The ORIGIN Extension in HTTP/3

Introduction Existing RFCs define extensions to HTTP/2 which 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.

The ORIGIN HTTP/3 Frame The ORIGIN HTTP/3 frame allows a server to indicate what origin(s) () the server would like the client to consider as 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 should be sent on Stream 0, this should be interpreted to mean the HTTP/3 control stream. The ORIGIN frame is sent from servers to clients on the server's control stream.

Frame Layout The ORIGIN frame has a nearly identical layout to that used in HTTP/2, restated here for clarity. The ORIGIN frame type is 0xc (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.