Autumn 2000John Kristoff1 Transport Layer Computer Networks.

Autumn 2000 John Kristoff 1

Transport Layer

Computer Networks

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Where are we?

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Recall

Network Layer Provides host-to-host communication Source and destination addresses

identify host interfaces Machine-to-machine networking

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Transport Protocols

Provide application-to-application communication

Need extended addressing mechanism to identify applications

Called end-to-end Optionally provide:

Reliability Flow Control Congestion Control

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Example Transport Layer:Transmission Control Protocol (TCP)

Standardized by IETF as RFC 793 Most popular layer 4 protocol Connection-oriented protocol Conceptually between applications and IP Full-duplex operation Byte-stream interface Of utmost importance for this class!

The book: TCP/IP Illustrated, Volume I - W.R. Stevens Also see: http://condor.depaul.edu/~jkristof/tcp.html

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TCP Feature Summary

Provides a completely reliable (no data duplication or loss), connection-

oriented, full-duplex stream transport service that allows two application programs to form a connection, send data in either

direction and then terminate the connection.

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Relationship Between TCP and Other Protocols

TCP on one computer uses IP to communicate with TCP on another computer

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Apparent Contradiction

IP offers best-effort (unreliable) delivery

TCP uses IP TCP provides completely reliable

transfer How is this possible?

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Achieving Reliability

Reliable connection setup Reliable data transmission Reliable connection shutdown

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Reliable Data Transmission

Positive Acknowledgement Receiver returns short message when data

arrives Call an acknowledgement

Retransmission Sender starts timer whenever message is

transmitted If timer expires before acknowledgement

arrives, sender retransmits message

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Retransmission Illustrated

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How Long Should TCP Wait Before Retransmitting?

Time for acknowledgement to arrive depends on Distance to destination Current traffic conditions

Multiple connections can be open simultaneously

Traffic conditions change rapidly

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Important Point

The delay required for data to reach a destination and an acknowledgement to return depends on traffic in the internet as well as the distance to

the destination. Since it allows multiple application programs to communicate with

multiple destinations concurrently, TCP must handle a variety of delays that can change

rapidly.

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Solving the Retransmission Problem

Keep estimate of round trip time on each connection

Use current estimate to set retransmission timer

Known as adaptive retransmission Key to TCPs success

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Adaptive Retransmission Illustrated

Timeout depends on current round-trip estimate

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TCP Flow Control

Receiver Advertises available buffer space Called the window

Sender Can send up to entire window before

ACK arrives

Also called a sliding window protocol

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Window Advertisement

Each acknowledgement carries new window information Called window advertisement Can be zero (called closed window)

Interpretation: I have received up through X and can take Y more octets

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Window Advertisement Illustrated

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Another View: Sliding Window Illustrated

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Startup and Shutdown

Connection Startup Must be reliable

Connection Shutdown Must be graceful

Difficult

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Why Startup/Shutdown is Difficult

Segments can be lost duplicated delayed delivered out of order either side can crash either side can reboot

Need to avoid duplicate shutdown "message from affecting later connection

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TCPs Startup Solution

Use three-message exchange Known as the 3-way handshake Necessary and sufficient for

unambiguous, reliable startup SYN messages used for connection

establishment

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3-Way Handshake Illustrated

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TCPs Shutdown Illustrated

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Byte Stream Sequencing

Segments are labeled with a sequence number

Protects from out-of-order delivery 32-bit number Limited size of byte stream? Initial Sequence Numbers (ISNs) must be

exchanged at TCP connection establishment

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More Complete Illustration of the 3-Way Handshake

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Application Multiplexing

Cannot extend IP address No unused bits

Cannot use OS dependent quantity Process ID Task number Job name

Must work on all computer systems

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Application Multiplexing Illustrated

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Protocol Ports

Each application assigned a unique integer

Server Follows standard Always uses same port number Usually uses lower port numbers

Client Obtains unused port from protocol software Usually uses higher port numbers

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Protocol Port Example

Web server application is assigned port 80 Web client application obtains port 32938 TCP segment sent from client to server has

source port number 32938 destination port number 80

When web server responds, TCP segment has source port number 80 destination port number 32938

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Standard Protocol Ports

See http://www.iana.org for standard protocol port assignments See /etc/services in UNIX systems and \winnt\system32\drivers\etc\services in Windows NT

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TCP Segment Format

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Example Transport Layer: User Datagram Protocol (UDP)

Unreliable message delivery Connectionless protocol No flow control (no window) No error recovery (no ACKs) Provides application multiplexing Error detection optional (checksum

field)

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UDP Message Format