





Network Working Group                                        S. Petrack
Request for Comments: 2848                                      MetaTel
Category: Standards Track                                     L. Conroy
                                            Siemens Roke Manor Research
                                                              June 2000


                       The PINT Service Protocol:
   Extensions to SIP and SDP for IP Access to Telephone Call Services

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document contains the specification of the PINT Service Protocol
   1.0, which defines a protocol for invoking certain telephone services
   from an IP network. These services include placing basic calls,
   sending and receiving faxes, and receiving content over the
   telephone. The protocol is specified as a set of enhancements and
   additions to the SIP 2.0 and SDP protocols.

Table of Contents

   1. Introduction .................................................  4
   1.1 Glossary ....................................................  6
   2. PINT Milestone Services ......................................  6
   2.1 Request to Call .............................................  7
   2.2 Request to Fax Content ......................................  7
   2.3 Request to Speak/Send/Play Content ..........................  7
   2.4 Relation between PINT milestone services and traditional
       telephone services ..........................................  7
   3. PINT Functional and Protocol Architecture ....................  8
   3.1. PINT Functional Architecture ...............................  8
   3.2. PINT Protocol Architecture .................................  9
   3.2.1. SDP operation in PINT .................................... 10
   3.2.2. SIP Operation in PINT .................................... 11
   3.3. REQUIRED and OPTIONAL elements for PINT compliance ......... 11
   3.4. PINT Extensions to SDP 2.0 ................................. 12



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   3.4.1. Network Type "TN" and Address Type "RFC2543" ............. 12
   3.4.2. Support for Data Objects within PINT ..................... 13
   3.4.2.1. Use of fmtp attributes in PINT requests ................ 15
   3.4.2.2. Support for Remote Data Object References in PINT ...... 16
   3.4.2.3. Support for GSTN-based Data Objects in PINT ............ 17
   3.4.2.4. Session Description support for included Data Objects .. 18
   3.4.3. Attribute Tags to pass information into the Telephone
          Network .................................................. 19
   3.4.3.1. The phone-context attribute ............................ 20
   3.4.3.2. Presentation Restriction attribute ..................... 22
   3.4.3.3. ITU-T CalledPartyAddress attributes parameters ......... 23
   3.4.4. The "require" attribute .................................. 24
   3.5. PINT Extensions to SIP 2.0 ................................. 25
   3.5.1. Multi-part MIME (sending data along with SIP request) .... 25
   3.5.2. Warning header ........................................... 27
   3.5.3. Mechanism to register interest in the disposition of a PINT
          service, and to receive indications on that disposition .. 27
   3.5.3.1. Opening a monitoring session with a SUBSCRIBE request .. 28
   3.5.3.2. Sending Status Indications with a NOTIFY request ....... 30
   3.5.3.3. Closing a monitoring session with an UNSUBSCRIBE request 30
   3.5.3.4. Timing of SUBSCRIBE requests ........................... 31
   3.5.4. The "Require:" header for PINT ........................... 32
   3.5.5. PINT URLs within PINT requests ........................... 32
   3.5.5.1. PINT URLS within Request-URIs .......................... 33
   3.5.6. Telephony Network Parameters within PINT URLs ............ 33
   3.5.7. REGISTER requests within PINT ............................ 34
   3.5.8. BYE Requests in PINT ..................................... 35
   4. Examples of PINT Requests and Responses ...................... 37
   4.1. A request to a call center from an anonymous user to receive
        a phone call ............................................... 37
   4.2. A request from a non anonymous customer (John Jones) to
        receive a phone call from a particular sales agent
        (Mary James) ............................................... 37
   4.3. A request to get a fax back ................................ 38
   4.4. A request to have information read out over the phone ...... 39
   4.5. A request to send an included text page to a friend's pager. 39
   4.6. A request to send an image as a fax to phone number
        +972-9-956-1867 ............................................ 40
   4.7. A request to read out over the phone two pieces of content
        in sequence ................................................ 41
   4.8. Request for the prices for ISDN to be sent to my fax
        machine .................................................... 42
   4.9. Request for a callback ..................................... 42
   4.10.Sending a set of information in response to an enquiry ..... 43
   4.11.Sportsline "headlines" message sent to your phone/fax/pager  44
   4.12.Automatically giving someone a fax copy of your phone bill . 45
   5. Security Considerations ...................................... 46
   5.1.  Basic Principles for PINT Use ............................. 46



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   5.1.1.  Responsibility for service requests ..................... 46
   5.1.2.  Authority to make requests .............................. 47
   5.1.3.  Privacy ................................................. 47
   5.1.4.  Privacy Implications of SUBSCRIBE/NOTIFY ................ 48
   5.2.  Registration Procedures ................................... 49
   5.3.  Security mechanisms and implications on PINT service ...... 50
   5.4.  Summary of Security Implications .......................... 52
   6. Deployment considerations and the Relationship PINT to I.N.
      (Informative) ................................................ 54
   6.1. Web Front End to PINT Infrastructure ....................... 54
   6.2. Redirects to Multiple Gateways ............................. 54
   6.3. Competing PINT Gateways REGISTERing to offer the same
        service .................................................... 55
   6.4. Limitations on Available Information and Request Timing for
        SUBSCRIBE .................................................. 56
   6.5. Parameters needed for invoking traditional GSTN Services
        within PINT................................................. 58
   6.5.1. Service Identifier ....................................... 58
   6.5.2. A and B parties .......................................... 58
   6.5.3. Other Service Parameters ................................. 59
   6.5.4. Service Parameter Summary ................................ 59
   6.6. Parameter Mapping to PINT Extensions........................ 60
   7. References ................................................... 62
   8. Acknowledgements ............................................. 64
   Appendix A: Collected ABNF for PINT Extensions .................. 65
   Appendix B: IANA Considerations ................................. 69
   Authors' Addresses .............................................. 72
   Full Copyright Statement ........................................ 73























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1. Introduction

   The desire to invoke certain telephone call services from the
   Internet has been identified by many different groups (users, public
   and private network operators, call center service providers,
   equipment vendors, see [7]). The generic scenario is as follows (when
   the invocation is successful):

      1. an IP host sends a request to a server on an IP network;
      2. the server relays the request into a telephone network;
      3. the telephone network performs the requested call service.

   As examples, consider a user who wishes to have a callback placed to
   his/her telephone. It may be that a customer wants someone in the
   support department of some business to call them back. Similarly, a
   user may want to hear some announcement of a weather warning sent
   from a remote automatic weather service in the event of a storm.

   We use the term "PSTN/Internet Interworking (PINT) Service" to denote
   such a complete transaction, starting with the sending of a request
   from an IP client and including the telephone call itself. PINT
   services are distinguished by the fact that they always involve two
   separate networks:

      an IP network to request the placement of a call, and the Global
      Switched Telephone Network (GSTN) to execute the actual call. It
      is understood that Intelligent Network systems, private PBXs,
      cellular phone networks, and the ISDN can all be used to deliver
      PINT services.  Also, the request for service might come from
      within a private IP network that is disconnected from the whole
      Internet.

   The requirements for the PINT protocol were deliberately restricted
   to providing the ability to invoke a small number of fixed telephone
   call services. These "Milestone PINT services" are specified in
   section 2.  Great care has been taken, however, to develop a protocol
   that is aligned with other Internet protocols where possible, so that
   future extensions to PINT could develop along with Internet
   conferencing.

   Within the Internet conference architecture, establishing media calls
   is done via a combination of protocols. SIP [1] is used to establish
   the association between the participants within the call (this
   association between participants within the call is called a
   "session"), and SDP [2] is used to describe the media to be exchanged
   within the session. The PINT protocol uses these two protocols
   together, providing some extensions and enhancements to enable SIP
   clients and servers to become PINT clients and servers.



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   A PINT user who wishes to invoke a service within the telephone
   network uses SIP to invite a remote PINT server into a session. The
   invitation contains an SDP description of the media session that the
   user would like to take place. This might be a "sending a fax
   session" or a "telephone call session", for example. In a PINT
   service execution session the media is transported over the phone
   system, while in a SIP session the media is normally transported over
   an internet.

   When used to invoke a PINT service, SIP establishes an association
   between a requesting PINT client and the PINT server that is
   responsible for invoking the service within the telephone network.
   These two entities are not the same entities as the telephone network
   entities involved in the telephone network service. The SIP messages
   carry within their SDP payloads a description of the telephone
   network media session.

   Note that the fact that a PINT server accepts an invitation and a
   session is established is no guarantee that the media will be
   successfully transported. (This is analogous to the fact that if a
   SIP invitation is accepted successfully, this is no guarantee against
   a subsequent failure of audio hardware).

   The particular requirements of PINT users lead to some new messages.
   When a PINT server agrees to send a fax to telephone B, it may be
   that the fax transmission fails after part of the fax is sent.
   Therefore, the PINT client may wish to receive information about the
   status of the actual telephone call session that was invoked as a
   result of the established PINT session. Three new requests,
   SUBSCRIBE, UNSUBSCRIBE, and NOTIFY, are added here to vanilla SIP to
   allow this.

   The enhancements and additions specified here are not intended to
   alter the behaviour of baseline SIP or SDP in any way. The purpose of
   PINT extensions is to extend the usual SIP/SDP services to the
   telephone world. Apart from integrating well into existing protocols
   and architectures, and the advantages of reuse, this means that the
   protocol specified here can handle a rather wider class of call
   services than just the Milestone services.

   The rest of this document is organised as follows: Section 2
   describes the PINT Milestone services; section 3 specifies the PINT
   functional and protocol architecture; section 4 gives examples of the
   PINT 1.0 extensions of SIP and SDP; section 5 contains some security
   considerations for PINT. The final section contains descriptions of
   how the PINT protocol may be used to provide service over the GSTN.





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   For a summary of the extensions to SIP and SDP specified in this
   document, Section 3.2 gives an combined list, plus one each
   describing the extensions to SIP and SDP respectively.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119. In addition,
   the construct "MUST .... OR ...." implies that it is an absolute
   requirement of this specification to implement one of the two
   possibilities stated (represented by dots in the above phrase). An
   implementation MUST be able to interoperate with another
   implementation that chooses either of the two possibilities.

1.1 Glossary

   Requestor - An Internet host from which a request for service
   originates

   PINT Service - A service invoked within a phone system in response to
   a request received from an PINT client.

   PINT Client - An Internet host that sends requests for invocation of
   a PINT Service, in accordance with this document.

   PINT Gateway - An Internet host that accepts requests for PINT
   Service and dispatches them onwards towards a telephone network.

   Executive System - A system that interfaces to a PINT Server and to a
   telephone network that executes a PINT service. It need not be
   directly associated with the Internet, and is represented by the PINT
   Server in transactions with Internet entities.

   Requesting User - The initiator of a request for service. This role
   may be distinct from that of the "party" to any telephone network
   call that results from the request.

   (Service Call) Party - A person who is involved in a telephone
   network call that results from the execution of a PINT service
   request, or a telephone network-based resource that is involved (such
   as an automatic Fax Sender or a Text-to-Speech Unit).

2. PINT Milestone Services

   The original motivation for defining this protocol was the desire to
   invoke the following three telephone network services from within an
   IP network:





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2.1 Request to Call

   A request is sent from an IP host that causes a phone call to be
   made, connecting party A to some remote party B.

2.2 Request to Fax Content

   A request is sent from an IP host that causes a fax to be sent to fax
   machine B. The request MAY contain a pointer to the fax data (that
   could reside in the IP network or in the Telephone Network), OR the
   fax data itself. The content of the fax MAY be text OR some other
   more general image data. The details of the fax transmission are not
   accessible to the IP network, but remain entirely within the
   telephone network.

   Note that this service does not relate to "Fax over IP": the IP
   network is only used to send the request that a certain fax be sent.
   Of course, it is possible that the resulting telephone network fax
   call happens to use a real-time IP fax solution, but this is
   completely transparent to the PINT transaction.

2.3 Request to Speak/Send/Play Content

   A request is sent from an IP host that causes a phone call to be made
   to user A, and for some sort of content to be spoken out. The request
   MUST EITHER contain a URL pointing to the content, OR include the
   content itself. The content MAY be text OR some other more general
   application data. The details of the content transmission are not
   accessible to the IP network, but remain entirely within the
   telephone network. This service could equally be called "Request to
   Hear Content"; the user's goal is to hear the content spoken to them.
   The mechanism by which the request is formulated is outside the scope
   of this document; however, an example might be that a Web page has a
   button that when pressed causes a PINT request to be passed to the
   PSTN, resulting in the content of the page (or other details) being
   spoken to the person.

2.4 Relation between PINT milestone services and traditional telephone
    services

   There are many different versions and variations of each telephone
   call service invoked by a PINT request. Consider as an example what
   happens when a user requests to call 1-800-2255-287 via the PINT
   Request-to-Call service.

   There may be thousands of agents in the call center, and there may be
   any number of sophisticated algorithms and pieces of equipment that
   are used to decide exactly which agent will return the call. And once



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   this choice is made, there may be many different ways to set up the
   call: the agent's phone might ring first, and only then the original
   user will be called; or perhaps the user might be called first, and
   hear some horrible music or pre-recorded message while the agent is
   located.

   Similarly, when a PINT request causes a fax to be sent, there are
   hundreds of fax protocol details to be negotiated, as well as
   transmission details within the telephone networks used.

   PINT requests do not specify too precisely the exact telephone-side
   service. Operational details of individual events within the
   telephone network that executes the request are outside the scope of
   PINT. This does not preclude certain high-level details of the
   telephone network session from being expressed within a PINT request.
   For example, it is possible to use the SDP "lang" attribute to
   express a language preference for the Request-to-Hear-Content
   Service.  If a particular PINT system wishes to allow requests to
   contain details of the telephone-network-side service, it uses the
   SDP attribute mechanism (see section 3.4.2).

3. PINT Functional and Protocol Architecture

3.1. PINT Functional Architecture

   Familiarity is assumed with SIP 2.0 [1] and with SDP [2].

   PINT clients and servers are SIP clients and servers. SIP is used to
   carry the request over the IP network to the correct PINT server in a
   secure and reliable manner, and SDP is used to describe the telephone
   network session that is to be invoked or whose status is to be
   returned.

   A PINT system uses SIP proxy servers and redirect servers for their
   usual purpose, but at some point there must be a PINT server with the
   means to relay received requests into a telephone system and to
   receive acknowledgement of these relayed requests. A PINT server with
   this capability is called a "PINT gateway". A PINT gateway appears to
   a SIP system as a User Agent Server. Notice that a PINT gateway
   appears to the PINT infrastructure as if it represents a "user",
   while in fact it really represents an entire telephone network
   infrastructure that can provide a set of telephone network services.









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   So the PINT system might appear to an individual PINT client as
   follows:

                           /\/\/\/\/\/\/\            /\/\/\/\/\/\/\/\
___________                \          __/___      ___\_             \
|  PINT   |      PINT      \   PINT  | PINT |     |Exec| Telephone  /
| client  |<-------------->|  server |gatewy|=====|Syst| Network    \
|_________|    protocol    /  cloud  |______|     |____|  Cloud     /
                           \            \            /              \
                           /\/\/\/\/\/\/\            \/\/\/\/\/\/\/\/

                 Figure 1: PINT Functional Architecture

   The system of PINT servers is represented as a cloud to emphasise
   that a single PINT request might pass through a series of location
   servers, proxy servers, and redirect servers, before finally reaching
   the correct PINT gateway that can actually process the request by
   passing it to the Telephone Network Cloud.

   The PINT gateway might have a true telephone network interface, or it
   might be connected via some other protocol or API to an "Executive
   System" that is capable of invoking services within the telephone
   cloud.

   As an example, within an I.N. (Intelligent Network) system, the PINT
   gateway might appear to realise the Service Control Gateway Function.
   In an office environment, it might be a server adjunct to the office
   PBX, connected to both the office LAN and the office PBX.

   The Executive System that lies beyond the PINT gateway is outside the
   scope of PINT.

3.2. PINT Protocol Architecture

   This section explains how SIP and SDP work in combination to convey
   the information necessary to invoke telephone network sessions.

   The following list summarises the extension features used in PINT
   1.0.  Following on from this the features are considered separately
   for SDP and then for SIP:

   1)  Telephony URLs in SDP Contact Fields
   2)  Refinement of SIP/SDP Telephony URLs
       *   Inclusion of private dialling plans
   3)  Specification of Telephone Service Provider (TSP) and/or phone-
       context URL-parameters
   4)  Data Objects as session media




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   4a) Protocol Transport formats to indicate the treatment of the media
       within the GSTN
   5)  Implicit (Indirect) media streams and opaque arguments
   6)  In-line data objects using multipart/mime
   7)  Refinement/Clarification of Opaque arguments passed onwards to
       Executive Systems
       *   Framework for Presentation Restriction Indication
       *   Framework for Q.763 arguments
   8)  An extension mechanism for SDP to specify strictures and force
       failure when a recipient does NOT support the specified
       extensions, using "require" headers.
   9)  Mandatory support for "Warning" headers to give more detailed
       information on request disposition.
   10) Mechanism to register interest in the disposition of a requested
       service, and to receive indications on that disposition.

   Both PINT and SIP rely on features of MIME[4]. The use of SIP 2.0 is
   implied by PINT 1.0, and this also implies compliance with version
   1.0 of MIME.

3.2.1. SDP operation in PINT

   The SDP payload contains a description of the particular telephone
   network session that the requestor wishes to occur in the GSTN. This
   information includes such things as the telephone network address
   (i.e.  the "telephone number") of the terminal(s) involved in the
   call, an indication of the media type to be transported (e.g. audio,
   text, image or application data), and an indication if the
   information is to be transported over the telephone network via
   voice, fax, or pager transport. An indication of the content to be
   sent to the remote telephone terminal (if there is any) is also
   included.

   SDP is flexible enough to convey these parameters independently. For
   example, a request to send some text via voice transport will be
   fulfilled by invoking some text-to-speech-over-the-phone service, and
   a request to send text via fax will be fulfilled by invoking some
   text-to-fax service.

   The following is a list of PINT 1.0 enhancements and additions to
   SDP.

      a. A new network type "TN" and address types "RFC2543" and "X-..."
         (section 3.4.1)
      b. New media types "text", "image", and "application", new
         protocol transport keywords "voice", "fax" and "pager" and the
         associated format types and attribute tags (section 3.4.2)




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      c. New format specific attributes for included content data
         (section 3.4.2.4)
      d. New attribute tags, used to pass information to the telephone
         network (section 3.4.3)
      e. A new attribute tag "require", used by a client to indicate
         that some attribute is required to be supported in the server
         (section 3.4.4)

3.2.2. SIP Operation in PINT

   SIP is used to carry the request for telephone service from the PINT
   client to the PINT gateway, and may include a telephone number if
   needed for the particular service. The following is a complete list
   of PINT enhancements and additions to SIP:

      f. The multipart MIME payloads (section 3.5.1)
      g. Mandatory support for "Warning:" headers (section 3.5.2)
      h. The SUBSCRIBE and NOTIFY, and UNSUBSCRIBE requests (section
         3.5.3)
      i. Require: headers (section 3.5.4)
      j. A format for PINT URLS within a PINT request (section 3.5.5)
      k. Telephone Network Parameters within PINT URLs (section 3.5.6)

   Section 3.5.8 contains remarks about how BYE requests are used within
   PINT. This is not an extension to baseline SIP; it is included here
   only for clarification of the semantics when used with telephone
   network sessions.

3.3. REQUIRED and OPTIONAL elements for PINT compliance

   Of these, only the TN network type (with its associated RFC2543
   address type) and the "require" attribute MUST be supported by PINT
   1.0 clients and servers. In practice, most PINT service requests will
   use other changes, of which references to Data Objects in requests
   are most likely to appear in PINT requests.

   Each of the other new PINT constructs enables a different function,
   and a client or server that wishes to enable that particular function
   MUST do so by the construct specified in this document. For example,
   building a PINT client and server that provide only the Request-to-
   Call telephone call service, without support for the other Milestone
   services, is allowed.

   The "Require:" SIP header and the "require" attribute provide a
   mechanism that can be used by clients and servers to signal their
   need and/or ability to support specific "new" PINT protocol elements.





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   It should be noted that many optional features of SIP and SDP make
   sense as specified in the PINT context. One example is the SDP
   a=lang:  attribute, which can be used to describe the preferred
   language of the callee. Another example is the use of the "t="
   parameter to indicate that the time at which the PINT service is to
   be invoked. This is the normal use of the "t=" field. A third example
   is the quality attributes.  Any SIP or SDP option or facility is
   available to PINT clients and servers without change.

   Conversely, support for Data Objects within Internet Conference
   sessions may be useful, even if the aim is not to provide a GSTN
   service request.  In this case, the extensions covering these items
   may be incorporated into an otherwise "plain" SIP/SDP invitation.
   Likewise, support for SDP "require" may be useful, as a framework for
   addition of features to a "traditional" SIP/SDP infrastructure.
   Again, these may be convenient to incorporate into SIP/SDP
   implementations that would not be used for PINT service requests.
   Such additions are beyond the scope of this document, however.

3.4. PINT Extensions to SDP

   PINT 1.0 adds to SDP the possibility to describe audio, fax, and
   pager telephone sessions. It is deliberately designed to hide the
   underlying technical details and complexity of the telephone network.
   The only network type defined for PINT is the generic "TN" (Telephone
   Network).  More precise tags such as "ISDN", "GSM", are not defined.
   Similarly, the transport protocols are designated simply as "fax",
   "voice", and "pager"; there are no more specific identifiers for the
   various telephone network voice, fax, or pager protocols. Similarly,
   the data to be transported are identified only by a MIME content
   type, such as "text" data, "image" data, or some more general
   "application" data. An important example of transporting
   "application" data is the milestone service "Voice Access to Web
   Content". In this case the data to be transported are pointed to by a
   URI, the data content type is application/URI, and the transport
   protocol would be "voice". Some sort of speech-synthesis facility,
   speaking out to a Phone, will have to be invoked to perform this
   service.

   This section gives details of the new SDP keywords.

3.4.1. Network Type "TN" and Address Type "RFC2543"

   The TN ("Telephone Network") network type is used to indicate that
   the terminal is connected to a telephone network.

   The address types allowed for network type TN are "RFC2543" and
   private address types, which MUST begin with an "X-".



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   Address type RFC2543 is followed by a string conforming to a subset
   of the "telephone-subscriber" BNF specified in figure 4  of SIP [1]).
   Note that this BNF is NOT identical to the BNF that defines the
   "phone-number" within the "p=" field of SDP.

   Examples:

       c= TN  RFC2543  +1-201-406-4090

       c= TN  RFC2543  12014064090

   A telephone-subscriber string is of one of two types:  global-phone-
   number or local-phone-number.  These are distinguished by preceeding
   a global-phone-number with a "plus" sign ("+"). A global-phone-number
   is by default to be interpreted as an internationally significant
   E.164 Number Plan Address, as defined by [6], whilst a local-phone-
   number is a number specified in the default dialling plan within the
   context of the recipient PINT Gateway.

   An implementation MAY use private addressing types, which can be
   useful within a local domain. These address types MUST begin with an
   "X-", and SHOULD contain a domain name after the X-, e.g. "X-
   mytype.mydomain.com".  An example of such a connection line is as
   follows:

         c= TN X-mytype.mydomain.com  A*8-HELEN

   where "X-mytype.mydomain.com" identifies this private address type,
   and "A*8-HELEN" is the number in this format. Such a format is
   defined as an "OtherAddr" in the ABNF of Appendix A. Note that most
   dialable telephone numbers are expressable as local-phone-numbers
   within address RFC2543; new address types SHOULD only be used for
   formats which cannot be so written.

3.4.2. Support for Data Objects within PINT

   One significant change over traditional SIP/SDP Internet Conference
   sessions with PINT is that a PINT service request may refer to a Data
   Object to be used as source information in that request. For example,
   a PINT service request may specify a document to be processed as part
   of a GSTN service by which a Fax is sent. Similarly, a GSTN service
   may be take a Web page and result in a vocoder processing that page
   and speaking the contents over a telephone.

   The SDP specification does not have explicit support for reference to
   or carriage of Data Objects within requests. In order to use SDP for
   PINT, there is a need to describe such media sessions as "a telephone




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   call to a certain number during which such-and-such an image is sent
   as a fax".

   To support this, two extensions to the session description format are
   specified. These are some new allowed values for the Media Field, and
   a description of the "fmtp" parameter when used with the Media Field
   values (within the context of the Contact Field Network type "TN").

   An addition is also made to the SIP message format to allow the
   inclusion of data objects as sub-parts within the request message
   itself. The original SDP syntax (from [2]) for media-field is given
   as:

      media-field =         "m=" media space port ["/" integer]
                            space proto 1*(space fmt) CRLF

   When used within PINT requests, the definition of the sub-fields is
   expanded slightly. The Media sub-field definition is relaxed to
   accept all of the discrete "top-level" media types defined in [4]. In
   the milestone services the discrete type "video" is not used, and the
   extra types "data" and "control" are likewise not needed. The use of
   these types is not precluded, but the behaviour expected of a PINT
   Gateway receiving a request including such a type is not defined
   here.

   The Port sub-field has no meaning in PINT requests as the destination
   terminals are specified using "TN" addressing, so the value of the
   port sub-field in PINT requests is normally set to "1". A value of
   "0" may be used as in SDP to indicate that the terminal is not
   receiving media.  This is useful to indicate that a telephone
   terminal has gone "on hold" temporarily.  Likewise, the optional
   integer sub-field is not used in PINT.

   As mentioned in [2], the Transport Protocol sub-field is specific to
   the associated Address Type. In the case that the Address Type in the
   preceeding Contact field is one of those defined for use with the
   Network Type "TN", the following values are defined for the Transport
   Protocol sub-field:

   "voice", "fax", and "pager".

   The interpretation of this sub-field within PINT requests is the
   treatment or disposition of the resulting GSTN service. Thus, for
   transport protocol "voice", the intent is that the service will
   result in a GSTN voice call, whilst for protocol "fax" the result
   will be a GSTN fax transmission, and protocol "pager" will result in
   a pager message being sent.




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   Note that this sub-field does not necessarily dictate the media type
   and subtype of any source data; for example, one of the milestone
   services calls for a textual source to be vocoded and spoken in a
   resulting telephone service call. The transport protocol value in
   this case would be "voice", whilst the media type would be "text".

   The Fmt sub-field is described in [2] as being transport protocol-
   specific. When used within PINT requests having one of the above
   protocol values, this sub-field consists of a list of one or more
   values, each of which is a defined MIME sub-type of the associated
   Media sub-field value. The special value "-" is allowed, meaning that
   there is no MIME sub-type. This sub-field retains (from [2]) its
   meaning that the list will contain a set of alternative sub-types,
   with the first being the preferred value.

   For experimental purposes and by mutual consent of the sender and
   recipient, a sub-type value may be specified as an <X-token>, i.e. a
   character string starting with "X-". The use of such values is
   discouraged, and if such a value is expected to find common use then
   it SHOULD be registered with IANA using the standard content type
   registration process (see Appendix C).

   When the Fmt parameter is the single character "-" ( a dash ), this
   is interpreted as meaning that a unspecified or default sub-type can
   be used for this service. Thus, the media field value "m=audio 1
   voice -<CRLF>" is taken to mean that a voice call is requested, using
   whatever audio sub type is deemed appropriate by the Executive
   System. PINT service is a special case, in that the request comes
   from the IP network but the service call is provided within the GSTN.
   Thus the service request will not normally be able to define the
   particular codec used for the resulting GSTN service call. If such an
   intent IS required, then the quality attribute may be used (see
   "Suggested Attributes" section of [2]).

3.4.2.1. Use of fmtp attributes in PINT requests

   For each element of the Fmt sub-field, there MUST be a following fmtp
   attribute. When used within PINT requests, the fmtp attribute has a
   general structure as defined here:

       "a=fmtp:" <subtype> <space> resolution
                          *(<space> resolution)
                          (<space> ";" 1(<attribute>)
                                       *(<space> <attribute>))
   where:
       <resolution> := (<uri-ref> | <opaque-ref> | <sub-part-ref>)





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   A fmtp attribute describes the sources used with a given Fmt entry in
   the Media field. The entries in a Fmt sub-field are alternatives
   (with the preferred one first in the list). Each entry will have a
   matching fmtp attribute. The list of resolutions in a fmtp attribute
   describes the set of sources that resolve the matching Fmt choice;
   all elements of this set will be used.

   It should be noted that, for use in PINT services, the elements in
   such a set will be sent as a sequence; it is unlikely that trying to
   send them in parallel would be successful.

   A fmtp attribute can contain a mixture of different kinds of element.
   Thus an attribute might contain a sub-part-ref indicating included
   data held in a sub-part of the current message, followed by an
   opaque-ref referring to some content on the GSTN, followed by a uri-
   ref pointing to some data held externally on the IP network.

   To indicate which form each resolution element takes, each of them
   starts with its own literal tag. The detailed syntax of each form is
   described in the following sub-sections.

3.4.2.2. Support for Remote Data Object References in PINT

   Where data objects stored elsewhere on the IP Network are to be used
   as sources for processing within a PINT service, they may be referred
   to using the uri-ref form. This is simply a Uniform Resource
   Identifier (URI), as described in [9].

   Note that the reference SHOULD be an absolute URI, as there may not
   be enough contextual information for the recipient server to resolve
   a relative reference; any use of relative references requires some
   private agreement between the sender and recipient of the message,
   and SHOULD be avoided unless the sender can be sure that the
   recipient is the one intended and the reference is unambiguous in
   context.

   This also holds for partial URIs (such
   as"uri:http://aNode/index.htm") as these will need to be resolved in
   the context of the eventual recipient of the message.

   The general syntax of a reference to an Internet-based external data
   object in a fmtp line within a PINT session description is:

       <uri-ref> := ("uri:" URI-reference)

   where URI-reference is as defined in Appendix A of [9]





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   For example:

         c= TN RFC2543 +1-201-406-4090
         m= text 1  fax plain
         a=fmtp:plain  uri:ftp://ftp.isi.edu/in-notes/rfc2468.txt
   or:
         c= TN RFC2543 +1-201-406-4090
         m= text 1  fax plain
         a=fmtp:plain
   uri:http://www.ietf.org/meetings/glance_minneapolis.txt

   means get this data object from the Internet and use it as a source
   for the requested GSTN Fax service.

3.4.2.3. Support for GSTN-based Data Objects in PINT

   PINT services may refer to data that are held not on the IP Network
   but instead within the GSTN. The way in which these items are
   indicated need have no meaning within the context of the Requestor or
   the PINT Gateway; the reference is merely some data that may be used
   by the Executive System to indicate the content intended as part of
   the request. These data form an opaque reference, in that they are
   sent "untouched" through the PINT infrastructure.

   A reference to some data object held on the GSTN has the general
   definition:

       <opaque-ref> := ("opr:" *uric)

   where uric is as defined in Appendix A of [9].

   For example:

         c= TN RFC2543 +1-201-406-4090
         m= text 1  fax plain
         a=fmtp:plain  opr:APPL.123.456

   means send the data that is indexed ON THE GSTN by the reference
   value "APPL.123.456" to the fax machine on +1-201-406-4090. The
   Executive System may also take the Telephone URL held in the To:
   field of the enclosing SIP message into account when deciding the
   context to be used for the data object dereference.

   Of course, an opaque reference may also be used for other purposes;
   it could, for example, be needed to authorise access to a document
   held on the GSTN rather than being required merely to disambiguate





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   the data object. The purpose to which an opaque reference is put,
   however, is out of scope for this document. It is merely an indicator
   carried within a PINT Request.

   An opaque reference may have no value in the case where the value to
   be used is implicit in the rest of the request. For example, suppose
   some company wishes to use PINT to implement a "fax-back service". In
   their current implementation, the image(s) to be faxed are entirely
   defined by the telephone number dialled. Within the PINT request,
   this telephone number would appear within the "To:" field of the PINT
   request, and so there is no need for an opaque reference value.

   If there are several resolutions for a PINT Service Request, and one
   of these is an opaque reference with no value, then that opaque
   reference MUST be included in the attribute line, but with an empty
   value field.

   For example:

         c= TN RFC2543 +1-201-406-4090
         m= text 1  fax plain
         a=fmtp:plain  uri:http://www.sun.com/index.html opr:

   might be used to precede some data to be faxed with a covering note.

   In the special case where an opaque reference is the sole resolution
   of a PINT Service Request, AND that reference needs no value, there
   is no need for a Fmt list at all; the intent of the service is
   unambiguous without any further resolution.

   For example:

         c= TN RFC2543 +1-201-406-4090
         m= text 1  fax -

   means that there is an implied content stored on the GSTN, and that
   this is uniquely identified by the combination of SIP To-URI and the
   Contact field of the session description.

3.4.2.4. Session Description support for included Data Objects

   As an alternative to pointing to the data via a URI or an opaque
   reference to a data item held on the GSTN, it is possible to include
   the content data within the SIP request itself. This is done by using
   multipart MIME for the SIP payload. The first MIME part contains the
   SDP description of the telephone network session to be executed. The
   other MIME parts contain the content data to be transported.




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   Format specific attribute lines within the session description are
   used to indicate which other MIME part within the request contains
   the content data. Instead of a URI or opaque reference, the format-
   specific attribute indicates the Content-ID of the MIME part of the
   request that contains the actual data, and is defined as:

       <sub-part-ref> := ("spr:" Content-ID)

   where Content-ID is as defined in Appendix A of [3] and in [10]).

   For example:

         c= TN RFC2543 +1-201-406-4090
         m= text 1  fax plain
         a=fmtp:plain  spr:<Content-ID>

   The <Content-ID> parameter is the Content-ID of one of the MIME parts
   inside the message, and this fragment means that the requesting user
   would like the data object held in the sub-part of this message
   labelled <Content-ID> to be faxed to the machine at phone number +1-
   201-406-4090.

   See also section 3.5.1 for a discussion on the support needed in the
   enclosing SIP request for included data objects.

3.4.3. Attribute Tags to pass information into the Telephone Network

   It may be desired to include within the PINT request service
   parameters that can be understood only by some entity in the
   "Telephone Network Cloud". SDP attribute parameters are used for this
   purpose. They MAY appear within a particular media description or
   outside of a media description.

   These attributes may also appear as parameters within PINT URLS (see
   section 3.5.6) as part of a SIP request.

   This is necessary so that telephone terminals that require the
   attributes to be defined can appear within the To: line of a PINT
   request as well as within PINT session descriptions.

   The purpose of these attributes is to allow the client to specify
   extra context within which a particular telephone number is to be
   interpreted.  There are many reasons why extra context might be
   necessary to interpret a given telephone number:







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      a. The telephone number might be reachable in many different ways
         (such as via competing telephone service providers), and the
         PINT client wishes to indicate its selection of service
         provider.
      b. The telephone number might be reachable only from a limited
         number of networks (such as an '800' freephone number).
      c. The telephone number might be reachable only within a single
         telephone network (such as the '152' customer service number of
         BT). Similarly, the number might be an internal corporate
         extension reachable only within the PBX.

   However, as noted above, it is not usually necessary to use SDP
   attributes to specify the phone context. URLs such as
   152@pint.bt.co.il within the To: and From: headers and/or Request-
   URI, normally offer sufficient context to resolve telephone numbers.

   If the client wishes the request to fail if the attributes are not
   supported, these attributes SHOULD be used in conjunction with the
   "require" attribute (section 3.4.4) and the
   "Require:org.ietf.sdp.require" header (section 3.5.4).

   It is not possible to standardise every possible internal telephone
   network parameter. PINT 1.0 attributes have been chosen for
   specification because they are common enough that many different PINT
   systems will want to use them, and therefore interoperability will be
   increased by having a single specification.

   Proprietary attribute "a=" lines, that by definition are not
   interoperable, may be nonetheless useful when it is necessary to
   transport some proprietary internal telephone network variables over
   the IP network, for example to identify the order in which service
   call legs are to be be made. These private attributes SHOULD BE,
   however, subject to the same IANA registration procedures mentioned
   in the SDP specification[2] (see also this Appendix C).

3.4.3.1. The phone-context attribute

   An attribute is specified to enable "remote local dialling". This is
   the service that allows a PINT client to reach a number from far
   outside the area or network that can usually reach the number. It is
   useful when the sending or receiving address is only dialable within
   some local context, which may be remote to the origin of the PINT
   client.

   For example, if Alice wanted to report a problem with her telephone,
   she might then dial a "network wide" customer care number; within the
   British Telecom network in the U.K., this is "152". Note that in this
   case she doesn't dial any trunk prefix - this is the whole dialable



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   number. If dialled from another operator's network, it will not
   connect to British Telecom's Engineering Enquiries service; and
   dialling "+44 152" will not normally succeed. Such numbers are called
   Network-Specific Service Numbers.

   Within the telephone network, the "local context" is provided by the
   physical connection between the subscriber's terminal and the central
   office. An analogous association between the PINT client and the PINT
   server that first receives the request may not exist, which is why it
   may be necessary to supply this missing "telephone network context".
   This attribute is defined as follows:

   a=phone-context: <phone-context-ident>
   phone-context-ident     =  network-prefix / private-prefix
   network-prefix          =  intl-network-prefix / local-network-prefix
   intl-network-prefix     =  "+" 1*DIGIT
   local-network-prefix    =  1*DIGIT
   excldigandplus          =  (0x21-0x2d,0x2f,0x40-0x7d))
   private-prefix          =  1*excldigandplus 0*uric

   An intl-network-prefix and local-network-prefix MUST be a bona fide
   network prefix, and a network-prefix that is an intl-network-prefix
   MUST begin with an E.164 service code ("country code").

   It is possible to register new private-prefixes with IANA so as to
   avoid collisions. Prefixes that are not so registered MUST begin with
   an "X-" to indicate their private, non-standard nature (see Appendix
   C).

   Example 1:

         c= TN   RFC2543  1-800-765-4321
         a=phone-context:+972

   This describes an terminal whose address in Israel (E.164 country
   code 972) is 1-800-765-4321.

   Example 2:

         c= TN   RFC2543  1-800-765-4321
         a=phone-context:+1

   This describes an terminal whose address in North America (E.164
   country code 1) is 1-800-765-4321.

   The two telephone terminals described by examples 1 and 2 are
   different; in fact they are located in different countries.




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   Example 3:

         c=TN RFC2543  123
         a=phone-context:+97252

   This describes a terminal whose address when dialled from within the
   network identified by +97252 is the string "123". It so happens that
   +97252 defines one of the Israeli cell phone providers, and 123
   reaches customer service when dialled within that network.

   It may well be useful or necessary to use the SDP "require" parameter
   in conjunction with the phone-context attribute.

   Example 4:

         c= TN  RFC2543  321
         a=phone-context:X-acme.com-23

   This might describe the telephone terminal that is at extension 321
   of PBX number 23 within the acme.com private PBX network. It is
   expected that such a description would be understandable by the
   acme.com PINT server that receives the request.

   Note that if the PINT server receiving the request is inside the
   acme.com network, the same terminal might be addressable as follows:

         c= TN  RFC2543 7-23-321

   (assuming that "7" is dialled in order to reach the private PBX
   network from within acme.com)

3.4.3.2. Presentation Restriction attribute

   Although it has no affect on the transport of the service request
   through the IP Network, there may be a requirement to allow
   originators of a PINT service request to indicate whether or not they
   wish the "B party" in the resulting service call to be presented with
   the "A party's" calling telephone number. It is a legal requirement
   in some jurisdictions that a caller be able to select whether or not
   their correspondent can find out the calling telephone number (using
   Automatic Number Indication or Caller Display or Calling Line
   Identity Presentation equipment). Thus an attribute may be needed to
   indicate the originator's preference.

   Whether or not the default behaviour of the Executive System is to
   present or not present a party's telephone number to the
   correspondent GSTN terminal is not specified, and it is not mandatory
   in all territories for a PINT Gateway or Executive System to act on



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   this attribute. It is, however, defined here for use where there are
   regulatory restrictions on GSTN operation, and in that case the
   Executive System can use it to honour the originator's request.

   The attribute is specified as follows:
       a=clir:<"true" | "false">

   This boolean value is needed within the attribute as it may be that
   the GSTN address is, by default, set to NOT present its identity to
   correspondents, and the originator wants to do so for this particular
   call. It is in keeping with the aim of this attribute to allow the
   originator to specify what treatment they want for the requested
   service call.

   The expected interpretation of this attribute is that, if it is
   present and the value is "false" then the Calling Line Identity CAN
   be presented to the correspondent terminal, whilst if it is "true"
   then if possible the Executive System is requested to NOT present the
   Calling Line Identity.

3.4.3.3. ITU-T CalledPartyAddress attributes parameters

   These attributes correspond to fields that appear within the ITU-T
   Q.763 "CalledPartyAddress" field (see [8] ,section 3.9). PINT clients
   use these attributes in order to specify further parameters relating
   to Terminal Addresses, in the case when the address indicates a
   "local-phone-number". In the case that the PINT request contains a
   reference to a GSTN terminal, the parameters may be required to
   correctly identify that remote terminal.

   The general form of this attribute is:  "a=Q763-<token>((":" <value>)
   |"")".  Three of the possible elements and their use in SDP
   attributes are described here. Where other Q763 elements are to be
   used, then these should be the subject of further specification to
   define the syntax of the attribute mapping. It is recommended that
   any such specification maintains the value sets shown in Q.763.

   The defined attributes are:

   a=Q763-nature:  - indicates the "nature of address indicator".
                       The value MAY be any number between 0 and 127.
                       The following values are specified:

                   "1" a subscriber number
                   "2" unknown
                   "3" a nationally significant number
                   "4" an internationally significant number




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   The values have been chosen to coincide with the values in Q.763.
   Note that other values are possible, according to national rules or
   future expansion of Q.763.

   a=Q763-plan:    - indicates the numbering plan to which the address
                       belongs. The value MAY be any number between 0
                       and 7. The following values are specified:

                   "1" Telephone numbering plan (ITU-T E.164)
                   "3" Data numbering plan (ITU-T X.121)
                   "4" Telex numbering plan (ITU-T F.69)

   The values have been chosen to coincide with the values in Q.763.
   Other values are allowed, according to national rules or future
   expansion of Q.763.

   a=Q763-INN      - indicates if routing to the Internal Network Number
                       is allowed. The value MUST be ONE of:

                   "0" routing to internal network number allowed
                   "1" routing to internal network number not
                                 allowed

   The values have been chosen to coincide with the values in Q.763.
   Note that it is possible to use a local-phone-number and indicate via
   attributes that the number is in fact an internationally significant
   E.164 number. Normally this SHOULD NOT be done; an internationally
   significant E.164 number is indicated by using a "global-phone-
   number" for the address string.

3.4.4. The "require" attribute

   According to the SDP specification, a PINT server is allowed simply
   to ignore attribute parameters that it does not understand. In order
   to force a server to decline a request if it does not understand one
   of the PINT attributes, a client SHOULD use the "require" attribute,
   specified as follows:

         a=require:<attribute-list>

   where the attribute-list is a comma-separated list of attributes that
   appear elsewhere in the session description.

   In order to process the request successfully the PINT server must
   BOTH understand the attribute AND ALSO fulfill the request implied by
   the presence of the attribute, for each attribute appearing within
   the attribute-list of the require attribute.




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   If the server does not recognise the attribute listed, the PINT
   server MUST return an error status code (such as 420 (Bad Extension)
   or 400 (Bad Request)), and SHOULD return suitable Warning: lines
   explaining the problem or an Unsupported: header containing the
   attribute it does not understand. If the server recognizes the
   attribute listed, but cannot fulfill the request implied by the
   presence of the attribute, the request MUST be rejected with a status
   code of (606 Not Acceptable), along with a suitable Unsupported:
   header or Warning: line.

   The "require" attribute may appear anywhere in the session
   description, and any number of times, but it MUST appear before the
   use of the attribute marked as required.

   Since the "require" attribute is itself an attribute, the SIP
   specification allows a server that does not understand the require
   attribute to ignore it. In order to ensure that the PINT server will
   comply with the "require" attribute, a PINT client SHOULD include a
   Require: header with the tag "org.ietf.sdp.require" (section 3.5.4)

   Note that the majority of the PINT extensions are "tagged" and these
   tags can be included in Require strictures. The exception is the use
   of phone numbers in SDP parts. However, these are defined as a new
   network and address type, so that a receiving SIP/SDP server should
   be able to detect whether or not it supports these forms. The default
   behaviour for any SDP recipient is that it will fail a PINT request
   if it does not recognise or support the TN and RFC2543 or X-token
   network and address types, as without the contents being recognised
   no media session could be created. Thus a separate stricture is not
   required in this case.

3.5. PINT Extensions to SIP 2.0

   PINT requests are SIP requests; Many of the specifications within
   this document merely explain how to use existing SIP facilities for
   the purposes of PINT.

3.5.1. Multi-part MIME (sending data along with SIP request)

   A PINT request can contain a payload which is multipart MIME. In this
   case the first part MUST contain an SDP session description that
   includes at least one of the format specific attribute tags for
   "included content data" specified above in section 3.4.3. Subsequent
   parts contain content data that may be transferred to the requested
   Telephone Call Service. As discussed earlier, within a single PINT
   request, some of the data MAY be pointed to by a URI within the
   request, and some of the data MAY be included within the request.




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   Where included data is carried within a PINT service request, the
   Content Type entity header of the enclosing SIP message MUST indicate
   this. To do so, the media type value within this entity header MUST
   be set to a value of "multipart". There is a content sub-type that is
   intended for situations like this in which sub-parts are to be
   handled together. This is the multipart/related type (defined in
   [19]), and it's use is recommended.

   The enclosed body parts SHOULD include the part-specific Content Type
   headers as appropriate ("application/sdp" for the first body part
   holding the session description, with an appropriate content type for
   each of the subsequent, "included data object" parts). This matches
   the standard syntax of MIME multipart messages as defined in [4].

   For example, in a multipart message where the string

   "------next-------" is the boundary, the first two parts might be as
   follows:

         ------next-------
         Content-Type: application/sdp
         ....
         c= TN RFC2543 +1-201-406-4090
         m= text 1 pager plain
         a=fmtp:plain spr:17@mymessage.acme.com

         ----------next-------
         Content-Type: text/plain
         Content-ID:  17@mymessage.acme.com

         This is the text that is to be paged to +1-201-406-4090

         ----------next-----------

   The ability to indicate different alternatives for the content to be
   transported is useful, even when the alternatives are included within
   the request. For example, a request to send a short message to a
   pager might include the message in Unicode [5] and an alternative
   version of the same content in text/plain, should the PINT server or
   telephone network not be able to process the unicode.

   PINT clients should be extremely careful when sending included data
   within a PINT request. Such requests SHOULD be sent via TCP, to avoid
   fragmentation and to transmit the data reliably. It is possible that
   the PINT server is a proxy server that will replicate and fork the
   request, which could be disastrous if the request contains a large
   amount of application data. PINT proxy servers should be careful not
   to create many copies of a request with large amounts of data in it.



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   If the client does not know the actual location of the PINT gateway,
   and is using the SIP location services to find it, and the included
   data makes the PINT request likely to be transported in several IP
   datagrams, it is RECOMMENDED that the initial PINT request not
   include the data object but instead hold a reference to it.

3.5.2. Warning header

   A PINT server MUST support the SIP "Warning:" header so that it can
   signal lack of support for individual PINT features. As an example,
   suppose the PINT request is to send a jpeg picture to a fax machine,
   but the server cannot retrieve and/or translate jpeg pictures from
   the Internet into fax transmissions.

   In such a case the server fails the request and includes a Warning
   such as the following:

         Warning:  305  pint.acme.com  Incompatible media format:  jpeg

   SIP servers that do not understand the PINT extensions at all are
   strongly encouraged to implement Warning: headers to indicate that
   PINT extensions are not understood.

   Also, Warning: headers may be included within NOTIFY requests if it
   is necessary to notify the client about some condition concerning the
   invocation of the PINT service (see next).

3.5.3. Mechanism to register interest in the disposition of a PINT
       service, and to receive indications on that disposition

   It can be very useful to find out whether or not a requested service
   has completed, and if so whether or not it was successful. This is
   especially true for PINT service, where the person requesting the
   service is not (necessarily) a party to it, and so may not have an
   easy way of finding out the disposition of that service. Equally, it
   may be useful to indicate when the service has changed state, for
   example when the service call has started.

   Arranging a flexible system to provide extensive monitoring and
   control during a service is non-trivial (see section 6.4 for some
   issues); PINT 1.0 uses a simple scheme that should nevertheless
   provide useful information. It is possible to expand the scheme in a
   "backwards compatible" manner, so if required it can be enhanced at a
   later date.

   The PINT 1.0 status registration and indication scheme uses three new
   methods; SUBSCRIBE, UNSUBSCRIBE, and NOTIFY. These are used to allow
   a PINT client to register an interest in (or "subscribe" to) the



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   status of a service request, to indicate that a prior interest has
   lapsed (i.e "unsubscribe" from the status), and for the server to
   return service indications. The state machine of
   SUBSCRIBE/UNSUBSCRIBE is identical to that of INVITE/BYE; just as
   INVITE signals the beginning and BYE signals the end of participation
   in a media session, SUBSCRIBE signals the beginning and UNSUBSCRIBE
   signals the end of participation in a monitoring session. During the
   monitoring session, NOTIFY messages are sent to inform the subscriber
   of a change in session state or disposition.

3.5.3.1. Opening a monitoring session with a SUBSCRIBE request

   When a SUBSCRIBE request is sent to a PINT Server, it indicates that
   a user wishes to receive information about the status of a service
   session. The request identifies the session of interest by including
   the original session description along with the request, using the
   SDP global-session-id that forms part of the origin-field to identify
   the service session uniquely.

   The SUBSCRIBE request (like any other SIP request about an ongoing
   session) is sent to the same server as was sent the original INVITE,
   or to a server which was specified in the Contact: field within a
   subsequent response (this might well be the PINT gateway for the
   session).

   Whilst there are situations in which re-use of the Call-ID used in
   the original INVITE that initiated the session of interest is
   possible, there are other situations in which it is not. In detail,
   where the subscription is being made by the user who initiated the
   original service request, the Call-ID may be used as it will be known
   to the receiver to refer to a previously established session.
   However, when the request comes from a user other than the original
   requesting user, the SUBSCRIBE request constitutes a new SIP call
   leg, so the Call-ID SHOULD NOT be used; the only common identifier is
   the origin-field of the session description enclosed within the
   original service request, and so this MUST be used.

   Rather than have two different methods of identifying the "session of
   interest" the choice is to use the origin-field of the SDP sub-part
   included both in the original INVITE and in this SUBSCRIBE request.

   Note that the request MUST NOT include any sub-parts other than the
   session description, even if these others were present in the
   original INVITE request. A server MUST ignore whatever sub-parts are
   included within a SUBSCRIBE request with the sole exception of the
   enclosed session description.





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   The request MAY contain a "Contact:" header, specifying the PINT User
   Agent Server to which such information should be sent.

   In addition, it SHOULD contain an Expires: header, which indicates
   for how long the PINT Requestor wishes to receive notification of the
   session status. We refer to the period of time before the expiration
   of the SUBSCRIBE request as the "subscription period". See section
   5.1.4.  for security considerations, particularly privacy
   implications.

   A value of 0 within the Expires: header indicates a desire to receive
   one single immediate response (i.e. the request expires immediately).
   It is possible for a sequence of monitoring sessions to be opened,
   exist, and complete, all relating to the same service session.

   A successful response to the SUBSCRIBE request includes the session
   description, according to the Gateway. Normally this will be
   identical to the last cached response that the Gateway returned to
   any request concerning the same SDP global session id (see [2],
   section 6, o= field). The t= line may be altered to indicate the
   actual start or stop time, however. The Gateway might add an i= line
   to the session description to indicate such information as how many
   fax pages were sent. The Gateway SHOULD include an Expires: header
   indicating how long it is willing to maintain the monitoring session.
   If this is unacceptable to the PINT Requestor, then it can close the
   session by sending an immediate UNSUBSCRIBE message (see 3.5.3.3).

   In principle, a user might send a SUBSCRIBE request after the
   telephone network service has completed. This allows, for example,
   checking up "the morning after" to see if the fax was successfully
   transmitted.  However, a PINT gateway is only required to keep state
   about a call for as long as it indicated previously in an Expires:
   header sent within the response to the original INVITE message that
   triggered the service session, within the response to the SUBSCRIBE
   message, within the response to any UNSUBSCRIBE message, or within
   its own UNSUBSCRIBE message (but see section 3.5.8, point 3).

   If the Server no longer has a record of the session to which a
   Requestor has SUBSCRIBEd, it returns "606 Not Acceptable", along with
   the appropriate Warning: 307 header indicating that the SDP session
   ID is no longer valid. This means that a requesting Client that knows
   that it will want information about the status of a session after the
   session terminates SHOULD send a SUBSCRIBE request before the session
   terminates.







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3.5.3.2. Sending Status Indications with a NOTIFY request

   During the subscription period, the Gateway may, from time to time,
   send a spontaneous NOTIFY request to the entity indicated in the
   Contact:  header of the "opening" SUBSCRIBE request. Normally this
   will happen as a result of any change in the status of the service
   session for which the Requestor has subscribed.

   The receiving user agent server MUST acknowledge this by returning a
   final response (normally a "200 OK"). In this version of the PINT
   extensions, the Gateway is not required to support redirects (3xx
   codes), and so may treat them as a failure.

   Thus, if the response code class is above 2xx then this may be
   treated by the Gateway as a failure of the monitoring session, and in
   that situation it will immediately attempt to close the session (see
   next).

   The NOTIFY request contains the modified session description. For
   example, the Gateway may be able to indicate a more accurate start or
   stop time.

   The Gateway may include a Warning: header to describe some problem
   with the invocation of the service, and may indicate within an i=
   line some information about the telephone network session itself.

   Example:
         NOTIFY  sip:petrack@pager.com SIP/2.0
         To: sip:petrack@pager.com
         From: sip:R2F.pint.com@service.com
         Call-ID: 19971205T234505.56.78@pager.com
         CSeq: 4711 SUBSCRIBE
         Warning: xxx  fax aborted, will try for the next hour.
         Content-Type:application/sdp

         c=...
         i=3 pages of 5 sent
         t=...

3.5.3.3. Closing a monitoring session with an UNSUBSCRIBE request

   At some point, either the Client's representative User Agent Server
   or the Gateway may decide to terminate the monitoring session. This
   is achieved by sending an UNSUBSCRIBE request to the correspondent
   server.  Such a request indicates that the sender intends to close
   the monitoring session immediately, and, on receipt of the final
   response from the receiving server, the session is deemed over.




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   Note that unlike the SUBSCRIBE request, which is never sent by a PINT
   gateway, an UNSUBSCRIBE request can be sent by a PINT gateway to the
   User Agent Server to indicate that the monitoring session is closed.
   (This is analogous to the fact that a gateway never sends an INVITE,
   although it can send a BYE to indicate that a telephone call has
   ended.)

   If the Gateway initiates closure of the monitoring session by sending
   an UNSUBSCRIBE message, it SHOULD include an "Expires:" header
   showing for how much longer after this monitoring session is closed
   it is willing to store information on the service session. This acts
   as a minimum time within which the Client can send a new SUBSCRIBE
   message to open another monitoring session; after the time indicated
   in the Expires: header the Gateway is free to dispose of any record
   of the service session, so that subsequent SUBSCRIBE requests can be
   rejected with a "606" response.

   If the subscription period specified by the Client has expired, then
   the Gateway may send an immediate UNSUBSCRIBE request to the Client's
   representative User Agent Server. This ensures that the monitoring
   session always completes with a UNSUBSCRIBE/response exchange, and
   that the representative User Agent Server can avoid maintaining state
   in certain circumstances.

3.5.3.4. Timing of SUBSCRIBE requests

   As it relies on the Gateway having a copy of the INVITEd session
   description, the SUBSCRIBE message is limited in when it can be
   issued.  The Gateway must have received the service request to which
   this monitoring session is to be associated, which from the Client's
   perspective happens as soon as the Gateway has sent a 1xx response
   back to it.

   However, once this has been done, there is no reason why the Client
   should not send a monitoring request. It does not have to wait for
   the final response from the Gateway, and it can certainly send the
   SUBSCRIBE request before sending the ACK for the Service request
   final response.  Beyond this point, the Client is free to send a
   SUBSCRIBE request when it decides, unless the Gateway's final
   response to the initial service request indicated a short Expires:
   time.

   However, there are good reasons (see 6.4) why it may be appropriate
   to start a monitoring session immediately before the service is
   confirmed by the PINT Client sending an ACK. At this point the
   Gateway will have decided whether or not it can handle the service
   request, but will not have passed the request on to the Executive
   System. It is therefore in a good position to ask the Executive



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   System to enable monitoring when it sends the service request
   onwards. In practical implementations, it is likely that more
   information on transient service status will be available if this is
   indicated as being important BEFORE or AS the service execution phase
   starts; once execution has begun the level of information that can be
   returned may be difficult to change.

   Thus, whilst it is free to send a SUBSCRIBE request at any point
   after receiving an Interim response from the Gateway to its service
   request, it is recommended that the Client should send such a
   monitoring request immediately prior to sending an ACK message
   confirming the service if it is interested in transient service
   status messages.

3.5.4. The "Require:" header for PINT

   PINT clients use the Require: header to signal to the PINT server
   that a certain PINT extension of SIP is required. PINT 1.0 defines
   two strings that can go into the Require header:

   org.ietf.sip.subscribe  -- the server can fulfill SUBSCRIBE requests
                              and associated methods (see section 3.5.3)

   org.ietf.sdp.require    -- the PINT server (or the SDP parser
                              associated to it) understands the "require"
                              attribute defined in (section 3.4.4)

   Example:
         Require:org.ietf.sip.subscribe,org.ietf.sdp.require

   A client SHOULD only include a Require: header where it truly
   requires the server to reject the request if the option is not
   supported.

3.5.5. PINT URLs within PINT requests

   Normally the hostnames and domain names that appear in the PINT URLs
   are the internal affair of each individual PINT system. A client uses
   the appropriate SDP payload to indicate the particular service it
   wishes to invoke; it is not necessary to use a particular URL to
   identify the service.

   A PINT URL is used in two different ways within PINT requests: within
   the Request-URI, and within the To: and From: headers. Use within the
   Request-URI requires clarification in order to ensure smooth
   interworking with the Telephone Network serviced by the PINT
   infrastructure, and this is covered next.




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3.5.5.1. PINT URLS within Request-URIs

   There are some occasions when it may be useful to indicate service
   information within the URL in a standardized way:

      a. it may not be possible to use SDP information to route the
         request if it is encrypted;
      b. it allows implementation that make use of I.N. "service
         indicators";
      c. It enables multiple competing PINT gateways to REGISTER with a
         single "broker" server (proxy or redirect) (see section 6.3)

   For these reasons, the following conventions for URLs are offered for
   use in PINT requests:

   1. The user portion of a sip URL indicates the service to be
   requested.  At present the following services are defined:

   R2C   (for Request-to-Call)
   R2F   (for Request-to-Fax)
   R2HC  (for Request-to-Hear-Content)

   The user portions "R2C", "R2F", and "R2HC" are reserved for the PINT
   milestone services. Other user portions MUST be used in case the
   requested service is not one of the Milestone services. See section
   6.2 for some related considerations concerning registrations by
   competing PINT systems to a single PINT proxy server acting as a
   service broker.

   2. The host portion of a sip URL contains the domain name of the PINT
   service provider.

   3. A new url-parameter is defined to be "tsp" (for "telephone service
   provider"). This can be used to indicate the actual telephone network
   provider to be used to fulfill the PINT request.

   Thus, for example:-
         INVITE sip:R2C@pint.pintservice.com SIP/2.0
         INVITE sip:R2F@pint.pintservice.com;tsp=telco.com SIP/2.0
         INVITE sip:R2HC@pint.mycom.com;tsp=pbx23.mycom.com SIP/2.0
         INVITE sip:13@pint.telco.com SIP/2.0

3.5.6. Telephony Network Parameters within PINT URLs

   Any legal SIP URL can appear as a PINT URL within the Request-URI or
   To:  header of a PINT request. But if the address is a telephone
   address, we indicated in section 3.4.3 that it may be necessary to
   include more information in order correctly to identify the remote



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   telephone terminal or service. PINT clients MAY include these
   attribute tags within PINT URLs if they are necessary or a useful
   complement to the telephone number within the SIP URL. These
   attribute tags MUST be included as URL parameters as defined in [1]
   (i.e. in the semi-colon separated manner).

   The following is an example of a PINT URL containing extra attribute
   tags:

sip:+9725228808@pint.br.com;user=phone;require=Q763-plan;a=Q763-plan:4

   As we noted in section 3.4.3, these extra attribute parameters will
   not normally be needed within a URL, because there is a great deal of
   context available to help the server interpret the phone number
   correctly. In particular, there is the SIP URL within the To: header,
   and there is also the Request-URI. In most cases this provides
   sufficient information for the telephone network.

   The SDP attributes defined in section 3 above will normally only be
   used when they are needed to supply necessary context to identify a
   telephone terminal.

3.5.7. REGISTER requests within PINT

   A PINT gateway is a SIP user agent server. A User Agent Server uses
   the REGISTER request to tell a proxy or redirect server that it is
   available to "receive calls" (i.e. to service requests). Thus a PINT
   Gateway registers with a proxy or redirect server the service that is
   accessible via itself, whilst in SIP, a user is registering his/her
   presence at a particular SIP Server.

   There may be competing PINT servers that can offer the same PINT
   service trying to register at a single PINT server. The PINT server
   might act as a "broker" among the various PINT gateways that can
   fulfill a request. A format for PINT URLs was specified in section
   3.5.5 that enables independent PINT systems to REGISTER an offer to
   provide the same service. The registrar can apply its own mechanisms
   and policies to decide how to respond to INVITEs from clients seeking
   service (See section 6.3 for some possible deployment options). There
   is no change between SIP and PINT REGISTER semantics or syntax.

   Of course, the information in the PINT URLs within the REGISTER
   request may not be sufficient to completely define the service that a
   gateway can offer. The use of SIP and SDP within PINT REGISTER
   requests to enable a gateway to specify in more detail the services
   it can offer is the subject of future study.





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3.5.8. BYE Requests in PINT

   The semantics of BYE requests within PINT requires some extra
   precision.  One issue concerns conferences that "cannot be left", and
   the other concerns keeping call state after the BYE.

   The BYE request [1] is normally used to indicate that the originating
   entity no longer wishes to be involved in the specified call. The
   request terminates the call and the media session. Applying this
   model to PINT, if a PINT client makes a request that results in
   invocation of a telephone call from A to B, a BYE request from the
   client, if accepted, should result in a termination of the phone
   call.

   One might expect this to be the case if the telephone call has not
   started when the BYE request is received. For example, if a request
   to fax is sent with a t= line indicating that the fax is to be sent
   tomorrow at 4 AM, the requestor might wish to cancel the request
   before the specified time.

   However, even if the call has yet to start, it may not be possible to
   terminate the media session on the telephone system side. For
   example, the fax call may be in progress when the BYE arrives, and
   perhaps it is just not possible to cancel the fax in session. Another
   possibility is that the entire telephone-side service might be
   completed before the BYE is received. In the above Request-to-Fax
   example, the BYE might be sent the following morning, and the entire
   fax has been sent before the BYE was received. It is too late to send
   the BYE.

   In the case where the telephone network cannot terminate the call,
   the server MUST return a "606 Not Acceptable" response to the BYE,
   along with a session description that indicates the telephone network
   session that is causing the problem.

   Thus, in PINT, a "Not Acceptable" response MAY be returned both to
   INVITE and BYE requests. It indicates that some aspect of the session
   description makes the request unacceptable.

   By allowing a server to return a "Not Acceptable" response to BYE
   requests, we are not changing its semantics, just enlarging its use.

   A combination of Warning: headers and i= lines within the session
   description can be used to indicate the precise nature of the
   problem.






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   Example:

         SIP/2.0 606 Not Acceptable
         From: ...
         To: .......
         .....
         Warning: 399 pint.mycom.com Fax in progress, service cannot be
            aborted
         Content-Type: application/sdp
         Content-Length: ...

         v=0
         ...
         ...
         i=3 of 5 pages sent OK
         c=TN  RFC2543  +12014064090
         m=image 1 fax tif
         a=fmtp:tif uri:http://tifsRus.com/yyyyyy.tif

   Note that the server might return an updated session description
   within a successful response to a BYE as well. This can be used, for
   example, to indicate the actual start times and stop times of the
   telephone session, or how many pages were sent in the fax
   transmission.

   The second issue concerns how long must a server keep call state
   after receiving a BYE. A question arises because other clients might
   still wish to send queries about the telephone network session that
   was the subject of the PINT transaction. Ordinary SIP semantics have
   three important implications for this situation:

   1. A BYE indicates that the requesting client will clear out all call
   state as soon as it receives a successful response. A client SHOULD
   NOT send a SUBSCRIBE request after it has sent a BYE.

   2. A server may return an Expires: header within a successful
   response to a BYE request. This indicates for how long the server
   will retain session state about the telephone network session. At any
   point during this time, a client may send a SUBSCRIBE request to the
   server to learn about the session state (although as explained in the
   previous paragraph, a client that has sent a BYE will not normally
   send a SUBSCRIBE).

   3. When engaged in a SUBSCRIBE/NOTIFY monitoring session, PINT
   servers that send UNSUBSCRIBE to a URL listed in the Contact: header
   of a client request SHOULD not clear session state until after the
   successful response to the UNSUBSCRIBE message is received. For
   example, it may be that the requesting client host is turned off (or



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   in a low power mode) when the telephone service is executed (and is
   therefore not available at the location previously specified in the
   Contact: attribute) to receive the PINT server's UNSUBSCRIBE. Of
   course, it is possible that the UNSUBSCRIBE request will simply time
   out.

4. Examples of PINT Requests and Responses

4.1. A request to a call center from an anonymous user to receive a
     phone call.

   C->S: INVITE  sip:R2C@pint.mailorder.com  SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         From: sip:anon-1827631872@chinet.net
         To: sip:+1-201-456-7890@iron.org;user=phone
         Call-ID: 19971205T234505.56.78@pager.com
         CSeq: 4711 INVITE
         Subject: Sale on Ironing Boards
         Content-type: application/sdp
         Content-Length: 174

         v=0
         o=- 2353687637 2353687637 IN IP4 128.3.4.5
         s=R2C
         i=Ironing Board Promotion
         e=anon-1827631872@chinet.net
         t=2353687637 0
         m=audio 1  voice -
         c=TN  RFC2543  +1-201-406-4090

   In this example, the context that is required to interpret the To:
   address as a telephone number is not given explicitly; it is
   implicitly known to the R2C@pint.mailorder.com server. But the
   telephone of the person who wishes to receive the call is explicitly
   identified as an internationally significant E.164 number that falls
   within the North American numbering plan (because of the "+1" within
   the c= line).

4.2. A request from a non anonymous customer (John Jones) to receive a
     phone call from a particular sales agent (Mary James) concerning
     the defective ironing board that was purchased

   C->S: INVITE  sip:marketing@pint.mailorder.com  SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         From: sip:john.jones.3@chinet.net
         To: sip:mary.james@mailorder.com
         Call-ID: 19971205T234505.56.78@pager.com
         CSeq: 4712 INVITE



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         Subject: Defective Ironing Board - want refund
         Content-type: application/sdp
         Content-Length: 150

         v=0
         o=- 2353687640 2353687640 IN IP4 128.3.4.5
         s=marketing
         e=john.jones.3@chinet.net
         c= TN RFC2543  +1-201-406-4090
         t=2353687640 0
         m=audio 1  voice -

   The To: line might include the Mary James's phone number instead of a
   email-like address. An implementation that cannot accept email-like
   URLs in the "To:" header must decline the request with a 606 Not
   Acceptable.  Note that the sending PINT client "knows" that the PINT
   Gateway contacted with the "marketing@pint.mailorder.com" Request-URI
   is capable of processing the client request as expected. (see 3.5.5.1
   for a discussion on this).

   Note also that such a telephone call service could be implemented on
   the phone side with different details. For example, it might be that
   first the agent's phone rings, and then the customer's phone rings,
   or it might be that first the customer's phone rings and he hears
   silly music until the agent comes on line. If necessary, such service
   parameter details might be indicated in "a=" attribute lines within
   the session description. The specification of such attribute lines
   for service consistency is beyond the scope of the PINT 1.0
   specifications.

4.3. A request from the same user to get a fax back on how to assemble
     the Ironing Board

C->S: INVITE  sip:faxback@pint.mailorder.com  SIP/2.0
      Via: SIP/2.0/UDP 169.130.12.5
      From: sip:john.jones.3@chinet.net
      To: sip:1-800-3292225@steam.edu;user=phone;phone-context=+1
      Call-ID: 19971205T234505.66.79@chinet.net
      CSeq: 4713 INVITE
      Content-type: application/sdp
      Content-Length: 218

      v=0
      o=- 2353687660 2353687660 IN IP4 128.3.4.5
      s=faxback
      e=john.jones.3@chinet.net
      t=2353687660 0
      m=application 1 fax URI



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RFC 2848               The PINT Service Protocol               June 2000


      c=TN  RFC2543  1-201-406-4091
      a=fmtp:URI uri:http://localstore/Products/IroningBoards/2344.html

   In this example, the fax to be sent is stored on some local server
   (localstore), whose name may be only resolvable, or that may only be
   reachable, from within the IP network on which the PINT server sits.
   The phone number to be dialled is a "local phone number" as well.
   There is no "phone-context" attribute, so the context (in this case,
   for which nation the number is "nationally significant") must be
   supplied by the faxback@pint.mailorder.com PINT server.

   If the server that receives it does not understand the number, it
   SHOULD decline the request and include a "Network Address Not
   Understood" warning.  Note that no "require" attribute was used here,
   since it is very likely that the request can be serviced even by a
   server that does not support the "require" attribute.

4.4. A request from same user to have that same information read out
     over the phone

C->S: INVITE  sip:faxback@pint.mailorder.com  SIP/2.0
      Via: SIP/2.0/UDP 169.130.12.5
      From: sip:john.jones.3@chinet.net
      To: sip:1-800-3292225@steam.edu;user=phone;phone-context=+1
      Call-ID: 19971205T234505.66.79@chinet.net
      CSeq: 4713 INVITE
      Content-type: application/sdp
      Content-Length: 220

      v=0
      o=- 2353687660 2353687660 IN IP4 128.3.4.5
      s=faxback
      e=john.jones.3@chinet.net
      t=2353687660 0
      m=application 1 voice URI
      c=TN  RFC2543  1-201-406-4090
      a=fmtp:URI uri:http://localstore/Products/IroningBoards/2344.html

4.5. A request to send an included text page to a friend's pager.

   In this example, the text to be paged out is included in the request.

C->S: INVITE  sip:R2F@pint.pager.com  SIP/2.0
      Via: SIP/2.0/UDP 169.130.12.5
      From: sip:scott.petrack@chinet.net
      To: sip:R2F@pint.pager.com
      Call-ID: 19974505.66.79@chinet.net
      CSeq: 4714 INVITE



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      Content-Type: multipart/related; boundary=--next

      ----next
      Content-Type: application/sdp
      Content-Length: 236
      v=0
      o=- 2353687680 2353687680 IN IP4 128.3.4.5
      s=R2F
      e=scott.petrack@chinet.net
      t=2353687680 0
      m=text 1 pager plain
      c= TN  RFC2543  +972-9-956-1867
      a=fmtp:plain spr:2@53655768


      ----next
      Content-Type: text/plain
      Content-ID: 2@53655768
      Content-Length:50

      Hi Joe! Please call me asap at 555-1234.

      ----next--

4.6. A request to send an image as a fax to phone number +972-9-956-1867

C->S: INVITE  sip:faxserver@pint.vocaltec.com  SIP/2.0
      Via: SIP/2.0/UDP 169.130.12.5
      From: sip:scott.petrack@chinet.net
      To: sip:faxserver@pint.vocaltec.com
      Call-ID: 19971205T234505.66.79@chinet.net
      CSeq: 4715 INVITE
      Content-type: application/sdp
      Content-Length: 267

      v=0
      o=- 2353687700 2353687700 IN IP4 128.3.4.5
      s=faxserver
      e=scott.petrack@chinet.net
      t=2353687700 0
      m=image  1 fax  tif gif
      c= TN  RFC2543  +972-9-956-1867
      a=fmtp:tif  uri:http://petrack/images/tif/picture1.tif
      a=fmtp:gif  uri:http://petrack/images/gif/picture1.gif







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   The image is available as tif or as gif. The tif is the preferred
   format. Note that the http server where the pictures reside is local,
   and the PINT server is also local (because it can resolve machine
   name "petrack")

4.7. A request to read out over the phone two pieces of content in
     sequence.

   First some included text is read out by text-to-speech. Then some
   text that is stored at some URI on the internet is read out.

C->S: INVITE  sip:R2HC@pint.acme.com  SIP/2.0
      Via: SIP/2.0/UDP 169.130.12.5
      From: sip:scott.petrack@chinet.net
      To: sip:R2HC@pint.acme.com
      Call-ID: 19974505.66.79@chinet.net
      CSeq: 4716 INVITE
      Content-Type: multipart/related; boundary=next

      --next
      Content-Type: application/sdp
      Content-Length: 316
      v=0
      o=- 2353687720 2353687720 IN IP4 128.3.4.5
      s=R2HC
      e=scott.petrack@chinet.net

      c= TN  RFC2543  +1-201-406-4091
      t=2353687720 0
      m=text  1  voice  plain
      a=fmtp:plain   spr:2@53655768
      m=text  1 voice plain
      a=fmtp:plain  uri:http://www.your.com/texts/stuff.doc

      --next
      Content-Type: text/plain
      Content-ID: 2@53655768
      Content-Length: 172

      Hello!! I am about to read out to you the document you
      requested, "uri:http://www.your.com/texts/stuff.doc".
      We hope you like acme.com's new speech synthesis server.
      --next--








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4.8. Request for the prices for ISDN to be sent to my fax machine

   INVITE sip:R2FB@pint.bt.co.uk  SIP/2.0
   Via: SIP/2.0/UDP 169.130.12.5
   To: sip:0345-12347-01@pint.bt.co.uk;user=phone;phone-context=+44
   From: sip:hank.wangford@newts.demon.co.uk
   Call-ID: 19981204T201505.56.78@demon.co.uk
   CSeq: 4716 INVITE
   Subject: Price List
   Content-type: application/sdp
   Content-Length: 169

   v=0
   o=- 2353687740 2353687740 IN IP4 128.3.4.5
   s=R2FB
   i=ISDN Price List
   e=hank.wangford@newts.demon.co.uk
   t=2353687740 0
   m=text 1  fax -
   c=TN  RFC2543  +44-1794-8331010

4.9. Request for a callback

   INVITE sip:R2C@pint.bt.co.uk  SIP/2.0
   Via: SIP/2.0/UDP 169.130.12.5
   To: sip:0345-123456@pint.bt.co.uk;user=phone;phone-context=+44
   From: sip:hank.wangford@newts.demon.co.uk
   Call-ID: 19981204T234505.56.78@demon.co.uk
   CSeq: 4717 INVITE
   Subject: It costs HOW much?
   Content-type: application/sdp
   Content-Length: 176

   v=0
   o=- 2353687760 2353687760 IN IP4 128.3.4.5
   s=R2C
   i=ISDN pre-sales query
   e=hank.wangford@newts.demon.co.uk
   c=TN  RFC2543  +44-1794-8331013
   t=2353687760 0
   m=audio 1  voice -










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4.10. Sending a set of information in response to an enquiry

   INVITE sip:R2FB@pint.bt.co.uk  SIP/2.0
   Via: SIP/2.0/UDP 169.130.12.5
   To: sip:0345-12347-01@pint.bt.co.uk;user=phone;phone-context=+44
   From: sip:colin.masterton@sales.hh.bt.co.uk
   Call-ID: 19981205T234505.56.78@sales.hh.bt.co.uk
   CSeq: 1147 INVITE
   Subject: Price Info, as requested
   Content-Type: multipart/related; boundary=next

   --next
   Content-type: application/sdp
   Content-Length: 325
   v=0
   o=- 2353687780 2353687780 IN IP4 128.3.4.5
   s=R2FB
   i=Your documents
   e=colin.masterton@sales.hh.bt.co.uk
   t=2353687780 0
   m=application 1  fax octet-stream
   c=TN  RFC2543  +44-1794-8331010
   a=fmtp:octet-stream uri:http://www.bt.co.uk/imgs/pipr.gif opr:
     spr:2@53655768

   --next
   Content-Type: text/plain
   Content-ID: 2@53655768
   Content-Length: 352

   Dear Sir,
     Thank you for your enquiry. I have checked availability in your
   area, and we can provide service to your cottage. I enclose a
   quote for the costs of installation, together with the ongoing
   rental costs for the line. If you want to proceed with this,
   please quote job reference isdn/hh/123.45.9901.
   Yours Sincerely,
      Colin Masterton
   --next--

   Note that the "implicit" faxback content is given by an EMPTY opaque
   reference in the middle of the fmtp line in this example.









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4.11. Sportsline "headlines" message sent to your phone/pager/fax

   (i) phone
         INVITE sip:R2FB@pint.wwos.skynet.com  SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         To:
   sip:1-900-123-456-7@wwos.skynet.com;user=phone;phone-context=+1
         From: sip:fred.football.fan@skynet.com
         Call-ID: 19971205T234505.56.78@chinet.net
         CSeq: 4721 INVITE
         Subject: Wonderful World Of Sports NFL Final Scores
         Content-type: application/sdp
         Content-Length: 220

         v=0
         o=- 2353687800 2353687800 IN IP4 128.3.4.5
         s=R2FB
         i=NFL Final Scores
         e=fred.football.fan@skynet.com
         c=TN  RFC2543 +44-1794-8331013
         t=2353687800 0
         m=audio 1 voice x-pay
         a=fmtp:x-pay opr:mci.com/md5:<crypto signature>

   (ii) fax
         INVITE sip:R2FB@pint.wwos.skynet.com  SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         To: sip:1-900-123-456-7@wwos.skynet.com;user=phone;
             phone-context=+1
         From: sip:fred.football.fan@skynet.com
         Call-ID: 19971205T234505.56.78@chinet.net
         CSeq: 4722 INVITE
         Subject: Wonderful World Of Sports NFL Final Scores
         Content-type: application/sdp
         Content-Length: 217

         v=0
         o=- 2353687820 2353687820 IN IP4 128.3.4.5
         s=R2FB
         i=NFL Final Scores
         e=fred.football.fan@skynet.com
         c=TN  RFC2543 +44-1794-8331010
         t=2353687820 0
         m=text 1 fax x-pay
         a=fmtp:x-pay opr:mci.com/md5:<crypto signature>






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   (iii) pager
         INVITE sip:R2FB@pint.wwos.skynet.com  SIP/2.0
         Via: SIP/2.0/UDP 169.130.12.5
         To: sip:1-900-123-456-7@wwos.skynet.com;user=phone;
             phone-context=+1
         From: sip:fred.football.fan@skynet.com
         Call-ID: 19971205T234505.56.78@chinet.net
         CSeq: 4723 INVITE
         Subject: Wonderful World Of Sports NFL Final Scores
         Content-type: application/sdp
         Content-Length: 219

         v=0
         o=- 2353687840 2353687840 IN IP4 128.3.4.5
         s=R2FB
         i=NFL Final Scores
         e=fred.football.fan@skynet.com
         c=TN  RFC2543 +44-1794-8331015
         t=2353687840 0
         m=text 1 pager x-pay
         a=fmtp:x-pay opr:mci.com/md5:<crypto signature>

   Note that these are all VERY similar.

4.12. Automatically giving someone a fax copy of your phone bill

      INVITE sip:BillsRUs@pint.sprint.com SIP/2.0
      Via: SIP/2.0/UDP 169.130.12.5
      To: sip:+1-555-888-1234@fbi.gov;user=phone
      From: sip:agent.mulder@fbi.gov
      Call-ID: 19991231T234505.56.78@fbi.gov
      CSeq: 911 INVITE
      Subject: Itemised Bill for January 98
      Content-type: application/sdp
      Content-Length: 247

      v=0
      o=- 2353687860 2353687860 IN IP4 128.3.4.5
      s=BillsRUs
      i=Joe Pendleton's Phone Bill
      e=agent.mulder@fbi.gov
      c=TN  RFC2543  +1-202-833-1010
      t=2353687860 0
      m=text 1  fax x-files-id
      a=fmtp:x-files-id opr:fbi.gov/jdcn-123@45:3des;base64,<signature>






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   Note: in this case the opaque reference is a collection of data used
   to convince the Executive System that the requester has the right to
   get this information, rather than selecting the particular content
   (the A party in the To: field of the SIP "wrapper" does that alone).

5.  Security Considerations

5.1.  Basic Principles for PINT Use

   A PINT Gateway, and the Executive System(s) with which that Gateway
   is associated, exist to provide service to PINT Requestors. The aim
   of the PINT protocol is to pass requests from those users on to a
   PINT Gateway so an associated Executive System can service those
   requests.

5.1.1.  Responsibility for service requests

   The facility of making a GSTN-based call to numbers specified in the
   PINT request, however, comes with some risks. The request can specify
   an incorrect telephone of fax number. It is also possible that the
   Requestor has purposely entered the telephone number of an innocent
   third party. Finally, the request may have been intercepted on its
   way through any intervening PINT or SIP infrastructure, and the
   request may have been altered.

   In any of these cases, the result may be that a call is placed
   incorrectly. Where there is intent or negligence, this may be
   construed as harassment of the person incorrectly receiving the call.
   Whilst the regulatory framework for misuse of Internet connections
   differs throughout the world and is not always mature, the rules
   under which GSTN calls are made are much more settled. Someone may be
   liable for mistaken or incorrect calls.

   Understandably, the GSTN Operators would prefer that this someone is
   not them, so they will need to ensure that any PINT Gateway and
   Executive System combination does not generate incorrect calls
   through some error in the Gateway or Executive system implementation
   or GSTN-internal communications fault. Equally, it is important that
   the Operator can show that they act only on requests that they have
   good reason to believe are correct. This means that the Gateway must
   not pass on requests unless it is sure that they have not been
   corrupted in transit from the Requestor.

   If a request can be shown to have come from a particular Requestor
   and to have been acted on in good faith by the PINT service provider,
   then responsibility for making requests may well fall to the
   Requestor rather than the Operator who executed these requests.




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RFC 2848               The PINT Service Protocol               June 2000


   Finally, it may be important for the PINT service provider to be able
   to show that they act only on requests for which they have some
   degree of assurance of origin. In many jurisdictions, it is a
   requirement on GSTN Operators that they place calls only when they
   can, if required, identify the parties to the call (such as when
   required to carry out a Malicious Call Trace). It is at least likely
   that the provider of PINT services will have a similar responsibility
   placed on them.

   It follows that the PINT service provider may require that the
   identity of the Requestor be confirmed. If such confirmation is not
   available, then they may be forced (or choose) not to provide
   service. This identification may require personal authentication of
   the Requesting User.

5.1.2.  Authority to make requests

   Where GSTN resources are used to provide a PINT service, it is at
   least possible that someone will have to pay for it. This person may
   not be the Requestor, as, for example, in the case of existing GSTN
   split-charging services like free phone in which the recipient of a
   call rather than the originator is responsible for the call cost.

   This is not, of course, the only possibility; for example, PINT
   service may be provided on a subscription basis, and there are a
   number of other models. However, whichever model is chosen, there may
   be a requirement that the authority of a Requestor to make a PINT
   request is confirmed.

   If such confirmation is not available, then, again, the PINT Gateway
   and associated Executive System may choose not to provide service.

5.1.3.  Privacy

   Even if the identity of the Requesting User and the Authority under
   which they make their request is known, there remains the possibility
   that the request is either corrupted, maliciously altered, or even
   replaced whilst in transit between the Requestor and the PINT
   Gateway.

   Similarly, information on the Authority under which a request is made
   may well be carried within that request. This can be sensitive
   information, as an eavesdropper might steal this and use it within
   their own requests. Such authority SHOULD be treated as if it were
   financial information (such as a credit card number or PIN).






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RFC 2848               The PINT Service Protocol               June 2000


   The data authorizing a Requesting User to make a PINT request should
   be known only to them and the service provider. However, this
   information may be in a form that does not match the schemes normally
   used within the Internet. For example, X.509 certificates[14] are
   commonly used for secured transactions on the Internet both in the IP
   Security Architecture[12] and in the TLS protocol[13], but the GSTN
   provider may only store an account code and PIN (i.e. a fixed string
   of numbers).

   A Requesting User has a reasonable expectation that their requests
   for service are confidential. For some PINT services, no content is
   carried over the Internet; however, the telephone or fax numbers of
   the parties to a resulting service calls may be considered sensitive.
   As a result, it is likely that the Requestor (and their PINT service
   provider) will require that any request that is sent across the
   Internet be protected against eavesdroppers; in short, the requests
   SHOULD to be encrypted.

5.1.4.  Privacy Implications of SUBSCRIBE/NOTIFY

   Some special considerations relate to monitoring sessions using the
   SUBSCRIBE and NOTIFY messages. The SUBSCRIBE message that is used to
   register an interest in the disposition of a PINT service transaction
   uses the original Session Description carried in the related INVITE
   message. This current specification does not restrict the source of
   such a SUBSCRIBE message, so it is possible for an eavesdropper to
   capture an unprotected session description and use this in a
   subsequent SUBSCRIBE request. In this way it is possible to find out
   details on that transaction that may well be considered sensitive.

   The initial solution to this risk is to recommend that a session
   description that may be used within a subsequent SUBSCRIBE message
   SHOULD be protected.

   However, there is a further risk; if the origin-field used is
   "guessable" then it might be possible for an attacker to reconstruct
   the session description and use this reconstruction within a
   SUBSCRIBE message.

   SDP (see section 6 of [2], "o=" field) does not specify the mechanism
   used to generate the sess-id field, and suggests that a method based
   on timestamps produced by Network Time Protocol [16] can be used.
   This is sufficient to guarantee uniqueness, but may allow the value
   to be guessed, particularly if other unprotected requests from the
   same originator are available.






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RFC 2848               The PINT Service Protocol               June 2000


   Thus, to ensure that the session identifier is not guessable the
   techniques described in section 6.3 of [17] can be used when
   generating the origin-field for a session description to be used
   inside a PINT INVITE message. If all requests from (and responses to)
   a particular PINT requesting entity are protected, then this is not
   needed. Where such a situation is not assured, AND where session
   monitoring is supported, then a method by which an origin-field
   within a session description is not guessable SHOULD be used.

5.2.  Registration Procedures

   Any number of PINT Gateways may register to provide the same service;
   this is indicated by the Gateways specifying the same "userinfo" part
   in the To: header field of the REGISTER request. Whilst such
   ambiguity would be unlikely to occur with the scenarios covered by
   "core" SIP, it is very likely for PINT; there could be any number of
   service providers all willing to support a "Request-To-Fax" service,
   for example.

   Unless a request specifies the Gateway name explicitly, an
   intervening Proxy that acts on a registration database to which
   several Gateways have all registered is in a position to select from
   the registrands using whatever algorithm it chooses; in principle,
   any Gateway that has registered as "R2F" would be appropriate.

   However, this opens up an avenue for attack, and this is one in which
   a "rogue" Gateway operator stands to make a significant gain. The
   standard SIP procedure for releasing a registration is to send a
   REGISTER request with a Contact field having a wildcard value and an
   expires parameter with a value of 0. It is important that a PINT
   Registrar uses authentication of the Registrand, as otherwise one
   PINT service provider would be able to "spoof" another and remove
   their registration. As this would stop the Proxy passing any requests
   to that provider, this would both increase requests being sent to the
   rogue and stop requests going to the victim.

   Another variant on this attack would be to register a Gateway using a
   name that has been registered by another provider; thus a rogue
   Operator might register its Gateway as "R2C@pint.att.com", thereby
   hijacking requests.

   The solution is the same; all registrations by PINT Gateways MUST be
   authenticated; this includes both new or apparent replacement
   registrations, and any cancellation of current registrations. This
   recommendation is also made in the SIP specification, but for the
   correct operation of PINT, it is very important indeed.





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RFC 2848               The PINT Service Protocol               June 2000


5.3.  Security mechanisms and implications on PINT service

   PINT is a set of extensions to SIP[1] and SDP[2], and will use the
   security procedures described in SIP. There are several implications
   of this, and these are covered here.

   For several of the PINT services, the To: header field of SIP is used
   to identify one of the parties to the resulting service call. The
   PINT Request-To-Call service is an example. As mentioned in the SIP
   specification, this field is used to route SIP messages through an
   infrastructure of Redirect and Proxy server between the corresponding
   User Agent Servers, and so cannot be encrypted. This means that,
   although the majority of personal or sensitive data can be protected
   whilst in transit, the telephone (or fax) number of one of the
   parties to a PINT service call cannot, and will be "visible" to any
   interception. For the PINT milestone services this may be acceptable,
   since the caller named in the To: service is typically a "well known"
   provider address, such as a Call Center.

   Another aspect of this is that, even if the Requesting User does not
   consider the telephone or fax numbers of the parties to a PINT
   service to be private, those parties might. Where PINT servers have
   reason to believe this might be the case they SHOULD encrypt the
   request, even if the Requestor has not done so. This could happen,
   for example, if a Requesting User within a company placed a PINT
   request and this was carried via the company's Intranet to their
   Proxy/firewall and thence over the Internet to a PINT Gateway at
   another location.

   If a request carries data that can be reused by an eavesdropper
   either to "spoof" the Requestor or to obtain PINT service by
   inserting the Requestor's authorization token into an eavesdropper's
   request, then this data MUST be protected. This is particularly
   important if the authorization token consists of static text (such as
   an account code and/or PIN).

   One approach is to encrypt the whole of the request, using the
   methods described in the SIP specification. As an alternative, it may
   be acceptable for the authorization token to be held as an opaque
   reference (see section 3.4.2.3 and examples 4.11 and 4.12), using
   some proprietary scheme agreed between the Requestor and the PINT
   service provider, as long as this is resistant to interception and
   re-use. Also, it may be that the authorization token cannot be used
   outside of a request cryptographically signed by the Requestor; if so
   then this requirement can be relaxed, as in this case the token
   cannot be re-used by another.  However, unless both the Requestor and
   the Gateway are assured that this is the case, any authorization
   token MUST be treated as sensitive, and so MUST be encrypted.



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RFC 2848               The PINT Service Protocol               June 2000


   A PINT request may contain data within the SDP message body that can
   be used more efficiently to route that request. For example, it may
   be that one Gateway and Executive System combination cannot handle a
   request that specifies one of the parties as a pager, whilst another
   can. Both gateways may have registered with a PINT/SIP Registrar, and
   this information may be available to intervening PINT/SIP Proxies.
   However, if the message body is encrypted, then the request cannot be
   decoded at the Proxy server, and so Gateway selection based on
   contained information cannot be made there.

   The result is that the Proxy may deliver the request to a Gateway
   that cannot handle it; the implication is that a PINT/SIP Proxy
   SHOULD consider its choice for the appropriate Gateway subject to
   correction, and, on receiving a 501 or 415 rejection from the first
   gateway chosen, try another. In this way, the request will succeed if
   at all possible, even though it may be delayed (and tie up resources
   in the inappropriate Gateways).

   This opens up an interesting avenue for Denial Of Service; sending a
   valid request that appears to be suitable for a number of different
   Gateways, and simply occupying those Gateways in decrypting a message
   requesting a service they cannot provide. As mentioned in section
   3.5.5.1, the choice of service name to be passed in the userinfo
   portion of the SIP Request-URI is flexible, and it is RECOMMENDED
   that names be chosen that allow a Proxy to select an appropriate
   Gateway without having to examine the SDP body part. Thus, in the
   example given here, the service might be called "Request-To-Page" or
   "R2P" rather than the more general use of "R2F", if there is a
   possibility of the SDP body part being protected during transit.

   A variation on this attack is to provide a request that is
   syntactically invalid but that, due to the encryption, cannot be
   detected without expending resources in decoding it. The effects of
   this form of attack can be minimised in the same way as for any SIP
   Invitation; the Proxy should detect the 400 rejection returned from
   the initial Gateway, and not pass the request onwards to another.

   Finally, note that the Requesting User may not have a prior
   relationship with a PINT Gateway, whilst still having a prior
   relationship with the Operator of the Executive System that fulfills
   their request. Thus there may be two levels of authentication and
   authorization; one carried out using the techniques described in the
   SIP specification (for use between the Requestor and the Gateway),
   with another being used between the Requesting User or the Requestor
   and the Executive System.






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   For example, the Requesting User may have an account with the PINT
   service provider. That provider might require that requests include
   this identity before they will be convinced to provide service. In
   addition, to counter attacks on the request whilst it is in transit
   across the Internet, the Gateway may require a separate X.509-based
   certification of the request. These are two separate procedures, and
   data needed for the former would normally be expected to be held in
   opaque references inside the SDP body part of the request.

   The detailed operation of this mechanism is, by definition, outside
   the scope of an Internet Protocol, and so must be considered a
   private matter. However, one approach to indicating to the Requestor
   that such "second level" authentication or authorization is required
   by their Service Provider would be to ask for this inside the textual
   description carried with a 401 response returned from the PINT
   Gateway.

5.4.  Summary of Security Implications

   From the above discussion, PINT always carries data items that are
   sensitive, and there may be financial considerations as well as the
   more normal privacy concerns. As a result, the transactions MUST be
   protected from interception, modification and replay in transit.

   PINT is based on SIP and SDP, and can use the security procedures
   outlined in [1] (sections 13 and 15). However, in the case of PINT,
   the SIP recommendation that requests and responses MAY be protected
   is not enough. PINT messages MUST be protected, so PINT
   Implementations MUST support SIP Security (as described in [1],
   sections 13 & 15), and be capable of handling such received messages.

   In some configurations, PINT Clients, Servers, and Gateways can be
   sure that they operate using the services of network level security
   [13], transport layer security [12], or physical security for all
   communications between them. In these cases messages MAY be exchanged
   without SIP security, since all traffic is protected already. Clients
   and servers SHOULD support manual configuration to use such lower
   layer security facilities.

   When using network layer security [13], the Security Policy Database
   MUST be configured to provide appropriate protection to PINT traffic.
   When using TLS, a port configured MUST NOT also be configured for
   non-TLS traffic. When TLS is used, basic authentication MUST be
   supported, and client-side certificates MAY be supported.







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   Authentication of the Client making the request is required, however,
   so if this is not provided by the underlying mechanism used, then it
   MUST be included within the PINT messages using SIP authentication
   techniques. In contrast with SIP, PINT requests are often sent to
   parties with which a prior communications relationship exists (such
   as a Telephone Carrier). In this case, there may be a shared secret
   between the client and the PINT Gateway. Such PINT systems MAY use
   authentication based on shared secrets, with HTTP "basic
   authentication". When this is done, the message integrity and privacy
   must be guaranteed by some lower layer mechanism.

   There are implications on the operation of PINT here though. If a
   PINT proxy or redirect server is used, then it must be able to
   examine the contents of the IP datagrams carried. It follows that an
   end-to-end approach using network-layer security between the PINT
   Client and a PINT Gateway precludes the use of an intervening proxy;
   communication between the Client and Gateway is carried via a tunnel
   to which any intervening entity cannot gain access, even if the IP
   datagrams are carried via this node. Conversely, if a "hop-by-hop"
   approach is used, then any intervening PINT proxies (or redirect
   servers) are, by implication, trusted entities.

   However, if there is any doubt that there is an underlying network or
   transport layer security association in place, then the players in a
   PINT protocol exchange MUST use encryption and authentication
   techniques within the protocol itself. The techniques described in
   section 15 of RFC2543 MUST be used, unless there is an alternative
   protection scheme that is agreed between the parties. In either case,
   the content of any message body (or bodies) carried within a PINT
   request or response MUST be protected; this has implications on the
   options for routing requests via Proxies (see 5.3).

   Using SIP techniques for protection, the Request-URI and To: fields
   headers within PINT requests cannot be protected. In  the baseline
   PINT services these fields may contain sensitive information. This is
   a consideration, and if these data ARE considered sensitive, then
   this will preclude the sole use of SIP techniques; in such a
   situation, transport [12] or network layer [13] protection mechanisms
   MUST be used.

   As a final point, this choice will in turn have an influence on the
   choice of transport layer protocol that can be used; if a TLS
   association is available between two nodes, then TCP will have to be
   used. This is different from the default behaviour of SIP (try UDP,
   then try TCP if that fails).






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6. Deployment considerations and the Relationship PINT to I.N.
   (Informative)

6.1. Web Front End to PINT Infrastructure

   It is possible that some other protocol may be used to communicate a
   Requesting User's requirements. Due to the high numbers of available
   Web Browsers and servers it seems likely that some PINT systems will
   use HTML/HTTP as a "front end". In this scenario, HTTP will be used
   over a connection from the Requesting User's Web Browser (WC) to an
   Intermediate Web Server (WS). This will be closely associated with a
   PINT Client (using some unspecified mechanism to transfer the data
   from the Web Server to the PINT Client). The PINT Client will
   represent the Requesting User to the PINT Gateway, and thus to the
   Executive System that carries out the required action.

    [WC]------[WS]
              [PC]
                \
                 \
                [PG]
                [XS]

                Figure 2: Basic "Web-fronted" Configuration

6.2. Redirects to Multiple Gateways

   It is quite possible that a given PINT Gateway is associated with an
   Executive System (or systems) that can connect to the GSTN at
   different places. Equally, if there is a chain of PINT Servers, then
   each of these intermediate or proxy servers (PP) may be able to route
   PINT requests to Executive Systems that connect at specific points to
   the GSTN. The result of this is that there may be more than one PINT
   Gateway or Executive System that can deal with a given request. The
   mechanisms by which the choice on where to deliver a request are
   outside the scope of this document.

    [WC]------[WS]                 [WC]------[WS]
              [PC]                           [PC]
                \                              \
                 \                              \
                [PG]                           [PP]
       .........[XS].........                  /  \
       :                    :                 /    \
                                           [PG]    [PG]
                                           [XS]    [XS]

                 Figure 3: Multiple Access Configurations



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   However, there do seem to be two approaches. Either a Server that
   acts as a proxy or redirect will select the appropriate Gateway
   itself and will cause the request to be sent on accordingly, or a
   list of possible Locations will be returned to the Requesting User
   from which they can select their choice.

   In SIP, the implication is that, if a proxy cannot resolve to a
   single unique match for a request destination, then a response
   containing a list of the choices should be returned to the Requesting
   User for selection. This is not too likely a scenario within the
   normal use of SIP.

   However, within PINT, such ambiguity may be quite common; it implies
   that there are a number of possible providers of a given service.

6.3. Competing PINT Gateways REGISTERing to offer the same service

   With PINT, the registration is not for an individual but instead for
   a service that can be handled by a service provider. Thus, one can
   envisage a registration by the PINT Server of the domain telcoA.com
   of its ability to support the service R2C as "R2C@telcoA.com", sent
   to an intermediary server that acts as registrar for the
   "broker.telcos.com" domain from "R2C@pint.telcoA.com" as follows:

         REGISTER sip:registrar@broker.telcos.com SIP/2.0
         To: sip:R2C@pint.telcoA.com
         From: sip:R2C@pint.telcoA.com
         ...

   This is the standard SIP registration service.

   However, what happens if there are a number of different Service
   Providers, all of whom support the "R2C" service? Suppose there is a
   PINT system at domain "broker.com". PINT clients requesting a
   Request-to-Call service from broker.com might be very willing to be
   redirected or proxied to any one of the various service providers
   that had previously registered with the registrar. PINT servers might
   also be interested in providing service for requests that did not
   specify the service provider explicitly, as well as those requests
   that were directed "at them".

   To enable such service, PINT servers would REGISTER at the broker
   PINT server registrations of the form:

         REGISTER sip:registrar@broker.com SIP/2.0
         To: sip:R2C@broker.com
         From: sip:R2C@pint.telcoA.com




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   When several such REGISTER messages appear at the registrar, each
   differing only in the URL in the From: line, the registrar has many
   possibilities, e.g.:

   (i)  it overwrites the prior registration for "R2C@broker.telcos.com"
        when the next comes in;

   (ii) it rejects the subsequent registration for
        "R2C@broker.telcos.com";

   (iii) it maintains all such registrations.

   In this last case, on receiving an Invitation for the "general"
   service, either:

       (iii.1) it passes on the invitation to all registered service
               providers, returning a collated response with all
               acceptances, using multiple Location: headers,
   or
       (iii.2) it silently selects one of the registrations (using, for
               example, a "round robin" approach) and routes the Invitation
               and response onwards without further comment.

   As an alternative to all of the above approaches, it:

   (iv) may choose to not allow registrations for the "general" service,
        rejecting all such REGISTER requests.

   The algorithm by which such a choice is made will be implementation-
   dependent, and is outside the scope of PINT. Where a behaviour is to
   be defined by requesting users, then some sort of call processing
   language might be used to allow those clients, as a pre-service
   operation, to download the behaviour they expect to the server making
   such decisions. This, however, is a topic for other protocols, not
   for PINT.

6.4. Limitations on Available Information and Request Timing for
     SUBSCRIBE

   A reference configuration for PINT is that service requests are sent,
   via a PINT Gateway, to an Executive System that fulfills the Service
   Control Function (SCF) of an Intelligent Network (see [11]). The
   success or failure of the resulting service call may be information
   available to the SCF and so may potentially be made available to the
   PINT Gateway. In terms of historical record of whether or not a
   service succeeded, a large SCF may be dealing with a million call
   attempts per hour. Given that volume of service transactions, there




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   are finite limits beyond which it cannot store service disposition
   records; expecting to find out if a Fax was sent last month from a
   busy SCF is unrealistic.

   Other status changes, such as that on completion of a successful
   service call, require the SCF to arrange monitoring of the service
   call in a way that the service may not do normally, for performance
   reasons. In most implementations, it is difficult efficiently to
   interrupt a service to change it once it has begun execution, so it
   may be necessary to have two different services; one that sets GSTN
   resources to monitor service call termination, and one that doesn't.
   It is unlikely to be possible to decide that monitoring is required
   once the service has started.

   These factors can have implications both on the information that is
   potentially available at the PINT Gateway, and when a request to
   register interest in the status of a PINT service can succeed. The
   alternative to using a general SCF is to provide a dedicated Service
   Node just for PINT services. As this node is involved in placing all
   service calls, it is in a position to collect the information needed.
   However, it may well still not be able to respond successfully to a
   registration of interest in call state changes once a service logic
   program instance is running.

   Thus, although a Requesting User may register an interest in the
   status of a service request, the PINT Gateway may not be in a
   position to comply with that request. Although this does not affect
   the protocol used between the Requestor and the PINT Gateway, it may
   influence the response returned. To avoid the problem of changing
   service logic once running, any registration of interest in status
   changes should be made at or before the time at which the service
   request is made.

   Conversely, if a historical request is made on the disposition of a
   service, this should be done within a short time after the service
   has completed; the Executive System is unlikely to store the results
   of service requests for long; these will have been processed as AMA
   (Automatic Message Accounting) records quickly, after which the
   Executive System has no reason to keep them, and so they may be
   discarded.

   Where the PINT Gateway and the Executive System are intimately
   linked, the Gateway can respond to status subscription requests that
   occur while a service is running. It may accept these requests and
   simply not even try to query the Executive System until it has
   information that a service has completed, merely returning the final
   status. Thus the PINT Requestor may be in what it believes is a
   monitoring state, whilst the PINT Gateway has not even informed the



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   Executive System that a request has been made. This will increase the
   internal complexity of the PINT Gateway in that it will have a
   complex set of interlocking state machines, but does mean that status
   registration and indication CAN be provided in conjunction with an
   I.N. system.

6.5. Parameters needed for invoking traditional GSTN Services within
     PINT

   This section describes how parameters needed to specify certain
   traditional GSTN services can be carried within PINT requests.

6.5.1. Service Identifier

   When a Requesting User asks for a service to be performed, he or she
   will, of course, have to specify in some way which service. This can
   be done in the URLs within the To: header and the Request-URI (see
   section 3.5.5.1).

6.5.2. A and B parties

   With the Request-to-Call service, they will also need to specify the
   A and B parties they want to be engaged in the resulting service
   call. The A party could identify, for example, the Call Center from
   which they want a call back, whilst the B party is their telephone
   number (i.e. who the Call Center agent is to call).

   The Request-to-Fax and Request-to-Hear-Content services require the B
   party to be specified (respectively the telephone number of the
   destination Fax machine or the telephone to which spoken content is
   to be delivered), but the A party is a Telephone Network based
   resource (either a Fax or speech transcoder/sender), and is implicit;
   the Requesting User does not (and cannot) specify it.

   With the "Fax-Back" variant of the Request-to-Fax service, (i.e.
   where the content to be delivered resides on the GSTN) they will also
   have specify two parties. As before, the B party is the telephone
   number of the fax machine to which they want a fax to be sent.
   However, within this variant the A party identifies the "document
   context" for the GSTN-based document store from which a particular
   document is to be retrieved; the analogy here is to a GSTN user
   dialling a particular telephone number and then entering the document
   number to be returned using "touch tone" digits. The telephone number
   they dial is that of the document store or A party, with the "touch
   tone" digits selecting the document within that store.






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6.5.3. Other Service Parameters

   In terms of the extra parameters to the request, the services again
   differ. The Request-to-Call service needs only the A and B parties.
   Also it is convenient to assert that the resulting service call will
   carry voice, as the Executive System within the destination GSTN may
   be able to check that assertion against the A and B party numbers
   specified and may treat the call differently.

   With the Request-to-Fax and Request-to-Hear-Content services, the
   source information to be transcoded is held on the Internet. That
   means either that this information is carried along with the request
   itself, or that a reference to the source of this information is
   given.

   In addition, it is convenient to assert that the service call will
   carry fax or voice, and, where possible, to specify the format for
   the source information.

   The GSTN-based content or "Fax-Back" variant of the Request-to-Fax
   service needs to specify the Document Store number and the Fax
   machine number to which the information is to be delivered. It is
   convenient to assert that the call will carry Fax data, as the
   destination Executive System may be able to check that assertion
   against the document store number and that of the destination Fax
   machine.

   In addition, the document number may also need to be sent. This
   parameter is an opaque reference that is carried through the Internet
   but has significance only within the GSTN. The document store number
   and document number together uniquely specify the actual content to
   be faxed.

6.5.4. Service Parameter Summary

   The following table summarises the information needed in order to
   specify fully the intent of a GSTN service request. Note that it
   excludes any other parameters (such as authentication or
   authorisation tokens, or Expires: or CallId: headers) that may be
   used in a request.

Service   ServiceID   AParty    BParty   CallFmt    Source   SourceFmt
-------   ---------   ------    ------   -------    ------    -------
  R2C         x         x         x       voice       -          -
  R2F         x         -         x        fax      URI/IL    ISF/ILSF
  R2FB        x         x         x        fax        OR         -
  R2HC        x         -         x       voice     URI/IL    ISF/ILSF




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   In this table, "x" means that the parameter is required, whilst "-"
   means that the parameter is not required.

   The Services listed are Request-to-Call (R2C), Request-to-Fax (R2F),
   the GSTN-based content or "Fax-back" Variant of Request-to-Fax
   (R2FB), and Request-to-Hear-Content (R2HC).

   The Call Format parameter values "voice" or "fax" indicate the kind
   of service call that results.

   The Source Indicator "URI/IL" implies that the information is either
   an Internet source reference (a Universal Resource Identifier, or
   URI) or is carried "in-line" with the message. The Source indicator
   "OR" means that the value passed is an Opaque Reference that should
   be carried along with the rest of the message but is to be
   interpreted only within the destination (GSTN) context. As an
   alternative, it could be given as a "local" reference with the "file"
   style, or even using a partial reference with the "http" style.
   However, the way in which such a reference is interpreted is a matter
   for the receiving PINT Server and Executive System; it remains, in
   effect, an opaque reference.

   The Source Format value "ISF/ILSF" means that the format of the
   source is specified either in terms of the URI or that it is carried
   "in-line".  Note that, for some data, the format either can be
   detected by inspection or, if all else fails, can be assumed from the
   URI (for example, by assuming that the file extension part of a URL
   indicates the data type). For an opaque reference, the Source Format
   is not available on the Internet, and so is not given.

6.6. Parameter Mapping to PINT Extensions

   This section describes the way in which the parameters needed to
   specify a GSTN service request fully might be carried within a "PINT
   extended" message. There are other choices, and these are not
   precluded. However, in order to ensure that the Requesting User
   receives the service that they expect, it is necessary to have some
   shared understanding of the parameters passed and the behaviour
   expected of the PINT Server and its attendant Executive System.

   The Service Identifier can be sent as the userinfo element of the
   Request-URI. Thus, the first line of a PINT Invitation would be of
   the form:

         INVITE <serviceID>@<pint-server>.<domain>  SIP/2.0






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   The A Party for the Request-to-Call and "Fax-back" variant of
   Request-to-Fax service can be held in the "To:" header field. In this
   case the "To:" header value will be different from the Request-URI.
   In the services where the A party is not specified, the "To:" field
   is free to repeat the value held in the Request-URI. This is the case
   for Request-to-Fax and Request-to-Hear-Content services.

   The B party is needed in all these milestone services, and can be
   held in the enclosed SDP sub-part, as the value of the "c=" field.

   The call format parameter can be held as part of the "m=" field
   value.  It maps to the "transport protocol" element as described in
   section 3.4.2 of this document.

   The source format specifier is held in the "m=", as a type and either
   "-" or sub-type. The latter is normally required for all services
   except Request-to-Call or "Faxback", where the "-" form may be used.
   As shown earlier, the source format and source are not always
   required when generating requests for services. However, the
   inclusion in all requests of a source format specifier can make
   parsing the request simpler and allows for other services to be
   specified in the future, and so values are always given. The source
   format parameter is covered in section 3.4.2 as the "media type"
   element.

   The source itself is identified by an "a=fmtp:" field value, where
   needed. With the exception of the Request-to-Call service, all
   invitations will normally include such a field. From the perspective
   of the SDP extensions, it can be considered as qualifying the media
   sub-type, as if to say, for example, "when I say jpeg, what I mean is
   the following".

   In summary, the parameters needed by the different services are
   carried in fields as shown in the following table:

Service   Svc Param    PINT/SIP or SDP field used      Example value
-------   ---------    --------------------------      -------------
  R2C
          ServiceID:   <SIP Request-URI userinfo>      R2C
          AParty:      <SIP To: field>                 sip:123@p.com
          BParty:      <SDP c= field>                  TN RFC2543 4567
          CallFormat:  <SDP transport protocol
                            sub-field of m= field>     voice
          SourceFmt:   <SDP media type sub-field
                            of m= field>               audio
                       (--- only "-" sub-type
                            sub-field value used)      ---
          Source:      (--- No source specified)       ---



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  R2F
          ServiceID:   <SIP Request-URI userinfo>      R2F
          AParty:      (--- SIP To: field not used) sip:R2F@pint.xxx.net
          BParty:      <SDP c= field>               TN RFCxxx +441213553
          CallFormat:  <SDP transport protocol
                            sub-field of m= field>     fax
          SourceFmt:   <SDP media type sub-field
                            of m= field>               image
                       <SDP media sub-type sub-field
                            of m= field>               jpeg
          Source:      <SDP a=fmtp: field qualifying
                            preceding m= field>    a=fmtp:jpeg<uri-ref>

  R2FB
          ServiceID:   <SIP Request-URI userinfo>      R2FB
          AParty:      <SIP To: field>              sip:1-730-1234@p.com
          BParty:      <SDP c= field>               TN RFCxxx +441213553
          CallFormat:  <SDP transport protocol
                            sub-field of m= field>     fax
          SourceFmt:   <SDP media type sub-field
                            of m= field>               image
                       <SDP media sub-type sub-field
                            of m= field>               jpeg
          Source:      <SDP a=fmtp: field qualifying
                            preceding m= field>     a=fmtp:jpeg opr:1234

  R2HC
          ServiceID:   <SIP Request-URI userinfo>      R2HC
          AParty:      (--- SIP To: field not used) sip:R2HC@pint.ita.il
          BParty:      <SDP c= field>               TN RFCxxx +441213554
          CallFormat:  <SDP transport protocol
                            sub-field of m= field>     voice
          SourceFmt:   <SDP media type sub-field
                            of m= field>               text
                       <SDP media sub-type sub-field
                            of m= field>               html
          Source:      <SDP a=fmtp: field qualifying
                            preceding m= field>     a=fmtp:html<uri-ref>

7. References

   [1]  Handley, M., Schooler, E., Schulzrinne, H. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [2]  Handley, M. and  V. Jacobsen, "SDP: Session Description
        Protocol", RFC 2327, April 1998.





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   [3]  Freed, N. and  N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [4]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November
        1996.

   [5]  The Unicode Consortium, "The Unicode Standard -- Version 2.0",
        Addison-Wesley, 1996.

   [6]  ITU-T Study Group 2, "E.164 - The International Public Network
        Numbering Plan", ITU-T, June 1997.

   [7]  Lu, H., Krishnaswamy, M., Conroy, L., Bellovin, S., Burg, F.,
        DeSimone, A., Tewani, K., Davidson, P., Schulzrinne, H. and K.
        Vishwanathan "Toward the PSTN/Internet Inter-Networking--Pre-
        PINT Implementations", RFC 2458, November 1998.

   [8]  ITU-T Study Group XI, "Q.763 - Formats and Codes for the ISDN
        User Part of SS No7" ITU-T, August 1994.

   [9]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

   [10] Crocker, D., "Standard for the format of ARPA Internet text
        messages", STD 11, RFC 822, August 1982.

   [11] ITU-T Study Group XI, "Q.1204 - IN Distributed Functional Plane
        Architecture", ITU-T, February 1994.

   [12] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

   [13] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [14] Housley, R., Ford, W., Polk W. and D. Solo, "Internet X.509
        Public Key Infrastructure Certificate and CRL Profile", RFC
        2459, January 1999.

   [15] Crocker, D. and P. Overall, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [16] Mills, D., "Network Time Protocol (version 3) specification and
        implementation", RFC 1305, March 1992.





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   [17] Eastlake, D., Crocker, S. and J.Schiller, "Randomness
        Recommendations for Security", RFC 1750, December 1994.

   [18] Mockapetris, P., "Domain Names - Implementation and
        Specification", STD 13, RFC 1035, November 1987.

   [19] Levinson, E., "The MIME Multipart/Related Content-type" RFC
        2387, August 1998.

8. Acknowledgements

   The authors wish to thank the members of the PINT working group for
   comments that were helpful to the preparation of this specification.
   Ian Elz's comments were extremely useful to our understanding of
   internal PSTN operations. The SUBSCRIBE and NOTIFY requests were
   first suggested by Henning Schulzrinne and Jonathan Rosenberg. The
   suggestion to use an audio port of 0 to express that the phone is "on
   hold" (i.e. not receiving voice) is due to Ray Zibman. Finally,
   thanks to Bernie Hoeneisen for his close proofreading.
































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Appendix A: Collected ABNF for PINT Extensions

;; --(ABNF is specified in RFC 2234 [15])

;; --Variations on SDP definitions

connection-field    = ["c=" nettype space addrtype space
                        connection-address CRLF]
; -- this is the original definition from SDP, included for completeness
; -- the following are PINT interpretations and modifications

nettype = ("IN"/"TN")
; -- redefined as a superset of the SDP definition

addrtype = (INAddrType / TNAddrType)
; -- redefined as a superset of the SDP definition

INAddrType = ("IP4"/"IP6")
; -- this non-terminal added to hold original SDP address types

TNAddrType = ("RFC2543"/OtherAddrType)

OtherAddrType = (<X-Token>)
; -- X-token is as defined in RFC2045

addr = (<FQDN> / <unicast-address> / TNAddr)
; -- redefined as a superset of the original SDP definition
; -- FQDN and unicast address as specified in SDP

TNAddr = (RFC2543Addr/OtherAddr)
; -- TNAddr defined only in context of nettype == "TN"

RFC2543Addr = (INPAddr/LDPAddr)

INPAddr = "+" <POS-DIGIT> 0*(("-" <DIGIT>)/<DIGIT>)
; -- POS-DIGIT and DIGIT as defined in SDP

LDPAddr = <DIGIT> 0*(("-" <DIGIT>)/<DIGIT>)

OtherAddr = 1*<uric>
; -- OtherAdd defined in the context of OtherAddrType
; -- uric is as defined in RFC2396

media-field = "m=" media <space> port <space> proto
                   1*(<space> fmt) <CRLF>
; -- NOTE redefined as subset/relaxation of original SDP definition
; -- space and CRLF as defined in SDP




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media = ("application"/"audio"/"image"/"text")
; -- NOTE redefined as a subset of the original SDP definition
; -- This could be any MIME discrete type; Only those listed are
; --  used in PINT 1.0

port = ("0" / "1")
; -- NOTE redefined from the original SDP definition;
; -- 0 retains usual sdp meaning of "temporarily no media"
; -- (i.e. "line is on hold")
; -- (1 means there is media)

proto = (INProto/TNProto)
; -- redefined as a superset of the original SDP definition

INProto = 1* (<alpha-numeric>)
; -- this is the "classic" SDP protocol, defined if nettype == "IN"
; -- alpha-numeric is as defined in SDP
TNProto = ("voice"/"fax"/"pager")
; -- this is the PINT protocol, defined if nettype == "TN"

fmt = (<subtype> / "-")
; -- NOTE redefined as a subset of the original SDP definition
; -- subtype as defined in RFC2046, or "-". MUST be a subtype of type
held
; --  in associated media sub-field or the special value "-".

attribute-fields = *("a=" attribute-list <CRLF>)
; -- redefined as a superset of the definition given in SDP
; -- CRLF is as defined in SDP

attribute-list = 1(PINT-attribute / <attribute>)
; -- attribute is as defined in SDP

PINT-attribute = (clir-attribute / q763-nature-attribute /
                   q763plan-attribute / q763-INN-attribute /
                   phone-context-attribute / tsp-attribute /
                   pint-fmtp-attribute / strict-attribute)

clir-attribute = clir-tag ":" ("true" / "false")

clir-tag = "clir"

q763-nature-attribute = Q763-nature-tag ":" q763-natures

q763-nature-tag = "Q763-nature"

q763-natures = ("1" / "2" / "3" / "4")




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q763-plan-attribute = Q763-plan-tag ":" q763-plans

q763-plan-tag = "Q763-plan"

q763-plans = ("1" / "2" / "3" / "4" / "5" / "6" / "7")
; -- of these, the meanings of 1, 3, and 4 are defined in the text

q763-INN-attribute = Q763-INN-tag ":" q763-INNs

q763-INN-tag = "Q763-INN"

q763-INNs = ("0" / "1")

phone-context-attribute = phone-context-tag ":" phone-context-ident

phone-context-tag = "phone-context"

phone-context-ident = network-prefix / private-prefix

network-prefix = intl-network-prefix / local-network-prefix

intl-network-prefix = "+" 1*<DIGIT>

local-network-prefix = 1*<DIGIT>

private-prefix = 1*excldigandplus 0*<uric>

excldigandplus = (0x21-0x2d,0x2f,0x40-0x7d))
tsp-attribute = tsp-tag "=" provider-domainname

tsp-tag = "tsp"

provider-domainname = <domain>
; -- domain is defined in RFC1035

; -- NOTE the following is redefined relative to the normal use in SDP
pint-fmtp-attribute = "fmtp:" <subtype> <space> resolution
                      *(<space> resolution)
                      (<space> ";" 1(<attribute>) *(<space>
<attribute>))
; -- subtype as defined in RFC2046.
; -- NOTE that this value MUST match a fmt on the ultimately preceeding
; --  media-field
; -- attribute is as defined in SDP

resolution = (uri-ref / opaque-ref / sub-part-ref)

uri-ref = uri-tag ":" <URI-Reference>



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; -- URI-Reference defined in RFC2396

uritag = "uri"

opaque-ref = opr-tag ":" 0*<uric>

opr-tag = "opr"

sub-part-ref = spr-tag ":" <Content-ID>
; -- Content-ID is as defined in RFC2046 and RFC822

spr-tag = "spr"

strict-attribute = "require:" att-tag-list

att-tag-list = 1(PINT-att-tag-list / <att-field> /
                    pint-fmtp-tag-list)
                  *(","
                    (PINT-att-tag-list / <att-field> /
                      pint-fmtp-tag-list)
                   )
; -- att-field as defined in SDP

PINT-att-tag-list = (phone-context-tag / clir-tag /
                       q763-nature-tag / q763-plan-tag /
                       q763-INN-tag)

pint-fmtp-tag-list = (uri-tag / opr-tag / spr-tag)

;; --Variations on SIP definitions

clir-parameter = clir-tag "=" ("true" / "false")

q763-nature-parameter = Q763-nature-tag "=" Q763-natures

q763plan-parameter = Q763-plan-tag "=" q763plans

q763-INN-parameter = Q763-INN-tag "=" q763-INNs

tsp-parameter = tsp-tag "=" provider-domainname

phone-context-parameter = phone-context-tag "=" phone-context-ident

SIP-param = ( <transport-param> / <user-param> / <method-param> /
                <ttl-param> / <maddr-param> / <other-param> )
; -- the values in this list are all as defined in SIP

PINT-param = ( clir-parameter / q763-nature-parameter /



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                q763plan-parameter / q763-INN-parameter/
                tsp-parameter / phone-context-parameter )

URL-parameter = (SIP-param / PINT-param)
; -- redefined SIP's URL-parameter to include ones defined in PINT

Require-header = "require:" 1(required-extensions)
                             *("," required-extensions)
; -- NOTE this is redefined as a subset of the SIP definition
; -- (from RFC2543/section 6.30)

required-extensions = ("org.ietf.sip.subscribe" /
                       "org.ietf.sdp.require")

Appendix B: IANA Considerations

   There are three kinds of identifier used in PINT extensions that
   SHOULD be registered with IANA, if a new value is specified. These
   are:

   *  Media Format sub-types, as described in section 3.4.2 of this
      document.
   *  Private Attributes as mentioned in section 3.4.3
   *  Private Phone Context values, as described in section 3.4.3.1.

   It should be noted that private Address Types (in section 3.4.1) have
   been explicitly excluded from this process, as they must be in the
   form of an X-Token.

B.1. Media Format Sub-types

   Taking these in turn, the media format sub-types are used within the
   PINT extensions to SDP to specify the attribute line that holds the
   data source definitions. In normal use, the values in this field are
   sub-types of MIME discrete types[4]. If a value other than an IANA-
   registered sub-type is to be used, then it should either be an X-
   Token (i.e. start with "X-") or it should be registered with IANA. if
   the intention is to describe a new MIME sub-type, then the procedures
   specified in RFC 2048 should be used. It is ASSUMED that any new MIME
   sub-type would follow the syntactic rules for interpretation of
   associated PINT fmtp lines defined in this document.

   Note that, in keeping with the SDP description, such registrations
   SHOULD include the "proto" field values within which they are
   defined; however, it is appropriate to specify only that they can be
   used with "all values of TNProto".





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   Conversely, if the intent is to define a new way of including data
   source definitions within PINT, then it will be necessary to specify,
   in the documentation supporting any such new "PINT Media Format Sub-
   type" registration, the syntax of the associated "fmtp" attribute
   line, as the identifier serves to indicate the interpretation that
   should be made of format specific attribute lines "tagged" with such
   a sub-type.

   If the fmtp interpretation follows the PINT default, then it is
   adequate to mention this in the defining document rather than
   repeating the syntax definition given here (although, in this case,
   it is unclear why such a new registration would be required). As
   before, the Media Format sub-type SHOULD specify the values of
   "proto" field within which it is defined, but this can be "all values
   of TNProto".

B.2. Private Attributes

   Any proprietary attribute lines that are added may be registered with
   IANA using the procedures mentioned in [2]; the mechanism is the same
   as that used in SDP. If the attribute is defined for use only within
   PINT, then it may be appropriate to mention this in the supporting
   documentation. Note that, in the PINT 1.0 specification covered here,
   there is no mechanism to add such freshly registered attribute lines
   to a "require:" clause.

B.3. Private phone-contexts

   Within the session description used for PINT requests, a phone-
   context attribute may be used to specify the prefix or context within
   which an associated telephone-number (in a connection line) should be
   interpreted.

   For "public" phone contexts the prefix to be used MUST start with
   either a DIGIT or a "+". Private phone contexts may be registered
   with IANA that do NOT start with either of these characters. Such a
   prefix may be useful to identify a private network, potentially with
   an associated numeric ID (see example 4 in section 3.4.3.1). In the
   example, the prefix acts as the context for X-acme.com's private
   network numbering plan.

   It is recommended that any private context to be registered have the
   general form of a token including a domain name, optionally followed
   by a digit string or other token. The appropriate form of the initial
   token name space will be similar to that used for private or vendor
   registrations for sub-types (e.g. vnd.acme.com). However, note that
   the registration will be used to specify a customer's private network
   numbering plan format rather than being used generally for all of



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   their equipment vendor's customer's; thus, fbi.gov would be
   appropriate, but lucent.com would not (unless the private network
   were to be that used by Lucent internally).

   In addition, the supporting documentation MUST either declare that
   there is no associated token, or define the syntax by which that
   token can be parsed (e.g. vnd.fbi.gov <space> 1*DIGIT). Note that the
   registration describes a format, not a value range; it is sufficient
   that the private context can be parsed, without the value being
   interpreted.

   In detail, the registration request SHOULD include:

   *  Kind of registration (i.e. private phone-context attribute to be
      used within the service description of PINT service requests)
   *  Contact details for the person responsible for the registration
      request (name, organisation, e-mail address, public telephone
      number)
   *  Private Prefix initial token name (e.g. vnd.fbi.gov)
   *  syntax for private context (e.g. "vnd.fbi.gov" <space> 1*DIGIT, or
      "vnd.gtn.gov.uk")
   *  Description of use (e.g. "This phone context declares an
      associated telephone number to be within the 'government
      telecommunications network'; the number is in an internal or
      private number plan form)
   *  Network Type and Address Type with which this private context is
      associated; If the "normal" telephone types (as specified in this
      document) are used, then the values would be shown as:
      "nettype=TN" , addrtype="RFC2543Addr". If, however, this context
      were to be used with another address type, then a reference to
      that address type name and the syntax of that address value would
      be required.

   In short, this context is the telephone equivalent of a "Net 10"
   address space behind a NAT, and the initial name (and contact
   information) shows the context within which that address is valid. It
   also specifies the format for the network and address types (and
   address value syntax) with which this context is associated.

   Of course, IANA may refer the requested registration to the IESG or
   an appropriate IETF working group for review, and may require
   revisions to be made before the registration is accepted.









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Authors' Addresses

   Scott Petrack
   MetaTel, Inc.
   45 Rumford Ave.
   Waltham MA 02453-3844

   Phone: +1 (781)-891-9000
   EMail: scott.petrack@metatel.com


   Lawrence Conroy
   Siemens Roke Manor Research
   Roke Manor
   Old Salisbury Lane
   Romsey, Hampshire
   U.K.    SO51 0ZN

   Phone: +44 (1794) 833666
   EMail: lwc@roke.co.uk































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Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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