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Chapter 24-25

Chapter 24-25. Internetworking Part 4 (Transport Protocols, UDP and TCP, Protocol Port Numbers). Transport Protocol. Separate layer of protocol stack Conceptually between Application and IP. Terminology. IP Provides computer-to-computer communication

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Chapter 24-25

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  1. Chapter 24-25 Internetworking Part 4 (Transport Protocols, UDP and TCP, Protocol Port Numbers)

  2. Transport Protocol • Separate layer of protocol stack • Conceptually between • Application and • IP

  3. Terminology • IP • Provides computer-to-computer communication • Source and destination addresses are computers • Called machine-to-machine • Transport Protocols • Provide application-to-application communication • Need extended addressing mechanism to identify applications • Called end-to-end

  4. Transport Protocol Functionality • Identify sending and receiving applications • Optionally provide • Reliability • Flow control • Congestion control • Note: not all transport protocols provide above facilities

  5. Two main Transport Protocols available • Transmission Control Protocol (TCP) • User Datagram Protocol (UDP) • Major differences • Interface to applications • Functionality

  6. Other Transport Protocols • Datagram Congestion Control Protocol (DCCP) • DCCP is useful for applications with timing constraints on the delivery of data that may become useless to the receiver if reliable in-order delivery combined with congestion avoidance is used. Such applications include streaming media, multiparty online games and Internet telephony.

  7. Other Transport Protocols • Stream Control Transmission Protocol (SCTP) Serving in a similar role as the popular protocols TCP and UDP. SCTP provides some of the same service features of both, ensuring reliable, in-sequence transport of messages with congestion control. SCTP is transaction-oriented and supports multi-streaming.

  8. User Datagram Protocol (UDP) • Provides unreliable transfer • Requires minimal • Overhead • Computation • Communication • Best for LAN applications

  9. UDP Details • Connectionless service paradigm • Message-oriented interface • Each message encapsulated in IP datagram • UDP header identifies • Sending application • Receiving application

  10. Identifying An Application • Cannot extend IP address • No unused bits • Cannot use OS-dependent quantity • Process ID • Task number • Job name • Must work on all computer systems

  11. Identifying an Application (continued) • Invent new abstraction • Used only with TCP/IP • Identifies sender and receiver unambiguously • Technique • Each application assigned unique integer • Called protocol port number

  12. Protocol Ports • Server • Follows standard • Always uses same port number • Uses lower port numbers • Client • Obtains unused port from protocol software • Uses higher port numbers

  13. Protocol Ports • The port numbers are divided into three ranges: the Well Known Ports, the Registered Ports, and the Dynamic and/or Private Ports. • Well Known Ports: 0 through 1023. For privileged users. • Registered Ports: 1024 through 49151. For normal users. • Dynamic and/or Private Ports: 49152 through 65535

  14. Well known Port number samples • ftp-data 20/tcp File Transfer [Default Data] • ftp-data 20/udp File Transfer [Default Data] • ftp 21/tcp File Transfer [Control] • ftp 21/udp File Transfer [Control] • ssh 22 Remote terminal • telnet 23 Remote terminal • DNS 53 Name resolution • http 80 Transfer of hypertext docs • pop3 110 Fetching email

  15. Protocol Port Example • Domain name server application is assigned port 53 • Application using DNS obtains port 28900 • UDP datagram sent from application to DNS server has • Source port number 28900 • Destination port number 53 • When DNS server replied, DP datagram has • Source port number 53 • Destination port number 28900

  16. Transmission Control Protocol (TCP) • Major transport protocol used in Internet • Heavily used • Completely (?) reliable transfer

  17. TCP Features • Connection-oriented service • Point-to-point • Full-duplex communication • Stream interface • Stream divided into segments for transmission • Each segment encapsulated in IP datagram • Uses protocol ports to identify applications

  18. TCP Feature Summary TCP 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.

  19. Relationship Between TCP And Other Protocols • TCP on one computer uses IP to communicate with TCP on another computer

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

  21. Achieving Reliability • Reliable connection startup • Reliable data transmission • Graceful connection shutdown

  22. Reliable Data Transmission • Positive acknowledgment • Receiver returns short message when data arrives • Called acknowledgment • Retransmission • Sender starts timer whenever message is transmitted • If timer expires before acknowledgment arrives, sender retransmits message

  23. Illustration of Retransmission

  24. How Long Should TCP WaitBefore Retransmitting? • Time for acknowledgment to arrive depends on • Distance to destination • Current traffic conditions • Multiple connections can be open simultaneously • Traffic conditions change rapidly

  25. Important Point The delay required for data to reach a destination and an acknowledgment to return depends on traffic in the internet as well as the distance to the destination. Because it allows multiple application programs to communication with multiple destinations concurrently, TCP must handle a variety of delays that can change rapidly

  26. 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

  27. Illustration of Adaptive Retransmission • Timeout depends on current round-trip estimate

  28. TCP Flow Control • Receiver • Advertises available buffer space • Called window • Sender • Can send up to entire window before Ack arrives

  29. Window Advertisement • Each acknowledgment 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

  30. Flow control and lost segments • If a segment is lost, the window does not advance until the segment is retransmitted and the sender receives the acknowledgment for the segment. • When the receiver gets a segment out of order, it can send an acknowledgment for the highest received data byte without sending individual acknowledgments for the intermediate data.

  31. Illustration of Window Advertisement

  32. Startup and Shutdown • Connection startup • Must be reliable • Connection shutdown • Must be graceful • Difficult

  33. Why Startup/Shutdown 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

  34. TCP’s Startup/Shutdown Solution • Uses three-message exchange • Known as 3-way handshake • Necessary and sufficient for • Unambiguous, reliable startup • Unambiguous, graceful shutdown • SYN used for startup • FIN used for shutdown

  35. Illustration of 3-Way Handshake

  36. Congestion Control • CongestionPacket loss Retransmission more Congestion ….. Congestion Collapse • Solution: If Packet loss, back off! • Send only one message (even if large window) • If successful, stepwise increase sent data until full window is used again

  37. TCP Segment Format • All TCP segments have same format • Data • Acknowledgment • SYN (startup) • FIN (shutdown) • Segment divided into two parts • Header • Payload area (zero or more bytes of data)

  38. TCP Segment Format (continued) • Header contains • Protocol port numbers to identify • Sending application • Receiving application • Bits to specify items such as • SYN • FIN • ACK • Fields for window advertisement, acknowledgment, etc.

  39. Illustration of TCP Segment • Sequence number specifies where in stream data belongs • Few segments contain options

  40. Summary • Transport protocols fit between applications and Internet Protocol • Two transport protocols in TCP/IP suite • User Datagram Protocol (UDP) • Transmission Control Protocol (TCP) • UDP • Unreliable • Message-oriented interface

  41. Summary (continued) • TCP • Major transport protocol used in Internet • Complete reliability • Stream-oriented interface • Uses adaptive retransmission

  42. Summary (continued) • Protocol ports • Integers • Used to identify sending and receiving applications • Allow unambiguous, simultaneous communication with multiple applications

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