Configuring Security Features189The web user interface pops up the dialog box to prompt ―Rebooting, pleasewait…‖.Secure Real-Time Transport Protocol (SRTP) provides means of encrypting the RTPstreams during VoIP phone calls to avoid interception and eavesdropping. The partiesparticipating in the call should enable the SRTP feature simultaneously. When thisfeature is enabled on both phones, the IP phone will negotiate with the other phonewhat type of encryption to utilize for the session. This negotiation process is compliantwith RFC 4568.When a user places a call on the enabled SRTP phone, the IP phone sends an INVITEmessage with the RTP encryption algorithm to the destination phone.The sample of the RTP encryption algorithm carried in the SDP of the INVITE messagefor reference:m=audio 11780 RTP/SAVP 0 8 18 9 101a=crypto:1 AES_CM_128_HMAC_SHA1_80inline:NzFlNTUwZDk2OGVlOTc3YzNkYTkwZWVkMTM1YWFja=crypto:2 AES_CM_128_HMAC_SHA1_32inline:NzkyM2FjNzQ2ZDgxYjg0MzQwMGVmMGUxMzdmNWFma=crypto:3 F8_128_HMAC_SHA1_80 inline:NDliMWIzZGE1ZTAwZjA5ZGFhNjQ5YmEANTMzYzA0a=rtpmap:0 PCMU/8000a=rtpmap:8 PCMA/8000a=rtpmap:18 G729/8000a=fmtp:18 annexb=noa=rtpmap:9 G722/8000a=fmtp:101 0-15a=rtpmap:101 telephone-event/8000a=ptime:20a=sendrecvThe callee receives the INVITE message with the RTP encryption algorithm. The calleeanswers the call and responses with a 200 OK message carrying the negotiated RTPencryption algorithm.
