TCP: Transmission Control Protocol

1. TCP: Transmission Control Protocol • Overview • Connection set-up and termination • Interactive • Bulk transfer • Timers • Improvements

2. TCP: Overview • Connection oriented, byte stream service • Full or half duplex service • Reliability (ARQ) • Sliding window with variable sized window • Stream is sent in segments (IP datagrams) • SN for bytes • Receiver buffer reorders bytes • Checksum on header and data • Discards duplicate data • Flow control

3. TCP: Overview 65535-20-20=65495 TCP segment IP Header TCP Header TCP Data Source port # Destination port # Sequence # Acknowledgement # HL reserv flags Window size Checksum Urgent pointer Options if any

4. TCP: Flags • URG: The urgent pointer is used • ACK: The acknowledgement number is valid • PSH: The receiver should pass this data to the • application as soon as possible • RST: Reset the connection • SYN: Synchronize sequence numbers to initiate • a connection. • FIN: The sender is finished sending data

5. TCP: Set-Up Syn=1 Ack = 0 A:SYN, MSS, SN=ISN A B B: SYN, MSS, SN=ISN Syn1 ack 1 B: ACK Full duplex A: ACK

6. TCP: Termination A:FIN A B B: ack of FIN B: FIN Both sides close A: ack of FIN

7. TCP windows Actually, min{offered window from receiver, cwnd} usable window 1 2 3 4 5 6 7 8 9 10 11 can send asap sent and acked sent but not acked

12. Transmission window: cwnd cwnd 1 1 ack1 2 2 3 ack2 ack3 4 5 6 7 4 Doubling every RT!

19. TCP Timeout and Retransmission • Each packet has a retransmission timer • It is initialized by the retransmission time out • (RTO) value • When the timer expires, a time out occurs and • the data is retransmitted • If a retransmission fails then the time-out doubles • i.e., exponential backoff. • It’s important to find a good RTO value

34. TCP • Slow start: cwnd =1 • cwnd exponentially increasing • Congestion avoidance: cwnd reaches ssthresh, • cwnd linearly increasing

42. TCP: Tahoe and Reno Tahoe: slow start + congestion avoidance Reno: fast retransmit + fast recovery