TCP IP Illustrated

Now well run the same example but modify the router

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Unformatted text preview: t; slip: (frag 47942:[email protected]) Figure 11.15 First datagram arriving at host slip from solaris. In this example the host solaris should not fragment the outgoing datagrams but should turn off the DF bit and let the router with the smaller MTU do the fragmentation. Now we'll run the same example but modify the router bsdi to return the next-hop MTU in the ICMP "can't fragment" error. Figure 11.16 shows the first six lines of the tcpdump output. 1 0.0 2 0.004199 (0.0042) 3 4.950193 (4.9460) 4 4.954325 (0.0041) 5 6 7 9.779855 (4.8255) 9.930018 (0.1502) 9.990170 (0.0602) solaris. 37974 > slip.discard: udp 650 (DF) bsdi > solaris: icmp: slip unreachable - need to frag, mtu = 296 (DF) solaris.37974 > slip.discard: udp 650 (DF) bsdi > solaris: icmp: slip unreachable - need to frag, mtu = 296 (DF) solaris.37974 > slip.discard: udp 650 (frag 35278:[email protected]+) solaris > slip: (frag 35278:[email protected]+) solaris > slip: (frag 35278:[email protected]) Figure 11.16 Path MTU discovery using UDP. Again, the first two datagrams are sent with the DF bit set, and both elicit the ICMP error. The ICMP error now specifies the next-hop MTU of 296. In lines 5, 6, and 7 we see the source host perform fragmentation, similar to Figure 11.14. But knowing the next-hop MTU, only three fragments are generated, compared to the four fragments generated by the router bsdi in Figure 11.15. 11.9 Interaction Between UDP and ARP file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (16 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol Using UDP we can see an interesting (and often unmentioned) interaction with UDP and typical implementations of ARP. We use our sock program to generate a single UDP datagram with 8192 bytes of data. We expect this to generate six fragments on an Ethernet (see Exercise 11.3). We also assure that the ARP cache is empty before running the program, so that an ARP request and reply must be exchanged before the first fragment is sent. verify ARP cache is empty bsdi % arp -a bsdi % sock -u -i -nl -w8192 svr4 discard We expect the first fragment to cause an ARP request to be sent. Five more fragments are gener...
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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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