up:
- “IP promises nothing — it is best-effort and never reports delivery outcome”
tags: - atomic
created: 2026-08-29 15:35
A router receives a datagram, consults a table, forwards it, and forgets it. It therefore cannot repair loss: it never knew a datagram was expected, loss usually happens at a router (asking the failing component to notice its own failure), and the path can change mid-conversation so there’s no fixed set of routers to hold state anyway.
The two endpoints are the only parties present for the whole conversation.
Every guarantee above IP is manufactured by state held at the two endpoints.
This is the end-to-end principle (Saltzer, Reed & Clark, 1984). Two consequences:
- Every guarantee costs endpoint memory and latency. Reliability costs a buffer of unacknowledged data plus a timeout wait; ordering costs buffered early arrivals plus the wait for gaps to fill. Nothing is free, because the network isn’t participating.
- Inventing a transport requires changing only the two machines talking — no router upgrade. This is what makes “write my own transport” physically sensible (though still constrained by deployment reality).
Where it leaks — three named asterisks, none of which change any derivation: ECN lets a router mark “congested” instead of dropping; ICMP carries some error reports; NAT holds per-flow state inside the network.
See also
- IP promises nothing — it is best-effort and never reports delivery outcome — this is the direct consequence of IP’s silence: guarantees can’t live in the network, so they live at the ends
- A network connection is identified by a 4-tuple, not a port — the connection state that endpoints hold is keyed by the 4-tuple; NAT (an end-to-end leak) works by rewriting exactly those coordinates
- Sequence numbers make a gap in a stream proof of loss that names what was lost — the first concrete “fix at the endpoints”: reliability built from endpoint-held state
- Single Writer Principle, one thread own all writes to a resource — same shape of idea: correctness comes from who holds the authoritative state, here the two endpoints rather than the network
References
Questions
flashcards/software-engineering/networking
Why can’t routers repair packet loss?::A router forwards and forgets — it never knew a datagram was expected, loss often happens at the router itself, and the path can change, so no fixed router holds the state
What is the end-to-end principle’s core claim about guarantees above IP?::Every guarantee above IP is manufactured by state held at the two endpoints, since only they are present for the whole conversation
Every guarantee above IP costs endpoint memory and latency — nothing is free because the network isn’t participating.