9.2. IPv4 Traceroute

9.2. IPv4 Traceroute

For IPv4 traceroute, we cannot follow the above procedure, since IPv4 ICMP Time Exceeded messages only include the invoking IP header and 8 octets that follow the IP header. Therefore, when a core router sends an IPv4 Time Exceeded message to an ITR, all the ITR has in the ICMP payload is the encapsulated header it prepended, followed by a UDP header. The original invoking IP header, and therefore the identity of the traceroute source, is lost.

The solution we propose to solve this problem is to cache traceroute IPv4 headers in the ITR and to match them up with corresponding IPv4 Time Exceeded messages received from core routers and the ETR. The ITR will use a circular buffer for caching the IPv4 and UDP headers of traceroute packets. It will select a 16-bit number as a key to find them later when the IPv4 Time Exceeded messages are received. When an ITR encapsulates an IPv4 traceroute packet, it will use the 16-bit number as the UDP source port in the encapsulating header. When the ICMP Time Exceeded message is returned to the ITR, the UDP header of the encapsulating header is present in the ICMP payload, thereby allowing the ITR to find the cached headers for the traceroute source. The ITR puts the cached headers in the payload and sends the ICMP Time Exceeded message to the traceroute source retaining the source address of the original ICMP Time Exceeded message (a core router or the ETR of the site of the traceroute destination).

The signature of a traceroute packet comes in two forms. The first form is encoded as a UDP message where the destination port is inspected for a range of values. The second form is encoded as an ICMP message where the IP identification field is inspected for a well-known value.