e a fax session, for instance. Once this format is defined, SIP is used as usual to establish a session. The only difference is that the session in question happens to be a fax over the PSTN rather than Real- time Transport Protocol (RTP) packets over the Internet. Once again we can see the strength of separating session description from session establish- ment. The same protocol, SIP, can be used to establish any kind of session. The only requirement is that a session description format that can be car- ried in a SIP body must be present to describe it. Figure 7-16 shows how Bob uses SIP to fax Laura. Bob does not have a fax machine in his office, but the information she needs is on ftp://ftp.com- pany.com/Bob/document.txt. Using SIP, he sends an INVITE to a PINT gateway that specifies the information to be sent (through a Universal Resource Identifier (URI)) and bears Laura's fax machine number (1-212- 555-5555). The PINT gateway interfaces the Internet and the PSTN. It downloads the information from the URI provided and sends it to Laura's fax machine, while it sends notifications to Bob about the status of the fax. The SDP syntax to describe this service is pretty simple. A new address type (c line) has been defined as Telephone Network (TN). It contains a PSTN phone number. The m lines describe the media stream (voice, fax, or pager). C= TN RFC2543 +1-212-555-5555 m= text 1 fax plain a=fmtp:plain uri:ftp://ftp.company.com/~Bob/document.txt Chapter 7 Bob's Laura's Figure 7-16 laptop gateway fax machine Fax session over the PSTN established (1) INVITE c=TN RFC2543+1-212-555-5555 by SIP. m=text 1 fax plain a=fmtp:plain ri:ftp://ftp.company.com/~Bob/document.txt (2) 200 OK (3) ACK (4) SUBSCRIBE (5) 200 OK Faxing 1st page (6) NOTIFY Faxed 1 out of 2 pages (7) 200 OK (8) NOTIFY Faxing 2nd page Faxed 2 out of 2 pages (9) 200 OK SIP for Conferencing We know that many services that have been implemented using SIP consist of point-to-point sessions. When Bob establishes a session with Laura, they engage in a two-party