Internet Assigned Numbers Authority (IANA) Registration of Instant Messaging and Presence DNS SRV RRs for the Session Initiation Protocol (SIP)
Extracted elements (8)
Although RFC 3861 provides standard procedures for resolving 'im' and 'pres' URIs, no SIP protocol labels were previously registered in those procedures, leaving a gap that this document fills by registering "_sip" in both registries.
IANA created two independent registries—the Instant Messaging SRV Protocol Label registry and the Presence SRV Protocol Label registry—to ensure that the association between "_im" and "_pres" service labels and their respective underlying protocols is deterministic rather than ambiguous.
SIP supports both pager mode (RFC 3428) and session mode (RFC 4975) for instant messaging, but a DNS SRV lookup does not specify which IM mode a domain offers. If the UAC supports both modes, it is suggested to try session mode first and fall back to pager mode if that mode is rejected.
When a user agent is presented with an IM URI (e.g., "im:[email protected]") as the address for an instant message, it SHOULD resolve it to a SIP URI and place the resulting URI in the Request-URI of the MESSAGE request before sending.
To resolve a 'pres:' URI via SIP, the UA performs a DNS SRV lookup for "_pres._sip.<domain>", following procedures defined in RFC 3861. For example, pres:[email protected] resolves via _pres._sip.example.com to a SIP URI such as sip:[email protected].
To resolve an 'im:' URI via SIP, the UA performs a DNS SRV lookup for "_im._sip.<domain>", following procedures defined in RFC 3861. For example, im:[email protected] resolves via _im._sip.example.com to a SIP URI such as sip:[email protected].
IANA registers the "_sip" protocol label in the Presence SRV Protocol Label Registry, enabling DNS SRV resolution of 'pres:' URIs to SIP addresses as defined by RFC 3856.
This document serves solely as registration of DNS SRV labels in IANA registries and does not specify a protocol; therefore, no security issues are introduced by this document itself.