Abstract:
A method and apparatus performs processing of the non-access stratus (NAS) layer (layer 3) in long term evolution (LTE) wireless transmit/receive units (WTRUs), which allows the NAS protocol layer to route layer 3 messages to the correct NAS entity, and to encode new NAS message types and information elements. A new architecture is presented that enables NAS security. When a NAS message is generated, a determination is made as to whether or not to cipher, de-cipher and/or integrity check the NAS message based on at least one of a protocol discriminator (PD) of the NAS message, an indicator field in a header of the NAS message, the type of the NAS message, a NAS security state variable, and an indication by an upper layer protocol. The NAS security state variable indicates whether NAS security is currently active or not and may comprise one bit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/021,436 filed Sep. 9, 2013, which is a continuation of U.S. application Ser. No. 12/254,265 filed Oct. 20, 2008, which issued as U.S. Pat. No. 8,532,614 on Sep. 10, 2013 which claims the benefit of U.S. Provisional Application No. 60/982,698 filed Oct. 25, 2007, which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     This application is related to wireless communications. 
     BACKGROUND 
     The current effort for the third generation partnership project (3GPP) long term evolution (LTE) program is to bring new technology, new architecture and new methods related to LTE settings and configurations in order to provide improved spectral efficiency, reduced latency, better utilization of radio resources to bring faster user experiences and richer applications and services with less cost. 
     The LTE layer 3 (L3) architecture may be considered as an evolution of the existing L3 architecture for a general packet radio service (GPRS) capable wireless transmit/receive unit (WTRU), (i.e., mobile station). LTE defines new mobility management (MM) concepts, (e.g., the concept of tracking areas replacing routing areas), and new MM procedures, (e.g., multiple tracking areas may be allocated in a tracking area update procedure). These new procedures will be described in more detail by new L3 protocols, (e.g., evolved mobility management (EMM)) and evolved session management (ESM)), that will be a part of the LTE non-access stratum (NAS). These new protocol entities are the LTE equivalent of GPRS mobility management (GMM), session management (SM), and the like. 
     Furthermore, as part of this evolution process, 3GPP will use a different security architecture in LTE than used is in universal mobile telecommunications system (UMTS) and global system for mobile communications (GSM). For the sake of comparison, the UMTS authentication and key agreement (AKA) procedures, (in the packet switched (PS) domain), may be considered to be the baseline for the new LTE procedures. The current UMTS AKA procedures and a brief description of the new LTE security architecture will now be described. 
     The UMTS AKA and ciphering procedures are spread over multiple protocol layers and use both NAS and radio resource control (RRC) signaling to accomplish their goal. In brief, identification and authentication of a WTRU is accomplished via NAS signaling. Once authentication at a NAS level is accomplished, ciphering and/or integrity protection is activated by the network using a security mode command, which is an RRC message. Once security is activated using the security mode command, the NAS layer in the WTRU first passes a ciphering key (CK) and an integrity key (IK) to the access stratum (AS) using a GMMAS-SECURITY-RESPONSE primitive over the GMMAS service access point (SAP), (defined between the GMM and the AS). The RRC receives these keys and passes them on to the radio link control (RLC) and the medium access control (MAC) using a CRLC-CONFIG primitive, (over the C-SAP between the RRC and RLC) and the CMAC-CONFIG primitive (over the C-SAP between the RRC and MAC). The C-SAP is a service access point for C-plane signaling between the RRC and lower layers. The actual ciphering and integrity protection is usually performed in the RLC, but is performed in the MAC in case of transparent RLC mode traffic. The lower layers, (i.e., MAC/RLC), are responsible for ensuring that messages intended for upper layers, (e.g., L3 NAS messages), have been integrity protected and/or ciphered correctly. If not, the lower layers ignore/drop the message. 
     For LTE, a radically different architecture for security has been proposed. The main difference is that instead of a single security layer, (i.e., in the MAC/RLC), there are now two levels of security—NAS security and AS security. NAS security terminates in the mobility management entity (MME), (i.e., core network), and the AS security terminates in the base station (i.e., eNode-B). In brief, the AKA procedures are completed in the NAS, the NAS security keys are derived first and upon completion, and the AS security parameters are derived from the NAS keys in a cryptographically separate manner, (i.e., knowledge of AS keys does not allow an attacker to determine the NAS keys). The main rationale for this decision was that in LTE, one might have base stations in vulnerable locations, (e.g., home Node-Bs), and since RRC (and therefore security) is terminated in the base station, this was considered to be a security risk. Hence two levels of security are required. 
       FIG. 1  shows the structure of a conventional LTE L3 header  100 . The first octet of the LTE L3 header  100  includes a transaction identifier or skip indicator field  105  and a protocol discriminator (PD) field  110 . The second octet of the LTE L3 header  100  includes a message type field  115 . Additional octets of the LTE L3 header  100  may include other information elements  120  as required. As previously described, new L3 protocol entities have been proposed, (e.g., EMM and ESM). However, the current LTE L3 header  100  does not support these new protocols. Specifically, the PD field  110  in the LTE L3 header  100  of  FIG. 1  is enhanced to distinguish these new protocols as options. 
       FIG. 2  shows the PD field  110  of the LTE L3 header  100  of  FIG. 1 . Referring to  FIGS. 1 and 2 , the last four bits (4321) of the first octet in the LTE L3 header  100  form the PD field  110 , which is used by the routing entity in the MM sub-layer of the NAS to route an L3 message including the LTE L3 header  100  to the appropriate NAS entity, (e.g., MM/GMM/SM currently). 
     The conventional NAS architecture  300  of a PS-only UMTS WTRU is shown in  FIG. 3 . The NAS architecture  300  includes a radio access bearer management (RABM) unit  305 , a connection management (CM) unit  310 , a mobility management (MM) unit  315  and an access stratum (AS) sublayer  320 . The RABM unit  305  includes a plurality of radio access bearer (RAB) entities  325 ,  330  and  335 , and an RAB control unit  340 . The CM unit  310  includes a session management (SM) unit  345 , a GPRS short message service (GSMS) entity  350 , and a supplemental service (SS) entity  355 . A packet data protocol (PDP)  360  is used as an interface between the CM unit  310  and the MM unit  315 . The MM unit  315  includes a GPRS MM (GMM) unit  365  and a PD unit  370 . Both the MM unit  315  and the RABM unit  305  interface with the AS sublayer  320 , which include a radio resource controller (RRC)  375 , a broadcast multicast controller (BMC)  380 , and a packet data conversion protocol (PDCP)  385 . The AS sublayer  320  provides services to the MM unit  315  and the RABM unit  305 . The MM unit  315  provides services to the entities of the CM unit  310 . 
     The RAB control unit  340  adds, modifies, deletes and/or reconfigures the RAB entities  325 ,  330  and  335 . The PD unit  370  is used for routing NAS message information elements (IEs) to various NAS entities. The SM unit  345  provides services to the RABM unit  305  and uses services of the MM unit  315 . The GSMS entity  350  is identical to the SMS entity for GPRS services in GSM, except it uses the services from the GMM unit  365 . The SS entity  355  is identical to the one for non-GPRS services, except it uses the services from the PS signaling connection. The RABM unit  305  hides the concepts of RABs that can be activated/released while a PDP context is active. If uplink (UL) data in the terminal is to be sent on an RAB (network service access point identifier (NSAPI)) that has been released, the RABM unit  305  will trigger a service request procedure in GMM unit  365 . 
     Usually, NAS message IEs are encoded in type/length/value (TLV) format. As shown in  FIG. 4 , NAS message IEs belong to one of five types of IEs  405 ,  410 ,  415 ,  420  and  425 . As shown in  FIG. 4 , IE  405  has a type (T) only format, IE  410  has a value (V) only format, IE  415  has a type and value (TV) format, IE  420  has a length and value (LV) format, and IE  425  has a type, length and value (TLV) format. As indicated in  FIG. 4 , an IE indicator (type) is present in IEs  405 ,  415  and  425 , but is not present in IEs  410  and  420 . A length indicator is present in IEs  420  and  425 , but is not present in IEs  405 ,  410  and  415 . A value indicator is present in IEs  410 ,  415 ,  420  and  425 , but is not present in IE  405 . 
     Some of the problems with using the NAS architecture  300  of  FIG. 3  is that the new NAS messages proposed do not have any message types defined in order to be identified. Also, some of the expected new NAS IEs have no defined format for their encoding. Furthermore, the NAS entities shown in  FIG. 3  do not support security, (i.e., it is difficult to implement security in the LTE NAS using the current NAS architecture). 
     In addition, in the NAS architecture  300 , the ciphering algorithms proposed for LTE are block ciphers, i.e., they work by using the CK and an indication of the length of the protocol data unit (PDU) to be ciphered to generate a keystream block, having a length equal to that of the unciphered PDU. This keystream block is then bitwise added (usually) to the unciphered PDU to generate the ciphered PDU. The procedure is also used at the receiver to generate the identical keystream block for deciphering. This keystream block is then bitwise added to the received ciphered PDU. 
     In LTE, ciphering of NAS messages has been agreed to. Therefore, the NAS layer has to indicate to the ciphering algorithm the length of the L3 NAS PDU to be ciphered. No functionality exists today for the NAS to do so. 
     Finally, if relocation of the MME is allowed, then it is possible that during handover an MME relocation may take place. An example of a handover procedure used to carry out the relocation of the MME is shown in  FIG. 5 . There is currently no procedure defined for the handling of an NAS sequence number (SN) and a hyper frame number (HFN) upon radio link failure and inter-MME handover. 
     SUMMARY 
     This application describes the features of an NAS layer (L3) in LTE WTRUs, whereby the NAS protocol layer is allowed to route layer 3 messages to the correct NAS entity, and to encode new NAS message types and information elements. A new architecture is presented that enables NAS security. When a NAS message is generated, a determination is made as to whether or not to cipher, de-cipher and/or integrity check the NAS message based on at least one of a protocol discriminator (PD) of the NAS message, an indicator field in a header of the NAS message, the type of the NAS message, a NAS security state variable, and an indication by an RRC protocol. The NAS security state variable indicates whether NAS security is currently active or not and may comprise one bit. 
     The NAS protocol layer is allowed to route L3 messages to the correct LTE NAS entity, (e.g., EMM and ESM). The encoding of new NAS message types and new NAS IEs is allowed. A new NAS architecture is provided to enable NAS security and allow the determination of the length of the NAS PDU for generation of a ciphering keystream of equal length. Furthermore, the NAS layer is allowed to handle SN and HFN upon radio link failure and handover. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
         FIG. 1  shows the structure of a conventional LTE L3 header; 
         FIG. 2  shows a PD field of the LTE L3 header of  FIG. 1 ; 
         FIG. 3  shows a conventional NAS architecture of a PS-only UMTS WTRU; 
         FIG. 4  shows the type/length/value (TLV) encoded format of NAS message IEs; 
         FIG. 5  shows an example of a conventional inter-MME handover procedure; and 
         FIG. 6  shows an example of a new NAS architecture. 
     
    
    
     DETAILED DESCRIPTION 
     When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
     Enhancement to Layer 3 Protocol Discriminator (PD) Field 
     Referring to  FIG. 1 , the L3 PD field  110  in the LTE L3 header  100  may be enhanced, such that a specific combination of the bits in the L3 PD field  110  indicates that the L3 message that follows the LTE LE header  100  is an upper layer LTE L3 message, (e.g., EMM, ESM). 
     The terms EMM and ESM are used to describe the LTE MM and SM entities and protocols, and their associated functions. If any additional entities/protocols are defined/modified in the NAS layer for LTE, their respective protocol may also be added to the L3 PD field. However, as shown in  FIG. 2 , there are very few bit combinations available to do so. Hence, the following options may be implemented:
         1) Define the spare PD values ‘0111’ and ‘1101’ to denote EMM and ESM protocols (in any order);   2) Extend the PD field to be one octet (or more) and map two values (of the available values that this increased PD field may take) to EMM and ESM protocols;   3) Re-use some existing PD values, (e.g., 1000 for GMM messages), to indicate the EMM and ESM protocols.       

     Enhancement to Message Type Description 
     Referring to  FIG. 1 , the second octet of the LTE L3 header  100  includes a message type field  115 . The different values of this octet map to different messages of the protocol layer identified by the protocol discriminator field  110 . 
     In order to define the new L3 NAS messages that are expected for LTE, additional values in the message type field  115  may be assigned for the following:
         1) Tracking Area Update Request;   2) Tracking Area Update Accept;   3) Tracking Area Update Complete;   4) Tracking Area Update Reject;   5) NAS Security Mode Command;   6) NAS Security Mode Command Complete; and   7) NAS Security Mode Command Failure.       

     New Integrity Check (IC) Information Element in NAS Messages 
     For integrity checking a NAS message, a new NAS integrity check (IC) IE may be appended to each NAS message. The receiver compares the value of this received IE with its own computation. Since the length of the IC bits is likely to be fixed, (as it is the output of a known algorithm), the IC IE may be encoded as a type 3 NAS message IE, (type and value (TV) only), as the length of the value part is fixed. Alternatively, it may be encoded as a type 2 or some other type. A new IE identifier may be defined to identify the NAS IC IE. 
     New Architecture for Securing the NAS Layer 
     The following different architectures are described for securing the NAS layer. 
     New NAS Entity/Protocol for Security 
       FIG. 6  shows an example of a new NAS L3 architecture  600 , which may reside within a WTRU, (i.e., a mobile station). The NAS L3 architecture  600  includes an evolved packet system bearer management (EPSBM) unit  605 , a CM unit  610 , an MM unit  615  and an LTE AS sublayer  620 . The EPSBM unit  605  includes a plurality of evolved packet system (EPS) bearer entities  625 ,  630  and  635 , and an EPS control unit  640 . The CM unit  610  includes an evolved session management (ESM) unit  645 , an evolved short message service (ESMS) entity  650 , and a supplemental service (SS) entity  655 . A PDP  660  is used as an interface between the CM unit  610  and the MM unit  615 . The MM unit  615  includes an evolved mobility management (EMM) unit  665  and a secure connectivity (SC) unit  370 . The SC unit  370  includes a PD unit  375 . Both the MM unit  615  and the EPSBM unit  605  interface with the LTE AS sublayer  620 , which includes an RRC  680  and a PDCP  685 . 
     The EPS bearer entities  625 ,  630  and  635  run between the WTRU and a packet data node (PDN) gateway and consists of the combination of a radio bearer (RB) and an EPS access bearer. The EPSBM unit  605  controls the modification and configuration of the EPS bearers. The LTE AS sublayer  620  provides services to the MM unit  615  and the EPSBM unit  605 . The ESM unit  645  provides services to the EPSBM unit  605  and uses services of the MM unit  615 . The ESMS entity  650  is identical to the SMS entity for GPRS services in GSM, except it uses the services from the EMM unit  665 . The SS entity  655  is identical to the one for non-GPRS services, except it uses the services from the PS signaling connection. 
     The SC unit  670  is used such that all messages from the EMM unit  665  and the ESM unit  645  pass through the SC unit  670  to the LTE AS sublayer  620 . The SC unit  670  may be viewed as a sub-entity of the MM unit  615  as shown in  FIG. 6 , or it may be viewed as an L3 entity separate from the MM unit  615 . The functionalities of the SC unit  670  may also reside in lower layers, (e.g., RRC  680 ), and it may also be defined to have its own protocol. 
     Functionally, any combination of the following may be implemented:
         1) The EMM, ESM and other entities (e.g., SMS) may send messages to the SC unit  670 .   2) SAPs and associated primitives may be defined for the EMM unit  665  and the SC unit  670 , and the ESM unit  645  and the SC unit  670 .   3) The SC unit  670 , (or more generally the NAS), may define a NAS security state variable, (e.g., 1 bit), that denotes whether currently NAS security is active or not. It is possible to have two such variables to distinguish between ciphering and integrity protection. Note that instead of 1 bit NAS security state variable, primitives may be exchanged between the NAS and the RRC to indicate presence or absence of ciphering. Also the one bit could also be considered a separate PDU that is used to indicate that ciphering has begun, (e.g., a separate NAS message may be used for this purpose). Alternatively, this one bit may be included in every NAS PDU.   4) The SC unit  670  may operate in either a transparent mode or a non-transparent mode. In the transparent mode, the SC unit  670  will not cipher/de-cipher and/or integrity check a particular NAS message. This decision may be based on any combination of the following factors: the protocol discriminator of the message, an indicator field in the L3 protocol header, (indicating requirement of ciphering and/or integrity protection for the particular PDU), the message type, the NAS security state variable, an indication by an upper layer protocol, (e.g., an indication from the EMM unit  665  and/or the ESM unit  645  to the SC unit  670  to not cipher/integrity check this message).   5) In non-transparent mode, the SC unit  670  may cipher/de-cipher and/or integrity check the relevant NAS PDU. This decision may also be based on any combination of the following factors: the protocol discriminator of the message, an indicator field in the L3 protocol header, (indicating a requirement of ciphering and/or integrity protection for the particular PDU), the message type, the NAS security state variable, an indication by an upper layer protocol, (e.g., an indication from the EMM unit  665  and/or the ESM unit  645  to the SC unit  670  to not cipher/integrity check this message).   6) On the receiving side, the SC unit  670  may receive a NAS PDU from lower layers and, if operating in the transparent mode, route it based on its protocol discriminator to the correct upper layer entity, (e.g., EMM/ESM). If the receiving SC unit  670  is in the non-transparent mode, it may cipher and/or integrity check the NAS PDU, increment its NAS SN and/or NAS HFN, and then route the message based on the PD field of the NAS header. The receive side may either de-cipher and then check the integrity of the message or vice versa depending on the order of these operations in the transmit side.       

     As previously described, it may be possible to have a separate protocol for the SC unit  670 . Such a protocol may be distinguished by having its own protocol discriminator field in the L3 header. The message types belonging to this protocol could be those related to ciphering, authentication, identification and key agreement among others. Examples include:
         1) Identity Request;   2) Identity Response;   3) Authentication Request;   4) Authentication Response;   5) Authentication Failure;   6) Authentication Reject;   7) NAS Security Mode Command;   8) NAS Security Mode Command Complete; and   9) NAS Security Mode Command Failure.       

     It may also be possible to define a state machine for this entity. As an example, the following describes possible states (other names for these states and other states may be possible):
         a) SECURITY INACTIVE: In this state, the WTRU is not authenticated to the network and no security context exists in the WTRU or in the network. As a sub-state of this state (or as separate states) one may have states denoting an ongoing AKA session, NAS security mode command exchange, and RRC security mode command exchange.   b) SECURITY ACTIVE: In this state, the WTRU is authenticated to the network and a security context exists in the WTRU and in the network. The WTRU may have some keys (e.g., K asme ) or all keys.       

     It may also be possible to implement this same SCE in a different, although in a more complicated, manner. For example, on the transmit side, the upper layer calling entities, (such as the EMM unit  665  or the ESM unit  645 ), having constructed the NAS PDU, may send it to the NAS SC unit  670 . The NAS SC unit  670  may then cipher and/or integrity check the PDU and then return it to the calling entity. The calling entity may then send the secured NAS PDU to the lower layer (i.e., RRC). This method has an advantage of reusing existing SAPs between the MM unit  615  and the LTE AS sublayer  620 . On the receive side, however, the message is de-ciphered and integrity checked before it can be routed to the correct upper-layer protocol entity. This functionality may be achieved by routing all received NAS PDUs to the SC unit  670 , which then makes the routing decision. 
     Another option is that each NAS layer does the ciphering/integrity-protection of the messages in its own protocol. Thus, the EMM unit  665  may cipher/integrity check/decipher EMM messages, and the ESM unit  645  may cipher/integrity check/decipher ESM messages, and so on. The role of the SC unit  670  in this case may be limited to providing sequence numbers to the EMM and ESM layers, (and any other layers), so that there is no collision of SNs being used. This problem may disappear by defining the SNs to increment, on a per protocol PDU basis, instead of on a per NAS PDU basis. In this case, each NAS sub-layer, (e.g., EMM/ESM), may have its own SN/HFN which is incremented for every NAS PDU belonging to this protocol. In this version, on the receive side, the routing entity in the NAS layer may route a received NAS PDU to the correct NAS entity based on the protocol discriminator field in the NAS header, (assuming that it is not ciphered), and then the respective protocol entity may de-cipher and/or integrity check the message. 
     Determination of the Length of the NAS PDU 
     The NAS layer may provide the length of the portion of the NAS PDU (to be de-ciphered) to the ciphering engine. The following are different options for determining this length.
         1) The transmit NAS entity may include an indication of the length in the L3 message, (e.g., in the protocol header). This length may be the length of the entire message, the ciphered portion of the message or some other value that allows the receiving NAS entity to determine the length of the keystream block that needs to be generated.   2) The transmit NAS entity may include an indication of the length of the L3 message to the RRC when it passes an L3 message for transmission. This length indication is received by the RRC layer in the receiver and passed on to the receiver NAS entity, where it is used to determine the length of the keystream block to be generated.   3) Currently, the NAS layer expects complete NAS PDUs without re-segmentation being performed at the NAS layer. As a result, when a NAS PDU is received, the receiving entity may be confident that it is complete, (its behavior if it receives a message missing expected values is currently to discard the message). Thus, it is possible to leave the determination of the length of the NAS PDU up to implementation.       

     An example implementation in the receiver is given below:
         1) a NAS PDU is received;   2) the NAS PDU is stored in a memory/buffer or equivalent;   3) the size of the message is determined, (e.g., memory/buffer space occupied);   4) the length of the ciphered portion of the message is determined by subtracting the length of the un-ciphered portion of the message from the size of the message. The length of the un-ciphered portion of the message may be fixed or may vary in a known pattern, (e.g., depending on the message type/protocol) or may be configured by the transmit NAS entity in the receiving NAS entity; and   5) the length of the ciphered portion is passed to the ciphering algorithm to determine the keystream to be used for de-ciphering.       

     Handling of NAS SN and HFN Upon Radio Link Failure and Handover 
     As part of these procedures it is necessary to determine how the current NAS SN and HFN number is processed in the network. 
     During handover, any of the following may be performed with the NAS SN and HFN currently in the WTRU and the source MME.
         1) They may both be reset to a default value.   2) The NAS HFN may not be reset but the NAS SN may be reset. The NAS HFN is passed by the source MME to the target as part of the handover (HO) request.   3) The NAS SN may not be reset but the NAS HFN may be reset. The NAS SN is passed by the source MME to the target as part of the HO request.   4) Neither the NAS HFN nor the NAS SN may be reset. They are both passed by the source MME to the target as part of the HO request.       

     The other question that needs to be answered is that, if any of the parameters, (NAS SN and/or HFN), are being reset, when this reset is triggered and how are the new NAS/ASME keys passed to the WTRU. 
     Therefore, during inter-MME handover, a NAS message is triggered from the source MME to the WTRU providing the parameters necessary for deriving the new NAS and ASME keys. Alternatively, the source MME may provide the WTRU with the new NAS and ASME keys in a secure way, (using existing ciphering/integrity protection). As an example, this message may be a NAS Security Mode Command/Reconfiguration. This NAS message may be sent prior to handover command, (e.g., in a separate downlink direct transfer message), or as an L3 message within the handover command. As another alternative, this information may be sent directly in a RRC message. 
     The reset of the NAS SN and/or HFN may be triggered by a lower layer indication of handover imminent, (e.g., when the RRC receives a handover command), or by the receipt of said separate NAS message. 
     Handling of NAS SN and HFN Upon Radio Link Failure and Inter-eNodeB Handover 
     For a radio link (RL) failure and inter-eNodeB handover in the uplink (UL) and downlink (DL). The NAS SN and/or the HFN may be reset. This may be triggered by the transmission or reception of a message, (e.g., an RRC message). 
     Handling of Retransmissions at NAS 
     In case of retransmissions, the NAS layer may detect whether the packet that is transmitted again is a duplicate PDU (a PDU that is already received) or a new PDU. The duplicate detection entity could be a common entity sitting below the EMM or the ESM, and monitoring whether the incoming packets have been sent before or not. In the event that an acknowledgement was not received for a transmitted NAS PDU, lower layers may provide an indication of this to the NAS layer. The NAS layer may use this indication to re-transmit the PDU, (or send a new PDU), with the same SN. 
     In case duplicate detection is not allowed, the same sequence number may be used for transmitting the data in case of a failed transmission as opposed to transmitting the data with a new sequence number. 
     Alternatively or in addition, the NAS transaction identifier may be used by the NAS to detect duplicate messages. This may require the NAS transaction ID to be part of the NAS header. Thus, the WTRU may be able to read this ID and discard any duplicate messages received. 
     Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided herein may be implemented in a computer program, software, or firmware incorporated in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
     Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. 
     A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) or Ultra Wide Band (UWB) module.