IP source routing questions

Steve Deering (deering@pescadero.stanford.edu)
2 Oct 1986 23:15-PDT

After looking through a couple of IP specs and a couple of IP
implementations, I still have a shaky understanding of how source
routing works. Assuming host A wants to send a datagram to host D,
source-routed through gateways B and C,

     host A gate B gate C host D
    +------+ +------+ +------+ +------+
    | | | | | | | |
    | A1|-------->|B1 B2|-------->|C2 C3|-------->|D3 |
    | | net 1 | | net 2 | | net 3 | |
    +------+ +------+ +------+ +------+

where A1, B1, B2, etc. are IP addresses. I think that the upper layer
protocol in host A should pass the following IP header values to the
IP layer:
             source:A1 destination:B1 route:B1,C2,D3 offset:4

where offset is the value of the third byte of the source route option.
Then I expect the datagram to contain the following values when it is

on net 1: source:A1 destination:B1 route:A1,C2,D3 offset:8

on net 2: source:A1 destination:C2 route:A1,B2,D3 offset:12

on net 3: source:A1 destination:D3 route:A1,B2,C3 offset:16

Is this right? Perhaps the source address changes at each hop? Or the
destination address remains constant? Or there should only be two
addresses in the route option? Or the offset values should be 4 less?

Methinks the specs could be clearer on this.

Steve Deering

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