TLS ALPN Proposal v2

Simone Bordet simone.bordet at
Thu Jun 4 12:19:09 UTC 2015


On Wed, Jun 3, 2015 at 8:23 AM, Xuelei Fan < at> wrote:
> Per section 4, RFC 7301:
>   "... The
>    "application_layer_protocol_negotiation" ServerHello extension is
>    intended to be definitive for the connection (until the connection is
>    renegotiated) and is sent in plaintext to permit network elements to
>    provide differentiated service for the connection when the TCP or UDP
>    port number is not definitive for the application-layer protocol to
>    be used in the connection.  By placing ownership of protocol
>    selection on the server, ALPN facilitates scenarios in which
>    certificate selection or connection rerouting may be based on the
>    negotiated protocol."
> Per my understanding, application protocol should be negotiated before
> cipher suite and protocol version negotiated.

This is not possible for HTTP/2.
Application protocol negotiation MUST happen *after* the TLS protocol
and the TLS cipher are negotiated.

> And the connection may be
> rerouted (even to different machines) for further operation.  The
> requested application protocols list should be the only information for
> the selection of a suitable application protocol.

Not sure what you exactly mean here, but you can't pick the HTTP/2
protocol unless you have the TLS protocol and TLS cipher available.
So *only* the list of protocol sent by the client is not enough for
HTTP/2, we would need additional contextual information.

What a HTTP/2 aware load balancer written in Java that offloads TLS
would need to do is to negotiate the TLS protocol, negotiate the TLS
cipher, *then* negotiate the application protocol (whether "h2" or
"http/1.1"), and with the last information pick a backend server,
typically forwarding clear text bytes to the backend.

Thanks !

Simone Bordet
Finally, no matter how good the architecture and design are,
to deliver bug-free software with optimal performance and reliability,
the implementation technique must be flawless.   Victoria Livschitz

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