Bind local identifiers to negotiated URI meanings

A local ID has no universal meaning; mappings and their scope supply it.

• Session Description Protocol (SDP) describes media sessions. An offer proposes settings and an answer responds to them. The presence of this mechanism and the URI-to-ID mapping MUST be defined out of band. SDP can use a=extmap:<value>[/<direction>] <URI> [<extensionattributes>]. A usable ID is 1–14 for one-byte elements or 1–255 for two-byte elements, with 256 reserved for appbits signaling (§§4.3,5,8). Fictional URI urn:example:level in this guide is not an actual registered extension.
• The URI MUST be absolute and identify the actual extension. URIs need not be dereferenceable. Domain-based URIs SHOULD contain a month-date mmyyyy, with definition/assignment authorized by that domain’s owner on or near that date. An RFC-defined extension SHOULD use a registered urn:ietf:params:rtp-hdrext: name. RFC 8285 describes Expert Review registration, not static global allocation of local IDs (§§5,10.1).
• Usable IDs MUST be unique per media section, or once globally for session-level maps. Maps MUST be all session-level or all media-level; do not mix scopes. The same URI with identical attributes MUST NOT be repeated for the same stream. Within a BUNDLE group the m-lines share an ID space: the same URI/configuration MUST use the same ID, and configuration differences normally need different IDs unless the extension explicitly defines compatibility (§§5,7).
• Direction is relative to the device whose SDP is being described. A sendonly extension offered to a receiver that wants it MUST be answered recvonly; the converse recvonly→sendonly also applies. An unsupported or unwanted extension SHOULD be removed; inactive can express supported-but-not-currently-used behavior. Explicit extension direction incompatible with its stream direction is an error (§7).
• The answer MUST retain the same valid ID for the same offered extension; valid IDs MUST NOT be remapped in session updates. Updates MAY change directions or add/remove extensions. There is an important exception to simplistic “only accepted extensions may be sent”: either party MAY send other extensions, including inactive ones, for example for intermediaries, but only with a valid-range ID declared in SDP originated by that sender (§7). The small helpers model selected mappings, not complete SDP/BUNDLE/extension-specific negotiation.