TCP
21
TCP Transport Control Protocol
description
TCP. Transport Control Protocol. Introduction. UDP provides the connection less, unreliable, datagram service TCP provides the connection oriented, reliable, byte stream service. TCP Services. Best sized chunks Adaptive timeout and retransmission strategy Acknowledgements Checksums - PowerPoint PPT Presentation
Transcript of TCP
TCP
Transport Control Protocol
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Introduction• UDP provides the connection less, unreliable,
datagram service• TCP provides the connection oriented,
reliable, byte stream service
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TCP Services• Best sized chunks• Adaptive timeout and retransmission strategy• Acknowledgements• Checksums• Re-sequencing• Discard duplicates• Flow control
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TCP Header
IPHeader
IPHeader
TCPHeader
TCPHeader TCP dataTCP data
TCP segment
IP datagram
20 bytes 20 bytes
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TCP Header
20 bytes
16-bit Source port number
32-bit sequence number
16-bit urgent pointer
Options (if any)
Data (if any)
16-bit Destination port number
32-bit acknowledgement number
16-bit window size
16-bit TCP checksum
header length
reservedURG
PSH
RST
SYN
FIN
ACK
0 15 16 31
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Connection establishment : 3-way handshake
SYN 12345:12345 (0)<mss 1024>
Client
Ack 67891
SYN 67890:67890 (0)ack 12346 <mss 1024>
Server
Segment 2
Segment 1
Segment 3
SYN 12345:12345 (0) <mss 1024>SYN seq# : (seq#+#bytes) (#bytes) option
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Best Sized Chunks• Option MSS (maximum segment size)• Announced on session establishment• Interactive data --> byte per byte (Nagle
algorithm)• Bulk data uses MSS
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Connection TerminationClient
ack 67891
FIN 67890:67890 (0)ack 12345 <mss 1024>
Server
Segment 2
Segment 4
Segment 3
FIN 12345:12345 (0)ack 67891 <mss 1024>
Ack 12346
Segment 4
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Reset a connection
<src port 1087 > <dst port 2000>
SYN 12345:12345 (0)win 4096
<mss 1024>
Client Server
Segment 2
Segment 1
<src port 2000 > <dst port 1087>
RST 0:0 (0)ACK 12346
Win 0
No ICMP !!!
Server isNOTRunning Service onPort 2000
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Half open connections
SYN 12345:12345 (0)<mss 1024>
Client
ack 67891
SYN 67890:67890 (0)ack 12346 <mss 1024>
Server
Segment 2
Segment 1
Segment 3
P 12346:12350 (4)ack 67891
P 67891:67899 (8)ack 12351
Segment 4
Segment 5
REBOOT ??????
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Adaptive timeout and retransmission strategy
• Exponential backoff– t, 2t, 4t, 8t, 16t, 32t, 64t seconds until 9
min• TCP calculates rrt --> used for timeout value
(t)
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Acknowledgements & Flow Control Overview
• Full duplex• Piggy backing• Controls for the receiver
– Sliding window protocol • Controls for the sender
– slow start : congestion window– Congestion avoidance :slow start
treshold
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Full duplexClient Server
Full duplex trafficpossible becausesequence numberand acknowledgenumber efficient
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Sliding window protocolClient
Fast SenderServer
Slow Receiver
Slow receiver controls the flow by resizing the Windowack 67891, win 0
PSH 67890, win 4096
PSH 55890, win 4096
PSH 44890, win 4096
ack 67891, win 4096
ack 67891, win 4096
PSH 99890, win 4096
PSH 88890, win 4096
PSH 77890, win 4096
Slow receiver shifts thewindow (same seq nr)
ack 99891, win 0 Slow receiver acknowledges (seq nr + 1)
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Slow Start - Window size – buffer size
ack 1
1
1
1
1
receiver
receiver
sender
sender
ack 1
ack 1
ack 1
ack 2 & 3
2
2
2
2
receiver
receiver
sender
sender
ack 2 & 3
ack 2 & 3
ack 2
time 0 time 8
time 1
time 2
time 3
time 4
time 5
time 6
time 7
time 10
time 12
time 13
time 15
time 14
3
3
3
3 time 9
ss-window=1 ss-window=2
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Slow Start - Window size – buffer size (continued)
ack 4&5&6
4
4
4
4
receiver
receiver
sender
sender
ack 4&5&6&7
ack 4&5
ack 4
8
8
8
8
receiver
receiver
sender
sender
ack 8-11
ack 8
time 16 time 24
time 17
time 18
time 20
time 21
time 22
time 23
time 25
time 28
time 26
time 31
time 30
9
9
9
9
time 29
ack 3
117
6 5
5
6 5
7 6 5
7 6
7
10
10
1012 11
1012 1113
12 111314
12131415
ack 5&6&7ss-window=4ss-window=8
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• Increasing the congestion window will not improve the system anymore, in effect, it will get worse !
receiver
sender time 32
time 33
time 34
ss-window=16
Slow Start - Window size – buffer size (continued)
13141516
14151617
15161718
16171819
. . .
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How big should the window be ?8 segments outstanding Andunacknowledged
Capacity (bits) = bandwidth (bits/sec) x round-trip-time (sec)
OR
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Capacity = bandwidh x delay
431 2
431 2 875 6
double RTT
RTT
431 2
RTT
431 2
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Congestion
124 3124 3 124 3
ACK
Bottleneck Congestion
R1 R2
Fast LAN Fast LANSlow WAN
ss-window=16 congestion-window=16 - no ack - time out
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0
5
10
15
20
25
30
35
40
45
0 10 20 30 40 50
Slow StartExponential Growth
Congestion Avoidance PhaseLinear Growth
Congestion Avoidance
