【OpenAirInterface5g】RRC NR解析之RrcSetupComplete

1. RRC SetupComplete介绍

RRC SetupComplete是终端收到网络侧分配到无线资源的"RRC SETUP“消息的应答。UE在应答消息中对分配资源进行了确认，并回复了其5G_S_TMSI_Value Part2的内容，消息中携带selectedPLMN-Identity、registeredAMF、snssai-list和NAS，NAS信息为上下文建立和注册请求。基站(gNB)收到后根据其AMF标识,与核心网一同进行认证和注册流程。

1.1 发送目的

通知网络侧,终端与无线网之间RRC连接建立完成

1.2 传输信道

承载：SRB1;
信道：DCCH;
方向：UE->gNB，其中的NAS最终会到达5GC

1.3 信令内容抓包

rrcSetupComplete信令抓包

• selectedPLMN-Identity：PLMN选择标识，指示UE选择的哪个PLMN，此处是1，表示选择了SIB1中的第一个PLMN，如果是2，则表示选择了SIB1中第二个PLMN
• registeredAMF：注册的AMF标识
• guami-Type：guami类型
• S-NSSAI：切片类型
• Security protected NAS 5GS message：已经进行加密和完整性保护的NAS消息，到达核心网AMF进行解码后可以看到详细内容

2.rrcSetupComplete解析处理流程

nr_rrc_gNB_decode_dcch()解码rrcSetupComplete流程如下：

1. 对于主函数传过来的buffer，调用uper_decode()进行per解码，获得消息ID为rrcSetupComplete。
2. 根据rnti查找UE上下文rrc_gNB_get_ue_context()，如果无法找到上下文，说明该rnti未进行接入，直接break跳出；正常情况能够找到UE上下文。
3. 对ng_5G_S_TMSI_Value类型进行判断，分两种情况：

• ng_5G_S_TMSI ：此处是完整的5g_s_tmsi，长度为48bit，。
• ng_5G_S_TMSI_Part2 ：在Rrc setup Request中存在是5G_S_TMSI_Part1，长度为39bit。此处Part2为5g_s_tmsi后9bit，两者合起来为48bit。

4. 根据收到的TMSI，更新UE上下文；
5. 进入rrc_gNB_process_RRCSetupComplete()函数，对解析后的rrcSetupComplete进行处理；
6. 由于SRB1已经建立，因此将上下文中SRB1的Active设为1，Srb_id设为1，StatusRrc（RRC状态）设为NR_RRC_CONNECTED；
7. 调用rrc_gNB_send_NGAP_NAS_FIRST_REQ()函数向NGAP模块发送初始NAS消息，该NAS信息通常为Register Request或Service Request，即UE开始向5GC进行注册。

3. 相关代码

3.1 rrcSetupComplete解析

     switch (ul_dcch_msg->message.choice.c1->present) {
    
      case NR_UL_DCCH_MessageType__c1_PR_rrcSetupComplete:
        if(!ue_context_p) {
    
          LOG_I(NR_RRC, "Processing NR_RRCSetupComplete UE %x, ue_context_p is NULL\n", ctxt_pP->rnti);
          break;
        }

        LOG_DUMPMSG(NR_RRC, DEBUG_RRC,(char *)Rx_sdu,sdu_sizeP,
                    "[MSG] RRC SetupComplete\n");
        MSC_LOG_RX_MESSAGE(
                MSC_RRC_GNB,
                MSC_RRC_UE,
                Rx_sdu,
                sdu_sizeP,
                MSC_AS_TIME_FMT" NR_RRCSetupComplete UE %x size %u",
                MSC_AS_TIME_ARGS(ctxt_pP),
                ue_context_p->ue_context.rnti,
                sdu_sizeP);
        LOG_D(NR_RRC,
                PROTOCOL_NR_RRC_CTXT_UE_FMT" RLC RB %02d --- RLC_DATA_IND %d bytes "
                "(RRCSetupComplete) ---> RRC_gNB\n",
                PROTOCOL_NR_RRC_CTXT_UE_ARGS(ctxt_pP),
                DCCH,
                sdu_sizeP);

        if (ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.present ==
              NR_RRCSetupComplete__criticalExtensions_PR_rrcSetupComplete) {
    
          if (ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.choice.
            rrcSetupComplete->ng_5G_S_TMSI_Value != NULL) {
    
            if (ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.choice.
            rrcSetupComplete->ng_5G_S_TMSI_Value->present == NR_RRCSetupComplete_IEs__ng_5G_S_TMSI_Value_PR_ng_5G_S_TMSI_Part2) {
    
            // ng-5G-S-TMSI-Part2 BIT STRING (SIZE (9))
              if (ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.choice.
                rrcSetupComplete->ng_5G_S_TMSI_Value->choice.ng_5G_S_TMSI_Part2.size != 2) {
    
                LOG_E(NR_RRC, "wrong ng_5G_S_TMSI_Part2 size, expected 2, provided %lu",
                            (long unsigned int)ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->
                            criticalExtensions.choice.rrcSetupComplete->
                            ng_5G_S_TMSI_Value->choice.ng_5G_S_TMSI_Part2.size);
                return -1;
              }

              if (ue_context_p->ue_context.ng_5G_S_TMSI_Part1 != 0) {
    
                ue_context_p->ue_context.ng_5G_S_TMSI_Part2 =
                                BIT_STRING_to_uint16(&ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->
                                    criticalExtensions.choice.rrcSetupComplete->
                                    ng_5G_S_TMSI_Value->choice.ng_5G_S_TMSI_Part2);
              }

            /* TODO */
            } else if (ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.choice.
              rrcSetupComplete->ng_5G_S_TMSI_Value->present == NR_RRCSetupComplete_IEs__ng_5G_S_TMSI_Value_PR_ng_5G_S_TMSI) {
    
              // NG-5G-S-TMSI ::= BIT STRING (SIZE (48))
              if (ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.choice.
                rrcSetupComplete->ng_5G_S_TMSI_Value->choice.ng_5G_S_TMSI.size != 6) {
    
                LOG_E(NR_RRC, "wrong ng_5G_S_TMSI size, expected 6, provided %lu",
                            (long unsigned int)ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->
                            criticalExtensions.choice.rrcSetupComplete->
                            ng_5G_S_TMSI_Value->choice.ng_5G_S_TMSI.size);
                return -1;
              }

              uint64_t fiveg_s_TMSI = bitStr_to_uint64(&ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->
                  criticalExtensions.choice.rrcSetupComplete->ng_5G_S_TMSI_Value->choice.ng_5G_S_TMSI);
              LOG_I(NR_RRC, "Received rrcSetupComplete, 5g_s_TMSI: 0x%lX, amf_set_id: 0x%lX(%ld), amf_pointer: 0x%lX(%ld), 5g TMSI: 0x%X \n",
                  fiveg_s_TMSI, fiveg_s_TMSI >> 38, fiveg_s_TMSI >> 38,
                  (fiveg_s_TMSI >> 32) & 0x3F, (fiveg_s_TMSI >> 32) & 0x3F,
                  (uint32_t)fiveg_s_TMSI);
              if (ue_context_p->ue_context.Initialue_identity_5g_s_TMSI.presence == TRUE) {
    
                  ue_context_p->ue_context.Initialue_identity_5g_s_TMSI.amf_set_id = fiveg_s_TMSI >> 38;
                  ue_context_p->ue_context.Initialue_identity_5g_s_TMSI.amf_pointer = (fiveg_s_TMSI >> 32) & 0x3F;
                  ue_context_p->ue_context.Initialue_identity_5g_s_TMSI.fiveg_tmsi = (uint32_t)fiveg_s_TMSI;
              }
            }
          }

          rrc_gNB_process_RRCSetupComplete(
                  ctxt_pP,
                  ue_context_p,
                  ul_dcch_msg->message.choice.c1->choice.rrcSetupComplete->criticalExtensions.choice.rrcSetupComplete);
          LOG_I(NR_RRC, PROTOCOL_NR_RRC_CTXT_UE_FMT" UE State = NR_RRC_CONNECTED \n",
              PROTOCOL_NR_RRC_CTXT_UE_ARGS(ctxt_pP));
        }

        ue_context_p->ue_context.ue_release_timer = 0;
        break;

3.2 rrcSetupComplete处理

void
rrc_gNB_process_RRCSetupComplete(
  const protocol_ctxt_t     *const ctxt_pP,
  rrc_gNB_ue_context_t      *ue_context_pP,
  NR_RRCSetupComplete_IEs_t *rrcSetupComplete
)
{
    
  LOG_I(NR_RRC, PROTOCOL_NR_RRC_CTXT_UE_FMT" [RAPROC] Logical Channel UL-DCCH, " "processing NR_RRCSetupComplete from UE (SRB1 Active)\n",
      PROTOCOL_NR_RRC_CTXT_UE_ARGS(ctxt_pP));
  ue_context_pP->ue_context.Srb1.Active = 1;
  ue_context_pP->ue_context.Srb1.Srb_info.Srb_id = 1;
  ue_context_pP->ue_context.StatusRrc = NR_RRC_CONNECTED;

  if (AMF_MODE_ENABLED) {
    
    rrc_gNB_send_NGAP_NAS_FIRST_REQ(ctxt_pP, ue_context_pP, rrcSetupComplete);
  } else {
    
    rrc_gNB_generate_SecurityModeCommand(ctxt_pP, ue_context_pP);
  }
}