t it the PSTN message bodies must be encrypted if they are transported over the public Internet, or used in a network with native SIP devices. This is because private information can be car- ried in PSTN messages because PSTN protocols assume a different trust model than an Internet protocol such as SIP. To prevent accidental disclosure of this information, the message bodies must be encrypted by the originating gateway and decrypted by the terminating gateway, which adds significant processing requirements and call setup delay. Encapsulated PSTN messages are carried as MIME bodies, which have been standardized for both ISUP and QSIG [17]. SIP: Understanding the Session Initiation Protocol 7.6 Universal Plug and Play Protocol The Universal Plug and Play Protocol (UPnP) [18] is a protocol developed to allow configuration and peer-to-peer networking of intelligent IP devices. UPnP works together with DHCP or Auto IP to configure and set up devices in small networks typical of home and small office networks [19]. The protocol has six basic functions: addressing, discovery, description, control, events, and presentation. Some SIP clients use UPnP to gain configuration information and to talk to firewalls and NATs that are UPnP enabled to open firewalls and learn private/public IP address mappings. For example, if a router or wireless bridge that encompasses a firewall and NAT function is UPnP enabled, an author- ized SIP client also UPnP enabled can automatically manage NAT and firewall traversal. In the future, proponents of UPnP see it playing a key role in home net- work configurations when multiple intelligent IP devices are connected in home or small office. However, the protocol is unlikey to be used in medium and large enterprises. UPnP references a number of IETF standards such as DHCP, HTTP Multicast UDP (HTTPMU), and Auto IP. It also uses W3C protocols such as XML and SOAP. References Handley, M., and V. Jacobson, "SDP: Session Description Protocol," RFC 2327, 1998. Handley, M., C. Perkins, and E. Whelan, "