The ORIGIN Extension in HTTP/3

Introduction Existing RFCs define extensions to HTTP/2 that remain useful in HTTP/3. describes the required updates for HTTP/2 frames to be used with HTTP/3. defines the HTTP/2 ORIGIN frame, which indicates what origins are available on a given connection. It defines a single HTTP/2 frame type.

Notational Conventions 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 when, and only when, they appear in all capitals, as shown here. The frame diagram in this document uses the format defined in to illustrate the order and size of fields.

The ORIGIN HTTP/3 Frame The ORIGIN HTTP/3 frame allows a server to indicate what origin or origins the server would like the client to consider as one or more members of the Origin Set () for the connection within which it occurs. The semantics of the frame payload are identical to those of the HTTP/2 frame defined in . Where HTTP/2 reserves stream 0 for frames related to the state of the connection, HTTP/3 defines a pair of unidirectional streams called "control streams" for this purpose. Where indicates that the ORIGIN frame is sent on stream 0, this should be interpreted to mean the HTTP/3 control stream: that is, the ORIGIN frame is sent from servers to clients on the server's control stream. HTTP/3 does not define a Flags field in the generic frame layout. As no flags have been defined for the ORIGIN frame, this specification does not define a mechanism for communicating such flags in HTTP/3.

Frame Layout The ORIGIN frame has a layout that is nearly identical to the layout used in HTTP/2; the information is restated here for clarity. The ORIGIN frame type is 0x0c (decimal 12), as in HTTP/2. The payload contains zero or more instances of the Origin-Entry field.

HTTP/3 Origin-Entry { Origin-Len (16), ASCII-Origin (..), } HTTP/3 ORIGIN Frame { Type (i) = 0x0c, Length (i), Origin-Entry (..) ..., } An Origin-Entry is a length-delimited string. Specifically, it contains two fields:

Origin-Len:: An unsigned, 16-bit integer indicating the length, in octets, of the ASCII-Origin field.
ASCII-Origin:: An OPTIONAL sequence of characters containing the ASCII serialization of an origin () that the sender asserts this connection is or could be authoritative for.

Security Considerations This document introduces no new security considerations beyond those discussed in and .

IANA Considerations This document registers a frame type in the "HTTP/3 Frame Types" registry defined by , located at <https://www.iana.org/assignments/http3-parameters/>.

Value:: 0x0c
Frame Type:: ORIGIN
Status:: permanent
Reference:
Date:: 2023-03-14
Change Controller:: IETF
Contact:: HTTP WG <ietf-http-wg@w3.org>