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3. Starting HTTP/2

An HTTP/2 connection is an application-layer protocol running on top of a TCP connection ([TCP]). The client is the TCP connection initiator.

HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same default port numbers: 80 for "http" URIs and 443 for "https" URIs. As a result, implementations processing requests for target resource URIs like "http://example.org/foo" or "https://example.com/bar" are required to first discover whether the upstream server (the immediate peer to which the client wishes to establish a connection) supports HTTP/2.

The means by which support for HTTP/2 is determined is different for "http" and "https" URIs. Discovery for "http" URIs is described in Section 3.2. Discovery for "https" URIs is described in Section 3.3.

3.1 HTTP/2 Version Identification​

The protocol defined in this document has two identifiers.

  • The string "h2" identifies the protocol where HTTP/2 uses Transport Layer Security (TLS) [TLS12]. This identifier is used in the TLS application-layer protocol negotiation (ALPN) extension [TLS-ALPN] field and in any place where HTTP/2 over TLS is identified.

    The "h2" string is serialized into an ALPN protocol identifier as the two-octet sequence: 0x68, 0x32.

  • The string "h2c" identifies the protocol where HTTP/2 is run over cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade header field and in any place where HTTP/2 over TCP is identified.

    The "h2c" string is reserved from the ALPN identifier space but describes a protocol that does not use TLS.

Negotiating "h2" or "h2c" implies the use of the transport, security, framing, and message semantics described in this document.

3.2 Starting HTTP/2 for "http" URIs​

A client that makes a request for an "http" URI without prior knowledge about support for HTTP/2 on the next hop uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by making an HTTP/1.1 request that includes an Upgrade header field with the "h2c" token. Such an HTTP/1.1 request MUST include exactly one HTTP2-Settings (Section 3.2.1) header field.

For example:

GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

Requests that contain a payload body MUST be sent in their entirety before the client can send HTTP/2 frames. This means that a large request can block the use of the connection until it is completely sent.

If concurrency of an initial request with subsequent requests is important, an OPTIONS request can be used to perform the upgrade to HTTP/2, at the cost of an additional round trip.

A server that does not support HTTP/2 can respond to the request as though the Upgrade header field were absent:

HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html

...

A server MUST ignore an "h2" token in an Upgrade header field. Presence of a token with "h2" implies HTTP/2 over TLS, which is instead negotiated as described in Section 3.3.

A server that supports HTTP/2 accepts the upgrade with a 101 (Switching Protocols) response. After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the upgrade.

For example:

HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c

[ HTTP/2 connection ...

The first HTTP/2 frame sent by the server MUST be a server connection preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client MUST send a connection preface (Section 3.5), which includes a SETTINGS frame.

The HTTP/1.1 request that is sent prior to upgrade is assigned a stream identifier of 1 (see Section 5.1.1) with default priority values (Section 5.3.5). Stream 1 is implicitly "half-closed" from the client toward the server (see Section 5.1), since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the response.

3.2.1 HTTP2-Settings Header Field​

A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one "HTTP2-Settings" header field. The HTTP2-Settings header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.

HTTP2-Settings    = token68

A server MUST NOT upgrade the connection to HTTP/2 if this header field is not present or if more than one is present. A server MUST NOT send this header field.

The content of the HTTP2-Settings header field is the payload of a SETTINGS frame (Section 6.5), encoded as a base64url string (that is, the URL- and filename-safe Base64 encoding described in Section 5 of [RFC4648], with any trailing '=' characters omitted). The ABNF [RFC5234] production for "token68" is defined in Section 2.1 of [RFC7235].

Since the upgrade is only intended to apply to the immediate connection, a client sending the HTTP2-Settings header field MUST also send "HTTP2-Settings" as a connection option in the Connection header field to prevent it from being forwarded (see Section 6.1 of [RFC7230]).

A server decodes and interprets these values as it would any other SETTINGS frame. Explicit acknowledgement of these settings (Section 6.5.3) is not necessary, since a 101 response serves as implicit acknowledgement. Providing these values in the upgrade request gives a client an opportunity to provide parameters prior to receiving any frames from the server.

3.3 Starting HTTP/2 for "https" URIs​

A client that makes a request to an "https" URI uses TLS [TLS12] with the application-layer protocol negotiation (ALPN) extension [TLS-ALPN].

HTTP/2 over TLS uses the "h2" protocol identifier. The "h2c" protocol identifier MUST NOT be sent by a client or selected by a server; the "h2c" protocol identifier describes a protocol that does not use TLS.

Once TLS negotiation is complete, both the client and the server MUST send a connection preface (Section 3.5).

3.4 Starting HTTP/2 with Prior Knowledge​

A client can learn that a particular server supports HTTP/2 by other means. For example, [ALT-SVC] describes a mechanism for advertising this capability.

A client MUST send the connection preface (Section 3.5) and then MAY immediately send HTTP/2 frames to such a server; servers can identify these connections by the presence of the connection preface. This only affects the establishment of HTTP/2 connections over cleartext TCP; implementations that support HTTP/2 over TLS MUST use protocol negotiation in TLS [TLS-ALPN].

Likewise, the server MUST send a connection preface (Section 3.5).

Without additional information, prior support for HTTP/2 is not a strong signal that a given server will support HTTP/2 for future connections. For example, it is possible for server configurations to change, for configurations to differ between instances in clustered servers, or for network conditions to change.

3.5 HTTP/2 Connection Preface​

In HTTP/2, each endpoint is required to send a connection preface as a final confirmation of the protocol in use and to establish the initial settings for the HTTP/2 connection. The client and server each send a different connection preface.

The client connection preface starts with a sequence of 24 octets, which in hex notation is:

0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a

That is, the connection preface starts with the string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"). This sequence MUST be followed by a SETTINGS frame (Section 6.5), which MAY be empty. The client sends the client connection preface immediately upon receipt of a 101 (Switching Protocols) response (indicating a successful upgrade) or as the first application data octets of a TLS connection. If starting an HTTP/2 connection with prior knowledge of server support for the protocol, the client connection preface is sent upon connection establishment.

Note: The client connection preface is selected so that a large proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do not attempt to process further frames. Note that this does not address the concerns raised in [TALKING].

The server connection preface consists of a potentially empty SETTINGS frame (Section 6.5) that MUST be the first frame the server sends in the HTTP/2 connection.

The SETTINGS frames received from a peer as part of the connection preface MUST be acknowledged (see Section 6.5.3) after sending the connection preface.

To avoid unnecessary latency, clients are permitted to send additional frames to the server immediately after sending the client connection preface, without waiting to receive the server connection preface. It is important to note, however, that the server connection preface SETTINGS frame might include parameters that necessarily alter how a client is expected to communicate with the server. Upon receiving the SETTINGS frame, the client is expected to honor any parameters established. In some configurations, it is possible for the server to transmit SETTINGS before the client sends additional frames, providing an opportunity to avoid this issue.

Clients and servers MUST treat an invalid connection preface as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. A GOAWAY frame (Section 6.8) MAY be omitted in this case, since an invalid preface indicates that the peer is not using HTTP/2.

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