 the receiver (MS or BS) should store the received data packets and must not forward them to the upper layers until the sender is authenticated. If the data packets belong to a service flow associated with a security architecture that supports data authentication, as indicated by the data authentication algorithm identifier in the cryptographic suite, the receiver can authenticate the sender by verifying that the cipher-text authentication code included in each data packet was produced with the TEK associated with this security architecture. If the data packets belong to a service flow associated with a security architecture that does not support data authentication the receiver can authenticate the sender when the RNG-REQ/RSP transaction completes successfully. In all cases, if the sender is authenticated, the decrypted data packets are forwarded to the upper layer in the recipient, and if the sender is not authenticated the data packets are discarded. The RNG-REQ/RSP messaging for seamless handover is shown in Figure 6-13. The MS initiates the RNG-REQ/RNG-RSP message exchange by transmitting a RNG-REQ message to the target BS before the deadline specified by the Seamless HO Ranging Initiation Deadline parameter included in the MOB_BSHO-REQ/RSP message during handover preparation. The RNG-REQ message must CHAPTER 6 The IEEE 802. 16m MAC CPS (Part I) include basic CID, C_KEY_COUNT, and a valid HMAC /CMAC tuple (see Chapter 8 for security aspects). When the BS receives the RNG-REQ message, the BS responds to the RNG-REQ message by transmitting a RNG-RSP message with a valid HMAC/CMAC tuple. The RNG-RSP message includes basic CID but is not required to include any CID update TLV or SA-TEK-Update TLV for unicast connections. When MS receives the RNG-RSP message from the target BS, the target BS becomes the serving BS of the MS, and the MS transmits a BR header with a zero bandwidth request. When BS receives the BR header, the seamless handover procedure completes successfully. 6.4.2 Handover Process The handover pro