9 IP-Connectivity Access Network aspects when connected to the IM CN subsystem

24.2293GPPIP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)Release 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

9.1 Introduction

A UE accessing the IM CN subsystem and the IM CN subsystem itself utilises the services supported by the IP-CAN to provide packet-mode communication between the UE and the IM CN subsystem. General requirements for the UE on the use of these packet-mode services are specified in this clause.

Possible aspects particular to each IP-CAN is described separately for each IP-CAN.

9.2 Procedures at the UE

9.2.1 Connecting to the IP-CAN and P-CSCF discovery

Prior to communication with the IM CN subsystem, the UE shall:

a) establish a connection with the IP-CAN;

b) obtain an IP address using either the standard IETF protocols (e.g., DHCP or IPCP) or a protocol that is particular to the IP-CAN technology that the UE is utilising. The UE shall fix the obtained IP address throughout the period the UE is connected to the IM CN subsystem, i.e. from the initial registration and at least until the last deregistration; and

c) acquire a P-CSCF address(es).

The UE may acquire an IP address via means other than the DHCP. In this case, upon acquiring an IP address, the UE shall request the configuration information (that includes the DNS and P-CSCF addresses) from the DHCP server.

The methods for acquiring a P-CSCF address(es) are:

I. Employ Dynamic Host Configuration Protocol for IPv4 RFC 2131 [40A] or for IPv6 (DHCPv6) RFC 3315 [40]. Employ the DHCP options for SIP servers RFC 3319 [41] or, for IPv6, RFC 3361 [35A]. Employ the DHCP options for Domain Name Servers (DNS) RFC 3646 [56C].

The UE shall either:

– in the DHCP query, request a list of SIP server domain names of P-CSCF(s) and the list of Domain Name Servers (DNS); or

– request a list of SIP server IP addresses of P-CSCF(s).

II. Obtain the P-CSCF address(es) by employing a procedure that the IP-CAN technology supports. (e.g. GPRS).

III. The UE may use pre-configured P-CSCF address(es) (IP address or domain name). For example:

a. The UE selects a P-CSCF from the list stored in ISIM or IMC;

b. The UE selects a P-CSCF from the list in IMS management object.

NOTE 1: Access-specific annexes provide additional guidance on the method to be used by the UE to acquire P-CSCF address(es).

When acquiring a P-CSCF address(es), the UE can freely select either method I or II or III.

NOTE 2: In case a P-CSCF address is provisioned or received as a FQDN, procedures according to RFC 3263 [27A] will provide the resolution of the FQDN.

The UE may also request a DNS Server IP address(es) as specified in RFC 3315 [40] and RFC 3646 [56C] or RFC 2131 [40A].

When:

– the UE obtains a connection with the IP-CAN by performing handover of the connection from another IP-CAN;

– IP address of the UE is not changed during the handover; and

– the UE already communicates with the IM CN subsystem via the connection with the other IP-CAN, e.g. the UE determines that its contact with host portion set to the UE IP address (or FQDN of the UE) associated with the connection with the other IP-CAN has been bound to a public user identity;

the UE shall continue using the P-CSCF address(es) acquired in the other IP-CAN.

9.2.2 Handling of the IP-CAN

The means to ensure that appropriate resources are available for the media flow(s) on the IP-CAN(s) related to a SIP session is dependant on the characteristics for each IP-CAN, and is described separately for each IP-CAN in question.

GPRS is described in annex B. xDSL is described in annex E. DOCSIS is described in annex H. EPS is described in annex L. cdma2000^® packet data subsystem is described in annex M. EPC via cdma2000^® HRPD is described in annex O. cdma2000^® Femtocell network is described in annex Q. Evolved Packet Core (EPC) via WLAN is described in annex R. DVB-RCS2 is described in annex S. 5GS is described in annex U. If a particular handling of the IP-CAN is needed for emergency calls, this is described in the annex for each access technology.

9.2.2A P-CSCF restoration procedure

The UE may support P-CSCF restoration procedures.

An IP-CAN may provide means for detecting a P-CSCF failure.

An UE supporting the P-CSCF restoration procedure should either use the keep-alive procedures described in RFC 6223 [143] or the procedure provided by a IP-CAN for monitoring the P-CSCF status.

NOTE 1: The UE can use other means to monitor the P-CSCF status, e.g. ICMP echo request/response. However, those other means are out of scope of this document.

NOTE 2: A UE registered through the procedures described in RFC 5626 [92] can use the keep-alive mechanism to monitor the status of the P-CSCF.

9.2.3 Special requirements applying to forked responses

Since the UE does not know that forking has occurred until a second provisional response arrives, the UE will request the radio/bearer resources as required by the first provisional response. For each subsequent provisional response that may be received, different alternative actions may be performed depending on the requirements in the SDP answer:

– the UE has sufficient radio/bearer resources to handle the media specified in the SDP of the subsequent provisional response, or

– the UE must request additional radio/bearer resources to accommodate the media specified in the SDP of the subsequent provisional response.

NOTE 1: When several forked responses are received, the resources requested by the UE is the "logical OR" of the resources indicated in the multiple responses to avoid allocation of unnecessary resources. The UE does not request more resources than proposed in the original INVITE request.

NOTE 2: When service-based local policy is applied, the UE receives the same authorization token for all forked requests/responses related to the same SIP session.

When an 199 (Early Dialog Terminated) response for the INVITE request is received for an early dialogue, the UE shall release reserved radio/bearer resources associated with that early dialogue.

When the first final 200 (OK) response for the INVITE request is received for one of the early dialogs, the UE proceeds to set up the SIP session using the radio/bearer resources required for this session. Upon the reception of the first final 200 (OK) response for the INVITE request, the UE shall release all unneeded radio/bearer resources.