The NSA looked at SSL VPN as a standard and left no stone unturned in exploring commercial SSL VPN for mobile, but found utter lack of interoperability across vendor products. Salter said NSA also was frustrated with the lack of interoperability in Unified Communications Systems (UCS) products, noting that buying one piece often meant buying several others, there being little evidence of multi-vendor interoperability. So with some frustration, NSA changed to go with an open-source Session Initiation Protocol (SIP) server for the present. NSA also switched its mobile security strategy toward IPSec VPN, where things looked better in terms of interoperability than SSL VPN, and selected the Secure Real-Time Transport Protocol for Voice App and Transport Layer Security (TLS) with keys. This all means "the voice call is doubly encrypted," Salter said. "There's VoIP encryption and IPsec encryption."
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