TCP IP Illustrated

G an ethernet has a smaller mtu than the end point

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Unformatted text preview: CP headers. If the older ICMP error is returned, the probable value of the next smallest MTU (Figure 2.5) must be tried. When a retransmission caused by this ICMP error occurs, the congestion window should not change, but slow start should be initiated. Since routes can change dynamically, when some time has passed since the last decrease of the path MTU, a larger value (up to the minimum of the MSS announced by the other end, or the outgoing interface MTU) can be tried. RFC 1191 recommends this time interval be about 10 minutes. (We saw in Section 11.8 that Solaris 2.2 uses a 30-sec-ond timer for this.) Given the normal default MSS of 536 for nonlocal destinations, path MTU discovery avoids fragmentation across intermediate links with an MTU of less than 576 (which is rare). It can also avoid fragmentation on local destinations when an intermediate link (e.g., an Ethernet) has a smaller MTU than the end-point networks (e.g., a token ring). But for path MTU discovery to be more useful, and take advantage of wide area file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (2 of 20) [12/09/2001 14.47.33] Chapter 24. TCP Futures and Performance networks with MTUs greater than 576, implementations must stop using a default MSS of 536 bytes for nonlocal destinations. A better choice for the MSS is the MTU of the outgoing interface (minus the size of the IP and TCP headers, of course). (In Appendix E we'll see that most implementations allow the system administrator to change this default MSS value.) An Example We can see how path MTU discovery operates when an intermediate router has an MTU less than either of the end point's interface MTUs. Figure 24.1 shows the topology for this example. Figure 24.1 Topology for path MTU example. We'll establish a connection from the host solaris (which supports the path MTU discovery mechanism) to the host slip. This setup is identical to the one used for our UDP path MTU discovery example (Figu...
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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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