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

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
CONGESTION CONTROL
TCP TIMERS
OPTIONS
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
Port
Protocol
Description 7
Echo
Echoes a received datagram back to the sender 9
Discard
Discards any datagram that is received 11
Users
Active users 13
Daytime
Returns the date and the time 17
Quote
Returns a quote of the day 19
Chargen
Returns a string of characters 20
FTP,data
File transfer Protocol for data 21
FTP,control
File transfer Protocol for control 23
TELNET
Terminal Network 25
SMTP
Simple Mail Transfer Protocol 53
DNS
Domain Name Server 67
BOOTP
Bootstrap protocol 79
Finger
finger 80
HTTP
Hypertext Transfer Protocol
111
RPC
Remote Procedure Call

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/tcp ftp-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 Full-Duplex 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. Stream delivery Sending TCP Receiving 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

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. UDP vs. TCP communication응용
UDP
역다중화
datagram
다중화

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 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. 12.3 SEGMENT (cont.) Control 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

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. 12.5 STATE TRANSITION DIAGRAM
finite state machine
At any moment, the machine is in one of the state
TCP states
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

38. STATE TRANSITION DIAGRAM
SYN
SYN +ACK
ACK
FIN
data
ack
client
server
- 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