Subject: Path verification contradictions in Connection migration

From: Magnus Westerlund <[email-masked]>
Date: Wed, 16 May 2018 12:00:59 +0200

Hi,

I was looking at the connection migration text in transport-11:

Section 6.8.2 says:

    Each endpoint validates its peer's address during connection
    establishment.  Therefore, a migrating endpoint can send to its peer
    knowing that the peer is willing to receive at the peer's current
    address.  Thus an endpoint can migrate to a new local address without
    first validating the peer's address.

    Receiving acknowledgments for data sent on the new path serves as
    proof of the peer's reachability from the new address.  Note that
    since acknowledgments may be received on any path, return
    reachability on the new path is not established.  To establish return
    reachability on the new path, an endpoint MAY concurrently initiate
    path validation Section 6.7 on the new path.

Then Section 6.8.3 says:

    Receiving a packet from a new peer address containing a non-probing
    frame indicates that the peer has migrated to that address.

    In response to such a packet, an endpoint MUST start sending
    subsequent packets to the new peer address and MUST initiate path
    validation (Section 6.7) to verify the peer's ownership of the
    unvalidated address.

It is especially the Note in 6.8.2 second of above paragraphs that I 
find contradicting.

When the server receives the clients non-probing frames from the 
client's new address, then the server MUST send the ACK for that packet 
towards the client's new address. That is at least how I interpret the 
second in Section 6.8.3 paragraph. Yes, from the server's perspective 
the serve->client path is unverified. However, I don't understand how 
the first part of the Note would be possible: "Note that since 
acknowledgments may be received on any path"?

Cheers

Magnus Westerlund

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