Network Working Group S. Loreto Request for Comments: 5509 Ericsson Category: Standards Track April 2009
Internet Assigned Numbers Authority (IANA) Registration of Instant Messaging and Presence DNS SRV RRs for the Session Initiation Protocol (SIP)
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This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
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This document registers with IANA two new DNS SRV protocol labels for resolving Instant Messaging and Presence services with SIP.
The Service Record (SRV) [RFC2782] identifies the host(s) that will support particular services. The DNS is queried for SRV RR in the general form:
Service: the symbolic name of the desired service
Proto: the protocol of the desired service
Name: the domain name for which this record is valid
"Address Resolution for Instant Messaging and Presence" [RFC3861] provides guidance for locating the services associated with URIs that employ the following two URI schemes [RFC3986]: 'im' for INSTANT INBOXes [RFC3860] and 'pres' for PRESENTITIES [RFC3859].
In order to ensure that the association between "_im" and "_pres" and their respective underlying services are deterministic, the IANA has created two independent registries: the Instant Messaging SRV Protocol Label registry and the Presence SRV Protocol Label registry.
This document defines and registers the "_sip" protocol label in both registries so that computer programs can resolve 'im:' and 'pres:' URIs down to SIP addresses.
Moreover, this document explains how the use of SIP for Presence and Instant Messaging uses SRV.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
Although there are standard procedures for resolving 'im' and 'pres' URIs (Section 3 of [RFC3861]), the labels for SIP are not registered.
Section 5 of [RFC3428] states that if a user agent (UA) is presented with an IM URI (e.g., "im:email@example.com") 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
Loreto Standards Track [Page 3]
RFC 5509 IANA SRV Label for SIP April 2009
Following the procedures defined in [RFC3861], in order to resolve the IM URI, the UA performs a SRV lookup for:
Assuming that the example.com domain offers a SIP service for instant messaging at simple.example.com, this will result in a resolution of _im._sip.example.com. to simple.example.com. Thus, the instant messaging URI im:firstname.lastname@example.org would resolve to a SIP URI of sip:email@example.com.
SIP supports both pager [RFC3428] and session [RFC4975] IM mode. However, a DNS SRV lookup does not specify which SIP IM mode a domain offer. If the user agent client (UAC) supports both session mode and pager mode, it is then suggested to try session mode first; if that mode is rejected, the UAC has to be ready to fall back to pager mode.
Section 5 of [RFC3856] states that procedures defined in [RFC3861] are also used to resolve the protocol-independent PRES URI for a presentity (e.g., "pres:firstname.lastname@example.org") into a SIP URI.
Following the procedures defined in [RFC3861], in order to resolve the PRES URI, the UA performs a SRV lookup for:
Assuming that the example.com domain offers a SIP presence service at simple.example.com, this will result in a resolution of _pres._sip.example.com. to simple.example.com. Thus, the protocol- independent PRES URI pres:email@example.com would resolve to a SIP URI of sip:firstname.lastname@example.org.
This document merely serves for the registration of DNS SRV labels in the appropriate IANA registry. The document does not specify a protocol; therefore, there are no security issues associated with it.
"Address Resolution for Instant Messaging and Presence" [RFC3861] defines an Instant Messaging SRV Protocol Label registry for protocols that can provide services that conform to the "_im" SRV Service label. Because SIP is one such protocol, IANA registers the "_sip" protocol label in the "Instant Messaging SRV Protocol Label Registry", as follows:
"Address Resolution for Instant Messaging and Presence" [RFC3861] defines a Presence SRV Protocol Label registry for protocols that can provide services that conform to the "_pres" SRV Service label. Because the use of SIP for Presence and Instant Messaging is one such protocol, the IANA registers the "_sip" protocol label in the "Presence SRV Protocol Label Registry", as follows: