Network Working Group K. Drage, Ed.
Internet-Draft Alcatel-Lucent
Intended status: Standards Track A. Johnston
Expires: March 23, 2012 Avaya
September 20, 2011
Interworking ISDN Call Control User Information with SIP
draft-drage-cuss-sip-uui-isdn-01
Abstract
The motivation and use cases for interworking and transporting ITU-T
DSS1 User-user information element data in SIP are described in the
"Problem Statement and Requirements for Transporting User to User
Call Control Information in SIP" document. As networks move to SIP
it is important that applications requiring this data can continue to
function in SIP networks as well as the ability to interwork with
this ISDN service for end-to- end transparency. This document
defines a usage of the User-to-User header field to enable
interworking with this ISDN service.
This document is covers the interworking with both public ISDN and
private ISDN capabilities, so the interworking with QSIG will also be
addressed.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 23, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . . . . . 6
5. Transition away from ISDN . . . . . . . . . . . . . . . . . . 6
6. ISDN Usage of the User-to-User Header Field . . . . . . . . . 7
7. UAC requirements . . . . . . . . . . . . . . . . . . . . . . . 7
8. UAS requirements . . . . . . . . . . . . . . . . . . . . . . . 9
9. UUI contents . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Considerations for ISDN interworking gateways . . . . . . . . 10
11. Coding requirements . . . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
13. Security Considerations . . . . . . . . . . . . . . . . . . . 12
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
15. Changes since previous versions . . . . . . . . . . . . . . . 12
16. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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 BCP 14, RFC 2119
[RFC2119].
2. Overview
This document describes a usage of the User-to-User header field
defined in [johnston-cuss-sip-uui] to enable the transport of User to
User Information (UUI) in ISDN interworking scenarios using SIP
[RFC3261]. Specifically, this document discuss the interworking of
call control related ITU-T DSS1 User-user information element [Q931],
[Q957.1] and ITU-T Q.763 User-to-user information parameter [Q763]
data in SIP. UUI is widely used in the PSTN today in contact centers
and call centers which 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, which 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 section 3.3 and section 7
[Q931]. It also defines a user-to-user signalling supplementary
service, which uses the information elements / parameters in the
signalling channel to carry additional data, but which 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 user-to-user signalling
supplementary service as follows:
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UUS1: User-to-user information exchanged during the setup and
clearing phases of a call, by transporting User-to-user
information element within call control messages. This in itself
has two subvariants, UUS1 implicit and UUS1 explicit. UUS1
explicit uses additional supplementary service control information
to control the request and granting of the service, as in USS2 and
UUS3. In UUS1 implicit, it is the presence of the user signalling
data itself that constitutes the request for the service. UUS1
explicit as a result 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: User-to-user information 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: User-to-user information 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 application 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 indeed the one that matches the requirements specified in
draft-johnston-cuss-sip-uui-reqs.
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 in 128 octets. 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
application does not prohibit longer data fields, the mechanism at
any interworking point is to discard 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 descriminator 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 user to user service, as the content is aligned
with the protocol discriminator that appears at the start of all
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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 a single user information package 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 CRC
mechanisms - this allows loss on a 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 payload itself,
although the limit on the size of the payload (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 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, this means that when an individual party clears, those
clearing messages can still contain user-to-user data. As a
conferee this is sent to the conference controller. As the
conference controller, as this effectively clears the conference,
it can be broadcast to all conferees, or sent to individual
conferees [OPEN ISSUE - CHECK THIS IN THE PROTOCOL - DOES IT
REQUIRE EXPLICIT].
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.
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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.
Contributors note: The above list needs to be studied further in
regard to private ISDN service definitions, e.g. for the interworking
of SIP and QSIG.
4. Relation to SIP-T
A method of transport of ISDN UUI 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, this
approach is not reasonable. Instead, the better approach is to
interwork the ISDN UUI 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 User Agent being an ISDN gateway while the other User Agent
is a native SIP endpoint. As networks transition away from ISDN, it
is possible that both User Agents 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 [johnston-cuss-sip-uui]. This
will be possible when both endpoints are aware of the actual
application using the UUI.
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 usage application for the UUI has been standardized,
the endpoint can carry the UUI both as ISDN and application encoding.
This will permit the other endpoint to utlize the UUI if it is an
ISDN gateway or a native SIP endpoint. When all the endpoints have
moved away from ISDN, the ISDN encoding usage can be discontinued.
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6. ISDN Usage of the User-to-User Header Field
This document defines the purpose usage of the ISDN interworking
application of UUI which is to interoperate with ISDN User to User
Signaling (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].
The general principals of this application of the UUI mechanism are
as follows:
That the sending application is expected limit their sending
requirements to the subset provided by the ISDN UUI service.
That the SIP UA will not allow the reception of more that one
User-to-User header field of the "isdn-uui" application in the
same SIP request or response, and 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 different
application is outside the scope of this document.
That an interworking point trying to interwork application data
that is too long will discard the application 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, in
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-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 UAC MUST meet the requirements of [johnston-cuss-sip-uui] in
addition to the requirements defined in this document.
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The UAC MUST only use this application of the UUI mechanism extension
in association with the initial INVITE method and 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.
If the UAC wishes to user 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 with an "app" header field
parameter set to "isdn-uui". This initial header field constitutes
the implicit request to use the UUI service, and is therefore
included even when there is no data to send at that point in time.
The UAC MUST NOT include the User-to-User header field with an "app"
header field parameter set to "isdn-uui" in any message of an INVITE
dialog if the original INVITE request did not include the User-to-
User header field with an "app" header field parameter set to "isdn-
uui"
When sending UUI for the ISDN application, the UAC MUST set the User-
to-User "app" header field parameter to "isdn-uui". The UAC MUST NOT
include more than one User-to-User header field for this application
in any SIP request or response.
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 payload information.
When receiving UUI, when multiple User-to-User header fields are
received in the same response with the "app" header field parameter
to "isdn-uui", the UAS MUST discard all these header fields. There
are no mechanisms for determining which was the intended data packet
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
can be imposed by the ISDN, and therefore payloads longer than this
will never reach the destination if such interworking occurs.
[johnston-cuss-sip-uui] defines a "uui" option tag for use with the
UUI mechanism extension. Because for the ISDN UUI service, the
service is service 1 implicit, the inclusion of the "uui" option tag
in a Supported header field conveys no additional information over
and above the presence of the User-to-User header field with the
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"app" header field parameter to "isdn-uui" in the INVITE request.
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 a usage is not precluded although it
does not form part of the application.
8. UAS requirements
The UAS MUST meet the requirements of [johnston-cuss-sip-uui] in
addition to the requirements defined in this document.
The UAS MUST only use this application of the UUI mechanism extension
in association with the initial INVITE method and 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.
The UAS MUST NOT include the User-to-User header field with an "app"
header field parameter set to "isdn-uui" in any message of an INVITE
dialog if the original INVITE request did not include the User-to-
User header field with an "app" header field parameter set to "isdn-
uui"
The UAS MAY include the User-to-User header field in responses to the
INVITE request, or subsequent BYE requests or responses within the
dialog, only where the original INVITE request included a User-to-
User header field with the "app" header field parameter to "isdn-
uui". When sending UUI for the ISDN application, the UAS MUST set
the User-to-User "app" header field parameter to "isdn-uui". The UAS
MUST NOT include more than one User-to-User header field for this
application in any SIP request or response.
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 "content"
header field parameter to "isdn-uui", the UAC MUST discard this
header fields.
When receiving UUI, when multiple User-to-User header fields are
received from the originating user in the same request with the
"content" header field parameter to "isdn-uui", the UAC MUST discard
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all these header fields. There are no mechanisms for determining
which was the intended data packet so all are discarded.
9. UUI contents
These requirements apply when the "app" header field parameter is set
to "isdn-uui". 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 application document for that
value applies.
When sending UUI, the sending application MAY, but need not, include
a "content" header field with a value set to "isdn-uui". A receiving
application MUST ignore a received User-to-User header field if the
"content" header field parameter is present and the value is some
other value that "isdn-uui".
When sending UUI, the sending application MAY, but need not, include
an "encoding" header field with a value set to "hex". A receiving
application MUST ignore a received User-to-User header field if the
"encoding" header field parameter is present and the value is some
other value that "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 payload information. It is up
to the receiving application what it does with this value.
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"
application on dialogs that are interworked.
When mapping data content from the ISDN to the 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, and it is for the application usage to sort out any
discrepancy. The same applies to the ISDN protocol discrimination
defined table 4-26 of ITU-T Recommendation Q.931 [Q931] as the first
octet of the payload information; the interworking gateway will not
perform any additional checking of this value.
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[johnston-cuss-sip-uui] defines a "uui" option tag for use with the
UUI mechanism extension. 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
"app" header field parameter.
This document defines "isdn-uui" as a new value of the User-to-User
"content" header field parameter.
12. IANA Considerations
This document adds the following row to the "UUI application values"
section of the SIP parameter registry:
Value: isdn-uui
Meaning: 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: RFCXXXX
Contact:
This document adds the following row to the "UUI content values"
section of the SIP parameter registry:
Value: isdn-uui
Meaning: The associated contents conforms to the content
associated with the ISDN user-to-user service. In the presence of
the "app" header field parameter set to "isdn-uui" this is the
default meaning and therefore need not be included in this case.
Reference: RFCXXXX
Contact:
Editor's Note: [RFCXXXX] should be replaced with the designation of
this document.
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13. Security Considerations
This document contains no specific requirements in regard to
security. The overlying use case will define the security measures
required. The underlying user-to-user extension provides a number of
tools that can meet certain security requirements. As a level of
guidance, data that is used to assist in selecting which SIP UA
should respond to the call would not be expected to carry any higher
level of security than a media feature tag. 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 application,
particularly taking into account the statement in the following
paragraph.
As this capability to is defined to interwork with the ISDN, if the
ISDN forms part of the route, any usage needs to assume that the
security level of the ISDN is the highest level of security
available. As the ISDN security is itself not definable on an end-
to-end basis, this can be an unknown quantity. This is because ISDN
security exists on a hop-by-hop basis, and is only as secure as the
least secure component. 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
user-to-user application, or indeed, a different method of data
transfer, needs to be selected by the application user.
14. Acknowledgements
Joanne McMillen was a major contributor and co-author of earlier
versions of this document.
Thanks to Spencer Dawkins, Vijay Gurbani, and Laura Liess for their
review of earlier versions of this document. The authors wish to
thank Francois Audet, Denis Alexeitsev, Paul Kyzivat, Cullen
Jennings, and Mahalingam Mani for their comments.
15. Changes since previous versions
Note to RFC editor: This section is to be deleted before final
publication.
Changes since made in the creation of the -01 version from the -00
version.
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Closure of a number of open issues identified in the -00 version
and the creation of appropriate procedures for the UAC, the UAS,
and the ISDN interworking gateway.
16. Normative References
[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.
[Q931] "ITU-T Recommendation Q.931: Digital subscriber Signalling
System No. 1 - Network layer; ISDN user-network interface
layer 3 specification for basic call control",
http://www.itu.int/rec/T-REC-Q.931-199805-I/en .
[Q957.1] "ITU-T Recommendation Q.957.1: 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)",
http://www.itu.int/rec/T-REC-Q.957.1-199607-I .
[Q763] "ITU-T Q.763 Signaling System No. 7 - ISDN user part
formats and codes",
http://www.itu.int/rec/T-REC-Q.931-199805-I/en .
[ANSII] "ANSI T1.643-1995, Telecommunications-Integrated Services
Digital Network (ISDN)-Explicit Call Transfer
Supplementary Service".
[ETSI] "ETSI ETS 300 207-1 Ed.1 (1994), Integrated Services
Digital Network (ISDN); Diversion supplementary
services".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol
for Telephones (SIP-T): Context and Architectures",
BCP 63, RFC 3372, September 2002.
[johnston-cuss-sip-uui-reqs]
Johnston, A., McMillen, J., and L. Liess, "Problem
Statement and Requirements for Transporting User to User
Call Control Information in SIP",
draft-johnston-cuss-sip-uui-reqs-00 .
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[johnston-cuss-sip-uui]
Johnston, A. and J. Rafferty, "A Mechanism for
Transporting User to User Call Control Information in
SIP", draft-johnston-cuss-sip-uui-00 .
Authors' Addresses
Keith Drage (editor)
Alcatel-Lucent
Quadrant, Stonehill Green, Westlea
Swindon
UK
Email: keith.drage@alcatel-lucent.com
Alan Johnston
Avaya
St. Louis, MO 63124
United States
Email: alan.b.johnston@gmail.com
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