This document is obsolete. Please
refer to RFC 7044.
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
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.
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.
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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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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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].
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).
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.
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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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.
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.
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]:
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.
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.
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.
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.
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
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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.
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
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has added, any hi-entry with a priv-value of "history" SHOULD be removed prior to forwarding.
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.
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
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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.
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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".
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.
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:
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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.
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.
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/* 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;\
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/* 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
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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
/* 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 --------------------------------------------------->|
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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.
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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.
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.
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7.2. Registration of "history" for SIP Privacy Header
This document defines a new priv-value for the SIP Privacy header: history
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
[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.
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,
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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.
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
Mark Watson Digital Fountain 39141 Civic Center Drive Suite 300 Fremont, CA 94538 U.S.A.
EMail: mark@digitalfountain.com
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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 4.2">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.
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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
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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
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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.
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.
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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].
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
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RFC 4244 SIP Request History Information November 2005
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