Internet Engineering Task Force (IETF) A. Johnston
Request for Comments: 7433 Avaya
Category: Standards Track J. Rafferty
ISSN: 2070-1721 Human Communications
A Mechanism for Transporting User-to-User Call Control Information
There is a class of applications that benefit from using SIP to
exchange User-to-User Information (UUI) data during session
establishment. This information, known as call control UUI data, is
a small piece of data inserted by an application initiating the
session and utilized by an application accepting the session. The
syntax and semantics for the UUI data used by a specific application
are defined by a UUI package. This UUI data is opaque to SIP and its
function is unrelated to any basic SIP function. This document
defines a new SIP header field, User-to-User, to transport UUI data,
along with an extension mechanism.
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
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements Discussion . . . . . . . . . . . . . . . . . . . 4
4. Normative Definition . . . . . . . . . . . . . . . . . . . . 5
4.1. Syntax for UUI Header Field . . . . . . . . . . . . . . . 6
4.2. Hex Encoding Definition . . . . . . . . . . . . . . . . . 7
4.3. Source Identity of UUI Data . . . . . . . . . . . . . . . 7
5. Guidelines for UUI Packages . . . . . . . . . . . . . . . . . 9
5.1. Extensibility . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1. Registration of User-to-User Header Field . . . . . . . . 11
6.2. Registration of User-to-User Header Field Parameters . . 11
6.3. Registration of UUI Packages . . . . . . . . . . . . . . 11
6.4. Registration of UUI Content Parameters . . . . . . . . . 12
6.5. Registration of UUI Encoding Parameters . . . . . . . . . 12
6.6. Registration of SIP Option Tag . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Other Possible Mechanisms . . . . . . . . . . . . . 17
A.1. Why INFO is Not Used . . . . . . . . . . . . . . . . . . 17
A.2. Why Other Protocol Encapsulation UUI Mechanisms Are Not
Used . . . . . . . . . . . . . . . . . . . . . . . . . . 17
A.3. MIME Body Approach . . . . . . . . . . . . . . . . . . . 17
A.4. URI Parameter . . . . . . . . . . . . . . . . . . . . . . 18
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
This document describes the transport of UUI data using SIP
[RFC3261]. It defines a mechanism for the transport of general
application UUI data and for the transport of the call control
related ITU-T Recommendation Q.931 User-user information element
[Q931] and ITU-T Recommendation Q.763 User-to-User information
parameter [Q763] data in SIP. UUI data is widely used in the Public
Switched Telephone Network (PSTN) today for contact centers and call
centers. There is also a trend for the related applications to
transition from ISDN to SIP. The UUI extension for SIP may also be
used for native SIP User Agents (UAs) implementing similar services
and to interwork with ISDN services. Note that in most cases, there
is an a priori understanding between the UAs in regard to what to do
with received UUI data. This document enables the definition of
packages and related attributes that can make such understandings
The UUI mechanism is designed to meet the use cases, requirements,
and call flows for SIP call control UUI detailed in [RFC6567]. All
references to requirement numbers (REQ-N) and figure numbers refer to
The mechanism is a new SIP header field, along with a new SIP option
tag. The header field carries the UUI data, along with parameters
indicating the encoding of the UUI data, the UUI package, and
optionally the content of the UUI data. The package definition
contains details about how a particular application can utilize the
UUI mechanism. The header field can be included (sometimes called
"escaped") into URIs supporting referral and redirection scenarios.
In these scenarios, the History-Info header field is used to indicate
the inserter of the UUI data. The SIP option tag can be used to
indicate support for the header field. Support for the UUI header
field indicates that a UA is able to extract the information in the
UUI data and pass it up the protocol stack. Individual packages
using the UUI mechanism can utilize SIP media feature tags to
indicate that a UA supports a particular UUI package. Guidelines for
defining UUI packages are provided.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
Note that the <allOneLine> tag convention from SIP Torture Test
Messages [RFC4475] is used to show that there are no line breaks in
the actual message syntax.
3. Requirements Discussion
This section describes how the User-to-User header field meets the
requirements in [RFC6567]. The header field can be included in
INVITE requests and responses and BYE requests and responses, meeting
REQ-1 and REQ-2.
For redirection and referral use cases and REQ-3, the header field is
included (escaped) within the Contact or Refer-To URI. The details
of this mechanism as it applies for redirection and referral use
cases are covered in Section 4.1.
Since SIP proxy forwarding and retargeting does not affect header
fields, the header field meets REQ-4.
The UUI header field will carry the UUI data and not a pointer to the
data, so REQ-5 is met.
Since the basic design of the UUI header field is similar to the ISDN
UUI service, interworking with PSTN protocols is straightforward and
is documented in a separate specification [RFC7434], meeting REQ-6.
Requirements REQ-7, REQ-8, and REQ-10 relate to discovery of the
mechanism and supported packages, and hence applications. REQ-7
relates to support of the UUI header field, while REQ-8 relates to
routing based on support of the UUI header field. REQ-7 is met by
defining a new SIP option tag "uui". The use of a Require:uui in a
request or Supported:uui in an OPTIONS response could be used to
require or discover support of the mechanism. The presence of a
Supported:uui or Require:uui header field can be used by proxies to
route to an appropriate UA, meeting REQ-8. However, note that only
UAs are expected to understand the UUI data -- proxies and other
intermediaries do not. REQ-10 is met by utilizing SIP feature tags
[RFC3840]. For example, the feature tag "sip.uui-isdn" could be used
to indicate support of the ISDN UUI package, or "sip.uui-pk1" could
be used to indicate support for a particular package, pk1.
Proxies commonly apply policy to the presence of certain SIP header
fields in requests by either passing them or removing them from
requests. REQ-9 is met by allowing proxies and other intermediaries
to remove UUI header fields in a request or response based on policy.
Carrying UUI data elements of at least 129 octets is trivial in the
UUI header field, meeting REQ-11. Note that avoiding having very
large UUI data elements is a good idea, as SIP header fields have
traditionally not been large.
To meet REQ-12 for the redirection and referral use cases, the
History-Info header field [RFC7044] can be used. In these
retargeting cases, the changed Request-URI will be recorded in the
History-Info header field along with the identity of the element that
performed the retargeting.
The requirement for integrity protection in REQ-13 could be met by
the use of an S/MIME signature over a subset of header fields, as
defined in "SIP Header Privacy and Integrity using S/MIME: Tunneling
SIP", Section 23.4 of RFC 3261. Note that the lack of deployment of
S/MIME with SIP means that, in general, REQ-13 is not met. The
requirement of REQ-14 for end-to-end privacy could be met using
S/MIME or using encryption at the application layer. Note that the
use of S/MIME to secure the UUI data will result in an additional
body being added to the request. Hop-wise Transport Layer Security
(TLS) [RFC5246] allows the header field to meet REQ-15 for hop-by-hop
4. Normative Definition
This document defines a new SIP header field "User-to-User" to
transport call control UUI data to meet the requirements in
To help tag and identify the UUI data used with this header field,
"purpose", "content", and "encoding" header field parameters are
defined. The "purpose" header field parameter identifies the package
that defines the generation and usage of the UUI data for a
particular application. The value of the "purpose" parameter is the
package name, as registered in the "UUI Packages" subregistry defined
in Section 6.3. For the case of interworking with the ISDN UUI
service, the ISDN UUI service interworking package is used. The
default value for the "purpose" header field is "isdn-uui" as defined
in [RFC7434]. If the "purpose" header field parameter is not
present, the ISDN UUI MUST be used. The "content" header field
parameter identifies the actual content of the UUI data. If not
present, the default content defined for the package MUST be used.
Newly defined UUI packages MUST define or reference at least a
default "content" value. The "encoding" header field parameter
indicates the method of encoding the information in the UUI data
associated with a particular "content" value. This specification
only defines "encoding=hex". If the "encoding" header field
parameter is not present, the default encoding defined for the
package MUST be used.
UUI data is considered an opaque series of octets. This mechanism
MUST NOT be used to convey a URL or URI, since the Call-Info header
field in [RFC3261] already supports this use case.
4.1. Syntax for UUI Header Field
The UUI header field can be present in INVITE requests and responses
and in BYE requests and responses. Note that when the UUI header is
used in responses, it can only be utilized in end-to-end responses,
e.g., 1xx (excluding 100), 2xx, and 3xx responses.
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) as described in RFC 5234 and extends RFC 3261 (where
token, quoted-string, and generic-param are defined).
UUI = "User-to-User" HCOLON uui-value *(COMMA uui-value)
uui-value = uui-data *(SEMI uui-param)
uui-data = token / quoted-string
uui-param = pkg-param / cont-param / enc-param / generic-param
pkg-param = "purpose" EQUAL pkg-param-value
pkg-param-value = token
cont-param = "content" EQUAL cont-param-value
cont-param-value = token
enc-param = "encoding" EQUAL enc-param-value
enc-param-value = token / "hex"
Each package defines how many User-to-User header fields of each
package may be present in a request or a response. A sender MAY
include multiple User-to-User header fields, and a receiver MUST be
prepared to receive multiple User-to-User header fields. Consistent
with the rules of SIP syntax, the syntax defined in this document
allows any combination of individual User-to-User header fields or
User-to-User header fields with multiple comma separated UUI data
elements. Any size limitations on the UUI data for a particular
purpose are to be defined by the related UUI package.
UAs SHALL ignore UUI data from packages or encoding that they do not
For redirection use cases, the header field is included (escaped)
within the Contact URI. For referral use cases, the header field is
included (escaped) within the Refer-To URI. For example, if a UA
supports this specification, it SHOULD include any UUI data included
in a redirection URI (if the UUI data and encoding is understood).
Note that redirection can occur multiple times to a request.
Currently, UAs that support attended transfer support the ability to
include a Replaces header field [RFC3891] into a Refer-To URI, and
when acting upon this URI, UAs add the Replaces header field to the
triggered INVITE. This sort of logic and behavior is utilized for
the UUI header field (that is, the UUI header field is included in
the triggered INVITE). The UA processing the REFER [RFC3515] or the
3xx response to the INVITE SHOULD support the UUI mechanism. If the
REFER or redirect target does not support UUI, the UUI header will be
discarded as per [RFC3261]. However, this may limit the utility of
use cases that depend upon the UUI being supported by all elements.
Here is an example of an included User-to-User header field from the
redirection response F2 of Figure 2 in [RFC6567]:
The resulting INVITE F4 would contain:
4.2. Hex Encoding Definition
This specification defines hex encoding of UUI data. When the value
of "hex" is used in the "encoding" parameter of a header field, the
data is encoded using base16 encoding according to Section 8 of
[RFC4648]. The hex-encoded value is normally represented using the
"token" construction from RFC 3261, although the "quoted-string"
construction is permitted, in which case the quotes MUST be ignored.
If a canonicalized version of a normally case-insensitive hex encoded
UUI data object is needed for a digital signature or integrity
checking, then the base16 encoding with all upper case MUST be used.
4.3. Source Identity of UUI Data
It is important for the recipient of UUI data to know the identity of
the UA that inserted the UUI data. In a request without a History-
Info header field, the identity of the entity that inserted the UUI
data will be assumed to be the source of the SIP message. For a SIP
request, typically this is the UA identified by the URI in the From
header field or a P-Asserted-Identity [RFC3325] header field. In a
request with a History-Info header field, the recipient needs to
parse the Targeted-to-URIs present (hi-targeted-to-uri defined in
[RFC7044]) to see if any included User-to-User header fields are
present. If an included User-to-User header field is present and
matches the UUI data in the request, this indicates that redirection
has taken place, resulting in the inclusion of UUI data in the
request. The inserter of the UUI data will be the UA identified by
the Targeted-to-URI of the History-Info element prior to the element
with the included UUI data. In a response, the inserter of the UUI
data will be the identity of the UA that generated the response.
Typically, this is the UA identified in the To header field of the
response. Note that any updates to this identity by use of the SIP
connected identity extension [RFC4916] or other identity modifiers
will update this information.
For an example of History-Info and redirection, consider Figure 2
from [RFC6567] where the Originating UA is Carol, the Redirector Bob,
and the Terminating UA Alice. The INVITE F4 containing UUI data
INVITE sips:email@example.com SIP/2.0
Via: SIP/2.0/TLS lab.example.com:5061
To: Bob <sips:firstname.lastname@example.org>
From: Carol <sips:email@example.com>;tag=323sf33k2
Without the redirection captured in the History-Info header field,
Alice would conclude that the UUI data was inserted by Carol.
However, the History-Info containing UUI data (index=1.1) indicates
that the inserter was Bob (index=1).
To enable maintaining a record of the inserter identity of UUI data,
UAs supporting this mechanism SHOULD support History-Info [RFC7044]
and include Supported: histinfo in all requests and responses.
If a border element such as a proxy or a Back-to-Back User Agent
(B2BUA) removes a History-Info header field containing a User-to-User
parameter, the UA consuming the UUI data may not be able at the SIP
level to identify the source of the UUI data.
5. Guidelines for UUI Packages
UUI packages defined using this SIP UUI mechanism MUST follow the
"Standards Action" guideline as defined in [RFC5226] and publish a
Standards Track RFC that describes the usage. The CUSS WG chose to
adopt this conservative policy while it considers other potential
registration policies. Note that this mechanism is not suitable for
the transport of arbitrary data between UAs. The following
guidelines are provided to help determine if this mechanism is
appropriate or not. The SIP UUI mechanism is applicable when all of
the following conditions are met:
1. The information is generated and consumed by an application
during session setup using SIP, but the application is not
necessarily SIP aware.
2. The behavior of SIP entities that support it is not significantly
changed (as discussed in Section 4 of [RFC5727]).
3. UAs are the generators and consumers of the UUI data. Proxies
and other intermediaries may route based on the presence of a
User-to-User header field or a particular package tag but do not
otherwise consume or generate the UUI data.
4. There are no privacy issues associated with the information being
transported (e.g., geolocation or emergency-related information
are examples of inappropriate UUI data).
5. The UUI data is not being utilized for User-to-User Remote
Procedure Calls (RPCs).
UUI packages define the semantics for a particular application usage
of UUI data. The content defines the syntax of the UUI data, while
the encoding defines the encoding of the UUI data for the content.
Each content is defined as a stream of octets, which allows multiple
encodings of that content. For example, packages may define:
1. The SIP methods and responses in which the UUI data may be
2. The maximum number of UUI data elements that may be inserted into
a request or response. The default is one per encoding. Note
that a UA may still receive a request with more than this maximum
number due to redirection. The package needs to define how to
handle this situation.
3. The default values for content and encoding if they are not
present. If the same UUI data may be inserted multiple times
with different encodings, the package needs to state this. A
package may support and define multiple contents and their
associated encodings and reuse contents defined by other
4. Any size limitations on the UUI data. Size needs to be specified
in terms of the octet stream output of the content, since the
size of the resulting uui-data element will vary depending on the
A package MUST define a "purpose" header field value to identify the
package in the coding. A package MUST describe the new application
that is utilizing the UUI data and provide some use case examples.
The default "content" value MUST be defined or referenced in another
document for the package. Additional allowed contents MAY also be
defined or referenced. Any restrictions on the size of the UUI data
MUST be described. In addition, a package MAY define a media feature
tag per [RFC3840] to indicate support for this UUI package. For
example, the media feature tag "sip.uui-pk1" could be defined to
indicate support for a UUI package named pk1. The definition of a
new SIP option tag solely to identify support for a UUI package is
NOT RECOMMENDED unless there are additional SIP behaviors needed to
implement this feature.
For an example UUI package definition, see [RFC7434].
New "content" values MUST describe the semantics of the UUI data and
valid encodings, and give some example use cases. A previously
defined UUI content value can be used in a new package. In this
case, the semantics and usage of the content by the new package is
defined within the new package. New UUI content types cannot be
added to existing packages -- instead, a new package would need to be
defined. New content values that are defined are added to the IANA
registry with a Standards Track RFC, which needs to discuss the
issues in this section. If no new encoding value is defined for a
content, the encoding defaults to "hex" as defined in this document.
In this case, the "hex" value will be explicitly stated via the
encoding parameter as the encoding for the content.
New "encoding" values associated with a new content MUST reference a
specific encoding scheme (such as "hex", which is defined in this
specification) or define the new encoding scheme. A previously
defined UUI encoding value can be used with a newly defined content.
In this case, the usage of the encoding is defined by the content
definition. New UUI encodings cannot be added to existing contents
-- instead, a new content would need to be defined. Newly defined
encoding values are added to the IANA registry with a Standards Track
RFC, which needs to discuss the issues in this section.
6. IANA Considerations
6.1. Registration of User-to-User Header Field
This document defines a new SIP header field named "User-to-User".
The following row has been added to the "Header Fields" section of
the SIP parameter registry:
| Header Name | Compact Form | Reference |
| User-to-User | | [RFC7433] |
6.2. Registration of User-to-User Header Field Parameters
This document defines the parameters for the header field defined in
the preceding section. The header field "User-to-User" can contain
the parameters "encoding", "content", and "purpose".
The following rows have been added to the "Header Field Parameters
and Parameter Values" section of the SIP parameter registry:
| Header Field | Parameter Name | Predefined Values | Reference |
| User-to-User | encoding | Yes | [RFC7433] |
| User-to-User | content | No | [RFC7433] |
| User-to-User | purpose | No | [RFC7433] |
6.3. Registration of UUI Packages
This specification establishes the "UUI Packages" subregistry under
The descriptive text for this subregistry is:
UUI packages provide information about the usage of the UUI data in a
User-to-User header field [RFC7433].
The registration policy for this registry is "Standards Action" as
defined in [RFC5226].
| Package | Description | Reference |
6.4. Registration of UUI Content Parameters
This specification establishes the "UUI Content Parameters"
subregistry under <http://www.iana.org/assignments/sip-parameters>.
The descriptive text for this subregistry is:
UUI content provides information about the content of the UUI data in
a User-to-User header field [RFC7433].
The registration policy for this registry is "Standards Action" as
defined in [RFC5226].
| Content | Description | Reference |
6.5. Registration of UUI Encoding Parameters
This specification establishes the "UUI Encoding Parameters"
subregistry under <http://www.iana.org/assignments/sip-parameters>
and initiates its population with the table below.
The descriptive text for this subregistry is:
UUI encoding provides information about the encoding of the UUI data
in a User-to-User header field [RFC7433].
The registration policy for this registry is "Standards Action" as
defined in [RFC5226].
| Encoding | Description | Reference |
| hex | The UUI data is encoded using hexadecimal | [RFC7433] |
6.6. Registration of SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of [RFC3261].
This document defines the SIP option tag "uui".
The following row has been added to the "Option Tags" section of the
SIP Parameter Registry:
| Name | Description | Reference |
| uui | This option tag is used to indicate that | [RFC7433] |
| | a UA supports and understands the | |
| | User-to-User header field. | |
7. Security Considerations
UUI data can potentially carry sensitive information that might
require confidentiality protection for privacy or integrity
protection from third parties that may wish to read or modify the UUI
data. The three security models described in [RFC6567] may be
applicable for the UUI mechanism.
One model treats the SIP layer as untrusted and requires end-to-end
integrity protection and/or encryption. This model can be achieved
by providing these security services at a layer above SIP. In this
case, applications are encouraged to use their own integrity and/or
encryption mechanisms before passing it to the SIP layer.
The second approach is for the application to pass the UUI without
any protection to the SIP layer and require the SIP layer to provide
this security. This approach is possible in theory, although its
practical use would be extremely limited. To preserve multi-hop or
end-to-end confidentiality and integrity of UUI data, approaches
using S/MIME or IPsec can be used, as discussed in the review of
REQ-13 and REQ-14 in Section 3 of this document. However, the lack
of deployment of these mechanisms means that applications cannot in
general rely on them being present.
The third model utilizes a trust domain and relies on perimeter
security at the SIP layer. This is the security model of the PSTN
and ISDN where UUI is commonly used today. This approach uses hop-
by-hop security mechanisms and relies on border elements for
filtering and application of policy. Standard deployed SIP security
mechanisms such as TLS transport offer privacy and integrity
protection properties on a hop-by-hop basis at the SIP layer.
If the UUI data was included by the UA originator of the SIP request
or response, normal SIP mechanisms can be used to determine the
identity of the inserter of the UUI data. If the UUI data was
included by a UA that was not the originator of the request, a
History-Info header field can be used to determine the identity of
the inserter of the UUI data. UAs can apply policy based on the
origin of the UUI data using this information. In short, the UUI
data included in an INVITE can be trusted as much as the INVITE
itself can be trusted.
Note that it is possible that this mechanism could be used as a
covert communication channel between UAs, conveying information
unknown to the SIP network.
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
[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>.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003,
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004,
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006,
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006,
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, June 2007,
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008, <http://www.rfc-editor.org/info/rfc5226>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
[RFC7044] Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
C. Holmberg, "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 7044,
February 2014, <http://www.rfc-editor.org/info/rfc7044>.
[RFC7434] Drage, K. and A. Johnston, "Interworking ISDN Call Control
User Information with SIP", RFC 7434, January 2015,
8.2. Informative References
[Q1980] ITU-T, "The Narrowband Signalling Syntax (NSS) - Syntax
Definition", ITU-T Recommendation Q.1980.1,
[Q763] ITU-T, "Signalling System No. 7 - ISDN User Part formats
and codes", ITU-T Recommendation Q.763,
[Q931] ITU-T, "ISDN user-network interface layer 3 specification
for basic call control", ITU-T Recommendation Q.931,
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002, <http://www.rfc-editor.org/info/rfc3325>.
[RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol
for Telephones (SIP-T): Context and Architectures", BCP
63, RFC 3372, September 2002,
[RFC4475] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
and H. Schulzrinne, "Session Initiation Protocol (SIP)
Torture Test Messages", RFC 4475, May 2006,
[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change Process
for the Session Initiation Protocol (SIP) and the Real-
time Applications and Infrastructure Area", BCP 67, RFC
5727, March 2010,
[RFC6086] Holmberg, C., Burger, E., and H. Kaplan, "Session
Initiation Protocol (SIP) INFO Method and Package
Framework", RFC 6086, January 2011,
[RFC6567] Johnston, A. and L. Liess, "Problem Statement and
Requirements for Transporting User-to-User Call Control
Information in SIP", RFC 6567, April 2012,
Appendix A. Other Possible Mechanisms
Two other possible mechanisms for transporting UUI data will be
described: MIME body and URI parameter transport.
A.1. Why INFO is Not Used
Since the INFO method [RFC6086] was developed for ISDN User Part
(ISUP) interworking of User-to-User Information, it might seem to be
the logical choice here. For non-call control User-to-User
Information, INFO can be utilized for end-to-end transport. However,
for transport of call control User-to-User Information, INFO can not
be used. As the call flows in [RFC6567] show, the information is
related to an attempt to establish a session and needs to be passed
with the session setup request (INVITE), responses to that INVITE, or
session termination requests. As a result, it is not possible to use
INFO in these cases.
A.2. Why Other Protocol Encapsulation UUI Mechanisms Are Not Used
Other protocols have the ability to transport UUI data. For example,
consider the ITU-T Recommendation Q.931 User-user information element
[Q931] and the ITU-T Recommendation Q.763 User-to-User information
parameter [Q763]. In addition, the Narrowband Signalling System
(NSS) [Q1980] is also able to transport UUI data. Should one of
these protocols be in use, and present in both User Agents, then
utilizing these other protocols to transport UUI data might be a
logical solution. Essentially, this is just adding an additional
layer in the protocol stack. In these cases, SIP is not transporting
the UUI data; it is encapsulating another protocol, and that protocol
is transporting the UUI data. Once a mechanism to transport that
other protocol using SIP exists, the UUI data transport function is
essentially obtained without any additional effort or work.
However, the CUSS working group believes, consistent with its
charter, that SIP needs to have its own native UUI data transport
mechanism. It is not reasonable for a SIP UA to have to implement
another entire protocol (either ISDN or NSS, for example) just to get
the very simple UUI data transport service. Of course, this work
does not preclude anyone from using other protocols with SIP to
transport UUI data.
A.3. MIME Body Approach
One method of transport is to use a MIME body. This is in keeping
with the Session Initiation Protocol for Telephones (SIP-T)
architecture [RFC3372] in which MIME bodies are used to transport
ISUP information. Since the INVITE will normally have a Session
Description Protocol (SDP) message body, the resulting INVITE with
SDP and UUI data will be multipart MIME. This is not ideal as many
SIP UAs do not support multipart MIME INVITEs.
A bigger problem is the insertion of a UUI message body by a redirect
server or in a REFER. The body would need to be encoded in the
Contact URI of the 3xx response or the Refer-To URI of a REFER.
Currently, the authors are not aware of any UAs that support this
capability today for any body type. As such, the complete set of
semantics for this operation would need to be determined and defined.
Some issues will need to be resolved, such as, do all the Content-*
header fields have to be included as well? And, what if the included
Content-Length does not agree with the included body?
Since proxies cannot remove a body from a request or response, it is
not clear how this mechanism could meet REQ-9.
The requirement for integrity protection could be met by the use of
an S/MIME signature over the body, as defined in "Securing MIME
bodies", Section 23.3 of RFC 3261. Alternatively, this could be
achieved using [RFC4474]. The requirement for end-to-end privacy
could be met using S/MIME encryption or using encryption at the
application layer. However, note that neither S/MIME or RFC 4474
enjoys deployment in SIP today.
As such, the MIME body approach meets REQ-1, REQ-2, REQ-4, REQ-5,
REQ-7, REQ-11, REQ-13, and REQ-14. Meeting REQ-12 seems possible,
although the authors do not have a specific mechanism to propose.
Meeting REQ-3 is problematic but not impossible for this mechanism.
However, this mechanism does not seem to be able to meet REQ-9.
A.4. URI Parameter
Another proposed approach is to encode the UUI data as a URI
parameter. This UUI parameter could be included in a Request-URI or
in the Contact URI or Refer-To URI. It is not clear how it could be
transported in a response that does not have a Request-URI, or in BYE
requests or responses.
An INVITE sent to this Contact URI would contain UUI data in the
Request-URI of the INVITE. The URI parameter has a drawback in that
a URI parameter carried in a Request-URI will not survive retargeting
by a proxy as shown in Figure 2 of [RFC6567]. That is, if the URI is
included with an Address of Record instead of a Contact URI, the URI
parameter in the Request-URI will not be copied over to the Contact
URI, resulting in the loss of the information. Note that if this
same URI was present in a Refer-To header field, the same loss of
information would occur.
The URI parameter approach would meet REQ-3, REQ-5, REQ-7, REQ-9, and
REQ-11. It is possible the approach could meet REQ-12 and REQ-13.
The mechanism does not appear to meet REQ-1, REQ-2, REQ-4, and
Joanne McMillen was a major contributor and coauthor of earlier
versions of this document. Thanks to Paul Kyzivat for his
contribution of hex encoding rules. Thanks to Spencer Dawkins, Keith
Drage, Vijay Gurbani, and Laura Liess for their review of the
document. The authors wish to thank Roland Jesske, Celine Serrut-
Valette, Francois Audet, Denis Alexeitsev, Paul Kyzivat, Cullen
Jennings, and Mahalingam Mani for their comments. Thanks to Scott
Kelly and Joel Halpern for their reviews.
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