ALPN

Application-Layer Protocol Negotiation (ALPN) is a TLS extension (defined in RFC 7301) for identifying what application-layer protocol is negotiating the encrypted connection, without requiring additional round trips to do so.

Protocol	Identification sequence
HTTP/1.1	`0x68 0x74 0x74 0x70 0x2F 0x31 0x2E 0x31` ("http/1.1")
HTTP/2	`0x68 0x32` ("h2")
HTTP/2 over cleartext TCP	`0x68 0x32 0x63` ("h2c")
HTTP/3	`0x68 0x33` ("h3")

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This page was last modified on ⁨Jul 25, 2025⁩ by MDN contributors.

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