TCP IP Illustrated

First recall that the ttl field in the ip header puts

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Unformatted text preview: lems needed to be worked out before including them in TCP. 3. We saw in Section 21.4 that many TCP implementations only measure one round- file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (7 of 20) [12/09/2001 14.47.33] Chapter 24. TCP Futures and Performance trip time per window. They do not measure the RTT of every segment. Better RTT measurements are required for operating on an LFN. The timestamp option, which we describe in Section 24.5, allows more segments to be timed, including retransmissions. 4. TCP identifies each byte of data with a 32-bit unsigned sequence number. What's to prevent a segment that gets delayed in the network from reappearing at a later time, after the connection that it was associated with has been terminated, and after a new connection has been established between the same two hosts and port numbers? First recall that the TTL field in the IP header puts an upper bound on the lifetime of any IP datagram-255 hops or 255 seconds, whichever comes first. In Section 18.6 we defined the maximum segment lifetime (MSL) as an implementation parameter used to prevent this scenario from happening. The recommended value of the MSL is 2 minutes (giving a 2MSL of 240 seconds), but we saw in Section 18.6 that many implementations use an MSL value of 30 seconds. A different problem with TCP's sequence numbers appears with LFNs. Since the sequence number space is finite, the same sequence number is reused after 4,294,967,296 bytes have been transmitted. What if a segment containing the byte with a sequence number N gets delayed in the network and then reappears later, while the connection is still up? This is only a problem if the same sequence number N is reused within the MSL period, that is, if the network is so fast that sequence number wrap occurs in less than MSL. On an Ethernet it takes almost 60 minutes to send this much data, so there is no chance of this happening, but the time required for the wrap to occur drops as the bandwidth...
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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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