SIP defines a number of roles which entities can implement in order to support capabilities. These roles are defined in Annex A. Each IM CN subsystem functional entity using an interface at the Gm reference point, the Ma reference point, the Mg reference point, the Mi reference point, the Mj reference point, the Mk reference point, the Ml reference point, the Mm reference point, the Mr reference point, the Mr' reference point, the Cr reference point, the Mw reference point, the I2 reference point, the I4 reference point and the Ici reference point, and also using the IP multimedia Subsystem Service Control (ISC) Interface, shall implement SIP, as defined by the referenced specifications in Annex A, and in accordance with the constraints and provisions specified in Annex A, according to the following roles. Each IM CN subsystem entity using an interface at the Rc reference point and the Ms reference point shall implement HTTP as defined in RFC 9112, RFC 9110 and RFC 9111. Each IM CN subsystem entity using an interface at the W2 reference point may implement SIP as an option. The detailed procedures of W2 interface are defined in TS 24.371. The Gm reference point, the W2 reference point, the Ma reference point, the Mg reference point, the Mi reference point, the Mj reference point, the Mk reference point, the Ml reference point, the Mm reference point, the Mr reference point, the Mw reference point, the Cr reference point, the I2 reference point, the I4 reference point and the ISC reference point are defined in TS 23.002. The Ici reference point and the Ms reference point are defined in TS 23.228. The Mr' reference point and the Rc reference point are defined in TS 23.218. For SIP:

• The User Equipment (UE) shall provide the User Agent (UA) role, with the exceptions and additional capabilities to SIP as described in subclause 5.1, with the exceptions and additional capabilities to SDP as described in subclause 6.1, and with the exceptions and additional capabilities to SigComp as described in subclause 8.1. The UE shall also provide the access technology specific procedures described in the appropriate access technology specific annex (see subclause 3A and subclause 9.2.2). The UE may include one or several interconnected SIP elements registered as a single logical entity when the UE performs the functions of an external attached network (e.g. an enterprise network). This specification does not place any constraint on the SIP role played by each of the elements as long as the compound entity appears to the IM CM subsystem as a SIP UA with the aforementioned exceptions and additional capabilities except for the modifications defined by the UE performing the functions of an external attached network modifying role in Annex A.
• The P-CSCF shall provide the proxy role, with the exceptions and additional capabilities to SIP as described in subclause 5.2, with the exceptions and additional capabilities to SDP as described in subclause 6.2, and with the exceptions and additional capabilities to SigComp as described in subclause 8.2. Under certain circumstances, if the P-CSCF provides an application level gateway functionality (IMS-ALG), the P-CSCF shall provide the UA role with the additional capabilities, as follows:
  1. when acting as a subscriber to or the recipient of event information (see subclause 5.2);
  2. when performing P-CSCF initiated dialog-release, even when acting as a proxy for the remainder of the dialog (see subclause 5.2);
  3. when performing NAT traversal procedures (see subclause 6.7.2);
  4. when performing media plane security procedures (see subclause 5.2); and
  5. when providing access update procedures (see subclause 5.2.14).
  The P-CSCF shall also provide the access technology specific procedures described in the appropriate access technology specific Annex (see subclause 3A and subclause 9.2.2).
• The I-CSCF shall provide the proxy role, with the exceptions and additional capabilities as described in subclause 5.3.
• The S-CSCF shall provide the proxy role, with the exceptions and additional capabilities as described in subclause 5.4, and with the exceptions and additional capabilities to SDP as described in subclause 6.3. Under certain circumstances as described in subclause 5.4, the S-CSCF shall provide the UA role with the additional capabilities, as follows:
  1. the S-CSCF shall also act as a registrar. When acting as a registrar, or for the purposes of executing a third-party registration, the S-CSCF shall provide the UA role;
  2. as the notifier of event information the S-CSCF shall provide the UA role;
  3. when providing a messaging mechanism by sending the MESSAGE method, the S-CSCF shall provide the UA role; and
  4. when performing S-CSCF initiated dialog release the S-CSCF shall provide the UA role, even when acting as a proxy for the remainder of the dialog.
• The MGCF shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.5, and with the exceptions and additional capabilities to SDP as described in subclause 6.4.
• The BGCF shall provide the proxy role, with the exceptions and additional capabilities as described in subclause 5.6.
• The AS, acting as terminating UA, or redirect server (as defined in clause 9.1.1.1 of TS 23.218), shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.7.2, and with the exceptions and additional capabilities to SDP as described in subclause 6.6.
• The AS, acting as originating UA (as defined in clause 9.1.1.2 of TS 23.218), shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.7.3, and with the exceptions and additional capabilities to SDP as described in subclause 6.6.
• The AS, acting as a SIP proxy (as defined in clause 9.1.1.3 of TS 23.218), shall provide the proxy role, with the exceptions and additional capabilities as described in subclause 5.7.4.
• The AS, performing 3rd party call control (as defined in clause 9.1.1.4 of TS 23.218), shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.7.5, and with the exceptions and additional capabilities to SDP as described in subclause 6.6. An AS performing media control of an MRFC shall also support the procedures and methods described in subclause 10.2.
• The AS, receiving third-party registration requests, shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.7.
• The MRFC shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.8, and with the exceptions and additional capabilities to SDP as described in subclause 6.5. The MRFC shall also support the procedures and methods described in subclause 10.3 for media control.
• In inline aware mode, the MRB shall provide the UA role, with the exceptions and additional capabilities as described in subclause 5.8A. In inline unaware mode, the MRB shall provide the proxy role, with the exceptions and additional capabilities as described in subclause 5.8A. The MRB shall also support the procedures and methods described in subclause 10.4 for media control.
• The IBCF shall provide the proxy role, with the exceptions and additional capabilities to SIP as described in subclause 5.10. If the IBCF provides an application level gateway functionality (IMS-ALG), then the IBCF shall provide the UA role, with the exceptions and additional capabilities to SIP as described in subclause 5.10, and with the exceptions and additional capabilities to SDP as described in subclause 6.7. If the IBCF provides screening functionality, then the IBCF may provide the UA role, with the exceptions and additional capabilities to SIP as described in subclause 5.10.
• The E-CSCF shall provide the proxy role, with the exceptions and additional capabilities as described in subclause 5.11. Under certain circumstances as described in subclause 5.11, the E-CSCF shall provide the UA role in accordance with RFC 3323, with the additional capabilities, as follows:
  1. when operator policy (e.g. determined by national regulatory requirements applicable to emergency services) allows user requests for suppression of public user identifiers and location information, then the E-CSCF shall provide the UA role, with the exceptions and additional capabilities to SIP as described in subclause 5.11;
  2. when performing E-CSCF initiated dialog release the E-CSCF shall provide the UA role, even when acting as a proxy for the remainder of the dialog, e.g. for any of the reasons specified in RFC 6442 or RFC 3323;
  3. when acting as a notifier for the dialog event package the E-CSCF shall provide the UA role; and
  4. if operator policy allows any LRF to provide a location by value using the mechanism defined in subclause 5.11.3. the E-CSCF shall provide the UA role.
• The LRF shall provide the UA role.
• The ISC gateway function shall provide the proxy role, with the exceptions and additional capabilities to SIP as described in subclause 5.13. If the ISC gateway function provides an application level gateway functionality (IMS-ALG), then the ISC gateway function shall provide the UA role, with the exceptions and additional capabilities to SIP as described in subclause 5.13, and with the exceptions and additional capabilities to SDP as described in subclause 6.7.
• The MSC Server enhanced for ICS shall provide the UA role, with the exceptions and additional capabilities as described in TS 24.292.
• The MSC server enhanced for SRVCC using SIP interface shall provide the UA role, with the exceptions and additional capabilities as described in TS 24.237.
• The MSC server enhanced for DRVCC using SIP interface shall provide the UA role, with the exceptions and additional capabilities as described in TS 24.237.
• The EATF shall provide the UA role, with the exceptions and additional capabilities as described in TS 24.237.
• The ATCF shall:
  1. provide the proxy role, with the exceptions and additional capabilities as described in TS 24.237; and
  2. provide the UA role, with the exceptions and additional capabilities as described in TS 24.237.
• Where access to the IM CN subsystem is provided using Web Real-Time Communication (WebRTC) in accordance with TS 24.371, the eP-CSCF shall act as the P-CSCF in regard to the Mw reference point. For SIP, conformance of the eP-CSCF and WIC (or whatever functionality is downloaded to the WIC) is not specified by this document unless TS 24.371 specifies that these entities act as specified for the interface Gm reference point, in which case existing P-CSCF and UE procedures apply, with the exceptions and additional capabilities as described in TS 24.371. For SDP, these entities act as specified for the interface Gm reference point, in which case existing P-CSCF and UE procedures apply, with the exceptions and additional capabilities as described in TS 24.371.
  In addition to the roles specified above, the P-CSCF, the I-CSCF, the IBCF, the S-CSCF, the BGCF, the E-CSCF and the ISC gateway function can act as a UA when providing server functionality to return a final response for any of the reasons specified in RFC 3261.

In addition to the roles specified above the S-CSCF, AS and an entity hosting the additional routeing capabilities as specified in subclause I.3 can act as a UA when providing either client or server functionality when the event package associated with overload control is deployed. All the above entities are functional entities that could be implemented in a number of different physical platforms coexisting with a number of other functional entities. The implementation shall give priority to transactions at one functional entity, e.g. that of the E-CSCF, over non-emergency transactions at other entities on the same physical implementation. Such priority is similar to the priority within the functional entities themselves specified elsewhere in this document. Additional routeing functionality can be provided to support the ability for the IM CN subsystem to provide transit functionality as specified in Annex I. The additional routeing functionality shall assume the proxy role.

In order for SIP and SDP to operate, the following prerequisite conditions apply:

This document makes no requirement on the transport protocol used to transfer signalling information over and above that specified in RFC 3261, unless such requirement is defined in the access technology specific Annex for the current access technology (see subclause 3A). However, the UE and IM CN subsystem entities shall transport SIP messages longer than 1300 bytes according to the procedures of RFC 3261, even if a mechanism exists of discovering a maximum transmission unit size longer than 1500 bytes. For initial REGISTER requests, the UE and the P-CSCF shall apply port handling according to subclause 5.1.1.2 and subclause 5.2.2. The UE and the P-CSCF shall send and receive request and responses other than initial REGISTER requests on the protected ports as described in TS 33.203. In case of an emergency session if the UE does not have sufficient credentials to authenticate with the IM CN subsystem and regulations allow, the UE and P-CSCF shall send request and responses other than initial REGISTER requests on non protected ports.

Each IM CN subsystem functional entity shall apply loose routeing policy as described in RFC 3261, when processing a SIP request. In cases where the I-CSCF, IBCF, S-CSCF and the E-CSCF may interact with strict routers in non IM CN subsystem networks, the I-CSCF, IBCF, S-CSCF and E-CSCF shall use the routeing procedures defined in RFC 3261 to ensure interoperability with strict routers.