b sends another DO to relock his door. In this scenario, one of the main services SIP provides is security. Every command sent by Bob has to be strongly authenticated before it is per- formed. Bob definitely does not want anyone else to use the cameras in his place or to unlock his door. The SIP security framework provides mecha- nisms to control networked appliances while ensuring the privacy of the user. Chapter 7 Bob's camera Audio system Figure 7-20 Door's lock system at Bob's door Bob controls Doorbell his networked (1) SUBSCRIBE appliances from Doorbell status his office. (2) 200 OK (3) NOTIFY Doorbell is ringing (2) 200 OK (5) INVITE (6) 200 OK (7) ACK Bob checks that the person ringing is the technician (8) INVITE (9) 200 OK (10) ACK Conversation between Bob and the technician (11) BYE (12) 200 OK (13) DO Unlock door (14) 200 OK (15) DO Lock door (16) 200 OK (17) BYE (18) 200 OK This chapter has provided some examples of architectures that use SIP as a signalling protocol. Now it is clear for the reader how to combine dif- ferent SIP extensions, and even different protocols in order to provide real- life services to real users. APPENDIX Finding Futher Information on SIP This appendix is for those of you that have finished this book and would like to read even more about SIP. The best way to find information about SIP or any other protocol developed by the IETF is to surf the Internet. However, if you are not familiar with IETF protocols, you might spend more time than you would like, looking for a particular piece of information on the Web. Finding a good Web page on a protocol is sometimes difficult. A regu- lar search engine will return too many hits if you just introduce the word "SIP," for instance. This appendix contains the most useful Web pages related to SIP. These Web pages will help readers find the information they want more rapidly. We also give information about the most interesting SIP-related mailing lists. As we mentioned before, mailing lists are one of the most important tools in the IET