Yesterday I wrote about RFC 791, but I didn’t mention how errors get handled or reported.

Any conforming implementation of Internet Protocol must support its companion, the Internet Control Message Protocol. Defined in RFC 792, ICMP provides a very lightweight mechanism for error reporting.

In essence, ICMP defines messages that are sent back to the source of a packet that fails delivery. These messages use the standard IP header format but contain specific data fields that define their purpose. The spec describes a dozen or so message types, but several of these have been deprecated. Some of the most notable types in active use are:

1. Destination Unreachable (Type 3): Sent when a packet cannot reach its destination.
2. Echo Request (Type 8) and Echo Reply (Type 0): Used by tools like ping to check network connectivity.
3. Time Exceeded (Type 11): Indicates that the packet time-to-live (TTL) has expired.

A good example of how the Time Exceeded message can be used is traceroute, which sends packets with successively increasing TTL values in order to probe the route(s) taken to a host. For example, when I run:

1traceroute 8.8.8.8

I get the following output:

traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 40 byte packets
 1  192.168.1.1 (192.168.1.1)  5.796 ms  4.522 ms  6.533 ms
 2  * * *
 3  lag-62-10.dtr02psdnca.netops.charter.com (96.34.102.37)  13.634 ms  13.846 ms  11.718 ms
 4  lag-30.crr03rvsdca.netops.charter.com (96.34.97.154)  18.296 ms  19.656 ms  26.183 ms
 5  lag-811.bbr01rvsdca.netops.charter.com (96.34.3.18)  15.703 ms  19.682 ms  16.463 ms
 6  lag-801.prr01lsanca.netops.charter.com (96.34.3.129)  16.699 ms  19.619 ms  16.894 ms
 7  sar02sghlga-eth-2-1.sghl.ga.charter.com (96.34.1.69)  18.305 ms
    72.14.219.250 (72.14.219.250)  17.759 ms  17.314 ms
 8  * * *
 9  dns.google (8.8.8.8)  20.087 ms  19.610 ms  17.278 ms

Here I see various intermediate nodes from my ISP all the way to Google’s DNS server. The asterisks here mean that there was no response. These nodes likely limit or disable ICMP responses.