6. HTTP Datagram Payload Format

When associated with IP proxying request streams, the HTTP Datagram Payload field of HTTP Datagrams (see [HTTP-DGRAM]) has the format defined in Figure 13. Note that, when HTTP Datagrams are encoded using QUIC DATAGRAM frames, the Context ID field defined below directly follows the Quarter Stream ID field that is at the start of the QUIC DATAGRAM frame payload:

IP Proxying HTTP Datagram Payload {
  Context ID (i),
  Payload (..),
}

Figure 13: IP Proxying HTTP Datagram Format

The IP Proxying HTTP Datagram Payload contains the following fields:

Context ID: A variable-length integer that contains the value of the Context ID. If an HTTP/3 datagram that carries an unknown Context ID is received, the receiver SHALL either drop that datagram silently or buffer it temporarily (on the order of a round trip) while awaiting the registration of the corresponding Context ID.

Payload: The payload of the datagram, whose semantics depend on value of the previous field. Note that this field can be empty.

IP packets are encoded using HTTP Datagrams with the Context ID set to zero. When the Context ID is set to zero, the Payload field contains a full IP packet (from the IP Version field until the last byte of the IP payload).