[quicwg/base-drafts] Migration before handshake completed is very messy (#2309)

[Christian Huitema <[e-mail]>]

In migration interop tests, the Picoquic client was starting to simulate NAT rebinding just after obtaining the 1-RTT read and write key. That turns out to be very messy, because at that point there is no proof that the server has received the client finished message, let alone acknowledged it. As far as tests are concerned, there is an easy fix: only start the migration tests after the client has received from the server an ACK of one of its 1-RTT packet. That's the moment when the client knows for sure that the handshake is successful.

The migration before handshake complete is messy because the client will try to re-transmit handshake messages that have not been acknowledged, and has some kind of dilemma. Should these re-transmissions aims at the original DCID used during the handshake, or at one of the new CID selected during the migration? What if the continuity of the new path has not been yet verified? What if the continuity test was lost?

Do we have an actual need to support migration before the handshake is fully completed? In practice, the handshake packets create a NAT mapping, and it is rather unlikely that this mapping will just disappear before the few RTT required to complete the handshake. In fact, it is possible, maybe likely, that packets received from an alternative address during the handshake comes from attackers, not from the actual client.

I think the implementations would be simpler and more robust if the spec stated that clients should not initiate "voluntary" migration before the handshake is fully complete. This means 1-RTT ACK received, or ACK of last handshake packet from the client, whichever comes first. Similarly, servers should not try to migrate connections before receiving at least one 1-RTT packet from the client through the same addresses and ports as the Initial messages.

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