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Unformatted text preview: 5 Mbits/sec phone lines used by the backbone.) The final network is
alter.net, the connection point to the Internet for aw.corn. 8.5 IP Source Routing Option
Normally IP routing is dynamic with each router making a decision about which next-hop
router to send the datagram to. Applications have no control of this, and are normally not
concerned with it. It takes tools such as Traceroute to figure out what the route really is.
The idea behind source routing is that the sender specifies the route. Two forms are
q q Strict source routing. The sender specifies the exact path that the IP datagram must
follow. If a router encounters a next hop in the source route that isn't on a directly
connected network, an ICMP "source route failed" error is returned.
Loose source routing. The sender specifies a list of IP address that the datagram must
traverse, but the datagram can also pass through other routers between any two
addresses in the list. Traceroute provides a way to look at source routing, as we can specify an option allowing
us to force a source route, and see what happens.
Some of the publicly available Traceroute source code packages contain patches to specify
loose source routing. But the standard versions normally don't include this option. A
comment in the patches is that "Van Jacobson's original traceroute (spring 1988) supported
this feature, but he removed it due to pressure from people with broken gateways." For the
examples shown in this section, the author installed these patches and modified them to
allow both loose and strict source routing.
Figure 8.6 shows the format of the source route option. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (9 of 17) [12/09/2001 14.46.48] Traceroute Program Figure 8.6 General format of the source route option in the IP header.
This format is nearly identical to the format of the record route option that we showed in
Figure 7.3. But with source routing we have to fill in the list of IP addresses before sending
the IP datagram, while with the record route option we allocate room and zer...
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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