Re: [quicwg/base-drafts] Description of the use of Preferred Address is unclear (#3353)

[Kazuho Oku <[e-mail]>]

@martinthomson 
>> Retirement of either of these connection IDs notifies the server of the address the client has chosen.
> 
> This implies a semantic to the retirement of connection IDs that is not already defined. It says that in addition to releasing the resource, the server can say definitively that those other network paths won't be used. But this is misleading because retiring CID 1 does not prevent CID 4 from being used on that path.

I think that the point being missed here is that under a given condition the server can determine the path being used for CID4.

If the CID4 is issued prior to the client choosing the path, it is true the server would not be able to determine the path on which CID4 will be used.

But if the server withholds NCID frames until the client chooses the path and notifies its choice by retiring one of the first two CIDs, then the server can tell for sure. Because either of the first or the second CID would be retired first, and that tells the server the path the client has chosen. As we agree, we already state that a client cannot come back to the original address, once it migrates to the preferred address.

The benefit of requiring such retirement is that the server can issue CIDs specific to the server address, as pointed out by https://github.com/quicwg/base-drafts/issues/3353#issuecomment-575384854. Assuming that such server deployment would be within our design scope, I think requiring such retirement is not a bad idea.

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