Internet Engineering Task Force (IETF) K. Drage, Ed.
Request for Comments: 7434 Alcatel-Lucent
Category: Standards Track A. Johnston
ISSN: 2070-1721 Avaya
December 2014
January 2015
Interworking ISDN Call Control User Information with SIP
Abstract
The motivation and use cases for interworking and transporting User-
to-User Information (UUI) from the ITU-T Digital Subscriber
Signalling System No. 1 (DSS1) User-user information element data in within
SIP are described in RFC 6567. As networks move to SIP, it is
important that applications requiring this data can continue to
function in SIP networks as well as have the ability to interwork
with this ISDN service for end-to-end transparency. This document
defines a usage (a new package called the ISDN UUI package) of the
User-to-User header field to enable interworking with this ISDN
service.
This document covers interworking with both public ISDN and private
ISDN capabilities, so the potential interworking with QSIG will also
be addressed.
The package is identified by the new value "isdn-uui" of the
"purpose" header field parameter.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7434.
Copyright Notice
Copyright (c) 2014 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Summary of the ISDN User-to-User Service . . . . . . . . . . 3
3.1. The Service . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Impacts of the ISDN Service on SIP Operation . . . . . . 5
4. Relation to SIP-T . . . . . . . . . . . . . . . . . . . . . . 5 6
5. Transition Away from ISDN . . . . . . . . . . . . . . . . . . 6
6. ISDN Usage of the User-to-User Header Field . . . . . . . . . 6
7. UAC Requirements . . . . . . . . . . . . . . . . . . . . . . 7
8. UAS Requirements . . . . . . . . . . . . . . . . . . . . . . 9
9. UUI Contents . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Considerations for ISDN Interworking Gateways . . . . . . . . 11
11. Coding Requirements . . . . . . . . . . . . . . . . . . . . . 11 12
12. Media Feature Tag . . . . . . . . . . . . . . . . . . . . . . 12
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
14. Security Considerations . . . . . . . . . . . . . . . . . . . 13
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
15.1. Normative References . . . . . . . . . . . . . . . . . . 14
15.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 16
1. Terminology
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 [RFC2119].
2. Overview
This document describes a usage of the User-to-User header field
defined in [RFC7433] to enable the transport of User-to-User
Information (UUI) UUI in ISDN
interworking scenarios using SIP [RFC3261]. Specifically, this
document discusses the interworking of the following items, which are
call control related related: ITU-T Recommendation Q.931 DSS1 User-user
information element Q.931 [Q931], ITU-T Recommendation Q.957.1 DSS1 User-
to-User Signalling (UUS) supplementary service [Q957.1], and ITU-T
Recommendation Q.763 User-to-user User-to-User information parameter [Q763] data
in SIP. Today, UUI is widely used in the Public Switched Telephone
Network (PSTN) in contact centers and call centers that are
transitioning away from ISDN to SIP.
This usage is not limited to scenarios where interworking will occur.
Rather it describes a usage where interworking is possible if
interworking is met. That does not preclude its usage directly
between two SIP terminals.
3. Summary of the ISDN User-to-User Service
3.1. The Service
ISDN defines a number of related services. Firstly, there is a user
signalling bearer service that uses the information elements /
parameters in the signalling channel to carry the data and does not
establish a related circuit-switched connection. For DSS1, this is
specified in ITU-T Recommendation Q.931, Sections 3.3 and 7 of
[Q931]. Secondly, it defines a User-to-User Signalling (UUS)
supplementary service that uses the information elements / parameters
in the signalling channel to carry additional data but that is used
in conjunction with the establishment of a related circuit-switched
connection. This reuses the same information elements / parameters
as the user signalling bearer service, with the addition of other
signalling information, and for DSS1 this is specified in ITU-T
Recommendation Q.957.1 [Q957.1].
ISDN defines three variants of the UUS supplementary service as
follows:
UUS1: User-to-User information Information exchanged during the setup and
clearing phases of a call by transporting DSS1 User-user
information elements within call control messages. This in itself
has two subvariants, UUS1 implicit and UUS1 explicit. In UUS1
implicit, it is the presence of the user signalling data itself
that constitutes the request for the service. UUS1 explicit uses
additional supplementary service control information to control
the request and granting of the service, as in UUS2 and UUS3. As
a result, UUS1 explicit also allows the requester to additionally
specify whether the parallel circuit-switched connection should
proceed if the UUS1 service cannot be provided (preferred or
required);
UUS2: DSS1 User-user information elements exchanged from the
sender's point of view during call establishment, between the DSS1
ALERTING and DSS1 CONNECT messages, within DSS1 USER INFORMATION
messages; and
UUS3: DSS1 User-user information elements exchanged while a call is
in the Active state, within DSS1 USER INFORMATION messages.
The service is always requested by the calling user.
This document defines only the provision of the ISDN UUS1 implicit
supplementary service to interworking scenarios, this being the most
widely deployed and used of the various ISDN User-to-User services,
and is indeed the one that matches the requirements specified in
[RFC6567].
The above comes from the ISDN specifications defined for public
networks. There is a parallel set of ISDN specifications defined for
private networks (QSIG). These specifications do not define a UUS1
implicit supplementary service. However, implementation of such a
UUS1 implicit supplementary service for private networks can readily
be constructed in a proprietary fashion based on the specifications
for public networks, and evidence suggests that some vendors have
done so. On this basis, there is no reason why this package cannot
also be used to support interworking with such a private network
service, on the assumption that the constraints are exactly the same
as those for the public network.
The ISDN UUS1 service has the following additional characteristics as
to the data that can be transported:
The maximum number of octets of user information that can be
transported is 128 octets plus a protocol discriminator. It is
noted that some early ISDN implementations had a limitation of 32
octets, but it is understood that these are not currently
deployed. While this package does not prohibit longer data
fields, the mechanism at any interworking point discards data
elements that are too long to handle. The handled length can
normally be assumed to be 128 octets.
The content of the user information octets is described by a
single octet protocol discriminator (see Table 4-26 of ITU-T
Recommendation Q.931) [Q931]. That protocol discriminator may
describe the protocol used within the user data, the structure of
the user data, or leave it entirely open. Note that not all
values within the protocol discriminator necessarily make sense
for use in the ISDN User-to-User service, as the content is
aligned with the protocol discriminator that appears at the start
of all DSS1 messages (see Table 4-1 of ITU-T Recommendation Q.931)
[Q931]. The protocol discriminator value has no impact on the
interworking capability.
Only single-user information a single piece of UUI data can be transported in each
message.
The ISDN service works without encryption or integrity protection.
The user trusts the intermediate network elements, and therefore
the operator of those elements, not to modify the data and to
deliver all the data to the remote user. On a link-by-link basis,
message contents are protected at Layer 2 by standard Cyclic
Redundancy Check (CRC) cyclic
redundancy check mechanisms -- this allows loss on a link-
level link-level
basis to be detected but does not guard against fraudulent attacks
on the link itself. This does not prevent the use of additional
encryption or integrity protection within the UUI data itself,
although the limit on the size of the UUI data (protocol
discriminator plus 128 octets) will restrict this.
3.2. Impacts of the ISDN Service on SIP Operation
The ISDN service has the following impacts that need to be understood
within the SIP environment.
Call transfer: ISDN call transfer cancels all ISDN User-to-User
supplementary services. In the ISDN, if User-to-User data is
required after call transfer, then UUS3 has to be renegotiated,
which is not provided by this SIP extension. The impact of this
restriction on the SIP environment is that UUI header fields
cannot be exchanged in transactions clearing down the SIP dialog
after call transfer has occurred.
Conference: ISDN conferencing allows the user to still exchange
User-to-User data after the conference is created. As far as UUS1
is concerned, it is not permitted. The ISDN three-party
supplementary service is similar in many ways to conferencing but
is signalled using a different mechanism. This means that on
clearing, the controller using UUS1 implicit does have the choice
of sending data to either or both remote users. Because SIP
conferencing cannot completely emulate the ISDN three-party
supplementary service at the served user, UUS1 implicit is not
possible.
Diversion: When ISDN diversion occurs, any UUS1 User-to-User data is
sent to the forwarded-to-user (assuming that the call meets
requirements for providing the service -- this is impacted by the
explicit service only). If the type of diversion is such that the
call is also delivered to the forwarding user, they will also
receive any UUS1 User-to-User data.
4. Relation to SIP-T
A method of transport of ISDN User-to-User data is to use SIP-T
[RFC3372] and transport the UUI information end-to-end (as part of an
ISUP message or QSIG message) as a MIME body. If the SIP-T method of
encapsulation of ISDN instead of interworking is used, this is a
reasonable mechanism and does not require any extensions to existing
SIP-T. However, if true ISDN interworking is being done, and
therefore SIP-T would not otherwise be used, this approach is not
reasonable because then implementation of the many elements of the
ISUP syntax would be required to understand one element of data.
Instead, the better approach is to interwork the ISDN User-to-User
data using the native SIP UUI transport mechanism, the User-to-User
header field. The rest of this document describes this approach.
5. Transition Away from ISDN
This interworking usage of the SIP UUI mechanism will likely begin
with one UA as an ISDN gateway while the other UA is a native SIP
endpoint. As networks transition away from ISDN, it is possible that
both UAs could become native SIP endpoints. In this case, there is
an opportunity to transition away from this ISDN usage to a more
general usage of [RFC7433].
The SIP UUI mechanism provides a way to achieve this transition. As
an endpoint moves from being an ISDN gateway to a native SIP
endpoint, and a future package for some form of enhanced UUI has been
standardized, the endpoint can carry the UUI data both as ISDN and as
the future package in parallel and in the same messages or in
different messages depending on the needs of the application. This
will permit the other endpoint to use the UUI according to the ISDN
UUI package if it is an ISDN gateway or according to the future
package if it is a native SIP endpoint.
6. ISDN Usage of the User-to-User Header Field
This document defines the package for the ISDN interworking of UUI
that interoperates with ISDN UUS, a supplementary service in which
the user is able to send/receive a limited amount of information to/
from another ISDN user over the signalling channel in association
with a call to the other ISDN user.
Two examples of ISDN UUI with redirection (transfer and diversion)
are defined in [ANSII] and [ETSI].
One objective of the design of this package has been to keep the
functionality at the interworking point as simple as possible. As a
result, there is also only one encoding value specified.
Responsibility for respecting the limits has been transferred to the
end UA. If an interworking point is reached, and the limitations of
the ISDN (see Section 3.1) are not met, then the UUI data will not be
transferred, although the SIP request will otherwise be interworked.
This is rather than have the interworking point attempt to resolve
the non-compliance with the limitations of ISDN.
The general principals of the UUI mechanism package are, therefore,
as follows:
The sending application is expected to limit their sending
requirements to the subset provided by the ISDN User-to-User
service.
The SIP UA will not allow the reception of more than one
User-to-User header field relating to the "isdn-uui" package in
the same SIP request or response; it will only allow it in a
request or response of the appropriate method (INVITE or BYE).
What happens to User-to-User header fields relating to other
packages is outside the scope of this document.
An interworking point trying to interwork UUI data that is too
long will discard the UUI data but proceed with the interworking.
There is no notification of such discard back to the sending user.
If the SIP user knows that it is interworking with the ISDN, then
the UUI application at the SIP endpoint should limit its
communication to 128-octet packets of 128 octets plus the protocol
discriminator, with the knowledge that discard will occur if it
does not. The UUI application at the SIP endpoint has complete
control over what occurs. It should be noted that this was
exactly the envisaged operation when early ISDN implementations
that only supported 32 octets interworked with those supporting
128 octets. It also corresponds to the interworking with ISDNs
that do not support the supplementary service at all, as discard
will occur in these circumstances as well. Note that failure to
include the User-to-User data into the ISDN SETUP message (when
discard occurs) will result in the service being unavailable for
the remainder of the call when UUS1 implicit operation is used.
7. UAC Requirements
The User Agent Client (UAC) MUST meet the requirements of [RFC7433]
in addition to the requirements defined in this document.
The UAC MUST only use this UUI mechanism extension package in
association with the initial INVITE method and the BYE method
relating to an INVITE dialog. Usage on transactions associated with
any other type of dialog, or on methods not associated with a dialog,
is precluded. Usage on other methods within the INVITE dialog, and
on re-INVITE transactions with the INVITE dialog, is also precluded.
If the UAC wishes to use or permit the sending of UUI data at any
point in the dialog, the UAC MUST include in the INVITE request for
that dialog a User-to-User header field. The UAC SHOULD set the
"purpose" header field parameter to "isdn-uui". Non-inclusion of the
"purpose" header field parameter is permitted, but this is primarily
to allow earlier implementations to support this package. This
initial header field constitutes the implicit request to use the UUI
service and is, therefore, included even when there is no data except
the protocol discriminator octet to send at that point in time.
The UAC MUST NOT include the User-to-User header field with a
"purpose" header field parameter set to "isdn-uui", or with no
"purpose" header field parameter, in any message of an INVITE dialog
if the original INVITE request did not include the User-to-User
header field, either with a "purpose" header field parameter set to
"isdn-uui" or with no "purpose" header field parameter included.
When sending UUI for the ISDN UUI package, if the "purpose" header
field is included, the UAC MUST set the User-to-User "purpose" header
field parameter to "isdn-uui". The UAC MUST NOT include more than
one User-to-User header field for this package in any SIP request or
response.
When receiving UUI, when multiple User-to-User header fields are
received in the same response with the "purpose" header field
parameter set to "isdn-uui", or with no "purpose" header field
parameter, or with some combination of these, the UAC MUST discard
all these header fields. There are no mechanisms for determining
which ones are the intended UUI data, so all are discarded.
The application designer will need to take into account the ISDN
service restrictions; failure to do so can result in information
being discarded at any interworking point with the ISDN. This
document makes no further normative requirements based on those
constraints because those constraints may vary from one ISDN to
another. It is reasonable to expect that a limitation of 128 octets
(plus a protocol discriminator) can be imposed by the ISDN;
therefore, UUI data longer than this will never reach the destination
if such interworking occurs. Note that the 128-octet limit (plus a
protocol discriminator) applies before the encoding (or after the
decoding) using the "hex" encoding. The "hex" encoding is defined in
[RFC7433].
A "uui" option tag for use with the UUI mechanism extension is
defined in [RFC7433]. Because the service is UUS1 implicit for the
ISDN User-to-User service, the inclusion of the "uui" option tag in a
Supported header field conveys no additional information over and
above the presence, in the INVITE request, of the User-to-User header
field with the "purpose" header field parameter set to "isdn-uui".
While there is no harm in including the "uui" option tag, and
strictly it should be included if the extension is supported, it
performs no function. The presence of the "uui" option tag in the
Require header field of an INVITE request will cause the request to
fail if it reaches a UAS or ISDN interworking gateway that does not
support this extension; such usage is not precluded although it does
not form part of allowed but will produce
results that are inconsistent with the package. mechanisms defined in the ISDN
UUS supplementary service.
8. UAS Requirements
The UAS MUST meet the requirements of [RFC7433] in addition to the
requirements defined in this document.
The UAS MUST only use this UUI mechanism extension package in
association with the initial INVITE method and the BYE method
relating to an INVITE dialog. Usage on transactions associated with
any other type of dialog, or on methods not associated with a dialog,
is precluded. Usage on other methods within the INVITE dialog, and
on re-INVITE transactions with the INVITE dialog, is also precluded.
The UAS MUST NOT include the User-to-User header field with a
"purpose" header field parameter set to "isdn-uui", or with no
"purpose" header field parameter, in any message of an INVITE dialog
if the original INVITE request did not include the User-to-User
header field, either with a "purpose" header field parameter set to
"isdn-uui" or with no "purpose" header field parameter included.
The UAS MAY include the User-to-User header field in responses to the
initial INVITE request, or the BYE requests or responses for the
dialog, only where the original INVITE request included a
User-to-User header field with the "purpose" header field parameter
set to "isdn-uui" or where no "purpose" header field parameter was
included. When sending UUI for the ISDN UUI package, the UAS SHOULD
set the User-to-User "purpose" header field parameter to "isdn-uui".
Non-inclusion of the "purpose" header field parameter is permitted,
but this is primarily to allow earlier implementations to support
this package.
When sending UUI for the ISDN UUI package, if the "purpose" header
field is included, the UAS MUST set the User-to-User "purpose" header
field parameter to "isdn-uui". The UAS MUST NOT include more than
one User-to-User header field for this package in any SIP request or
response.
The "isdn-interwork" value for the "purpose" header field parameter
was used in documents that led to the publication of the present
document. Although these documents had no other status than "Work in
Progress", this value is implemented by some vendors. While not
defined by this document, implementations could find it useful for
interoperability purposes to support parsing and interpreting "isdn-
interwork" the same way as "isdn-uui" when receiving messages.
Where the UAS is acting as a redirect server, the UAS MUST NOT
include the User-to-User header field in the header URI parameter in
a 3xx response to an incoming request.
When receiving UUI, when a User-to-User header field is received in a
request that is not from the originating user with the "purpose"
header field parameter set to "isdn-uui", or with no "purpose" header
field parameter, the UAS MUST discard this header field.
When receiving UUI, when multiple User-to-User header fields are
received from the originating user in the same request with the
"purpose" header field parameter set to "isdn-uui", or with no
"purpose" header field parameter, or with some combination of these,
the UAS MUST discard all these header fields. There are no
mechanisms for determining which ones are the intended UUI data, so
all are discarded.
9. UUI Contents
These requirements apply when the "purpose" header field parameter is
set to "isdn-uui" or when there is no "purpose" header field
parameter.
Processing for User-to-User header fields sent or received with
values other than this value are outside the scope of this document,
and the appropriate package document for that value applies.
The default and only content defined for this package is "isdn-uui".
When sending UUI, the sending SIP entity MAY, but need not, include a
"content" header field with a value set to "isdn-uui". A receiving
SIP entity MUST ignore a received User-to-User header field if the
"content" header field parameter is present and the value is some
other value than "isdn-uui".
The default and only encoding defined for this package is "hex".
When sending UUI, the sending SIP entity MAY, but need not, include
an "encoding" header field with a value set to "hex". A receiving
SIP entity MUST ignore a received User-to-User header field if the
"encoding" header field parameter is present and the value is some
other value than "hex".
When sending UUI, the sending application MUST include a protocol
discriminator octet, conforming to Table 4-26 of ITU-T Recommendation
Q.931 [Q931], as the first octet of the UUI data. It is up to the
receiving application what it does with this value. This document
places no other normative requirement on the use of the protocol
discriminator; it is required at interworking gateways to allow
mapping into the appropriate fields in the ISDN protocols; otherwise,
the usage is entirely up to the application and is outside the scope
of this document. Valid values are identified and documented by ITU-
T, and there is no IANA registry for these values.
10. Considerations for ISDN Interworking Gateways
ISDN interworking gateways MUST support the requirements defined for
UAS and UAC operation.
ISDN interworking gateways MUST support only the "isdn-uui" package
on dialogs that are interworked.
ISDN interworking gateways will take octet-structured data from the
ISDN side and encode it using the "hex" encoding scheme defined in
[RFC7433] for inclusion as the UUI data in the User-to-User header
field. In the reverse direction, it will take valid UUI data
according to the "hex" encoding scheme and decode it to octet-
structured data to send to the ISDN side.
When mapping data content from the ISDN to SIP signalling, or from
SIP signalling to the ISDN, the gateway needs to assume that all
content is octet-structured binary, irrespective of the value of the
received protocol discriminator. There are no requirements in the
ISDN to ensure that the content matches the value of the protocol
discriminator; the application usage sorts out any discrepancy. The
same applies to the ISDN protocol discrimination defined Table 4-26
of ITU-T Recommendation Q.931 [Q931] discriminator as the first octet of
the UUI
data; data, as defined in Table 4-26 of ITU-T Recommendation Q.931
[Q931]; the interworking gateway will not perform any additional
checking of this value.
A "uui" option tag for use with the UUI mechanism extension is
defined in [RFC7433]. The option tag is not interworked at an ISDN
interworking gateway. The ISDN interworking gateways MUST NOT take
the omission of the "uui" option tag in a received INVITE request to
indicate that interworking of a received header field is not to be
performed.
11. Coding Requirements
This document defines "isdn-uui" as a new value of the User-to-User
"purpose" header field parameter. The following ABNF adds to the
production in [RFC7433]:
pkg-param-value =/ "isdn-uui"
This document defines "isdn-uui" as a new value of the User-to-User
"content" header field parameter. A content value of "isdn-uui"
indicates that the contents have a first octet that is a protocol
discriminator (see Table 4-26 of ITU-T Recommendation Q.931 [Q931])
followed by UUI data that can be subject to a length limitation
(before encoding or after decoding) that is generally 128 octets.
The following ABNF adds to the production in [RFC7433].
cont-param-value =/ "isdn-uui"
12. Media Feature Tag
This document defines the new media feature tag "sip.uui-isdn". This
feature tag indicates that this ISDN UUI package is supported by the
sender, and its usage is entirely in accordance with [RFC3840]. This
document makes no additional provisions for the use of this feature
tag.
13. IANA Considerations
Per this document, the following row has been added to the "UUI
Packages" subregistry of the SIP parameter registry:
Value: isdn-uui
Description: The associated application is being used with
constraints suitable for interworking with the ISDN User-to-User
service, and therefore can be interworked at ISDN gateways.
Reference: RFC 7434
Contact: Keith Drage
Per this document, the following row has been added to the "UUI
Content" subregistry of the SIP parameter registry:
Value: isdn-uui
Description: The associated contents conform to the content
associated with the ISDN User-to-User service. In the presence of
the "purpose" header field parameter set to "isdn-uui" (or the
absence of any "purpose" header field parameter), this is the
default meaning and therefore need not be included in this case.
Reference: RFC 7434
Contact: Keith Drage
This document defines the following media feature tag, which has been
added to the features.sip-tree of the Media feature tags registry:
Media feature tag name: sip.uui-isdn
ASN.1 Identifier: 1.3.6.1.8.4.x
Summary of the media feature indicated by this tag: This media
feature tag when used in a Contact header field of a SIP request
or a SIP response indicates that the entity sending the SIP
message supports the package "uui-isdn".
Values appropriate for use with this feature tag: none
Examples of typical use: Indicating that a mobile phone supports
Single Radio Voice call Continuity (SRVCC) for calls in the
alerting phase.
Related standards or documents: RFC 7434
Security Considerations: Security considerations for this media
feature tag are discussed in Section 11.1 of [RFC3840]
14. Security Considerations
This document contains no specific requirements in regard to security
over and above those specified in [RFC7433]. However, since this
capability is designed to interwork with the ISDN, the general
security considerations of SIP to ISDN User Part (ISUP) interworking
defined in [RFC3398] apply. Any SIP/PSTN gateway implementing the
ISDN User-to-User service should not blindly trust ISUP from the
PSTN. In general, the overlying use case will define the security
measures required. The underlying User-to-User header field
extension provides a number of tools that can meet certain security
requirements.
Information that might otherwise reveal private information about an
individual, or where a level of authenticity needs to be guaranteed,
may need a higher level of protection and may indeed not be suitable
for this package, particularly taking into account the statement in
the following paragraph.
As this capability is defined to interwork with the ISDN, if the ISDN
forms part of the route, any usage needs to be aware that the
security level of the ISDN service may be lower than the security of
the SIP service. The ISDN security is itself not definable on an
end-to-end basis and exists on a hop-by-hop basis. This can be high
in some places (e.g., it can require physical access to a secure
building) and in other places it can be low (e.g., the point where an
ISDN access enters a building). If this level of security is not
sufficient, then either a different package or indeed a different
method of data transfer needs to be selected by the application user.
15. References
15.1. Normative References
[Q931] ITU-T, "ISDN user-network interface layer 3 specification
for basic call control", ITU-T Recommendation Q.931,
<http://www.itu.int/rec/T-REC-Q.931-199805-I/en>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002, <http://www.rfc-editor.org/info/rfc3261>.
[RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol
for Telephones (SIP-T): Context and Architectures", BCP
63, RFC 3372, September 2002,
<http://www.rfc-editor.org/info/rfc3372>.
[RFC3398] Camarillo, G., Roach, A., Peterson, J., and L. Ong,
"Integrated Services Digital Network (ISDN) User Part
(ISUP) to Session Initiation Protocol (SIP) Mapping", RFC
3398, December 2002,
<http://www.rfc-editor.org/info/rfc3398>.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004,
<http://www.rfc-editor.org/info/rfc3840>.
[RFC7433] Johnston, A. and J. Rafferty, "A Mechanism for
Transporting User-to-User Call Control Information in
SIP", RFC 7433, December 2014.
15.2. Informative References
[ANSII] ANSI, "Integrated Services Digital Network (ISDN) -
Explicit Call Transfer Supplementary Service", ANSI-
T1.643A - SUP A, December 1996.
[ETSI] ETSI, "Integrated Services Digital Network (ISDN);
Diversion supplementary services; Digital Subscriber
Signalling System No. one (DSS1) protocol; Part 1:
Protocol specification", ETSI ETS 300 207-1, Ed. 1,
December 1994.
[Q763] ITU-T, "Signalling System No. 7 - ISDN User Part formats
and codes", ITU-T Recommendation Q.763,
<http://www.itu.int/rec/T-REC-Q.763-199912-I/en>.
[Q957.1] ITU-T, "Digital subscriber Signalling System No. 1 - Stage
3 description for supplementary services using DSS 1;
Stage 3 description for additional information transfer
supplementary services using DSS 1: User-to-User
Signalling (UUS)", ITU-T Recommendation Q.957.1,
<http://www.itu.int/rec/T-REC-Q.957.1-199607-I>.
[RFC6567] Johnston, A. and L. Liess, "Problem Statement and
Requirements for Transporting User-to-User Call Control
Information in SIP", RFC 6567, April 2012,
<http://www.rfc-editor.org/info/rfc6567>.
Appendix A. Acknowledgments
Joanne McMillen was a major contributor and coauthor of earlier
versions of this document.
Thanks to Spencer Dawkins, Vijay Gurbani, Laura Liess, and Roland
Jesske for their reviews of this document. The authors wish to thank
Francois Audet, Denis Alexeitsev, Paul Kyzivat, Cullen Jennings,
Mahalingam Mani, and Celine Serrut-Valette for their comments.
The death of Francois Audet occurred before this document was
finalized, and the authors would like to identify the significant
contribution of Francois to this and a number of important RFCs and
to express their condolences to his family. It was always a pleasure
to work with Francois.
Authors' Addresses
Keith Drage (editor)
Alcatel-Lucent
Quadrant, Stonehill Green, Westlea
Swindon
United Kingdom
EMail: keith.drage@alcatel-lucent.com
Alan Johnston
Avaya
St. Louis, MO
United States
EMail: alan.b.johnston@gmail.com