent returns an ACK
n Recipient sends duplicate ACK if a packet is lost
n Sender re-transmits the lost packets
first transmission
retransmission
duplicate acknowledgements(DUPACK)
sender recipient
1
5
34
2
ack1ack1
ack1ack1
2345
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TCP transmission window
n 1) Increase the window exponentially to determine the available bandwidth
n 2) When the source fails to receive an acknowledgement, the window size is reduced by half.
n 3) The source increases its window size by one unit every average round trip time
W
W/2
Window Size
slope = 1 /round-trip time
time t
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Wireless LAN using TCP
Base StationHost Server
Mobile terminalRouter
(1) Host serverestablishs a TCPconnection withmobile terminaland starts tosend data
(2) High bit errorrate in thewireless channel
(3) Missingacknowledgementstrigger congestioncontrol at hostserver (transmissionwindow is reduced)
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Adding the Snoop Agent
Base Station with Snoop AgentHost Server
Mobile terminalRouter
(1) Host serverestablishs a TCPconnection withmobile terminaland starts tosend data
(2) High bit errorrate in thewireless channel
(3) Snoop Agentre-transmits lostpackets locally
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The Snoop Protocoln Snoop copies packets to its cache n Starts a retransmission timern Re-transmits the packets if a DupAck is
received or the timer popsn When an Ack is received, deletes the cache
entry
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Snoop_Data()
new packet? packet seq num > lastack num?
1) Forwardpacket2) Reset lcoalretransmissiontimer
discard packet
in sequence? forward packet
1)cache packet2)forward packet
yes
no
no
yes
no
yescommon case
congestionloss
Senderretranmission
packetarrives
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Snoop_Ack()
new ack?
1) Free cacherecord2) Forward Ackto sender
discard packetDuplicate Ack?
discard packet
Retransmit lostpackets
yes
common case
packet loss
ackarrives
First DupAck?
false ack
RepeatedDupAck
no
ye
sye
s
no
no
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Opnet Implementation
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2 extra protocol layers
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Snoop State Transition Diagram
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The Cachetypedef struct
{unsigned int src_ip;unsigned int dest_ip;int src_port;int dest_port;unsigned int seq_num;unsigned int ack_num;unsigned int rcv_win;int urgent_pointer;int data_len;int urg;int ack;int push;int rst;int syn;
int fin;} TcpInfo;
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Cache FunctionsFunction DescriptionsnCacheInit Initialize the cache
recordssnCachedPkt Copies a packet into the
cachesnCacheRetrieve Retrieves a packet from
the cachesnCacheDestroy Deletes a packet from
the cache
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Packet Error Generatorn Used to create packet lossn Packet lost are uniformly distributed
P a c k e t s D r o p p e d b y P E G a t P E R 3 0 %
0
2 0
4 0
6 0
8 0
1 0 0
0 5 0 0 1 0 0 0 1 5 0 0
T i m e ( s e c o n d s )
To
tal
Pa
ck
ets
Dro
pp
ed
T o t a l S e n d
T o t a l D r o p
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PEG State Transition Diagram
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Scenario 1 – Single Mobile Upload
n To study how the Snoop Protocol improves the performance
n Upload 100, 000 byte file from Workstation 1
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Results
n Upload Response Time
n Improve 68 times at error rate of 30%
Upload Reponse Time For Scenario 1
0200400600800
100012001400
0 20 40 60 80 100
PER (%)
Tim
e (s
eco
nd
s)
Snoop Enabled
Snoop Disabled
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Congestion Window Size
n Congestion Window Size
TCP Congestion Window Size at Packet Error Rate 20%
010000200003000040000500006000070000
0 20 40 60 80
Time (seconds)
Cong
estio
n W
indo
w Si
ze (b
its)
Snoop Disabled
Snoop Enabled
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Sent Sequence Number
n Each data byte is represented by a sequence number
Sent Sequence Number at PEG 20%
0
50000
100000
0 20 40 60 80Time (seconds)
Tran
smitt
ed S
eque
nce
Num
ber
Snoop Disabled
Snoop Enabled
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Packet Cache
n Study how are the packet cached
n Transmission Window: 4 packetsn Number of packets cached at the end is 0
Number of Cached Packets
012345
0 5 10 15
Time (seconds)
Num
ber o
f cac
hed
pack
ets
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Difficulty of Project
n Develop Snoop and PEG model from scratch. Need significant amount of time for developing and debugging the code
n Need to place the two models between the ARP and IP nodes. Need to study the source code of ARP and IP node and separate them
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Future Work
n Vary the retransmission timer based on calculated round trip delay in the wireless link
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Referencesn [1] IEEE 802.11 Workgroupn http://grouper.ieee.org/groups/802/11/index.htmln [2] Performance Enchancing Proxy (PEP) Request for Comments (RFC)n http://community.roxen.com/developers/idocs/drafts/draft-ietf-pilc-pep-
04.htmln [3] Improving TCP/IP Performance over Wireless Networksn http://www.cs.berkeley.edu/~ss/papers/mobicom95/html/mobicom-
final.htmln [4] W.Richard Stevens, TCP/IP Illustrated Volume 1, Addison Wesley,
Professional Computing Series, 1984n [5] Andrew S. Tanenbaum, Computer Networks Third Edition, Prentice-
Hall Press, 1996n [6] Wireless LAN Model Description, Opnet Manual.