1 Transport Layer Computer Networks

2 Where are we?

3 Recall Network Layer Provides host-to-host communication Source and destination addresses identify host interfaces Machine-to-machine networking

4 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

5 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:

6 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.

7 Relationship Between TCP and Other Protocols TCP on one computer uses IP to communicate with TCP on another computer

8 Apparent Contradiction IP offers best-effort (unreliable) delivery TCP uses IP TCP provides completely reliable transfer How is this possible?

9 Achieving Reliability Reliable connection setup Reliable data transmission Reliable connection shutdown

10 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

11 Retransmission Illustrated

12 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

13 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.

14 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 TCP ’ s success

15 Adaptive Retransmission Illustrated Timeout depends on current round-trip estimate

16 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

17 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

18 Window Advertisement Illustrated

19 Another View: Sliding Window Illustrated

20 Startup and Shutdown Connection Startup Must be reliable Connection Shutdown Must be graceful Difficult

21 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

22 TCP’s 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

23 3-Way Handshake Illustrated

24 TCP’s Shutdown Illustrated

25 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

26 More Complete Illustration of the 3-Way Handshake

27 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

28 Application Multiplexing Illustrated

29 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

30 Protocol Port Example Web server application is assigned port 80 Web client application obtains port TCP segment sent from client to server has source port number destination port number 80 When web server responds, TCP segment has source port number 80 destination port number 32938

31 Standard Protocol Ports See for standard protocol port assignments See /etc/services in UNIX systems and \winnt\system32\drivers\etc\services in Windows NT

32 TCP Segment Format

33 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)

34 UDP Message Format