TCP/IP Illustrated Vol 1TCP/IP Illustrated Vol 1

TCP Timeout andTCP Timeout andRetransmissionRetransmission

2005. 5. 2( 월 )이 병 희

icebyung@hufs.ac.kr

IntroductionIntroduction TCP provides a reliable transport

Each end send acknowledgment to the other end when receive data

Data and acknowledgment can get lost TCP handles this by setting a timeout when it sends

data and retransmits data

TCP manages four different timers retransmission timer persist timer keepalive timer 2MSL timer

RTT MeasurementRTT Measurement Timeout should be based on Round-trip time

(RTT) RTT measured by observing difference between

time of transmission and arrival of acknowledgment

SRTT = αSRTT + (1 – α)RTTRTO = β SRTT

It can’t keep up with wide fluctuations in the RTT causing unnecessary retransmissions

RTO measurement in RFC1122 Err = RTT – SRTT SRTT = SRTT + gErr D = D + h( |Err| - D) RTO = SRTT + 4D

RTT Measurement ExampleRTT Measurement Example The timer increment a counter

every time the 500ms The starting Seq number is

remembered

Karn’s AlgorithmKarn’s Algorithm When timeout occurs and packet is

retransmitted 송신 호스트는 RTT 연산에 대해 혼선 Is the ACK for the first transmission or second ?

Do not update the RTT until an ACK is received for a segment that was not retransmitted

if TCP timestamp option is used, we don’t need to apply Karn’s Algorithm

Segment1,Seq =1010

ACK, Seq=1010

Segment1,Seq =1010Segment1

Retransmission

Congestion Avoidance AlgorithmCongestion Avoidance Algorithm assumption of the algorithm

packet loss caused by damage is very small the loss of a packet signals congestion

Congestion Avoidance and Slow Start algorithm Initialization of the connection sets cwnd to one and ssthresh to 65535bytes

When congestion occurs, one-half of the current window size is saved in ssthresh. If congestion is indicated by timeout, cwnd is set to one.

When new data is acknowledged by the other end, cwnd is increased

Congestion Avoidance AlgorithmCongestion Avoidance Algorithm If cwnd is less than or equal to ssthresh, doing slow

start. otherwise doing congestion avoidance.

slow start has cwnd start at one segment, and be incremented by one segment every time an ACK is received

congestion avoidance dictates that cwnd be incremented by 1/cwnd each time an ACK is received

Fast Retransmit and Fast RecoveryFast Retransmit and Fast Recovery Fast Retransmission

If Sender receive three or more duplicate ACK in a row, Sender retransmit the missing segment without waiting for a retransmission timer to expire

Fast Recovery After fast retransmission, sender do congestion

avoidance Set ssthresh to one-half of the minimum of current cwnd and

receiver’s advertised window Set cwnd to ssthresh + 3(segment size) Each time another duplicate ACK arrives, increment cwnd by

the segment size and transmit packet When the next ACK arrives that acknowledges new data set

cwnd to ssthresh

Congestion exampleCongestion example segment 45 got lost

Receiver send duplicated ACK Sender get three duplicated

ACKs in a row Sender starts fast retransmission

TCP/IP Illustrated Vol 1TCP/IP Illustrated Vol 1

TCP Persist TimerTCP Persist Timer

TCP Persist TimerTCP Persist Timer When window size goes to ‘0’, Sender stops

transmitting data until the window becomes nonzero

If an ACK is lost, both sender and receiver are waiting for the other

To prevent deadlock, sender use persist timer

7169:8193(1024) ack1 win 4096

ACK 9217 win 0

8193:9217(1024) ack1 win 4096

ACK 9217 win 4096

TCP Persist TimerTCP Persist Timer After segment13, sender set persist timer If sender doesn’t receive window update when timer

expire, it send window probes

persist timer start

send window probes

svr4bsdi

(11)

(12)

(13)

(14)

(15)

Silly Window SyndromeSilly Window Syndrome It can be caused by either side

Receiver advertise small size windows Sender transmit small amount of data

Ways to prevent silly window syndrome Receiver side

must not advertise small window use delay ACK

Sender side must not transmit data until full-size or at least one-half of

maximum window size sender can send everything when we are not excepting an

ACK

SWS exampleSWS example

sun.1069

PSH 1:1025(1024) ack 1, win 4096

PSH 1025:2049(1024) ack 1, win 4096

bsdi.7777

PSH 2049:3073(1024) ack 1, win 4096

PSH 3073:4097(1024) ack 1, win 4096

ack 4097, win 0

1 2

3

4

54097:4098(1) ack 1, win 40966

ack 4098, win 0 74098:4099(1) ack 1, win 40968

ack 4099, win 0 9

4099:4100(1) ack 1, win 409610

ack 4100, win 1533 11

0.0

0.002026(0.0020)

0.003737(0.0017)

0.005361(0.0016)

0.170306(0.1649)5.151768(4.9815)

5.170308(0.0185)10.151592(4.9813)

10.170299(0.0187)

15.151466(4.9812)

15.170296(0.0188)

SWS exampleSWS example

12

13

14

15

16

17

18

19

20

21

4100:5124(1024) ack 1, win 4096

ack 5124, win 509

5124:5633(509) ack 1, win 4096

ack 5633, win 0

5633:5634(1) ack 1, win 4096

ack 5634, win 1279

FIN, PSH 5634:6145(511) ack 1, win 4096

ack 6146, win 767

ack 6146, win 2816

FIN 1:1(0) ack 6146, win 4096

ack 2, win 409622

sun.1069 bsdi.7777

15.172006(0.0017)

15.370307(0.1983)20.151782(4.7815)

20.170297(0.0185)25.151162(4.9809)

25.170302(0.0191)25.171801(0.0015)

25.174401(0.0026)39.991658(14.8173)

51.991775(12.0001)

51.992665(0.0009)

SWS exampleSWS example

Action Receiver bufferTime Segment#

(Fig. 22.2) Send TCP Receive TCP Application data available

0.000 1 1:1025(1024) 1024 30720.002 2 1025:2049(1024) 2048 2048

0.003 3 2049:3073(1024) 3072 1024

0.005 4 3073:4097(1024) 4096 0

0.170 5 ACK 4097, win 0

3.99 read 256 3840 256

5.151 6 4097:4098(1) 3841 255

5.17 7 ACK 4098, win 0

5.99 read 256 3585 511

7.99 read 256 3329 767

9.99 read 256 3073 1023

10.151 8 4098:4099(1) 3074 1022

10.170 9 ACK 4099, win 0

11.99 read 256 2818 1278

13.99 read 256 2562 1534

15.151 10 4099:4100(1) 2563 1533

15.170 11 ACK 4100, win 1533

15.172 12 4100:5124(1024) 3587 509

15.370 13 ACK 5124, win 509

SWS exampleSWS example

Action Receiver bufferTime Segment#

(Fig. 22.2) Send TCP Receive TCP Application Data Available

15.370 13 ACK 5124, win 50915.99 read 256 3331 765

17.99 read 256 3075 1021

19.99 read 256 2819 1277

20.151 14 5124:5633(509) 3328 768

20.17 15 ACK 5633, win 0

21.99 read 256 3072 1024

23.99 read 256 2816 1280

25.151 16 5633:5634(1) 2817 1279

25.170 17 ACK 5634, win 1279

25.171 18 5634:6145(511) 3328 768

25.174 19 ACK 6146, win 767

25.99 read 256 3072 1024

39.99 read 256 1280 2816

39.99 20 ACK 6146, win 2816

51.99 read 256(EOF) 0 4096

51.991 21 ACK 6146, win 4096

51.992 22 ACK 2

TCP/IP Illustrated Vol 1TCP/IP Illustrated Vol 1

TCP Keepalive TimerTCP Keepalive Timer

TCP Keepalive TimerTCP Keepalive Timer Introduction

If there are no data exchange between TCP connection, the connection remains established

Even if intermediate routers are crashed or lines are down, the connection remains established

Keepalive option provides capability that server can know whether client is down

If intermediate router has crashed, keepalive will think that the client is down and terminate connection

DescriptionDescription States of client and server

If there is no activity for two hours, the server sends a probe segment to the client

State of Client’s host and Server Server’s Action

The client’s host is running and reachable from the server

Server’s TCP will reset keepalive timer for 2 hours.

If data across the TCP connection before expire, timer is reset for 2 hours again.

The client’s host has crashed and is either down or in the process of rebooting

Server will not receive response in 75 second. Server sends total 10 probes, 75 second apart. If no response, server will terminate the connection

The client’s host has just rebooted

Because client has just rebooted, server will receive response but it will be a reset. So, server terminates the connection

The client’s host is running, but unreachable from the server

Same as client’s down. Server terminates the connection after 10 probes, 75 second apart.

Keepalive ExampleKeepalive Example

bsdi % sock –K svr4 echo hello, world hello, world

- disconnect cable –(4 hours later) read error: Connection timed out

first keepalive probe(2 hours later)

second keepalive probe(4 hours later)

• Other End Crashes

Keepalive ExampleKeepalive Example• Other End Crashes and Reboot

bsdi % sock –K svr4 echo hi there hi there

read error: Connection reset by peer

first keepalive probe(2 hours later)

Keepalive ExampleKeepalive Example• Other End is Unreachable

slip % sock –K vangogh.cs.berkeley.edu echo testing testing

- link is down (3 hours later) - read error: No route to host

ClientServer

Terminate the connection

Router(alive)

Router(down)

first keepalive probe(2 hours later)

second keepalive probe(4 hours later)