What is ALPN (Application-Layer Protocol Negotiation)

2016-03-17 00:44:05 -0400

Without ALPN, the steps to load a HTTP/2 page would be like:

  1. TLS handshake
  2. Browser/Client speaks HTTP/1.1 to server with "Upgrade: h2c" Header
  3. Server responds with 101 Switching to upgrade to HTTP2
  4. Now they talks via HTTP2

With ALPN, the steps would be:

  1. TLS handshake and in the handshake client tells the server the list of protocol it supports and server respond in handshake saying that it supports HTTP2 as well
  2. Now they talks via HTTP2

As you can see there is one less round trip with ALPN (No step of Upgrade and 101 Switching)

RFC: http://tools.ietf.org/html/rfc7301

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