TCP IP Illustrated

G at the serial driver also newer modems have large

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Unformatted text preview: interactive traffic may already be buffered in the modem. Waiting 533 ms is unacceptable for interactive response. Human factors studies have found that an interactive response time longer than 100-200 ms is perceived as bad [Jacobson 1990a]. This is the round-trip time for an interactive packet to be sent and something to be returned (normally a character echo). Reducing the MTU of the SLIP link to 256 means the maximum amount of time the link can be busy with a single frame is 266 ms, and half of this (our average wait) is 133 ms. This is better, but still not perfect. The reason we choose this value (as compared to 64 or 128) is to provide good utilization of the line for bulk data transfers (such as large file transfers). Assuming a 5-byte CSLIP header, 256 bytes of data in a 261-byte frame gives 98.1% of the line to data and 1.9% to headers, which is good utilization. Reducing the MTU below 256 reduces the maximum throughput that we can achieve for bulk data transfers. The MTU value listed in Figure 2.5, 296 for a point-to-point link, assumes 256 bytes of data and the 40-byte TCP and IP headers. Since the MTU is a value that IP queries the link layer for, the value must include the normal TCP and IP headers. This is how IP makes its fragmentation decision. IP knows nothing about the header compression that CSLIP performs. Our average wait calculation (one-half the time required to transfer a maximum sized frame) only applies when a SLIP link (or PPP link) is used for both interactive traffic and bulk data transfer. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (10 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm When only interactive traffic is being exchanged, 1 byte of data in each direction (assuming 5-byte compressed headers) takes around 12.5 ms for the round trip at 9600 bits/sec. This is well within the 100-200 ms range mentioned earlier. Also notice...
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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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