TCP IP Illustrated

The echo reply needs to be fragmented but the df flag

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Unformatted text preview: echo reply. This is what causes the problem. The echo reply is the same size as the echo request (just over 600 bytes), but the MTU on sun's outgoing SLIP interface is 552. The echo reply needs to be fragmented, but the DF flag is set. This causes sun to generate the ICMP unreachable error back to bsdi (where it's discarded). This is why we never saw any echo replies on solaris. The replies never got past sun. Figure 11.12 shows the path of the packets. Figure 11.12 Packets exchanged in example. Finally, the notation mtu=0 in lines 3 and 6 of Figure 11.11 indicates that sun does not return the MTU of the outgoing interface in the ICMP unreachable message, as shown in Figure 11.9. (In Section 25.9 we return to this problem and use SNMP to determine that the MTU of the SLIP interface on netb is 1500.) 11.7 Determining the Path MTU Using Traceroute Although most systems don't support the path MTU discovery feature, we can easily modify a version of traceroute (Chapter 8) to let us determine the path MTU. What we'll do is send file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (11 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol packets with the "don't fragment" bit set. The size of the first packet we send will equal the MTU of the outgoing interface, and whenever we receive an ICMP "can't fragment" error (which we described in the previous section) we'll reduce the size of the packet. If the router sending the ICMP error sends the newer version that includes the MTU of the outgoing interface, we'll use that value; otherwise we'll try the next smallest MTU. As RFC 1191 [Mogul and Deering 1990] states, there are a limited number of MTUs, so our program has a table of the likely values and moves to the next smallest value. Let's first try it from our host sun to the host slip, knowing that the SLIP link has an MTU of 296: sun % traeeroute.pmtu slip traceroute to slip (, 30 hops max outgoing MTU = 1500 1 bsdi ( 15 ms 6 ms 6 ms 2 bsdi ( 6 ms fragmentation required and DF set, trying new MTU fragmentation required and DF set, trying new MTU fragmentation required and DF set, trying new MTU fragmentation required and DF set, trying new MTU fragmentation required an...
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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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