brescia@PARK-STREET.BBN.COM (Mike Brescia) (03/18/88)
I have found source routing useful for tracing routes, both for local net access problems and routing problems between gateways. Today I had to bemoan the fact that a particular gateway would not source-route, even though it did not crash, because they had called to see why they could ping a core gateway but not establish an EGP connection. With source routing I send a packet from gateway A to B and back to myself, and vice versa. If it fails, I may get ICMP net-unreachable for route problems, or host-unreachable for host access problems. One additional feature would make problem tracking useful; that is the ability to ask a gateway to give back its opinion of the next hop for a particular destination net. This is the info that RR gives you, but only if the packet gets back to you. If gateway A wants to send to B and B is a black hole, you would like to get the info from A instead. I have been able to do source routing through 4.3BSD systems. I think they may do it even if IP_FORWARDING is off. LSI11 core gateways do source-and-record-route but not record-route. Butterfly gateways do both. In both gateways, the alignment of the IP address fields in the options is not critical. Mike Brescia BBNCC Gateway Development