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RFC 2848


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

Part 2 of 4, p. 8 to 37
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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

   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

                           /\/\/\/\/\/\/\            /\/\/\/\/\/\/\/\
___________                \          __/___      ___\_             \
|  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

   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

   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

      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
      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
      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

   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.


       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-".  An example of such a connection line is as

         c= TN  A*8-HELEN

   where "" 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

      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

   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]). 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>))
       <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. 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

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

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

       <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: 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. 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-

   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
      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 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). 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

   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

   Example 1:

         c= TN   RFC2543  1-800-765-4321

   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

   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

   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

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

   Note that if the PINT server receiving the request is inside the 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 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. 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

   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:


   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

         Content-Type: application/sdp
         c= TN RFC2543 +1-201-406-4090
         m= text 1 pager plain

         Content-Type: text/plain

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


   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  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. 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

   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

   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

   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

Top      Up      ToC       Page 30 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

   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.

         NOTIFY SIP/2.0
         CSeq: 4711 SUBSCRIBE
         Warning: xxx  fax aborted, will try for the next hour.

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

   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. 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:

   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)


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

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.

Top      Up      ToC       Page 33 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
      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/2.0
         INVITE; SIP/2.0
         INVITE; SIP/2.0
         INVITE 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

   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

   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

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

         i=3 of 5 pages sent OK
         c=TN  RFC2543  +12014064090
         m=image 1 fax tif
         a=fmtp:tif uri:

   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

   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

(page 37 continued on part 3)

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