[Captive-portals] Requirements for "captive portal closed" notifications

Lorenzo Colitti <lorenzo@google.com> Tue, 20 March 2018 15:29 UTC

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From: Lorenzo Colitti <lorenzo@google.com>
Date: Tue, 20 Mar 2018 15:29:04 +0000
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Subject: [Captive-portals] Requirements for "captive portal closed" notifications
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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.