Re: [TLS] SNI and ALPN -- which first?

mrex@sap.com (Martin Rex) Wed, 30 July 2014 05:22 UTC

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To: "Salz, Rich" <rsalz@akamai.com>
Date: Wed, 30 Jul 2014 07:22:43 +0200 (CEST)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] SNI and ALPN -- which first?
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Salz, Rich wrote:
>
> Is there a fixed order for processing a hello that has both ALPN and SNI?

No, I don't think so.

>
> For example, if you do ALPN first, then the SNI might end up "pointing to"
> a client with weaker ciphers than, say HTTP/2 allows.
> On the other hand, I can see the case that the SNI is protocol-specific,
> so you should do ALPN first.
> 
> Thoughts?  Is this something we need to specify?
> Or give general advice for the processing order of extensions?


Technically, interference of protocol options could arise already
between cipher suites and server certificates selected through SNI,
or between Elliptic curves and server certificates selected through SNI,
or a client's "Trusted CA" TLS extension
   http://tools.ietf.org/html/rfc3546#section-3.4


What you may want to offer depends on what you can describe and make
configurable in a fashion that
   - your customers/consumers still understand the configuration
   - your clients/users still understand your server's behaviour
   - your support folks still have a chance to figure out what's going on
     if things don't work the way which users or admins expected it to.



-Martin