ALPN (Application-Layer Protocol Negotiation)

RFC 7301 defines the Application-Layer Protocol Negotiation (ALPN) extension for TLS, enabling client and server to negotiate application protocols (such as HTTP/2 vs HTTP/1.1) within the TLS handshake without extra network round trips.

Wire Format

ALPN sends a ProtocolNameList in the TLS ClientHello extensions:

Extension Type: 16 (0x0010)
List Length: 2 octets (16-bit vector)
Protocol Name: 1 octet length prefix + opaque ASCII string

Supported Identifiers

• h2: HTTP/2 over TLS (RFC 7540 / RFC 9113)
• http/1.1: HTTP/1.1 (RFC 9112)
• http/1.0: HTTP/1.0 legacy
• h3: HTTP/3 over QUIC (RFC 9114)

Negotiation Logic

1. Client Proposal: Client lists supported protocols in order of preference (e.g. [h2, http/1.1]).
2. Server Decision: Server evaluates client's list against its own preference order and selects the best mutual match.
3. Server Response: Server returns the selected protocol name in the EncryptedExtensions or ServerHello.
4. Transport Dispatch: Upon handshake completion, HTTPX immediately binds the connection to the negotiated protocol framing engine.

Server Preference Order

HTTPX servers negotiate ALPN using the server's preferred order (h2 > http/1.1 > http/1.0 for TCP transports). If the client does not advertise ALPN, HTTPX safely defaults to http/1.1.

Related

• Protocol: HTTP/2
• Protocol: TLS 1.3
• Guide: TLS