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Unformatted text preview: o out the list of
IP addresses, letting the routers fill in the next entry in the list. Also, with source routing we
only allocate room for and initialize the number of IP addresses required, normally fewer
than nine. With the record route option we allocated as much room as we could, for up to
The code is 0x83 for loose source routing, and 0x89 for strict source routing. The len and
ptr fields are identical to what we described in Section 7.3.
The source route options are actually called "source and record route" (LSRR and SSRR,
for loose and strict) since the list of IP addresses is updated as the datagram passes along the
path. What happens is as follows:
q q q The sending host takes the source route list from the application, removes the first
entry (it becomes the destination address of the datagram), moves all the remaining
entries left by one entry (where left is as in Figure 8.6), and places the original
destination address as the final entry in the list. The pointer still points to the first
entry in the list (e.g., the value of the pointer is 4).
Each router that handles the datagram checks whether it is the destination address of
the datagram. If not, the datagram is forwarded as normal. (In this case loose source
routing must have been specified, or we wouldn't have received the datagram.)
If the router is the destination, and the pointer is not greater than the length, then (1)
the next address in the list (where ptr points) becomes the destination address of the
datagram, (2) the IP address corresponding to the outgoing interface replaces the
source address just used, and (3) the pointer is incremented by 4. This is best explained with an example. In Figure 8.7 we assume that the sending
application on host S sends a datagram to D, specifying a source route of R1, R2, and R3. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (10 of 17) [12/09/2001 14.46.48] Traceroute Program Figure 8.7 Example of IP source routing.
In this figure t...
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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