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Unformatted text preview: ted. But if we use Telnet
and connect to the daytime server on aix, the ICMP redirect is generated, and the routing table
on aix is updated. If we then execute ping with the record route option enabled, the route
shows that the datagrams go from netb to aix and back to netb, without the extra hop to the
router gateway. We'll look at these ICMP redirects in more detail in Section 9.5. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (11 of 13) [12/09/2001 14.46.45] Chapter 7. Ping Program 7.4 IP Timestamp Option
The IP timestamp option is similar to the record route option. Figure 7.7 shows the format of the
IP timestamp option (compare with Figure 7.3). Figure 7.7 General format of timestamp option in IP header.
The code field is 0x44 for the timestamp option. The two fields len and ptr are the same as for
the record route option: the total length of the option (normally 36 or 40) and a pointer to the
next available entry (5,9, 13, etc.).
The next two fields are 4-bit values: OF is the overflow field and FL is a flags field. The
operation of the timestamp option is driven by the flags field, as shown in Figure 7.8.
flags Description 0 Record only timestamps. This is what we show in Figure 7.7. 1 Each router records its IP address and its timestamp. There is room for only four of
these pairs in the options list. 3 The sender initializes the options list with up to four pairs of IP addresses and a 0
timestamp. A router records its timestamp only if the next IP address in the list
matches the router's.
Figure 7.8 Meaning of the flags value for timestamp option. If a router can't add a timestamp because there's no room left, it just increments the overflow
The preferred value for the timestamps is the number of milliseconds past midnight, UTC,
similar to the ICMP timestamp request and reply (Section 6.4). If this format is not available to
a router, it can insert whatever time representation that it uses, but must then turn on the highorder bit of the timestamp to indicate the nonstandard value.
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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.
- Spring '12