| Rfc | 2848 |
|---|
| Title | The PINT Service Protocol: Extensions to SIP and SDP for IP Access
to Telephone Call Services |
|---|
| Author | S. Petrack, L. Conroy |
|---|
| Date | June 2000 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
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
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
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
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.
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.
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:
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
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.
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
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)
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.
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-".
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
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.
Note that this sub-field does not necessarily dictate the media type
and subtype of any source data; for example, one of the milestone
services calls for a textual source to be vocoded and spoken in a
resulting telephone service call. The transport protocol value in
this case would be "voice", whilst the media type would be "text".
The Fmt sub-field is described in [2] as being transport protocol-
specific. When used within PINT requests having one of the above
protocol values, this sub-field consists of a list of one or more
values, each of which is a defined MIME sub-type of the associated
Media sub-field value. The special value "-" is allowed, meaning that
there is no MIME sub-type. This sub-field retains (from [2]) its
meaning that the list will contain a set of alternative sub-types,
with the first being the preferred value.
For experimental purposes and by mutual consent of the sender and
recipient, a sub-type value may be specified as an <X-token>, i.e. a
character string starting with "X-". The use of such values is
discouraged, and if such a value is expected to find common use then
it SHOULD be registered with IANA using the standard content type
registration process (see Appendix C).
When the Fmt parameter is the single character "-" ( a dash ), this
is interpreted as meaning that a unspecified or default sub-type can
be used for this service. Thus, the media field value "m=audio 1
voice -<CRLF>" is taken to mean that a voice call is requested, using
whatever audio sub type is deemed appropriate by the Executive
System. PINT service is a special case, in that the request comes
from the IP network but the service call is provided within the GSTN.
Thus the service request will not normally be able to define the
particular codec used for the resulting GSTN service call. If such an
intent IS required, then the quality attribute may be used (see
"Suggested Attributes" section of [2]).
3.4.2.1. Use of fmtp attributes in PINT requests
For each element of the Fmt sub-field, there MUST be a following fmtp
attribute. When used within PINT requests, the fmtp attribute has a
general structure as defined here:
"a=fmtp:" <subtype> <space> resolution
*(<space> resolution)
(<space> ";" 1(<attribute>)
*(<space> <attribute>))
where:
<resolution> := (<uri-ref> | <opaque-ref> | <sub-part-ref>)
A fmtp attribute describes the sources used with a given Fmt entry in
the Media field. The entries in a Fmt sub-field are alternatives
(with the preferred one first in the list). Each entry will have a
matching fmtp attribute. The list of resolutions in a fmtp attribute
describes the set of sources that resolve the matching Fmt choice;
all elements of this set will be used.
It should be noted that, for use in PINT services, the elements in
such a set will be sent as a sequence; it is unlikely that trying to
send them in parallel would be successful.
A fmtp attribute can contain a mixture of different kinds of element.
Thus an attribute might contain a sub-part-ref indicating included
data held in a sub-part of the current message, followed by an
opaque-ref referring to some content on the GSTN, followed by a uri-
ref pointing to some data held externally on the IP network.
To indicate which form each resolution element takes, each of them
starts with its own literal tag. The detailed syntax of each form is
described in the following sub-sections.
3.4.2.2. Support for Remote Data Object References in PINT
Where data objects stored elsewhere on the IP Network are to be used
as sources for processing within a PINT service, they may be referred
to using the uri-ref form. This is simply a Uniform Resource
Identifier (URI), as described in [9].
Note that the reference SHOULD be an absolute URI, as there may not
be enough contextual information for the recipient server to resolve
a relative reference; any use of relative references requires some
private agreement between the sender and recipient of the message,
and SHOULD be avoided unless the sender can be sure that the
recipient is the one intended and the reference is unambiguous in
context.
This also holds for partial URIs (such
as"uri:http://aNode/index.htm") as these will need to be resolved in
the context of the eventual recipient of the message.
The general syntax of a reference to an Internet-based external data
object in a fmtp line within a PINT session description is:
<uri-ref> := ("uri:" URI-reference)
where URI-reference is as defined in Appendix A of [9]
For example:
c= TN RFC2543 +1-201-406-4090
m= text 1 fax plain
a=fmtp:plain uri:ftp://ftp.isi.edu/in-notes/rfc2468.txt
or:
c= TN RFC2543 +1-201-406-4090
m= text 1 fax plain
a=fmtp:plain
uri:http://www.ietf.org/meetings/glance_minneapolis.txt
means get this data object from the Internet and use it as a source
for the requested GSTN Fax service.
3.4.2.3. Support for GSTN-based Data Objects in PINT
PINT services may refer to data that are held not on the IP Network
but instead within the GSTN. The way in which these items are
indicated need have no meaning within the context of the Requestor or
the PINT Gateway; the reference is merely some data that may be used
by the Executive System to indicate the content intended as part of
the request. These data form an opaque reference, in that they are
sent "untouched" through the PINT infrastructure.
A reference to some data object held on the GSTN has the general
definition:
<opaque-ref> := ("opr:" *uric)
where uric is as defined in Appendix A of [9].
For example:
c= TN RFC2543 +1-201-406-4090
m= text 1 fax plain
a=fmtp:plain opr:APPL.123.456
means send the data that is indexed ON THE GSTN by the reference
value "APPL.123.456" to the fax machine on +1-201-406-4090. The
Executive System may also take the Telephone URL held in the To:
field of the enclosing SIP message into account when deciding the
context to be used for the data object dereference.
Of course, an opaque reference may also be used for other purposes;
it could, for example, be needed to authorise access to a document
held on the GSTN rather than being required merely to disambiguate
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.
Format specific attribute lines within the session description are
used to indicate which other MIME part within the request contains
the content data. Instead of a URI or opaque reference, the format-
specific attribute indicates the Content-ID of the MIME part of the
request that contains the actual data, and is defined as:
<sub-part-ref> := ("spr:" Content-ID)
where Content-ID is as defined in Appendix A of [3] and in [10]).
For example:
c= TN RFC2543 +1-201-406-4090
m= text 1 fax plain
a=fmtp:plain spr:<Content-ID>
The <Content-ID> parameter is the Content-ID of one of the MIME parts
inside the message, and this fragment means that the requesting user
would like the data object held in the sub-part of this message
labelled <Content-ID> to be faxed to the machine at phone number +1-
201-406-4090.
See also section 3.5.1 for a discussion on the support needed in the
enclosing SIP request for included data objects.
3.4.3. Attribute Tags to pass information into the Telephone Network
It may be desired to include within the PINT request service
parameters that can be understood only by some entity in the
"Telephone Network Cloud". SDP attribute parameters are used for this
purpose. They MAY appear within a particular media description or
outside of a media description.
These attributes may also appear as parameters within PINT URLS (see
section 3.5.6) as part of a SIP request.
This is necessary so that telephone terminals that require the
attributes to be defined can appear within the To: line of a PINT
request as well as within PINT session descriptions.
The purpose of these attributes is to allow the client to specify
extra context within which a particular telephone number is to be
interpreted. There are many reasons why extra context might be
necessary to interpret a given telephone number:
a. The telephone number might be reachable in many different ways
(such as via competing telephone service providers), and the
PINT client wishes to indicate its selection of service
provider.
b. The telephone number might be reachable only from a limited
number of networks (such as an '800' freephone number).
c. The telephone number might be reachable only within a single
telephone network (such as the '152' customer service number of
BT). Similarly, the number might be an internal corporate
extension reachable only within the PBX.
However, as noted above, it is not usually necessary to use SDP
attributes to specify the phone context. URLs such as
152@pint.bt.co.il within the To: and From: headers and/or Request-
URI, normally offer sufficient context to resolve telephone numbers.
If the client wishes the request to fail if the attributes are not
supported, these attributes SHOULD be used in conjunction with the
"require" attribute (section 3.4.4) and the
"Require:org.ietf.sdp.require" header (section 3.5.4).
It is not possible to standardise every possible internal telephone
network parameter. PINT 1.0 attributes have been chosen for
specification because they are common enough that many different PINT
systems will want to use them, and therefore interoperability will be
increased by having a single specification.
Proprietary attribute "a=" lines, that by definition are not
interoperable, may be nonetheless useful when it is necessary to
transport some proprietary internal telephone network variables over
the IP network, for example to identify the order in which service
call legs are to be be made. These private attributes SHOULD BE,
however, subject to the same IANA registration procedures mentioned
in the SDP specification[2] (see also this Appendix C).
3.4.3.1. The phone-context attribute
An attribute is specified to enable "remote local dialling". This is
the service that allows a PINT client to reach a number from far
outside the area or network that can usually reach the number. It is
useful when the sending or receiving address is only dialable within
some local context, which may be remote to the origin of the PINT
client.
For example, if Alice wanted to report a problem with her telephone,
she might then dial a "network wide" customer care number; within the
British Telecom network in the U.K., this is "152". Note that in this
case she doesn't dial any trunk prefix - this is the whole dialable
number. If dialled from another operator's network, it will not
connect to British Telecom's Engineering Enquiries service; and
dialling "+44 152" will not normally succeed. Such numbers are called
Network-Specific Service Numbers.
Within the telephone network, the "local context" is provided by the
physical connection between the subscriber's terminal and the central
office. An analogous association between the PINT client and the PINT
server that first receives the request may not exist, which is why it
may be necessary to supply this missing "telephone network context".
This attribute is defined as follows:
a=phone-context: <phone-context-ident>
phone-context-ident = network-prefix / private-prefix
network-prefix = intl-network-prefix / local-network-prefix
intl-network-prefix = "+" 1*DIGIT
local-network-prefix = 1*DIGIT
excldigandplus = (0x21-0x2d,0x2f,0x40-0x7d))
private-prefix = 1*excldigandplus 0*uric
An intl-network-prefix and local-network-prefix MUST be a bona fide
network prefix, and a network-prefix that is an intl-network-prefix
MUST begin with an E.164 service code ("country code").
It is possible to register new private-prefixes with IANA so as to
avoid collisions. Prefixes that are not so registered MUST begin with
an "X-" to indicate their private, non-standard nature (see Appendix
C).
Example 1:
c= TN RFC2543 1-800-765-4321
a=phone-context:+972
This describes an terminal whose address in Israel (E.164 country
code 972) is 1-800-765-4321.
Example 2:
c= TN RFC2543 1-800-765-4321
a=phone-context:+1
This describes an terminal whose address in North America (E.164
country code 1) is 1-800-765-4321.
The two telephone terminals described by examples 1 and 2 are
different; in fact they are located in different countries.
Example 3:
c=TN RFC2543 123
a=phone-context:+97252
This describes a terminal whose address when dialled from within the
network identified by +97252 is the string "123". It so happens that
+97252 defines one of the Israeli cell phone providers, and 123
reaches customer service when dialled within that network.
It may well be useful or necessary to use the SDP "require" parameter
in conjunction with the phone-context attribute.
Example 4:
c= TN RFC2543 321
a=phone-context:X-acme.com-23
This might describe the telephone terminal that is at extension 321
of PBX number 23 within the acme.com private PBX network. It is
expected that such a description would be understandable by the
acme.com PINT server that receives the request.
Note that if the PINT server receiving the request is inside the
acme.com network, the same terminal might be addressable as follows:
c= TN RFC2543 7-23-321
(assuming that "7" is dialled in order to reach the private PBX
network from within acme.com)
3.4.3.2. Presentation Restriction attribute
Although it has no affect on the transport of the service request
through the IP Network, there may be a requirement to allow
originators of a PINT service request to indicate whether or not they
wish the "B party" in the resulting service call to be presented with
the "A party's" calling telephone number. It is a legal requirement
in some jurisdictions that a caller be able to select whether or not
their correspondent can find out the calling telephone number (using
Automatic Number Indication or Caller Display or Calling Line
Identity Presentation equipment). Thus an attribute may be needed to
indicate the originator's preference.
Whether or not the default behaviour of the Executive System is to
present or not present a party's telephone number to the
correspondent GSTN terminal is not specified, and it is not mandatory
in all territories for a PINT Gateway or Executive System to act on
this attribute. It is, however, defined here for use where there are
regulatory restrictions on GSTN operation, and in that case the
Executive System can use it to honour the originator's request.
The attribute is specified as follows:
a=clir:<"true" | "false">
This boolean value is needed within the attribute as it may be that
the GSTN address is, by default, set to NOT present its identity to
correspondents, and the originator wants to do so for this particular
call. It is in keeping with the aim of this attribute to allow the
originator to specify what treatment they want for the requested
service call.
The expected interpretation of this attribute is that, if it is
present and the value is "false" then the Calling Line Identity CAN
be presented to the correspondent terminal, whilst if it is "true"
then if possible the Executive System is requested to NOT present the
Calling Line Identity.
3.4.3.3. ITU-T CalledPartyAddress attributes parameters
These attributes correspond to fields that appear within the ITU-T
Q.763 "CalledPartyAddress" field (see [8] ,section 3.9). PINT clients
use these attributes in order to specify further parameters relating
to Terminal Addresses, in the case when the address indicates a
"local-phone-number". In the case that the PINT request contains a
reference to a GSTN terminal, the parameters may be required to
correctly identify that remote terminal.
The general form of this attribute is: "a=Q763-<token>((":" <value>)
|"")". Three of the possible elements and their use in SDP
attributes are described here. Where other Q763 elements are to be
used, then these should be the subject of further specification to
define the syntax of the attribute mapping. It is recommended that
any such specification maintains the value sets shown in Q.763.
The defined attributes are:
a=Q763-nature: - indicates the "nature of address indicator".
The value MAY be any number between 0 and 127.
The following values are specified:
"1" a subscriber number
"2" unknown
"3" a nationally significant number
"4" an internationally significant number
The values have been chosen to coincide with the values in Q.763.
Note that other values are possible, according to national rules or
future expansion of Q.763.
a=Q763-plan: - indicates the numbering plan to which the address
belongs. The value MAY be any number between 0
and 7. The following values are specified:
"1" Telephone numbering plan (ITU-T E.164)
"3" Data numbering plan (ITU-T X.121)
"4" Telex numbering plan (ITU-T F.69)
The values have been chosen to coincide with the values in Q.763.
Other values are allowed, according to national rules or future
expansion of Q.763.
a=Q763-INN - indicates if routing to the Internal Network Number
is allowed. The value MUST be ONE of:
"0" routing to internal network number allowed
"1" routing to internal network number not
allowed
The values have been chosen to coincide with the values in Q.763.
Note that it is possible to use a local-phone-number and indicate via
attributes that the number is in fact an internationally significant
E.164 number. Normally this SHOULD NOT be done; an internationally
significant E.164 number is indicated by using a "global-phone-
number" for the address string.
3.4.4. The "require" attribute
According to the SDP specification, a PINT server is allowed simply
to ignore attribute parameters that it does not understand. In order
to force a server to decline a request if it does not understand one
of the PINT attributes, a client SHOULD use the "require" attribute,
specified as follows:
a=require:<attribute-list>
where the attribute-list is a comma-separated list of attributes that
appear elsewhere in the session description.
In order to process the request successfully the PINT server must
BOTH understand the attribute AND ALSO fulfill the request implied by
the presence of the attribute, for each attribute appearing within
the attribute-list of the require attribute.
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.
Where included data is carried within a PINT service request, the
Content Type entity header of the enclosing SIP message MUST indicate
this. To do so, the media type value within this entity header MUST
be set to a value of "multipart". There is a content sub-type that is
intended for situations like this in which sub-parts are to be
handled together. This is the multipart/related type (defined in
[19]), and it's use is recommended.
The enclosed body parts SHOULD include the part-specific Content Type
headers as appropriate ("application/sdp" for the first body part
holding the session description, with an appropriate content type for
each of the subsequent, "included data object" parts). This matches
the standard syntax of MIME multipart messages as defined in [4].
For example, in a multipart message where the string
"------next-------" is the boundary, the first two parts might be as
follows:
------next-------
Content-Type: application/sdp
....
c= TN RFC2543 +1-201-406-4090
m= text 1 pager plain
a=fmtp:plain spr:17@mymessage.acme.com
----------next-------
Content-Type: text/plain
Content-ID: 17@mymessage.acme.com
This is the text that is to be paged to +1-201-406-4090
----------next-----------
The ability to indicate different alternatives for the content to be
transported is useful, even when the alternatives are included within
the request. For example, a request to send a short message to a
pager might include the message in Unicode [5] and an alternative
version of the same content in text/plain, should the PINT server or
telephone network not be able to process the unicode.
PINT clients should be extremely careful when sending included data
within a PINT request. Such requests SHOULD be sent via TCP, to avoid
fragmentation and to transmit the data reliably. It is possible that
the PINT server is a proxy server that will replicate and fork the
request, which could be disastrous if the request contains a large
amount of application data. PINT proxy servers should be careful not
to create many copies of a request with large amounts of data in it.
If the client does not know the actual location of the PINT gateway,
and is using the SIP location services to find it, and the included
data makes the PINT request likely to be transported in several IP
datagrams, it is RECOMMENDED that the initial PINT request not
include the data object but instead hold a reference to it.
3.5.2. Warning header
A PINT server MUST support the SIP "Warning:" header so that it can
signal lack of support for individual PINT features. As an example,
suppose the PINT request is to send a jpeg picture to a fax machine,
but the server cannot retrieve and/or translate jpeg pictures from
the Internet into fax transmissions.
In such a case the server fails the request and includes a Warning
such as the following:
Warning: 305 pint.acme.com Incompatible media format: jpeg
SIP servers that do not understand the PINT extensions at all are
strongly encouraged to implement Warning: headers to indicate that
PINT extensions are not understood.
Also, Warning: headers may be included within NOTIFY requests if it
is necessary to notify the client about some condition concerning the
invocation of the PINT service (see next).
3.5.3. Mechanism to register interest in the disposition of a PINT
service, and to receive indications on that disposition
It can be very useful to find out whether or not a requested service
has completed, and if so whether or not it was successful. This is
especially true for PINT service, where the person requesting the
service is not (necessarily) a party to it, and so may not have an
easy way of finding out the disposition of that service. Equally, it
may be useful to indicate when the service has changed state, for
example when the service call has started.
Arranging a flexible system to provide extensive monitoring and
control during a service is non-trivial (see section 6.4 for some
issues); PINT 1.0 uses a simple scheme that should nevertheless
provide useful information. It is possible to expand the scheme in a
"backwards compatible" manner, so if required it can be enhanced at a
later date.
The PINT 1.0 status registration and indication scheme uses three new
methods; SUBSCRIBE, UNSUBSCRIBE, and NOTIFY. These are used to allow
a PINT client to register an interest in (or "subscribe" to) the
status of a service request, to indicate that a prior interest has
lapsed (i.e "unsubscribe" from the status), and for the server to
return service indications. The state machine of
SUBSCRIBE/UNSUBSCRIBE is identical to that of INVITE/BYE; just as
INVITE signals the beginning and BYE signals the end of participation
in a media session, SUBSCRIBE signals the beginning and UNSUBSCRIBE
signals the end of participation in a monitoring session. During the
monitoring session, NOTIFY messages are sent to inform the subscriber
of a change in session state or disposition.
3.5.3.1. Opening a monitoring session with a SUBSCRIBE request
When a SUBSCRIBE request is sent to a PINT Server, it indicates that
a user wishes to receive information about the status of a service
session. The request identifies the session of interest by including
the original session description along with the request, using the
SDP global-session-id that forms part of the origin-field to identify
the service session uniquely.
The SUBSCRIBE request (like any other SIP request about an ongoing
session) is sent to the same server as was sent the original INVITE,
or to a server which was specified in the Contact: field within a
subsequent response (this might well be the PINT gateway for the
session).
Whilst there are situations in which re-use of the Call-ID used in
the original INVITE that initiated the session of interest is
possible, there are other situations in which it is not. In detail,
where the subscription is being made by the user who initiated the
original service request, the Call-ID may be used as it will be known
to the receiver to refer to a previously established session.
However, when the request comes from a user other than the original
requesting user, the SUBSCRIBE request constitutes a new SIP call
leg, so the Call-ID SHOULD NOT be used; the only common identifier is
the origin-field of the session description enclosed within the
original service request, and so this MUST be used.
Rather than have two different methods of identifying the "session of
interest" the choice is to use the origin-field of the SDP sub-part
included both in the original INVITE and in this SUBSCRIBE request.
Note that the request MUST NOT include any sub-parts other than the
session description, even if these others were present in the
original INVITE request. A server MUST ignore whatever sub-parts are
included within a SUBSCRIBE request with the sole exception of the
enclosed session description.
The request MAY contain a "Contact:" header, specifying the PINT User
Agent Server to which such information should be sent.
In addition, it SHOULD contain an Expires: header, which indicates
for how long the PINT Requestor wishes to receive notification of the
session status. We refer to the period of time before the expiration
of the SUBSCRIBE request as the "subscription period". See section
5.1.4. for security considerations, particularly privacy
implications.
A value of 0 within the Expires: header indicates a desire to receive
one single immediate response (i.e. the request expires immediately).
It is possible for a sequence of monitoring sessions to be opened,
exist, and complete, all relating to the same service session.
A successful response to the SUBSCRIBE request includes the session
description, according to the Gateway. Normally this will be
identical to the last cached response that the Gateway returned to
any request concerning the same SDP global session id (see [2],
section 6, o= field). The t= line may be altered to indicate the
actual start or stop time, however. The Gateway might add an i= line
to the session description to indicate such information as how many
fax pages were sent. The Gateway SHOULD include an Expires: header
indicating how long it is willing to maintain the monitoring session.
If this is unacceptable to the PINT Requestor, then it can close the
session by sending an immediate UNSUBSCRIBE message (see 3.5.3.3).
In principle, a user might send a SUBSCRIBE request after the
telephone network service has completed. This allows, for example,
checking up "the morning after" to see if the fax was successfully
transmitted. However, a PINT gateway is only required to keep state
about a call for as long as it indicated previously in an Expires:
header sent within the response to the original INVITE message that
triggered the service session, within the response to the SUBSCRIBE
message, within the response to any UNSUBSCRIBE message, or within
its own UNSUBSCRIBE message (but see section 3.5.8, point 3).
If the Server no longer has a record of the session to which a
Requestor has SUBSCRIBEd, it returns "606 Not Acceptable", along with
the appropriate Warning: 307 header indicating that the SDP session
ID is no longer valid. This means that a requesting Client that knows
that it will want information about the status of a session after the
session terminates SHOULD send a SUBSCRIBE request before the session
terminates.
3.5.3.2. Sending Status Indications with a NOTIFY request
During the subscription period, the Gateway may, from time to time,
send a spontaneous NOTIFY request to the entity indicated in the
Contact: header of the "opening" SUBSCRIBE request. Normally this
will happen as a result of any change in the status of the service
session for which the Requestor has subscribed.
The receiving user agent server MUST acknowledge this by returning a
final response (normally a "200 OK"). In this version of the PINT
extensions, the Gateway is not required to support redirects (3xx
codes), and so may treat them as a failure.
Thus, if the response code class is above 2xx then this may be
treated by the Gateway as a failure of the monitoring session, and in
that situation it will immediately attempt to close the session (see
next).
The NOTIFY request contains the modified session description. For
example, the Gateway may be able to indicate a more accurate start or
stop time.
The Gateway may include a Warning: header to describe some problem
with the invocation of the service, and may indicate within an i=
line some information about the telephone network session itself.
Example:
NOTIFY sip:petrack@pager.com SIP/2.0
To: sip:petrack@pager.com
From: sip:R2F.pint.com@service.com
Call-ID: 19971205T234505.56.78@pager.com
CSeq: 4711 SUBSCRIBE
Warning: xxx fax aborted, will try for the next hour.
Content-Type:application/sdp
c=...
i=3 pages of 5 sent
t=...
3.5.3.3. Closing a monitoring session with an UNSUBSCRIBE request
At some point, either the Client's representative User Agent Server
or the Gateway may decide to terminate the monitoring session. This
is achieved by sending an UNSUBSCRIBE request to the correspondent
server. Such a request indicates that the sender intends to close
the monitoring session immediately, and, on receipt of the final
response from the receiving server, the session is deemed over.
Note that unlike the SUBSCRIBE request, which is never sent by a PINT
gateway, an UNSUBSCRIBE request can be sent by a PINT gateway to the
User Agent Server to indicate that the monitoring session is closed.
(This is analogous to the fact that a gateway never sends an INVITE,
although it can send a BYE to indicate that a telephone call has
ended.)
If the Gateway initiates closure of the monitoring session by sending
an UNSUBSCRIBE message, it SHOULD include an "Expires:" header
showing for how much longer after this monitoring session is closed
it is willing to store information on the service session. This acts
as a minimum time within which the Client can send a new SUBSCRIBE
message to open another monitoring session; after the time indicated
in the Expires: header the Gateway is free to dispose of any record
of the service session, so that subsequent SUBSCRIBE requests can be
rejected with a "606" response.
If the subscription period specified by the Client has expired, then
the Gateway may send an immediate UNSUBSCRIBE request to the Client's
representative User Agent Server. This ensures that the monitoring
session always completes with a UNSUBSCRIBE/response exchange, and
that the representative User Agent Server can avoid maintaining state
in certain circumstances.
3.5.3.4. Timing of SUBSCRIBE requests
As it relies on the Gateway having a copy of the INVITEd session
description, the SUBSCRIBE message is limited in when it can be
issued. The Gateway must have received the service request to which
this monitoring session is to be associated, which from the Client's
perspective happens as soon as the Gateway has sent a 1xx response
back to it.
However, once this has been done, there is no reason why the Client
should not send a monitoring request. It does not have to wait for
the final response from the Gateway, and it can certainly send the
SUBSCRIBE request before sending the ACK for the Service request
final response. Beyond this point, the Client is free to send a
SUBSCRIBE request when it decides, unless the Gateway's final
response to the initial service request indicated a short Expires:
time.
However, there are good reasons (see 6.4) why it may be appropriate
to start a monitoring session immediately before the service is
confirmed by the PINT Client sending an ACK. At this point the
Gateway will have decided whether or not it can handle the service
request, but will not have passed the request on to the Executive
System. It is therefore in a good position to ask the Executive
System to enable monitoring when it sends the service request
onwards. In practical implementations, it is likely that more
information on transient service status will be available if this is
indicated as being important BEFORE or AS the service execution phase
starts; once execution has begun the level of information that can be
returned may be difficult to change.
Thus, whilst it is free to send a SUBSCRIBE request at any point
after receiving an Interim response from the Gateway to its service
request, it is recommended that the Client should send such a
monitoring request immediately prior to sending an ACK message
confirming the service if it is interested in transient service
status messages.
3.5.4. The "Require:" header for PINT
PINT clients use the Require: header to signal to the PINT server
that a certain PINT extension of SIP is required. PINT 1.0 defines
two strings that can go into the Require header:
org.ietf.sip.subscribe -- the server can fulfill SUBSCRIBE requests
and associated methods (see section 3.5.3)
org.ietf.sdp.require -- the PINT server (or the SDP parser
associated to it) understands the "require"
attribute defined in (section 3.4.4)
Example:
Require:org.ietf.sip.subscribe,org.ietf.sdp.require
A client SHOULD only include a Require: header where it truly
requires the server to reject the request if the option is not
supported.
3.5.5. PINT URLs within PINT requests
Normally the hostnames and domain names that appear in the PINT URLs
are the internal affair of each individual PINT system. A client uses
the appropriate SDP payload to indicate the particular service it
wishes to invoke; it is not necessary to use a particular URL to
identify the service.
A PINT URL is used in two different ways within PINT requests: within
the Request-URI, and within the To: and From: headers. Use within the
Request-URI requires clarification in order to ensure smooth
interworking with the Telephone Network serviced by the PINT
infrastructure, and this is covered next.
3.5.5.1. PINT URLS within Request-URIs
There are some occasions when it may be useful to indicate service
information within the URL in a standardized way:
a. it may not be possible to use SDP information to route the
request if it is encrypted;
b. it allows implementation that make use of I.N. "service
indicators";
c. It enables multiple competing PINT gateways to REGISTER with a
single "broker" server (proxy or redirect) (see section 6.3)
For these reasons, the following conventions for URLs are offered for
use in PINT requests:
1. The user portion of a sip URL indicates the service to be
requested. At present the following services are defined:
R2C (for Request-to-Call)
R2F (for Request-to-Fax)
R2HC (for Request-to-Hear-Content)
The user portions "R2C", "R2F", and "R2HC" are reserved for the PINT
milestone services. Other user portions MUST be used in case the
requested service is not one of the Milestone services. See section
6.2 for some related considerations concerning registrations by
competing PINT systems to a single PINT proxy server acting as a
service broker.
2. The host portion of a sip URL contains the domain name of the PINT
service provider.
3. A new url-parameter is defined to be "tsp" (for "telephone service
provider"). This can be used to indicate the actual telephone network
provider to be used to fulfill the PINT request.
Thus, for example:-
INVITE sip:R2C@pint.pintservice.com SIP/2.0
INVITE sip:R2F@pint.pintservice.com;tsp=telco.com SIP/2.0
INVITE sip:R2HC@pint.mycom.com;tsp=pbx23.mycom.com SIP/2.0
INVITE sip:13@pint.telco.com SIP/2.0
3.5.6. Telephony Network Parameters within PINT URLs
Any legal SIP URL can appear as a PINT URL within the Request-URI or
To: header of a PINT request. But if the address is a telephone
address, we indicated in section 3.4.3 that it may be necessary to
include more information in order correctly to identify the remote
telephone terminal or service. PINT clients MAY include these
attribute tags within PINT URLs if they are necessary or a useful
complement to the telephone number within the SIP URL. These
attribute tags MUST be included as URL parameters as defined in [1]
(i.e. in the semi-colon separated manner).
The following is an example of a PINT URL containing extra attribute
tags:
sip:+9725228808@pint.br.com;user=phone;require=Q763-plan;a=Q763-plan:4
As we noted in section 3.4.3, these extra attribute parameters will
not normally be needed within a URL, because there is a great deal of
context available to help the server interpret the phone number
correctly. In particular, there is the SIP URL within the To: header,
and there is also the Request-URI. In most cases this provides
sufficient information for the telephone network.
The SDP attributes defined in section 3 above will normally only be
used when they are needed to supply necessary context to identify a
telephone terminal.
3.5.7. REGISTER requests within PINT
A PINT gateway is a SIP user agent server. A User Agent Server uses
the REGISTER request to tell a proxy or redirect server that it is
available to "receive calls" (i.e. to service requests). Thus a PINT
Gateway registers with a proxy or redirect server the service that is
accessible via itself, whilst in SIP, a user is registering his/her
presence at a particular SIP Server.
There may be competing PINT servers that can offer the same PINT
service trying to register at a single PINT server. The PINT server
might act as a "broker" among the various PINT gateways that can
fulfill a request. A format for PINT URLs was specified in section
3.5.5 that enables independent PINT systems to REGISTER an offer to
provide the same service. The registrar can apply its own mechanisms
and policies to decide how to respond to INVITEs from clients seeking
service (See section 6.3 for some possible deployment options). There
is no change between SIP and PINT REGISTER semantics or syntax.
Of course, the information in the PINT URLs within the REGISTER
request may not be sufficient to completely define the service that a
gateway can offer. The use of SIP and SDP within PINT REGISTER
requests to enable a gateway to specify in more detail the services
it can offer is the subject of future study.
3.5.8. BYE Requests in PINT
The semantics of BYE requests within PINT requires some extra
precision. One issue concerns conferences that "cannot be left", and
the other concerns keeping call state after the BYE.
The BYE request [1] is normally used to indicate that the originating
entity no longer wishes to be involved in the specified call. The
request terminates the call and the media session. Applying this
model to PINT, if a PINT client makes a request that results in
invocation of a telephone call from A to B, a BYE request from the
client, if accepted, should result in a termination of the phone
call.
One might expect this to be the case if the telephone call has not
started when the BYE request is received. For example, if a request
to fax is sent with a t= line indicating that the fax is to be sent
tomorrow at 4 AM, the requestor might wish to cancel the request
before the specified time.
However, even if the call has yet to start, it may not be possible to
terminate the media session on the telephone system side. For
example, the fax call may be in progress when the BYE arrives, and
perhaps it is just not possible to cancel the fax in session. Another
possibility is that the entire telephone-side service might be
completed before the BYE is received. In the above Request-to-Fax
example, the BYE might be sent the following morning, and the entire
fax has been sent before the BYE was received. It is too late to send
the BYE.
In the case where the telephone network cannot terminate the call,
the server MUST return a "606 Not Acceptable" response to the BYE,
along with a session description that indicates the telephone network
session that is causing the problem.
Thus, in PINT, a "Not Acceptable" response MAY be returned both to
INVITE and BYE requests. It indicates that some aspect of the session
description makes the request unacceptable.
By allowing a server to return a "Not Acceptable" response to BYE
requests, we are not changing its semantics, just enlarging its use.
A combination of Warning: headers and i= lines within the session
description can be used to indicate the precise nature of the
problem.
Example:
SIP/2.0 606 Not Acceptable
From: ...
To: .......
.....
Warning: 399 pint.mycom.com Fax in progress, service cannot be
aborted
Content-Type: application/sdp
Content-Length: ...
v=0
...
...
i=3 of 5 pages sent OK
c=TN RFC2543 +12014064090
m=image 1 fax tif
a=fmtp:tif uri:http://tifsRus.com/yyyyyy.tif
Note that the server might return an updated session description
within a successful response to a BYE as well. This can be used, for
example, to indicate the actual start times and stop times of the
telephone session, or how many pages were sent in the fax
transmission.
The second issue concerns how long must a server keep call state
after receiving a BYE. A question arises because other clients might
still wish to send queries about the telephone network session that
was the subject of the PINT transaction. Ordinary SIP semantics have
three important implications for this situation:
1. A BYE indicates that the requesting client will clear out all call
state as soon as it receives a successful response. A client SHOULD
NOT send a SUBSCRIBE request after it has sent a BYE.
2. A server may return an Expires: header within a successful
response to a BYE request. This indicates for how long the server
will retain session state about the telephone network session. At any
point during this time, a client may send a SUBSCRIBE request to the
server to learn about the session state (although as explained in the
previous paragraph, a client that has sent a BYE will not normally
send a SUBSCRIBE).
3. When engaged in a SUBSCRIBE/NOTIFY monitoring session, PINT
servers that send UNSUBSCRIBE to a URL listed in the Contact: header
of a client request SHOULD not clear session state until after the
successful response to the UNSUBSCRIBE message is received. For
example, it may be that the requesting client host is turned off (or
in a low power mode) when the telephone service is executed (and is
therefore not available at the location previously specified in the
Contact: attribute) to receive the PINT server's UNSUBSCRIBE. Of
course, it is possible that the UNSUBSCRIBE request will simply time
out.
4. Examples of PINT Requests and Responses
4.1. A request to a call center from an anonymous user to receive a
phone call.
C->S: INVITE sip:R2C@pint.mailorder.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:anon-1827631872@chinet.net
To: sip:+1-201-456-7890@iron.org;user=phone
Call-ID: 19971205T234505.56.78@pager.com
CSeq: 4711 INVITE
Subject: Sale on Ironing Boards
Content-type: application/sdp
Content-Length: 174
v=0
o=- 2353687637 2353687637 IN IP4 128.3.4.5
s=R2C
i=Ironing Board Promotion
e=anon-1827631872@chinet.net
t=2353687637 0
m=audio 1 voice -
c=TN RFC2543 +1-201-406-4090
In this example, the context that is required to interpret the To:
address as a telephone number is not given explicitly; it is
implicitly known to the R2C@pint.mailorder.com server. But the
telephone of the person who wishes to receive the call is explicitly
identified as an internationally significant E.164 number that falls
within the North American numbering plan (because of the "+1" within
the c= line).
4.2. A request from a non anonymous customer (John Jones) to receive a
phone call from a particular sales agent (Mary James) concerning
the defective ironing board that was purchased
C->S: INVITE sip:marketing@pint.mailorder.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:john.jones.3@chinet.net
To: sip:mary.james@mailorder.com
Call-ID: 19971205T234505.56.78@pager.com
CSeq: 4712 INVITE
Subject: Defective Ironing Board - want refund
Content-type: application/sdp
Content-Length: 150
v=0
o=- 2353687640 2353687640 IN IP4 128.3.4.5
s=marketing
e=john.jones.3@chinet.net
c= TN RFC2543 +1-201-406-4090
t=2353687640 0
m=audio 1 voice -
The To: line might include the Mary James's phone number instead of a
email-like address. An implementation that cannot accept email-like
URLs in the "To:" header must decline the request with a 606 Not
Acceptable. Note that the sending PINT client "knows" that the PINT
Gateway contacted with the "marketing@pint.mailorder.com" Request-URI
is capable of processing the client request as expected. (see 3.5.5.1
for a discussion on this).
Note also that such a telephone call service could be implemented on
the phone side with different details. For example, it might be that
first the agent's phone rings, and then the customer's phone rings,
or it might be that first the customer's phone rings and he hears
silly music until the agent comes on line. If necessary, such service
parameter details might be indicated in "a=" attribute lines within
the session description. The specification of such attribute lines
for service consistency is beyond the scope of the PINT 1.0
specifications.
4.3. A request from the same user to get a fax back on how to assemble
the Ironing Board
C->S: INVITE sip:faxback@pint.mailorder.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:john.jones.3@chinet.net
To: sip:1-800-3292225@steam.edu;user=phone;phone-context=+1
Call-ID: 19971205T234505.66.79@chinet.net
CSeq: 4713 INVITE
Content-type: application/sdp
Content-Length: 218
v=0
o=- 2353687660 2353687660 IN IP4 128.3.4.5
s=faxback
e=john.jones.3@chinet.net
t=2353687660 0
m=application 1 fax URI
c=TN RFC2543 1-201-406-4091
a=fmtp:URI uri:http://localstore/Products/IroningBoards/2344.html
In this example, the fax to be sent is stored on some local server
(localstore), whose name may be only resolvable, or that may only be
reachable, from within the IP network on which the PINT server sits.
The phone number to be dialled is a "local phone number" as well.
There is no "phone-context" attribute, so the context (in this case,
for which nation the number is "nationally significant") must be
supplied by the faxback@pint.mailorder.com PINT server.
If the server that receives it does not understand the number, it
SHOULD decline the request and include a "Network Address Not
Understood" warning. Note that no "require" attribute was used here,
since it is very likely that the request can be serviced even by a
server that does not support the "require" attribute.
4.4. A request from same user to have that same information read out
over the phone
C->S: INVITE sip:faxback@pint.mailorder.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:john.jones.3@chinet.net
To: sip:1-800-3292225@steam.edu;user=phone;phone-context=+1
Call-ID: 19971205T234505.66.79@chinet.net
CSeq: 4713 INVITE
Content-type: application/sdp
Content-Length: 220
v=0
o=- 2353687660 2353687660 IN IP4 128.3.4.5
s=faxback
e=john.jones.3@chinet.net
t=2353687660 0
m=application 1 voice URI
c=TN RFC2543 1-201-406-4090
a=fmtp:URI uri:http://localstore/Products/IroningBoards/2344.html
4.5. A request to send an included text page to a friend's pager.
In this example, the text to be paged out is included in the request.
C->S: INVITE sip:R2F@pint.pager.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:scott.petrack@chinet.net
To: sip:R2F@pint.pager.com
Call-ID: 19974505.66.79@chinet.net
CSeq: 4714 INVITE
Content-Type: multipart/related; boundary=--next
----next
Content-Type: application/sdp
Content-Length: 236
v=0
o=- 2353687680 2353687680 IN IP4 128.3.4.5
s=R2F
e=scott.petrack@chinet.net
t=2353687680 0
m=text 1 pager plain
c= TN RFC2543 +972-9-956-1867
a=fmtp:plain spr:2@53655768
----next
Content-Type: text/plain
Content-ID: 2@53655768
Content-Length:50
Hi Joe! Please call me asap at 555-1234.
----next--
4.6. A request to send an image as a fax to phone number +972-9-956-1867
C->S: INVITE sip:faxserver@pint.vocaltec.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:scott.petrack@chinet.net
To: sip:faxserver@pint.vocaltec.com
Call-ID: 19971205T234505.66.79@chinet.net
CSeq: 4715 INVITE
Content-type: application/sdp
Content-Length: 267
v=0
o=- 2353687700 2353687700 IN IP4 128.3.4.5
s=faxserver
e=scott.petrack@chinet.net
t=2353687700 0
m=image 1 fax tif gif
c= TN RFC2543 +972-9-956-1867
a=fmtp:tif uri:http://petrack/images/tif/picture1.tif
a=fmtp:gif uri:http://petrack/images/gif/picture1.gif
The image is available as tif or as gif. The tif is the preferred
format. Note that the http server where the pictures reside is local,
and the PINT server is also local (because it can resolve machine
name "petrack")
4.7. A request to read out over the phone two pieces of content in
sequence.
First some included text is read out by text-to-speech. Then some
text that is stored at some URI on the internet is read out.
C->S: INVITE sip:R2HC@pint.acme.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
From: sip:scott.petrack@chinet.net
To: sip:R2HC@pint.acme.com
Call-ID: 19974505.66.79@chinet.net
CSeq: 4716 INVITE
Content-Type: multipart/related; boundary=next
--next
Content-Type: application/sdp
Content-Length: 316
v=0
o=- 2353687720 2353687720 IN IP4 128.3.4.5
s=R2HC
e=scott.petrack@chinet.net
c= TN RFC2543 +1-201-406-4091
t=2353687720 0
m=text 1 voice plain
a=fmtp:plain spr:2@53655768
m=text 1 voice plain
a=fmtp:plain uri:http://www.your.com/texts/stuff.doc
--next
Content-Type: text/plain
Content-ID: 2@53655768
Content-Length: 172
Hello!! I am about to read out to you the document you
requested, "uri:http://www.your.com/texts/stuff.doc".
We hope you like acme.com's new speech synthesis server.
--next--
4.8. Request for the prices for ISDN to be sent to my fax machine
INVITE sip:R2FB@pint.bt.co.uk SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:0345-12347-01@pint.bt.co.uk;user=phone;phone-context=+44
From: sip:hank.wangford@newts.demon.co.uk
Call-ID: 19981204T201505.56.78@demon.co.uk
CSeq: 4716 INVITE
Subject: Price List
Content-type: application/sdp
Content-Length: 169
v=0
o=- 2353687740 2353687740 IN IP4 128.3.4.5
s=R2FB
i=ISDN Price List
e=hank.wangford@newts.demon.co.uk
t=2353687740 0
m=text 1 fax -
c=TN RFC2543 +44-1794-8331010
4.9. Request for a callback
INVITE sip:R2C@pint.bt.co.uk SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:0345-123456@pint.bt.co.uk;user=phone;phone-context=+44
From: sip:hank.wangford@newts.demon.co.uk
Call-ID: 19981204T234505.56.78@demon.co.uk
CSeq: 4717 INVITE
Subject: It costs HOW much?
Content-type: application/sdp
Content-Length: 176
v=0
o=- 2353687760 2353687760 IN IP4 128.3.4.5
s=R2C
i=ISDN pre-sales query
e=hank.wangford@newts.demon.co.uk
c=TN RFC2543 +44-1794-8331013
t=2353687760 0
m=audio 1 voice -
4.10. Sending a set of information in response to an enquiry
INVITE sip:R2FB@pint.bt.co.uk SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:0345-12347-01@pint.bt.co.uk;user=phone;phone-context=+44
From: sip:colin.masterton@sales.hh.bt.co.uk
Call-ID: 19981205T234505.56.78@sales.hh.bt.co.uk
CSeq: 1147 INVITE
Subject: Price Info, as requested
Content-Type: multipart/related; boundary=next
--next
Content-type: application/sdp
Content-Length: 325
v=0
o=- 2353687780 2353687780 IN IP4 128.3.4.5
s=R2FB
i=Your documents
e=colin.masterton@sales.hh.bt.co.uk
t=2353687780 0
m=application 1 fax octet-stream
c=TN RFC2543 +44-1794-8331010
a=fmtp:octet-stream uri:http://www.bt.co.uk/imgs/pipr.gif opr:
spr:2@53655768
--next
Content-Type: text/plain
Content-ID: 2@53655768
Content-Length: 352
Dear Sir,
Thank you for your enquiry. I have checked availability in your
area, and we can provide service to your cottage. I enclose a
quote for the costs of installation, together with the ongoing
rental costs for the line. If you want to proceed with this,
please quote job reference isdn/hh/123.45.9901.
Yours Sincerely,
Colin Masterton
--next--
Note that the "implicit" faxback content is given by an EMPTY opaque
reference in the middle of the fmtp line in this example.
4.11. Sportsline "headlines" message sent to your phone/pager/fax
(i) phone
INVITE sip:R2FB@pint.wwos.skynet.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To:
sip:1-900-123-456-7@wwos.skynet.com;user=phone;phone-context=+1
From: sip:fred.football.fan@skynet.com
Call-ID: 19971205T234505.56.78@chinet.net
CSeq: 4721 INVITE
Subject: Wonderful World Of Sports NFL Final Scores
Content-type: application/sdp
Content-Length: 220
v=0
o=- 2353687800 2353687800 IN IP4 128.3.4.5
s=R2FB
i=NFL Final Scores
e=fred.football.fan@skynet.com
c=TN RFC2543 +44-1794-8331013
t=2353687800 0
m=audio 1 voice x-pay
a=fmtp:x-pay opr:mci.com/md5:<crypto signature>
(ii) fax
INVITE sip:R2FB@pint.wwos.skynet.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:1-900-123-456-7@wwos.skynet.com;user=phone;
phone-context=+1
From: sip:fred.football.fan@skynet.com
Call-ID: 19971205T234505.56.78@chinet.net
CSeq: 4722 INVITE
Subject: Wonderful World Of Sports NFL Final Scores
Content-type: application/sdp
Content-Length: 217
v=0
o=- 2353687820 2353687820 IN IP4 128.3.4.5
s=R2FB
i=NFL Final Scores
e=fred.football.fan@skynet.com
c=TN RFC2543 +44-1794-8331010
t=2353687820 0
m=text 1 fax x-pay
a=fmtp:x-pay opr:mci.com/md5:<crypto signature>
(iii) pager
INVITE sip:R2FB@pint.wwos.skynet.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:1-900-123-456-7@wwos.skynet.com;user=phone;
phone-context=+1
From: sip:fred.football.fan@skynet.com
Call-ID: 19971205T234505.56.78@chinet.net
CSeq: 4723 INVITE
Subject: Wonderful World Of Sports NFL Final Scores
Content-type: application/sdp
Content-Length: 219
v=0
o=- 2353687840 2353687840 IN IP4 128.3.4.5
s=R2FB
i=NFL Final Scores
e=fred.football.fan@skynet.com
c=TN RFC2543 +44-1794-8331015
t=2353687840 0
m=text 1 pager x-pay
a=fmtp:x-pay opr:mci.com/md5:<crypto signature>
Note that these are all VERY similar.
4.12. Automatically giving someone a fax copy of your phone bill
INVITE sip:BillsRUs@pint.sprint.com SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:+1-555-888-1234@fbi.gov;user=phone
From: sip:agent.mulder@fbi.gov
Call-ID: 19991231T234505.56.78@fbi.gov
CSeq: 911 INVITE
Subject: Itemised Bill for January 98
Content-type: application/sdp
Content-Length: 247
v=0
o=- 2353687860 2353687860 IN IP4 128.3.4.5
s=BillsRUs
i=Joe Pendleton's Phone Bill
e=agent.mulder@fbi.gov
c=TN RFC2543 +1-202-833-1010
t=2353687860 0
m=text 1 fax x-files-id
a=fmtp:x-files-id opr:fbi.gov/jdcn-123@45:3des;base64,<signature>
Note: in this case the opaque reference is a collection of data used
to convince the Executive System that the requester has the right to
get this information, rather than selecting the particular content
(the A party in the To: field of the SIP "wrapper" does that alone).
5. Security Considerations
5.1. Basic Principles for PINT Use
A PINT Gateway, and the Executive System(s) with which that Gateway
is associated, exist to provide service to PINT Requestors. The aim
of the PINT protocol is to pass requests from those users on to a
PINT Gateway so an associated Executive System can service those
requests.
5.1.1. Responsibility for service requests
The facility of making a GSTN-based call to numbers specified in the
PINT request, however, comes with some risks. The request can specify
an incorrect telephone of fax number. It is also possible that the
Requestor has purposely entered the telephone number of an innocent
third party. Finally, the request may have been intercepted on its
way through any intervening PINT or SIP infrastructure, and the
request may have been altered.
In any of these cases, the result may be that a call is placed
incorrectly. Where there is intent or negligence, this may be
construed as harassment of the person incorrectly receiving the call.
Whilst the regulatory framework for misuse of Internet connections
differs throughout the world and is not always mature, the rules
under which GSTN calls are made are much more settled. Someone may be
liable for mistaken or incorrect calls.
Understandably, the GSTN Operators would prefer that this someone is
not them, so they will need to ensure that any PINT Gateway and
Executive System combination does not generate incorrect calls
through some error in the Gateway or Executive system implementation
or GSTN-internal communications fault. Equally, it is important that
the Operator can show that they act only on requests that they have
good reason to believe are correct. This means that the Gateway must
not pass on requests unless it is sure that they have not been
corrupted in transit from the Requestor.
If a request can be shown to have come from a particular Requestor
and to have been acted on in good faith by the PINT service provider,
then responsibility for making requests may well fall to the
Requestor rather than the Operator who executed these requests.
Finally, it may be important for the PINT service provider to be able
to show that they act only on requests for which they have some
degree of assurance of origin. In many jurisdictions, it is a
requirement on GSTN Operators that they place calls only when they
can, if required, identify the parties to the call (such as when
required to carry out a Malicious Call Trace). It is at least likely
that the provider of PINT services will have a similar responsibility
placed on them.
It follows that the PINT service provider may require that the
identity of the Requestor be confirmed. If such confirmation is not
available, then they may be forced (or choose) not to provide
service. This identification may require personal authentication of
the Requesting User.
5.1.2. Authority to make requests
Where GSTN resources are used to provide a PINT service, it is at
least possible that someone will have to pay for it. This person may
not be the Requestor, as, for example, in the case of existing GSTN
split-charging services like free phone in which the recipient of a
call rather than the originator is responsible for the call cost.
This is not, of course, the only possibility; for example, PINT
service may be provided on a subscription basis, and there are a
number of other models. However, whichever model is chosen, there may
be a requirement that the authority of a Requestor to make a PINT
request is confirmed.
If such confirmation is not available, then, again, the PINT Gateway
and associated Executive System may choose not to provide service.
5.1.3. Privacy
Even if the identity of the Requesting User and the Authority under
which they make their request is known, there remains the possibility
that the request is either corrupted, maliciously altered, or even
replaced whilst in transit between the Requestor and the PINT
Gateway.
Similarly, information on the Authority under which a request is made
may well be carried within that request. This can be sensitive
information, as an eavesdropper might steal this and use it within
their own requests. Such authority SHOULD be treated as if it were
financial information (such as a credit card number or PIN).
The data authorizing a Requesting User to make a PINT request should
be known only to them and the service provider. However, this
information may be in a form that does not match the schemes normally
used within the Internet. For example, X.509 certificates[14] are
commonly used for secured transactions on the Internet both in the IP
Security Architecture[12] and in the TLS protocol[13], but the GSTN
provider may only store an account code and PIN (i.e. a fixed string
of numbers).
A Requesting User has a reasonable expectation that their requests
for service are confidential. For some PINT services, no content is
carried over the Internet; however, the telephone or fax numbers of
the parties to a resulting service calls may be considered sensitive.
As a result, it is likely that the Requestor (and their PINT service
provider) will require that any request that is sent across the
Internet be protected against eavesdroppers; in short, the requests
SHOULD to be encrypted.
5.1.4. Privacy Implications of SUBSCRIBE/NOTIFY
Some special considerations relate to monitoring sessions using the
SUBSCRIBE and NOTIFY messages. The SUBSCRIBE message that is used to
register an interest in the disposition of a PINT service transaction
uses the original Session Description carried in the related INVITE
message. This current specification does not restrict the source of
such a SUBSCRIBE message, so it is possible for an eavesdropper to
capture an unprotected session description and use this in a
subsequent SUBSCRIBE request. In this way it is possible to find out
details on that transaction that may well be considered sensitive.
The initial solution to this risk is to recommend that a session
description that may be used within a subsequent SUBSCRIBE message
SHOULD be protected.
However, there is a further risk; if the origin-field used is
"guessable" then it might be possible for an attacker to reconstruct
the session description and use this reconstruction within a
SUBSCRIBE message.
SDP (see section 6 of [2], "o=" field) does not specify the mechanism
used to generate the sess-id field, and suggests that a method based
on timestamps produced by Network Time Protocol [16] can be used.
This is sufficient to guarantee uniqueness, but may allow the value
to be guessed, particularly if other unprotected requests from the
same originator are available.
Thus, to ensure that the session identifier is not guessable the
techniques described in section 6.3 of [17] can be used when
generating the origin-field for a session description to be used
inside a PINT INVITE message. If all requests from (and responses to)
a particular PINT requesting entity are protected, then this is not
needed. Where such a situation is not assured, AND where session
monitoring is supported, then a method by which an origin-field
within a session description is not guessable SHOULD be used.
5.2. Registration Procedures
Any number of PINT Gateways may register to provide the same service;
this is indicated by the Gateways specifying the same "userinfo" part
in the To: header field of the REGISTER request. Whilst such
ambiguity would be unlikely to occur with the scenarios covered by
"core" SIP, it is very likely for PINT; there could be any number of
service providers all willing to support a "Request-To-Fax" service,
for example.
Unless a request specifies the Gateway name explicitly, an
intervening Proxy that acts on a registration database to which
several Gateways have all registered is in a position to select from
the registrands using whatever algorithm it chooses; in principle,
any Gateway that has registered as "R2F" would be appropriate.
However, this opens up an avenue for attack, and this is one in which
a "rogue" Gateway operator stands to make a significant gain. The
standard SIP procedure for releasing a registration is to send a
REGISTER request with a Contact field having a wildcard value and an
expires parameter with a value of 0. It is important that a PINT
Registrar uses authentication of the Registrand, as otherwise one
PINT service provider would be able to "spoof" another and remove
their registration. As this would stop the Proxy passing any requests
to that provider, this would both increase requests being sent to the
rogue and stop requests going to the victim.
Another variant on this attack would be to register a Gateway using a
name that has been registered by another provider; thus a rogue
Operator might register its Gateway as "R2C@pint.att.com", thereby
hijacking requests.
The solution is the same; all registrations by PINT Gateways MUST be
authenticated; this includes both new or apparent replacement
registrations, and any cancellation of current registrations. This
recommendation is also made in the SIP specification, but for the
correct operation of PINT, it is very important indeed.
5.3. Security mechanisms and implications on PINT service
PINT is a set of extensions to SIP[1] and SDP[2], and will use the
security procedures described in SIP. There are several implications
of this, and these are covered here.
For several of the PINT services, the To: header field of SIP is used
to identify one of the parties to the resulting service call. The
PINT Request-To-Call service is an example. As mentioned in the SIP
specification, this field is used to route SIP messages through an
infrastructure of Redirect and Proxy server between the corresponding
User Agent Servers, and so cannot be encrypted. This means that,
although the majority of personal or sensitive data can be protected
whilst in transit, the telephone (or fax) number of one of the
parties to a PINT service call cannot, and will be "visible" to any
interception. For the PINT milestone services this may be acceptable,
since the caller named in the To: service is typically a "well known"
provider address, such as a Call Center.
Another aspect of this is that, even if the Requesting User does not
consider the telephone or fax numbers of the parties to a PINT
service to be private, those parties might. Where PINT servers have
reason to believe this might be the case they SHOULD encrypt the
request, even if the Requestor has not done so. This could happen,
for example, if a Requesting User within a company placed a PINT
request and this was carried via the company's Intranet to their
Proxy/firewall and thence over the Internet to a PINT Gateway at
another location.
If a request carries data that can be reused by an eavesdropper
either to "spoof" the Requestor or to obtain PINT service by
inserting the Requestor's authorization token into an eavesdropper's
request, then this data MUST be protected. This is particularly
important if the authorization token consists of static text (such as
an account code and/or PIN).
One approach is to encrypt the whole of the request, using the
methods described in the SIP specification. As an alternative, it may
be acceptable for the authorization token to be held as an opaque
reference (see section 3.4.2.3 and examples 4.11 and 4.12), using
some proprietary scheme agreed between the Requestor and the PINT
service provider, as long as this is resistant to interception and
re-use. Also, it may be that the authorization token cannot be used
outside of a request cryptographically signed by the Requestor; if so
then this requirement can be relaxed, as in this case the token
cannot be re-used by another. However, unless both the Requestor and
the Gateway are assured that this is the case, any authorization
token MUST be treated as sensitive, and so MUST be encrypted.
A PINT request may contain data within the SDP message body that can
be used more efficiently to route that request. For example, it may
be that one Gateway and Executive System combination cannot handle a
request that specifies one of the parties as a pager, whilst another
can. Both gateways may have registered with a PINT/SIP Registrar, and
this information may be available to intervening PINT/SIP Proxies.
However, if the message body is encrypted, then the request cannot be
decoded at the Proxy server, and so Gateway selection based on
contained information cannot be made there.
The result is that the Proxy may deliver the request to a Gateway
that cannot handle it; the implication is that a PINT/SIP Proxy
SHOULD consider its choice for the appropriate Gateway subject to
correction, and, on receiving a 501 or 415 rejection from the first
gateway chosen, try another. In this way, the request will succeed if
at all possible, even though it may be delayed (and tie up resources
in the inappropriate Gateways).
This opens up an interesting avenue for Denial Of Service; sending a
valid request that appears to be suitable for a number of different
Gateways, and simply occupying those Gateways in decrypting a message
requesting a service they cannot provide. As mentioned in section
3.5.5.1, the choice of service name to be passed in the userinfo
portion of the SIP Request-URI is flexible, and it is RECOMMENDED
that names be chosen that allow a Proxy to select an appropriate
Gateway without having to examine the SDP body part. Thus, in the
example given here, the service might be called "Request-To-Page" or
"R2P" rather than the more general use of "R2F", if there is a
possibility of the SDP body part being protected during transit.
A variation on this attack is to provide a request that is
syntactically invalid but that, due to the encryption, cannot be
detected without expending resources in decoding it. The effects of
this form of attack can be minimised in the same way as for any SIP
Invitation; the Proxy should detect the 400 rejection returned from
the initial Gateway, and not pass the request onwards to another.
Finally, note that the Requesting User may not have a prior
relationship with a PINT Gateway, whilst still having a prior
relationship with the Operator of the Executive System that fulfills
their request. Thus there may be two levels of authentication and
authorization; one carried out using the techniques described in the
SIP specification (for use between the Requestor and the Gateway),
with another being used between the Requesting User or the Requestor
and the Executive System.
For example, the Requesting User may have an account with the PINT
service provider. That provider might require that requests include
this identity before they will be convinced to provide service. In
addition, to counter attacks on the request whilst it is in transit
across the Internet, the Gateway may require a separate X.509-based
certification of the request. These are two separate procedures, and
data needed for the former would normally be expected to be held in
opaque references inside the SDP body part of the request.
The detailed operation of this mechanism is, by definition, outside
the scope of an Internet Protocol, and so must be considered a
private matter. However, one approach to indicating to the Requestor
that such "second level" authentication or authorization is required
by their Service Provider would be to ask for this inside the textual
description carried with a 401 response returned from the PINT
Gateway.
5.4. Summary of Security Implications
From the above discussion, PINT always carries data items that are
sensitive, and there may be financial considerations as well as the
more normal privacy concerns. As a result, the transactions MUST be
protected from interception, modification and replay in transit.
PINT is based on SIP and SDP, and can use the security procedures
outlined in [1] (sections 13 and 15). However, in the case of PINT,
the SIP recommendation that requests and responses MAY be protected
is not enough. PINT messages MUST be protected, so PINT
Implementations MUST support SIP Security (as described in [1],
sections 13 & 15), and be capable of handling such received messages.
In some configurations, PINT Clients, Servers, and Gateways can be
sure that they operate using the services of network level security
[13], transport layer security [12], or physical security for all
communications between them. In these cases messages MAY be exchanged
without SIP security, since all traffic is protected already. Clients
and servers SHOULD support manual configuration to use such lower
layer security facilities.
When using network layer security [13], the Security Policy Database
MUST be configured to provide appropriate protection to PINT traffic.
When using TLS, a port configured MUST NOT also be configured for
non-TLS traffic. When TLS is used, basic authentication MUST be
supported, and client-side certificates MAY be supported.
Authentication of the Client making the request is required, however,
so if this is not provided by the underlying mechanism used, then it
MUST be included within the PINT messages using SIP authentication
techniques. In contrast with SIP, PINT requests are often sent to
parties with which a prior communications relationship exists (such
as a Telephone Carrier). In this case, there may be a shared secret
between the client and the PINT Gateway. Such PINT systems MAY use
authentication based on shared secrets, with HTTP "basic
authentication". When this is done, the message integrity and privacy
must be guaranteed by some lower layer mechanism.
There are implications on the operation of PINT here though. If a
PINT proxy or redirect server is used, then it must be able to
examine the contents of the IP datagrams carried. It follows that an
end-to-end approach using network-layer security between the PINT
Client and a PINT Gateway precludes the use of an intervening proxy;
communication between the Client and Gateway is carried via a tunnel
to which any intervening entity cannot gain access, even if the IP
datagrams are carried via this node. Conversely, if a "hop-by-hop"
approach is used, then any intervening PINT proxies (or redirect
servers) are, by implication, trusted entities.
However, if there is any doubt that there is an underlying network or
transport layer security association in place, then the players in a
PINT protocol exchange MUST use encryption and authentication
techniques within the protocol itself. The techniques described in
section 15 of RFC2543 MUST be used, unless there is an alternative
protection scheme that is agreed between the parties. In either case,
the content of any message body (or bodies) carried within a PINT
request or response MUST be protected; this has implications on the
options for routing requests via Proxies (see 5.3).
Using SIP techniques for protection, the Request-URI and To: fields
headers within PINT requests cannot be protected. In the baseline
PINT services these fields may contain sensitive information. This is
a consideration, and if these data ARE considered sensitive, then
this will preclude the sole use of SIP techniques; in such a
situation, transport [12] or network layer [13] protection mechanisms
MUST be used.
As a final point, this choice will in turn have an influence on the
choice of transport layer protocol that can be used; if a TLS
association is available between two nodes, then TCP will have to be
used. This is different from the default behaviour of SIP (try UDP,
then try TCP if that fails).
6. Deployment considerations and the Relationship PINT to I.N.
(Informative)
6.1. Web Front End to PINT Infrastructure
It is possible that some other protocol may be used to communicate a
Requesting User's requirements. Due to the high numbers of available
Web Browsers and servers it seems likely that some PINT systems will
use HTML/HTTP as a "front end". In this scenario, HTTP will be used
over a connection from the Requesting User's Web Browser (WC) to an
Intermediate Web Server (WS). This will be closely associated with a
PINT Client (using some unspecified mechanism to transfer the data
from the Web Server to the PINT Client). The PINT Client will
represent the Requesting User to the PINT Gateway, and thus to the
Executive System that carries out the required action.
[WC]------[WS]
[PC]
\
\
[PG]
[XS]
Figure 2: Basic "Web-fronted" Configuration
6.2. Redirects to Multiple Gateways
It is quite possible that a given PINT Gateway is associated with an
Executive System (or systems) that can connect to the GSTN at
different places. Equally, if there is a chain of PINT Servers, then
each of these intermediate or proxy servers (PP) may be able to route
PINT requests to Executive Systems that connect at specific points to
the GSTN. The result of this is that there may be more than one PINT
Gateway or Executive System that can deal with a given request. The
mechanisms by which the choice on where to deliver a request are
outside the scope of this document.
[WC]------[WS] [WC]------[WS]
[PC] [PC]
\ \
\ \
[PG] [PP]
.........[XS]......... / \
: : / \
[PG] [PG]
[XS] [XS]
Figure 3: Multiple Access Configurations
However, there do seem to be two approaches. Either a Server that
acts as a proxy or redirect will select the appropriate Gateway
itself and will cause the request to be sent on accordingly, or a
list of possible Locations will be returned to the Requesting User
from which they can select their choice.
In SIP, the implication is that, if a proxy cannot resolve to a
single unique match for a request destination, then a response
containing a list of the choices should be returned to the Requesting
User for selection. This is not too likely a scenario within the
normal use of SIP.
However, within PINT, such ambiguity may be quite common; it implies
that there are a number of possible providers of a given service.
6.3. Competing PINT Gateways REGISTERing to offer the same service
With PINT, the registration is not for an individual but instead for
a service that can be handled by a service provider. Thus, one can
envisage a registration by the PINT Server of the domain telcoA.com
of its ability to support the service R2C as "R2C@telcoA.com", sent
to an intermediary server that acts as registrar for the
"broker.telcos.com" domain from "R2C@pint.telcoA.com" as follows:
REGISTER sip:registrar@broker.telcos.com SIP/2.0
To: sip:R2C@pint.telcoA.com
From: sip:R2C@pint.telcoA.com
...
This is the standard SIP registration service.
However, what happens if there are a number of different Service
Providers, all of whom support the "R2C" service? Suppose there is a
PINT system at domain "broker.com". PINT clients requesting a
Request-to-Call service from broker.com might be very willing to be
redirected or proxied to any one of the various service providers
that had previously registered with the registrar. PINT servers might
also be interested in providing service for requests that did not
specify the service provider explicitly, as well as those requests
that were directed "at them".
To enable such service, PINT servers would REGISTER at the broker
PINT server registrations of the form:
REGISTER sip:registrar@broker.com SIP/2.0
To: sip:R2C@broker.com
From: sip:R2C@pint.telcoA.com
When several such REGISTER messages appear at the registrar, each
differing only in the URL in the From: line, the registrar has many
possibilities, e.g.:
(i) it overwrites the prior registration for "R2C@broker.telcos.com"
when the next comes in;
(ii) it rejects the subsequent registration for
"R2C@broker.telcos.com";
(iii) it maintains all such registrations.
In this last case, on receiving an Invitation for the "general"
service, either:
(iii.1) it passes on the invitation to all registered service
providers, returning a collated response with all
acceptances, using multiple Location: headers,
or
(iii.2) it silently selects one of the registrations (using, for
example, a "round robin" approach) and routes the Invitation
and response onwards without further comment.
As an alternative to all of the above approaches, it:
(iv) may choose to not allow registrations for the "general" service,
rejecting all such REGISTER requests.
The algorithm by which such a choice is made will be implementation-
dependent, and is outside the scope of PINT. Where a behaviour is to
be defined by requesting users, then some sort of call processing
language might be used to allow those clients, as a pre-service
operation, to download the behaviour they expect to the server making
such decisions. This, however, is a topic for other protocols, not
for PINT.
6.4. Limitations on Available Information and Request Timing for
SUBSCRIBE
A reference configuration for PINT is that service requests are sent,
via a PINT Gateway, to an Executive System that fulfills the Service
Control Function (SCF) of an Intelligent Network (see [11]). The
success or failure of the resulting service call may be information
available to the SCF and so may potentially be made available to the
PINT Gateway. In terms of historical record of whether or not a
service succeeded, a large SCF may be dealing with a million call
attempts per hour. Given that volume of service transactions, there
are finite limits beyond which it cannot store service disposition
records; expecting to find out if a Fax was sent last month from a
busy SCF is unrealistic.
Other status changes, such as that on completion of a successful
service call, require the SCF to arrange monitoring of the service
call in a way that the service may not do normally, for performance
reasons. In most implementations, it is difficult efficiently to
interrupt a service to change it once it has begun execution, so it
may be necessary to have two different services; one that sets GSTN
resources to monitor service call termination, and one that doesn't.
It is unlikely to be possible to decide that monitoring is required
once the service has started.
These factors can have implications both on the information that is
potentially available at the PINT Gateway, and when a request to
register interest in the status of a PINT service can succeed. The
alternative to using a general SCF is to provide a dedicated Service
Node just for PINT services. As this node is involved in placing all
service calls, it is in a position to collect the information needed.
However, it may well still not be able to respond successfully to a
registration of interest in call state changes once a service logic
program instance is running.
Thus, although a Requesting User may register an interest in the
status of a service request, the PINT Gateway may not be in a
position to comply with that request. Although this does not affect
the protocol used between the Requestor and the PINT Gateway, it may
influence the response returned. To avoid the problem of changing
service logic once running, any registration of interest in status
changes should be made at or before the time at which the service
request is made.
Conversely, if a historical request is made on the disposition of a
service, this should be done within a short time after the service
has completed; the Executive System is unlikely to store the results
of service requests for long; these will have been processed as AMA
(Automatic Message Accounting) records quickly, after which the
Executive System has no reason to keep them, and so they may be
discarded.
Where the PINT Gateway and the Executive System are intimately
linked, the Gateway can respond to status subscription requests that
occur while a service is running. It may accept these requests and
simply not even try to query the Executive System until it has
information that a service has completed, merely returning the final
status. Thus the PINT Requestor may be in what it believes is a
monitoring state, whilst the PINT Gateway has not even informed the
Executive System that a request has been made. This will increase the
internal complexity of the PINT Gateway in that it will have a
complex set of interlocking state machines, but does mean that status
registration and indication CAN be provided in conjunction with an
I.N. system.
6.5. Parameters needed for invoking traditional GSTN Services within
PINT
This section describes how parameters needed to specify certain
traditional GSTN services can be carried within PINT requests.
6.5.1. Service Identifier
When a Requesting User asks for a service to be performed, he or she
will, of course, have to specify in some way which service. This can
be done in the URLs within the To: header and the Request-URI (see
section 3.5.5.1).
6.5.2. A and B parties
With the Request-to-Call service, they will also need to specify the
A and B parties they want to be engaged in the resulting service
call. The A party could identify, for example, the Call Center from
which they want a call back, whilst the B party is their telephone
number (i.e. who the Call Center agent is to call).
The Request-to-Fax and Request-to-Hear-Content services require the B
party to be specified (respectively the telephone number of the
destination Fax machine or the telephone to which spoken content is
to be delivered), but the A party is a Telephone Network based
resource (either a Fax or speech transcoder/sender), and is implicit;
the Requesting User does not (and cannot) specify it.
With the "Fax-Back" variant of the Request-to-Fax service, (i.e.
where the content to be delivered resides on the GSTN) they will also
have specify two parties. As before, the B party is the telephone
number of the fax machine to which they want a fax to be sent.
However, within this variant the A party identifies the "document
context" for the GSTN-based document store from which a particular
document is to be retrieved; the analogy here is to a GSTN user
dialling a particular telephone number and then entering the document
number to be returned using "touch tone" digits. The telephone number
they dial is that of the document store or A party, with the "touch
tone" digits selecting the document within that store.
6.5.3. Other Service Parameters
In terms of the extra parameters to the request, the services again
differ. The Request-to-Call service needs only the A and B parties.
Also it is convenient to assert that the resulting service call will
carry voice, as the Executive System within the destination GSTN may
be able to check that assertion against the A and B party numbers
specified and may treat the call differently.
With the Request-to-Fax and Request-to-Hear-Content services, the
source information to be transcoded is held on the Internet. That
means either that this information is carried along with the request
itself, or that a reference to the source of this information is
given.
In addition, it is convenient to assert that the service call will
carry fax or voice, and, where possible, to specify the format for
the source information.
The GSTN-based content or "Fax-Back" variant of the Request-to-Fax
service needs to specify the Document Store number and the Fax
machine number to which the information is to be delivered. It is
convenient to assert that the call will carry Fax data, as the
destination Executive System may be able to check that assertion
against the document store number and that of the destination Fax
machine.
In addition, the document number may also need to be sent. This
parameter is an opaque reference that is carried through the Internet
but has significance only within the GSTN. The document store number
and document number together uniquely specify the actual content to
be faxed.
6.5.4. Service Parameter Summary
The following table summarises the information needed in order to
specify fully the intent of a GSTN service request. Note that it
excludes any other parameters (such as authentication or
authorisation tokens, or Expires: or CallId: headers) that may be
used in a request.
Service ServiceID AParty BParty CallFmt Source SourceFmt
------- --------- ------ ------ ------- ------ -------
R2C x x x voice - -
R2F x - x fax URI/IL ISF/ILSF
R2FB x x x fax OR -
R2HC x - x voice URI/IL ISF/ILSF
In this table, "x" means that the parameter is required, whilst "-"
means that the parameter is not required.
The Services listed are Request-to-Call (R2C), Request-to-Fax (R2F),
the GSTN-based content or "Fax-back" Variant of Request-to-Fax
(R2FB), and Request-to-Hear-Content (R2HC).
The Call Format parameter values "voice" or "fax" indicate the kind
of service call that results.
The Source Indicator "URI/IL" implies that the information is either
an Internet source reference (a Universal Resource Identifier, or
URI) or is carried "in-line" with the message. The Source indicator
"OR" means that the value passed is an Opaque Reference that should
be carried along with the rest of the message but is to be
interpreted only within the destination (GSTN) context. As an
alternative, it could be given as a "local" reference with the "file"
style, or even using a partial reference with the "http" style.
However, the way in which such a reference is interpreted is a matter
for the receiving PINT Server and Executive System; it remains, in
effect, an opaque reference.
The Source Format value "ISF/ILSF" means that the format of the
source is specified either in terms of the URI or that it is carried
"in-line". Note that, for some data, the format either can be
detected by inspection or, if all else fails, can be assumed from the
URI (for example, by assuming that the file extension part of a URL
indicates the data type). For an opaque reference, the Source Format
is not available on the Internet, and so is not given.
6.6. Parameter Mapping to PINT Extensions
This section describes the way in which the parameters needed to
specify a GSTN service request fully might be carried within a "PINT
extended" message. There are other choices, and these are not
precluded. However, in order to ensure that the Requesting User
receives the service that they expect, it is necessary to have some
shared understanding of the parameters passed and the behaviour
expected of the PINT Server and its attendant Executive System.
The Service Identifier can be sent as the userinfo element of the
Request-URI. Thus, the first line of a PINT Invitation would be of
the form:
INVITE <serviceID>@<pint-server>.<domain> SIP/2.0
The A Party for the Request-to-Call and "Fax-back" variant of
Request-to-Fax service can be held in the "To:" header field. In this
case the "To:" header value will be different from the Request-URI.
In the services where the A party is not specified, the "To:" field
is free to repeat the value held in the Request-URI. This is the case
for Request-to-Fax and Request-to-Hear-Content services.
The B party is needed in all these milestone services, and can be
held in the enclosed SDP sub-part, as the value of the "c=" field.
The call format parameter can be held as part of the "m=" field
value. It maps to the "transport protocol" element as described in
section 3.4.2 of this document.
The source format specifier is held in the "m=", as a type and either
"-" or sub-type. The latter is normally required for all services
except Request-to-Call or "Faxback", where the "-" form may be used.
As shown earlier, the source format and source are not always
required when generating requests for services. However, the
inclusion in all requests of a source format specifier can make
parsing the request simpler and allows for other services to be
specified in the future, and so values are always given. The source
format parameter is covered in section 3.4.2 as the "media type"
element.
The source itself is identified by an "a=fmtp:" field value, where
needed. With the exception of the Request-to-Call service, all
invitations will normally include such a field. From the perspective
of the SDP extensions, it can be considered as qualifying the media
sub-type, as if to say, for example, "when I say jpeg, what I mean is
the following".
In summary, the parameters needed by the different services are
carried in fields as shown in the following table:
Service Svc Param PINT/SIP or SDP field used Example value
------- --------- -------------------------- -------------
R2C
ServiceID: <SIP Request-URI userinfo> R2C
AParty: <SIP To: field> sip:123@p.com
BParty: <SDP c= field> TN RFC2543 4567
CallFormat: <SDP transport protocol
sub-field of m= field> voice
SourceFmt: <SDP media type sub-field
of m= field> audio
(--- only "-" sub-type
sub-field value used) ---
Source: (--- No source specified) ---
R2F
ServiceID: <SIP Request-URI userinfo> R2F
AParty: (--- SIP To: field not used) sip:R2F@pint.xxx.net
BParty: <SDP c= field> TN RFCxxx +441213553
CallFormat: <SDP transport protocol
sub-field of m= field> fax
SourceFmt: <SDP media type sub-field
of m= field> image
<SDP media sub-type sub-field
of m= field> jpeg
Source: <SDP a=fmtp: field qualifying
preceding m= field> a=fmtp:jpeg<uri-ref>
R2FB
ServiceID: <SIP Request-URI userinfo> R2FB
AParty: <SIP To: field> sip:1-730-1234@p.com
BParty: <SDP c= field> TN RFCxxx +441213553
CallFormat: <SDP transport protocol
sub-field of m= field> fax
SourceFmt: <SDP media type sub-field
of m= field> image
<SDP media sub-type sub-field
of m= field> jpeg
Source: <SDP a=fmtp: field qualifying
preceding m= field> a=fmtp:jpeg opr:1234
R2HC
ServiceID: <SIP Request-URI userinfo> R2HC
AParty: (--- SIP To: field not used) sip:R2HC@pint.ita.il
BParty: <SDP c= field> TN RFCxxx +441213554
CallFormat: <SDP transport protocol
sub-field of m= field> voice
SourceFmt: <SDP media type sub-field
of m= field> text
<SDP media sub-type sub-field
of m= field> html
Source: <SDP a=fmtp: field qualifying
preceding m= field> a=fmtp:html<uri-ref>
7. References
[1] Handley, M., Schooler, E., Schulzrinne, H. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[2] Handley, M. and V. Jacobsen, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[3] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[5] The Unicode Consortium, "The Unicode Standard -- Version 2.0",
Addison-Wesley, 1996.
[6] ITU-T Study Group 2, "E.164 - The International Public Network
Numbering Plan", ITU-T, June 1997.
[7] Lu, H., Krishnaswamy, M., Conroy, L., Bellovin, S., Burg, F.,
DeSimone, A., Tewani, K., Davidson, P., Schulzrinne, H. and K.
Vishwanathan "Toward the PSTN/Internet Inter-Networking--Pre-
PINT Implementations", RFC 2458, November 1998.
[8] ITU-T Study Group XI, "Q.763 - Formats and Codes for the ISDN
User Part of SS No7" ITU-T, August 1994.
[9] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[10] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC 822, August 1982.
[11] ITU-T Study Group XI, "Q.1204 - IN Distributed Functional Plane
Architecture", ITU-T, February 1994.
[12] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[13] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[14] Housley, R., Ford, W., Polk W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and CRL Profile", RFC
2459, January 1999.
[15] Crocker, D. and P. Overall, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[16] Mills, D., "Network Time Protocol (version 3) specification and
implementation", RFC 1305, March 1992.
[17] Eastlake, D., Crocker, S. and J.Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[18] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[19] Levinson, E., "The MIME Multipart/Related Content-type" RFC
2387, August 1998.
8. Acknowledgements
The authors wish to thank the members of the PINT working group for
comments that were helpful to the preparation of this specification.
Ian Elz's comments were extremely useful to our understanding of
internal PSTN operations. The SUBSCRIBE and NOTIFY requests were
first suggested by Henning Schulzrinne and Jonathan Rosenberg. The
suggestion to use an audio port of 0 to express that the phone is "on
hold" (i.e. not receiving voice) is due to Ray Zibman. Finally,
thanks to Bernie Hoeneisen for his close proofreading.
Appendix A: Collected ABNF for PINT Extensions
;; --(ABNF is specified in RFC 2234 [15])
;; --Variations on SDP definitions
connection-field = ["c=" nettype space addrtype space
connection-address CRLF]
; -- this is the original definition from SDP, included for completeness
; -- the following are PINT interpretations and modifications
nettype = ("IN"/"TN")
; -- redefined as a superset of the SDP definition
addrtype = (INAddrType / TNAddrType)
; -- redefined as a superset of the SDP definition
INAddrType = ("IP4"/"IP6")
; -- this non-terminal added to hold original SDP address types
TNAddrType = ("RFC2543"/OtherAddrType)
OtherAddrType = (<X-Token>)
; -- X-token is as defined in RFC2045
addr = (<FQDN> / <unicast-address> / TNAddr)
; -- redefined as a superset of the original SDP definition
; -- FQDN and unicast address as specified in SDP
TNAddr = (RFC2543Addr/OtherAddr)
; -- TNAddr defined only in context of nettype == "TN"
RFC2543Addr = (INPAddr/LDPAddr)
INPAddr = "+" <POS-DIGIT> 0*(("-" <DIGIT>)/<DIGIT>)
; -- POS-DIGIT and DIGIT as defined in SDP
LDPAddr = <DIGIT> 0*(("-" <DIGIT>)/<DIGIT>)
OtherAddr = 1*<uric>
; -- OtherAdd defined in the context of OtherAddrType
; -- uric is as defined in RFC2396
media-field = "m=" media <space> port <space> proto
1*(<space> fmt) <CRLF>
; -- NOTE redefined as subset/relaxation of original SDP definition
; -- space and CRLF as defined in SDP
media = ("application"/"audio"/"image"/"text")
; -- NOTE redefined as a subset of the original SDP definition
; -- This could be any MIME discrete type; Only those listed are
; -- used in PINT 1.0
port = ("0" / "1")
; -- NOTE redefined from the original SDP definition;
; -- 0 retains usual sdp meaning of "temporarily no media"
; -- (i.e. "line is on hold")
; -- (1 means there is media)
proto = (INProto/TNProto)
; -- redefined as a superset of the original SDP definition
INProto = 1* (<alpha-numeric>)
; -- this is the "classic" SDP protocol, defined if nettype == "IN"
; -- alpha-numeric is as defined in SDP
TNProto = ("voice"/"fax"/"pager")
; -- this is the PINT protocol, defined if nettype == "TN"
fmt = (<subtype> / "-")
; -- NOTE redefined as a subset of the original SDP definition
; -- subtype as defined in RFC2046, or "-". MUST be a subtype of type
held
; -- in associated media sub-field or the special value "-".
attribute-fields = *("a=" attribute-list <CRLF>)
; -- redefined as a superset of the definition given in SDP
; -- CRLF is as defined in SDP
attribute-list = 1(PINT-attribute / <attribute>)
; -- attribute is as defined in SDP
PINT-attribute = (clir-attribute / q763-nature-attribute /
q763plan-attribute / q763-INN-attribute /
phone-context-attribute / tsp-attribute /
pint-fmtp-attribute / strict-attribute)
clir-attribute = clir-tag ":" ("true" / "false")
clir-tag = "clir"
q763-nature-attribute = Q763-nature-tag ":" q763-natures
q763-nature-tag = "Q763-nature"
q763-natures = ("1" / "2" / "3" / "4")
q763-plan-attribute = Q763-plan-tag ":" q763-plans
q763-plan-tag = "Q763-plan"
q763-plans = ("1" / "2" / "3" / "4" / "5" / "6" / "7")
; -- of these, the meanings of 1, 3, and 4 are defined in the text
q763-INN-attribute = Q763-INN-tag ":" q763-INNs
q763-INN-tag = "Q763-INN"
q763-INNs = ("0" / "1")
phone-context-attribute = phone-context-tag ":" phone-context-ident
phone-context-tag = "phone-context"
phone-context-ident = network-prefix / private-prefix
network-prefix = intl-network-prefix / local-network-prefix
intl-network-prefix = "+" 1*<DIGIT>
local-network-prefix = 1*<DIGIT>
private-prefix = 1*excldigandplus 0*<uric>
excldigandplus = (0x21-0x2d,0x2f,0x40-0x7d))
tsp-attribute = tsp-tag "=" provider-domainname
tsp-tag = "tsp"
provider-domainname = <domain>
; -- domain is defined in RFC1035
; -- NOTE the following is redefined relative to the normal use in SDP
pint-fmtp-attribute = "fmtp:" <subtype> <space> resolution
*(<space> resolution)
(<space> ";" 1(<attribute>) *(<space>
<attribute>))
; -- subtype as defined in RFC2046.
; -- NOTE that this value MUST match a fmt on the ultimately preceeding
; -- media-field
; -- attribute is as defined in SDP
resolution = (uri-ref / opaque-ref / sub-part-ref)
uri-ref = uri-tag ":" <URI-Reference>
; -- URI-Reference defined in RFC2396
uritag = "uri"
opaque-ref = opr-tag ":" 0*<uric>
opr-tag = "opr"
sub-part-ref = spr-tag ":" <Content-ID>
; -- Content-ID is as defined in RFC2046 and RFC822
spr-tag = "spr"
strict-attribute = "require:" att-tag-list
att-tag-list = 1(PINT-att-tag-list / <att-field> /
pint-fmtp-tag-list)
*(","
(PINT-att-tag-list / <att-field> /
pint-fmtp-tag-list)
)
; -- att-field as defined in SDP
PINT-att-tag-list = (phone-context-tag / clir-tag /
q763-nature-tag / q763-plan-tag /
q763-INN-tag)
pint-fmtp-tag-list = (uri-tag / opr-tag / spr-tag)
;; --Variations on SIP definitions
clir-parameter = clir-tag "=" ("true" / "false")
q763-nature-parameter = Q763-nature-tag "=" Q763-natures
q763plan-parameter = Q763-plan-tag "=" q763plans
q763-INN-parameter = Q763-INN-tag "=" q763-INNs
tsp-parameter = tsp-tag "=" provider-domainname
phone-context-parameter = phone-context-tag "=" phone-context-ident
SIP-param = ( <transport-param> / <user-param> / <method-param> /
<ttl-param> / <maddr-param> / <other-param> )
; -- the values in this list are all as defined in SIP
PINT-param = ( clir-parameter / q763-nature-parameter /
q763plan-parameter / q763-INN-parameter/
tsp-parameter / phone-context-parameter )
URL-parameter = (SIP-param / PINT-param)
; -- redefined SIP's URL-parameter to include ones defined in PINT
Require-header = "require:" 1(required-extensions)
*("," required-extensions)
; -- NOTE this is redefined as a subset of the SIP definition
; -- (from RFC2543/section 6.30)
required-extensions = ("org.ietf.sip.subscribe" /
"org.ietf.sdp.require")
Appendix B: IANA Considerations
There are three kinds of identifier used in PINT extensions that
SHOULD be registered with IANA, if a new value is specified. These
are:
* Media Format sub-types, as described in section 3.4.2 of this
document.
* Private Attributes as mentioned in section 3.4.3
* Private Phone Context values, as described in section 3.4.3.1.
It should be noted that private Address Types (in section 3.4.1) have
been explicitly excluded from this process, as they must be in the
form of an X-Token.
B.1. Media Format Sub-types
Taking these in turn, the media format sub-types are used within the
PINT extensions to SDP to specify the attribute line that holds the
data source definitions. In normal use, the values in this field are
sub-types of MIME discrete types[4]. If a value other than an IANA-
registered sub-type is to be used, then it should either be an X-
Token (i.e. start with "X-") or it should be registered with IANA. if
the intention is to describe a new MIME sub-type, then the procedures
specified in RFC 2048 should be used. It is ASSUMED that any new MIME
sub-type would follow the syntactic rules for interpretation of
associated PINT fmtp lines defined in this document.
Note that, in keeping with the SDP description, such registrations
SHOULD include the "proto" field values within which they are
defined; however, it is appropriate to specify only that they can be
used with "all values of TNProto".
Conversely, if the intent is to define a new way of including data
source definitions within PINT, then it will be necessary to specify,
in the documentation supporting any such new "PINT Media Format Sub-
type" registration, the syntax of the associated "fmtp" attribute
line, as the identifier serves to indicate the interpretation that
should be made of format specific attribute lines "tagged" with such
a sub-type.
If the fmtp interpretation follows the PINT default, then it is
adequate to mention this in the defining document rather than
repeating the syntax definition given here (although, in this case,
it is unclear why such a new registration would be required). As
before, the Media Format sub-type SHOULD specify the values of
"proto" field within which it is defined, but this can be "all values
of TNProto".
B.2. Private Attributes
Any proprietary attribute lines that are added may be registered with
IANA using the procedures mentioned in [2]; the mechanism is the same
as that used in SDP. If the attribute is defined for use only within
PINT, then it may be appropriate to mention this in the supporting
documentation. Note that, in the PINT 1.0 specification covered here,
there is no mechanism to add such freshly registered attribute lines
to a "require:" clause.
B.3. Private phone-contexts
Within the session description used for PINT requests, a phone-
context attribute may be used to specify the prefix or context within
which an associated telephone-number (in a connection line) should be
interpreted.
For "public" phone contexts the prefix to be used MUST start with
either a DIGIT or a "+". Private phone contexts may be registered
with IANA that do NOT start with either of these characters. Such a
prefix may be useful to identify a private network, potentially with
an associated numeric ID (see example 4 in section 3.4.3.1). In the
example, the prefix acts as the context for X-acme.com's private
network numbering plan.
It is recommended that any private context to be registered have the
general form of a token including a domain name, optionally followed
by a digit string or other token. The appropriate form of the initial
token name space will be similar to that used for private or vendor
registrations for sub-types (e.g. vnd.acme.com). However, note that
the registration will be used to specify a customer's private network
numbering plan format rather than being used generally for all of
their equipment vendor's customer's; thus, fbi.gov would be
appropriate, but lucent.com would not (unless the private network
were to be that used by Lucent internally).
In addition, the supporting documentation MUST either declare that
there is no associated token, or define the syntax by which that
token can be parsed (e.g. vnd.fbi.gov <space> 1*DIGIT). Note that the
registration describes a format, not a value range; it is sufficient
that the private context can be parsed, without the value being
interpreted.
In detail, the registration request SHOULD include:
* Kind of registration (i.e. private phone-context attribute to be
used within the service description of PINT service requests)
* Contact details for the person responsible for the registration
request (name, organisation, e-mail address, public telephone
number)
* Private Prefix initial token name (e.g. vnd.fbi.gov)
* syntax for private context (e.g. "vnd.fbi.gov" <space> 1*DIGIT, or
"vnd.gtn.gov.uk")
* Description of use (e.g. "This phone context declares an
associated telephone number to be within the 'government
telecommunications network'; the number is in an internal or
private number plan form)
* Network Type and Address Type with which this private context is
associated; If the "normal" telephone types (as specified in this
document) are used, then the values would be shown as:
"nettype=TN" , addrtype="RFC2543Addr". If, however, this context
were to be used with another address type, then a reference to
that address type name and the syntax of that address value would
be required.
In short, this context is the telephone equivalent of a "Net 10"
address space behind a NAT, and the initial name (and contact
information) shows the context within which that address is valid. It
also specifies the format for the network and address types (and
address value syntax) with which this context is associated.
Of course, IANA may refer the requested registration to the IESG or
an appropriate IETF working group for review, and may require
revisions to be made before the registration is accepted.
Authors' Addresses
Scott Petrack
MetaTel, Inc.
45 Rumford Ave.
Waltham MA 02453-3844
Phone: +1 (781)-891-9000
EMail: scott.petrack@metatel.com
Lawrence Conroy
Siemens Roke Manor Research
Roke Manor
Old Salisbury Lane
Romsey, Hampshire
U.K. SO51 0ZN
Phone: +44 (1794) 833666
EMail: lwc@roke.co.uk
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