INTERNET-DRAFT Scott Petrack,
Internet Engineering Task Force Metatel
PINT Working Group Lawrence Conroy,
Issued: 14 October 1999 Siemens Roke Manor Research
Expires: 14 April 2000
The PINT Service Protocol:
Extensions to SIP and SDP for IP Access to Telephone Call Services
<draft-ietf-pint-protocol-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (c) The Internet Society (1999). 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.
This document is intended for the PSTN-Internet Interworking (PINT) working
group of the Internet Engineering Task Force. Comments are solicited and
should be addressed to the working group's mailing list at
pint@lists.research.bell-labs.com and/or the authors.
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Contents
1. Introduction ......................................................... 4
1.1 Glossary ............................................................ 5
2. PINT Milestone Services .............................................. 6
2.1 Request to Call ................................................. 6
2.2 Request to Fax .................................................. 6
2.3 Request to Hear Content ......................................... 6
2.4 Relation between PINT milestone services and traditional
telephone services ............................................... 6
3. PINT Functional and Protocol Architecture ............................. 7
3.1. PINT Functional Architecture .................................... 7
3.2. PINT Protocol Architecture ...................................... 8
3.2.1. SDP operation in PINT ..................................... 9
3.2.2. SIP Operation in PINT ..................................... 9
3.3. REQUIRED and OPTIONAL elements for PINT compliance ............. 10
3.4. PINT Extensions to SDP 2.0 ..................................... 10
3.4.1. Network Type "TN" and Address Type "RFC2543" ............. 11
3.4.2. Support for Data Objects within PINT ..................... 11
3.4.2.1. Use of fmtp attributes in PINT requests ............ 13
3.4.2.2. Support for Remote Data Object References in PINT .. 13
3.4.2.3. Support for GSTN-based Data Objects in PINT ........ 14
3.4.2.4. Session Description support for included Data Objects 15
3.4.3. Attribute Tags to pass information into the Telephone
Network .................................................. 16
3.4.3.1. The phone-context attribute ........................ 17
3.4.3..2. Presentation Restriction attribute ................. 19
3.4.3.3. ITU-T CalledPartyAddress attributes parameters ..... 19
3.4.4. The "require" attribute .................................. 20
3.5. PINT Extensions to SIP 2.0 ..................................... 21
3.5.1. Multi-part MIME (sending data along with SIP request) .... 21
3.5.2. Warning header ........................................... 22
3.5.3. Mechanism to register interest in the disposition of a PINT
service, and to receive indications on that disposition .. 23
3.5.3.1. Opening a monitoring session with a SUBSCRIBE request 23
3.5.3.2. Sending Status Indications with a NOTIFY request ... 24
3.5.3.3. Closing a monitoring session with an UNSUBSCRIBE
request ............................................ 25
3.5.3.4. Timing of SUBSCRIBE requests ....................... 25
3.5.4. The "Require:" header for PINT ........................... 26
3.5.5. PINT URLs within PINT requests ........................... 26
3.5.5.1. PINT URLS within Request-URIs ...................... 27
3.5.6. Telephony Network Parameters within PINT URLs ............ 27
3.5.7. REGISTER requests within PINT ............................ 28
3.5.8. BYE Requests in PINT ..................................... 28
4. Examples of PINT Requests and Responses .............................. 30
4.1. A request to a call centre from an anonymous user to receive a
phone call ..................................................... 30
4.2. A request from a non anonymous customer (John Jones) to receive a
phone call from a particular sales agent (Mary James) .......... 30
4.3. A request to get a fax back .................................... 31
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4.4. A request to have information read out over the phone .......... 32
4.5. A request to send an included text page to a friend's pager .... 32
4.6. A request to send an image as a fax to phone number
+972-9-956-1867 ................................................ 33
4.7. A request to read out over the phone two pieces of content in
sequence ....................................................... 33
4.8. Request for the prices for ISDN to be sent to my fax machine ... 34
4.9. Request for a callback ......................................... 34
4.10.Sending a set of information in response to an enquiry ......... 35
4.11.Sportsline "headlines" message sent to your phone/fax/pager .... 35
4.12.Automatically giving someone a fax copy of your phone bill ..... 37
5. Security Considerations .............................................. 38
5.1. Basic Principles for PINT Use ................................. 38
5.1.1. Responsibility for service requests ..................... 38
5.1.2. Authority to make requests .............................. 38
5.1.3. Privacy ................................................. 39
5.1.4. Privacy Implications of SUBSCRIBE/NOTIFY ................ 39
5.2. Registration Procedures ....................................... 40
5.3. Security mechanisms and implications on PINT service .......... 40
5.4. Summary of Security Implications .............................. 42
6. Deployment considerations and the Relationship PINT to I.N.
(Informative) ........................................................ 44
6.1. Web Front End to PINT Infrastructure ........................... 44
6.2. Redirects to Multiple Gateways ................................. 44
6.3. Competing PINT Gateways REGISTERing to offer the same service .. 45
6.4. Limitations on Available Information and Request Timing for
SUBSCRIBE ...................................................... 46
6.5. Parameters needed for invoking traditional GSTN Services within
PINT ........................................................... 47
6.5.1. Service Identifier ....................................... 47
6.5.2. A and B parties .......................................... 47
6.5.3. Other Service Parameters ................................. 48
6.5.4. Service Parameter Summary ................................ 48
6.6. Parameter Mapping to PINT Extensions............................ 49
7. Open Issues and Draft State .......................................... 51
7.1. Open Issues .................................................... 51
7.2. Draft State .................................................... 51
8. References ........................................................... 51
9. Acknowledgements ..................................................... 52
Appendix A: Collected ABNF for PINT Extensions .......................... 53
Appendix B: IANA Considerations ......................................... 58
Appendix C: Authors' Addresses .......................................... 60
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1. Introduction
The desire to invoke certain telephone call services from the Internet has
been identified by many different groups (users, public and private network
operators, call center service providers, equipment vendors, see [7]). The
generic scenario is as follows (when the invocation is successful):
1. an IP host sends a request to a server on an IP network;
2. the server relays the request into a telephone network;
3. the telephone network performs the requested call service.
As examples, consider a user who wishes to have a call placed to his/her
telephone. It may be that a customer wishes to get a call from the support
department of some business, or a user wishes to hear some remote automatic
weather service via recorded or synthesised speech. Within a local
environment such a request might result in the placement of a call between
employees over the internal PBX.
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.
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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, UNSIUBSCRIBE, 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 extension
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.
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 services 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.
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Executive System - A system that interfaces to a telephone network that
executes a PINT service, and to a PINT Server. It is not directly associated
with the Internet, and is represented by the PINT Server.
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
A request is sent from an IP host that causes a fax to be sent to fax
machine B. The request MUST contain a pointer to the fax data (that
could reside in the IP network or in the Telephone Network), OR
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.
The PINT Request to Fax service does not involve "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 Hear 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.
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.
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There may be thousands of agents in the call centre, and there may be any
number of sophisticated algorithms and equipments 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
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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.
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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.
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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 is identified only as
a MIME type, such as "text" data, "image" data, or some more general
"application" data, etc. An important example of transporting "application"
data is the milestone service "Voice Access to Web Content". In this case
the data to be transported is pointed to by a URI, the data 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.
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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 RFC2543, (this is 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.
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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.
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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 should 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 to included data held in a
sub-part of the current message, followed by an opaque-ref 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].
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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://aMachine/index.html")
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 is 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; it
is merely some data that may be used by the Executive System to indicate the
content intended as part of the request. This data forms an opaque
reference, in that it is 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 me the data that is indexed ON THE GSTN by the reference value
"APPL.123.456"; 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.
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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 spr:<Content-ID> opr:
might be used to precede some unambiguous "faxed back" data with a covering
note (see next sub-section for details of the sub-part reference).
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)
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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).
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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 should 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
confrontation. Prefixes that are not so registered MUST begin with an "X-"
to indicate their private, non-standard nature (see Appendix C).
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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)
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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 it 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.
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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 fail 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.
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In order to process the request successfully the PINT server must BOTH
understand the attribute AND ALSO fulfil 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 fulfil the
request implied by the presence of the attribute, the request MUST fail
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
"ietf.org.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. All subsequent parts contain content data
that is to be transferred to the requested Telephone Call Service. As
discussed earlier, within a single PINT request, some of the data MAY be
pointed to by a URI within the request, and some of the data MAY be included
within the request.
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Where included data is carried within a PINT service request, the Content
Type entity header of the enclosing SIP message MUST indicate this. To do
so, the media type value within this entity header MUST be set to a value of
"multipart".
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 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.
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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. Such enhancement would be
expected to be the subject of a separate document.
The PINT 1.0 status registration and indication scheme uses three new
methods; SUBSCRIBE, UNSUBSCRIBE, and NOTIFY. These are used to allow a PINT
Requesting entity to register an interest in (or "subscribe" to) the status
of a service request, to indicate that this monitoring session is over,
and for the gateway to return service indications. All of these messages
follow the same procedure as used for all the SIP requests other than
INVITE; the recipient MUST acknowledge the request with a final response
message, otherwise the request will be repeated.
3.5.3.1. Opening a monitoring session with a SUBSCRIBE request
The SUBSCRIBE request indicates that a user wishes to receive information
about the status of a session. The request identifies the session of
interest normally by including the original session description along
with the request. 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. (When the request comes from a user other than the original
requesting user, the request constitutes a new SIP call leg, so the
Call-ID should not be used; instead the origin-field of the session
description enclosed within the original service request must be used).
The request MUST NOT include whatever content was present in the original
request other than the session description, and a server MUST ignore
whatever content is included within a SUBSCRIBE request with the sole
exception of the enclosed session description.
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The request MAY contain a "Contact:" header, specifying the PINT User Agent
Server to which such information should be sent. In addition, it SHOULD
contain an Expires: header, which indicates for how long the PINT Requestor
wishes to receive notification of the session status. 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). We refer
to the period of time before the expiration of the SUBSCRIBE request as the
"subscription period".
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 BYE (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 a Expires: header within the response to the
original INVITE message that triggered the service session, within the
response to the SUBSCRIBE message, within the response to the BYE
message, or within its own BYE 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.
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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.
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.
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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 fail the request if the option is not supported.
3.5.5. PINT URLs within PINT requests
Normally the hostnames and domain names that appear in the PINT URLs are the
internal affair of each individual PINT system. A client uses the
appropriate SDP payload to indicate the particular service it wishes to
invoke; it is not necessary to use a particular URL to identify the service.
A PINT URL is used in two different ways within PINT requests: within the
Request-URI, and within the To: and From: headers. Use within the
Request-URI requires clarification in order to ensure smooth interworking
with the Telephone Network serviced by the PINT infrastructure, and this
is covered next.
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3.5.5.1. PINT URLS within Request-URIs
There are some occasions when it may be useful to indicate service
information within the URL in a standardized way:
a. it may not be possible to use SDP information to route the request if
it is encrypted;
b. it allows implementation that make use of I.N. "service indicators";
c. It enables multiple competing PINT gateways to REGISTER with a single
"broker" server (proxy or redirect) (see section 6.3)
For these reasons, the following conventions for URLs are offered for use
in PINT requests:
1. The user portion of a sip URL indicates the service to be requested. At
present the following services are defined:
R2C (for Request-to-Call)
R2F (for Request-to-Fax)
R2HC (for Request-to-Hear-Content)
The user portions "R2C", "R2F", and "R2HC" are reserved for the PINT
milestone services. Other user portions MUST be used in case the requested
service is not one of the Milestone services. See section 6.2 for some
related considerations concerning registrations by competing PINT systems to
a single PINT proxy server acting as a service broker.
2. The host portion of a sip URL contains the domain name of the PINT
service provider.
3. A new url-parameter is defined to be "tsp" (for "telephone service
provider"). This can be used to indicate the actual telephone network
provider to be used to fulfil 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
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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 the 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 fulfil 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.
A question arises when the telephone call might not have even started at the
time 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.
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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 can be returned to INVITE or
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 may 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:
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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.
3. When engaged in a SUBSCRIBE/NOTIFY monitoring session, PINT servers that
send BYE to a URL listed in the Contact: header of a client request SHOULD
not clear session state until after the successful response to the BYE is
received. For example, it may be that the requesting client host is turned
off 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 BYE. Of course, it is possible that the BYE request will
simply time out.
4. Examples of PINT Requests and Responses
4.1. A request to a call centre 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
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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 fail 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:1-800-3292225K@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
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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 does not understand the number, it should fail
the request with 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/mixed; 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
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----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/mixed; 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
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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 -
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4.10.Sending a set of information in response to an enquiry
INVITE sip:R2FB@pint.bt.co.uk SIP/2.0
Via: SIP/2.0/UDP 169.130.12.5
To: sip:0345-12347-01@pint.bt.co.uk;user=phone;phone-context=+44
From: sip:colin.masterton@sales.hh.bt.co.uk
Call-ID: 19981205T234505.56.78@sales.hh.bt.co.uk
CSeq: 1147 INVITE
Subject: Price Info, as requested
Content-Type: multipart/mixed; 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
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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.
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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 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).
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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 harrasment 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 will 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.
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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 data 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.
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SDP (see section 6 of [2], "o=" field) does not specify the mechansim 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.
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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 Centre.
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).
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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 fulfils 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.
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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).
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6. Deployment considerations and the Relationship PINT to I.N. (Informative)
6.1. Web Front End to PINT Infrastructure
It is possible that some other protocol may be used to communicate a
Requesting User's requirements. Due to the high numbers of available Web
Browsers and servers it seems likely that some PINT systems will use
HTML/HTTP as a "front end". In this scenario, HTTP will be used over a
connection from the Requesting User's Web Browser (WC) to an Intermediate
Web Server (WS). This will be closely associated with a PINT Client (using
some unspecified mechanism to transfer the data from the Web Server to the
PINT Client). The PINT Client will represent the Requesting User to the PINT
Gateway, and thus to the Executive System that carries out the required
action.
[WC]------[WS]
[PC]
\
\
[PG]
[XS]
Figure 2: Basic "Web-fronted" Configuration
6.2. Redirects to Multiple Gateways
It is quite possible that a given PINT Gateway is associated with an
Executive System (or systems) that can connect to the GSTN at different
places. Equally, if there is a chain of PINT Servers, then each of these
intermediate or proxy servers (PP) may be able to route PINT requests to
Executive Systems that connect at specific points to the GSTN. The result of
this is that there may be more than one PINT Gateway or Executive System that
can deal with a given request. The mechanisms by which the choice on where
to deliver a request are outside the scope of this document.
[WC]------[WS] [WC]------[WS]
[PC] [PC]
\ \
\ \
[PG] [PP]
.........[XS]......... / \
: : / \
[PG] [PG]
[XS] [XS]
Figure 3: Multiple Access Configurations
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.
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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.
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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 fulfils 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
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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 Centre from which they want a call
back, whilst the B party is their telephone number (i.e. who the Call Centre
agent is to call).
The Request-to-Fax and Request-to-Hear-Content services require the B party
to be specified (respectively the telephone number of the destination Fax
machine or the telephone to which spoken content is to be delivered), but
the A party is a Telephone Network based resource (either a Fax or speech
transcoder/sender), and is implicit; the Requesting User does not (and
cannot) specify it.
With the "Fax-Back" variant of the Request-to-Fax service, (i.e. where the
content to be delivered resides on the GSTN) they will also have specify two
parties. As before, the B party is the telephone number of the fax machine
to which they want a fax to be sent. However, within this variant the A
party identifies the "document context" for the GSTN-based document store
from which a particular document is to be retrieved; the analogy here is to
a GSTN user dialling a particular telephone number and then entering the
document number to be returned using "touch tone" digits. The telephone
number they dial is that of the document store or A party, with the "touch
tone" digits selecting the document within that store.
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6.5.3. Other Service Parameters
In terms of the extra parameters to the request, the services again differ.
The Request-to-Call service needs only the A and B parties. Also it is
convenient to assert that the resulting service call will carry voice, as
the Executive System within the destination GSTN may be able to check that
assertion against the A and B party numbers specified and may treat the call
differently.
With the Request-to-Fax and Request-to-Hear-Content services, the source
information to be transcoded is held on the Internet. That means either that
this information is carried along with the request itself, or that a
reference to the source of this information is given. In addition, it is
convenient to assert that the service call will carry fax or voice, and,
where possible, to specify the format for the source information.
The GSTN-based content or "Fax-Back" variant of the Request-to-Fax service
needs to specify the Document Store number and the Fax machine number to
which the information is to be delivered. It is convenient to assert that
the call will carry Fax data, as the destination Executive System may be
able to check that assertion against the document store number and that of
the destination Fax machine.
In addition, the document number may also need to be sent. This parameter is
an opaque reference that is carried through the Internet but has
significance only within the GSTN. The document store number and document
number together uniquely specify the actual content to be faxed.
6.5.4. Service Parameter Summary
The following table summarises the information needed in order to specify
fully the intent of a GSTN service request. Note that it excludes any other
parameters (such as authentication or authorisation tokens, or Expires: or
CallId: headers) that may be used in a request.
Service ServiceID AParty BParty CallFmt Source SourceFmt
------- --------- ------ ------ ------- ------ -------
R2C x x x voice - -
R2F x - x fax URI/IL ISF/ILSF
R2FB x x x fax OR -
R2HC x - x voice URI/IL ISF/ILSF
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 either that the data is either an
Internet source reference (a Universal Resource Identifier, or URI) or is
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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.
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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
BParty: <SIP To: field> sip:123@p.com
AParty: <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
BParty: (--- SIP To: field not used) sip:R2F@pint.xxx.net
AParty: <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
BParty: <SIP To: field> sip:1-730-1234@p.com
AParty: <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
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R2HC
ServiceID: <SIP Request-URI userinfo> R2HC
BParty: (--- SIP To: field not used) sip:R2HC@pint.ita.il
AParty: <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. Open Issues and Draft State
7.1. Open Issues
Thre are no current technical open issues.
7.2. Draft State
This draft reflects all changes resulting from the WG "last call" phase.
8. References
[1] M. Handley, E. Schooler, H. Schulzrinne, & J. Rosenberg,
"SIP: Session Initiation Protocol", RFC2543,
Internet Engineering Task Force, March 1999.
[2] M. Handley & V. Jacobsen,
"SDP: Session Description Protocol", RFC2327,
Internet Engineering Task Force, April 1998.
[3] N. Freed & N. Borenstein,
"Multipurpose Internet Mail Extensions (MIME)
Part One: Format of Internet Message Bodies",
RFC2045, November 1996.
[4] N. Freed & N. Borenstein,
"Multipurpose Internet Mail Extensions (MIME)
Part Two: Media Types",
RFC2046, 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] H. Lu et al,
"Toward the PSTN/Internet Inter-Networking--Pre-PINT Implementations",
Informational RFC2458, Internet Engineering Task Force, Nov 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] T. Berners-Lee, R. Fielding, & L. Masinter,
"Uniform Resource Identifiers (URI): Generic Syntax", RFC2396,
Internet Engineering Task Force, August 1998.
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[10] D. Crocker,
"Standard for the format of ARPA Internet text messages",RFC822,
Internet Engineering Task Force, August 1982.
[11] ITU-T Study Group XI,
"Q.1204 - IN Distributed Functional Plane Architecture",
ITU-T, February 1994.
[12] T. Dierks & C. Allen,
"The TLS Protocol Version 1.0", RFC2246,
Internet Engineering Task Force, January 1999.
[13] S. Kent, R. Atkinson,
"Security Architecture for the Internet Protocol", RFC2401,
Internet Engineering Task Force, November 1998.
[14] R. Housley, W. Ford, W. Polk & D. Solo,
"Internet X.509 Public Key Infrastructure Certificate and CRL Profile",
RFC2459, Internet Engineering Task Force, January 1999.
[15] D. Crocker & P. Overall,
"Augmented BNF for Syntax Specifications: ABNF", RFC2234,
Internet Engineering Task Force, November 1997.
[16] D. Mills, "Network Time Protocol (version 3) specification and
implementation", RFC1305, Internet Engineering Task Force, March 1992.
[17] D. Eastlake, S. Crocker & J.Schiller,
"Randomness Recommendations for Security", Informational RFC 1305,
Internet Engineering Task Force, March 1992.
[18] P. Mockapetris,
"Domain Names - Implementation and Specification" RFC 1035,
Inernet Engineering Task Force November 1987.
9. 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. Finally, thanks to Bernie
Hoeneisen for his close proofreading.
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Appendix A: Collected ABNF for PINT Extensions
;; --(ABNF is specified in RFC 2234 [15])
;; --Variations on SDP definitions
connection-field = ["c=" nettype space addrtype space
connection-address CRLF]
; -- this is the original definition from SDP, included for completeness
; -- the following are PINT interpretations and modifications
nettype = ("IN"/"TN")
; -- redefined as a superset of the SDP definition
addrtype = (INAddrType / TNAddrType)
; -- redefined as a superset of the SDP definition
INAddrType = ("IP4"/"IP6")
; -- this non-terminal added to hold original SDP address types
TNAddrType = ("RFC2543"/OtherAddrType)
OtherAddrType = (<X-Token>)
; -- X-token is as defined in RFC2045
addr = (<FQDN> / <unicast-address> / TNAddr)
; -- redefined as a superset of the original SDP definition
; -- FQDN and unicast address as specified in SDP
TNAddr = (RFC2543Addr/OtherAddr)
; -- TNAddr defined only in context of nettype == "TN"
RFC2543Addr = (INPAddr/LDPAddr)
INPAddr = "+" <POS-DIGIT> 0*(("-" <DIGIT>)/<DIGIT>)
; -- POS-DIGIT and DIGIT as defined in SDP
LDPAddr = <DIGIT> 0*(("-" <DIGIT>)/<DIGIT>)
OtherAddr = 1*<uric>
; -- OtherAdd defined in the context of OtherAddrType
; -- uric is as defined in RFC2396
media-field = "m=" media <space> port <space> proto
1*(<space> fmt) <CRLF>
; -- NOTE redefined as subset/relaxation of original SDP definition
; -- space and CRLF as defined in SDP
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
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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"
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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>
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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
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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")
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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 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 approapriate 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.
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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 connextion 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.
Petrack & Conroy [Page 59]
<draft-ietf-pint-protocol-02.txt> PSP:Extensions to SIP and SDP October, 1999
Appendix C: Author's Addresses
Scott Petrack
MetaTel, Inc.
284 North Ave.
Weston, MA 02493
scott.petrack@metatel.com
+1 (781)-891-9000
Lawrence Conroy
Siemens Roke Manor Research
Roke Manor
Old Salisbury Lane
Romsey, Hampshire
U.K. SO51 0ZN
lwc@roke.co.uk
+44 (1794) 833666
Petrack & Conroy [Page 60]
