diameter server assignment request

Server Assignment Messages

The Oracle Communications Core Session Manager sends a Server Assignment Request (SAR) to the HSS requesting to confirm the SIP or SIPS URI of the SIP server that is currently serving the user. The SAR message also serves the purpose of requesting that the Diameter server send the user profile to the SIP server. The SAR's AVPs are populated as follows:

• Public-User-Identity—the SIP AOR of the endpoint being registered (same as UAR)
• Private-User-Identity—the username from the SIP authorization header, if it is present. If not, this value is the public User ID. (Same as UAR)
• Server-Name—the home server route parameter in the sip-registrar configuration element. It is the FQDN or IP address used to identify and route to this Oracle Communications Core Session Manager sent as a URI.
• REGISTRATION (1)—for all new and refreshing registrations.
• Set to TIMEOUT_DEREGISTRATION (4)—when the contact is unregistered due to expiration. This occurs if the force-unregistration option is configured in the sip config.
• USER_DEREGISTRATION (5)—when the contact is unregistered by the user (contact parameter expires=0).
• User-Data-Already-Available—always set to USER_DATA_ALREADY_AVAILABLE (1)

Diameter Result Codes

The Diameter Result Codes presented on this page will be seen in one of two AVPs: the Diameter Base Result-Code AVP or the Vendor-Specific Experimental-Result-Code AVP.

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Nick vs Networking

Telco network engineering.

Diameter and SIP: User-Authorization-Request/Answer

These posts focus on the use of Diameter and SIP in an IMS / VoLTE context, however these practices can be equally applied to other networks.

The Diameter User-Authorization-Request and User-Authorization-Answer commands are used as the first line of authorization of a user and to determine which Serving-CSCF to forward a request to.

When a SIP Proxy (I-CSCF) receives an incoming SIP REGISTER request, it sends a User-Authorization-Request to a Diameter server to confirm if the user exists on the network, and which S-CSCF to forward the request to.

When the Diameter server receives the User-Authorization-Request it looks at the User-Name (1) AVP to determine if the Domain / Realm is served by the Diameter server and the User specified exists.

Assuming the user & domain are valid, the Diameter server sends back a User-Authorization-Answer , containing a Server-Capabilities (603) AVP with the Server-Name of the S-CSCF the user will be served by.

I always find looking at the packets puts everything in context, so here’s a packet capture of both the User-Authorization-Request and the User-Authorization-Answer .

First Registration

If this is the first time this Username / Domain combination (Referred to in the RFC as an AOR – Address of Record) is seen by the Diameter server in the User-Authorization-Request it will allocate a S-CSCF address for the subscriber to use from it’s pool / internal logic.

The Diameter server will store the S-CSCF it allocated to that Username / Domain combination (AoR) for subsequent requests to ensure they’re routed to the same S-CSCF.

The Diameter server indicates this is the first time it’s seen it by adding the DIAMETER_FIRST_REGISTRATION (2001) AVP to the User-Authorization-Answer .

Subsequent Registration

If the Diameter server receives another User-Authorization-Request for the same Username / Domain (AoR) it has served before, the Diameter server returns the same S-CSCF address as it did in the first User-Authorization-Answer .

It indicates this is a subsequent registration in much the same way the first registration is indicated, by adding an DIAMETER_SUBSEQUENT_REGISTRATION (2002) AVP to the User-Authorization-Answer .

User-Authorization-Type (623) AVP

An optional User-Authorization-Type (623) AVP is available to indicate the reason for the User-Authorization-Request. The possible values / reasons are:

• Creating / Updating / Renewing a SIP Registration (REGISTRATION (0))
• Establishing Server Capabilities & Registering (CAPABILITIES (2))
• Terminating a SIP Registration (DEREGISTRATION (1))

If the User-Authorization-Type is set to DEREGISTRATION (1) then the Diameter server returns the S-CSCF address in the User-Authorization-Answer and then removes the S-SCSF address it had associated with the AoR from it’s own records.

Other Diameter Cx (IMS) Calls

User-Authorization-Request / User-Authorization-Answer Server-Assignment-Request / Server-Assignment-Answer Location-Info-Request / Location-Info-Answer Multimedia-Auth-Request / Multimedia-Auth-Answer Registration-Termination-Request / Registration-Termination-Answer Push-Profile-Request / Push-Profile-Answer

References:

3GPP Specification #: 29.229

RFC 4740 – Diameter Session Initiation Protocol (SIP) Application

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