RFC 2824 (rfc2824) - Page 3 of 25

Call Processing Language Framework and Requirements

RFC 2824                         CPL-F                          May 2000


   A number of Internet applications have such customization
   environments -- the web has CGI [3], for instance, and e-mail has
   Sieve [4] or procmail. To create such an open customization
   environment for Internet telephony, we need a standardized, safe way
   for these new service creators to describe the desired behavior of
   network servers.

   This document describes an architecture in which network devices
   respond to call signalling events by triggering user-created programs
   written in a simple, static, non-expressively-complete language. We
   call this language a call processing language.

   The development of this document has been substantially informed by
   the development of a particular call processing language, as
   described in [5]. In general, when this document refers to "a call
   processing language," it is referring to a generic language that
   fills this role; "the call processing language" or "the CPL" refers
   to this particular language.

2 Terminology

   In this section we define some of the terminology used in this
   document.

   SIP [1] terminology used includes:

      invitation: The initial INVITE request of a SIP transaction, by
           which one party initiates a call with another.

      redirect server: A SIP device which responds to invitations and
           other requests by informing the request originator of an
           alternate address to which the request should be sent.

      proxy server: A SIP device which receives invitations and other
           requests, and forwards them to other SIP devices. It then
           receives the responses to the requests it forwarded, and
           forwards them back to the sender of the initial request.

      user agent: A SIP device which creates and receives requests, so
           as to set up or otherwise affect the state of a call. This
           may be, for example, a telephone or a voicemail system.

      user agent client: The portion of a user agent which initiates
           requests.

      user agent server: The portion of a user agent which responds to
           requests.




Lennox & Schulzrinne         Informational