The Transmission Control Protocol (TCP)

Application Services

(Telnet, FTP, e-mail, WWW)

Reliable Stream

Transport (TCP)

Connectionless Packet Delivery Service

(IP)

Unreliable Transport

Service (UDP)

The Transmission Control Protocol (TCP)

• TCP is a protocol that specifies:

– How to distinguish among multiple destinations on a given machine

– How to initiate and terminate a stream transfer

– Format of the data and acknowledgments that two computers exchange to achieve a reliable transfer

– Procedures the computers use to ensure that the data arrives correctly

Distinguishing AmongMultiple Destinations

• Like UDP, TCP uses protocol ports to identify the ultimate destination within a machine

• Unlike UDP, TCP defines a connection as the fundamental abstraction for data transfer

• Connection:– Virtual circuit– Identified by a pair of endpoints

Initiating Stream Transfers

• Virtual Circuit - simulate a circuit switched network over a packet-switched network– Sender and receiver interact before transfer begins to

set up the connection– Once the connection is established a stream transfer can

begin– During the transfer, protocol software on the two

machines communicate to ensure that data is delivered correctly

– Upon completion of the transfer the connection is closed

Initiating Stream Transfers (cont)

• An endpoint for a TCP connection is defined by a (host, port) pair– Host = the IP address of for a host

– Port = a TCP port on that host

• A TCP connection is defined by a pair of endpoints:– Port 1037 on www.whitehouse.gov and port 76 on

viper.cs.virginia.edu:• (198.137.240.91, 1037) and (128.143.137.17, 76)

Initiating Stream Transfers (cont)

• Multiple connections to same host:

– Port 1037 on www.whitehouse.gov and port 76 on viper.cs.virginia.edu:

• (198.137.240.91, 1037) and (128.143.137.17, 76)

– Port 355 on falcon.cs.jmu.edu and port 801 on viper.cs.virginia.edu:

• (134.126.10.30, 355) and (128.143.137.17, 801)

Initiating Stream Transfers (cont)

• Multiple connections to same port:

– Port 1037 on www.whitehouse.gov and port 444 on viper.cs.virginia.edu:

• (198.137.240.91, 1037) and (128.143.137.17, 444)– Port 355 on falcon.cs.jmu.edu and port 444 on

viper.cs.virginia.edu:• (134.126.10.30, 355) and (128.143.137.17, 444)

• No ambiguity - connection identified by both endpoints

Initiating Stream Transfers (cont)

• Both endpoints must agree to participate:– Passive open - one application program contacts its

O.S. and indicates that it will accept an incoming connection

– Active open - application program at other end contacts its O.S. and requests a connection

• Both TCP software modules cooperate to establish a connection

• Application programs can transmit data (TCP provides reliability)

TCP Data Stream Format

• Data from application programs is a sequence of octets (with no type associated)

• TCP divides data into segments for transmission

• Usually, each segment travels across the internet in a single IP datagram

Review: Unreliable PacketDelivery Service

• Benefits:– Fast

– Cost-effective

• Drawbacks:– Packet loss, corruption, delay, duplication, out-of-order

delivery

– Sender might transmit faster than receiver can receive

– Dictates a set packet size

The Need for a Reliable Stream Delivery Service

• Application-level programmers don’t want:– To have to provide reliability in each application

– To be constrained by fixed-size packets

• Application-level programmers do want:– Reliability

– Unstructured, stream-oriented service

– Full duplex virtual circuit connection

– Buffered transfer

Providing Reliability with Acknowledgments and Retransmissions

Sender Network Receiver

Receive ACK 2

Send packet 1

Send ACK 1

Receive packet 1

Send packet 2

Receive ACK 1

Receive packet 2

Send ACK 2

Packet Loss

Sender Network ReceiverSend packet 1

Send packet 1

Receive ACK 1

Receive packet 1

Send ACK 1

(Timeout)

ACK Loss

Sender Network ReceiverSend packet 1

Send packet 1

Receive ACK 1

(Timeout)

Send ACK 1

Receive packet 1

Receive packet 1

Send ACK 1(Discard)

ACK Delayed

Sender Network ReceiverSend packet 1

Receive ACK 1

Receive ACK 1

(Timeout)

Send packet 1

(Discard)

Send ACK 1

Receive packet 1

Receive packet 1

Send ACK 1

(Discard)

A Problem

A simple positive acknowledgment protocol wastes a substantial amount of network bandwidth because it must delay sending a new packet until it receives an acknowledgment for the previous packet.

A Solution

Sender Network Receiver

1

2

3 ACK1

ACK2

ACK3

Sliding Windows

1 2 3 4 5 6 7 8 9 10

All packets inside the window can be transmitted

When an acknowledgment for packet 1 is received the window slides to the right and allows the transmission of packet 9:

1 2 3 4 5 6 7 8 9 10

window

Sliding Windows (cont)

• Receiver must keep a sliding window as well:

• After packet 1 is received and acknowledged:

1 2 3 4 5 6 7 8 9 10

window

1 2 3 4 5 6 7 8 9 10

Sliding Windows (cont)

• Conceptually partitions the packets into three classes:– Transmitted, received, and acknowledged packets

(outside and left of sliding window)– Packets being transmitted (inside the window)– Packets waiting to be transmitted (outside and right of

sliding window)

1 2 3 4 5 6 7 8 9 10

Sliding Windows (cont)

• Only unacknowledged packets are retransmitted– Keep separate timer for each packet

• Performance:– Depends on the window size and the speed of the

underlying network

– Note: sliding window protocol with window size = 1 is the same as the simple positive acknowledgment protocol we saw earlier

Sliding Windows: Performance

• Tuning the sliding window protocol

– Increase window size so that sender is transmitting packets as fast as the network can carry them

– Eliminates network idle time

– Increases throughput (as compared to the simple positive acknowledgment protocol)

Ideal Sliding Window Performance

Sender Network Receiver

TCP - Efficient Transmission and Flow Control

• Uses a specialized sliding window mechanism• Multiple segments can be sent before an

acknowledgment arrives– Efficient transmission

• Allows receiver to restrict transmission until it has sufficient buffer space to accommodate more data– Flow control

TCP Sliding Windows

• Operates at the octet level• Octets of the data stream are numbered sequentially

1 2 3 4 5 6 7 8 9 10

Octets 1&2 have been sent and acknowledged

Octets 3-6 sent but

not acknowledged

Octets 7&8 can be sent

TCP Sliding Windows

• Four per (full duplex) connection:– A sends to B– A receives from B– B sends to A– B receives from A

TCP Sliding Windows - Flow Control

• Each window can vary in size over time– Each ACK contains a window advisement– Specifies how many additional octets of data

the receiver is willing to accept– Sender increases or decreases sending window

sized based on the receiver’s advise

• Provides end-to-end flow control

TCP Sliding Windows - Flow Control Example

• Sender transmits 3 octets

• Receives an ACK 3 with window advisement = 3

• Sender transmits 1 octet

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14

TCP Sliding Windows - Flow Control Example (cont)

• Receives an ACK 4 with window advisement = 7

• Sender transmits 4 octets

• Receives an ACK 8 with window advisement = 4

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Summary

• TCP is a transport protocol that specifies:

– How to distinguish among multiple destinations on a given machine

– How to initiate and terminate a stream transfer

– Format of the data and acknowledgments that two computers exchange to achieve a reliable transfer

– Procedures the computers use to ensure that the data arrives correctly