VoLTE基础自学系列 | 3GPP规范解读之Rx接口（上集）

Posted 2022-08-12 COCOgsta

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Rx接口概述

Rx接口AF到PCRF的接口，是触发VoLTE专有承载建立的重要接口。V噢LTE网络中，实际上是P-CSCF与PCRF的接口。

Rx接口基于Diameter协议，承载部分在RFC3588.应用部分在TS29.214中定义。通过Diameter包头中的Application-ID为16777236区分。vendor identifier取值为10415代表3GPP。

4.4.1 会话的初始化

1）VoLTE呼叫中的FSI和PSI

1）If the AF provides service information that has been fully negotiated （e.g. based on the SDP answer）, the AF may include the Service-Info-Status AVP set to FINAL_SERVICE_INFORMATION. In this case the PCRF shall authorize the session and provision the corresponding PCC/QoS rules to the PCEF.

2）The AF may additionally provide preliminary service information not fully negotiated yet （e.g. based on the SDP offer） at an earlier stage. To do so, the AF shall include the Service-Info-Status AVP with the value set to PRELIMINARY SERVICE INFORMATION. Upon receipt of such preliminary service information, the PCRF shall perform an early authorization check of the service information.

解读：

简单来说，就是完成SDP编码协商（收到被叫183）应设置为FSI，没有完成则设置为PSI（也就是收到主叫的INVITE）

第一个AAR和第二个AAR的Service-Info-Status AVP

2）Rx与Gx的会话绑定

1）When the PCRF received an Initial AA-Request from the AF, the PCRF shall perform session binding as described in 3GPP TS 29.213.

2）If the PCRF fails in executing session binding, the PCRF responds to the AF with an AA-Answer including the Experimental-Result-Code AVP set to the value IP-CAN_SESSION_NOT_AVAILABLE.

解读：

也就是说，当PCRF收到AF发过来的AAR-I消息时，需要完成和Gx接口的会话绑定。

如果绑定失败，在AAA消息的Experimental-Result-Code AVP中设置原因代码IP-CAN_SESSION_NOT_AVAILABLE。

3）业务信息的授权

1）If the service information provided in the AA-Request command is rejected （e.g. the subscribed guaranteed bandwidth for a particular user is exceeded）, the PCRF shall indicate in the AA-Answer command the cause for the rejection with the Experimental-Result-Code AVP set to the value REQUESTED_SERVICE_NOT_AUTHORIZED.

2）If the service information provided in the AA-Request command is rejected by the PCRF due to a temporary condition in the network （e.g. the user plane in the cell the user is located is congested）, the PCRF may indicate in the AA-Answer the cause for the rejection with the Experimental-Result-Code AVP set to the value REQUESTED_SERVICE_TEMPORARY_NOT_AUTHORIZED （4XX1）.

3）The PCRF may also provide a retry-interval within the Retry-Interval AVP in the AA-Answer command to the AF. When the AF receives the re-try interval within the Retry-Interval AVP, the AF shall not send the same service information to the PCRF again （for the same IP-CAN session） until the re-try interval has elapsed.

解读：

如果AAR中的业务信息被拒绝（如申请的带宽超过签约的带宽），则AAA消息的Experimental-Result-Code AVP中设置原因代码REQUESTED_SERVICE_NOT_AUTHORIZED。代表请求的业务未授权。

如果AAR中的业务信息因为临时的原因（如用户所在小区拥塞）被拒绝，则AAA消息的Experimental-Result-Code AVP中设置原因代码REQUESTED_SERVICE_TEMPORARILY_NOT_AUTHORIZED （4XX1）。代表请求的业务暂时不能完成授权，请稍后再试。

PCRF如果在AAA消息携带了Retry-Interval AVP。则AF在时间间隔内不能重发相同的业务信息授权请求。

4）IMS对EPC网络的事件订阅

1）The AF may request notifications of specific IP-CAN session events through the usage of the Specific-Action AVP in the AA-Request command. The PCRF shall make sure to inform the AF of the requested notifications in the event that they take place.

2）The AF may also include the Specific-Action AVP to request notification for certain user plane events, e.g. bearer termination.

解读：

IMS网元不了解EPS承载状态，但某些场景又必须了解（比如EPS承载丢失，IMS也需要在高层释放资源）。所以，AF可以通过AAR中的Specific-Action AVP向PCRF订阅EPS承载状态等事件。

PCRF把AAR中的Specific-Action AVP转成Gx接口的Event-Trigger AVP向PGW发起订阅。

5）IMS和EPC网络的计费关联

1）The AF may include the AF-Application-Identifier AVP into the AA-Request in order to indicate the particular service the AF session belongs to.

2）The AF may include the AF-Charging-Identifier AVP into the AA-Request for charging correlation purposes.

3）The PCRF shall reply with an AA-Answer to the AF. The acknowledgement towards the AF should take place before or in parallel with any required PCC Rule provisioning towards the PCEF and shall include the Access-Network-Charging-Identifier（s） and may include the Access-Network-Charging-Address AVP, if they are available.

解读：

IMS有自己的计费机制，采用时长计费，会产生一个ICID的计费标识。但VoLTE要消耗4G流量，因此EPC也会产生自己的计费标识ECID。这两个ID需要关联，以实现在BOSS中的话单关联，完成EPC流量扣减，避免重复收费。

那AF页就可以通过AF-Charging-Identifier AVP将IMS的计费标识传递给PCRF再给PGW，体现在EPC的CDR话单中。

同样，PCRF也需要上报EPC的计费ID，通过Access-Network-Charging-Identifier AVP上报。

6）AAA消息中的其他重要AVP

1）The AA-Answer message shall also include the IP-CAN-Type AVP, if such information is available. In that case, the AA-Answer message shall also include the RAT type information within the RAT-Type AVP.

解读：

AAA消息中应包含用户的接入类型，报告给IMS域。

7）P-CSCF如果没有收到AAA怎么办？

The behaviour when the AF does not receive the AA Answer, or when it arrives after the internal timer waiting for it has expired, or when it arrives with an indication different than DIAMETER_SUCCESS, are outside the scope of this specification and based on operator policy.

解读：

AF可能出现以下几种针对AAA的异常情况：
- 压根没收到AAA
- 内部AAA消息等待计时器超时之后收到AAA
- 收到的AAA消息指示出错

那么，这几种场景，AF的处理取决于运营商的策略，不在本规范中制定，或者看下一个版本。

8）如果资源分配不足导致VoLTE专载建立失败，PCRF该怎么办？

If the PCRF fails in installing PCC/QoS rules based on the provided service information due to resource allocation failures as specified in 3GPP TS 29.212 and if requested by the AF, the PCRF shall send an RAR command to the AF with the Specific-Action AVP set to the value INDICATION_OF_FAILED_RESOURCES_ALLOCATION to report the resource allocation failure. The AF shall send an RAA command to acknowledge the RAR command.

解读：

如果PCRF因为资源分配失败（专载建立失败）的原因导致不能为上层业务（例如VoLTE语音），那么PCRF需要给AF发送RAR消息，Specific-Action AVP中取值

4.4.2 会话的修改

1）AAR-U的触发

The AF may modify the session information at any time （e.g. due to an AF session modification or internal AF trigger） by sending an AA-Request command to the PCRF containing the Media-Component-Description AVP（s） with the updated Service Information.

The AF may include the Rx-Request-Type AVP set to UPDATE_REQUEST in the AAR.

If the AF provides service information that has been fully negotiated （e.g. based on the SDP answer）, the AF may include the Service-Info-Status AVP set to FINAL_SERVICE_INFORMATION. In this cause the PCRF shall authorize the session and provision the corresponding PCC rules to the PCRF.

The AF may additionally provide preliminary service information not fully negotiated yet （e.g. based on the SDP offer） at an earlier stage. To do so, the AF shall include the Service-Info-Status AVP with the value set to PRELIMINARY SERVICE INFORMATION.

解读：

这段跟会话建立（AAR-I）的时候的业务重授权差不多。

2）业务信息的重授权

1）The PCRF shall process the received Service Information according the operator policy and may decide whether the request is accepted or not. If the updated Service Information is not acceptable （e.g. subscribed guaranteed bandwidth for a particular user is exceeded）, the PCRF shall indicate in the AA-Answer command the cause for the rejection with the Experimental-Result-Code AVP set to the value REQUESTED_SERVICE_NOT_AUTHORIZED. If the service information provided in the AA-Request command is rejected by the PCRF due to a temporary condition in the network （e.g. the user plane in the cell the user is located is congested）, the PCRF may indicate in the AA-Answer the cause for the rejection with the Experimental-Result-Code AVP set to the value REQUESTED_SERVICE_TEMPORARY_NOT_AUTHORIZED （4xx1）. The PCRF may also provide a retry-interval within the Retry-Interval AVP in the AA-Answer command to the AF. When the AF receives the re-try interval within the Retry-Interval AVP, the AF shall not send the same service information to the PCRF again. The PCRF may additionally provide the acceptable bandwidth within the Acceptable-Service-Info AVP in the AA-Answer command.

2）If accepted, the PCRF shall update the Service Information with the new information received. Due to the updated Service Information, the PCRF may need to create, modify or delete the related PCC rules as specified in 3GPP TS 29.213 and provide the updated information towards the PCEF following the corresponding procedures specified at 3GPP TS 29.212.

解读：

这块和4.4.1 会话建立时的业务授权基本一致。区别只是AAR-I变成了AAR-U。

如果PCRF接受了AF下发的AAR-U，则需要根据收到的信息更新业务信息，并触发Gx接口的更新

3）IMS和EPC网络的计费关联

The PCRF shall reply with an AA-Answer to the AF. The acknowledgement towards the AF should take place before or in parallel with any required PCC Rule provisioning towards the PCEF and shall include the Access-Network-Charging-Identifier（s） and may include the Access-Network-Charging-Address AVP, if they are available at this moment and have not been yet supplied earlier to the AF.

The AA-Answer message shall include the IP-CAN-Type AVP if such information is available and has not yet been supplied earlier to the AF.

解读：

和前面完全一样，只不过最后加了个补充说明：如果会话建立的时候没发ANCI AVP，那么现在发。

同样，如果会话建立时没有给IMS提供IP-CAN类型，那么现在提供。

4）更新会话的异常场景

If the PCRF does not have an existing session for the Rx session being modified （such as after a PCRF failure）, the PCRF may reject the request with an AA-Answer with the result code set to DIAMETER_UNKNOWN_SESSION_ID.

If the PCRF modifies existing PCC/QoS rules based on the updated service information and the modification fails due to resource allocation failure as specified in 3GPP TS 29.212 and if requested by the AF, the PCRF shall send an RAR command to the AF with the Specific-Action AVP set to the value INDICATION_OF_FAILED_RESOURCES_ALLOCATION to report the modification failure. The AF shall send an RAA command to acknowledge the RAR command.

解读：

和前面完全一样，找不到会话回复结果代码“DIAMETER_UNKNOWN_SESSION_ID”。

和前面一样，因资源分配失败导致会话更新失败回复原因“INDICATION_OF_FAILED_RESOURCES_ALLOCATION”

4.4.3 门控相关流程（Gate Related Procedures）

门控的基本概念

Depending on the application, in the Service Information provision, the AF may instruct the PCRF when the IP flow（s） are to be enabled or disabled to pass through the IP-CAN. The AF does this by sending the AA-Request message containing the Media-Component-Description AVP（s） that contains the flow status information （in the Flow-Status AVP） for the flows to be enabled or disabled.

In response to this action the PCRF shall set the appropriate gate status for the corresponding active PCC rule（s）.

The PCRF forwards the AF decision to enable or disable the authorized IP flows.

解读：

门控，就是指啥时候开门和关门。比如大学宿舍的门控就是晚上11点关门。当关门后，信令中指定的IP流量就不允许通过，反过来就可以通过。AF通过发送携带Flow-Status AVP的AAR消息通知PCRF是否要对该流量进行门控管理。

作为响应，PCRF应根据AF要求，为相应已经激活的PCC规则设置合适的门控状态。

4.4.4 AF会话释放 AF Session Termination

AF会话（Rx会话）释放流程

When an AF session is terminated, if the AF had received a successful AA-Answer for the initial AA-Request, the AF shall send a Session-Termination-Request command to the PCRF. Otherwise, the AF shall wait for the initial AA-Answer to be received prior to sending the Session-Termination-Request command to the PCRF.

When the PCRF receives a ST-Request from the AF, including an AF session termination, it shall acknowledge that request by sending a ST-Answer to the AF. Afterwards, it shall free the resources allocated for the corresponding Service Data Flow（s）. In order to do that, the PCRF shall initiate the request for the removal of any related PCC/QoS rules from the PCEF and for the update of the Authorized QoS for the affected IP-CAN bearer following the corresponding procedures specified at 3GPP TS 29.212.

解读：

AF发送STR给PCRF发起Rx会话释放流程，PCRF回复STA。

接下来，PCRF需要释放之前分配给该SDF（例如VoLTE音频、视频）的资源，并按照29.212的要求发起IP-CAN（EPS承载+Gx会话）释放流程。

4.4.5 信令通路状态通知的订阅 Subscription to Notification of Signaling Path Status

信令通路状态订阅基本原理

An AF may subscribe to notifications of the status of the AF Signalling transmission path. To do so, the AF shall open an Rx Diameter session with the PCRF for the AF signaling using an AA-Request command. The AF shall provide the UE's IP address （using either the Framed-IP-Address AVP or the Framed-Ipv6-Prefix AVP） and the Specific-Action AVP requesting the subscription to "INDICATION_OF_LOSS_OF BEARER" and/or "INDICATION_OF_RELEASE_OF_BEARER".

The AF shall additionally provide a Media-Component-Description AVP including a single Media-Sub-Component AVP with the Flow-Usage AVP set to the value "AF_SIGNALLING". The Media-Component-Description AVP shall contain the Media-Component-Number AVP set to "0".

解读：

IMS不了解EPC承载状态，需要通过PCRF发起对EPC承载状态的订阅，以判断是否需要触发应用层的对应处理。

本例中，AF向PCRF订阅了两个事件：EPS承载丢失或者EPS承载释放，发生此类事件请上报给我。

这个功能好像没看到在现网使用。个人觉得原因是信令通路（SIP/SDP）是由缺省承载提供，无需订阅。只需要订阅专有承载（RTP/RTCP）的相关事件即可。

4.4.6 转发平面的事件 Traffic Plane Events

IP-CAN会话终结

When an IP-CAN session is terminated, the PCRF shall inform the AF about the IP-CAN session termination by sending an ASR （abort session request） command to the AF on each active Rx Diameter session.

When the AF receives the ASR command, it shall acknowledge the command by sending an ASA （abort session answer） command to the PCRF.

After that the AF shall initiate an AF session termination procedure as defined in subclause 4.4.4.

解读：

当IP-CAN（EPS承载+Gx口会话）终结时，PCRF给AF通知ASR。

AF回复AAA。并且AF将发起Rx接口会话终结流程（STR/STA）

业务数据流（SDF）的去激活

It may happen that one or more PCC/QoS Rules （i.e. Service Data Flows） are deactivated at the PCEF at a certain time, either permanently or temporarily. When the PCRF gets the knowledge that one or more SDFs have been deactivated, （e.g. due to a bearer release or loss of bearer or out of credit condition）, the PCRF shall inform the AF accordingly if the AF has previously subscribed using the Specific-Action AVP in the AAR command.

When not all the service data flows within the AF session area affected, the PCRF shall inform the AF by sending an RAR （re-authorization request） command. The RAR command shall include the deactivated IP Flows encoded in the Flows AVP and the cause encoded in the Specific-Action AVP.

The AF may then decide to terminate the Rx Diameter session used for the notification of the status of the AF Signalling transmission path.

解读：

SDF去激活的场景可能是因为Gx接口的PCC规则可能需要永久或临时去激活（例如EPS承载释放或丢失、欠费等），如果这些场景AF之前通过AAR消息里的Specific-Action AVP进行了订阅的话，那PCRF需要通知AF。

如果不是Rx接口所有的SDF需要去激活，PCRF发送RAR通知AF，并将需要去激活的业务在Flows AVP中指明，并在Specific-Action AVP注明去激活原因。

AF收到后，发起Rx接口终结流程（STR/STA）。

IP-CAN类型变更的通知

If the AF has successfully subscribed to change notifications in UE's IP-CAN type and RAT type, the PCRF shall provide the UE's IP-CAN type and RAT type information （if applicable） in the AA-Answer if already known by the PCRF. The PCRF shall also send an RAR command when a corresponding event occurs, i.e. when the UE's IP-CAN type or RAT type changes or becomes available. In this cause the RAR from the PCRF shall include the Specific-Action AVP for the subscribed event and include the IP-CAN-Type AVP, RAT-Type AVP （if applicable） for the UE's new IP-CAN/RAT.

解读：

如果AF之前向PCRF订阅了IP-CAN、RAT类型变更的事件，那么该事件触发时（比如LTE到2/3G的切换），那么PCRF需要发送RAR进行上报，报告实时的IP-CAN-Type AVP，RAT-Type AVP取值。

IP-CAN-Type AVP取值在29.212中定义。主要取值为3GPP-GPRS、3GPP2、3GPP-EPS、xDSL等。

RAT-TYPE的取值主要包括WLAN、UTRAN、GERAN、EUTRAN、HSPA_EVOLUTION、HRPD、CMDA2000_1X、EHRPD等。另外还有一个取值Virtual代表RAT未知。

接入网络计费信息（ACNI）的通知

If the AF has subscribed to a notification about Access Network Charging Information, the PCRF shall provide the Access Network Charging Information in the response, if already known by the PCRF. If not available, the PCRF shall provide the Access Network Charging Information by sending a RAR command when the Access Network Charging Information is received from the PCEF.

The RAR shall include the Specific-Action AVP set to the value "CHARGING_CORRELATION_EXCHAGE" and shall include the assigned Access-Network-Charging-Identifer（s） and may include the Access-Network-Charging-Address AVP.

解读：

当IMS用户发起VoLTE呼叫时，AF向PCRF发起AAR消息，且该消息中携带Specific-Action AVP，指示订阅的事件为CHARGING_CORRELATION_EXCHAGE。

PCRF会通过RAR消息向PCEF下发CHARGING_CORRELATION_EXCHANGE事件。

PCEF向PCRF发送RAA消息上报CHARGING_CORRELATION_EXCHANGE事件，同时携带计费标识。

PCRF通过AAA消息将CHARGING_CORRELATION_EXCHANGE事件和此AVP携带的计费标识及其值上报给AF。

PS：Access-Network-Charging-Identifier的取值为EPC计费标识ECID，用于实现和IMS计费关联。

报告接入网络信息（NETLOC功能）

If the AF requests the PCRF to report the access network information （e.g. user location）, the AF shall subscribe to the "ACCESS_NETWORK_INFO_REPORT" within the Specific-Action AVP and shall include the required access network information within the Required-Access-Info AVP.

When the PCRF receives a request to report the access network information from the AF in an AAR command or in an STR command triggered by the AF, if the PCRF determines that the access network does not support the access network information reporting based on the currently used IP-CAN type or the values of the RAT-Type AVP or the PCEF does not support the access network information reporting based on the Supported-Feature AVP, the PCRF shall respond to AF with an AAA or STA command including the NetLoc-Access-Support AVP set to the value of 0 （NETLOC_ACCESS_NOT_SUPPORTED）; otherwise, it shall immediately configure the PCEF to provide such access network information.

When the PCRF then receives the access information from the PCEF, the PCRF shall provide the corresponding access network information to the AF within the 3GPP-User-Location-Info AVP, TWAN-Identifier AVP, User-Location-Info-Time AVP, UE-Local-IP-Address AVP, 3GPP-SGSN-MCC-MNC AVP （if location info is not available） and/or 3GPP-MS-TimeZone AVP in the RAR command. If the information is provided in the RAR command, PCRF shall also provide the ACCESS_NETWORK_INFO_REPORT within Specific-Action AVP.

解读：

现网被叫号码补全场景，IMS需要EPC上报接入网络信息。

AF通过Specific-Action AVP 取值为ACCESS_NETWORK_INFO_REPORT向PCRF订阅。

PCRF在AAR/STR消息中，如果看到AF进行了订阅，除非不支持，否则找PGW去要用户接入网络信息。

PGW通过Gx接口向PCRF上报用户接入网络信息，放到3GPP User-Location-Info AVP等AVP中上报。

TS29.214的附录部分

P-CSCF收到SDP offer时的业务下发

P-CSCF也（MAY）可以在收到带有SDP offer payload的SIP消息（例如初始INVITE），向PCRF下发业务信息，用于PCRF侧对该业务信息进行早期带宽授权检查（专载预建立）、或者被叫UE侧IMS会话建立的预授权、或者需要向PCRF获取接入网络信息等目的。

如果是为被叫UE侧IMS会话建立的预授权，P-CSCF可以（MAY）从SDP offer中提取业务信息发给PCRF。前提是SDP offer中未知名precondition或者precondition信息中指明了本地precondition条件已经满足。本例中，P-CSCF可以将Service-Info-Status值设置为PRELIMINARY SERVICE INFORMATION。

P-CSCF收到SDP answer时的业务下发

P-CSCF只要收到带有SDP answer payload的SIP消息（例如被叫183），就应（SHALL）向PCRF下发业务信息（AAR），用于PCRF侧对该业务信息进行QoS授权（建立专载），除非SDP payload表示这是个电路域的承载（c line或m line等于PSTN）。

业务信息可以用SDP offer和SDP answer中提取。这样能保证PCRF接收到正确的业务信息，从而完成Qos授权。

P-CSCF可以（MAY）将Service-Info-Status值设置为FINAL_SERVICE_INFORMATION。

P-CSCF如何下发业务信息？- Flow-Description AVP

P-CSCF应（SHALL）从SDP导出业务信息，置于Flow-Description AVP中。

上行Flow-Description取值规则如下：

目的地址应从P-CSCF下行方向收到的SDP信息中提取；

源地址可以从P-CSCF上行方向收到的SDP信息中提取；

如果是IPv6地址的话只考虑前面的64位前缀；

源端口和目的端口按TS29.213规则提取；

例如：UE A发SDP给UE B，那么UE B的PCRF将使用SDP中的地址作为UE B上行Flow-Description AVP的目的地址；UE A的PCRF则使用相同地址的64位前缀作为UE A上行Flow-Description AVP的源地址。如果源地址不是由64位前级组成，那源地址应该是通配符。

下行Flow-Description取值规则如下：

目的地址应从P-CSCF上行方向收到的SDP信息中提取；

源地址可以从P-CSCF下行方向收到的SDP信息的目的地址提取；

如果是IPv6地址的话只考虑前面的64位前缀；

源端口和目的端口按TS29.213规则提取；

例如：UE A发SDP给UE B，那么UE A的PCRF将使用SDP中的地址作为UE A下行Flow-Description AVP中的目的地址；UE B的PCRF则使用相同地址的64位前缀作为UE B下行Flow-Description AVP中的源地址。如果源地址不是由64位前缀组成，那源抛址应该是通配符。

P-CSCF如何下发业务信息？- 带宽信息获取

如果支持29.213的E2EMTSIQOS特性，P-CSCF应（SHALL）从SDP的b=AS和a=bw-info中提取带宽信息。

如果有关联的RTCP流，则P-CSCF应使用SDP中的b=AS，b=RR，b=RS参数。

IP流的开门 Enabling of IP Flows

媒体流的开门与关门

在SIP会话建立完成之前（也就是收到INVITE的200OK或者其他2xx的响应。），P-CSCF可以根据运营商策略决定是否对媒体IP流执行开门或关门处理，也就是是否允许或禁止早期媒体在前向或后向转发。

当2xx响应收到后，P-CSCF应根据最后收到的SDP中的方向属性，enable（开门）所有媒体IP流。

如果P-CSCF在SIP会话建立完成之后收到了SDP answer，那么P-CSCF应根据最后收到的SDP answer信息来设置Flow-Status AVP的值。

怎么从SDP信息中提取信息，映射到Flow-Status AVP的规则在 29.213 clause 6.2中定义。

SIP分叉的支持 Support for SIP forking

SIP Forking场景下的PCC规则

Forking场景下，P-CSCF在收到最终响应前可能收到多个中间响应（1xx）。这个过程中可能会建立多个早期媒体会话（如彩铃）

在第一个早期媒体会话建立后，针对后续建立的早期媒体会话的中间响应，P-CSCF应使用已有的Diameter会话发送AAR给PCRF，并携带SIP-Forking-Indication AVP并设置值为SEVERAL_DIALOGUES，并且包含从最后收到的中间响应中提取的业务信息。

P-CSCF还应从后续的early dialogue（例如PRACK和OK for PRACK、UPDATE和OK for UPDATE）中的SDP offer-answer交互对中提取业务信息，并通过已有的Diameter会话发送AAR给PC RF来下发业务信息，并携带SIP-Forking-Indication AVP并设置值为SEVERAL_DIALOGUES。

PCRF收到后，应根据Flow Description AVPs中的要求，给PCEF下发额外的PCC规则。

还有很多细节，请参考规范。此处省略若干。。。