Re: [quicwg/base-drafts] Server's preferred address (#1251)

[Christian Huitema <[e-mail]>]

huitema commented on this pull request.



> +time during the connection after the handshake is completed.  If this packet
+contains a PATH_CHALLENGE frame, the server sends a PATH_RESPONSE frame as per
+{{migrate-validate}}, but the server MUST continue sending all other packets
+from its original IP address.
+
+Once the server has received a non-probing packet on its preferred address which
+is the largest packet number seen so far, the server begins sending to the
+client exclusively from its preferred IP address.
+
+
+## Interaction of Client and Server Migration
+
+A client might need to perform a connection migration before the server's
+connection migration has completed.  In this case, the client SHOULD perform
+path validation to both the original and preferred server address from the
+client's new address concurrently.

Let's keep it simple. This PR is about triggering a migration at the end of the handshake. Given the prevalence of NAT, the path challenge sent by the client to a new server address will very often arrive at the server with a different source address than the source address of the client initial packet, and there is not much the client can do about that. The default source address of the client might also change, but from a server point of view that's just the same as any client migration. So in most cases we will see "challenge from client, response and challenge from server, response from client." Both server and clients will have to consider whether sending data before the verification completes, but that's what they always do.

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