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that sequence number. This is Jacobson's fast retransmit algorithm, which is followed by his
fast recovery algorithm. We discuss both algorithms in Section 21.7.
Notice that after the retransmission (segment 63), the sender continues normal data
transmission (segments 67, 69, and 71). TCP does not wait for the other end to acknowledge
Let's examine what happens at the receiver. When normal data is received in sequence
(segment 43), the receiving TCP passes the 256 bytes of data to the user process. But the next
segment received (segment 46) is out of order: the starting sequence number of the data
(6913) is not the next expected sequence number (6657). TCP saves the 256 bytes of data
and responds with an ACK of the highest sequence number successfully received, plus one
(6657). The next seven segments received by vangogh (48, 50, 52, 54, 55, 57, and 59) are
also out of order. The data is saved by the receiving TCP, and duplicate ACKs are generated.
Currently there is no way for TCP to tell the other end that a segment is missing. Also, TCP
cannot acknowledge out-of-order data. All vangogh can do at this point is continue sending
the ACKs of 6657.
When the missing data arrives (segment 63), the receiving TCP now has data bytes 66578960 in its buffer, and passes these 2304 bytes to the user process. All 2304 bytes are
acknowledged in segment 72. Also notice that this ACK advertises a window of 5888 (8192 2304), since the user process hasn't had a chance to read the 2304 bytes that are ready for it.
If we look in detail at the tcpdump output for the dips around times 14 and 21 in Figure
21.6, we see that they too were caused by the receipt of three duplicate ACKs, indicating that
a packet had been lost. In each of these cases only a single packet was retransmitted.
We'll continue this example in Section 21.8, after describing more about the congestion
avoidance algorithms. 21.6 Congestion Avoidance Algorithm file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (14 of 29) [12/09/2001 14.47.27] Chapte...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12