23.txt?number=3323 Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks (RFC-3325). http://www.ietf.org/rfc/rfc3325.txt?number=3325. HTTP Authentication: Basic and Digest Access Authentication (RFC-2617). http://www.ietf.org/rfc/rfc2617.txt?number=2617. The Secure Real-Time Transport Protocol (SRTP) (RFC-3711). http://www.ietf.org/rfc/rfc3711.txt?number=3711. A DNS RR for specifying the location of services (DNS SRV) (RFC-2782). http://www.ietf.org/rfc/rfc2782.txt?number=2782. SIP: Session Initiation Protocol (RFC-3261). http://www.ietf.org/rfc/rfc3261.txt?number=3261. A Message Summary and Message Waiting Indication Event Package for the Session Initiation Protocol (SIP) (RFC-3842). http://www.ietf.org/rfc/rfc3842.txt?number=3842. Chapter Interworking This chapter covers the following topics: Protocols Media Encryption interworking Chapter 7, "Design and Implementation Considerations," discussed a series of network design and implementation considerations you should work through when connecting a service provider Session Initiation Protocol (SIP) trunk into your network. This chapter explores an additional set of considerations regarding interworking. When all external Public Switched Telephone Network (PSTN) calls to the enterprise were Time Division Multiplexing (TDM) and only internal calls used IP, the enterprise network designer was in complete control of the following: Which IP user endpoints communicated with each other and the compatibility of the capabilities among these endpoints The H.323 and SIP protocols and vintages used between network elements The methods of interworking such as dual tone multi-frequency (DTMF), fax and codec choice, and packetization When external PSTN calls arrive as IP calls using a SIP trunk, the enterprise no longer has direct control over any of these network elements. The protocol variations and media encoding methods of externally originated IP calls (such as the codec or DTMF-relay method chosen) might be incompatible 