TCP IP Illustrated

The quantity enough data is 16 windows of data this

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Unformatted text preview: is also triggered by duplicate ACKs. We receive three more duplicates after the retransmission, so we see three additional increases in cwnd, followed by a decrease to 1280. For the remainder of the transfer cwnd increases linearly to a final value of 3615. 21.9 Per-Route Metrics Newer TCP implementations maintain many of the metrics that we've described in this chapter in the routing table entry. When a TCP connection is closed, if enough data was sent to obtain meaningful statistics, and if the routing table entry for the destination is not a default route, the following information is saved in the routing table entry, for the next use of the entry: the smoothed RTT, the smoothed mean deviation, and the slow start threshold. The quantity "enough data" is 16 windows of data. This gives 16 RTT samples, which allows the smoothed RTT filter to converge within 5% of the correct value. Additionally, the route(8) command can be used by the administrator to set the metrics for a given route: the three values mentioned in the preceding paragraph, along with the MTU, the outbound bandwidth-delay product (Section 20.7), and the inbound bandwidth-delay product. When a new TCP connection is established, either actively or passively, if the routing table entry being used for the connection has values for these metrics, the corresponding variable is initialized from the metrics. 21.10 ICMP Errors Let's see how TCP handles ICMP errors that are returned for a given connection. The most common ICMP errors that TCP can encounter are source quench, host unreach-able, and network unreachable. Current Berkeley-based implementations handle these ICMP errors as follows: q A received source quench causes the congestion window, cwnd, to be set to one segment to initiate slow start, but the slow start threshold, ssthresh, is not changed, so the window will open until it's either open all the way (limited by the window size and round-trip time) or until congestion occurs. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (22 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmis...
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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.

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