RFC3298: Service in the Public Switched Telephone Network/Intelligent Network (PSTN/IN) Requesting InTernet Service (SPIRITS) Protocol Requirements
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Network Working Group I. Faynberg, Editor
Request for Comments: 3298 Lucent Technologies
Category: Informational J. Gato
Vodaphone
H. Lu
Lucent Technologies
L. Slutsman
AT&T
August 2002
Service in the Public Switched Telephone Network/Intelligent Network
(PSTN/IN) Requesting InTernet Service (SPIRITS) Protocol Requirements
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes the SPIRITS protocol requirements, based on
the architecture presented in RFC 3136. (SPIRITS stands for "Service
in the PSTN/IN Requesting InTernet Service".) The purpose of the
protocol is to support services that originate in the Public Switched
Telephone Network (PSTN) and necessitate the interactions between the
PSTN and the Internet. Similarly, such services are called SPIRITS
services. (Internet Call Waiting, Internet Caller-ID Delivery, and
Internet Call Forwarding are examples of SPIRIT services, but the
protocol is to define the building blocks from which many other
services can be built.) On the PSTN side, the SPIRITS services are
initiated from the Intelligent Network (IN) entities; the earlier
IETF work on the PSTN/Internet Interworking (PINT) resulted in the
protocol (RFC 2848) in support of the services initiated the other
way around--from the Internet to PSTN.
To this end, this document lists general requirements for the SPIRITS
protocol as well as those pertinent to IN, Wireless IN, and PINT
building blocks. The document also presents the SPIRITS WG consensus
on the choice of the SPIRITS signaling protocol.
Faynberg, et al. Informational [Page 1]
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1. Conventions used in this document
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.
Unless otherwise qualified, the term PINT is used here not to refer
to the present PINT services and protocol, but in reference to the
scope of the generic PINT (vs. SPIRITS) service characteristics--
services being invoked from an IP network (vs. PSTN).
2. Introduction
This document describes the SPIRITS protocol requirements, based on
the architecture presented in RFC 3136. (SPIRITS stands for "Service
in the PSTN/IN Requesting InTernet Service.") The purpose of the
protocol is to support services that originate in the Public Switched
Telephone Network (PSTN) and necessitate the interactions between the
PSTN and the Internet. Such services are called SPIRITS services.
(Internet Call Waiting, Internet Caller-ID Delivery, and Internet
Call Forwarding are examples of SPIRIT services, but the protocol is
to define the building blocks from which many other services can be
built.) On the PSTN side, the SPIRITS services are initiated from
the Intelligent Network (IN) entities; the earlier IETF work on the
PSTN/Internet Interworking (PINT) resulted in the protocol (RFC 2848)
in support of the services initiated the other way around--from the
Internet to PSTN.
To this end, this document lists general requirements for the SPIRITS
protocol as well as those pertinent to IN, Wireless IN, and PINT
building blocks. The document also presents the SPIRITS WG consensus
on the choice of the SPIRITS signaling protocol. The joint
PINT/SPIRITS architecture (described in [1]) is depicted in Figure 1.
It is assumed that the Spirits Client is either co-located with the
IN Service Control Function (SCF) or communicates with it (over the
PSTN-specific interface D) in such a way so as to act on behalf of
the PSTN/IN. (This assumption is confirmed by current
implementations, as reported in [2].)
The SPIRITS services are invoked (and, subsequently, the SPIRITS
protocol is initiated) when a message from a SPIRITS Client (located
in the IN Service Control Point [SCP] or Service Node [SN]) arrives
on interface C to the SPIRITS gateway. The Spirits gateway processes
the message and, in turn, passes it on over the Interface B to the
SPIRITS server. In most practically important cases, the request
from a SPIRITS client is ultimately caused by a request from a
Central Office (i.e., a telephone switch) sent to either the SCP or
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SN, although the Internet-based service initiation by these elements
that had not been triggered by the Central Office is theoretically
possible. (Definitely, none of the SPIRITS benchmark services are
initiated in such a way, so, for the purposes of the SPIRITS protocol
development, it should be assumed that the service invocation was a
direct result of an earlier action by the Service Switching
Function.)
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......................
+----------------+ . .
| +------------+ | . +------------+ .
| | | | A . | | .
| | PINT Client|********************|PINT Server/|********
| | | | . | Gateway | *
| +------------+ | . +------------+ . *
| | . . *
| Subscriber's | . . *
| | . . *
| IP Host | . . *
| | . +------------+ . *
| +------------+ | . | SPIRITS | . *
| | SPIRITS | | B . | Gateway | . *
| | Server |********************| | . * E
| | | | . +------------+ . *
| +------------+ | . * . *
+----------------+ . * . *
...........*.......... *
* *
* *
Subscriber's * C *
Telephone * *
* *
(---) * *
* * *
* * * *
++++++++++++++++++++++++++ PSTN ++++++++++++++++++++++++++
* * *
* * *
* +------------------+ *
* Line | SPIRITS Client | *
* | | *
+--------------------+ +---+----- D ---------+-*+
| | INAP/SS7 | |
|Service Switching ************Service Control Function |
| Function | | |
| | +-------------------------+
| |
| |
+--------------------+
Figure 1. Joint PINT/SPIRITS Architecture
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With PINT (and that also applies to the PINT architecture and
protocol as described in [3]), the service request to the PINT Server
is always initiated by the PINT Client over the interface A. The PINT
Server can either be co-located with the IN Service Control or a
similar entity (referred to as "Executive System" by [3]) or
communicate with it over the PSTN-specific interface E.
As Figure 1 shows, the PINT Client and SPIRITS Server are co-located
in Subscriber's IP Host. In fact, both can be implemented to run as
one process. No provision is made for interactions between the PINT
Client and Spirits Server. Similarly, the PINT Server/PINT Gateway
and SPIRITS gateway are assumed to be co-located, too. This
assumption is convenient but not essential; the PINT Server could
also be co-located with the SPIRITS Client. In either case, no
specific provision is made to define interworking between either the
PINT Server and Spirits Gateway or PINT Server and SPIRITS Client
other than by listing the overall PINT-related requirements.
Since the currently deployed worldwide wireless networks are based on
circuit switching, they are considered PSTN networks for the SPIRITS
purposes. Adding SPIRITS type of services to wireless networks can
allow new services to be developed (for example geolocation
information can be handled in the IP network).
Nevertheless, there are certain peculiarities of wireless networks,
which force considerations to be made in the protocol
requirements and in the SPIRITS architecture.
A particular Wireless IN standard development being considered here
is CAMEL phase 3, standardized by the Third Generation Partnership
group (3GPP). The relevant service and architectural considerations
and protocol requirements are presented later in this document. As
far as the architecture is concerned, certain wireless events are
generated by Home Location Register (HLR), which may, but does not
have to, be part of the Mobile Switching Center (MSC) (a wireless
equivalent of the SSP). These events are communicated to Service
Control, at which point they use the same mechanism for invoking
SPIRITS services that the IN would.
The rest of this document addresses the general requirements,
IN Requirements, specific Wireless IN requirements, PINT
Requirements, the protocol development methodology, and security
issues, in that order.
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3. General Requirements
Based on the success of extending SIP for PINT ([3]) and, especially,
the results of pre-SPIRITS implementations reported in [2], the
Session Initiation Protocol (SIP) [7] has been chosen as the
signaling base protocol for SPIRITS.
Thus, it is a requirement that specific SPIRITS-related parameters be
carried in a manner consistent with SIP practices. In particular,
either Session Description Protocol (SDP) [8] or Multi-purpose
Internet Mail Extensions MIME [5-6] may be used for this purpose.
Except for the proposed new SUBSCRIBE/NOTIFY mechanism [4], and
extensions already defined in PINT, no new SIP extensions are
foreseen; instead the SPIRITS protocol is to rely on the above
extension mechanisms.
It is by no means a requirement that any SPIRITS implementation
automatically support PINT services. The SPIRITS protocol must be
defined in a manner where, as the minimum, it can support only the
basic notification mechanism without relying on PINT services or
otherwise relying on persistent interactions with PSTN.
Nevertheless, it has been demonstrated [2] that combining PINT
building blocks with those of SPIRITS is beneficial to building rich,
enhanced PSTN/Internet services, so the SPIRITS protocol must meet
the PINT-related requirements listed in section 7 of this document.
One specific example demonstrating the application of the latter
requirement, which is elaborated on further in this document, is as
follows: Implementation of SUBSCRIBE/NOTIFY is not mandatory as far
as the minimum SPIRITS protocol is concerned. Thus, the initial PSTN
(Detection Point) notification will always arrive via the SIP INVITE
method; however, to implement persistent interactions with the PSTN,
the SUBSCRIBE method may be used to obtain further notifications of
the PSTN events. Subsequently, these events will be reported on by
means of the NOTIFY method.
4. IN Requirements
The interface immediately relevant to IN is that between the SPIRITS
Client and SPIRITS Gateway (interface C). A typical message (which
starts a SPIRITS service) looks like this:
C -> G: <Event Notification>, <Parameter-List (DP)>
The relevant events correspond to the detection points (DPs) of the
IN Basic Call State Model (BCSM). The <Parameter-List> is a function
of a specific DP; it contains the parameters relevant to it. The
following requirements apply:
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1) The list of the DPs to be covered encompasses those defined in the
IN Capability Set 3 BCSM as well as those which relate to the
Wireless IN (WIN) specified by the IMT 2000 project in ITU-T.
2) Not all parameters associated with such DPs are needed by the
SPIRITS benchmark services, nor may all the parameters be needed
in SPIRITS. The selection of the relevant parameters is part of
the SPIRITS protocol definition.
3) It is desirable to avoid semantic overload of protocol messages.
(One way to achieve that is to match each type of an event with a
message that corresponds to it.) As the SPIRITS protocol is
designed as a set of extensions to another (existing) protocol
with the defined message set, the syntax and semantics of the
extensions should be defined with this requirement in mind.
4) The ITU-T Recommendations use the abstract syntax notation (ASN.1)
to specify the semantics of the IN Application Protocol (INAP)
parameters, which are expected to be binary-encoded. Neither the
use of the ASN.1, nor the requirement for binary encoding are the
typical requirements for the IETF application protocols.
Recognizing that, provisions must be made for careful
specification of the conversion of the INAP parameters to text,
which must preserve their original semantics. The actual
conversion of the parameters is the function of the SPIRITS
Client.
In order to issue an initial query (or a notification) to service
control, a switch must have such a DP set. This can be done
statically via service management (this particular action should
be left to implementation and thus is considered outside of the
scope of SPIRITS Protocol) or dynamically--but only for the
purpose of a particular call--from the service control. In the
latter case, it is part of the SPIRITS (or PINT) protocol to
request the event notification from the service control. The SIP
specific event notification scheme [4] should be specifically
considered. This function can be performed by either the Spirits
Client or PINT Server, the distinction being further discussed in
the next section. Assuming that it is performed by the SPIRITS
Client, the relevant message should look like:
G->C: SUBSCRIBE <Event> <Mode> <DP-specific parameters>,
where <Event> refers to a particular DP; <Mode> determines whether
the Event Detection Point (EDP) is to be armed as EDP Request
(EDP-R), EDP Notification (EDP-N), or TDP-R (the need for TDP-N is
not foreseen because it would not provide any additional
capability for SPIRITS); and the <DP-specific parameters> is the
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list of the values of the parameters associated with the EDP (for
example, if the DP in question is O_No_Answer, then the value of
the appropriate timer should be included in the list). Note that
such a subscription may also originate at a) PINT Client or b)
SPIRITS Gateway, either of which may (but does not have to) have a
locally significant definition of the <Event>. In either case, it
is the function of the SPIRITS Client to translate the definition
of the Event into a particular DP (or set of DPs) when passing the
message to Service Control. To summarize, for the case when PINT
and SPIRITS events are defined in a way where they do not refer to
the BCSM DPs, it is the function of the SPIRITS Client to define a
mapping:
Event -> DP List,
for each event for which the PSTN notification is needed.
The list of CS-3 DPs envisioned in SPIRITS is:
- origination_attempt_authorized (the SPIRITS service can control
call attempts, (for example, to limit calls during specific
time periods)
- collected_information and analyzed_information (for SPIRITS
outgoing call screening)
- o_answer, o_term_seized, and t_answer (to release SPIRITS
resources after the call is complete and perform relevant OA&M
actions such as creating a record of attempts to reach a party
via various means like land-line phone, cell phone, SMS, or
paging.)
- o_no_answer, route_select_failure, and t_no_answer (to re-route
a call)
- o_called_party_busy (to re-route a call and for Internet Call
Waiting)
- o_mid_call and t_mid_call (to assist a midcall action)
- o_abandon, o_disconnect, t_abandon, and t_disconnect (to
terminate a SPIRITS service and release the resources and
perform relevant OA&M actions such as creating a record of
attempts to reach a party via various means like land-line
phone, cell phone, SMS, or paging.)
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In addition, the following DPs are relevant to the present SPIRITS
milestone services:
- termination_attempt_authorized
- facility_selected_and_available (could be used in SPIRITS
Internet Caller-ID)
- t_busy (for Internet Call Waiting and Call Forwarding).
5. Wireless-IN-related Requirements
Wireless IN covers several types of "calls," which are neither
circuit switched nor have an effect on circuit switched calls. For
this reason, those are not considered in SPIRITS requirements. To
further clarify this point, the types of "calls" not considered are:
- USSD (Unstructured Supplementary Service Data)
- GPRS (General Packet Radio System)
- SMS (Short Message System)
The types of calls relevant to SPIRITS are as follows:
a) Voice Calls. In this case no new DP is needed since CAMEL DPs
are included in CS2. The only special case is "Not Reachable"
(when it is detected that the mobile user is out of coverage or
has switched off), which is mapped as a special cause in the
Busy DP. Since the Busy DP parameters would be received (if a
SPIRITS service has subscribed to Busy), it would be possible
to distinguish a "busy" from a "not reachable" situation.
This translates into the requirement that one of the parameters
in the Event Notification message (from SPIRITS Client to
SPIRITS Gateway, over the interface C) denotes the "cause" for
the Busy Detection Point.
Another aspect of difference, when compared to PSTN, is setting
of static DPs. In CAMEL networks, this is done in the Home
Location Register (HLR) (and copied to the VLR during location
update). It is important to note this difference, even though
it has no effect on SPIRITS protocol.
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RFC 3298 SPIRITS Protocol Requirements August 2002
b) Mobility Management events. This allows a SPIRITS server to be
notified of changes of location of a mobile user. The events
would only be applicable to mobile users reachable through a
Circuit-Switched network. To provide for this function, the
subscription marks must be set in the subscriber's HLR. This
is equivalent to setting TDPs in the SSP. In this case, the
marks in the HLR (which are copied to the Visitor Location
Register [VLR] on location update) are not mapped into Trigger
Detection Points.
As with TDP setting, this is outside of the scope of SPIRITS
protocol.
In order to support this function in SPIRITS, the SPIRITS
protocol should be able to map the CAMEL specific operations
into events notification to the SPIRITS client. Since the SCP
receives the information about the mobility state, this
involves the C interface. (This is just an extension of the DP
notification mechanism from the SPIRITS client to the SPIRITS
gateway).
The events (which are not DP-related) which need notifications
are:
- Location Update in the same VLR service area
- Location Update in another VLR service area
- IMSI attach
- MS initiated IMSI detach
- Network initiated IMSI detach.
With this mechanism, the SPIRITS services can use the user-
profile-based location information. For example, the Internet
Call Waiting service can re-direct the call to a mobile phone.
c) Supplementary Services Notification.
This mechanism makes a SPIRITS server aware of a subscriber
having invoked one of the following supplementary services:
Explicit Call Transfer, Call Deflection, Call Completion on
Busy Subscriber, or Multi-Party.
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6. PINT-related Requirements
Before a SPIRITS service can be invoked, the relevant IP Host must be
registered. Thus, Registration is an essential service, which is
initiated from the IP side. The registration information is
ultimately used by the PSTN to authenticate the subscriber.
Depending on the model, this can be done in two ways with the present
architecture:
1) The PINT Client issues the appropriate Register message over the
interface A, which is then passed by the PINT server to the SPIRITS
Gateway and SPIRITS Client:
PINT C.: -- Register --> PINT S. [--> SPIRITS Gateway --> SPIRITS
C.]. In this case the SPIRITS Client (co-located with the service
control) is responsible for record keeping and the authentication.
2) The PINT Client issues the appropriate Register message to the
PINT Server, which then passes this information to the PSTN service
control "by magic".
The second model is much easier to handle, because it involves only
one relevant interface ("A"); however it assumes no interworking
between PINT and SPIRITS except that the SPIRITS Client finds "by
magic" that a friendly and expecting IP Host is alive and well.
Finally, in the event PINT is not implemented, the SIP SUBSCRIBE
mechanism can be used.
As noted in the previous section, the existing SUBSCRIBE/NOTIFY PINT
building blocks [3] must be extended for their use in SPIRITS for the
purposes of setting DPs/getting DP event notifications. (A more
general SIP mechanism for the same PINT-introduced block is described
in [4]; it provides the necessary mechanism for specifying relevant
events.) Conversely, the same building blocks for the functional
capabilities can be used in both PINT and SPIRITS protocols. Note,
however, that in SPIRITS the PSTN notification may arrive without a
particular subscription to an event (in the case of a statically set
DP).
7. Follow-up on Event Notifications
The requirements of this section are neither PINT-specific, nor IN-
specific; their role is to outline the remaining element necessary
for the delivery of the SPIRITS service, which is the reaction to the
notification received.
Faynberg, et al. Informational [Page 11]
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In a particular scenario where:
a) The IP subscriber registers a SPIRITS service;
b) A call triggering the SPIRITS service is received (and
notification is sent); and
c) The call disposition is performed by the end user, the
signalling flow is demonstrated in Figure 2.
|----> Registration ----->|
SPIRITS |<-- Event Notification <-- | SPIRITS
Gateway |---> Call Disposition ---->| Client
| |
|
|
|
V
Service Control
|
|
V
SSP
Figure 2: Sequence of SPIRITS actions
One of the following actions is required by benchmark services:
a) Accept the incoming call
b) Reject the incoming call
c) Redirect the incoming call
d) Accept the call via VoIP (this particular item is outside of
the scope of SPIRITS WG).
Accordingly, the SPIRITS protocol should define the following message
types:
a) S->G: <Accept Call>
b) S->G: <[Reject Call],[Cause]>
c) S->G: <[Redirect Call],[Redirection Destination]>
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8. Methodology
To determine the MINIMUM SPIRITS protocol vocabulary (i.e., the set
of messages), the PSTN events associated with each detection point of
the Basic Call State Model should be examined. To date, the CS-3
BSCM has the richest set of DPs, although not all switching exchanges
have implemented it.
To determine the MINIMUM information available to the SPIRITS client
(this information is to be carried by the SPIRITS protocol from
SPIRITS client to SPIRITS server), each DP-specific information
elements needs to be examined.
Parameters should be event-specific, the following generic types of
parameters are expected to be mandatory:
- timer (for no answer)
- midcall control info (for mid_call)
- number of digits (for collected_information)
9. Security Considerations
Overall, the basic aspects of security apply to SPIRITS protocol:
- Authentication:
In the communications between the SPIRITS Client and SPIRITS
Gateway as well as the SPIRITS Gateway and SPIRITS Server, it is
required that the information be sent between known and trusted
partners.
- Integrity:
It is a requirement that no exchanged data be modified in transit.
- Confidentiality:
It is a requirement that any private user information or
confidential network data be protected by the protocol (typically
through encryption, for which the protocol should allow a choice
in the algorithm selection.
- Availability:
It is a requirement that the communicating endpoints remain in
service for authorized use only.
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In addition, the protocol should support non-repudiation for those
control messages pertinent to charging the PSTN subscriber.
As Figure 1 demonstrates, there are two distinct communications
interfaces, B and C. The B interface is, in general, across the
public Internet and is thus most vulnerable to security attacks
resulting in theft or denial of service. The C interface, on the
other hand is likely to be implemented across a service providers
intranet, where the security measures should be applied at the
discretion of the service provider. Even then, because at least one
IP host (the PINT gateway) is connected to the Internet, special
measures (e.g., installation of firewalls, although this particular
measure alone may be insufficient) need to be taken to protect the
interface C and the rest of the network from security attacks.
The assumption that the PINT Client and SPIRITS server are co-
located, dictates that the security considerations for the A and B
interfaces are exactly same. Detailed security requirements and
solutions for interface A (and, consequently, B) can be found in RFC
2848 [3].
Possible security attacks can result in both theft and denial of
services. In addition, such attacks may violate the privacy of a
PSTN subscriber. For example, with Internet Call Waiting, a
fraudulent registration (or a manipulation of integrity of a valid
registration) may force a network operator to provide to an
authorized party a full log of attempted telephone calls (accompanied
by the identification of callers). Furthermore, the calls may be
diverted to wrong recipients (who may further defraud the
unsuspecting calling party). In this case, the calling party is
using only the PSTN and thus expecting the security of communications
that are typical of the PSTN. The PSTN service providers may be
liable for the consequences of establishing wrong connections. In
addition, the PSTN service providers may be liable for inadvertent
divulging of the private information of the subscriber.
The service and network providers need to review the possibilities of
the security attacks and prepare the means of protection from them.
Some of this may be achieved by using the means outside of those
provided by the protocol itself. For example, administrative
information (such as statistics collected by PINT MIB or SPIRITS MIB)
can help in determining violations and thwarting them. As far as the
protocol is concerned, it must provide the means for authenticating a
subscriber as well as a session. It must also provide a capability
to carry encrypted information in its body.
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10. Acknowledgements
The authors are grateful to all participants in the SPIRITS group for
the discussion that has been shaping this work. Many thanks go to
Jorgen Bjorkner, Alec Brusilovsky, Jim Buller, Lawrence Conroy, Soren
Nyckelgard, and John Voelker for their incisive comments. Special
thanks are to Vijay Gurbani, Dave Hewins, and Kumar Vemuri, whose
careful, detailed reviews of several versions of this document have
been particularly helpful in improving its quality.
11. References
[1] Slutsman, L., Faynberg, I., Lu, H. and M. Weissman, "The Spirits
Architecture", RFC 3136, June 2001.
[2] Lu, H. (Editor), Faynberg, I., Voelker, J., Weissman, M., Zhang,
W., Rhim, S., Hwang, J., Ago, S., Moeenuddin, S., Hadvani, S.,
Nyckelgard, S., Yoakum, J. and L. Robart, "Pre-SPIRITS
Implementations of PSTN-Initiated Services", RFC 2995, November
2000.
[3] Petrack, S. and L. Conroy, "The PINT Service Protocol: Extensions
to SIP and SDP for IP Access to Telephone Call Services", RFC
2848, June 2000.
[4] Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[5] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[7] Handley, M., Schooler, E., Schulzrinne, H. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[8] Handley, M. and V. Jacobsen, "SDP: Session Description
Protocol", RFC 2327, April 1998.
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12. Authors' Addresses
Lev Slutsman
AT&T Laboratories
200 Laurel Ave.
Middletown, New Jersey, 07748
Phone: (732) 420-3752
EMail: lslutsman@att.com
Igor Faynberg
Bell Labs/Lucent Technologies
Room 4D-601A, 101 Crawfords Corner Road
Holmdel, New Jersey, 07733
Phone: (732) 949-0137
EMail: faynberg@lucent.com
Jorge Gato
Vodaphone
Avda de Europa, 1.
28108 Alcobendas (Madrid). Spain
Phone: +34 607 13 31 10
Fax: +34 607 13 30 57
EMail: jgato@airtel.es
Hui-Lan Lu
Bell Labs/Lucent Technologies
Room 4C-607A, 101 Crawfords Corner Road
Holmdel, New Jersey, 07733
Phone: (732) 949-0321
EMail: huilanlu@lucent.com
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RFC 3298 SPIRITS Protocol Requirements August 2002
13. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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