TCP: reliable data transfer

00 sendbase = initial_sequence number

01 nextseqnum = initial_sequence number

03 loop (forever) {

04 switch(event)

05 event: data received from application above

06 create TCP segment with sequence number nextseqnum

07 start timer for segment nextseqnum

08 pass segment to IP

09 nextseqnum = nextseqnum + length(data)

10 event: timer timeout for segment with sequence number y

11 retransmit segment with sequence number y

12 compue new timeout interval for segment y

13 restart timer for sequence number y

14 event: ACK received, with ACK field value of y

15 if (y > sendbase) { /* cumulative ACK of all data up to y */

16 cancel all timers for segments with sequence numbers < y

17 sendbase = y

18 }

19 else { /* a duplicate ACK for already ACKed segment */

20 increment number of duplicate ACKs received for y

21 if (number of duplicate ACKS received for y == 3) {

22 /* TCP fast retransmit */

23 resend segment with sequence number y

24 restart timer for segment y

25 }

26 } /* end of loop forever */

Simplified

TCP

sender