introduction to TCP/IP

2. Introduction
 TCP: Major Transport Protocol in the TCP/IP suite
 Uses unreliable datagram service offered by IP when
sending data to another computer
 Provides reliable data delivery service to applications

3. The Need for Reliable Transport
 Reliability is fundamental in a computer system
 Software in the internet must provide the same level
of reliability as a computer system
 Software must guarantee prompt and reliable
communication without any loss, duplication, and
change in the order

4. Transmission Control Protocol
 Reliability is the responsibility of the Transport Layer
 In TCP/IP, TCP provides reliable transport service
 Most internet applications use TCP as no other
protocol has proved to work better

5. Service provided by TCP
 Connection-Oriented service
 Point-to-point
 Complete Reliability
 Full-duplex communication
 Stream interface
 Reliable Connection Startup
 Graceful Connection Shutdown

6. End-to-End Service and Datagrams
 Applications can request a connection
 TCP connections are called Virtual Connections
 Created by Software only
 Internet does not provide software or hardware support
for the connections
 TCP software modules on two computers create an
illusion of a connection

7. End-to-End Service and Datagrams
 TCP uses IP to carry messages
 TCP message encapsulated in IP datagram and sent to
the destination
 On the destination host, IP passes the contents to TCP

8. End-to-End Service and Datagrams

9. Achieving Reliability
 The major problems in the reliable delivery are:
 Unreliable delivery by the underlying communication
system
 System reboot

10. Packet Loss and Retransmission
 How does TCP achieve Reliability?
 Retransmission
 An Acknowledgement is used to verify that data has
arrived successfully.
 If Acknowledgement does not arrive, the previous data is
retransmitted.

11. Packet Loss and Retransmission

12. How Long Should TCP Wait Before Retransmitting?
 Time for acknowledgement to arrive depends on
 Distance to destination
 Current traffic conditions
 Multiple connections can be opened simultaneously.
 Traffic conditions change rapidly.

13. Adaptive Retransmission
 Set a timer. It sounds so easy, but …What time interval?
 Too long?
– You are spending time waiting for something that is just not
going to happen.
 Too short?
– You will resend needlessly.

14. Adaptive Retransmission
 Keep estimate of round trip time on each connection
 Use current estimate to set transmission timer
 Know as Adaptive Retransmission
 Key to TCP’s success

15. Comparison of Retransmission Times

16. Buffers, Flow Control & Windows

17. Three-Way Handshake to Close a Connection

18. Three-way Handshake to Begin a
Connection
 This will not mean much until you look at the
packets, but
 SYN
– Randomly chosen sequence number, S1:0
 SYN + ACK
– S2:S1+1
 ACK
– S1+1:S2+1

19. Congestion Control
 The goal is to avoid adding retransmissions to an
already congested network
 Reduce window size quickly in response to lost
messages
 Assumption: loss is due to congestion
 Resume carefully. Otherwise the network will swing
wildly between congestion and under utilization

20. TCP Segment Format