【5G核心网】free5GC Path Switch Request源码分析

朴灿烈づ我的快乐病毒、 2022-12-22 09:35 274阅读 0赞

Path Switch Request 过程的目的是请求将 NG-U 传输承载的下行链路终结点切换到新的终结点。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70][] Figure 8.4.4.2-1: Path switch request: successful operation

NG-RAN 节点通过向 AMF 发送 PATH SWITCH REQUEST 消息来启动该过程。一旦接收到路径切换请求消息，AMF 就应为 PDU 会话 ID IE 中指示的每个 PDU 会话，将路径切换请求传输 IE 透明地传输到与所关注的 PDU 会话相关联的 SMF。

在 5GC 中成功完成所有必要的更新（包括UP路径切换）后，对于 PATH SWITCH REQUEST 中至少包括的 PDU 会话资源之一，AMF 将向 NG-RAN 节点发送 PATH SWITCH REQUEST ACKNOWLEDGE 消息。

接受的 QoS 流列表应包括在路径切换请求传输IE内的路径切换请求消息中。 SMF 应按照 TS 23.502 \[10\]中的规定处理此信息。 对于每个PDU会话，在 PATH SWITCH REQUEST 消息的请求传输 IE 中包括每个 ***TNL Information IE 中的 DL QoS Flow***，SMF 可以使用包含的 UP 传输层信息作为该 PDU 会话在不同隧道中拆分的关联 QoS 流的下行链路终结点。

# 1. NG-RAN 向 AMF 发起 PATH SWITCH REQUEST #

NG-RAN 节点发送此消息，以通知 AMF 新服务的 NG-RAN 节点，并通过 AMF 将一个或多个 PDU 会话资源的一些 NG-U DL 隧道终止点传输到 SMF。 NGAP 消息 Present 为 NGAPPDUPresentInitiatingMessage，initiatingMessage 的 ProcedureCode 设置为 ProcedureCodePathSwitchRequest， Present 设置为 InitiatingMessagePresentPathSwitchRequest

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 1][] PATH SWITCH REQUEST

N2 Path Switch Request

>  *  List of PDU Sessions To Be Switched with N2 SM Information,
>  *  List of PDU Sessions that failed to be established with the failure cause given in the N2 SM information element, 
>  *  UE Location Information

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 2][] NGAP PathSwitchRequest

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 3][] 9.3.4.8 Path Switch Request Transfer

###    1.1 AMF 处理 PATH SWITCH REQUEST ###

// TS 23.502 4.9.1
    func HandlePathSwitchRequest(ran *context.AmfRan, message *ngapType.NGAPPDU) {
    
    	var rANUENGAPID *ngapType.RANUENGAPID
    	var sourceAMFUENGAPID *ngapType.AMFUENGAPID
    	var userLocationInformation *ngapType.UserLocationInformation
    	var uESecurityCapabilities *ngapType.UESecurityCapabilities
    	var pduSessionResourceToBeSwitchedInDLList *ngapType.PDUSessionResourceToBeSwitchedDLList
    	var pduSessionResourceFailedToSetupList *ngapType.PDUSessionResourceFailedToSetupListPSReq

**    1.1.1 AMF 向 SMF 发送 UpdateSmContext**

n2SmInfoType 设置为 N2SmInfoType\_PATH\_SWITCH\_REQ

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 4][] AMF->SMF， Update SM Context

###    1.2 SMF 处理 **UpdateSmContext** ###

func HandlePDUSessionSMContextUpdate(smContextRef string, body models.UpdateSmContextRequest) *http_wrapper.Response {
    	//GSM State
    	//PDU Session Modification Reject(Cause Value == 43 || Cause Value != 43)/Complete
    	//PDU Session Release Command/Complete
    	logger.PduSessLog.Infoln("In HandlePDUSessionSMContextUpdate")
    	smContext := smf_context.GetSMContext(smContextRef)

**    1.2.1 根据 n2SmInfoType 为 N2SmInfoType\_PATH\_SWITCH\_REQ**

switch smContextUpdateData.N2SmInfoType {
    case models.N2SmInfoType_PATH_SWITCH_REQ:
    	smContext.SMContextState = smf_context.ModificationPending
    	logger.CtxLog.Traceln("SMContextState Change State: ", smContext.SMContextState.String())
    
    	if err := smf_context.HandlePathSwitchRequestTransfer(body.BinaryDataN2SmInformation, smContext); err != nil {
    		logger.PduSessLog.Errorf("Handle PathSwitchRequestTransfer: %+v", err)
    	}
    
    	if n2Buf, err := smf_context.BuildPathSwitchRequestAcknowledgeTransfer(smContext); err != nil {
    		logger.PduSessLog.Errorf("Build Path Switch Transfer Error(%+v)", err)
    	} else {
    		response.BinaryDataN2SmInformation = n2Buf
    	}
    
    	response.JsonData.N2SmInfoType = models.N2SmInfoType_PATH_SWITCH_REQ_ACK
    	response.JsonData.N2SmInfo = &models.RefToBinaryData{
    		ContentId: "PATH_SWITCH_REQ_ACK",
    	}
    
    	smContext.PendingUPF = make(smf_context.PendingUPF)
    	for _, dataPath := range tunnel.DataPathPool {
    
    		if dataPath.Activated {
    			ANUPF := dataPath.FirstDPNode
    			DLPDR := ANUPF.DownLinkTunnel.PDR
    
    			pdrList = append(pdrList, DLPDR)
    			farList = append(farList, DLPDR.FAR)
    
    			if _, exist := smContext.PendingUPF[ANUPF.GetNodeIP()]; !exist {
    				smContext.PendingUPF[ANUPF.GetNodeIP()] = true
    			}
    		}
    	}
    
    	sendPFCPModification = true
    	smContext.SMContextState = smf_context.PFCPModification
    	logger.CtxLog.Traceln("SMContextState Change State: ", smContext.SMContextState.String())

HandlePathSwitchRequestTransfer 函数将发来的 PathSwitchRequestTransfer ，根据 gTPTunnel 设置 FAR，更新 N3 RAN 地址

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 5][] PFCP Sessino Modification request

BuildPathSwitchRequestAcknowledgeTransfer 函数向 AMF 响应请求

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 6][] 9.3.4.9 Path Switch Request Acknowledge Transfer

# 2. AMF 向 RAN 发送 PathSwitchRequestAcknowledge #

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3pob25nbGluemhhbmc_size_16_color_FFFFFF_t_70 7][] PathSwitchRequestAcknowledge

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