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Unformatted text preview: he limitations that we encountered with the record route option, things get worse with
the timestamp option. If we record both IP addresses and timestamps (a flags of 1), we can store
only four of these pairs. Recording only timestamps is next to useless because we have no
indication regarding which timestamp corresponds to which router (unless we have a fixed
topology that never changes). A flags of 3 is better, as we can then select which routers insert
their timestamp. A more fundamental problem is that you probably have no control over how
accurate the timestamp is at any given router. This makes it fruitless to try to measure hop times
between routers using this IP option. We'll see that the traceroute program (Chapter 8) provides file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (12 of 13) [12/09/2001 14.46.45] Chapter 7. Ping Program a better way of measuring hop times between routers. 7.5 Summary
The ping program is the basic connectivity test between two systems running TCP/IP. It uses
the ICMP echo request and echo reply messages and does not use a transport layer (TCP or
UDP). The Ping server is normally part of the kernel's ICMP implementation.
We looked at the normal ping output for a LAN, WAN, and SLIP links (dialup and
hardwired), and performed some serial line throughput calculations for a dedicated SLIP link.
ping also let us examine and use the IP record route option. We used this IP option to see how
default routes are often used, and will return to this topic in Chapter 9. We also looked at the IP
timestamp option, but it is of limited practical use.
7.1 Draw a time line for the ping output for the SLIP link in Section 7.2.
7.2 Calculate the RTT if the SLIP link between bsdi and slip is set to 9600 bits/sec. Assume
the default of 56 bytes of data.
7.3 The current BSD ping program allows us to specify a pattern for the data portion of the
ICMP message. (The first 8 bytes of the data portion are not filled with the pattern, since the
time at which the packet is sent is stored here.) If we spec...
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