TCP IP Illustrated

Since we are watching these packets on the host sun

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Unformatted text preview: se packets on the host sun (see Figure 7.4) we only see the ICMP echo request with the empty list, and the ICMP echo reply with three addresses in the list. We have deleted the remaining lines in the tcpdump output, since they are nearly identical to what we show in Figure 7.5. The notation EOL at the end of the record route information indicates the IP option "end of list" value appeared. The EOL option has a value of 0. What's happening is that 39 bytes of RR data are in the 40 bytes of option space in the IP header. Since the option space is set to 0 before the datagram is sent, this final byte of 0 that follows the 39 bytes of RR data is interpreted as an EOL. That is what we want to have happen. If there are multiple options in the option field of the IP header, and pad bytes are needed before the next option starts, the other special character NOP ("no operation"), with a value of 1, can be used. In Figure 7.5, SVR4 sets the TTL field of the echo request to 32, and BSD/386 sets it to 255. (It prints as 254 since the router bsdi has already decremented it by one.) Newer systems are setting the TTL of ICMP messages to the maximum (255). It turns out that of the three TCP/IP implementations used by the author, both BSD/386 and SVR4 support the record route option. That is, they correctly update the RR list when forwarding a datagram, and they correctly reflect the RR list from an incoming ICMP echo request to the outgoing ICMP echo reply. SunOS 4.1.3, however, updates the RR list when forwarding a datagram, but does not reflect the RR list. Solaris 2.x corrects this problem. Abnormal Output The following example was seen by the author and provides a starting point for our description of the ICMP redirect message in Chapter 9. We ping the host aix on the 140.252.1 subnet (accessible through the dialup SLIP connection on the host sun) with the record route option. We get the following output, when run on the host slip: slip % ping -R aix PING aix (
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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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