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PROPOSED STANDARD
Errata Exist
Internet Engineering Task Force (IETF) M. Barnes
Request for Comments: 7044 Polycom
Obsoletes: 4244 F. Audet
Category: Standards Track Skype
ISSN: 2070-1721 S. Schubert
NTT
J. van Elburg
Detecon International Gmbh
C. Holmberg
Ericsson
February 2014
An Extension to the Session Initiation Protocol (SIP) for
Request History Information
Abstract
This document defines a standard mechanism for capturing the history
information associated with a Session Initiation Protocol (SIP)
request. This capability enables many enhanced services by providing
the information as to how and why a SIP request arrives at a specific
application or user. This document defines an optional SIP header
field, History-Info, for capturing the history information in
requests. The document also defines SIP header field parameters for
the History-Info and Contact header fields to tag the method by which
the target of a request is determined. In addition, this
specification defines a value for the Privacy header field that
directs the anonymization of values in the History-Info header field.
This document obsoletes RFC 4244.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7044.
Barnes, et al. Standards Track [Page 1]
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Copyright Notice
Copyright (c) 2014 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
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Without obtaining an adequate license from the person(s) controlling
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outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Barnes, et al. Standards Track [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. History-Info Header Field Protocol Structure . . . . . . . . 7
5.1. History-Info Header Field Example Scenario . . . . . . . 10
6. User Agent Handling of the History-Info Header Field . . . . 12
6.1. User Agent Client (UAC) Behavior . . . . . . . . . . . . 12
6.2. User Agent Server (UAS) Behavior . . . . . . . . . . . . 12
6.3. Back-to-Back User Agent (B2BUA) Behavior . . . . . . . . 12
7. Proxy/Intermediary Handling of History-Info Header Fields . . 13
8. Redirect Server Handling of History-Info Header Fields . . . 13
9. Handling of History-Info Header Fields in Requests and
Responses . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Receiving a Request . . . . . . . . . . . . . . . . . . . 14
9.2. Sending a Request with History-Info . . . . . . . . . . . 14
9.3. Receiving a Response with History-Info or Request
Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 15
9.4. Sending History-Info in Responses . . . . . . . . . . . . 16
10. Processing the History-Info Header Field . . . . . . . . . . 16
10.1. Privacy in the History-Info Header Field . . . . . . . . 16
10.1.1. Indicating Privacy . . . . . . . . . . . . . . . . . 16
10.1.2. Applying Privacy . . . . . . . . . . . . . . . . . . 17
10.2. Reason in the History-Info Header Field . . . . . . . . 18
10.3. Indexing in the History-Info Header Field . . . . . . . 19
10.4. Mechanism for Target Determination in the History-Info
Header Field . . . . . . . . . . . . . . . . . . . . . . 21
11. Application Considerations . . . . . . . . . . . . . . . . . 22
12. Application-Specific Usage . . . . . . . . . . . . . . . . . 24
12.1. PBX Voicemail . . . . . . . . . . . . . . . . . . . . . 24
12.2. Consumer Voicemail . . . . . . . . . . . . . . . . . . . 25
13. Security Considerations . . . . . . . . . . . . . . . . . . . 25
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
14.1. Registration of New SIP History-Info Header Field . . . 26
14.2. Registration of "history" for SIP Privacy Header Field . 27
14.3. Registration of Header Field Parameters . . . . . . . . 27
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
16. Changes from RFC 4244 . . . . . . . . . . . . . . . . . . . . 28
16.1. Backwards Compatibility . . . . . . . . . . . . . . . . 29
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
17.1. Normative References . . . . . . . . . . . . . . . . . . 31
17.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Request History Requirements . . . . . . . . . . . . 33
A.1. Security Requirements . . . . . . . . . . . . . . . . . . 34
A.2. Privacy Requirements . . . . . . . . . . . . . . . . . . 35
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1. Introduction
Many services that SIP is anticipated to support require the ability
to determine why and how a SIP request arrived at a specific
application. Examples of such services include (but are not limited
to) sessions initiated to call centers via "click to talk" SIP
Uniform Resource Locators (URLs) on a web page, "call history/
logging"-style services within intelligent "call management" software
for SIP user agents (UAs), and calls to voicemail servers. Although
SIP implicitly provides the retarget capabilities that enable SIP
requests to be routed to chosen applications, there is a need for a
standard mechanism within SIP for communicating the retargeting
history of the requests. This request history information allows the
receiving application to obtain information about how and why the SIP
request arrived at the application/user.
This document defines a SIP header field, History-Info, to provide a
standard mechanism for capturing the request history information to
enable a wide variety of services for networks and end-users. SIP
header field parameters are defined for the History-Info and Contact
header fields to tag the method by which the target of a request is
determined. This specification also defines a value, "history", for
the Privacy header field. In addition, a SIP option tag, "histinfo",
is defined.
The History-Info header field provides a building block for
development of SIP-based applications and services. The requirements
for the solution described in this specification are included in
Appendix A. Example scenarios using the History-Info header field
are available in [CALLFLOWS].
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The term "retarget" is used in this specification to refer to the
process of a SIP entity changing the Request-URI (Section 7.1 of
[RFC3261]) in a request based on the rules for determining request
targets as described in Section 16.5 of [RFC3261] and of the
subsequent forwarding of that request as described in step 2 in
Section 16.6 of [RFC3261]. 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 the forwarding of the request.
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The terms "location service", "forward", "redirect", and "AOR"
(address-of-record) are used consistently with the terminology in
[RFC3261].
The term "target user" is used in this specification as the human
user associated with one or more particular AORs (in case the human
user has multiple aliases).
The references to "domain for which the SIP entity/proxy/intermediary
is responsible" are consistent with and intended to convey the same
context as the usage of that terminology in [RFC3261]. The
applicability of History-Info to architectures or models outside the
context of [RFC3261] is outside the scope of this specification.
3. Background
SIP implicitly provides retargeting capabilities that enable SIP
requests to be routed to specific applications as defined in
[RFC3261]. The motivation for capturing the request history is that
in the process of retargeting a request, old routing information can
be forever lost. This lost information may be important history that
allows elements to which the request is retargeted to process the
request in a locally defined, application-specific manner. This
document defines a mechanism for transporting the request history.
Application-specific behavior is outside the scope of this
specification.
Current network applications for other protocols provide the ability
for elements involved with the request to obtain additional
information relating to how and why the request was routed to a
particular destination. The following are examples of such
applications:
1. Web "referral" applications, whereby an application residing
within a web server determines that a visitor to a website has
arrived at the site via an "associate" site that will receive
some "referral" commission for generating this traffic.
2. Email relaying whereby the recipient obtains a detailed "trace of
the path" of the message from originator to receiver, including
the time of each relay.
3. Traditional telephony services such as voicemail, call-center
"automatic call distribution", and "follow me"-style services.
Several of the aforementioned applications currently define
application-specific mechanisms through which it is possible to
obtain the necessary history information.
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In addition, request history information could be used to enhance
basic SIP functionality by providing the following:
o Some diagnostic information for debugging SIP requests.
o Capturing aliases and Globally Routable User Agent URIs (GRUUs)
[RFC5627], which can be overwritten by a registrar or a "home
proxy" (a proxy serving as the terminal point for routing an
address-of-record) upon receipt of the initial request.
o Facilitating the use of limited use addresses (minted on demand)
and sub-addressing.
o Preserving service-specific URIs that can be overwritten by a
downstream proxy, such as those defined in [RFC3087], and control
of network announcements and Interactive Voice Response (IVR) with
a SIP URI [RFC4240].
4. Overview
The fundamental functionality provided by the request history
information is the ability to inform proxies and user agents (UAs)
involved in processing a request about the history or progress of
that request. The solution is to capture the Request-URIs, as a
request is retargeted, in a SIP header field: History-Info. This
allows for the capturing of the history of a request that would be
lost with the normal SIP processing involved in the subsequent
retargeting of the request.
The History-Info header field is added to a request when a new
request is created by a User Agent Client (UAC) or forwarded by a
proxy, or when the target of a request is changed. It is possible
for the target of a request to be changed by the same proxy/SIP
intermediary multiple times (referred to as 'internal retargeting').
A SIP entity changing the target of a request in response to a
redirect also propagates any History-Info header field from the
initial request in the new request. The ABNF and detailed
description of the History-Info header field parameters, along with
examples, are provided in Section 5. Sections 6, 7, and 8 provide
the detailed handling of the History-Info header field by SIP user
agents, proxies, and redirect servers, respectively.
This specification also defines three new SIP header field
parameters, "rc", "mp", and "np", for the History-Info and Contact
header fields to tag the method by which the target of a request is
determined. Further detail on the use of these header field
parameters is provided in Section 5.
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This specification also defines a priv-value for the Privacy header,
"history"; it requires anonymization of all the History-Info header
field entries in a request or to a specific History-Info header field
value (hi-entry) as described below. Further detail is provided in
Section 10.1.
In addition, a SIP option tag, "histinfo", is defined. The use of
this option tag is described in Section 6.1.
5. History-Info Header Field Protocol Structure
The History-Info header field defined in this specification defines
the usage in out-of-dialog requests or initial requests for a dialog
(e.g., INVITE, REGISTER, MESSAGE, REFER and OPTIONS, PUBLISH and
SUBSCRIBE, etc.) and any non-100 provisional or final responses to
these requests.
The following provides details for the information that is captured
in the History-Info header field entries for each target used for
forwarding a request.
o hi-targeted-to-uri: A mandatory parameter for capturing the
Request-URI for the specific request as it is forwarded.
o hi-index: A mandatory parameter for History-Info reflecting the
chronological order of the information, indexed to reflect the
forking and retargeting of requests. The format for this
parameter is a sequence of nonnegative integers, separated by dots
to indicate the number of forward hops and retargets. This
results in a tree representation of the history of the request,
with the lowest-level index reflecting a leaf. By adding the new
entries in chronological order (i.e., following existing entries
per the details in Section 10.3), including the index and sending
the messages using a secure transport, the ordering of the
History-Info header fields in the request is assured. In
addition, applications may extract a variety of metrics (total
number of retargets, total number of retargets from a specific
branch, etc.) based upon the index values.
o hi-target-param: An optional parameter reflecting the mechanism by
which the Request-URI captured in the hi-targeted-to-uri in the
History-Info header field value (hi-entry) was determined. This
parameter is either an "rc", "mp", or "np" header field parameter,
which is interpreted as follows:
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"rc": The hi-targeted-to-URI represents a change in
Request-URI, while the target user remains the same. This
occurs, for example, when the user has multiple AORs as an
alias. The "rc" header field parameter contains the value of
the hi-index in the hi-entry with an hi-targeted-to-uri that
reflects the Request-URI that was retargeted.
"mp": The hi-targeted-to-URI represents a user other than the
target user associated with the Request-URI in the incoming
request that was retargeted. This occurs when a request is
statically or dynamically retargeted to another user
represented by an AOR unassociated with the AOR of the original
target user. The "mp" header field parameter contains the
value of the hi-index in the hi-entry with an
hi-targeted-to-uri that reflects the Request-URI that was
retargeted, thus identifying the "mapped from" target.
"np": The hi-targeted-to-URI represents that there was no
change in the Request-URI. This would apply, for example, when
a proxy merely forwards a request to a next-hop proxy and loose
routing is used. The "np" header field parameter contains the
value of the hi-index in the hi-entry with an
hi-targeted-to-uri that reflects the Request-URI that was
copied unchanged into the request represented by this hi-entry.
That value will usually be the hi-index of the parent hi-entry
of this hi-entry.
o Extension (hi-extension): A parameter to allow for future optional
extensions. As per [RFC3261], any implementation not
understanding an extension MUST ignore it.
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
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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].
This document also extends the "contact-params" for the Contact
header field as defined in [RFC3261] with the "rc", "mp", and "np"
header field parameters defined above.
In addition to the parameters defined by the ABNF, an hi-entry may
also include a Reason header field and/or a Privacy header field,
which are both included in the "headers" component of the
hi-targeted-to-uri as described below:
o Reason: An optional parameter for History-Info, reflected in the
History-Info header field by including the Reason header field
[RFC3326] included in the hi-targeted-to-uri. A reason is
included in the hi-targeted-to-uri of an hi-entry to reflect
information received in a response to the request sent to that
URI.
o Privacy: An optional parameter for History-Info, reflected in the
History-Info header field values by including the Privacy header
[RFC3323] with a priv-value of "history", as defined in this
document, included in the hi-targeted-to-uri or by adding the
Privacy header field with a priv-value of "history" to the
request. The latter case indicates that the History-Info entries
for all History-Info entries whose hi-targeted-to-uri has the same
domain as the domain for which the SIP entity processing the
message is responsible MUST be anonymized prior to forwarding,
whereas the use of the Privacy header field included in the hi
-targeted-to-uri means that a specific hi-entry MUST be
anonymized.
Note that since both the Reason and Privacy parameters are included
in the hi-targeted-to-uri, these fields will not be available in the
case that the hi-targeted-to-uri is a Tel-URI [RFC3966].
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The following provides examples of the format for the History-Info
header field. Note that the backslash, CRLF, and whitespace between
the lines in the examples below are inserted for readability purposes
only. Note, however, that History-Info can be broken into multiple
lines due to the SWS (sep whitespace) that is part of HCOLON, COMMA,
and SEMI, and there can be multiple History-Info header fields due to
the rule of Section 7.3 of [RFC3261]. Additional detailed examples
are available in [CALLFLOWS].
History-Info: <sip:UserA@ims.example.com>;index=1;foo=bar
History-Info: <sip:UserA@ims.example.com?Reason=SIP%3B\
cause%3D302>;index=1.1,\
<sip:UserB@example.com?Privacy=history&Reason=SIP%3B\
cause%3D486>;index=1.2;mp=1.1,\
<sip:45432@192.168.0.3>;index=1.3;rc=1.2
5.1. History-Info Header Field Example Scenario
The following is an illustrative example of usage of History-Info.
In this example, Alice (sip:alice@atlanta.example.com) calls Bob
(sip:bob@biloxi.example.com). Alice's proxy in her home domain
(sip:atlanta.example.com) forwards the request to Bob's proxy
(sip:biloxi.example.com). When the request arrives at
sip:biloxi.example.com, it does a location service lookup for
bob@biloxi.example.com and changes the target of the request to Bob's
Contact URIs that were provided as part of normal SIP registration.
In this example, Bob is simultaneously contacted on a PC client and
on a phone, and Bob answers on the PC client.
One important thing illustrated by this call flow is that without
History-Info, Bob would "lose" the original target information or the
initial Request-URI, including any parameters in the Request-URI.
Bob can recover that information by locating the last hi-entry with
an "rc" header field parameter. This "rc" header field parameter
contains the index of the hi-entry containing the lost target
information, i.e., the sip:bob@biloxi.example.com hi-entry with
index=1.1. Note that in the 200 response to Alice, an hi-entry is
not included for the fork to sip:bob@192.0.2.7 (index 1.1.1) since
biloxi.example.com had not received a response from that fork at the
time it sent the 200 OK that ultimately reached Alice.
Additional detailed examples are available in [CALLFLOWS].
Note: This example uses loose routing procedures.
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Alice atlanta.example.com biloxi.example.com Bob@pc Bob@phone
| | | | |
| INVITE sip:bob@biloxi.example.com;p=x | |
|--------------->| | | |
| Supported: histinfo | | |
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1 |
| | | | |
| | INVITE sip:bob@biloxi.example.com;p=x |
| |--------------->| | |
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1 |
| History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
| | | | |
| | | INVITE sip:bob@192.0.2.3|
| | |--------------->| |
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
| History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
| History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
| | | | |
| | | INVITE sip:bob@192.0.2.7|
| | |-------------------------->|
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
| History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
| History-Info: <sip:bob@192.0.2.7>;index=1.1.2;rc=1.1
| | | 200 | |
| | |<---------------| |
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
| History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
| History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
| | | | |
| | 200 | | |
| |<---------------| | |
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
| History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
| History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
| | | | |
| | | Proxy Cancels INVITE |
| | |<=========================>|
| 200 | | | |
|<---------------| | | |
| History-Info: <sip:bob@biloxi.example.com;p=x>;index=1
| History-Info: <sip:bob@biloxi.example.com;p=x>;np=1;index=1.1
| History-Info: <sip:bob@192.0.2.3>;index=1.1.1;rc=1.1
| ACK | | | |
|--------------->| ACK | | |
| |--------------->| ACK | |
| | |--------------->| |
Figure 1: Basic Call
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6. User Agent Handling of the History-Info Header Field
This section describes the processing specific to UAs -- User Agent
Clients (UACs), User Agent Servers (UASs), and Back-to-Back User
Agents (B2BUAs) -- for the History-Info header.
6.1. User Agent Client (UAC) Behavior
The UAC MUST include the "histinfo" option tag in the Supported
header field in any out-of-dialog requests or initial requests for a
dialog for which the UAC would like the History-Info header field in
the response. When issuing a request, the UAC MUST follow the
procedures in Section 9.2. In the case of an initial request, except
where the UAC is part of a B2BUA, there is no cache of hi-entries
with which to populate the History-Info header field, and the
hi-index is set to 1 per Section 10.3. When receiving a response,
the UAC MUST follow the procedures in Section 9.3.
If the UAC generates further forks of the initial request (either due
to acting on a 3xx response or internally directed forking to
multiple destinations), the successive requests will add hi-entries
with hi-indexes of 2, 3, etc.
6.2. User Agent Server (UAS) Behavior
When receiving a request, a UAS MUST follow the procedures defined in
Section 9.2. When sending a response other than a 3xx response, a
UAS MUST follows the procedures defined in Section 9.4. When sending
a 3xx response, the UAS MUST follow the procedures defined for a
redirect server per Section 8. An application at the UAS can make
use of the cached hi-entries as described in Section 11.
6.3. Back-to-Back User Agent (B2BUA) Behavior
A B2BUA MAY follow the behavior of a SIP intermediary, per Section 7,
as an alternative to following the behavior of a UAS per Section 6.2
or a UAC per Section 6.1. In behaving as an intermediary, a B2BUA
carries forward hi-entries received in requests at the UAS to
requests being forwarded by the UAC, as well as carrying forward
hi-entries in responses received at the UAC to the responses
forwarded by the UAS, subject to privacy considerations per
Section 10.1.
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7. Proxy/Intermediary Handling of History-Info Header Fields
This section describes the procedures for proxies and other SIP
intermediaries for the handling of the History-Info header fields for
each of the following scenarios:
Receiving a Request: An intermediary MUST follow the procedures in
Section 9.1 for the handling of hi-entries in incoming SIP
requests.
Sending a Request: For each outgoing request relating to a target in
the target set, the intermediary MUST follow the procedures of
Section 9.2.
Receiving a Response or Timeout: An intermediary MUST follow the
procedures of Section 9.3 when a SIP response is received or a
request times out.
Sending a Response: An intermediary MUST follow the procedures of
Section 9.4 for the handling of the hi-entries when sending a SIP
response.
In some cases, an intermediary may retarget a request more than once
before forwarding, i.e., a request is retargeted to a SIP entity that
is "internal" to the intermediary before the same intermediary
retargets the request to an external target. A typical example would
be a proxy that retargets a request first to a different user (i.e.,
it maps to a different AOR) and then forwards it to a registered
contact bound to the same AOR. In this case, the intermediary MUST
add an hi-entry for (each of) the internal target(s) per the
procedures in Section 9.2. The intermediary MAY include a Reason
header field in the hi-entry with the hi-targeted-to-uri that has
been retargeted. Note that this is shown in the INVITE (F6) in the
example entitled "Sequentially Forking (History-Info in Response)" in
[CALLFLOWS].
8. Redirect Server Handling of History-Info Header Fields
A redirect server MUST follow the procedures in Section 9.1 when it
receives a SIP request. A redirect server MUST follow the procedures
in Section 9.4 when it sends a SIP response. When generating the
Contact header field in a 3xx response, the redirect server MUST add
the appropriate "mp", "np", or "rc" header field parameter to each
Contact header field as described in Section 10.4, if applicable.
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9. Handling of History-Info Header Fields in Requests and Responses
This section describes the procedures for SIP entities for the
handling of the History-Info header field in SIP requests and
responses.
9.1. Receiving a Request
When receiving a request, a SIP entity MUST keep a copy of the
hi-entries from the incoming request. This document describes this
copy in terms of a cache containing the hi-entries associated with
the request. The hi-entries MUST be added to the cache in the order
in which they were received in the request.
If the Request-URI of the incoming request does not match the hi
-targeted-to-uri in the last hi-entry (i.e., the previous SIP entity
that sent the request did not include a History-Info header field),
the SIP entity MUST add an hi-entry to the end of the cache, on
behalf of the previous SIP entity. This is done as follows, before
proceeding to Section 9.2.
The SIP entity MUST set the hi-targeted-to-uri to the value of the
Request-URI in the incoming request. If the Request-URI is a
Tel-URI, it SHOULD be transformed into a SIP URI (per
Section 19.1.6 of [RFC3261]) before being added as an
hi-targeted-to-uri.
If privacy is required, the SIP entity MUST follow the procedures
of Section 10.1.
The SIP entity MUST set the hi-index parameter as described in
Section 10.3.
The SIP entity MUST NOT include an "rc", "mp", or "np" header
field parameter.
9.2. Sending a Request with History-Info
When sending a request, a SIP entity MUST include all the hi-entries
from the cache that was created per Section 9.1. In addition, the
SIP entity MUST add a new hi-entry to the outgoing request, but the
SIP entity MUST NOT add the hi-entry to the cache at this time. The
hi-entries in the outgoing request's History-Info header field
represent the preorder of the tree of hi-entries, that is, by the
lexicographic ordering of the hi-indexes. The new hi-entry is
populated as follows:
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hi-targeted-to-uri: The hi-targeted-to-uri MUST be set to the value
of the Request-URI of the current (outgoing) request. If the
Request-URI is a Tel-URI, it SHOULD be transformed into a SIP URI
(per Section 19.1.6 of [RFC3261]) before being added as an
hi-targeted-to-uri.
privacy: If privacy is required, the procedures of Section 10.1 MUST
be followed.
hi-index: The SIP entity MUST include an hi-index for the hi-entry
as described in Section 10.3.
rc/mp/np: The SIP entity MUST include an "rc", "mp", or "np" header
field parameter in the hi-entry, if applicable, per the procedures
in Section 10.4.
9.3. Receiving a Response with History-Info or Request Timeouts
When a SIP entity receives a non-100 response or a request times out,
the SIP entity performs the following steps:
Step 1: Add hi-entry to cache
The SIP entity MUST add the hi-entry that was added to the request
that received the non-100 response or timed out to the cache, if
it was not already cached. The hi-entry MUST be added to the
cache in ascending order as indicated by the values in the
hi-index parameters of the hi-entries (e.g., 1.2.1 comes after 1.2
but before 1.2.2 or 1.3).
Step 2: Add Reason header field
If the response is not a 100 or 2xx response, the SIP entity adds
one or more Reason header fields to the hi-targeted-to-uri in the
(newly) cached hi-entry reflecting the SIP response code in the
non-100 or non-2xx response, per the procedures of Section 10.2.
Step 3: Add additional hi-entries
The SIP entity MUST also add to the cache any hi-entries received
in the response that are not already in the cache. This situation
can occur when the entity that generated the non-100 response
retargeted the request before generating the response. As per
Step 1, the hi-entries MUST be added to the cache in ascending
order as indicated by the values in the hi-index parameters of the
hi-entries.
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It is important to note that the cache (and the request or response)
does not contain hi-entries for requests that have not yet received a
non-100 response, so there can be gaps in indices (e.g., 1.2 and 1.4
could be present but not 1.3).
Note that in the case that a request has traversed one or more
intermediaries that do not support RFC 4244 or this document, there
can be duplicate indices (due to forking), which would be added to
the appropriate position in the cache in the order in which they are
received.
9.4. Sending History-Info in Responses
When sending a response other than a 100, a SIP entity MUST include
all the cached hi-entries in the response, subject to the privacy
consideration in Section 10.1.2, and with the following exception: If
the received request contained no hi-entries and there is no
"histinfo" option tag in the Supported header field, the SIP entity
MUST NOT include History-Info in the response.
10. Processing the History-Info Header Field
The following subsections describe the procedures for processing the
History-Info header field. These procedures are applicable to SIP
entities such as proxies/intermediaries, redirect servers, or user
agents.
10.1. Privacy in the History-Info Header Field
The privacy requirements for this document are described in
Appendix A.2. Section 10.1.1 describes the insertion of the Privacy
header field (defined in [RFC3323]) to indicate the privacy to be
applied to the History-Info header field entries. Section 10.1.2
describes how to apply privacy to a request or response that is being
forwarded, based on the presence of the Privacy header field.
10.1.1. Indicating Privacy
As with other SIP headers described in [RFC3323], the
hi-targeted-to-uris in the History-Info header field can
inadvertently reveal information about the initiator of the request.
Thus, the UAC needs a mechanism to indicate that the
hi-targeted-to-uris in the hi-entries need to be privacy protected.
The Privacy header field is used by the UAC to indicate that privacy
is to be applied to all the hi-entries in the request as follows:
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o If the UAC is including a Privacy header field with a priv-value
of "header" in the request, then the UAC SHOULD NOT include a
priv-value of "history" in the Privacy header field in the
request.
o If the UAC is including any priv-values other than "header" in the
Privacy header field, then the UAC MUST also include a priv-value
of "history" in the Privacy header field in the request.
o If the UAC is not including any priv-values in the Privacy header
field in the request, then the UAC MUST add a Privacy header
field, with a priv-value of "history", to the request. The UAC
MUST NOT include a priv-value of "critical" in the Privacy header
field in the request in this case.
In addition, the History-Info header field can reveal general routing
and diverting information that is within an intermediary and that the
intermediary wants to privacy protect. In this case, the
intermediary MUST construct a Privacy header field with the single
priv-value of "history" and include the Privacy header field in the
hi-targeted-to-uri, for each new hi-entry created by the intermediary
whose hi-targeted-to-uri it wishes to privacy protect. Note that the
priv-value in the Privacy header for the incoming request does not
necessarily influence whether the intermediary includes a Privacy
header field in the hi-entries. For example, even if the Privacy
header for the incoming request contained a priv-value of "none", the
proxy can still set a priv-value of "history" in the Privacy header
field included in the hi-targeted-to-uri.
Finally, the UAS may not want to reveal the final reached target to
the originator. In this case, the UAS MUST include a Privacy header
field with a priv-value of "history" in the hi-targeted-to-uri in the
last hi-entry, in the response. As noted above, the UAS of the
request MUST NOT use any other priv-values in the Privacy header
field included in the hi-entry.
10.1.2. Applying Privacy
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 "headers"
component of the hi-targeted-to-uri in the individual hi-entries.
If there is a Privacy header field in the message header of a request
or response, 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
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Privacy Service. The Privacy Service MUST change any
hi-targeted-to-uris in these hi-entries that have not been anonymized
(evidenced by their domain not being "anonymous.invalid") to
anonymous URIs containing a domain of anonymous.invalid as
recommended in Section 4.1.1.3 of [RFC3323]. As defined in
Section 4.1.1.2 of [RFC3323], the recommendations of [RFC3261] for
anonymizing the URI Username SHOULD be followed (i.e., "anonymous" in
the user portion of the URI). If there is a Privacy header field in
the "headers" component of the hi-targeted-to-uri in the hi-entries,
then the Privacy header field value MUST be removed from the
hi-entry. Once all the appropriate hi-entries have been anonymized,
the Privacy Service MUST remove the priv-value of "history" from the
Privacy header field in the message header of the request or
response. If there are no remaining priv-values in the Privacy
header field, the Privacy Service MUST remove the Privacy header
field from the request or response per [RFC3323].
If there is not a Privacy header field in the message header of the
request or response that is being forwarded, but there is a Privacy
header field with a priv-value of "history" in the "headers"
component in any of the hi-targeted-uris in the hi-entries associated
with the domain for which a SIP intermediary is responsible, then the
Privacy Service MUST update those hi-targeted-to-uris as described
above. Any other priv-values in the Privacy header field in the
"headers" component of the hi-targeted-to-uris in the hi-entries MUST
be ignored. In any case, the Privacy Service MUST remove the Privacy
header field from the "headers" component of the hi-targeted-to-uris
in the hi-entries prior to forwarding.
10.2. Reason in the History-Info Header Field
A Reason header field is added when the hi-entry is added to the
cache based upon the receipt of a SIP response that is neither a 100
nor a 2xx response, as described in Section 9.3. The SIP entity MUST
include a Reason header field, containing the SIP Response Code, in
the "headers" component of the hi-targeted-to-uri in the last
hi-entry added to the cache, unless the hi-targeted-to-uri is a
Tel-URI. In addition, if the response contains any Reason header
fields (see [RFC3326]), then the SIP entity MUST also include the
Reason header fields in the "headers" component of the
hi-targeted-to-uri in the last hi-entry added to the cache.
If a request has timed out (instead of being explicitly rejected),
the SIP entity MUST update the cache as if the request received a SIP
error response code of 408 "Request Timeout".
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A request can receive multiple responses that are neither 100 nor 2xx
responses and that carry or imply (for responses without Reason
headers, and for timeouts) multiple, possibly duplicated,
reason-values to be applied to an hi-targeted-to-uri. In these
situations, the SIP entity creating the History-Info header value
would choose the appropriate Reason header field value.
A SIP entity MAY also include a Reason header field (in the "headers"
component of an hi-targeted-to-uri) that contains the URI of a
request that was retargeted as a result of internal retargeting.
If additional Reason header field parameters are defined in the
future per [RFC3326], the use of these Reason header field parameters
for the History-Info header field MUST follow the same rules as
described above.
10.3. Indexing in the History-Info Header Field
In order to maintain ordering and accurately reflect the retargeting
of the request, the SIP entity MUST add an hi-index to each hi-entry.
Per the syntax in Section 5, the hi-index consists of a series of
nonnegative integers separated by dots (e.g., 1.1.2). Each dot
reflects a SIP forwarding hop. The nonnegative integer following
each dot reflects the order in which a request was retargeted at the
hop. The highest nonnegative integer at each hop reflects the number
of entities to which the request has been retargeted at the specific
hop (i.e., the number of branches) at the time that the request
represented by this hi-entry was generated. Thus, the indexing
results in a logical tree representation for the history of the
request and the hi-entries are given in the preorder of the tree.
The first index in a series of History-Info entries MUST be set to 1.
In the case that a SIP entity (intermediary or UAS) adds a first
hi-entry on behalf of the previous hop, the hi-index MUST be set to
1. For each forward hop (i.e., each new level of indexing), the last
integers of the hi-indexes of the new requests MUST be generated
starting at 1 and incrementing by 1 for each additional request.
The basic rules for adding the hi-index are summarized as follows:
1. Forwarding a request without changing the target: In the case of
a request that is being forwarded without changing the target,
the hi-index reflects the increasing length of the branch. In
this case, the SIP entity MUST read the value from the History-
Info header field in the received request and MUST add another
level of indexing by appending the dot delimiter followed by an
initial value of 1 for the new level. For example, if the
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hi-index in the last History-Info header field in the received
request is 1.1, a proxy would add an hi-entry with an hi-index of
1.1.1 and forward the request.
2. Retargeting within a processing entity - first instance: For the
first instance of retargeting within a processing entity, the SIP
entity MUST calculate the hi-index as prescribed for basic
forwarding.
3. Retargeting within a processing entity - subsequent instance: For
each subsequent retargeting of a request by the same SIP entity,
the SIP entity MUST calculate and add the hi-index for each new
branch by incrementing the rightmost value from the hi-index in
the last hi-entry. Per the example above, the hi-index in the
next request forwarded by this same SIP entity would be 1.1.2.
4. Retargeting based upon a response: In the case of retargeting due
to a specific response (e.g., 302), the SIP entity MUST calculate
the hi-index calculated per rule 3. That is, the rightmost value
of the hi-index MUST be incremented (i.e., a new branch is
created). For example, if the hi-index in the History-Info
header field of the sent request is 1.2 and the response to the
request is a 302, then the hi-index in the History-Info header
field for the new hi-targeted-to-URI would be 1.3.
5. Forking requests: If the request forwarding is done in multiple
forks (sequentially or in parallel), the SIP entity MUST set the
hi-index for each hi-entry for each forked request per the rules
above, with each new request having a unique index. Each index
MUST be sequentially assigned. For example, if the index in the
last History-Info header field in the received request is 1.1,
this processing entity would initialize its index to 1.1.1 for
the first fork, 1.1.2 for the second, and so forth. (See
Figure 1 for an example.) Note that, in the case of parallel
forking, only the hi-entry corresponding to the fork is included
in the request because no response can yet have been received for
any of the parallel forked requests.
6. 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 an hi-entry MUST add a single index with a value of "0"
(i.e., the nonnegative integer zero) prior to adding the
appropriate index for the action to be taken. For example, if
the index of the last hi-entry in the request received was 1.1.2
and there was a missing hi-entry and the request was being
forwarded to the next hop, the resulting index will be 1.1.2.0.1.
In the case of requests that are forked by a proxy that does not
support History-Info, it is possible for hi-entries generated by
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different entities to have the same index, i.e., each entity
supporting History-Info would receive a forked request with the
same hi-index to which they would add the value of ".0" prior to
adding the appropriate index. Thus, in the previous example,
each of the next-hop entities would generate an hi-index of
1.1.2.0.1.
10.4. Mechanism for Target Determination in the History-Info Header
Field
This specification 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 is the hi-index of the
hi-entry with an hi-targeted-to-uri that represents the Request-URI
that was retargeted.
The SIP entity MUST determine the specific parameter field to be
included in the hi-target-param, in the History-Info header field, as
the targets are added to the target set per the procedures in
Section 16.5 of [RFC3261] or per Section 8.1.3.4 of [RFC3261] in the
case of retargeting to a Contact URI received in a 3xx response. In
the latter case, the specific header field parameter in the Contact
header field becomes the header field parameter that is used in the
hi-entry when the request is retargeted. If the Contact header field
does not contain an "rc" or "mp" header field parameter, then the SIP
entity MUST NOT include an "rc" or "mp" header field parameter in the
hi-target-param in the hi-entry when the request is retargeted to a
Contact URI received in a 3xx response. This is because the redirect
server is the only element with any knowledge on how the target was
determined. Note that the "np" header field parameter is not
applicable in the case of redirection.
Based on the following criteria, the SIP entity (intermediary or
redirect server) determines the specific header field parameter
("rc", "mp", or "np") to be used.
o "rc": The Request-URI has changed while the target user associated
with the original Request-URI prior to retargeting has been
retained.
o "mp": The target was determined based on a mapping to a user other
than the target user associated with the Request-URI being
retargeted.
o "np": The target hasn't changed, and the associated Request-URI
remained the same.
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Note that there are two scenarios by which the "mp" header field
parameter can be derived.
o The mapping was done by the receiving entity on its own authority,
in which case the mp-value is the parent index of the hi-entry's
index.
o The mapping was done due to receiving a 3xx response, in which
case the mp-value is an earlier sibling or descendant of an
earlier sibling of the hi-entry's index; the index is that of the
downstream request that received the 3xx response.
11. Application Considerations
History-Info provides a very flexible building block that can be used
by intermediaries and UAs for a variety of services. 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 the hi-entries. The SIP
entity MUST be prepared to process effectively messages whose
hi-entries show evidence of "gaps", that is, situations that reveal
that not all of the forks of the request have been recorded in the
hi-entries. Gaps are possible if the request is forwarded through
intermediaries that do not support the History-Info header field and
are reflected by the existence of hi-entries with a nonnegative
integer of "0", e.g., "1.1.0.1". Gaps are also possible in the case
of parallel forking if there is an outstanding request at the time
the SIP entity sends a message. In addition, gaps may introduce the
possibility of duplicate values for the hi-index in the case that a
proxy that does not support History-Info forks a request. If gaps
are detected, the SIP entity MUST NOT treat this as an error but
SHOULD indicate to any applications that there are gaps. The
interpretation of the information in the History-Info header field
depends upon the specific application; an application might need to
provide special handling in some cases where there are gaps.
The following describes some categories of information that
applications can use:
1. Complete history information, e.g., for debugging or other
operational and management aspects, optimization of determining
targets to avoid retargeting to the same URI, etc. This
information is relevant to proxies, UACs, and UASs.
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2. Hi-entry with the index that matches the value of the "rc" header
field parameter in the last hi-entry with an "rc" header field
parameter in the request received by a UAS, i.e., the last AOR
that was retargeted to a contact based on an AOR-to-contact
binding.
3. Hi-entry with the index that matches the value of the "mp" header
field parameter in the last hi-entry with an "mp" header field
parameter in the hi-target-param in the request received by a
UAS, i.e., the last Request-URI that was mapped to reach the
destination.
4. Hi-entry with the index that matches the value of the "rc" header
field parameter in the first hi-entry with an "rc" header field
parameter in the request received by a UAS. Note that this would
be the original AOR if all the entities involved support the
History-Info header field and there is an absence of an "mp"
header field parameter prior to the "rc" header field parameter
in the hi-target-param in the History-Info header field.
However, there is no guarantee that all entities will support
History-Info; thus, the hi-entry that matches the value of the
"rc" header field parameter of the first hi-entry with an "rc"
header field parameter in the hi-target-param within the domain
associated with the target URI at the destination is more likely
to be useful.
5. Hi-entry with the index that matches the value of the "mp" header
field parameter in the first hi-entry with an "mp" header field
parameter in the request received by a UAS. Note that this would
be the original mapped URI if all entities supported the History-
Info header field. However, there is no guarantee that all
entities will support History-Info; thus, the hi-entry that
matches the value of the "mp" header field parameter of the first
hi-entry with an "mp" header field parameter within the domain
associated with the target URI at the destination is more likely
to be useful.
In many cases, applications are most interested in the information
within one or more particular domains; thus, only a subset of the
information is required.
Some applications may use multiple types of information. For
example, an Automatic Call Distribution (ACD) / call center
application that utilizes the hi-entry with an index that matches the
value of the "mp" header field parameter in the first hi-entry with
an "mp" header field parameter may also display other agents,
reflected by hi-entries prior to hi-entries with an "rc" header field
parameter, to whom the call was targeted prior to its arrival at the
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current agent. This could allow the agent the ability to decide how
they might forward or reroute the call if necessary (avoiding agents
that were not previously available for whatever reason, etc.).
Since support for History-Info header field is optional, a service
MUST define default behavior for requests and responses not
containing History-Info header fields. For example, an entity may
receive an incomplete set of hi-entries or hi-entries that are not
tagged appropriately with an hi-target-param in the case of entries
added by entities that are only compliant to RFC 4244. This may not
impact some applications (e.g., debug); however, it could require
some applications to make some default assumptions in this case. For
example, in an ACD scenario, the application could select the oldest
hi-entry with the domain associated with the ACD system and display
that as the original called party. Depending upon how and where the
request may have been retargeted, the complete list of agents to whom
the call was targeted may not be available.
12. Application-Specific Usage
The following are possible (non-normative) application-specific
usages of History-Info.
12.1. PBX Voicemail
A voicemail system (VMS) typically requires the original called party
information to determine the appropriate mailbox so an appropriate
greeting can be provided and the appropriate party notified of the
message.
The original target is determined by finding the first hi-entry
tagged with "rc" and using the hi-entry referenced by the index of
the "rc" header field parameter as the target for determining the
appropriate mailbox. This hi-entry is used to populate the "target"
URI parameter as defined in [RFC4458]. The VMS can look at the last
hi-entry and find the target of the mailbox by looking at the URI
entry in the "target" URI parameter in the hi-entry.
This example usage does not work properly in the presence of
forwarding that takes place before the call reaches the company. In
that case, not the first hi-entry with an "rc" value, but the first
hi-entry with an "rc" value following an "mp" entry needs to be
picked. Further detail for this example can be found in the call
flow entitled "PBX Voicemail Example" in [CALLFLOWS].
Note that in the case where there is no entry tagged with "rc", a VMS
can follow the procedures, as defined in [RFC4458], for the
"Interaction with Request History Information".
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12.2. Consumer Voicemail
The voicemail system in this environment typically requires the last
called party information to determine the appropriate mailbox so an
appropriate greeting can be provided and the appropriate party
notified of the message.
The last target is determined by finding the hi-entry referenced by
the index of the last hi-entry tagged with "rc" for determining the
appropriate mailbox. This hi-entry is used to populate the "target"
URI parameter as defined in [RFC4458]. The VMS can look at the last
hi-entry and find the target of the mailbox by looking for the
"target" URI parameter in the hi-entry. Further detail for this
example can be found in the call flow entitled "Consumer Voicemail
Example" in [CALLFLOWS].
In the case where there is no entry tagged with "rc", a VMS can
follow the procedures, as defined in [RFC4458], for the "Interaction
with Request History Information".
13. Security Considerations
The security requirements for this specification are specified in
Appendix A.1.
This document defines a header field for SIP. The use of the
Transport Layer Security (TLS) protocol [RFC5246] as a mechanism to
ensure the overall confidentiality of the History-Info header fields
(SEC-req-4) is strongly RECOMMENDED. If TLS is NOT used, the
intermediary MUST ensure that the messages are only sent within an
environment that is secured by other means or that the messages don't
leave the intermediary's domain. This results in History-Info's
having at least the same level of security as other headers in SIP
that are inserted by intermediaries. With TLS, History-Info header
fields are no less, nor no more, secure than other SIP header fields,
which generally have even more impact on the subsequent processing of
SIP sessions than the History-Info header field.
Note that while using the SIPS scheme (as per [RFC5630]) protects
History-Info from tampering by arbitrary parties outside the SIP
message path, all the intermediaries on the path are trusted
implicitly. A malicious intermediary could arbitrarily delete,
rewrite, or modify History-Info. This specification does not attempt
to prevent or detect attacks by malicious intermediaries.
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In terms of ensuring the privacy of hi-entries, the same security
considerations as those described in [RFC3323] apply. The Privacy
Service that's defined in [RFC3323] MUST also support the new Privacy
header field priv-value of "history" and anonymize hi-entries in the
case of a priv-value of "header" as described in Section 10.1.2.
14. IANA Considerations
IANA registrations have been implemented or updated as detailed in
the following subsections.
This document obsoletes [RFC4244] but uses the same SIP header field
name, Privacy header field, and Option tag. References to [RFC4244]
in the IANA "Session Initiation Protocol (SIP) Parameters" registry
(<http://www.iana.org/assignments/sip-parameters>) have been replaced
with references to this document.
14.1. Registration of New SIP History-Info Header Field
This document defines a SIP header field name, History-Info; and an
option tag, histinfo. The following updates have been made to
<http://www.iana.org/assignments/sip-parameters>.
The following row has been updated in the "Header Fields" sub-
registry:
Header Name Compact Form Reference
----------- ------------ ---------
History-Info none [RFC7044]
The following has been updated in the "Option Tags" sub-registry:
Name Description Reference
---- ----------- ---------
histinfo When used with the Supported header field, [RFC7044]
this option tag indicates the UAC supports
the History Information to be captured for
requests and returned in subsequent
responses. This tag is not used in a
Proxy-Require or Require header field,
since support of History-Info is optional.
Barnes, et al. Standards Track [Page 26]
RFC 7044 History-Info February 2014
14.2. Registration of "history" for SIP Privacy Header Field
This document defines a priv-value for the SIP Privacy header field:
history. The following updates have been made to the "SIP Privacy
Header Field Values" sub-registry in <http://www.iana.org/assignments
/sip-parameters> for the registration of the SIP Privacy header
field:
Privacy
Type Description Registrant Reference
------ ----------- ---------- ---------
history Privacy requested for Mary Barnes [RFC7044]
History-Info header mary.ietf.barnes@gmail.com
field(s)
14.3. Registration of Header Field Parameters
This specification defines the following new SIP header field
parameters in the "Header Field Parameters and Parameter Values" sub-
registry in <http:/www.iana.org/assignments/sip-parameters>.
Header Field Parameter Name Predefined Values Reference
------------- -------------- ----------------- ---------
History-Info mp No [RFC7044]
History-Info rc No [RFC7044]
History-Info np No [RFC7044]
Contact mp No [RFC7044]
Contact rc No [RFC7044]
Contact np No [RFC7044]
15. Acknowledgements
Jonathan Rosenberg et al. produced the document that provided
additional use cases precipitating the requirement for the new header
parameters to capture the method by which a Request-URI is
determined. The authors would like to acknowledge the constructive
feedback provided by Ian Elz, Paul Kyzivat, John Elwell, Hadriel
Kaplan, Marianne Mohali, Brett Tate, and Dale Worley. John Elwell
also provided excellent suggestions in terms of document structure.
Dan Romascanu performed the Gen-ART review.
Mark Watson, Cullen Jennings, and Jon Peterson provided significant
input into the initial work that resulted in the development of
[RFC4244]. The authors would like to acknowledge the constructive
feedback provided by Robert Sparks, Paul Kyzivat, Scott Orton, John
Elwell, Nir Chen, Palash Jain, Brian Stucker, Norma Ng, Anthony
Brown, Jayshree Bharatia, Jonathan Rosenberg, Eric Burger, Martin
Barnes, et al. Standards Track [Page 27]
RFC 7044 History-Info February 2014
Dolly, Roland Jesske, Takuya Sawada, Sebastien Prouvost, and
Sebastien Garcin in the development of [RFC4244].
The authors would like to acknowledge the significant input from
Rohan Mahy on some of the normative aspects of the ABNF for
[RFC4244], particularly regarding security and the index (the need
for it as well as its format).
16. Changes from RFC 4244
This RFC replaces [RFC4244].
Deployment experience with [RFC4244] over the years has shown a
number of issues, warranting an update:
o In order to make [RFC4244] work in "real life", one needs to make
"assumptions" on how History-Info is used. For example, numerous
implementations filter out many entries and only leave specific
entries corresponding, for example, to first and last redirection.
Since vendors use different rules, this causes significant
interoperability issues.
o [RFC4244] is overly permissive and evasive about recording
entries, causing interoperability issues.
o The examples in the call flows had errors and were confusing
because they often assume "loose routing".
o [RFC4244] has lots of repetitive and unclear text due to the
combination of requirements with the solution.
o [RFC4244] gratuitously mandates the use of TLS on every hop. No
existing implementation enforces this rule, and instead, whether
to use TLS is a general SIP issue, not an issue with [RFC4244]
per se.
o [RFC4244] does not include clear procedures on how to deliver
current target URI information to the UAS when the Request-URI is
replaced with a contact.
o [RFC4244] does not allow for marking History-Info entries for easy
processing by user agents.
Barnes, et al. Standards Track [Page 28]
RFC 7044 History-Info February 2014
The following summarizes the functional changes between this
specification and [RFC4244]:
1. Added header field parameters to capture the specific method by
which a target is determined to facilitate processing by users of
the History-Info header field entries. A specific header field
parameter is captured for each of the target URIs as the target
set is determined (per Section 16.5 of [RFC3261]). The header
field parameter is used in both the History-Info and the Contact
header fields.
2. Added a way to indicate a gap in History-Info by adding a
nonnegative integer of "0".
3. Rather than recommending that entries be removed in the case of
certain values of the Privacy header field, the entries are
anonymized.
4. Updated the security section to be equivalent to the security
recommendations for other SIP header fields inserted by
intermediaries.
5. Removed Appendix B ("Voicemail") since a separate call flow
document is being published as a companion to this document.
The first two changes are intended to facilitate application usage of
the History-Info header field and eliminate the need to make
assumptions based upon the order of the entries and ensure that the
most complete set of information is available to the applications.
In addition, editorial changes were done to both condense and clarify
the text, moving the requirements to an appendix and removing the
inline references to the requirements. The examples were simplified
and updated to reflect the protocol changes. Several of the call
flows in the appendix were removed and put into a separate document
that includes additional use cases that require the new header field
parameters.
16.1. Backwards Compatibility
This specification is backwards compatible because [RFC4244] allows
for the addition of new optional parameters. This specification adds
an optional SIP header field parameter to the History-Info and
Contact header fields. Entities that have not implemented this
specification will ignore these parameters; however, per [RFC4244],
an entity will not remove these parameters from an hi-entry. While
entities compliant to this document and [RFC4244] must be able to
recognize gaps in the hi-entries, this document requires that an
Barnes, et al. Standards Track [Page 29]
RFC 7044 History-Info February 2014
index of "0" be used in this case. In comparison, [RFC4244]
recommended (but did not require) the use of "1". However, since the
ABNF in [RFC4244] defines the index as a DIGIT, "0" would be a valid
value; thus, an [RFC4244] implementation should not have an issue if
it receives hi-entries added by intermediaries compliant to this
document.
As for the behavior of the UACs, UASs, and intermediaries, the
following additional normative changes have been made:
UAC behavior
1. Inclusion of option tag by UAC has changed from SHOULD to MUST.
2. Inclusion of hi-target-entry along with hi-index has changed from
MAY/RECOMMEND to MUST/MUST.
3. Behavior surrounding the addition of hi-target-entry based on a
3xx response has changed from MAY/SHOULD to MUST.
None of the behavior changes will cause any backward or forward
compatibility issues.
UAS behavior
1. Inclusion of hi-entry in response has changed from SHOULD to
MUST.
As the entity receiving response with hi-entry expected it with
SHOULD, this change will not cause any backward compatibility issues.
Proxy/redirect server behavior
1. Inclusion of the History-Info header field when forwarding the
request has changed from SHOULD to MUST.
2. Association of Reason with timeout/internal reason has changed
from MAY to MUST.
3. Inclusion of hi-index has changed from RECOMMENDED to MUST.
4. Inclusion of hi-entries in the response has changed from SHOULD
to MUST.
None of the above behavior changes impact backwards compatibility
since they only strengthen normative behavior to improve
interoperability.
Barnes, et al. Standards Track [Page 30]
RFC 7044 History-Info February 2014
In cases where an entity that is compliant to this document receives
a request that contains hi-entries compliant only to RFC 4244 (i.e.,
the hi-entries do not contain any of the new header field
parameters), the entity MUST NOT add any of the new header field
parameters to the hi-entries. The hi-entries MUST be cached and
forwarded as any other entries are, as specified in Section 9.1. As
with entities that are compliant to RFC 4244, applications must be
able to function in cases of missing information, as specified in
Section 11.
17. References
17.1. 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,
June 2002.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, December 2002.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC4244] Barnes, M., "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC
4244, November 2005.
17.2. Informative References
[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable
User Agent URIs (GRUUs) in the Session Initiation
Protocol (SIP)", RFC 5627, October 2009.
[RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630, October 2009.
Barnes, et al. Standards Track [Page 31]
RFC 7044 History-Info February 2014
[RFC3087] Campbell, B. and R. Sparks, "Control of Service Context
using SIP Request-URI", RFC 3087, April 2001.
[RFC4240] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC
3966, December 2004.
[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, April 2006.
[CALLFLOWS] Barnes, M., Audet, F., Schubert, S., Elburg, H., and C.
Holmberg, "Session Initiation Protocol (SIP) History-Info
Header Call Flow Examples", Work in Progress, November
2013.
Barnes, et al. Standards Track [Page 32]
RFC 7044 History-Info February 2014
Appendix A. Request History Requirements
The following list constitutes a set of requirements for a "Request
History" capability.
1. CAPABILITY-req: The "Request History" capability provides a
capability to inform proxies and UAs involved in processing a
request about the history/progress of that request. Although
this is inherently provided when the retarget is in response to a
SIP redirect, it is deemed useful for non-redirect retargeting
scenarios, as well.
2. GENERATION-req: "Request History" information is generated when
the request is retargeted.
A. In some scenarios, it might be possible for more than one
instance of retargeting to occur within the same proxy. A
proxy MUST also generate "Request History" information for
the 'internal retargeting'.
B. An entity (UA or proxy) retargeting in response to a redirect
or REFER MUST include any "Request History" information from
the redirect/REFER in the new request.
3. ISSUER-req: "Request History" information can be generated by a
UA or proxy. It can be passed in both requests and responses.
4. CONTENT-req: The "Request History" information for each
occurrence of retargeting shall include the following:
A. the new URI or address to which the request is in the process
of being retargeted,
B. the URI or address from which the request was retargeted, and
whether the retarget URI was an AOR,
C. the mechanism by which the new URI or address was determined,
D. the reason for the Request-URI or address modification, and
E. chronological ordering of the "Request History" information.
5. REQUEST-VALIDITY-req: "Request History" is applicable to requests
not sent within an early or established dialog (e.g., INVITE,
REGISTER, MESSAGE, and OPTIONS).
Barnes, et al. Standards Track [Page 33]
RFC 7044 History-Info February 2014
6. BACKWARDS-req: "Request History" information may be passed from
the generating entity backwards towards the UAC. This is needed
to enable services that inform the calling party about the dialog
establishment attempts.
7. FORWARDS-req: "Request History" information may also be included
by the generating entity in the request, if it is forwarded
onwards.
A.1. Security Requirements
The "Request History" information is being inserted by a network
element retargeting a request, resulting in a slightly different
problem than the basic SIP header problem, thus requiring specific
consideration. It is recognized that these security requirements can
be generalized to a basic requirement of being able to secure
information that is inserted by proxies.
The potential security problems include the following:
1. A rogue application could insert a bogus Request History-Info
entry by either adding an additional hi-entry as a result of
retargeting or entering invalid information.
2. A rogue application could rearrange the "Request History"
information to change the nature of the end application or to
mislead the receiver of the information.
3. A rogue application could delete some or all of the "Request
History" information.
Thus, a security solution for "Request History" must meet the
following requirements:
1. SEC-req-1: The entity receiving the "Request History" must be
able to determine whether any of the previously added "Request
History" content has been altered.
2. SEC-req-2: The ordering of the "Request History" information must
be preserved at each instance of retargeting.
3. SEC-req-3: The entity receiving the information conveyed by the
"Request History" must be able to authenticate the entity
providing the request.
4. SEC-req-4: To ensure the confidentiality of the "Request History"
information, only entities that process the request SHOULD have
visibility to the information.
Barnes, et al. Standards Track [Page 34]
RFC 7044 History-Info February 2014
It should be noted that these security requirements apply to any
entity making use of the "Request History" information.
A.2. Privacy Requirements
Since the Request-URI that is captured could inadvertently reveal
information about the originator, there are general privacy
requirements that MUST be met:
1. PRIV-req-1: The entity retargeting the request must ensure that
it maintains the network-provided privacy (as described in
[RFC3323]) associated with the request as it is retargeted.
2. PRIV-req-2: The entity receiving the "Request History" must
maintain the privacy associated with the information. In
addition, local policy at a proxy may identify privacy
requirements associated with the Request-URI being captured in
the "Request History" information.
3. PRIV-req-3: "Request History" information subject to privacy
shall not be included in outgoing messages unless it is protected
as described in [RFC3323].
Barnes, et al. Standards Track [Page 35]
RFC 7044 History-Info February 2014
Authors' Addresses
Mary Barnes
Polycom
TX
US
EMail: mary.ietf.barnes@gmail.com
Francois Audet
Skype
EMail: francois.audet@skype.net
Shida Schubert
NTT
EMail: shida@ntt-at.com
Hans Erik van Elburg
Detecon International Gmbh
Sternengasse 14-16
Cologne
Germany
EMail: ietf.hanserik@gmail.com
Christer Holmberg
Ericsson
Hirsalantie 11, Jorvas
Finland
EMail: christer.holmberg@ericsson.com
Barnes, et al. Standards Track [Page 36]
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