Re: Question about IP options


William Westfield (BILLW@MATHOM.CISCO.COM)
Thu 17 Sep 87 17:53:55-PDT


[ Indented quotes from Jonathan Biggar, ">"ed quotes from Mike Brescia ]

     2) When using either loose or strict source routing, does the next hop
         or the final destination go in the destination address field of the
         IP header?

>Next hop. A rationale for this choice, based on processing power needed, is
>that a gateway does not have to look at the options unless the IP destination
>address is the gateway. Then you look for options, and, finding a source
>route, send it back out again.

Unfortunately this rationale is completely false, since there are many
IP options that must be processed whether or not the IP destination is
the gateway's address. (These include record route, timestamping, and
security.)

A better reason is the behavior that you get when some gateway does
not implement source routing is much more sociable this way.

Consider a packet sent from A to D with source route B:C:D, and where
B does not implement source routing. If the original destination D
stays in the header through the entire trip, then the packet will
arrive at B, and be forwarded on to some gateway (perhaps C) without
processing the source route option. C will get the packet, notice
that it has an incomplete source route, and forward it to the next
gateway in the list, which is still B. The packet will loop between
B and C until it dies. Depending on the type of link between them,
this could be very expensive in some sense.

When the IP destination is replaced at each hop, all that happens is
that B receives the packet, and probably throws it out due to some
higher level protocol checksum faliure...

Bill Westfield
cisco Systems
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