00 sendbase= initial sequence number TcP:可 01 nextseqnum= initial sequence number 03 loop(forever)i 靠数据传 04 switch(event) 05 event: data received from application above create TCP segment with sequence number nextseqnum 输 start timer for segment nextseqnur 08 pass segment to IP 09 nextseqnum= nextseqnum length(data 10 event: timer timeout for segment with sequence number retransmit segment with sequence number compue new timeout interval for segment y 简化的 restart timer for sequence number TCP 14 event: ACK received with acK field value of 15 if y >sendbase)(/* cumulative ACK of all data up to y * 发送方 16 cancel all timers for segments with sequence numbers <y sendbase=y 18 19 else/ a duplicate ACK for already ACKed segment*/ increment number of duplicate ACKs received fory 21 if(number of duplicate ACKS received fory = 3) 22 / tCP fast retransmit * 23 resend segment with sequence number restart timer for segment y 25 3/end of loop forever */ 主讲人:西安交通大学程向前 第4讲传输层之二46主讲人：西安交通大学 程向前 第4讲 传输层之二 4-6 TCP: 可 靠数据传 输 00 sendbase = initial_sequence number 01 nextseqnum = initial_sequence number 02 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 */ 简化的 TCP 发送方