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Security header type (octet 1)

8 7 6 5
0 0 0 0 Plain NAS message, not security protected

Security protected NAS message:
0 0 0 1 Integrity protected
0 0 1 0 Integrity protected and ciphered
0 0 1 1 Integrity protected with new EPS security context (NOTE 1)
0 1 0 0 Integrity protected and ciphered with new EPS security context (NOTE 2)

Non-standard L3 message:
1 1 0 0 Security header for the SERVICE REQUEST message

1 1 0 1 These values are not used in this version of the protocol.
to If received they shall be interpreted as ‘1100’. (NOTE 3)
1 1 1 1

All other values are reserved.

NOTE 1: This codepoint may be used only for a SECURITY MODE COMMAND message.
NOTE 2: This codepoint may be used only for a SECURITY MODE COMPLETE message.
NOTE 3: When bits 7 and 8 are set to '11', bits 5 and 6 can be used for future extensions of the SERVICE REQUEST message.