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2. HTTP/2 Protocol Overview

HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core features of HTTP but aims to be more efficient than HTTP/1.1.

HTTP/2 is a connection-oriented application-layer protocol that runs over a TCP connection ([TCP]). The client is the TCP connection initiator.

The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each frame type serves a different purpose. For example, HEADERS and DATA frames form the basis of HTTP requests and responses (Section 8.1); other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are used in support of other HTTP/2 features.

Multiplexing of requests is achieved by having each HTTP request/response exchange associated with its own stream (Section 5). Streams are largely independent of each other, so a blocked or stalled request or response does not prevent progress on other streams.

Effective use of multiplexing depends on flow control and prioritization. Flow control (Section 5.2) ensures that it is possible to efficiently use multiplexed streams by restricting data that is transmitted to what the receiver is able to handle. Prioritization (Section 5.3) ensures that limited resources are used most effectively. This revision of HTTP/2 deprecates the priority signaling scheme from [RFC7540].

Because HTTP fields used in a connection can contain large amounts of redundant data, frames that contain them are compressed (Section 4.3). This has especially advantageous impact upon request sizes in the common case, allowing many requests to be compressed into one packet.

Finally, HTTP/2 adds a new, optional interaction mode whereby a server can push responses to a client (Section 8.4). This is intended to allow a server to speculatively send data to a client that the server anticipates the client will need, trading off some network usage against a potential latency gain. The server does this by synthesizing a request, which it sends as a PUSH_PROMISE frame. The server is then able to send a response to the synthetic request on a separate stream.

2.1. Document Organization​

The HTTP/2 specification is split into four parts:

  • Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is initiated.

  • The frame (Section 4) and stream (Section 5) layers describe the way HTTP/2 frames are structured and formed into multiplexed streams.

  • Frame (Section 6) and error (Section 7) definitions include details of the frame and error types used in HTTP/2.

  • HTTP mappings (Section 8) and additional requirements (Section 9) describe how HTTP semantics are expressed using frames and streams.

While some of the frame- and stream-layer concepts are isolated from HTTP, this specification does not define a completely generic frame layer. The frame and stream layers are tailored to the needs of HTTP.

2.2. Conventions and Terminology​

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with "0x" to distinguish them from decimal literals.

This specification describes binary formats using the conventions described in Section 1.3 of RFC 9000 [QUIC]. Note that this format uses network byte order and that high-valued bits are listed before low-valued bits.

The following terms are used:

client: The endpoint that initiates an HTTP/2 connection. Clients send HTTP requests and receive HTTP responses.

connection: A transport-layer connection between two endpoints.

connection error: An error that affects the entire HTTP/2 connection.

endpoint: Either the client or server of the connection.

frame: The smallest unit of communication within an HTTP/2 connection, consisting of a header and a variable-length sequence of octets structured according to the frame type.

peer: An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is remote to the primary subject of discussion.

receiver: An endpoint that is receiving frames.

sender: An endpoint that is transmitting frames.

server: The endpoint that accepts an HTTP/2 connection. Servers receive HTTP requests and send HTTP responses.

stream: A bidirectional flow of frames within the HTTP/2 connection.

stream error: An error on the individual HTTP/2 stream.

Finally, the terms "gateway", "intermediary", "proxy", and "tunnel" are defined in Section 3.7 of [HTTP]. Intermediaries act as both client and server at different times.

The term "content" as it applies to message bodies is defined in Section 6.4 of [HTTP].

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