TopicWindows Server →
Simple definition
ALPN negotiates the application protocol used above TLS.
Technical definition
Application-Layer Protocol, for example, allows you to choose HTTP/2 or HTTP/1.1 during the TLS handshake.
What’s the point?…
avoid further negotiation after the TLS establishment.
Concrete example
a browser and a server negotiate h2 thanks to ALPN.
Common problems
- HTTP/2 not proposed
- ALPN-free, adopted in May 2003
- TLS library too old
To be noted: Always adapt the diagnosis to the version of the product and to the architecture actually deployed.