TCP IP Illustrated

The technique used was to run ping on the host

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Unformatted text preview: ection also. (In Chapter 25 we'll see how to determine this using SNMP.) On a point-to-point link, it is not required that the MTU be the same in both directions. The technique used was to run ping on the host solaris, to the host bsdi, increasing the size of the data packets until fragmentation was seen on the incoming packets. This is shown in Figure 11.10. Figure 11.10 Systems being used to determine MTU of SLIP link from netb to sun. tcpdump was run on the host sun, watching the SLIP link, to see when fragmentation occurred. No fragmentation was observed and everything was fine until the size of the data portion of the ping packet was increased from 500 to 600 bytes. The incoming echo requests were seen (there was still no fragmentation), but the echo replies disappeared. To track this down, tcpdump was also run on bsdi, to see what it was receiving and sending. Figure 11.11 shows the output. 1 2 3 4 5 0.0 solaris > bsdi: icmp: echo request (DF) 0.000000 (0.0000) bsdi > solaris: icmp: echo reply (DF) sun > bsdi: icmp: solaris unreachable 0.000000 (0.0000) need to frag, mtu = 0 (DF) 0.738400 (0.7384) solaris > bsdi: icmp: echo request (DF) 0.748800 (0.0104) bsdi > solaris: icmp: echo reply (DF) file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (10 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol 6 0.748800 (0.0000) sun > bsdi: icmp: solaris unreachable need to frag, mtu = 0 (DF) Figure 11.11 tcpdump output for ping of bsdi from solaris with 600-byte IP datagram. First, the notation (DF) in each line means the don't fragment bit is turned on in the IP header. It turns out that Solaris 2.2 normally turns this bit on, as part of its implementation of the path MTU discovery mechanism. Line 1 shows that the echo request got through the router netb to sun without being fragmented, and with the DF bit set, so we know that the SLIP MTU of netb has not been reached yet. Next, notice in line 2 that the DF flag is copied into the...
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