Per discussion at the mike today on what we should do with the ICMP unreachable draft - here are some properties I think are necessary in a hint to the UE that the captive portal is closed.
1. The notification should not be easy to spoof. This is easiest to do by making it a hint to the UE that it should talk to the API.
- An ICMP message by itself is not secure. For example, it's trivial for an off-path attacker to generate ICMP messages for sessions from legitimate UEs to <popularwebsite>:443. Getting a UE to trust such a message only requires getting the ephemeral port right, and many OSes have a quite limited range of ephemeral ports.
- Tero points out that if we do want to secure such a message, then we should not roll our own security but should use an existing, secure protocol such as IPsec.
2. It should be possible to send the notification *before* the captive portal closes, to facilitate seamless connectivity. Ideally the user should be able to re-up the captive portal without having to wait until the network is dead or the device has switched to another network.
3. The notification should not be on a per-destination basis. A hint that conveys the information "you can reach facebook, but to reach CNN you need to upgrade to another service plan" is not technically infeasible but is unlikely ever to reach WG and IETF consensus and therefore I think we should not spend our time talking about it.
4. I'm not sure whether it's possible for the hint to be anything more than a binary "you are or will very soon be captive". Saying things like "an upgrade opportunity is available" may be hard to encode.