Chapter 12 Transmission Control Protocol (TCP)

1. Chapter 12 Transmission Control Protocol (TCP) Mi-Jung Choi Dept. of Computer Science and Engineering mjchoi@postech.ac.kr

2. Contents 12.1 TCP SERVICES 12.2 TCP FEATURES 12.3 SEGMENT 12.4 A TCP CONNECTION 12.5 STATE TRANSITION DIAGRAM 12.6 FLOW CONTROL 12.7 ERROR CONTROL 12.8 CONGESTION CONTROL 12.9 TCP TIMERS 12.10 OPTIONS 12.12 TCP PACKAGE

3. Objectives • Be able to name and understand the services offered by TCP • Understand TCP’s flow and error control and congestion control • Be familiar with the fields in a TCP segment • Understand the phases in a connection-oriented connection • Understand the TCP transition state diagram • Be able to name and understand the timers used in TCP • Be familiar with the TCP options

4. TCP/IP Protocols

5. TCP/IP Functions • To create a process-to-process communication (using port numbers) • To create a flow control mechanism at the transport level (using sliding window) • To create a error control mechanism at the transport level (using Ack packet, time-out, retransmission) • Sequence control mechanism • A connection oriented, reliable transport protocol

6. 12.1 TCP SERVICES • We explain the services offered by TCP to the processes at the application layer • The topics discussed in this section include: • Process-to-Process Communication • Stream Delivery Service • Full-Duplex Communication • Connection-Oriented Service • Reliable Service

7. Process-to-Process Communication • For client/server communication we must define the • Local host • Local client program • Remote host • Remote server program

8. PROCESS-TO-PROCESS COMMUNICATION (cont.) • Port numbers : ~ mentioned in UDP chapter

9. PROCESS-TO-PROCESS COMMUNICATION (cont.) • Well-known port in TCP

10. Example 1 As we said in Chapter 11, in UNIX, the well-known ports are stored in a file called /etc/services. Each line in this file gives the name of the server and the well-known port number. We can use the grep utility to extract the line corresponding to the desired application. The following shows the ports for FTP. $ grep ftp /etc/services ftp-data 20/tcpftp-control 21/tcp

11. PROCESS-TO-PROCESS COMMUNICATION (cont.) • Socket addresses ~ a combination of IP address and port number ~ to make a connection for each end ~ to need a pair of socket addresses: client and server socket address • These four pieces of information are part of the IP header (IP address) and TCP header (port number)

12. 12.1 TCP SERVICES (cont.) • Stream delivery service • Sending and receiving buffers • Segments • Full-Duplex service • piggybacking • Connection-Oriented service • A virtual connection (not physical connection) • Reliable service • Reply acknowledge packet

13. Sending TCP ~ receives data as a stream of bytes from application process using sending buffer ~ make data to appropriate segments and transfer to network Receiving TCP ~ receives segments using receiving buffer ~ reassemble segments to data and send data as a stream of bytes to application process Stream delivery

14. Sending and receiving buffers The sending process and the receiving process may not produce and consume data at the same speed, TCP needs buffers for storage

15. TCP segments IP layer, as a service provider for TCP, needs to send data in packets, not as a stream of bytes. TCP groups a number of bytes together into a packet called segment

16. 응용 응용 응용 응용 UDP UDP 다중화 역다중화 datagram datagram datagram datagram UDP vs. TCP communication

17. 12.2 TCP FEATURES • To provide the services mentioned in the previous section, TCP has several features that are briefly summarized in this section. • The topics discussed in this section include: • Numbering System • Flow Control • Error Control • Congestion Control

18. Numbering system • TCP keeps track of the segment being transmitted or received using sequence number and acknowledge number • These number is used for flow and error control • The bytes of data being transferred in each connection are numbered by TCP • The numbering starts with a randomly generated number (b/w 0 ~ 232-1)

19. Numbering system • When TCP receives bytes of data from the process and stores them in sending buffer • After numbering the bytes, TCP assigns sequence number to each segment that is being sent • The value of the sequence number field in a segment defines the number of the first data byte contained in that segment • The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receives. The acknowledgment number is cumulative

20. Example 2 • Suppose a TCP connection is transferring a file of 5000 bytes. The first byte is numbered 10001. What are the sequence numbers for each segment if data is sent in five segments, each carrying 1000 bytes? • Solution: The following shows the sequence number for each segment: Segment 1➡ Sequence Number: 10,001 (range: 10,001 to 11,000) Segment 2➡ Sequence Number: 11,001 (range: 11,001 to 12,000) Segment 3➡ Sequence Number: 12,001 (range: 12,001 to 13,000) Segment 4➡ Sequence Number: 13,001 (range: 13,001 to 14,000) Segment 5➡ Sequence Number: 14,001 (range: 14,001 to 15,000)

21. 12.3 SEGMENT A packet in TCP is called a segment The topics discussed in this section include: • Format • Encapsulation

22. 12.3 SEGMENT • The unit of data transfer b/w 2 devices using TCP is a segment

23. 12.3 SEGMENT (cont.) • Segment • source port address : the port number of the application program in the sender’s host • destination port address : the port number of the application program in the receiver’s host • sequence number : the number of the 1st byte of data in this segment • acknowledgement number : the byte number that the receiver is expecting to receive from the other party • header length : 4 bytes • control : • window size: 16 bits • checksum : all segment including pseudo-header • urgent pointer : • option :

24. Flag Description ----- -------------- URG Urgent pointer field is valid ACK Acknowledgment field is valid PSH Push the data RST Connection must be reset SYN Synchronize sequence numbers FIN Terminate the connection 12.3 SEGMENT (cont.) • Control

25. 12.3 SEGMENT (cont.) • Control • URG: 긴급 포인터가 유효함 • ACK: 확인 응답 번호가 유효함 • PSH: 수신측은 데이터를 가능한 빨리 응용으로 보내야 함 • RST: 연결을 재설정 • SYN: 연결을 초기화히기 위해 순서 번호를 동기화 • FIN: 송신측이 데이터 전송을 종료함

26. Pseudoheader added to the TCP datagram The inclusion of the checksum in TCP is mandatory

27. Encapsulation and decapsulation

28. 12.4 TCP CONNECTION • TCP is connection-oriented. A connection-oriented transport protocol establishes a virtual path between the source and destination. All of the segments belonging to a message are then sent over this virtual path. A connection-oriented transmission requires three phases: connection establishment, data transfer, and connection termination. • The topics discussed in this section include: • Connection Establishment • Data Transfer • Connection Termination • Connection Reset

29. 12.4 TCP CONNECTION • TCP: connection oriented • Establishment the VC b/w source TCP and destination • Connection establishment and termination • Connection establishment • 3 단계 수행 • 호스트 A는 호스트 B에게 연결 설정 세그먼트를 전송(초기화 정보) • 호스트 B는 호스트 A에게 확인응답 세그먼트 와 초기화 정보 세그먼트 전송 • 호스트 A는 호스트 B에게 확인응답 세그먼트 전송 • Connection termination • 4단계 수행 • 호스트 A는 연결 종료를 알리고 세그먼트 전송 • 호스트 B는 A의 요구에 대해 확인 응답하는 세그먼트 전송 • 호스트 B는 연결 종료를 알리는 세그먼트 전송 • 호스트 A는 B의 요구에 대해 확인응답

30. Connection establishment using three-way handshaking

31. Connection establishment using three-way handshaking • A SYN segment cannot carry data, but it consumes one sequence number. • A SYN + ACK segment cannot carry data, but does consume one sequence number. • An ACK segment, if carrying no data, consumes no sequence number.

32. Data transfer The FIN segment consumes one sequence number if it does not carry data

33. Connection termination using three-way handshaking The FIN + ACK segment consumes one sequence number if it does not carry data

34. Half Close • One end stops sending data while still receiving data. • Normally initiated by client. • It can occur when the server needs all data before processing can begin • Sorting example • The client, after sending all data to be sorted, can close the connection in the outbound direction. • However, the inbound direction must remain open to receive the sorted data.

35. Connection Reset • The TCP at end may deny a connection request, may abort a connection, or may terminate an idle connection. All of these are done with the RST (reset) flag. • Denying a connection • Aborting a connection • Terminating an idle connection

36. 12.5 STATE TRANSITION DIAGRAM To keep track of all the different events happening during connection establishment, connection termination, and data transfer, the TCP software is implemented as a finite state machine. The topics discussed in this section include: • Scenarios

37. State Description ----- -------------- CLOSED There is no connection LISTEN The server is waiting for calls from the client SYN-SENT A connection request is sent; waiting for acknowledgment SYN-RCVD A connection request is received ESTABLISHED Connection is established FIN-WAIT-1 The application has requested the closing of the connection FIN-WAIT-2 The other side has accepted the closing of the connection CLOSING Both sides have decided to close simultaneously TIME-WAIT Waiting for retransmitted segments to die CLOSE-WAIT The server is waiting for the application to close LAST-ACK The server is waiting for the last acknowledgment 12.5 STATE TRANSITION DIAGRAM • finite state machine • At any moment, the machine is in one of the state • TCP states

38. server client SYN SYN +ACK ACK data ack FIN ACK FIN ACK STATE TRANSITION DIAGRAM - state : ovals - Transition from one to another state : directed line - values on line input / output - Dotted line : server - Solid line : client - Thin line : unusual situation

39. Connection scenario The common value for MSL (Maximum Segment Lifetime) is between 30 seconds and 1 minute

40. STATE TRANSITION DIAGRAM Client states Server states

41. Connection termination using three-way handshake

42. Simultaneous Open

43. Simultaneous Close

44. Denying a Connection

45. Aborting a connection

46. TCP OPERATION • Encapsulation and decapsulation

47. TCP OPERATION (cont.) • Buffering

48. TCP OPERATION (cont.) • Multiplexing and demultiplexing