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 Petrack & Conroy Standards Track [Page 1] RFC 2848 The PINT Service Protocol June 2000 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 Petrack & Conroy Standards Track [Page 2] RFC 2848 The PINT Service Protocol June 2000 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 Petrack & Conroy Standards Track [Page 3] RFC 2848 The PINT Service Protocol June 2000 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. Petrack & Conroy Standards Track [Page 4] RFC 2848 The PINT Service Protocol June 2000 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. Petrack & Conroy Standards Track [Page 5] RFC 2848 The PINT Service Protocol June 2000 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: Petrack & Conroy Standards Track [Page 6] RFC 2848 The PINT Service Protocol June 2000 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 Petrack & Conroy Standards Track [Page 7] RFC 2848 The PINT Service Protocol June 2000 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. Petrack & Conroy Standards Track [Page 8] RFC 2848 The PINT Service Protocol June 2000 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 Petrack & Conroy Standards Track [Page 9] RFC 2848 The PINT Service Protocol June 2000 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) Petrack & Conroy Standards Track [Page 10] RFC 2848 The PINT Service Protocol June 2000 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. Petrack & Conroy Standards Track [Page 11] RFC 2848 The PINT Service Protocol June 2000 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-". Petrack & Conroy Standards Track [Page 12] RFC 2848 The PINT Service Protocol June 2000 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 Petrack & Conroy Standards Track [Page 13] RFC 2848 The PINT Service Protocol June 2000 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. Petrack & Conroy Standards Track [Page 14] RFC 2848 The PINT Service Protocol June 2000 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 , 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 -" 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:" resolution *( resolution) ( ";" 1() *( )) where: := ( | | ) Petrack & Conroy Standards Track [Page 15] RFC 2848 The PINT Service Protocol June 2000 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:" URI-reference) where URI-reference is as defined in Appendix A of [9] Petrack & Conroy Standards Track [Page 16] RFC 2848 The PINT Service Protocol June 2000 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: := ("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 Petrack & Conroy Standards Track [Page 17] RFC 2848 The PINT Service Protocol June 2000 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. Petrack & Conroy Standards Track [Page 18] RFC 2848 The PINT Service Protocol June 2000 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: := ("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: The 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 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: Petrack & Conroy Standards Track [Page 19] RFC 2848 The PINT Service Protocol June 2000 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 so