Independent Submission M. Mohali

Independent Submission M. Mohali
Request for Comments: 7544 Orange
Obsoletes: 6044 August 2015
Category: Informational
ISSN: 2070-1721

Mapping and Interworking of Diversion Information between Diversion and
History-Info Header Fields in the Session Initiation Protocol (SIP)

Abstract

Although the SIP History-Info header field described in RFC 7044 is
the solution adopted in IETF, the non-standard Diversion header field
described, as Historic, in RFC 5806 is nevertheless already
implemented and used for conveying call-diversion-related information
in Session Initiation Protocol (SIP) signaling.

RFC 7044 obsoletes the original RFC 4244 and redefines the History-
Info header field for capturing the history information in requests.

Since the Diversion header field is used in existing network
implementations for the transport of call diversion information, its
interworking with the SIP History-Info standardized solution is
needed. This document describes a recommended interworking guideline
between the Diversion header field and the History-Info header field
to handle call diversion information. This work is intended to
enable the migration from non-standard implementations toward IETF
specification-based implementations.

This document obsoletes RFC 6044, which describes the interworking
between the Diversion header field defined in RFC 5806 and the
obsoleted History-Info header field defined on RFC 4244.

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Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see 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/rfc7544.

Copyright Notice

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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.

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Table of Contents

1. Introduction ....................................................4
1.1. Overview ...................................................4
1.2. Background .................................................4
1.3. From RFC 4244 to RFC 7044 ..................................5
2. Problem Statement ...............................................5
3. Interworking Recommendations ....................................7
3.1. General Recommendations ....................................7
3.2. Privacy Considerations .....................................8
3.3. Headers in SIP Method .....................................10
3.4. SIP Network/Terminal Using Diversion Header Field
to SIP Network/Terminal Using History-Info Header Field ...10
3.5. SIP Network/Terminal Using History-Info Header
Field to SIP Network/Terminal Using Diversion
Header Field ..............................................12
4. Reminder of the Syntax for Header Fields .......................13
4.1. History-Info Header Field Syntax ..........................13
4.2. Diversion Header Field Syntax .............................16
5. Diversion Header Field to History-Info Header Field ............16
6. History-Info Header Field to Diversion Header Field ............20
7. Examples .......................................................22
7.1. Example with Diversion Header Field Changed into
History-Info Header Field .................................22
7.2. Example with History-Info Header Field Changed into
Diversion Header Field ....................................22
7.3. Example with Two SIP Networks Using History-Info Header
Field Interworking with a SIP Network Using Diversion
Header Field ..............................................22
7.4. Additional Interworking Cases .............................24
8. Backward Compatibility .........................................26
9. Security Considerations ........................................26
10. References ....................................................26
10.1. Normative References .....................................26
10.2. Informative References ...................................27
Appendix A. Interworking between Diversion Header Field and
Voicemail URI ........................................29
A.1. Diversion Header Field to Voicemail URI ...................29
A.2. Voicemail URI to Diversion Header Field ...................29
Acknowledgements ..................................................30
Author's Address ..................................................30

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1. Introduction

1.1. Overview

For some services based on VoIP (Voice over IP) services (e.g.,
voicemail, Interactive Voice Recognition (IVR), or automatic call
distribution), it is helpful for the called SIP user agent to
identify from whom and why the session was diverted. For this
information to be used by various service providers or by
applications, it needs to pass through the network. This is possible
with two different SIP header fields: the History-Info header field
defined in [RFC7044] and the historic Diversion header field defined
in [RFC5806]. Both of these header fields are able to transport
diversion information in SIP signaling.

Although the Diversion header field is not standardized, it has been
widely implemented. Therefore, it is useful to have guidelines to
make this header field interwork with the standard History-Info
header field.

Note that new implementation and deployment of the Diversion header
field are strongly discouraged.

This document provides a mechanism for the translation of header
field content between the Diversion header field and the History-Info
header field.

This document obsoletes [RFC6044].

1.2. Background

The obsoleted History-Info header field [RFC4244] and its extension
for forming SIP service URIs (including Voicemail URI) [RFC4458] used
to be recommended by IETF to convey redirection information. They
also used to be recommended in the Communication Diversion (CDIV)
3GPP specification [TS_24.604].

The Diversion header field was originally described in a document
that was submitted to the SIP Working Group and was eventually
published as an Independent Submission as [RFC5806] for the
historical record; it serves as a reference for this RFC.

This header field contains a list of diverting URIs and associated
information providing specific information as the reason for the call
diversion. Most of the first SIP-based implementations have
implemented the Diversion header field when no standard solution was
ready to deploy. The IETF has standardized the History-Info header
field partly because it can transport general history information.

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This allows the receiving party to determine how and why the session
is received. As the History-Info header field may contain further
information than call diversion information, it is critical to avoid
losing information and to be able to extract the relevant data using
the retargeting cause URI parameter described in [RFC4458] for the
transport of the call forwarding reason.

The Diversion header field and the History-Info header field have
different syntaxes, which are described in this document. Note that
the main difference is that the History-Info header field is a
chronological writing header whereas the Diversion header field
applies a reverse chronology (i.e., the first diversion entry read
corresponds to the last diverting user).

Appendix A provides an interworking guideline between the Diversion
header field and the Voicemail URI, which is another way to convey
diversion information without using the History-Info header field.
The Voicemail URI is defined in [RFC4458].

1.3. From RFC 4244 to RFC 7044

The details of why and how [RFC4244] was obsoleted by [RFC7044] are
provided in Section 16 of [RFC7044].

The main changes for implementation of the History-Info header field
are as follows:

1. The header field parameters "mp", "rc", and "np" were added to
capture the specific method by which a target is determined.

2. A way to indicate a gap in the History-Info header field was
added by using a "0" in the index.

3. To apply privacy, entries were anonymized rather than removed.

4. Many SHOULDs were changed into MUSTs to have a more reliable
header.

Backward-compatibility aspects are discussed in Section 8 of this
document.

2. Problem Statement

This section provides the baseline terminology used in the rest of
the document and defines the scope of interworking between the
Diversion header field and the History-Info header field.

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There are many ways in which SIP signaling can be used to modify a
session destination before it is established and many reasons for
doing so. The behavior of the SIP entities that will have to further
process the session downstream will sometimes vary depending on the
reasons that led to changing the destination, for example, whether it
is for a simple proxy to route the session or for an application
server (AS) to provide a supplementary service. The Diversion header
field and the History-Info header field differ in the approach and
scope of addressing this problem.

For clarity, the following vocabulary is used in this document:

o Retarget/redirect: these terms refer to the process of a Proxy
Server/User Agent Client (UAC) changing a Request-URI (Section 7.1
of [RFC3261]) in a request and thus changing the target of the
request. This includes changing the Request-URI due to a location
service lookup and redirect processing. This also includes
internal (to a proxy/SIP intermediary) changes of the URI prior to
forwarding of the request. The term "retarget" is defined in
[RFC7044].

o Call forwarding/call diversion/communication diversion: these
terms are equivalent and refer to the Communications Diversion
(CDIV) supplementary services, based on the ISDN Communication
diversion supplementary services and defined in 3GPP [TS_24.604].
They are applicable to entities that are intended to modify the
original destination of an IP multimedia session during or prior
to the session establishment.

This document does not intend to describe when or how History-Info or
Diversion header fields should be used. Hereafter is provided
clarification on the context in which the interworking is required.

The Diversion header field has exactly the same scope as the call
diversion service, and each header field entry reflects a call
diversion invocation. The Diversion header field is used for
recording call forwarding information that could be useful to network
entities downstream. Today, this SIP header field is implemented by
several manufacturers and deployed in networks.

The History-Info header field is used to store all retargeting
information, including call diversion information. As such, the
History-Info header field [RFC7044] is used to convey call-diversion-
related information by using a cause URI parameter [RFC4458] in the
relevant entry.

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Note, however, that the use of cause URI parameter [RFC4458] in a
History-Info entry for a call diversion is specific to the 3GPP
specification [TS_24.604]. [RFC4458] focuses on retargeting toward a
voicemail server and does not specify whether the cause URI parameter
should be added in a URI for other cases. As a consequence,
implementations that do not use the cause URI parameter for call
forwarding information are not considered for the mapping described
in this document. Nevertheless, some recommendations are given in
the next sections on how to avoid the loss of non-mapped information
at the boundary between a network region using the History-Info
header field and one using the Diversion header field.

[RFC7044] defines three header field parameters: "rc", "mp", and
"np". The header field parameters "rc" and "mp" indicate the
mechanism by which a new target for a request is determined. The
header field "np" reflects that the target has not changed. All
parameters contain an index whose value refers to the hi-index of the
hi-entry, which contains a hi-targeted-to-uri that represents the
Request-URI that was retargeted.

Since both header fields address call forwarding needs, diverting
information could be mixed up or be inconsistent if both are present
in an uncoordinated fashion in the INVITE request. So, Diversion and
History-Info header fields must not independently coexist in the same
session signaling. This document addresses how to convert
information between the Diversion header field and the History-Info
header field and when and how to preserve both header fields to cover
additional cases.

For the transportation of consistent diversion information
downstream, it is necessary to make the two header fields interwork.
Interworking between the Diversion header field and the History-Info
header field is introduced in Sections 5 and 6. Since the
coexistence scenario may vary from one use case to another,
guidelines regarding interaction of header fields are proposed in
Section 3.

3. Interworking Recommendations

3.1. General Recommendations

Interworking function (IWF):

In a normal case, the network topology assumption is that the
interworking described in this document should be performed by a
specific SIP border device that is aware, by configuration, that
it is at the border between two regions, one using the History-
Info header field and one using the Diversion header field.

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As the History-Info header field is a standard solution, a network
using the Diversion header field must be able to provide information
to a network using the History-Info header field. In this case, to
avoid coexistence of header fields, it is required to replace, as
often as possible, the Diversion header field with the History-Info
header field in the INVITE request during the interworking.

Since, the History-Info header field has a wider scope than the
Diversion header field, it may be used for needs and services other
than call diversion. In addition, to trace call diversion
information, the History-Info header field also acts as a session
history and can store all successive Request-URI values.
Consequently, even if it should be better to remove the History-Info
header field after the creation of the Diversion header field to
avoid confusion, the History-Info header field must remain unmodified
in the SIP signaling if it contains supplementary (non-diversion)
information. It is possible to have History-Info header fields that
do not have values that can be mapped into the Diversion header
field. In this case, no interworking with the Diversion header field
should be performed, and it must be defined per implementation what
to do in this case. This point is out of the scope of this document.

In conclusion, it is recommended to have local policies minimizing
the loss of information and find the best way to keep it up to the
terminating user agent.

The following sections describe the basic use cases. Additional
interworking cases are described in Section 7.4.

3.2. Privacy Considerations

When a SIP message is forwarded to a domain for which the SIP
intermediary is not responsible, a Privacy Service at the boundary of
the domain applies the appropriate privacy based on the value of the
Privacy header field in the message header or in the privacy
parameter within the concerned header:

1. For the History-Info header field, it is the Privacy header field
included as the "headers" component of the hi-targeted-to-uri in
the individual hi-entries with the priv-value "history".

2. For the Diversion header field, it is the diversion-privacy
parameter "privacy" in each Diversion header field.

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For the History-Info header field, as recommended in [RFC7044]:

o If there is a Privacy header field in the message header of a
request with a priv-value of "header" or "history", then all the
hi-targeted-to-uris (in the hi-entries associated with the domain
for which the SIP intermediary is responsible) are anonymized by
the Privacy Service. The Privacy Service must change any
hi-targeted-to-uri in these hi-entries that have not been
anonymized to the anonymous SIP URI "[email protected]"
as recommended in Sections 4.1.1.2 and 4.1.1.3 of [RFC3323].

o If there is a Privacy header field in the "headers" component of a
hi-targeted-to-uri with a priv-value of "history", then all the
concerned hi-entries must be anonymized as described above prior
to forwarding.

The Privacy Service must remove the Privacy header field from the
"headers" component of the hi-targeted-to-uris of the concerned
hi-entries and the priv-value of "history" from the Privacy header
field in the message header of the request prior to forwarding. If
there are no remaining priv-values in the Privacy header field, the
Privacy Service must remove the Privacy header field from the
request.

For the Diversion header field:

o If there is a Privacy header field in the message header of a
request with a priv-value of "header", then all the addresses in
the Diversion header fields (associated with the domain for which
the SIP intermediary is responsible) are anonymized by the Privacy
Service by changing the address to the anonymous SIP URI
"[email protected]" as recommended in Sections 4.1.1.2
and 4.1.1.3 of [RFC3323] prior to forwarding.

o For each Diversion header field or each entry in the Diversion
header field, if there is a diversion-privacy parameter with a
value set to "full", "uri", or "name", then the concerned
Diversion header field address must be anonymized as described
above prior to forwarding.

In the concerned Diversion header field entries, the diversion-
privacy parameter must be removed from the header.

The privacy information interworking as described in Sections 5 and 6
must only be considered within a trusted domain that ensures correct
application of the privacy requirements.

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3.3. Headers in SIP Method

The recommended interworking presented in this document should apply
only for INVITE requests.

In 3xx responses:

Both History-Info and Diversion header fields could be present in
3xx responses.

When a proxy wants to interwork with a network supporting the
other header field, it should apply the interworking between
Diversion header field and History-Info header field in the 3xx
response.

When a recursing proxy redirects an initial INVITE after receiving
a 3xx response, it should add as a last entry either a Diversion
header field or a History-Info header field (according to its
capabilities) in the forwarded INVITE. Local policies could apply
regarding whether or not to send the received header field in the
next INVITE.

In SIP responses other than 100:

All SIP responses where the History-Info header field could be
present are not used for the call forwarding service and should
not be changed into the Diversion header field. The destination
network must be transparent to the received History-Info header
field.

Note: The following mapping is inspired by the ISDN User Part (ISUP)
to SIP interworking described in [TS_29.163].

3.4. SIP Network/Terminal Using Diversion Header Field to SIP Network/
Terminal Using History-Info Header Field

When the Diversion header field is used to create a History-Info
header field, the Diversion header field must be removed in the
outgoing INVITE. It is assumed that all the information present in
the Diversion header field is transferred in the History-Info header
field.

If a History-Info header field is also present in the incoming INVITE
(in addition to the Diversion header field), the Diversion header
field and History-Info header field present must be mixed, and only
the diversion information not yet present in the History-Info header

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field must be inserted as a last entry (most recent) in the existing
History-Info header field, following the creation process recommended
in [RFC7044].

As an example, this could be the case of an INVITE coming from
network_2 using the Diversion header field but previously passed
through network_1 using the History-Info header field (or the
network_2 uses History-Info header field to transport successive URI
information) and going to network_3 using the History-Info header
field.

IWF* IWF*
network_1 | network_2 |network_3
History-Info | Diversion |using
| |History-
| |Info
UA A P1 AS B | P2 AS C UA C AS D | UA E
| | | | | | | | | |
|INVITE | | | | | | | | |
|------>| | | | | | | | |
| | | | | | | | | |
| |INVITE | | | | | | | |
| |------>| | | | | | | |
| |Supported: histinfo | | | | | |
| | History-Info: | | | | | |
| | <sip:proxyP1>;index=1,| | | | | |
| | <sip:userB>;index=1.1;rc=1 | | | | |
| | | | | | | | | |
| | |INVITE | | | | | | |
| | |------>| | | | | | |
| | |History-Info: | | | | | |
| | |<sip:proxyP1>;index=1, | | | | |
| | |<sip:userB>;index=1.1;rc=1, | | | |
| | |<sip:userC;cause=302>;index=1.1.1;mp=1.1 | |

In this case, the incoming INVITE contains a Diversion header field
and a History-Info header field. Therefore, as recommended in this
document, it is necessary to create, for network_3, a single History-
Info header field gathering existing information from both the
History-Info and the Diversion header fields received. Anyway, it is
required that network_2 (i.e., IWF) remove the Diversion header field
when the message is going to a network not using the Diversion header
field. Then, network_3 could use call forwarding information that is
present in a single header field and add its own diversion
information if necessary.

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Notes:

1. If a network is not able either to use only one header field each
time or to maintain both header fields up to date, the
chronological order cannot be certified.

2. It is not possible to have only a Diversion header field when the
History-Info header field contains more than call diversion
information. If previous policy recommendations are applied, the
chronological order is respected as Diversion entries are
inserted at the end of the History-Info header field taking into
account the Diversion internal chronology.

3.5. SIP Network/Terminal Using History-Info Header Field to SIP
Network/Terminal Using Diversion Header Field

When the History-Info header field is interpreted to create a
Diversion header field, some precautions must be taken.

If the History-Info header field contains only call forwarding
information, then it must be deleted after the interworking.

If the History-Info header field contains other information, then
only the information of concern to the diverting user must be used to
create entries in the Diversion header field, and the History-Info
header field must be kept as received in the INVITE and forwarded
downstream.

Note: The History-Info header field could be used for reasons other
than call diversion services, for example, by a service that needs to
know if a specific AS has yet been invoked in the signaling path. If
the call is later forwarded to a network using the History-Info
header field, it would be better not to lose history information due
to passing though the network that only supports the Diversion header
field. A recommended solution must not disrupt the standard
behavior, and networks that do not implement the History-Info header
field must be transparent to a received History-Info header field.

If a Diversion header field is present in the incoming INVITE (in
addition to the History-Info header field), only diversion
information present in the History-Info header field but not in the
Diversion header field must be inserted from the last entry (most
recent) into the existing Diversion header field as recommended in
[RFC5806].

Note that the chronological order could not be certified. If
previous policy recommendations are respected, this case should not
happen.

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Forking case:

The History-Info header field enables the recording of sequential
forking for the same served user. During an interworking from the
History-Info header field to the Diversion header field, the
History-Info entries containing a forking situation (with an
incremented "index" parameter) could possibly be mapped if they
contain a call forwarding "cause" parameter. The interworking
entity could choose to create only a Diversion entry or not apply
the interworking. The choice could be done according a local
policy.

The same logic is applied for an interworking with Voicemail URI (see
Appendix A).

4. Reminder of the Syntax for Header Fields

4.1. History-Info Header Field Syntax

The ABNF syntax [RFC5234] for the History-Info header field and
header field parameters is as follows.

History-Info = "History-Info" HCOLON hi-entry *(COMMA hi-entry)
hi-entry = hi-targeted-to-uri *(SEMI hi-param)
hi-targeted-to-uri = name-addr
hi-param = hi-index/hi-target-param/hi-extension
hi-index = "index" EQUAL index-val
index-val = number *("." number)
number = [ %x31-39 *DIGIT ] DIGIT
hi-target-param = rc-param / mp-param / np-param
rc-param = "rc" EQUAL index-val
mp-param = "mp" EQUAL index-val
np-param = "np" EQUAL index-val
hi-extension = generic-param

The ABNF definitions for "generic-param", "name-addr", "HCOLON",
"COMMA", "SEMI", and "EQUAL" are from [RFC3261].

The History-Info header field is specified in [RFC7044]. The top-
most History-Info entry (first in the list) corresponds to the oldest
history information.

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Cause URI parameter:

A hi-entry may contain a cause URI parameter expressing the
diversion reason. This cause URI parameter is defined in
[RFC4458]. The ABNF grammar [RFC5234] for the cause-param
parameter is shown below as it has been subject to Erratum ID 1409
[Err1409] for [RFC4458]. The Status-Code is defined in [RFC3261].

cause-param = "cause=" Status-Code

The cause-param parameter is a SIP/SIPS URI parameter and should
be inserted in the History-Info entry (URI) of the diverted-to
user in case of call diversion as recommended in the 3GPP CDIV
specification [TS_24.604]. The cause values used in the cause-
param for the diverting reason are listed in [RFC4458]. Because
it is a parameter dedicated to call forwarding service, its
presence is used to determine that a hi-entry is a diverting user.
More precisely, each diverting user is located in the hi-entry
before the one containing a cause-param with cause value as listed
in [RFC4458].

Reason header field:

The Reason header field defined in [RFC3326] should be escaped in
the hi-entry of the diverting user when the call diversion is due
to a received SIP response. The Reason header field contains a
cause parameter set to the true SIP response code received
(Status-Code).

Therefore, in case of call diversion due to a SIP response, both
cause parameters should be used. The complexity is that these
parameters could be used at the same time in the History-Info
header field but not in the same hi-entry and not with the same
meaning. Only the cause-param is dedicated to call diversion
service. The 'cause' Reason header field parameter is not taken
into account in the mapping with a Diversion header field.

Target URI parameter:

[RFC4458] also defines the 'target' URI parameter, which could be
inserted in a Request-URI and consequently in the
hi-targeted-to-uri. This parameter is used to keep the diverting
user address in the downstream INVITE request in Voicemail URI
implementation. As this information is already present in the hi-
entries, the 'target' URI parameter is not taken into account
regarding the interworking with the Diversion header field. From
the Diversion header field, it could be possible to create the

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'target' URI parameter in the hi-entries and/or in the Request-
URI, but this possibility is based on local policies not described
in this document.

Privacy header field:

A Privacy header field as defined in [RFC3323] could also be
embedded in hi-entries with the 'history' value defined in
[RFC7044].

Index header field parameter:

The index parameter is a string of digits, separated by dots, to
indicate the number of forward hops and retargets.

Note: A history entry could contain the "gr" parameter. Regardless
of the rules concerning the "gr" parameter defined in [TS_24.604],
which must be applied, this parameter has no impact on the mapping
and must only be copied with the served user address.

Missing entry:

If the request clearly has a gap in the hi-entry (i.e., the last
hi-entry and Request-URI differ), the entity adding a hi-entry
must add a single index with a value of "0" (i.e., the non-
negative integer zero) prior to adding the appropriate index for
the action to be taken (e.g., Index=1.1.2.0.1). Prior to any
application usage of the History-Info header field parameters, the
SIP entity that processes the hi-entries must evaluate the
hi-entries and determine if there are any gaps in them.

"histinfo" option tag:

According to [RFC7044], a proxy that receives a Request with the
"histinfo" option tag in the Supported header field should return
captured History-Info in subsequent, provisional, and final
responses to the Request. The behavior depends upon whether or
not the local policy supports the capture of History-Info.

Example:

History-Info:
<sip:diverting_user1_addr?Privacy=none&Reason=SIP%3Bcause%3D302>;
index=1,
<sip:diverting_user2_addr;cause=480?Privacy=history>;index=1.1;mp=1,
<sip:last_diversion_target;cause=486>;index=1.1.1;mp=1.1

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4.2. Diversion Header Field Syntax

The following text is restating the exact syntax that the production
rules in [RFC5806] define, but using ABNF [RFC5234]:

Diversion = "Diversion" HCOLON diversion-params
*(COMMA diversion-params)
diversion-params = name-addr *(SEMI (diversion-reason /
diversion-counter / diversion-limit /
diversion-privacy / diversion-screen /
diversion-extension))
diversion-reason = "reason" EQUAL ("unknown" / "user-busy" /
"no-answer" / "unavailable" / "unconditional"
/ "time-of-day" / "do-not-disturb" /
"deflection" / "follow-me" / "out-of-service"
/ "away" / token / quoted-string)
diversion-counter = "counter" EQUAL 1*2DIGIT
diversion-limit = "limit" EQUAL 1*2DIGIT
diversion-privacy = "privacy" EQUAL ("full" / "name" / "uri" /
"off" / token / quoted-string)
diversion-screen = "screen" EQUAL ("yes" / "no" / token /
quoted-string)
diversion-extension = token [EQUAL (token / quoted-string)]

Note: The Diversion header field could be used in the comma-separated
format as described below and in a header-separated format. Both
formats could be combined in a received INVITE as recommended in
[RFC3261].

Example:

Diversion:
<sip:diverting_user2_addr>;reason=user-busy;counter=1;privacy=full,
<sip:diverting_user1_addr>;reason=unconditional;counter=1;privacy=off

5. Diversion Header Field to History-Info Header Field

The following text is valid only if no History-Info header field is
present in the INVITE request. If at least one History-Info header
field is present, the interworking function must adapt its behavior
to respect the chronological order. For more information, see
Section 3.

Concerning the privacy information in the Diversion header field, the
following mapping only applies within a trusted domain; for other
domains, see the privacy considerations in Section 3.2.

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For N Diversion entries, N+1 History-Info entries must be created.
To create the History-Info entries in the same order as during a
session establishment, the Diversion entries must be mapped from the
bottom-most to the top-most. Each Diversion entry shall be mapped
into a History-Info entry. An additional History-Info entry (the
last one) must be created with the diverted-to party address present
in the Request-URI of the received INVITE. The mapping is described
in the table below.

The first entry created in the History-Info header field contains:

o a hi-targeted-to-uri with the name-addr parameter of the bottom-
most Diversion header field.

o if a privacy parameter is present in the bottom-most Diversion
entry, then a Privacy header field must be escaped in the History-
Info header field as described in the table below.

o a hi-index set to 1.

For each of the following Diversion entries (from bottom to top), the
History-Info entries are created as follows (from top to bottom):

Source Destination
Diversion header component: History-Info header component:
=======================================================================
name-addr hi-targeted-to-uri

=======================================================================
reason of the previous cause URI parameter
Diversion entry A cause-param "cause" is
added in each hi-entry
(except the first one)
"unknown"----------------------------------404 (default 'cause' value)
"unconditional"----------------------------302
"user-busy"--------------------------------486
"no-answer"--------------------------------408
"deflection "------------------------------480 or 487
"unavailable"------------------------------503
"time-of-day"------------------------------404 (default)
"do-not-disturb"---------------------------404 (default)
"follow-me"--------------------------------404 (default)
"out-of-service"---------------------------404 (default)
"away"-------------------------------------404 (default)

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======================================================================
counter hi-index
"1" or parameter ------------------------The previous created index
not present is extended with ".1"
Superior to "1" -------------------------Create N-1 placeholder History
(i.e., N) entry with the previous index
extended with ".1"
Then the History-Info header
created with the Diversion
entry with the previous index
extended with ".1"
======================================================================
privacy Privacy header escaped in the
hi-targeted-to-uri
"full"-----------------------------------"history"
"Off"------------------------------------Privacy header field
absent or "none"
"name"-----------------------------------"history"
"uri"------------------------------------"history"
======================================================================
hi-target-param
An mp-param "mp" is added in
each created hi-entry
(except the first one)
The "mp" parameter is set to
the index value of the
preceding hi-entry.
=======================================================================

A last History-Info entry is created and contains:

o a hi-targeted-to-uri with the Request-URI of the INVITE request.

o a cause-param from the top-most Diversion entry, mapped from the
diversion-reason as described above.

o an index set to the previous created index extended with a new
level ".1" added at the end.

o a hi-target-param set to "mp" equals to the index value of the
previous hi-entry.

Notes:

1. For other optional Diversion parameters, there is no
recommendation as the History-Info header field does not provide
equivalent parameters.

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2. For values of the diversion-reason that are mapped with a
recommended default value, it could also be possible to choose
another value. The cause-param URI parameter offers fewer
possible values than the diversion-reason parameter. However, it
has been considered that the cause-param values list was
sufficient to implement CDIV service as defined in 3GPP
[TS_24.604] as it covers a large portion of cases.

3. The Diversion header field can contain a "tel" URI as defined in
[RFC3966] in the name-addr parameter. The History-Info header
field can also contain an address that is a "tel" URI, but if
this hi-entry has to be completed with either a SIP header field
(e.g., Reason or Privacy) or a SIP URI parameter (e.g., 'cause'
or 'target'), the "tel" URI must be converted into a SIP URI.
[RFC3261] gives an indication as to the mapping between sip: and
tel: URIs, but in this particular case, it is difficult to assign
a valid hostport as the diversion occurred in a previous network
and a valid hostport is difficult to determine. So, it is
suggested that in case of "tel" URI in the Diversion header
field, the History-Info header field should be created with a SIP
URI with user=phone and a domain set to "unknown.invalid".

4. The Diversion header field allows carrying of a counter that
retains the information about the number of successive
redirections. History-Info does not have an equivalent because
to trace and count the number of diversions, it is necessary to
count the cause parameter containing a value associated to a call
diversion listed in [RFC4458]. Reading the index value is not
enough. With the use of the "placeholder" entry the History-Info
header field, entries can reflect the real number of diversions
that occurred, thanks to the cause-param.

Example of placeholder entry in the History-Info header field:

<sip:[email protected];cause=xxx>;index=1.1

<sip:bob_addr;cause=404>;index=1.1.1;mp=1.1

"cause=xxx" reflects the diverting reason of a previous diverting
user. For a placeholder hi-entry, the value "404" must be taken for
the cause-param and so, located in the next hi-entry.

For recommendations for local policies regarding the coexistence of
header fields in the INVITE request, see Sections 3 and 7.4.

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6. History-Info Header Field to Diversion Header Field

Concerning the privacy information for the History-Info header field,
the following mapping only applies within a trusted domain; for other
domains, see the privacy considerations in Section 3.2.

To create the Diversion entries in the same order as during a session
establishment, the History-Info entries must be mapped from the top-
most to the bottom-most. The first History-Info header field entry
selected will be mapped into the last Diversion header field entry
and so on. One Diversion header field entry must be created for each
History-Info entry that has cause-param with a value listed in
[RFC4458].

Diversion information:

The definitions of "Target_entry" and "Diverting_entry" are included
below to help readers understand the mapping of the History-Info
header field.

The diversion information can be identified by finding the following
hi-entries:

o Target_entry: hi-entries containing a cause-param URI parameter
with a value listed in [RFC4458] will contain the diversion reason
and the address of the target of the concerned call forwarding.
Per [RFC7044], these hi-entries may also contain a hi-target-param
set to "mp".

o Diverting_entry: For each previously identified hi-entry:

* If there is an "mp" header field parameter, the hi-entry whose
hi-index matches the value of the hi-target-param "mp" will
contain the diverting party address, its possible privacy, and/
or SIP reason when the retargeting has been caused by a
received SIP response.

* If there is no "mp" header field parameter, the information of
the diverting party address, privacy and/or SIP reason will be
found in the hi-entry that precede this identified hi-entry.

Note: Per [RFC7044], all retargeting entries must point to a hi-entry
that contains an "mp" parameter, but for backward-compatibility
reasons, it may be absent from some of the received hi-entries. See
Section 8 for more information on backward compatibility.

The History-Info header field must be mapped into the Diversion
header field as follows:

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Source Destination
History-Info header component: Diversion header component:
=====================================================================
hi-targeted-to-uri name-addr
of the Diverting_entry.

=====================================================================
cause-param reason
of the Target_entry
404---------------------------------------"unknown" (default value)
302---------------------------------------"unconditional"
486---------------------------------------"user-busy"
408---------------------------------------"no-answer"
480 or 487--------------------------------"deflection "
503---------------------------------------"unavailable"
=====================================================================
hi-index counter
Mandatory parameter for-------------------The counter is set to "1".
History-Info reflecting
the chronological order
of the information.
=====================================================================
Privacy header field escaped privacy
in the hi-targeted-to-uri
of the Diverting_entry
"history"----------------------------------"full"
Privacy header field ----------------------"Off"
Absent or "none"
=====================================================================

Note: For other optional History-Info parameters, there is no
recommendation as the Diversion header field does not provide
equivalent parameters.

For recommendations for local policies regarding the coexistence of
header fields in the INVITE request, see Section 3.

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7. Examples

7.1. Example with Diversion Header Field Changed into History-Info
Header Field

INVITE sip:last_diverting_target
Diversion:
<sip:diverting_user3_address>;reason=unconditional;counter=1;
privacy=off,
<sip:diverting_user2_address>;reason=user-busy;counter=1;
privacy=full,
<sip:diverting_user1_address>;reason=no-answer;counter=1;
privacy=off

Mapped into:

History-Info:
<sip:diverting_user1_address?Privacy=none>;index=1,
<sip:diverting_user2_address;
cause=408?Privacy=history>;index=1.1;mp=1,
<sip:diverting_user3_address;
cause=486?Privacy=none>;index=1.1.1;mp=1.1,
<sip:last_diverting_target;cause=302>;index=1.1.1.1;mp=1.1.1

7.2. Example with History-Info Header Field Changed into Diversion
Header Field

INVITE sip:last_diverting_target; cause=486
History-Info:
<sip:diverting_user1_address?Privacy=history>;index=1,
<sip:diverting_user2_address;cause=302?Privacy=none>;index=1.1;mp=1,
<sip:last_diverting_target;cause=486>;index=1.1.1;mp=1.1

Mapped into:

Diversion:
<sip:diverting_user2_address>;reason=user-busy;counter=1;privacy=off,
<sip:diverting_user1_address>;reason=unconditional;counter=1;privacy=
full

7.3. Example with Two SIP Networks Using History-Info Header Field
Interworking with a SIP Network Using Diversion Header Field

A -> P1 -> B -> C -> P2 -> D-> E
A, B, C, D and E are users.
B, C and D have call forwarding service invoked.
P1 and P2 are proxies.
Only relevant information is shown on the following call flow.

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IWF* IWF*
SIP network using | SIP network using |SIP net.
History-Info | Diversion |using
| History-Info
| |
UA A P1 AS B | P2 AS C UA C AS D | UA E
| | | | | | | | | |
|INV B | | | | | | | | |
|------>| | | | | | | | |
| | | | | | | | | |
| |INV B | | | | | | | |
| |------>| | | | | | | |
| |Supported: histinfo | | | | | |
| | History-Info: | | | | | |
| | <sip:proxyP1>;index=1, | | | | |
| | <sip:userB>;index=1.1;rc=1 | | | | |
| | | | | | | | | |
| | |INV C | | | | | | |
| | |------>| | | | | | |
| | |History-Info: | | | | | |
| | <sip:proxyP1>;index=1, | | | | |
| | <sip:userB>;index=1.1;rc=1, | | | |
| | <sip:proxyP2;cause=302>;index=1.1.1;mp=1.1 | |
| | | | | | | | | |
| | | |INV C | | | | | |
| | | |----->| | | | | |
| | | Diversion: | | | | | |
| | <sip:userB>;reason=unconditional;counter=1;privacy=off|
| | | |History-Info: | | | | |
| | | <sip:proxyP1>;index=1, | | | |
| | | <sip:userB>;index=1.1;rc=1, | | |
| | | <sip:proxyP2;cause=302>;index=1.1.1;mp=1.1 |
| | | | | | | | | |
| | | | |INV C | | | | |
| | | | |------>| | | | |
| | | | No modification of Diversion header |
| | | | | | | | | |
| | | | | |INV C | | | |
| | | | | |------>| | | |
| | | | | | | | | |
| | | | | |<--180-| | | |
| | | | | | | | | |
| | | | | No response timer expires | |
| | | | | |---INV D --->| | |
| | |Diversion: | | |
| | <sip:userC>;reason=no-answer;counter=1;privacy=full, |
| | <sip:userB>;reason=unconditional;counter=1;privacy=off|
| | | History-Info: | | |

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| | | <sip:proxyP1>;index=1, | | |
| | | <sip:userB>;index=1.1;rc=1, | | |
| | | <sip:proxyP2;cause=302>;index=1.1.1;mp=1.1 |
| | | | | | | | | |
| | | | | | | |INV E | |
| | | | | | | |----->| |
| | |Diversion: | |
| | <sip:userD>;reason=time-of-day;counter=1;privacy=off, |
| | <sip:userC>;reason=no-answer;counter=1;privacy=full, |
| | <sip:userB>;reason=unconditional;counter=1;privacy=off|
| | | History-Info: | |
| | | <sip:proxyP1>;index=1, | |
| | | <sip:userB>;index=1.1;rc=1, | |
| | | <sip:proxyP2;cause=302>;index=1.1.1;mp=1.1 |
| | | | | | | | | |
| | | | | | | | | INV E |
| | | | | | | | |------>|
| | History-Info: | | | | | |
| | <sip:proxyP1>;index=1, | | | | |
| | <sip:userB>;index=1.1;rc=1, | | | |
| | <sip:proxyP2;cause=302>;index=1.1.1;mp=1.1, | |
| | <sip:userC ?Privacy=history>;index=1.1.1.0.1, | |
|<sip:userD;cause=408?Privacy=none>;index=1.1.1.0.1.1;mp=1.1.1.0.1, |
| |<sip:userE;cause=404>;index=1.1.1.0.1.1.1;mp=1.1.1.0.1.1 |
| | | | | | | | | |
| | | | | | | | | |

Note: The IWF is an interworking function that could be a stand-alone
equipment not defined in this document (it could be a proxy).

7.4. Additional Interworking Cases

Even for particular cases in which both header fields could coexist,
it should be the responsibility of the network local policy to make
it work together. This section describes some situations and some
recommendations on behavior.

In the case where there is one network that includes different nodes,
some of them supporting the Diversion header field and other ones
supporting the History-Info header field, there is a problem when any
node handling a message does not know the next node that will handle
the message. This case can occur when the network has new and old
nodes, the older ones using the Diversion header field and the most
recent using the History-Info header field.

While a network replacement may be occurring, there will be a time
when both nodes coexist in the network. If the different nodes are
being used to support different subscriber types due to different

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node capabilities, then the problem is more important. In this case,
there is a need to pass both the History-Info header field and the
Diversion header field within the core network.

These header fields need to be equivalent to ensure that, whatever
the node receiving the message, the correct diversion information is
received. This requires that, whatever the received header field,
there is a requirement to be able to compare the header fields and to
convert the header fields. Depending upon the node capability, it
may be possible to make assumptions as to how this is handled.

o If it is known that the older Diversion header field supporting
nodes does not pass on any received History-Info header field,
then the interworking becomes easier. If a message is received
with only Diversion header fields, then it has originated from an
old node. The equivalent History-Info entries can be created, and
these can then be passed as well as the Diversion header field.

o If the node creates a new History-Info header field for a call
diversion, then an additional Diversion header field must be
created.

o If the next node is an old node, then the Diversion header field
will be used by that node, and the History-Info entries will be
removed from the message when it is passed on.

o If the next node is a new node, then the presence of both the
Diversion header field and History-Info header field means that
interworking has already occurred and the Diversion and History-
Info entries must be considered equivalent.

o If both nodes pass on both the History-Info header field and
Diversion header field but only actively use one, then both types
of nodes need to perform the interworking and must maintain
equivalence between the header fields. This will eventually
result in the use of the Diversion header field being deprecated
when all nodes in the network support the History-Info header
field.

o If a gap is identified in the History-Info header field by a node
that would create a new entry, it shall add a single index with a
value of "0" prior to adding the appropriate index for the action
to be taken.

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8. Backward Compatibility

Issues with backward compatibility are due to the evolution of the
History-Info header field from [RFC4244] to [RFC7044], as described
in Section 1.3 of this document. Backward compatibility is taken
into account throughout this document for the interworking with the
Diversion header field. More details are provided in the "Backwards
Compatibility" section of [RFC7044].

9. Security Considerations

The security considerations in [RFC7044] and [RFC5806] apply.

The privacy considerations described in Section 3.2 apply.

The use of the Diversion header field or History-Info header field
requires application of the requested privacy and integrity requested
by each diverting user or entity. Without integrity, the requested
privacy functions could be downgraded or eliminated, potentially
exposing identity information. Without confidentiality,
eavesdroppers on the network (or any intermediaries between the user
and the Privacy Service) could see the very personal information that
the user has asked the Privacy Service to obscure. Unauthorized
insertion and deletion/modification of those header fields can
provide misleading information to users and applications. A SIP
entity that can provide a redirection reason in a History-Info header
field or Diversion header field should be able to suppress this in
accordance with privacy requirements of the user concerned.

10. References

10.1. 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,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.

[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323,
DOI 10.17487/RFC3323, November 2002,
<http://www.rfc-editor.org/info/rfc3323>.

[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, DOI 10.17487/RFC3326, December 2002,
<http://www.rfc-editor.org/info/rfc3326>.

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[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, DOI 10.17487/RFC3966, December 2004,
<http://www.rfc-editor.org/info/rfc3966>.

[RFC4244] Barnes, M., Ed., "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 4244,
DOI 10.17487/RFC4244, November 2005,
<http://www.rfc-editor.org/info/rfc4244>.

[RFC5806] Levy, S. and M. Mohali, Ed., "Diversion Indication in
SIP", RFC 5806, DOI 10.17487/RFC5806, March 2010,
<http://www.rfc-editor.org/info/rfc5806>.

[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,
DOI 10.17487/RFC7044, February 2014,
<http://www.rfc-editor.org/info/rfc7044>.

10.2. Informative References

[Err1409] RFC Errata, Erratum ID 1409, RFC 4458.

[RFC4458] Jennings, C., Audet, F., and J. Elwell, "Session
Initiation Protocol (SIP) URIs for Applications such as
Voicemail and Interactive Voice Response (IVR)", RFC 4458,
DOI 10.17487/RFC4458, April 2006,
<http://www.rfc-editor.org/info/rfc4458>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC6044] Mohali, M., "Mapping and Interworking of Diversion
Information between Diversion and History-Info Headers in
the Session Initiation Protocol (SIP)", RFC 6044,
DOI 10.17487/RFC6044, October 2010,
<http://www.rfc-editor.org/info/rfc6044>.

[TS_24.604]
3rd Generation Partnership Project, "Communication
Diversion (CDIV) using IP Multimedia (IM) Core Network
(CN) subsystem; Protocol specification", Release 13.1,
3GPP TS 24.604, June 2015.

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[TS_29.163]
3rd Generation Partnership Project, "Interworking between
the IP Multimedia (IM) Core Network (CN) subsystem and
Circuit Switched (CS) networks", Release 13.2, 3GPP TS
29.163, June 2015.

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Appendix A. Interworking between Diversion Header Field and Voicemail
URI

Voicemail URI is a mechanism described in [RFC4458] to provide a
simple way to transport only one redirecting user address and the
reason why the diversion occurred in the Request-URI of the INVITE
request. This mechanism is mainly used for call diversion to a
voicemail.

A.1. Diversion Header Field to Voicemail URI

Received:
Diversion: userA-address;reason=user-busy;counter=1;privacy=full

Sent (Voicemail URI created in the R-URI line of the INVITE):
sip: [email protected];target=userA-address;cause=486 SIP/2.0

Mapping of the Redirection Reason is the same as for History-Info
header field with a default value set to 404.

If the Diversion header field contains more than one Diversion entry,
the choice of the redirecting user information inserted in the URI is
in charge of the network local policy. For example, the choice
criterion of the redirecting information inserted in the URI could be
the destination of forwarded INVITE request (whether or note the
voicemail serves this user).

Note: This interworking could be done in addition to the interworking
of the Diversion header field into the History-Info header field.

A.2. Voicemail URI to Diversion Header Field

In case of real voicemail, this way of interworking should not
happen. However, if for any reason it occurs, it is recommended to
do it as follows:

Received:
INVITE sip: [email protected];\
target=sip:+33145454500%40example.com;user=phone;\
cause=302 SIP/2.0

Sent in the forwarded INVITE:
Diversion: sip:+33145454500%40example.com;user=phone;
reason=unconditional;counter=1

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Acknowledgements

The author would like to acknowledge the constructive feedback and
support provided by Steve Norreys, Jan Van Geel, Martin Dolly,
Francisco Silva, Guiseppe Sciortino, Cinza Amenta, Christer Holmberg,
Ian Elz, Jean-Francois Mule, Mary Barnes, Francois Audet, Erick
Sasaki, Shida Schubert, Joel M. Halpern, Bob Braden, Robert Sparks,
Merci a Lionel Morand, and Xavier Marjou et Philippe Fouquart.

Author's Address

Marianne Mohali
Orange
38-40 rue du General Leclerc
Issy-Les-Moulineaux Cedex 9 92794
France

Phone: +33 1 45 29 45 14
Email: [email protected]

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