ed for this purpose. The INFO method is used to carry mid-call PSTN messages in its body. Figure 7-15 shows MIME media types and INFO in an interworking situation. INVITE and the 200 OK carry in their bodies, apart from the session description, a PSTN signalling message. ACK might carry another PSTN signalling message, but it typically does not because PSTN signalling pro- tocols rarely use a three-way handshake. Once a session is established, the ingress gateway receives a PSTN sig- nalling message. It uses the INFO method to transmit such signalling to the egress gateway. The INFO method does not have to modify any para- meter of the SIP session to enable the exchange of PSTN messages between gateways. Ingress Egress Figure 7-15 MIME media types exchange gateway gateway exchange and INFO used for (1) PSTN signalling (2) INVITE SIP bridging. PSTN signalling message (3) PSTN signalling (5)200 OK (4) PSTN signalling (6) PSTN signiling PSTN signalling message (7) ACK (8) PSTN signalling (9) INFO PSTN signalling message (10) PSTN signalling (11) 200 OK Building Applications with the SIP Toolkit The PINT Service Protocol Another type of interworking with the PSTN is defined by PINT. The IETF PSTN and Internet Interworking (PINT) working group studies how Inter- net devices request PSTN telephony services. For instance, a user can click on a link in a Web page to request that someone from the support depart- ment call him or her on the PSTN phone. Or a user can request that a fax be sent to a certain machine on the PSTN providing the data, possibly though a pointer, to be faxed. The PINT working group developed the PINT protocol [RFC 2848] in order to provide this kind of service. The protocol consists of SIP and SDP with extensions. Among other extensions, PINT uses the SUBSCRIBE/ NOTIFY methods to receive status reports about the services invoked. PINT's goal is to use SIP in a limited way to establish only those sessions that do not fall into the category of Internet sessions. PINT defines the SDP format to describ