tokens: ['If', 'the', 'UE', 'has', 'an', 'already', 'active', 'TLS', 'session', 'then', 'it', 'shall', 'use', 'this', 'to', 'protect', 'the', 'REGISTER', 'message', 'for', 'reregistration', 'When', 'the', 'PCSCF', 'receives', 'a', 'REGISTER', 'message', 'protected', 'by', 'a', 'TLS', 'session', 'whose', 'TLS', 'Session', 'ID', 'is', 'associated', 'with', 'an', 'IMPI', 'from', 'a', 'previously', 'successful', 'registration', 'cf', 'O', 'then', 'the', 'PCSCF', 'shall', 'proceed', 'as', 'follows', '', 'If', 'the', 'IMPI', 'is', 'present', 'in', 'the', 'REGISTER', 'message', 'the', 'PCSCF', 'shall', 'verify', 'that', 'the', 'IMPI', 'in', 'the', 'REGISTER', 'matches', 'the', 'IMPI', 'associated', 'with', 'the', 'TLS', 'Session', 'ID', 'If', 'the', 'IMPIs', 'match', 'then', 'the', 'PCSCF', 'shall', 'forward', 'this', 'REGISTER', 'message', 'together', 'with', 'a', 'TLS', 'integrity', 'protection', 'indicator', 'indicating', 'the', 'logical', 'value', 'authentication', 'complete', '', 'If', 'the', 'IMPI', 'is', 'not', 'present', 'in', 'the', 'REGISTER', 'message', 'the', 'PCSCF', 'shall', 'not', 'include', 'any', 'TLS', 'integrity', 'protection', 'indicator', 'When', 'the', 'SCSCF', 'receives', 'a', 'REGISTER', 'message', 'with', 'a', 'TLS', 'integrity', 'protection', 'indicator', 'indicating', 'the', 'logical', 'value', 'authentication', 'complete', 'it', 'may', 'authenticate', 'the', 'user', 'by', 'means', 'of', 'SIP', 'Digest', 'according', 'to']