Connection Management

Networked Systems 3Lecture 13

Lecture Outline

• Modelling protocol behaviour

• Managing transport connections• Connection establishment

• Reliable data transfer

• Connection tear down

2

Modelling Protocol Behaviour

• Can model protocols using a finite state machine• A set of states with transitions between

them

• The current state indicates what the system is doing at any time

• Transitions show occurrence of events and the response of the system

• Can be used to define the behaviour of a protocol

Disconnected

Request sent

Connected

Send “connection requested”

Get “accepted”

Sen

d “c

lose

con

nect

ion”

Example: partial state machine for opening and closing a connection

Get“declined”

3

Managing Connections

• One role of transport layer: provide reliability

• How to reliably manage transport connections?• Setup a reliable connection over an unreliable connectionless network

• Transport data without loss over an unreliable network

• Agree to tear down a connection

4

Disconnected

Connection Establishment

• How do two hosts agree to communicate?

Host A

Tim

e

Host B

Tim

e

Connectionrequested

Connectionaccepted

Request sent

Connected

Listening

Connected

Disconnected Listening

Request sent

Connected

Connected

5

Disconnected

Connection Establishment

• What if the initial request is lost?

Host A

Tim

e

Host B

Tim

e

Connectionrequested

Connectionaccepted

Request sent

Connected

Listening

Connected

Disconnected Listening

Request sent

Connected

Connected

x

Timeout &retransmit

6

Disconnected

Connection Establishment

• What if the “connection accepted” reply is lost?

Host A

Tim

e

Host B

Tim

e

Connectionrequested

Connectionaccepted

Request sent

Connected

Listening

Connected

Disconnected Listening

Request sent

Connected

Connected

xTimeout &retransmit

Error: already connected!

7

Disconnected

Connection Establishment

• What if the “connection accepted” reply is lost?• Sequence number in messages; random initial value

Host A

Tim

e

Host B

Tim

e

Connectionrequested

Connectionaccepted

Request sent

Connected

Listening

Connected

Disconnected Listening

Request sent

Connected

Connected

xTimeout &retransmit

Duplicate requestRetransmit “connection accepted”

seq = x

seq = x

Connected

8

Connection Establishment

• What if data from an old connection is still in the network?

Disconnected

Host A

Tim

e

Host B

Tim

e

Connectionrequested

Connectionaccepted

Request sent

Connected

Listening

Connected

Disconnected Listening

Request sent

Connected

Connected

Delayed connectionaccepted message

Error: alreadyconnected!

seq = x

9

Accepting connectionAccepting connection

Connection Establishment

• Solution for robust connection establishment: use a three-way handshake

Disconnected

Host A

Tim

e

Host B

Tim

e

Connectionrequested

Connectionaccepted

Request sent

Connected

Listening

Connected

Disconnected Listening

Request sent

Connected

seq = x

ack = x, seq = y

ack = y

Connected

10

Three Way Handshake

• Three way handshake ensures robustness• Delayed control messages generate an acknowledgement with incorrect

sequence number

• This is detected, and stops the connection establishment

• Hosts cannot reuse initial sequence number until the maximum packet lifetime passed

• Requires hosts to keep state regarding previous connections, to avoid reuse

• Randomly chosen initial sequence number makes collisions unlikely if a host crash causes state to be lost

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SYN, ACK = x, seq = y

SYN, seq = x

Example: TCP Connections

• TCP connections use a three way handshake• Use the SYN and ACK flags in the

TCP header to signal connection progress

• Packets contain sequence number and acknowledgement number

Host A

Tim

e

Host B

Tim

e

ACK = y

12

Example: TCP Connections

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31Version = 4 Header Len DSCP ECN Total Length

Packet Identifier DF MF Fragment OffsetTTL Upper Layer Protocol Header Checksum

Source AddressDestination Address

Source Port Destination Port

Sequence Number

Acknowledgement Number

Data Offset Reserved Urg Ack Psh Rst Syn Fin Window

Checksum Urgent Pointer

[options - variable length]

[data - variable length]

IP

TCP

Data

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TCP State Machine

Established

Listen

Closed

SYN recv’d SYN sent

Connect/SYN

Close/-Close/-Listen/-

SYN/SYN+ACK

RST/- Send/SYN

SYN/SYN+ACK

SYN+ACK/ACKACK/-

(simultaneous open)

Action/Effect

• Shows client and server on same state diagram

• Additional transitions allow simultaneous open

14

Reliable Data Transfer

• Two approaches to reliable data transfer at the transport layer• End-to-end ARQ

• Positive or negative acknowledgements

• End-to-end FEC

• Within each network layer packet

• Across several network layer packets

• Conceptually identical to operation at data link layer

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Example: TCP

• TCP packets include sequence number and an (optional) acknowledgement number• Sequence number counts bytes

transmitted

• Send cumulative positive acknowledgements• Acknowledgement specifies the next byte expected

• Only acknowledge contiguously data packets (sliding window protocol, so several data packets in flight)

• Duplicate acknowledgement implies loss

• Retransmit lost packets

Host A

Tim

e

Host B

Tim

e

x

seq = 5seq = 6seq = 7seq = 8seq = 9

seq = 10seq = 11

ack = 6ack = 7

ack = 8

ack = 8ack = 8

ack = 8seq = 8seq = 9

16

Example: TCP

• Packet reordering also causes duplicate ACKs• Gives appearance of loss, when

the data was merely delayed

• TCP uses triple duplicate ACK to indicate loss• Three identical ACKs in a row

• Slightly delays response to loss, but makes TCP more robust to reordering

Host ATi

me

Host B

Tim

e

seq = 5seq = 6seq = 7seq = 8seq = 9

seq = 10seq = 11

ack = 6ack = 7ack = 7

ack = 10ack = 11ack = 12

ack = 9

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Example: TCP

• Problem with cumulative ACKs: don’t signal any packets received after the highest contiguously received packet• E.g. if packets 1, 2, 3, 5, 6, and 7 are received, the ACK will show 4 as the

next packet outstanding, but won’t mention packets 5, 6, and 7

• Leads to unnecessary retransmissions

• Solution: Selective ACK (“SACK”) option to TCP

• But only supported by 68% of web servers, ten years after standardisation... [Medina, Allman, & Floyd, ACM CCR, Apr 2005]

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Connection Tear Down

• Three way handshake to tear down a connection

• What happens if the last ACK is lost?• A has closed the connection, so cannot

resend the ACK; B is still waiting

• Unavoidable problem → B must eventually give up, without knowing if the last packet arrived

• Data sent on last packet is potentially lost

FIN, ACK = x, seq = y

FIN, seq = x

Host A

Tim

e

Host B

Tim

e

ACK = y

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TCP State Machine

Established

SYN recv’d

Closing

Time Wait

Close wait

Last ACK

FIN wait 1

FIN wait 2

Closed

Close/FIN Close/FIN FIN/ACK

Close/FIN

FIN/ACK

FIN+ACK/ACK

FIN/ACKACK/- ACK/-

ACK/-

Timeout/-

(passive close)(active close)

• TCP uses a three way handshake to close connection

• Signalled by the FIN bit in the packet header

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TCP State Machine

Established

Listen

Closed

SYN recv’d SYN sent

Closing

Time Wait

Close wait

Last ACK

FIN wait 1

FIN wait 2

Closed

Connect/SYN

Close/-Close/-Listen/-

SYN/SYN+ACK

RST/- Send/SYN

SYN/SYN+ACK

SYN+ACK/ACKACK/-

Close/FIN Close/FIN FIN/ACK

Close/FIN

FIN/ACK

FIN+ACK/ACK

FIN/ACKACK/- ACK/-

ACK/-

Timeout/-

(passive close)(active close)

(simultaneous open)

Complete TCP finite state machine: connection establishment and tear down

Note: RFC 793 has a mistake in this diagram 21

TCP Connection Progress

Open connection:192.168.0.4.49159 > 130.209.240.1.80: S 1033471698:1033471698(0) win 65535130.209.240.1.80 > 192.168.0.4.49159: S 3518203430:3518203430(0) ack 1033471699 win 5792192.168.0.4.49159 > 130.209.240.1.80: . ack 3518203431 win 65535Send “GET /index.html HTTP/1.1”:192.168.0.4.49159 > 130.209.240.1.80: P 1033471699:1033471725(26) ack 3518203431 win 65535130.209.240.1.80 > 192.168.0.4.49159: . ack 1033471725 win 5792Send “Host: www.dcs.gla.ac.uk”:192.168.0.4.49159 > 130.209.240.1.80: P 1033471725:1033471746(21) ack 3518203431 win 65535130.209.240.1.80 > 192.168.0.4.49159: . ack 1033471746 win 5792Send blank line:192.168.0.4.49159 > 130.209.240.1.80: P 1033471746:1033471748(2) ack 3518203431 win 65535130.209.240.1.80 > 192.168.0.4.49159: . ack 1033471748 win 5792Receive web page:130.209.240.1.80 > 192.168.0.4.49159: . 3518203431:3518204879(1448) ack 1033471748 win 5792130.209.240.1.80 > 192.168.0.4.49159: . 3518204879:3518206327(1448) ack 1033471748 win 5792192.168.0.4.49159 > 130.209.240.1.80: . ack 3518206327 win 65160Server closes connection:130.209.240.1.80 > 192.168.0.4.49159: FP 3518206327:3518207344(1017) ack 1033471748 win 5792192.168.0.4.49159 > 130.209.240.1.80: . ack 3518207345 win 64143130.209.240.1.80 > 192.168.0.4.49159: . ack 1033471748 win 5792192.168.0.4.49159 > 130.209.240.1.80: F 1033471748:1033471748(0) ack 3518207345 win 65535130.209.240.1.80 > 192.168.0.4.49159: . ack 1033471749 win 5792

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Questions?

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