The Sy interface - between PCRF and OCS

Now I have more work to do then I had and it's all focused on GGSN node part and additional equipment not covered by 3GPP specs.Few months ago, in comments some of you asked me to look deeper into the Sy interface. Have to admit that hadn't have much time lately and.. let's be honest, some kind of a mood to research, put together and publish new articles.

Back to the point where we should start.
     

The Sy interface overview

The Sy reference point is defined between the PCRF and the OCS. Big picture of the relationships between functional entities are shown in Fig 1. below.

Fig. 1. Sy overview

As you may (even should) notice on Fig 1. above are simoultaneously SPR and UDR. Those entities work separetly, that means if there is SPR with Sp interface, there is no UDR with Ud interface in the network. Those were put one one figure to save space. Cojunction or means there is the one OR the other. When UDC architecture is used, the SPR with Sp interface, whenever mentioned in this document, is replaced by UDR and Ud.

Overview

The Sy reference point is located between the Policy and Charging Rules Function (PCRF) and the Online Charging System (OCS). The Sy reference point enables transfer of information relating to subscriber spending from OCS to PCRF and supports the following functions:

•     Request of policy counter status reporting from PCRF to OCS.
•     Notification of policy counter status change from OCS to PCRF.
•     Cancellation of policy counter status reporting from PCRF to OCS.

Since the Sy interface resides between PCRF and OCS in the HPLMN, roaming with home routed or visited access as well as non-roaming scenarios are supported in the same manner.

Diameter as base protocol

The Diameter Base Protocol as specified in IETF RFC 3588 shall apply except as modified by the defined support of the methods and the defined support of the commands and AVPs, result and error codes as specified in this specification. Unless otherwise specified, the procedures (including error handling and unrecognized information handling) shall be used unmodified.

With regard to the Diameter protocol defined over the Sy interface, the OCS acts as a Diameter server, in the sense that it is the network element that handles policy counter status requests for a particular realm. The PCRF acts as the Diameter client, in the sense that is the network element requesting policy counter status to the OCS.

A Diameter routing table entry can have a different destination based on the application identifier of the command. The application identifier stored in the command header must match the value of any application identifier AVPs in the command body. Diameter agents (relay, proxy, redirection, translation agents) should use the application identifier in the command header to route to a suitable destination.
Subscriber Spending limits

Policy decisions based on spending limits is a function that allows PCRF to make policy decisions based on the status of policy counters that are maintained in the OCS. The PCRF uses the policy counter statuses received from the OCS as input to its policy decisions, e.g. downgrade the QoS (e.g. APN-AMBR) or modify the PCC/QoS/ADC Rules.

When the status of policy counters is first required to make a policy decision for a subscriber, the PCRF uses the Initial Spending Limit Report Request procedure. The PCRF may request specific or all policy counter statuses to be reported by the OCS for the user. The OCS provides the status to the PCRF of the requested policy counters, and will notify the PCRF of any changes in the status of those policy counters.

The PCRF may request reporting for specific policy counter(s) that it is not currently subscribed and/or cancel reporting for specific policy counter status(es) using the Intermediate Spending Limit Report Request. The PCRF may cancel spending limit reporting for all policy counter(s) using the Final Spending Limit Report Request.

Functional elements

PCRF

The Policy Control and Charging Rules Function (PCRF) is a functional element that encompasses policy control decision and flow based charging control functionalities.
The PCRF may take information on the subscriber's spending status into account in its policy decisions. The PCRF may request spending limit reporting for policy counters from the OCS using the Initial or Intermediate Spending Limit Report Request procedure. The PCRF may cancel spending limit reporting for specific policy counter(s) using the Intermediate Spending Limit Report Request procedure, or for all policy counter(s) using the Final Spending Limit Report Request procedure. Both described below.

The PCRF shall have at least one active IP-CAN session to be able to initiate an Sy session to be used when required for spending limit reporting for that subscriber. The PCRF shall terminate the Sy session when the last IP-CAN session for that subscriber is terminated or no IP-CAN session for the same user depends on the spending status information provided over Sy reference point.

The PCRF may use the status of each relevant policy counter as input to its policy decision as required by the decision logic.

More about PCRF can be found here:http://www.lteandbeyond.com/2012/01/gx-interface-sitting-between-pcrf-and.html

OCS

The Online Charging System (OCS), for the purpose of policy decisions based on the subscriber's spending, shall:

    maintain the policy counter statuses applicable for a subscriber
    report the policy counter status values for the subscriber when requested to the PCRF
    when a policy counter status changes, report the change to the PCRF

More about OCS can be found here:
http://www.lteandbeyond.com/2012/01/online-charging-system-ocs-how-gy.html or here.

Spending Limits procedures over Sy

Initial/Intermediate Spending Limit Report Req/Res

This procedure shall be used by the PCRF to request the status of policy counters available at the OCS, and to subscribe or unsubscribe to updates of policy counters by the OCS.

Table 1. Initial-Intermediate Spending Limit Report Request

Table 2. Initial-Intermediate Spending Limit Report Response

Detailed behaviour of the PCRF

The PCRF shall make use of this procedure when it determines for a subscriber that

• The status of policy counter(s) to which the PCRF does not have an existing subscription for status change notifications.
• The status of one or more, but not all, policy counter(s) to which the PCRF has an existing subscription for status change notifications are no longer required.

The Final Spending Limit Request described below is used to remove all subscriptions.
In the initial request, i.e. when the request is sent for the first time for the Subscriber, the PCRF shall set the SL-Request-Type (where SL stands for Spending Limit) AVP to the value INITIAL_REQUEST (0). For subsequent requests for the same Subscriber, the PCRF shall set the SL-Request-Type AVP to INTERMEDIATE_REQUEST (1).
For each policy counter that the PCRF requires the current status and notifications of future status changes, the PCRF shall indicate the concerned policy counter identifiers in the request. Alternatively, the policy counter identifiers may be omitted if the PCRF requires the current status and notifications of future status changes of all available policy counters.

Detailed behaviour of the OCS

Upon reception of the request from the PCRF, the OCS shall check if there is an ongoing Sy session associated with the received Session-Id AVP. If there is no Sy session and the SL-Request-Type AVP is set to INITIAL_REQUEST, an Sy session is created on the OCS. If there is an Sy session and the SL-Request-Type AVP is not set to INTERMEDIATE_REQUEST (1), the OCS shall return a response with the Result-Code set to DIAMETER_INVALID_AVP_VALUE and with the Failed-AVP AVP containing the SL-Request-Type AVP. If there is no Sy session and the SL-Request-Type AVP is not set to INITIAL_REQUEST (0), the OCS shall return a response with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID.
If all the policy counter identifiers are known to the OCS, it shall store the policy counter identifiers against the Sy session such that the OCS can subsequently notify the PCRF of policy counter state changes.
If a policy counter identifier is known by the OCS, but is not applicable to the subscriber (e.g. not provisioned), the OCS may use an operator configured policy counter status to indicate this to the PCRF.
If an OCS treats unknown policy counter identifiers as valid, an operator configured policy counter status may be used to indicate that the policy counter is unknown. The status of known policy counters shall be returned to the PCRF in the same procedure in this case.
If the OCS determines that any policy counter identifiers are invalid, the OCS shall return a response with the Result-Code AVP set to DIAMETER_INVALID_AVP_VALUE and with the Failed-AVP AVP indicating the invalid policy counter identifiers. If the SL-Request-Type AVP is set to INITIAL_REQUEST when this failure occurs, the Sy session is not created. If the SL-Request-Type AVP is set to INTERMEDIATE_REQUEST (1) when this failure occurs, then none of the changes in the request take effect but the Sy session is maintained. The PCRF may resend the request omitting the invalid policy counter identifiers.
When the PCRF provides a new subscribed policy counter identifier list, the OCS shall remove any policy counter identifiers no longer in the list from association with the Sy session such that the OCS will no longer notify the PCRF of those policy counter state changes.
If an initial or intermediate request contains no policy counter identifiers, the OCS shall store all available policy counter identifiers against the Sy session such that the OCS can subsequently notify the PCRF of policy counter state changes.
The OCS shall include the current status of all subscribed policy counters (if any) in the response and set the Result-Code to DIAMETER_SUCCESS.

Spending Limit Report

This procedure shall be used by the OCS to notify the PCRF of changes in the status of subscribed policy counter(s).

Table 3. Spending Limit Report Request

Table 4. Spending Limit Report Response

Detailed behaviour of the OCS

When the status of a specific policy counter changes, the OCS shall determine the Sy sessions impacted by the change (i.e. those Sy sessions that have subscribed to staus change notifications for the changed policy counter) and send a Spending Limit Report request to the PCRF associated with each affected Sy session.

If several policy counters change status at the same time, the OCS may group the status change notifications into a single Spending Limit Report request to the PCRF by sending multiple Policy-Counter-Status-Report AVPs in the request.

Detailed behaviour of the PCRF

The PCRF shall acknowledge the request by sending a response with a Result-Code AVP set to DIAMETER_SUCCESS and use the status of the received policy counter(s) as input to its policy decision to apply operator defined actions, e.g. downgrade the QoS.

The PCRF shall ignore an unknown policy counter status report for all unknown policy counter identifiers in an SLA or in an SNR (Status Notification Request) from the OCS. oce

Final Spending Limit Report Request

This procedure shall be used by the PCRF to unsubscribe to any future updates of policy counters for a given subscriber by the OCS.

Table 5. Final Spending Limit Report Request

Table 6. Final Spending Limit Report Response

Detailed behaviour of the PCRF

When the PCRF decides that policy decisions for a given user no longer depend on policy counter(s) to which the PCRF has existing subscriptions for status change notifications, the PCRF shall send the Final Spending Limit Report Request to the OCS.

Detailed behaviour of the OCS

Upon reception of the request from the PCRF, the OCS shall check that there is an ongoing Sy session associated with the received Session-Id AVP. If there is no Sy session, the OCS shall return a response with the Result-Code AVP set to DIAMETER_UNKNOWN_SESSION_ID.
The OCS shall remove all policy counter subscriptions associated with the Sy session such that the OCS will no longer notify the PCRF of policy counter state changes and close the session by returning a response with the Result-Code AVP set to DIAMETER_SUCCESS.


Source(s):
More about Sy interface, especially on Sy messages can be found in 3GPP TS 29.219