Re: [TLS] ALPN - specifying client preference

[Ashok Kumar <[e-mail]>]

Hi Andrei,

I was thinking in terms of having options to prevent strict preference
orders. Let's say in near future, browsers use ALPN to negotiate between
HTTP/1.1, SPDY/3, HTTP/2.0 and client doesn't mind between SPDY/3, HTTP/2.0
but prefers either of those over HTTP/1.1. I was just thinking that the
server need not pick one of the two just because it appears first and
thinks the client prefers it.

Now that I read your mail, "server SHOULD select protocol that the _server
prefers_ among the protocols advertised by the client", I see that it's
completely server's choice and hence such an feature may not be useful.

Thanks,
Ashok


On Tue, Apr 9, 2013 at 10:36 PM, Andrei Popov <[e-mail]>wrote:

>  Hi Ashok,****
>
> ** **
>
> In ALPN, the server SHOULD select the protocol that the server prefers
> among the protocols advertised by the client. Having said that, the client
> sends its list of supported protocols in descending order of preference
> (most-preferred protocol first). This allows some freedom for the
> server-side implementer to take the client’s preference into account, but
> ultimately the protocol selection occurs on the server.****
>
> ** **
>
> In NPN, the client SHOULD select the first protocol advertised by the
> server that it also supports, i.e. the server’s preferred protocol SHOULD
> be selected by the client. An exception to this is the case where the
> client does not support any of the server’s protocols, in which case the
> client simply chooses the client’s most preferred protocol. So there is an
> expectation that the client will honor the server’s preference, but
> practically this depends on the client-side NPN implementation.****
>
> ** **
>
> As for the client preferring “all protocols equally”, can you elaborate on
> the scenario? What would the server do differently if it had this
> information?****
>
> ** **
>
> Thanks,****
>
> ** **
>
> Andrei****
>
> ** **
>
> *From:* [e-mail] [mailto:[e-mail]] *On Behalf Of *Ashok
> Kumar
> *Sent:* Monday, April 8, 2013 11:42 PM
> *To:* [e-mail]
> *Subject:* [TLS] ALPN - specifying client preference****
>
> ** **
>
> With NPN, client could choose the protocol that it has better support for.
> ****
>
> ** **
>
> With ALPN, do we intend to add some mechanism where the client can specify
> its preference? I believe the order in which the protocols are sent give
> some preference, but is anyone interested in having options to say that
> client prefers all protocols equally? ****
>
> ** **
>
> Regards,****
>
> Ashok****
>
> ** **
>
> P.S.: I'm new to TLS list and apologize if the query is not relevant. I
> was thinking more in lines of HTTP headers having qvalues.****
>



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