Network Working Group M. Barnes, Ed.
Request for Comments: 4244 Nortel
Category: Standards Track November 2005
An Extension to the Session Initiation Protocol (SIP)
for Request History Information
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
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 call arrives at a specific
application or user. This document defines a new optional SIP
header, History-Info, for capturing the history information in
requests.
Table of Contents
1. Introduction ....................................................2
1.1. Overview ...................................................2
1.2. Conventions Used in This Document ..........................3
1.3. Background: Why define a Generic "Request History"
capability? ................................................3
2. "Request History" Requirements ..................................4
2.1. Security Requirements ......................................6
2.2. Privacy Requirements .......................................7
3. Request History Information Description .........................7
3.1. Optionality of History-Info ................................8
3.2. Securing History-Info ......................................8
3.3. Ensuring the Privacy of History-Info .......................9
4. Request History Information Protocol Details ....................9
4.1. Protocol Structure of History-Info ........................10
4.2. Protocol Examples .........................................11
4.3. Protocol Usage ............................................12
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4.3.1. User Agent Client (UAC) Behavior ...................12
4.3.2. User Agent Server (UAS) Behavior ...................13
4.3.3. Proxy Behavior .....................................13
4.3.4. Redirect Server Behavior ...........................18
4.4. Security for History-Info .................................18
4.5. Example Applications Using History-Info ...................19
4.5.1. Example with Privacy Header for Entire
Request at Proxy2 ..................................21
4.5.2. Example with Privacy Header for Specific
URI (UA4) at Proxy2 ................................22
5. Application Considerations .....................................24
6. Security Considerations ........................................25
7. IANA Considerations ............................................25
7.1. Registration of New SIP History-Info Header ...............25
7.2. Registration of "history" for SIP Privacy Header ..........26
8. Normative References ...........................................26
9. Informative References .........................................26
10. Acknowledgements ..............................................26
11. Contributors' Addresses .......................................27
Appendix. Example Scenarios........................................28
Appendix A. Sequentially forking (History-Info in Response).....28
Appendix B. Voicemail...........................................34
Appendix C. Automatic Call Distribution Example.................39
Appendix D. Session via Redirect and Proxy Servers..............41
1. Introduction
1.1. Overview
Many services that SIP is anticipated to support require the ability
to determine why and how the call 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 redirect/retarget capabilities that enable calls to be
routed to chosen applications, there is currently no standard
mechanism within SIP for communicating the history of such a request.
This "request history" information allows the receiving application
to determine hints about how and why the call arrived at the
application/user.
This document defines a new SIP header, 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. The
History-Info header provides a building block for development of new
services.
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Section 1.3 provides additional background motivation for the Request
History capability. Section 2 identifies the requirements for a
solution, with Section 3 providing an overall description of the
solution.
Section 4 provides the details of the additions to the SIP protocol.
Example uses of the new header are included in Section 4.5, with
additional scenarios included in the Appendix.
Section 5 summarizes the application considerations identified in the
previous sections. Section 6 summarizes the security solution.
1.2. Conventions Used in This Document
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 RFC 2119 [RFC2119].
1.3. Background: Why define a Generic "Request History" capability?
SIP implicitly provides redirect/retarget capabilities that enable
calls to be routed to specific applications as defined in [RFC3261].
The term 'retarget' will be used henceforth in this document to refer
to the process of a Proxy Server/User Agent Client (UAC) changing a
Uniform Resource Identifier (URI) in a request and thus changing the
target of the request. This term is chosen to avoid associating this
request history only with the specific SIP Redirect Server capability
that provides for a response to be sent back to a UAC requesting that
the UAC should retarget the original request to an alternate URI.
The rules for determining request targets as described in Section
16.5 of [RFC3261] are consistent with the use of the retarget term in
this document.
The motivation for the request history is that in the process of
retargeting, old routing information can be forever lost. This lost
information may be important history that allows elements to which
the call is retargeted to process the call in a locally defined,
application-specific manner. The proposal in this document is to
provide a mechanism for transporting the request history. It is not
proposing any application-specific behavior for a Proxy or UA upon
receipt of the information. Indeed, such behavior should be a local
decision for the recipient application.
Current network applications provide the ability for elements
involved with the call to exchange additional information relating to
how and why the call was routed to a particular destination. The
following are examples of such applications:
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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 forwarding whereby the forwarded-to user obtains a "history"
of who sent the email to whom and at what time
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.
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. (Note that
the diagnostic utility of this mechanism is limited by the fact
that its use by entities that retarget is optional.)
o A stronger security solution for SIP. A side effect is that each
proxy that captures the "request history" information in a secure
manner provides an additional means (without requiring signed keys)
for the original requestor to be assured that the request was
properly retargeted.
2. "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) OPTIONALITY-req: The "Request History" information is optional.
2.1) In many cases, it is anticipated that whether the history is
added to the Request would be a local policy decision
enforced by the specific application; thus, no specific
protocol element is needed.
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2.2) Due to the capability being "optional" from the SIP protocol
perspective, the impact to an application of not having the
"Request History" must be described. Applicability
guidelines to be addressed by applications using this
capability must be provided as part of the solution to these
requirements.
3) GENERATION-req: "Request History" information is generated when
the request is retargeted.
3.1) In some scenarios, it might be possible for more than one
instance of retargeting to occur within the same Proxy. A
proxy should also generate Request History information for
the 'internal retargeting'.
3.2) An entity (UA or proxy) retargeting in response to a redirect
or REFER should include any Request History information from
the redirect/REFER in the new request.
4) ISSUER-req: "Request History" information can be generated by a UA
or proxy. It can be passed in both requests and responses.
5) CONTENT-req: The "Request History" information for each
occurrence of retargeting shall include the following:
5.1) The new URI or address to which the request is in the process
of being retargeted,
5.2) The URI or address from which the request was retargeted,
5.3) The reason for the Request-URI or address modification,
5.4) Chronological ordering of the Request History information.
6) REQUEST-VALIDITY-req: Request History is applicable to requests
not sent within an established dialog (e.g., INVITE, REGISTER,
MESSAGE, and OPTIONS).
7) 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.
8) FORWARDS-req: Request History information may also be included by
the generating entity in the request, if it is forwarded onwards.
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2.1. 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 entry
either by adding an additional entry as a result of retargeting or
entering invalid information.
2) A rogue application could re-arrange 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.
It should be noted that these security requirements apply to any
entity making use of the Request History information, either by
retargeting and capturing the information, or as an application
making use of the information received in either a Request or
Response.
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2.2. 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
requirements shall not be included in outgoing messages unless it
is protected as described in [RFC3323].
3. Request History Information Description
The fundamental functionality provided by the request history
information is the ability to inform proxies and UAs involved in
processing a request about the history or progress of that request
(CAPABILITY-req). The solution is to capture the Request-URIs as a
request is forwarded in a new header for SIP messages: History-Info
(CONTENT-req). This allows for the capturing of the history of a
request that would be lost with the normal SIP processing involved in
the subsequent forwarding of the request. This solution proposes no
changes in the fundamental determination of request targets or in the
request forwarding as defined in Sections 16.5 and 16.6 of the SIP
protocol specification [RFC3261].
The History-Info header can appear in any request not associated with
an established dialog (e.g., INVITE, REGISTER, MESSAGE, REFER and
OPTIONS, PUBLISH and SUBSCRIBE, etc.) (REQUEST-VALIDITY-req) and any
valid response to these requests (ISSUER-req).
The History-Info header is added to a Request when a new request is
created by a UAC or forwarded by a Proxy, or when the target of a
request is changed. The term 'retarget' is introduced to refer to
this changing of the target of a request and the subsequent
forwarding of that request. It should be noted that retargeting only
occurs when the Request-URI indicates a domain for which the
processing entity is responsible. In terms of the SIP protocol, the
processing associated with retargeting is described in Sections 16.5
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and 16.6 of [RFC3261]. As described in Section 16.5 of [RFC3261], it
is possible for the target of a request to be changed by the same
proxy multiple times (referred to as 'internal retargeting' in
Section 2), as the proxy MAY add targets to the target set after
beginning Request Forwarding. Section 16.6 of [RFC3261] describes
Request Forwarding. It is during this process of Request Forwarding
that the History Information is captured as an optional, additional
header field. Thus, the addition of the History-Info header does not
impact fundamental SIP Request Forwarding. An entity (UA or proxy)
changing the target of a request in response to a redirect or REFER
SHOULD also propagate any History-Info header from the initial
Request in the new request (GENERATION-req, FORWARDS-req).
3.1. Optionality of History-Info
The History-Info header is optional in that neither UAs nor Proxies
are required to support it. A new Supported header, "histinfo", is
included in the Request to indicate whether the History-Info header
is returned in Responses (BACKWARDS-req). In addition to the
"histinfo" Supported header, local policy determines whether or not
the header is added to any request, or for a specific Request-URI,
being retargeted. It is possible that this could restrict the
applicability of services that make use of the Request History
Information to be limited to retargeting within domain(s) controlled
by the same local policy, or between domain(s) which negotiate
policies with other domains to ensure support of the given policy, or
services for which complete History Information isn't required to
provide the service (OPTIONALITY-req). All applications making use
of the History-Info header MUST clearly define the impact of the
information not being available and specify the processing of such a
request.
3.2. Securing History-Info
This document defines a new header for SIP. The use of the Transport
Layer Security (TLS) protocol [RFC2246] as a mandatory mechanism to
ensure the overall confidentiality of the History-Info headers (SEC-
req-4) is strongly RECOMMENDED. This results in History-Info having
at least the same level of security as other headers in SIP that are
inserted by intermediaries. If TLS is not available for the
connection over which the request is being forwarded, then the
request MUST NOT include the History-Info header or the request MUST
be redirected to the client, including the History-Info header, so
that the request can be retargeted by the client.
With the level of security provided by TLS (SEC-req-3), the
information in the History-Info header can thus be evaluated to
determine if information has been removed by evaluating the indices
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RFC 4244 SIP Request History Information November 2005
for gaps (SEC-req-1, SEC-req-2). It would be up to the application
to define whether it can make use of the information in the case of
missing entries.
Note that while using the SIPS scheme 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.
3.3. Ensuring the Privacy of History-Info
Since the History-Info header can inadvertently reveal information
about the requestor as described in [RFC3323], the Privacy header
SHOULD be used to determine whether an intermediary can include the
History-Info header in a Request that it receives and forwards
(PRIV-req-2) or that it retargets (PRIV-req-1). Thus, the History-
Info header SHOULD NOT be included in Requests where the requestor
has indicated a priv-value of Session- or Header-level privacy.
In addition, the History-Info header can reveal general routing
information, which may be viewed by a specific intermediary or
network, to be subject to privacy restrictions. Thus, local policy
MAY also be used to determine whether to include the History-Info
header at all, whether to capture a specific Request-URI in the
header, or whether it be included only in the Request as it is
retargeted within a specific domain (PRIV-req-3). In the latter
case, this is accomplished by adding a new priv-value, history, to
the Privacy header [RFC3323] indicating whether any or a specific
History-Info header(s) SHOULD be forwarded.
It is recognized that satisfying the privacy requirements can impact
the functionality of this solution by overriding the request to
generate the information. As with the optionality and security
requirements, applications making use of History-Info SHOULD address
any impact this may have or MUST explain why it does not impact the
application.
4. Request History Information Protocol Details
This section contains the details and usage of the proposed new SIP
protocol elements. It also discusses the security aspects of the
solution.
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RFC 4244 SIP Request History Information November 2005
4.1. Protocol Structure of History-Info
History-Info is a header field as defined by [RFC3261]. It is an
optional header field and MAY appear in any request or response not
associated with a dialog or which starts a dialog. For example,
History-Info MAY appear in INVITE, REGISTER, MESSAGE, REFER, OPTIONS,
SUBSCRIBE, and PUBLISH and any valid responses, plus NOTIFY requests
that initiate a dialog.
This document adds the following entry to Table 2 of [RFC3261]. The
additions to this table are also provided for extension methods at
the time of publication of this document. This is provided as a
courtesy to the reader and is not normative in any way.
Header field where proxy ACK BYE CAN INV OPT REG MSG
------------ ----- ----- --- --- --- --- --- --- ---
History-Info amdr - - - o o o o
SUB NOT REF INF UPD PRA PUB
--- --- --- --- --- --- ---
History-Info amdr o o o - - - o
The History-Info header carries the following information, with the
mandatory parameters required when the header is included in a
request or response:
o Targeted-to-URI (hi-targeted-to-uri): A mandatory parameter for
capturing the Request-URI for the specific Request as it is
forwarded.
o Index (hi-index): A mandatory parameter for History-Info
reflecting the chronological order of the information, indexed to
also reflect the forking and nesting of requests. The format for
this parameter is a string of digits, 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 branch of the tree. By adding
the new entries in order (i.e., following existing entries per
the details in Section 4.3.3.1), including the index and securing
the header, the ordering of the History-Info headers in the
request is assured (SEC-req-2). 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 Reason: An optional parameter for History-Info, reflected in the
History-Info header by including the Reason Header [RFC3326]
escaped in the hi-targeted-to-uri. A reason is not included for
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a hi-targeted-to-uri when it is first added in a History-Info
header, but rather is added when the retargeting actually occurs.
Note that this does appear to complicate the security problem;
however, retargeting only occurs when the hi-targeted-to-uri
indicates a domain for which the processing entity is
responsible. Thus, it would be the same processing entity that
initially added the hi-targeted-to-URI to the header that would
be updating it with the Reason.
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" escaped in the hi-
targeted-to-uri or by adding the Privacy header with a priv-value
of "history" to the Request. The use of the Privacy Header with
a priv-value of "history" indicates whether a specific or all
History-Info headers should not be forwarded.
o Extension (hi-extension): An optional parameter to allow for
future optional extensions. As per [RFC3261], any implementation
not understanding an extension should ignore it.
The following summarizes the syntax of the History-Info header, based
upon the standard SIP syntax [RFC3261]:
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-extension
hi-index = "index" EQUAL 1*DIGIT *(DOT 1*DIGIT)
hi-extension = generic-param
4.2. Protocol Examples
The following provides some examples of the History-Info header.
Note that the backslash and CRLF between the fields in the examples
below are for readability purposes only.
History-Info:<sip:UserA@ims.example.com?Reason=SIP%3B\
cause%3D302>;index=1;foo=bar
History-Info: <sip:UserA@ims.example.com?Reason=SIP%3B \
cause%3D302>; index=1.1,
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<sip:UserB@example.com?Privacy=history&Reason=SIP%3B\
cause%3D486>;index=1.2,
<sip:45432@vm.example.com>;index=1.3
4.3. Protocol Usage
This section describes the processing specific to UAs and Proxies for
the History-Info header, the "histinfo" option tag, and the priv-
value of "history". As discussed in Section 1.3, the fundamental
objective is to capture the target Request-URIs as a request is
forwarded. This allows for the capturing of the history of a request
that would be lost due to subsequent (re)targeting and forwarding.
To accomplish this for the entire history of a request, either the
UAC must capture the Request-URI in a History-Info header in the
initial request or a proxy must add a History-Info header with both a
hi-entry for the Request-URI in the initial request and a hi-entry
for the target Request-URI as the request is forwarded. The basic
processing is for each entity forwarding a request to add a hi-entry
for the target Request-URI, updating the index and adding the Reason
as appropriate for any retargeted Request-URI.
4.3.1. User Agent Client (UAC) Behavior
The UAC SHOULD include the "histinfo" option tag in the Supported
header in any request not associated with an established dialog for
which the UAC would like the History-Info header in the response. In
addition, the UAC MAY improve the diagnostic utility of its request
by adding a History-Info header, using the Request-URI of the request
as the hi-target-to-uri and initializing the index to the RECOMMENDED
value of 1 in the hi-entry. As a result, intermediaries and the UAS
will know at least the original Request-URI, and if the Request-URI
was modified by a previous hop.
In the case where the request is routed to a redirect server and the
UAC receives a 3xx response with a Contact header, the UAC MAY
maintain the previous hi-entry(s) in the request. In this case, the
reason header SHOULD be associated with the hi-targeted-to-uri in the
previous (last) hi-entry, as described in Section 4.3.3.1.2. A new
hi-entry MAY then be added for the URI from the Contact header (which
becomes the new Request-URI). In this case, the index is created by
reading and incrementing the value of the index from the previous
hi-entry, thus following the same rules as those prescribed for a
proxy in retargeting, described in Section 4.3.3.1.3. An example of
this scenario can be found in Appendix D.
A UAC that does not want the History-Info header added due to privacy
considerations SHOULD include a Privacy header with a priv-value(s)
of "session", "header", or "history" in the request.
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With the exception of the processing of a 3xx response described
above, the processing of the History-Info header received in the
Response is application specific and outside the scope of this
document. However, the validity of the information SHOULD be ensured
prior to any application usage. For example, the entries MAY be
evaluated to determine gaps in indices, which could indicate that an
entry has been maliciously removed or removed for privacy reasons.
Either way, an application MAY want to be aware of potentially
missing information.
4.3.2. User Agent Server (UAS) Behavior
The processing of the History-Info header by a UAS in a Request
depends upon local policy and specific applications at the UAS that
might make use of the information. Prior to any application usage of
the information, the validity SHOULD be ascertained. For example,
the entries MAY be evaluated to determine gaps in indices, which
could indicate that an entry has been maliciously removed or removed
for privacy reasons. Either way, an application MAY want to be aware
of potentially missing information.
If the "histinfo" option tag is received in a request, the UAS SHOULD
include any History-Info received in the request in the subsequent
response.
4.3.3. Proxy Behavior
The inclusion of the History-Info header in a Request does not alter
the fundamental processing of proxies for determining request targets
as defined in Section 16.5 of [RFC3261]. Whether a proxy adds the
History-Info header or a new hi-entry as it forwards a Request
depends upon the following considerations:
1. Whether the Request contains the "histinfo" option tag in the
Supported header.
2. Whether the proxy supports the History-Info header.
3. Whether the Request contains a Privacy header with a priv-value
of "session", "header", or "history".
4. Whether any History-Info header added for a proxy/domain should
go outside that domain. An example being the use of the
History-Info header within the specific domain in which it is
retargeted, however, policies (for privacy, user and network
security, etc.) would prohibit the exposure of that information
outside that domain. To accommodate such a scenario, a proxy
MAY insert the Privacy header with a priv-value of "history"
when the request is being forwarded within the same domain. An
example of such an application is provided in Appendix C.
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5. Whether a hi-entry is added for a specific Request-URI due to
local privacy policy considerations. A proxy MAY add the
Privacy header with a priv-value of "history" associated with
the specific hi-targeted-to-uri.
An example policy would be a proxy that only adds the History-Info
header if the "histinfo" option tag is in the Supported header.
Other proxies may have a policy that they always add the header, but
never forward it outside a particular domain, accomplishing this by
adding a Privacy header with a priv-value of "history" to each hi-
entry to allow the information to be collected for internal
retargeting only.
Each application making use of the History-Info header SHOULD address
the impacts of the local policies on the specific application (e.g.,
what specification of local policy is optimally required for a
specific application and any potential limitations imposed by local
policy decisions).
Consistent with basic SIP processing of optional headers, proxies
SHOULD maintain the History-Info header(s), received in messages
being forwarded, independent of whether local policy supports
History-Info.
The specific processing by proxies for adding the History-Info
headers in Requests and Responses, to accommodate the considerations
outlined above, is described in detail in the following sections.
4.3.3.1. Adding the History-Info Header to Requests
Upon evaluation of the considerations under which the History-Info
header is to be included in requests (e.g., no Privacy header
overriding inclusion, local policy supports, etc.), detailed in
Section 4.3.3, a proxy SHOULD add a hi-entry as it forwards a
Request. Section 16.6 of [RFC3261] defines the steps to be followed
as the proxy forwards a Request. Step 5 prescribes the addition of
optional headers. Although this would seem the appropriate step for
adding the History-Info header, the interaction with Step 6,
"Postprocess routing information", and the impact of a strict route
in the Route header could result in the Request-URI being changed;
thus, adding the History-Info header between Steps 8 (adding Via
header) and 9 (adding Content-Length) is RECOMMENDED. Note that in
the case of loose routing, the Request-URI does not change during the
forwarding of a Request; thus, the capturing of History-Info for such
a request would result in duplicate Request-URIs with different
indices. The hi-entry MUST be added following any hi-entry received
in the request being forwarded. Additionally, if a request is
received that doesn't include a History-Info header, the proxy MAY
Barnes Standards Track [Page 14]RFC 4244 SIP Request History Information November 2005
add a History-Info header with a hi-entry preceding the one being
added for the current request being forwarded. The index for this
hi-entry is RECOMMENDED to start at 1. The following subsections
define the details of creating the information associated with each
hi-entry.
4.3.3.1.1. Privacy in the History-Info Header
If there is a Privacy header in the request with a priv-value of
"session", "header", or "history", a hi-entry MAY be added, if the
request is being forwarded to a Request-URI associated with a domain
for which the processing entity is responsible (and provided local
policy supports the History-Info header, etc.). If a request is
being forwarded to a Request-URI associated with a domain for which
the proxy is not responsible and there is a Privacy header in the
request with a priv-value of "session", "header", or "history", the
proxy SHOULD remove any hi-entry(s) prior to forwarding, depending
upon local policy and whether the proxy might know a priori that it
can rely on a downstream privacy service to apply the requested
privacy.
For the scenario where there is no Privacy header in the request and
the request is being forwarded to a Request-URI associated with the
domain(s) for which this entity is responsible, there are several
additional considerations:
o If there is no local policy associated with privacy, then a hi-
entry MAY be added to the Request.
o If the proxy's local policies, per consideration 4 in section
4.3.3, indicate that the History-Info header should not be
forwarded beyond the domain for which this intermediary is
responsible, then a Privacy header with a priv-value of "history"
SHOULD be associated with each hi-entry added by that proxy in
this scenario.
o If the proxy's policy, per consideration 5 in Section 4.3.3,
indicates that History-Info for a specific Request-URI should not
be forwarded beyond the domain for which this intermediary is
responsible, then a Privacy header with a priv-value of "history"
SHOULD be associated with the specific hi-entry, for that
specific hi-targeted-to-uri, added by that proxy in this
scenario.
If a request is being forwarded to a Request-URI associated with a
domain for which the proxy is not responsible and local policy
requires privacy associated with any, or with specific, hi-entries it
Barnes Standards Track [Page 15]RFC 4244 SIP Request History Information November 2005
has added, any hi-entry with a priv-value of "history" SHOULD be
removed prior to forwarding.
4.3.3.1.2. Reason in the History-Info Header
For retargets that are the result of an explicit SIP response, a
Reason MUST be associated with the hi-targeted-to-uri. If the SIP
response does not include a Reason header, the SIP Response Code that
triggered the retargeting MUST be included as the Reason associated
with the hi-targeted-to-uri that has been retargeted. If the
response contains a non-SIP Reason header (e.g., Q.850), it MUST be
captured as an additional Reason associated with the hi-targeted-to-
uri that has been retargeted, along with the SIP Response Code. If
the Reason header is a SIP reason, then it MUST be used as the Reason
associated with the hi-targeted-to-uri rather than the SIP response
code.
For retargets as a result of timeouts or internal events, a Reason
MAY be associated with the hi-targeted-to-uri that has been
retargeted.
The addition of the Reason should occur prior to the forwarding of
the request (which may add a new hi-entry with a new hi-targeted-to-
uri) as it is associated with the hi-targeted-to-uri that has been
retargeted, since it reflects the reason why the Request to that
specific URI was not successful.
4.3.3.1.3. Indexing in the History-Info Header
In order to maintain ordering and accurately reflect the nesting and
retargeting of the request, an index MUST be included along with the
Targeted-to-URI being captured. Per the syntax in Section 4.1, the
index consists of a dot-delimited series of digits (e.g., 1.1.2).
Each dot reflects a hop or level of nesting; thus, the number of hops
is determined by the total number of dots. Within each level, the
integer reflects the number of peer entities to which the request has
been routed. Thus, the indexing results in a logical tree
representation for the history of the Request. It is recommended
that for each level of indexing, the index start at 1. It is
recommended that an increment of 1 is used for advancing to a new
branch.
The basic rules for adding the index are summarized as follows:
1. Basic Forwarding: In the case of a Request that is being
forwarded, the index is determined by adding another level of
indexing since the depth/length of the branch is increasing. To
accomplish this, the proxy reads the value from the History-Info
Barnes Standards Track [Page 16]RFC 4244 SIP Request History Information November 2005
header in the received request, if available, and adds another
level of indexing by appending the dot delimiter followed by an
initial index for the new level RECOMMENDED to be 1. For
example, if the index in the last History-Info header field in
the received request is 1.1, this proxy would initialize its
index to 1.1.1 and forward the request.
2. Retargeting within a Proxy - 1st instance: For the first
instance of retargeting within a Proxy, the calculation of the
index follows that prescribed for basic forwarding.
3. Retargeting within a Proxy - subsequent instance: For each
subsequent retargeting of a request by the same proxy, another
branch is added. With the index for each new branch calculated
by incrementing the last/lowest digit at the current level, the
index in the next request forwarded by this same proxy,
following the example above, 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 index would be
calculated per rule 3. That is, the lowest/last digit of the
index is incremented (i.e., a new branch is created), with the
increment RECOMMENDED to be 1. For example, if the index in the
History-Info header of the received request was 1.2, then the
index in the History-Info header field for the new hi-targeted-
to-URI would be 1.3.
5. Retargeting the request in parallel (forking): If the request
forwarding is done in parallel, the index MUST be captured for
each forked request per the rules above, with each new Request
having a unique index. The only difference in the messaging for
this scenario and the messaging produced per basic proxy
retargeting in rules 2 and 3 is these forwarded requests do not
have History-Info entries associated with their peers. The
proxy builds the subsequent response (or request) using the
aggregated information associated with each of those requests
and including the header entries in the order indicated by the
indexing. Responses are processed as described in Section 16.7
of [RFC3261] with the aggregated History-Info entries processed
similar to Step 7 "Aggregate Authentication Header Field
Values". Section 4.5 provides an example of a parallel request
scenario, highlighting this indexing mechanism.
Barnes Standards Track [Page 17]RFC 4244 SIP Request History Information November 2005
4.3.3.2. Processing History-Info in Responses
A proxy that receives a Request with the "histinfo" option tag in the
Supported header, and depending upon a local policy supporting the
capture of History-Info, SHOULD return captured History-Info in
subsequent, provisional, and final responses to the Request, subject
to the following considerations for privacy:
o If the response is being forwarded to a Request-URI associated
with a domain for which the proxy is not responsible and there
was a Privacy header, in the request received by the proxy, with
a priv-value of "session", "header", or "history", the proxy MUST
remove the History-Info header (i.e., all hi-entries) prior to
forwarding.
o If a request is being forwarded to a Request-URI associated with
a domain for which the proxy is not responsible and local policy
requires privacy associated with any or all hi-entry(s) it has
added, any hi-entry with a priv-value of "history" MUST be
removed prior to forwarding.
o If a proxy receives a response from another intermediary
associated with a domain for which it is responsible, including
hi-entry(s) with privacy headers, and that response is to be
forwarded to a domain for which it is not responsible, then those
hi-entry(s) MUST be removed.
The processing of History-Info in responses follows the methodology
described in Section 16.7 of [RFC3261], with the processing of
History-Info headers adding an additional step, just before Step 9,
"Forwarding the Response".
4.3.4. Redirect Server Behavior
A redirect server SHOULD NOT add any new History-Info, as that would
be done by the entity receiving the 3xx response. However, a
redirect server MAY include History-Info in responses by adding any
History-Info headers received in a request to a subsequent response.
4.4. Security for History-Info
As discussed in Section 3, the security requirements are met by
recommending the use of TLS (a basic SIP requirement per [RFC3261])
for hop-by-hop security. If TLS is not available on the connection
over which a request containing a History-Info header is being
forwarded, then either of the following two options MUST be
implemented:
Barnes Standards Track [Page 18]RFC 4244 SIP Request History Information November 2005
o The History-Info header MUST be removed prior to forwarding the
request, or
o The request MUST be redirected, including the History-Info header
in the response, to allow the UAC to securely issue the request,
including the History-Info header.
4.5. Example Applications Using History-Info
This scenario highlights an example where the History-Info in the
response is primarily of use in not retrying routes that have already
been tried by another proxy. Note that this is just an example and
that there may be valid reasons why a Proxy would want to retry the
routes, and thus, this would likely be a local proxy or even user-
specific policy.
UA1 sends a call to Bob to proxy 1. Proxy 1 forwards the request to
Proxy 2. Proxy 2 sends the requests in parallel and tries several
places (UA2, UA3, and UA4) before sending a response to Proxy 1 that
all the places are busy. Proxy 1, without the History-Info, would
try some of the same places (e.g., UA3) based upon registered
contacts for Bob, before completing at UA5. However, with the
History-Info, Proxy 1 determines that UA3 has already received the
invite; thus, the INVITE goes directly to UA5.
Section 4.5.1 provides this same scenario using one of the privacy
mechanisms, with Proxy2 (P2) adding the Privacy header indicating
that the History-Info header is not to be propagated outside P2's
domain. This scenario highlights the potential functionality lost
with the use of "history" privacy in the Privacy header for the
entire request and the need for careful consideration on the use of
privacy for History-Info.
Section 4.5.2 also provides the same scenario using one of the
privacy mechanisms, however, due to local policy at Proxy2, only one
of the Request-URIs (UA4) in the History-Info contains a priv-value
of "history", thus allowing some optimized functionality in the
routing of the request, but still maintaining privacy for specific
URIs.
The formatting in these scenarios is for visual purposes; thus,
backslash and CRLF are used between the fields for readability and
the headers in the URI are not shown properly formatted for escaping.
Refer to Section 4.2 examples for the proper formatting. Additional
detailed scenarios are available in the appendix.
Barnes Standards Track [Page 19]RFC 4244 SIP Request History Information November 2005
UA1 Proxy1 Proxy2 UA2 UA3 UA4 UA5
| | | | | | |
|--INVITE -->| | | | | |
| |-INVITE->| | | | |
Supported: histinfo
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1
| | | | | | |
| | |-INVITE>| | | |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User2@UA2.example.com>;index=1.1.1
| | | | | | |
| | |-----INVITE ---->| | |
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User3@UA3.example.com>;index=1.1.2
| | | | | | |
| | |-------INVITE------------>| |
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User4@UA4.example.com>;index=1.1.3
/* All Responses from the INVITEs indicate non-success/non-
availability*/
| | | | | | |
| |<-480 ---| | | | |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User2@UA2.example.com?Reason=SIP;\
cause=408;text="RequestTimeout">;index=1.1.1,
<sip:User3@UA3.example.com?Reason=SIP; \
cause=487;text="Request Terminated">; index=1.1.2,
<sip:User4@UA4.example.com?Reason=SIP;\
cause=603;text="Decline">; index=1.1.3
| | | | | | |
/* Upon receipt of the response, P1 determines another route for the
INVITE, but finds that it matches a route already attempted
(e.g., UA3), thus the INVITE is only forwarded to UA5, where
the session is successfully established */
| | | | | | |
| |----------------INVITE --------------------->|
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User2@UA2.example.com?Reason=SIP;cause=408;\
text="RequestTimeout">;index=1.1.1,
<sip:User3@UA3.example.com?Reason=SIP;cause=487;\
Barnes Standards Track [Page 20]RFC 4244 SIP Request History Information November 2005
text="Request Terminated">; index=1.1.2,
<sip:User4@UA4.example.com?Reason=SIP;cause=603;\
text="Decline">; index=1.1.3
<sip:User5@UA5.example.com>;index=1.2
| | | | | | |
| |<-----200 OK---------------------------------|
|<--200 OK---| | | | | |
| | | | | | |
|--ACK --------------------------------------------------->|
4.5.1. Example with Privacy Header for Entire Request at Proxy2
UA1 Proxy1 Proxy2 UA2 UA3 UA4 UA5
| | | | | | |
|--INVITE -->| | | | | |
| |-INVITE->| | | | |
Supported: histinfo
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1
| | | | | | |
| | |-INVITE>| | | |
Privacy: history
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User2@UA2.example.com>;index=1.1.1
| | | | | | |
| | |-----INVITE ---->| | |
Privacy: history
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User3@UA3.example.com>;index=1.1.2
| | | | | | |
| | |-------INVITE------------>| |
Privacy: history
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User4@UA4.example.com>;index=1.1.3
/* All Responses from the INVITEs indicate non-success/non-
availability and only the initial, received History-Info entries
are NOT returned to P1 due to the Privacy header value.*/
| | | | | | |
| |<-480 ---| | | | |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1
| | | | | | |
/* Upon receipt of the response, P1 determines another route for the
Barnes Standards Track [Page 21]RFC 4244 SIP Request History Information November 2005
INVITE, including UA3, which was attempted by P2, but due to
Privacy P1 is not aware of this, so UA3 is re-attempted prior to
forwarding the INVITE to UA5, where the session is successfully
established */
| | | | | | |
| |--------------INVITE ----->| | |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User3@UA3.example.com>; index=1.2
| | | | | | |
| |<-- 486 -------------------| | |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User3@UA3.example.com>; index=1.2
| | | | | | |
| |----------------INVITE --------------------->|
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User3@UA3.example.com?Reason=SIP;cause=486;\
text="Busy Here">;index=1.2,
<sip:User5@UA5.example.com>;index=1.3
| | | | | | |
| |<-----200 OK---------------------------------|
|<--200 OK---| | | | | |
| | | | | | |
|--ACK --------------------------------------------------->|
4.5.2. Example with Privacy Header for Specific URI (UA4) at Proxy2
UA1 Proxy1 Proxy2 UA2 UA3 UA4 UA5
| | | | | | |
|--INVITE -->| | | | | |
| |-INVITE->| | | | |
Supported: histinfo
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1
| | | | | | |
| | |-INVITE>| | | |
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User2@UA2.example.com>;index=1.1.1
| | | | | | |
| | |-----INVITE ---->| | |
History-Info:<sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User3@UA3.example.com>;index=1.1.2
| | | | | | |
Barnes Standards Track [Page 22]RFC 4244 SIP Request History Information November 2005
| | |-------INVITE------------>| |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>;index=1.1,
<sip:User4@UA4.example.com?\
Privacy=history>; index=1.1.3
/* All Responses from the INVITEs indicate non-success/non-
availability. The History-Info associated with UA4 is not returned
in the response due to the privacy header associated with that URI */
| | | | | | |
| |<-480 ---| | | | |
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User2@UA2.example.com?Reason=SIP;\
cause=408;text="RequestTimeout">;index=1.1.1,
<sip:User3@UA3.example.com?Reason=SIP; \
cause=487;text="Request Terminated">; index=1.1.2,
| | | | | | |
/* Upon receipt of the response, P1 determines another route for the
INVITE, but finds that it matches a route already attempted
(e.g., UA3), thus the INVITE is only forwarded to UA5, where
the session is successfully established */
| | | | | | |
| |----------------INVITE --------------------->|
History-Info: <sip:Bob@P1.example.com>;index=1,
<sip:Bob@P2.example.com>; index=1.1,
<sip:User2@UA2.example.com?Reason=SIP;cause=408;\
text="RequestTimeout">;index=1.1.1,
<sip:User3@UA3.example.com?Reason=SIP;cause=487;\
text="Request Terminated">; index=1.1.2,
<sip:User5@UA5.example.com>;index=1.2
| | | | | | |
| |<-----200 OK---------------------------------|
|<--200 OK---| | | | | |
| | | | | | |
|--ACK --------------------------------------------------->|
Barnes Standards Track [Page 23]RFC 4244 SIP Request History Information November 2005
5. Application Considerations
As seen by the example scenarios in the appendix, History-Info
provides a very flexible building block that can be used by
intermediaries and UAs for a variety of services. As such, any
services making use of History-Info must be designed with the
following considerations:
1) History-Info is optional; thus, a service MUST define default
behavior for requests and responses not containing History-Info
headers.
2) History-Info may be impacted by privacy considerations.
Applications requiring History-Info need to be aware that if
Header-, Session-, or History-level privacy is requested by a UA
(or imposed by an intermediary) that History-Info may not be
available in a request or response. This would be addressed by an
application in the same manner as the previous consideration by
ensuring there is reasonable default behavior should the
information not be available.
3) History-Info may be impacted by local policy. Each application
making use of the History-Info header SHOULD address the impacts
of the local policies on the specific application (e.g., what
specification of local policy is optimally required for a specific
application and any potential limitations imposed by local policy
decisions). Note that this is related to the optionality and
privacy considerations identified in 1 and 2 above, but goes
beyond that. For example, due to the optionality and privacy
considerations, an entity may receive only partial History-Info
entries; will this suffice? Note that this would be a limitation
for debugging purposes, but might be perfectly satisfactory for
some models whereby only the information from a specific
intermediary is required.
4) The security associated with the History-Info header requires the
use of TLS. In the case of TLS not being available for a
connection over which a request is being forwarded, the History-
Info header may be removed from a request. The impact of lack of
having the information depends upon the nature of the specific
application (e.g., Is the information something that appears on a
display or is it processed by automata which could have negative
impacts on the subsequent processing of a request?). It is
suggested that the impact of an intermediary not supporting the
security recommendations should be evaluated by the application to
ensure that the impacts have been sufficiently addressed by the
application.
Barnes Standards Track [Page 24]RFC 4244 SIP Request History Information November 2005
6. Security Considerations
The threat model and related security and privacy requirements for
the History-Info header are described in Sections 2.1 and 2.2 of this
document. Sections 3.2, 3.3, and 4.4 provide normative
recommendations related to security and privacy fulfilling these
requirements. The use of TLS is mandated between the entities (i.e.,
UAC to Proxy, Proxy to Proxy, and Proxy to UAS) that use the
History-Info header. The appropriate handling of a request in the
case that TLS is not available for a specific connection is described
in Section 5.
With TLS, History-Info headers are no less, nor no more, secure than
other SIP headers, which generally have even more impact on the
subsequent processing of SIP sessions than the History-Info header.
7. IANA Considerations
7.1. Registration of New SIP History-Info Header
This document defines a new SIP header field name: History-Info and a
new option tag: histinfo.
The following changes have been made to
http:///www.iana.org/assignments/sip-parameters
The following row has been added to the header field section:
Header Name Compact Form Reference
----------- ------------ ---------
History-Info none [RFC4244]
The following has been added to the Options Tags section:
Name Description Reference
---- ----------- ---------
histinfo When used with the Supported header, [RFC4244]
this option tag indicates support
for 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 Standards Track [Page 25]RFC 4244 SIP Request History Information November 2005
7.2. Registration of "history" for SIP Privacy Header
This document defines a new priv-value for the SIP Privacy header:
history
The following changes have been made to
http://www.iana.org/assignments/sip-priv-values
The following has been added to the registration for the SIP Privacy
header:
Name Description Registrant Reference
---- ----------- ---------- ---------
history Privacy requested for Mary Barnes [RFC4244]
History-Info header(s) mary.barnes@nortel.com
8. 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.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
9. Informative References
[RFC3665] Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and
K. Summers, "Session Initiation Protocol (SIP) Basic Call
Flow Examples", BCP 75, RFC 3665, December 2003.
10. Acknowledgements
The editor would like to acknowledge the constructive feedback
provided by Robert Sparks, Paul Kyzivat, Scott Orton, John Elwell,
Nir Chen, Francois Audet, Palash Jain, Brian Stucker, Norma Ng,
Anthony Brown, Jayshree Bharatia, Jonathan Rosenberg, Eric Burger,
Barnes Standards Track [Page 26]RFC 4244 SIP Request History Information November 2005
Martin Dolly, Roland Jesske, Takuya Sawada, Sebastien Prouvost, and
Sebastien Garcin.
The editor would like to acknowledge the significant input from Rohan
Mahy on some of the normative aspects of the ABNF, particularly
around the need for and format of the index and around the security
aspects.
11. Contributors' Addresses
Cullen, Mark, and Jon contributed to the development of the initial
requirements.
Cullen and Mark provided substantial input in the form of email
discussion in the development of the initial version of the
individual solution document.
Cullen Jennings
Cisco Systems
170 West Tasman Dr
MS: SJC-21/3
Phone: +1 408 421 9990
EMail: fluffy@cisco.com
Jon Peterson
NeuStar, Inc.
1800 Sutter Street, Suite 570
Concord, CA 94520
USA
Phone: +1 925-363-8720
EMail: Jon.Peterson@NeuStar.biz
Mark Watson
Digital Fountain
39141 Civic Center Drive Suite 300
Fremont, CA 94538
U.S.A.
EMail: mark@digitalfountain.com
Barnes Standards Track [Page 27]
RFC 4244 SIP Request History Information November 2005
Appendix. Example Scenarios
The scenarios in Appendices A-D provide sample use cases for the
History-Info header for informational purposes only. They are not
intended to be normative and the formatting is for visual purposes;
thus, the headers in the URI are not shown properly formatted for
escaping. Refer to Section 4.2 examples with the proper formatting.
Appendix A. Sequentially Forking (History-Info in Response)
This scenario highlights an example where the History-Info in the
response is useful to an application or user that originated the
request.
Alice at UA1 sends a call to Bob via Proxy1. Proxy1 sequentially
tries several places (UA2, UA3 and UA4) unsuccessfully before sending
a response to Alice.
This scenario is provided to show that by providing the History-Info
to UA1, the end-user or an application at UA1 could make a decision
on how best to attempt finding Bob. Without this mechanism, UA1
might well attempt UA3 (and thus UA4) and then re-attempt UA4 on a
third manual attempt at reaching Bob. With this mechanism, either
the end-user or application could know that Bob is busy on his home
phone and is physically not in the office. If there were an
alternative address for Bob known to this end-user or application,
that hasn't been attempted, then either the application or the end-
user could attempt that. The intent here is to highlight an example
of the flexibility of this mechanism that enables applications well
beyond SIP as it is certainly well beyond the scope of this document
to prescribe detailed applications.
In this scenario, since UA1 has not included the original Request-URI
in the INVITE, the proxy adds a hi-entry to capture the original
Request-URI to provide the complete set of information, as discussed
in Section 4.3.3.1.
Barnes Standards Track [Page 28]
RFC 4244 SIP Request History Information November 2005
UA1 Proxy1 UA2 UA3 UA4
| | | | |
|-INVITE F1->| | | |
| | | | |
| |--INVITE F2------>| | |
|<--100 F3---| | | |
| |<-302 F4----------| | |
| | | | |
| |-------INVITE F5 --------->| |
| | | | |
| |<-------180 F6 ------------| |
|<---180 F7--| | | |
| . . |---retransmit INVITE ----->| |
| | | | |
| | ( timeout ) | | |
| | | | |
| |------INVITE F8 ------------------->|
|<--100 F9 --| | | |
| | | | |
| |<-486 F10 --------------------------|
| | | | |
| |-- ACK F11------------------------->|
|<--486 F12--| | | |
| | | | |
|--ACK F13-->| | | |
| | | | |
Message Details
F1 INVITE UA1 ->Proxy1
INVITE sip:UserA@example.com SIP/2.0
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Contact: Alice <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=UserA 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
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RFC 4244 SIP Request History Information November 2005
/*Client for UA1 prepares to receive data on port 49170
from the network. */
F2 INVITE Proxy1 ->UA2
INVITE sip:UserA@ims.example.com SIP/2.0
Via: SIP/2.0/UDP ims.example.com:5060;branch=1
Via: SIP/2.0/UDP example.net:5060
Record-Route: <sip:UserA@example.com>
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
History-Info: <sip:UserA@example.com>; index=1,
<sip:UserA@ims.example.com>; index=1.1
Contact: Alice <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=UserA 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F3 100 Trying Proxy1 ->UA1
SIP/2.0 100 Trying
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
F4 302 Moved Temporarily UA2 ->Proxy1
SIP/2.0 302 Moved Temporarily
Via: SIP/2.0/UDP ims.example.com:5060;branch=1
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>;tag=3
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Contact: <sip:UserB@example.com>
Content-Length: 0
Barnes Standards Track [Page 30]RFC 4244 SIP Request History Information November 2005
F5 INVITE Proxy1 -> UA3
INVITE sip:UserB@example.com SIP/2.0
Via: SIP/2.0/UDP ims.example.com:5060;branch=2
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
History-Info: <sip:UserA@example.com>; index=1,
<sip:UserA@ims.example.com?Reason=SIP;\
cause=302; text="Moved Temporarily">; index=1.1,
<sip:UserB@example.com>;index=1.2
CSeq: 1 INVITE
Contact: Alice <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=User1 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F6 180 Ringing UA3 ->Proxy1
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>;tag=5
Call-ID: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
F7 180 Ringing Proxy1 -> UA1
SIP/2.0 180 Ringing
SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
/* User B is not available. INVITE is sent multiple
times until it times out. */
Barnes Standards Track [Page 31]RFC 4244 SIP Request History Information November 2005
/* The proxy forwards the INVITE to UA4 after adding the
additional History Information entry. */
F8 INVITE Proxy1 -> UA4
INVITE sip:UserC@example.com SIP/2.0
Via: SIP/2.0/UDP ims.example.com:5060;branch=3
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
History-Info: <sip:UserA@example.com>; index=1,
<sip:UserA@ims.example.com?Reason=SIP;\
cause=302; text="Moved Temporarily">;index=1.1,
<sip:UserB@example.com?Reason=SIP;cause=480;\
text="Temporarily Unavailable" >;index=1.2,
<sip:UserC@example.com>;index=1.3
CSeq: 1 INVITE
Contact: Alice <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=User1 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
F9 100 Trying Proxy1 ->UA1
SIP/2.0 100 Trying
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
F10 486 Busy Here UA4 -> Proxy1
SIP/2.0 486 Busy Here
Via: SIP/2.0/UDP ims.example.com:5060;branch=3
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
Barnes Standards Track [Page 32]RFC 4244 SIP Request History Information November 2005
CSeq: 1 INVITE
Content-Length: 0
F11 ACK Proxy1 -> UA4
ACK sip:UserC@example.com SIP/2.0
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 ACK
Content-Length: 0
/* The proxy forwards the 486 to Alice after adding the
associated History Information entries from the series of
INVITES */
F12 486 Busy Here Proxy1 -> UA1
SIP/2.0 486 Busy Here
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
History-Info: <sip:UserA@example.com>; index=1,
<sip:UserA@ims.example.com?Reason=SIP;\
cause=302; text="Moved Temporarily">;index=1.1,
<sip:UserB@example.com?Reason=SIP;cause=480;\
text="Temporarily Unavailable" >;index=1.2,
<sip:UserC@example.com>;index=1.3
CSeq: 1 INVITE
Content-Length: 0
F13 ACK Alice -> Proxy 1
ACK sip:UserA@example.com SIP/2.0
Via: SIP/2.0/UDP example.net:5060
From: Alice <sip:User1@example.net>
To: Bob <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 ACK
Content-Length: 0
Barnes Standards Track [Page 33]RFC 4244 SIP Request History Information November 2005
Appendix B. Voicemail
This scenario highlights an example where the History-Info in the
request is primarily of use by an edge service (e.g., voicemail
server). It should be noted that this isn't intended to be a
complete specification for this specific edge service as it is quite
likely that additional information is needed by the edge service.
History-Info is just one building block that this service makes use
of.
UA1 called UA A, which had been forwarded to UA B, which forwarded to
a UA VM (voicemail server). Based upon the retargeted URIs and
Reasons (and other information) in the INVITE, the VM server makes a
policy decision about what mailbox to use, which greeting to play,
etc.
UA1 Proxy UA-A UA-B UA-VM
| | | | |
|--INVITE F1-->| | | |
| | | | |
| |--INVITE F2-->| | |
|<--100 F3-----| | | |
| |<-302 F4------| | |
| | | | |
| |--------INVITE F5---------->| |
| | | | |
| |<--------180 F6-------------| |
|<---180 F7----| | | |
| . . . | | | |
| |------retransmit INVITE---->| |
| . . . | | | |
| | (timeout) | |
| | | | |
| |-------INVITE F8---------------------->|
| | | | |
| |<-200 F9-------------------------------|
| | | | |
|<-200 F10-----| | | |
| | | | |
|--ACK F11-------------------------------------------->|
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RFC 4244 SIP Request History Information November 2005
Message Details
INVITE F1 UA1->Proxy
INVITE sip:UserA@example.com SIP/2.0
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Contact: BigGuy <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=UserA 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
/*Client for UA1 prepares to receive data on port 49170
from the network. */
INVITE F2 Proxy->UA-A
INVITE sip:UserA@ims.example.com SIP/2.0
Via: SIP/2.0/UDPims.example.com:5060;branch=1
Via: SIP/2.0/UDP example.net:5060
Record-Route: <sip:UserA@example.com>
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
History-Info: <sip:UserA@ims.example.com>; index=1
Contact: BigGuy <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=UserA 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
100 Trying F3 Proxy->UA1
Barnes Standards Track [Page 35]RFC 4244 SIP Request History Information November 2005
SIP/2.0 100 Trying
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
302 Moved Temporarily F4 UserA->Proxy
SIP/2.0 302 Moved Temporarily
Via: SIP/2.0/UDP ims.example.com:5060;branch=1
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy<sip:UserA@example.com>;tag=3
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Contact: <sip:UserB@example.com>
Content-Length: 0
INVITE F5 Proxy-> UA-B
INVITE sip:UserB@example.com SIP/2.0
Via: SIP/2.0/UDP ims.example.com:5060;branch=2
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>
Call-Id: 12345600@example.net
History-Info: <sip:UserA@ims.example.com?Reason=SIP;\
cause=302; text="Moved Temporarily">; index=1,
<sip:UserB@example.com>;index=2
CSeq: 1 INVITE
Contact: BigGuy <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=User1 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
180 Ringing F6 UA-B ->Proxy
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
Barnes Standards Track [Page 36]RFC 4244 SIP Request History Information November 2005
To: LittleGuy <sip:UserA@example.com>;tag=5
Call-ID: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
180 Ringing F7 Proxy-> UA1
SIP/2.0 180 Ringing
SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Content-Length: 0
/* User B is not available. INVITE is sent multiple
times until it times out. */
/* The proxy forwards the INVITE to UA-VM after adding the
additional History Information entry. */
INVITE F8 Proxy-> UA-VM
INVITE sip:VM@example.com SIP/2.0
Via: SIP/2.0/UDP ims.example.com:5060;branch=3
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>
Call-Id: 12345600@example.net
History-Info:<sip:UserA@ims.example.com?Reason=SIP;\
cause=302; text="Moved Temporarily">;index=1,
<sip:UserB@example.com?Reason=SIP;cause=480;\
text="Temporarily Unavailable" >;index=2,
<sip:VM@example.com>;index=3
CSeq: 1 INVITE
Contact: BigGuy <sip:User1@example.net>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=User1 2890844526 2890844526 IN IP4 client.example.net
s=Session SDP
c=IN IP4 192.0.2.3
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
200 OK F9
Barnes Standards Track [Page 37]RFC 4244 SIP Request History Information November 2005
SIP/2.0 200 OK UA-VM->Proxy
Via: SIP/2.0/UDP ims.example.com:5060;branch=3
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>;tag=3
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Contact: TheVoiceMail <sip:VM@example.com>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=UserA 2890844527 2890844527 IN IP4 vm.example.com
s=Session SDP
c=IN IP4 192.0.2.4
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
200 OK F10 Proxy->UA1
SIP/2.0 200 OK
Via: SIP/2.0/UDP ims.example.com:5060;branch=3
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy <sip:UserA@example.com>;tag=3
Call-Id: 12345600@example.net
CSeq: 1 INVITE
Contact: TheVoiceMail <sip:VM@example.com>
Content-Type: application/sdp
Content-Length: <appropriate value>
v=0
o=UserA 2890844527 2890844527 IN IP4 vm.example.com
s=Session SDP
c=IN IP4 192.0.2.4
t=0 0
m=audio 3456 RTP/AVP 0
a=rtpmap:0 PCMU/8000
ACK F11 UA1-> UA-VM
ACK sip:VM@example.com SIP/2.0
Via: SIP/2.0/UDP example.net:5060
From: BigGuy <sip:User1@example.net>
To: LittleGuy<sip:UserA@example.com>;tag=3
Call-Id: 12345600@example.net
Barnes Standards Track [Page 38]
RFC 4244 SIP Request History Information November 2005
CSeq: 1 ACK
Content-Length: 0
/* RTP streams are established between UA1 and
UA-VM. UA-VM starts announcement for UA1 */
Appendix C. Automatic Call Distribution Example
This scenario highlights an example of an Automatic Call Distribution
service, where the agents are divided into groups based upon the type
of customers they handle. In this example, the Gold customers are
given higher priority than Silver customers, so a Gold call would get
serviced even if all the agents servicing the Gold group (ACDGRP1)
were busy, by retargeting the request to the Silver Group. Upon
receipt of the call at the agent assigned to handle the incoming
call, based upon the History-Info header in the message, the
application at the agent can provide an indication that this is a
Gold call, from how many groups it might have overflowed before
reaching the agent, etc. and thus can be handled appropriately by the
agent.
For scenarios whereby calls might overflow from the Silver to the
Gold, clearly the alternate group identification, internal routing,
or actual agent that handles the call SHOULD not be sent to UA1.
Thus, for this scenario, one would expect that the Proxy would not
support the sending of the History-Info in the response, even if
requested by the calling UA.
As with the other examples, this is not prescriptive of how one would
do this type of service but an example of a subset of processing that
might be associated with such a service. In addition, this example
is not addressing any aspects of Agent availability, which might also
be done via a SIP interface.
Barnes Standards Track [Page 39]
RFC 4244 SIP Request History Information November 2005
UA1 Proxy ACDGRP1 Svr ACDGRP2 Svr UA2-ACDGRP2
| | | | |
|--INVITE F1-->| | | |
Supported:histinfo
| | | | |
| |--INVITE F2-->| | |
Supported:histinfo
History-Info: <sip:Gold@example.com>; index=1
History-Info: <sip:ACDGRP1@example.com>; index=1.1
| | | | |
| |<-302 F3------| | |
Contact: <sip:ACDGRP2@ACD.com>
| | | | |
| |--------INVITE F4---------->| |
History-Info: <sip:Gold@example.com>; index=1
History-Info: <sip:ACDGRP1@example.com>; index=1.1
History-Info: <sip:ACDGRP2@example.com>; index=1.2
| | | | |
| | | | |
| | | |INVITE F5>|
History-Info: <sip:Gold@example.com>; index=1
History-Info: <sip:ACDGRP1@example.com>; index=1.1
History-Info: <sip:ACDGRP2@example.com>; index=1.2
| | | | |
| | | |<-200 F6--|
| | | | |
| |<-200 F7--------------------| |
History-Info: <sip:Gold@example.com>; index=1
History-Info: <sip:ACDGRP1@example.com>; index=1.1
History-Info: <sip:ACDGRP2@example.com>; index=1.2
|<-200 F8------| | | |
< No History-Info included in the response due to Local Policy>
| | | | |
|--ACK F9--------------------------------------------->|
Barnes Standards Track [Page 40]RFC 4244 SIP Request History Information November 2005
Appendix D. Session via Redirect and Proxy Servers
In this scenario, Alice places a call to Bob using first a Redirect
server then a Proxy Server. The INVITE message is first sent to the
Redirect Server. The Server returns a 302 Moved Temporarily response
(F2) containing a Contact header with Bob's current SIP address.
Alice then generates a new INVITE with Bob's current SIP address
included in another History-Info entry. The INVITE is then sent to
Bob via the Proxy Server, with Bob receiving the complete History
information; the call then proceeds normally. The complete call flow
for this scenario, without the use of History-Info, is described in
Section 3.6 of the SIP Basic Call Flow Examples [RFC3665].
Alice Redirect Server Proxy 3 Bob
| | | |
| INVITE F1 | | |
|--------------->| | |
| 302 F2 | | |
|<---------------| | |
| ACK F3 | | |
|--------------->| | |
| INVITE F4 | |
|-------------------------------->| INVITE F5 |
| 100 F6 |--------------->|
Message Details
F1 INVITE Alice -> Redirect Server
INVITE sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 1 INVITE
History-Info: <sip:bob@biloxi.example.com>; index=1
Contact: <sip:alice@client.atlanta.example.com>
Content-Length: 0
F2 302 Moved Temporarily Redirect Proxy -> Alice
SIP/2.0 302 Moved Temporarily
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
;received=192.0.2.1
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=53fHlqlQ2
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
Barnes Standards Track [Page 41]RFC 4244 SIP Request History Information November 2005
CSeq: 1 INVITE
History-Info: <sip:bob@biloxi.example.com>; index=1
Contact: <sip:bob@chicago.example.com;transport=tcp>
Content-Length: 0
F3 ACK Alice -> Redirect Server
ACK sip:bob@biloxi.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com:5060;branch=z9hG4bKbf9f44
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>;tag=53fHlqlQ2
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 1 ACK
Content-Length: 0
F4 INVITE Alice -> Proxy 3
INVITE sip:bob@chicago.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 2 INVITE
History-Info: <sip:bob@biloxi.example.com?Reason=SIP;cause=302>\
text="Moved Temporarily">; index=1,
<sip:bob@chicago.example.com>; index=2
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Content-Length: 0
F5 INVITE Proxy 3 -> Bob
INVITE sip:bob@client.chicago.example.com SIP/2.0
Via: SIP/2.0/TCP ss3.chicago.example.com:5060;branch=z9hG4bK721e.1
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.1
Max-Forwards: 69
Record-Route: <sip:ss3.chicago.example.com;lr>
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@biloxi.example.com>
Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com
CSeq: 2 INVITE
History-Info: <sip:bob@biloxi.example.com?Reason=SIP;cause=302>\
text="Moved Temporarily">; index=1,
<sip:bob@chicago.example.com>; index=2,
<sip:bob@client.chicago.example.com>; index=2.1
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Barnes Standards Track [Page 42]RFC 4244 SIP Request History Information November 2005
Content-Length: 0
Detailed Call Flow continues per section 6.3 in [RFC3665].
Editor's Address
Mary Barnes
Nortel
2201 Lakeside Blvd
Richardson, TX USA
Phone: 1-972-684-5432
EMail: mary.barnes@nortel.com
Barnes Standards Track [Page 43]
RFC 4244 SIP Request History Information November 2005
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Barnes Standards Track [Page 44]
