An Error Is a Packet About a Packet
An ICMP error has to tell the original sender what went wrong — but the sender doesn't hand ICMP any context. Where does the diagnosis come from?
Open Session 01Sessions
ICMP track sessions
Unreachable Has Flavors
"Destination Unreachable" sounds like one failure. Why does it split into four codes, and why does the sender of the message change depending on which one you get?
Open Session 02TTL Is a Hop Budget
Nothing on the network guarantees a packet won't loop forever between routers. What stops it, and who reports back when it doesn't make it?
Open Session 03Time Exceeded Draws the Map
What decides which router answers a probe, and why is an "error" reply the probe's success signal instead of its failure?
Open Session 04Build the Toy Traceroute Loop
If traceroute has no query that asks "where does this path go," how does looping over ttl and reading the wreckage assemble the answer — and what tells the loop when to stop?