TCP IP Illustrated

Also with source routing we only allocate room for

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 nine addresses. 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...
View Full Document

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.

Ask a homework question - tutors are online