August 5, 2026
Final: Thursday, August 6
Next week: review session for final exam study guide. No new material.
Reverse Path Filtering and the Merry-go-round
Strict RPF means that the router checks the route to the source address, and if that doesn't match the arrival interface, the packet is dropped. Loose RPF means that the source address is at least reachable.
For the MerryGoRound, a router will not forward the packet if it looks like it was the source. So if r1 sends a packet, it will be dropped when it cycles around to r1 again.
This Linux command can tell you how many packets were dropped:
nstat -rsz | grep IPReversePathFilter
RPF is particularly important for multicast, which is really only a thing within individual sites, and not on the internet as a whole. But it also prevents spoofing, like me trying to send out a packet with spoofed source address on the internet from within Loyola.