【5G NR】RRC Reject解析

1. RRC Reject描述

2. UE接收RRCReject处理流程

当UE收到RRC Reject后，按如下流程处理：

1. 停止计时器T300，T319，T302
2. 重置MAC并释放默认的MAC Cell Group配置;
3. 如果在RRCReject中配置了waitTime，则启动定时器T302，将定时器值设置为waitTime;
4. 如果收到的RRCReject是为了响应上层的请求，通知上层访问受限;
5. 如果收到的RRCReject是用来回复RRCSetupRequest，通知上层RRC连接失败，程序结束;
   
   
   
6. 如果收到RRCReject是用来响应RRCResumeRequest：
   
   
   

• 如果上层触发resume，告知上层RRC连接失败;
• 如果由于RNA更新而触发恢复，将变量pendingRnaUpdate设置为true，丢弃当前的KgNB密钥，KRRCenc密钥，KRRCint密钥，KUPint密钥和根据5.3.13.3导出的KUPenc密钥;
• 暂停SRB1，程序结束;

当定时器T302运行时，RRC_INACTIVE状态的UE将继续监视寻呼。

3. 消息定义

RRCReject消息用于拒绝RRC连接建立或RRC连接重建。

-- ASN1START -- TAG-RRCREJECT-START RRCReject ::= SEQUENCE { 
     criticalExtensions CHOICE { 
     rrcReject RRCReject-IEs, criticalExtensionsFuture SEQUENCE { 
    } } } RRCReject-IEs ::= SEQUENCE { 
     waitTime RejectWaitTime OPTIONAL, -- Need N lateNonCriticalExtension OCTET STRING OPTIONAL, nonCriticalExtension SEQUENCE{ 
    } OPTIONAL } -- TAG-RRCREJECT-STOP -- ASN1STOP 

RejectWaitTime用于为计时器T302提供以秒为单位的值，取值范围1~16。

-- ASN1START -- TAG-REJECTWAITTIME-START RejectWaitTime ::= INTEGER (1..16) -- TAG-REJECTWAITTIME-STOP -- ASN1STOP 

4. OAI RRC Reject发送判断

在开源OAI代码中，基站收到rrcSetupRequest后，会对其携带的UE Identity类型进行判断，如果既不是随机值也不是TMSI，则会向UE发送RRC Reject。

if (NR_InitialUE_Identity_PR_randomValue == rrcSetupRequest->ue_Identity.present) { 
     ...... } else if (NR_InitialUE_Identity_PR_ng_5G_S_TMSI_Part1 == rrcSetupRequest->ue_Identity.present) { 
     ...... } else { 
     rrc_gNB_generate_RRCReject(ctxt_pP, rrc_gNB_get_ue_context(gnb_rrc_inst, ctxt_pP->rnti), CC_id); } 

5. OAI RRC Reject编码

熟悉ASN用法的都清楚，option选项比较多，为了适配灵活多变的数据结构，需要大量在结构体中使用指针。在实际赋值前，需要使用malloc或者calloc给指针变量申请内存。RRC Reject承载于CCCH信道，因此编码时外层需要进行DL_CCCH编码。

uint8_t do_RRCReject(uint8_t Mod_id, uint8_t *const buffer) { 
     asn_enc_rval_t enc_rval;; NR_DL_CCCH_Message_t dl_ccch_msg; NR_RRCReject_t *rrcReject; NR_RejectWaitTime_t waitTime = 1; memset((void *)&dl_ccch_msg, 0, sizeof(NR_DL_CCCH_Message_t)); dl_ccch_msg.message.present = NR_DL_CCCH_MessageType_PR_c1; //指定逻辑信道消息类型为CCCH dl_ccch_msg.message.choice.c1 = CALLOC(1, sizeof(struct NR_DL_CCCH_MessageType__c1)); dl_ccch_msg.message.choice.c1->present = NR_RRCReject__criticalExtensions_PR_rrcReject; //指定dl_ccch消息类型为rrcReject dl_ccch_msg.message.choice.c1->choice.rrcReject = CALLOC(1,sizeof(NR_RRCReject_t)); rrcReject = dl_ccch_msg.message.choice.c1->choice.rrcReject; rrcReject->criticalExtensions.choice.rrcReject = CALLOC(1, sizeof(struct NR_RRCReject_IEs)); rrcReject->criticalExtensions.choice.rrcReject->waitTime = CALLOC(1, sizeof(NR_RejectWaitTime_t)); //为waitTime申请内存 rrcReject->criticalExtensions.present = NR_RRCReject__criticalExtensions_PR_rrcReject; //设置rrcReject内部present为rrcReject rrcReject->criticalExtensions.choice.rrcReject->waitTime = &waitTime; //waitTime赋值 if ( LOG_DEBUGFLAG(DEBUG_ASN1) ) { 
     xer_fprint(stdout, &asn_DEF_NR_DL_CCCH_Message, (void *)&dl_ccch_msg); } //消息体进行per编码 enc_rval = uper_encode_to_buffer(&asn_DEF_NR_DL_CCCH_Message, NULL, (void *)&dl_ccch_msg, buffer, 100); if(enc_rval.encoded == -1) { 
     LOG_E(NR_RRC, "[gNB AssertFatal]ASN1 message encoding failed (%s, %lu)!\n", enc_rval.failed_type->name, enc_rval.encoded); return -1; } LOG_D(NR_RRC,"RRCReject Encoded %zd bits (%zd bytes)\n", enc_rval.encoded,(enc_rval.encoded+7)/8); return((enc_rval.encoded+7)/8); }