[5g NR] RRC reject analysis

author ： No.7 Huazai
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1. RRC Reject describe

The trigger condition ： Base station receives from UE Of RRC Connection Request, According to the admission control algorithm , Don't allow UE Access .
send content ：wait time, Timer T302 Duration

2. UE receive RRCReject Processing flow

When UE received RRC Reject after , Follow the following procedure ：

1. Stop timer T300,T319,T302

2. Reset MAC And release the default MAC Cell Group To configure ;

3. If in RRCReject Configuration of the waitTime, Then start the timer T302, Set the timer value to waitTime;

4. If you receive RRCReject It is to respond to the request of the upper layer , Notify the upper layer that access is restricted ;

5. If you receive RRCReject It is used to reply RRCSetupRequest, Inform the top RRC The connection fails , Program end ;
   

6. If you receive RRCReject In response to RRCResumeRequest：
   

• If the upper layer triggers resume, Inform the upper level RRC The connection fails ;
• If because of RNA Update and trigger recovery , Put the variable pendingRnaUpdate Set to true, Discard the current KgNB secret key ,KRRCenc secret key ,KRRCint secret key ,KUPint Key and basis 5.3.13.3 Derived KUPenc secret key ;
• Pause SRB1, Program end ;

Timer T302 Runtime ,RRC_INACTIVE State of UE Will continue to monitor paging .

3. Message definition

RRCReject Messages are used to reject RRC Connection establishment or RRC Connection reconstruction .

Signaling wireless bearer ：SRB0
transmission mode : TM
Logical channel ：CCCH
Direction ： Network to UE

RRCReject news

-- 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 Used for timer T302 Provide a value in seconds , Value range 1~16.

RejectWaitTime information content

-- ASN1START
-- TAG-REJECTWAITTIME-START

RejectWaitTime ::=                  INTEGER (1..16)

-- TAG-REJECTWAITTIME-STOP
-- ASN1STOP

4. OAI RRC Reject Send judgment

Open source OAI In the code , Base station received rrcSetupRequest after , Will carry it UE Identity Type to judge , If it is neither random nor TMSI, Will go to UE send out 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 code

be familiar with ASN The usage is clear ,option There are many options , In order to adapt to flexible data structures , You need a lot of pointers in the structure . Before the actual assignment , Need to use malloc perhaps calloc Apply memory for pointer variables .RRC Reject Carried on CCCH channel , Therefore, the outer layer needs to be encoded DL_CCCH code .

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; // Specifies that the logical channel message type is 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; // Appoint dl_ccch The message type is 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)); // by waitTime Application memory 

    rrcReject->criticalExtensions.present = NR_RRCReject__criticalExtensions_PR_rrcReject; // Set up rrcReject Inside present by rrcReject
    rrcReject->criticalExtensions.choice.rrcReject->waitTime = &waitTime; //waitTime assignment 

    if ( LOG_DEBUGFLAG(DEBUG_ASN1) ) {
    
        xer_fprint(stdout, &asn_DEF_NR_DL_CCCH_Message, (void *)&dl_ccch_msg);
    }
	// Message body per code 
    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);
}