9. Traceroute Considerations

9. Traceroute Considerations

When a source host in a LISP site initiates a traceroute to a destination host in another LISP site, it is highly desirable for it to see the entire path. Since packets are encapsulated from the ITR to the ETR, the hop across the tunnel could be viewed as a single hop. However, LISP traceroute will provide the entire path so the user can see 3 distinct segments of the path from a source LISP host to a destination LISP host:

Segment 1 (in source LISP site based on EIDs):

source host ---> first hop ... next hop ---> ITR

Segment 2 (in the core network based on RLOCs):

ITR ---> next hop ... next hop ---> ETR

Segment 3 (in the destination LISP site based on EIDs):

ETR ---> next hop ... last hop ---> destination host

For segment 1 of the path, ICMP Time Exceeded messages are returned in the normal manner as they are today. The ITR performs a TTL decrement and tests for 0 before encapsulating. Therefore, the ITR's hop is seen by the traceroute source as having an EID address (the address of the site-facing interface).

For segment 2 of the path, ICMP Time Exceeded messages are returned to the ITR because the TTL decrement to 0 is done on the outer header, so the destinations of the ICMP messages are the ITR RLOC address and the source RLOC address of the encapsulated traceroute packet. The ITR looks inside of the ICMP payload to inspect the traceroute source so it can return the ICMP message to the address of the traceroute client and also retain the core router IP address in the ICMP message. This is so the traceroute client can display the core router address (the RLOC address) in the traceroute output. The ETR returns its RLOC address and responds to the TTL decrement to 0, as the previous core routers did.

For segment 3, the next-hop router downstream from the ETR will be decrementing the TTL for the packet that was encapsulated, sent into the core, decapsulated by the ETR, and forwarded because it isn't the final destination. If the TTL is decremented to 0, any router on the path to the destination of the traceroute, including the next-hop router or destination, will send an ICMP Time Exceeded message to the source EID of the traceroute client. The ICMP message will be encapsulated by the local ITR and sent back to the ETR in the originated traceroute source site, where the packet will be delivered to the host.