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just the V.42bis data compression our average is 330 ms. If we turn off the V.42 error control
(which also turns off the V.42bis data compression) our average is 300 ms. These modem
parameters do affect the RTTs, and using the error control and data compression appears to be
the best. 7.3 IP Record Route Option
The ping program gives us an opportunity to look at the IP record route (RR) option. Most
versions of ping provide the -R option that enables the record route feature. It causes ping to set
the IP RR option in the outgoing IP datagram (which contains the ICMP echo request message).
This causes every router that handles the datagram to add its IP address to a list in the options
field. When the datagram reaches the final destination, the list of IP addresses should be copied
into the outgoing ICMP echo reply, and all the routers on the return path also add their IP
addresses to the list. When ping receives the echo reply it prints the list of IP addresses.
As simple as this sounds, there are pitfalls. Generation of the RR option by the source host,
processing of the RR option by the intermediate routers, and reflection of the incoming RR list
in an ICMP echo request into the outgoing ICMP echo reply are all optional features.
Fortunately, most systems today do support these optional features, but some systems don't
reflect the IP list.
The biggest problem, however, is the limited room in the IP header for the list of IP addresses.
We saw in Figure 3.1 that the header length in the IP header is a 4-bit field, limiting the entire file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (7 of 13) [12/09/2001 14.46.45] Chapter 7. Ping Program IP header to 15 32-bit words (60 bytes). Since the fixed size of the IP header is 20 bytes, and the
RR option uses 3 bytes for overhead (which we describe below), this leaves 37 bytes (60-20-3)
for the list, allowing up to nine IP addresses. In the early days of the ARPANET, nine IP
addresses seemed like a lot, but since this is a round-trip list (in the case of the -R option for
ping), it's of limited use today. (In Chapter 8 we'...
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