1. Transmission Control Protocol (TCP)
Recall, UDP is a connectionless unreliable datagram transport service
IP, the Network Layer, also only provides a connectionless unreliable delivery
Some applications still have a need a more reliable service
A different protocol is included in the TCP/IP suite to provide applications with a reliable connection oriented transport

2. Position of TCP in TCP/IP protocol suite

3. Properties of TCP Stream Oriented Virtual Circuit Connection
Data from the source process is passed to destination process as a sequenced stream
Bytes arrive to the application in the order send by the source process
Virtual Circuit Connection
Provides the application with the appearance of a dedicated connection between the two processes
There is a call setup
Data transfer phase
Call disconnect
Reliable Stream
TCP performs all error detection and recovery
Applications not concerned with error recovery

4. Stream delivery
Reliable Stream

5. Properties of TCP Buffered transfer Unstructured Stream
Application process generates octets in any size
Transport (TCP) collects bytes to create (possibly) larger datagrams for transport
Bytes collected (buffered) at destination for delivery to destination process
Unstructured Stream
Appears like a continuous stream of bytes
No record boundaries
Full duplex connection
Data may flow in both directions concurrently
Provides for one side to terminate flow in one direction and reduce to half duplex

6. TCP Stream Identification
TCP uses port numbers like UDP
TCP connections are identified by (host, port) pairs or (IP address, port)
Like with UDP, some applications are assigned to well known ports

7. Some Well Known TCP Ports

8. Port numbers
Web
Browser
Web
Server 80
53,125

9. Reliable Transport Recall Data Link reliable service like HDLC
Received data accepted with a positive acknowledgement
A timer is associated with every datagram
If timer expires before acknowledgement arrives, datagram is retransmitted
To improve performance, multiple datagrams can be sent before receiving an acknowledgement
Sliding Window Protocols
Maximum number of unacked datagrams allowed is window size

10. TCP Stream Flow TCP stream is a continuous stream of bytes
Stream is broken down in chunks called segments which vary in size by application
TCP user a timer for each segment sent
Acks are sent for received data, but only after a short delay
Checksum is included in TCP header and covers TCP header, pseudo header, and all data
TCP segments are carried over IP and can arrive out of order and can have duplicates

11. TCP Stream Flow Like HDLC TCP uses a sliding window
Sliding window is measured in bytes, not frames or buffers
TCP references the byte number relative to the beginning of a stream
Acknowledgements reference byte numbers being acknowledged
Acknowledgments do not refer to segments

12. TCP Windows
……. n-1 n ……………. n + w-1 …..
Bytes acknowledged
Sending Window
Sender may send from byte n for up to w bytes before receiving an ack
Window size is w bytes
Sender may break up w bytes into any number of segments
The byte number of the first byte in each segment is included with each segment
The acknowledgment field references the next byte number expected in sequence

13. Sending and receiving buffers

14. TCP segments

15. Sender buffer

16. Receiver window

17. Sender buffer and sender window

18. Sliding the Sender Window
ACK (203)

19. Flow Control When TCP acks data, it indicates the next byte expected
In order to increase or decrease the flow of data TCP also sends a window size
Window size represents the number of bytes the receiver is prepared to receive before any acknowledgement is sent
To slow things down, the receiver can decrease window size
Window size change dynamically during a connection
Receiver must always maintain a buffer of window size

20. Expanding the sender window
ACK (205)
Window 10

21. Shrinking the sender window
ACK (210)
Window 6

22. TCP Flow Control
The TCP sender window is totally controlled by the receiver
Sender does not have to send a full window’s worth of data
Size of sending window can increase or decrease dynamically
Receiver can send an acknowledgment and new window size at any time
Window size can be reduced to zero

23. TCP segment format
IP Header TCP Header Data

24. Sequence Numbers Acknowledgement Numbers
When a connection is established, an initial starting sequence number is agreed upon, usually not 1.
Subsequent segments reference data bytes relative to this starting sequence number
Acknowledgments are always the next bytes expected in the sequence

25. Control field

26. CHECKSUM Calculations
Pseudoheader added to the TCP datagram
CHECKSUM Calculations

27. Making a TCP Connection
Three steps are required to set up a connection
This is called three-way handshake
A segment is sent from A to B
SYN bit is set
Sequence number set to some initial value X
A segment is sent from B to A
ACK bit is set (ack = X +1)
Sequence number set to some value Y
ACK bit is set (ack = Y+1)
Connection is now established

28. Making a TCP Connections
Host A
Host B
SYN seq X
SYN ack(X + 1) seq Y
ack( Y + 1)
Connection Established

29. Closing a Connection Remember a TCP connection is full duplex
A connection must be closed in both directions
If A has no more data to send
A finishes sending data if any
A sends a last segment
FIN bit set
Seq number set to Z
B send segment to A
ACK bit set (ack = Z + 1)
B sends end of file indication to application
Connection now closed in one direction
Data can continue flowing in the other direction

30. Closing a Connection
When it is time to close the connection in the other direction
B sends a segment to A
FIN bit is set
Seq number set to V
A sends a segment to B
ACK bit set (ack = V +1)
A send end of file indication to application
Connection is now closed in both directions

31. Closing a TCP Connection
Host A
Host B
FIN seq Z
ack( Z + 1)
FIN seq V
ack( V + 1)
Connection Closed

32. Resetting a Connection
Closing a connection with a four-way handshake is the normal way of closing TCP connections
There are situations where it is necessary to close connections abnormally or RESET
A sends a segment to B
RST bit is set
B acks the RST segment immediately and closes the connection
B informs application program of termination

33. Examples of Resets
First datagram of the three-way handshake (SYN) to a non existing port
Abnormal condition encountered
Extensively long idle time

34. SYN Flooding Attacks
The three way handshake requires that the server side buffer the initial SYN while waiting for final ACK segment to be received
Servers can handle some fixed number of such pending connections
One common denial of service attack is for to send many initial SYN segments but never send the final ACK segment
This results in many pending connections filling up all possible buffers
Additional legitimate request are denied
Most modern TCPs have mechanisms to protect from this situation

35. Encapsulation and decapsulation

36. Multiplexing and demultiplexing

37. Comparison of UDP/TCP TCP UDP Byte oriented protocol Message oriented
No message boundaries
One stream of data
Reliable protocol
Retransmission of lost data
Flow control
Congestion control
UDP
Message oriented
Message boundaries preserved
Each message independent
Unreliable protocol
No error recovery
No flow control
No congestion control