A Second Look at Detecting Third-Party Addresses in Traceroute Traces with the IP Timestamp Option

Artifacts in traceroute measurement output can lead to false inferences of AS-level links and paths when used to deduce AS topology. One traceroute artifact is caused by routers that respond to traceroute probes with a source address not in the path towards the destination, i.e. an off-path address. The most well-known traceroute artifact, the third-party address, is caused by off-path addresses that map to ASes not in the corresponding BGP path. In PAM 2013, Marchetta

et al.

proposed a technique to detect off-path addresses in traceroute paths [14]. Their technique assumed that a router IP address reported in a traceroute path towards a destination was off-path if, in a subsequent probe towards the same destination, the router did not insert a timestamp into a pre-specified timestamp option in the probe’s IP header. However, no standard precisely defines how routers should handle the pre-specified timestamp option, and implementations are inconsistent. Marchetta

et al.

claimed that most IP addresses in a traceroute path are off-path, and that consecutive off-path addresses are common. They reported no validation of their results. We cross-validate their approach with a first-principles approach, rooted in the assumption that subnets between connected routers are often /30 or /31 because routers are often connected with point-to-point links. We infer if an address in a traceroute path corresponds to the interface on a router that received the packet (the in-bound interface) by attempting to infer if its /30 or /31 subnet mate is an alias of the previous hop. We traceroute from 8 Ark monitors to 80K randomly chosen destinations, and find that most observed addresses are configured on the in-bound interface on a point-to-point link connecting two routers, i.e. are on-path. Because the technique from [14] reports 70.9%–74.9% of these addresses as being off-path, we conclude it is not reliable at inferring which addresses are off-path or third-party.