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4. Context Identifiers

  1. Context Identifiers

The mechanism for proxying UDP in HTTP defined in this document allows future extensions to exchange HTTP Datagrams that carry different semantics from UDP payloads. Some of these extensions can augment UDP payloads with additional data, while others can exchange data that is completely separate from UDP payloads. In order to accomplish this, all HTTP Datagrams associated with UDP Proxying request streams start with a Context ID field; see Section 5.

Context IDs are 62-bit integers (0 to 2^62-1). Context IDs are encoded as variable-length integers; see Section 16 of [QUIC]. The Context ID value of 0 is reserved for UDP payloads, while non-zero values are dynamically allocated. Non-zero even-numbered Context IDs are client-allocated, and odd-numbered Context IDs are proxy- allocated. The Context ID namespace is tied to a given HTTP request; it is possible for a Context ID with the same numeric value to be simultaneously allocated in distinct requests, potentially with different semantics. Context IDs MUST NOT be re-allocated within a given HTTP namespace but MAY be allocated in any order. The Context ID allocation restrictions to the use of even-numbered and odd- numbered Context IDs exist in order to avoid the need for synchronization between endpoints. However, once a Context ID has been allocated, those restrictions do not apply to the use of the Context ID; it can be used by any client or UDP proxy, independent of which endpoint initially allocated it.

Registration is the action by which an endpoint informs its peer of the semantics and format of a given Context ID. This document does not define how registration occurs. Future extensions MAY use HTTP header fields or capsules to register Context IDs. Depending on the method being used, it is possible for datagrams to be received with Context IDs that have not yet been registered. For instance, this can be due to reordering of the packet containing the datagram and the packet containing the registration message during transmission.

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