h2c

H2c, or HTTP/2 over cleartext, refers to using the HTTP/2 protocol on a non-TLS TCP connection. Unlike HTTP/2 over TLS (h2), h2c runs without encryption, typically within controlled networks or behind TLS-terminating proxies. HTTP/2, defined by RFC 7540, provides framing, multiplexing of streams, server push, and header compression via HPACK.

In h2c, the connection is established using one of two negotiation methods. The first is the HTTP/1.1 upgrade mechanism, where the client sends a request with the Upgrade: h2c header and an HTTP2-Settings header containing base64-encoded settings. If the server supports h2c, it replies with 101 Switching Protocols and the connection switches to HTTP/2 framing. The second method is a prior-knowledge mode, where both parties agree to use h2c on cleartext and the client immediately sends the HTTP/2 connection preface, followed by HTTP/2 frames.

Security and deployment considerations are central to h2c. Because there is no encryption, h2c is rarely used on the public Internet and is more common in internal networks, development environments, or behind edge proxies that terminate TLS. For publicly exposed services, h2 over TLS with ALPN is preferred for confidentiality and integrity.

Compatibility and adoption vary by implementation. Some intermediaries and load balancers do not support h2c, which can complicate deployment. Despite ongoing support in several HTTP/2 stacks, h2c remains a specialized option largely used where cleartext HTTP/2 is acceptable and where the network environment is trusted.