1. CS4470 Computer Networking Protocols
11/30/2018
CS Computer Networking Protocols
14. TCP
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline TCP services TCP segment format
11/30/2018
Outline
TCP services
TCP segment format
TCP connection establishment and termination
TCP state diagram
14. TCP CS4470

3. TCP services Process-to-Process communication
11/30/2018
TCP services
Process-to-Process communication
Using port number
Full-duplex communication
Data can flow in both directions at the same time
Stream delivery service
Connection-oriented service
14. TCP CS4470

4. Stream delivery service
TCP is a stream oriented protocol
It allows the sending process to deliver data as a stream of bytes and the receiving process to obtain data as stream of bytes
The two processes seem to be connected by an imaginary tube that carries their data across the Internet
Note: most of the following diagrams are from chap12 , “TCP/IP Protocol Suite, 3rded”
14. TCP CS4470

5. Stream delivery service
Both the sender and the receiver keep two buffers: sending buffer and receiving buffer
They may not write or read data at the same speed
TCP need buffers for storage
14. TCP CS4470

6. Stream delivery service
At the sending site
The buffer has 3 types of chambers
The white area empty chambers that can be filled by the sending process
The blue area holds bytes that have been sent but not yet acknowledged.
TCP keeps these bytes in the buffer until it receives an ACK
After the bytes are acked, the chambers are recycled and available for use by the the sender
The pink area contains bytes to be sent by the sending TCP
At the receiver site
The white area contains empty chambers to be filled by bytes received from the network
The colored sections contain received bytes that can be read by the receiving process
When a byte is read by the receiving process, the chamber is recycled and added to the pool of empty chambers
14. TCP CS4470

7. Stream delivery service
The IP layter needs to send data in packets, not as a stream of bytes
TCP groups a number of bytes together into a packet called a segment
The segments are not necessarily the same size
TCP adds a header to each segment and delivers the segment to the IP layer for transmission
The segments are encapsulated in an IP datagram and transmitted
The entire operation is transparent to the receiving process
14. TCP CS4470

8. TCP segment structure
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9. Seq. Num. and Ack. Num. Byte number Sequence number
TCP numbers all data bytes that that are transmitted in a connection
Numbering is independent in each direction
The numbering does not necessarily start from 0
TCP generates a random number for the first byte
Sequence number
After bytes are numbered, TCP assigns a sequence number to each segment that is being sent.
The sequence number for each segment is the byte number of the first byte carried in that segment
14. TCP CS4470

10. Example
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?
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)
14. TCP CS4470

11. Seq. Num. and Ack. Num. Acknowledge number for a segment
The sequence number of the next byte the receiver is expecting from the other party
Ex. A receives a segment (byes ) from B
A puts 536 in the ack. Num. field of the segment it sends to B
TCP provides cumulative acknowledgments
Ex. A receives a segment(0-535) and another segment (900-1,000) from B
A’s next segment to B will contain 536 in the ack. Num. field
14. TCP CS4470

12. Example: Telnet
A popular application-layer protocol used for remote login
It runs over TCP
It’s an interactive application
The user at the client side typed a character
The character is sent to the server
The server sends the same character back to the client
The “echo back” ensures that the characters seen by the telnet user have already been received and processed at the remote site
Each character traverses the network twice
14. TCP CS4470

13. simple telnet scenario
Seq. Num. and Ack. Num.
Host A
Host B
User
types
‘C’
Seq=42, ACK=79, data = ‘C’
host ACKs
receipt of
‘C’, echoes
back ‘C’
Seq=79, ACK=43, data = ‘C’
host ACKs
receipt
of echoed
‘C’
Seq=43, ACK=80
time
simple telnet scenario
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14. Exercise Host A Host B Seq=42, ACK=79 100 bytes
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15. TCP segment structure
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16. Control field The field defines 6 different control bits
These bits enable flow control, connection establishment and termination, connection abortion, and the mode of data transfer
14. TCP CS4470

17. TCP is connected-oriented
When a process A at site A wants to send and receive data from another process at B
The two TCP establish a connection between them
Data are exchanged in both direction
The connection is terminated
The connection is a virtual connection
Both parties keep buffers and some state information (variables)
The connection information is invisible to the intermediate routers
14. TCP CS4470

18. Connection establishment
TCP transmits data in full-duplex mode
Each party must initialize communication and get approval from the other party before any data is transferred
Server and client
The connection establishment is TCP is called three-way handshaking
Server request for a passive open
The server tells its TCP that it’s ready to accept a connection
Client issues a request for an active open
TCP can now starts a three-way handshake process
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19. Three-way handshake
1. The client sends the first segment, a SYN segment
SYN is set to 1
specifies initial seq. number: client_isn (8000)
no data
2. Server receives SYN
server allocates buffers and variables
replies with SYNACK segment
Both ACK and SYN are set to 1
specifies server initial seq. number: server_isn (15000)
ack = client_isn+1
No data
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20. Three-way handshake 3. client receives SYNACK
allocate buffers and variables
replies with ACK segment
ACK is set to 1
SYN is set to 0
ack =server_isn+1
The segment may contain data
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21. Three-way handshake
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22. Connection terminate Any of the two parties can close the connection
Two options for connection termination
Three-way handshake
Normal situation
Four-way handshake with a half-close option
One end can stop sending data while still receiving data
Either the server or the client can issue a half-close requestion.
14. TCP CS4470

23. Connection termination using three-way handshake
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24. Connection termination using four-way handshake (half close)
Seg: y-1
The 2nd seg consumes no sequence number
Though the client has received sequence number y-1 and is expecting y, the server sequence number is still y-1
Then the connection finally closes, the sequence number of the last ACK is still x, because no sequence number are consumed during data transfer in that direction
14. TCP CS4470

25. TCP State 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
A finite state machine is a machine that goes through a limited number of states
At any moment, the machine is in one of the states
It remains in that state until an event happens
The event can take the machine to a new state and can also make the machine perform some actions
14. TCP CS4470

26. TCP State diagram
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27. TCP state transition diagram
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28. TCP state transition diagram
The ovals represent the states
The directed lines represent the transition from one state to another
Each line has two strings separated by a slash
The first string is the input, what TCP receives
The second string is the output, what TCP sends
The diagram is for both client and server
The dotted black lines represent the transition that a server normally goes through
The solid black lines shows the transitions that a client normally goes through
In some situations, a server transitions through a solid line or a client transitions through a dotted line
The colored lines show special situations
14. TCP CS4470

29. Common scenario 1 Connection termination using 4-way handshake
The common value for MSL is between 30 seconds and 1 minute
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30. Common scenario 2 Connection termination using 3-way handshake
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31. Common scenario 3:Simultaneous close
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32. Common scenario 4: Denying a connection
14. TCP CS4470