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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 (126.96.36.199...
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- Spring '12