1. COMPUTER NETWORKS
CS610
Lecture-36
Hammad Khalid Khan

2. Review Lecture 35 UDP datagram Format UDP Encapsulation
TCP: Reliable Transport Service
The Need For Reliable Transport
Service Provided by TCP
End-to-End Service and Datagrams
Achieving Reliablility

3. 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.

4. Packet Loss and Retransmission

5. 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.

6. 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.

7. 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

8. Comparison of Retransmission Times

9. Buffers, Flow Control & Windows

10. Three-Way Handshake to Close a Connection

11. 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

12. Summary Packet Loss and Retransmission Adaptive Retransmission
Buffers, Flow Control & Windows
Three-Way Handshake