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This discusses problems associated with automatic call handling (ACH) when using the Session Initiation Protocol (SIP).
This work is being discussed on the bliss@ietf.org mailing list.
1.
Introduction
2.
Examples of ACH
3.
Known problem areas with ACH
3.1.
Conflict between proxy and UA
3.2.
Conflict between UAs
3.3.
Obtaining information from UA for ACH at proxy
3.4.
Informing the calling UA
3.5.
Scope of conditions
3.6.
Configuring the proxy
4.
Discussion
4.1.
Proxy versus UA
4.2.
Avoiding inconsistent configurations
4.3.
Enterprise and carrier environments
5.
Conclusions
6.
IANA considerations
7.
Security considerations
8.
Ackowledgements
9.
Informative References
§
Author's Address
§
Intellectual Property and Copyright Statements
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The Session Initiation Protocol (SIP) (RFC 3261 [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) establishes calls or sessions for real-time communication between users. When a call is targeted at a called user, often the call is subject to some automatic treatment to determine whether to present the call to the user or take some alternative action such as forwarding to voicemail. Similarly, if some condition arises after presenting a call to the called user but before answer, automatic treatment can lead to some alternative action. Automatic treatment is in accordance with policy determined in advance by the user or the user's organization. This automatic treatment of incoming calls is referred to as automatic call handling (ACH) in this document.
In order to encourage innovation, ACH is deliberately not specified in RFC 3261 or in RFCs that specify extensions to SIP. However, the flexibility that this affords has sometimes led to problems, where different implementations have approached the issue in different ways, leading to unexpected and often unwanted behavior when those implementations are deployed together. This document analyses the sources of problems with ACH.
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ACH can occur prior to or instead of presenting an incoming call to a called user or after presentation but before the called user answers the call. The particular treatment applied to a call is generally dependent on a number of factors, examples of which are as follows:
Examples of particular treatment to be applied to a call if appropriate conditions are met are as follows:
A user can specify quite complex sets of rules for ACH. For example, "if presence indicates I am in a meeting, or if my desk phone is busy, or if I do not reply within 15 seconds, forward calls to my assistant between the hours of 09.00 and 17.00, Monday to Friday, but at other times forward to my voicemail, unless the call is from my home or my partner's mobile phone, in which case forward to my mobile phone".
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A significant problem area with ACH is interactions between proxies (or B2BUAs) that perform ACH and UAs that perform ACH. The domain proxy for a user is configured to treat incoming calls in a certain way under certain conditions. One of the user's UAs is configured to treat incoming calls in a different way under the same or overlapping conditions. If the condition can be detected by the proxy without presenting the call to the UA, the proxy will win and the user may wonder why the action configured at the UA is not being taken. For example, if the proxy detects a DND condition from a presence server and forwards calls to voicemail, any script at the UA to forward private calls to a mobile phone would never execute. This may or may not be what the user (or his/her organization) desires to happen.
Alternatively, if a condition is detected by a UA before it is detected at the proxy, the action determined by the UA will "win", unless the proxy is somehow able to figure out what has happened and apply its own action. For example, if a phone determines it is busy and returns a 302 response code to forward the call elsewhere or performs "call waiting" action, this might prevent the proxy taking whatever action it would have taken on receipt of a 486 response. This may or may not be what the user (or his/her organization) desires to happen.
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Where an incoming call is forked to multiple UAs, there is potential for different UAs to be configured to perform different actions under the same or overlapping conditions. With parallel forking (where the INVITE request is sent to each UA at approximately the same time), results can be indeterminate and are likely to depend on which UA responds first. With serial forking, this is likely to be more deterministic, but UAs would need to be configured taking into account the order in which the proxy presents calls to the UAs.
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When ACH is performed at a proxy, it sometimes requires information from the UA, in response to the INVITE request. If this information does not arrive in the form expected by the proxy (e.g, a particular response code), ACH will be adversely impacted. For example, if the proxy is configured to perform forwarding on DND and relies on the DND condition to be indicated in the INVITE response, it depends on the UA indicating the condition in the form expected by the proxy. As there is no standardized means of indicating DND in a response (see [draft‑dnd] (Elwell, J. and M. Vakil, “SIP: Session Initiation Protocol,” November 2007.)), this can be a problem.
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A related problem is informing the calling UA, and hence the caller, what has happened. In the case where ACH results in rejection of the call, this might be just a case of sending back an appropriate response code. Considerations are similar to those for a proxy in Section 3.3 (Obtaining information from UA for ACH at proxy), except that privacy might require the proxy to send a different response code rather than the one reflecting the condition encountered. For example, the user might not wish the caller to know about his absence.
The choice of response code might not be an interoperability issue if the calling UA is relatively dumb, but might be an issue if there is an application that takes the response code into account.
Where ACH results in forwarding (to a different AoR or a different contact for the same AoR), this can be achieved by retargeting or redirection. In the case of retargeting, the calling UA receives no information, apart from a final response and perhaps identity from the retargeted-to user. On the other hand, if redirection is used, the calling UA will receive a 3xx response, the contact URI in which could indicate the source of the redirection and the reason, in accordance with RFC 4458 [RFC4458] (Jennings, C., Audet, F., and J. Elwell, “Session Initiation Protocol (SIP) URIs for Applications such as Voicemail and Interactive Voice Response (IVR),” April 2006.).
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When an INVITE request is forked to multiple UAs, the user may or may not require a condition at one UA to be considered as applying to other branches. This includes branches already active (through parallel forking) or branches yet to be activated (through serial forking). This can impact when to invoke ACH at the proxy, i.e., whether to perform ACH when one UA reports an appropriate condition (cancelling other active branches if necessary) or to wait for the outcome on other branches.
Although to a large extent this issue can be handled by appropriate scripting at the proxy, an important consideration is how to treat the 6xx class of responses. For example, if a UA issues a 600 Busy Everywhere response (as opposed to a 486 Busy response), what is the scope of "everywhere"? A simple interpretation is that it literally means "everywhere", and all other branches should be abandoned and the 6xx response passed back to the caller if no other ACH is prescribed for this condition. However, this interpretation is not always reasonable. If a user has several phones, it might be reasonable to interpret a 600 response from one phone as meaning that all other phones are busy, but if the user also has voicemail it is unlikely that that too should be treated as busy. Also, if ACH requires forwarding to a different user (different AoR) on busy, it might be expected that this would take place even on receipt of a 600 response from a UA.
Another example is the 603 Decline response code. This is often is intended to be applied everywhere.
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If ACH is performed at the proxy, the user needs a means to configure the proxy with the required rules. There is no SIP means of doing this, but a number of mechanisms can perform the basis for this task, e.g.:
Without a single standardized way of configuring a proxy, there is a danger that the UA and proxy might not support a common method, requiring the user to employ other means (e.g., using a different device, contacting a support centre). Furthermore, it might lead the user to configuring ACH at the UA when in practice ACH at the proxy would serve the user's needs better.
Related to this is the means by which a UA (and hence the user) can discover how the proxy is configured.
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The end-to-end principle of SIP would suggest that ACH at the UA is more appropriate than ACH at the proxy. However, certain considerations make ACH at the proxy more viable or even essential.
ACH in the event that there is no registered contact obviously can only be performed by the proxy.
A proxy is more easily able to take account of the state of other UAs, e.g., by waiting for all branches of a forked call to respond before invoking ACH. Although a UA can use techniques such as the registration event package (RFC 3680 [RFC3680] (Rosenberg, J., “A Session Initiation Protocol (SIP) Event Package for Registrations,” March 2004.)) in combination with the dialog event package (RFC 4235 [RFC4235] (Rosenberg, J., Schulzrinne, H., and R. Mahy, “An INVITE-Initiated Dialog Event Package for the Session Initiation Protocol (SIP),” November 2005.)) to determine the state of other UAs, this is complex, may not yield the information required, and may suffer from timing-related inconsistencies.
A proxy needs to be configured once and can perform ACH independently of the number of UAs involved. Obtaining consistent behaviour using ACH at the UA may involve configuring multiple UAs and keeping their configurations aligned. The UA configuration framework [draft‑config] (Petrie, D. and S. Channabasappa, “A Framework for Session Initiation Protocol User Agent Profile Delivery,” November 2007.) may be a suitable mechanism for this and would require a means for the user to configure the profile delivery server. However, there can be no guarantee that all UAs will download a revised configuration at the same time, so it can lead to a time window when inconsistent behaviour may occur.
With these considerations in mind, a proxy will often turn out to be a more suitable place for performing ACH.
On the other hand, there may be situations in which UA-specific ACH may be required, and it may not be feasible to configure the proxy to provide this level of granularity. For example, it may be required to take one action if the desk UA is busy but a different action if the mobile UA is busy. Convincing use cases for this are hard to find, but it cannot be ruled out. A possible approach here is to use proxy-based ACH as the default handling for all UAs and UA-based ACH for any UA-specific exceptions.
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Given that there is frequently a need to perform ACH at the proxy, problems can be avoided by turning off ACH at all UAs. There may be exceptions to this, e.g., where there is need for a specific UA to perform actions different from default actions carried out by the proxy, or where there is a requirement for behavior not supported by the proxy. Where ACH does need to be configured at one or more UAs, care must be taken to avoid unintentional conflicts. Use of the SIP configuration framework can help to ensure consistent handling at all UAs. One consideration during the work on profiles for use with the SIP configuration framework might be the downloading of policy relating to ACH, such that ACH could be suppressed in order to ensure that proxy-based ACH operates correctly.
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Considerations for ACH will often differ between enterprise and carrier environments. In enterprise environments, enterprise policy will often govern what a user can and cannot do. This does not necessarily mean that ACH will be done at a proxy, because the enterprise will probably manage UAs too and ensure that they behave in line with policy, although proxy-based ACH will often be easier to accomplish.
In a carrier environment, everything can be expected to be under the control of the user. Proxy-based ACH is still relevant, however, particularly for mobile devices that are often out of reach or turned off.
Handling such as team calls (where any team member can be selected according to availability) is perhaps more likely in enterprise, although in a residential environment it could be used for finding any family member.
Despite these different considerations, requirements are similar to a large extent and the same solution should be sought for both environments.
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The discussions above raise a number of questions, which need to be addressed by the BLISS WG:
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None.
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This document just discusses issues relating to DND. It does not define any new protocol or practices and therefore does not introduce any security issues, other than the possible user desire not to disclose a DND condition to callers.
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The author would like to acknowledge the assistance of Jason Fischl, Shida Schubert and Srivatsa Srinivasan in writing this draft, and also input on specific implementations from various members of the BLISS WG.
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| [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. |
| [RFC3680] | Rosenberg, J., “A Session Initiation Protocol (SIP) Event Package for Registrations,” RFC 3680, March 2004. |
| [RFC3880] | Lennox, J., Wu, X., and H. Schulzrinne, “Call Processing Language (CPL): A Language for User Control of Internet Telephony Services,” RFC 3880, October 2004. |
| [RFC4235] | Rosenberg, J., Schulzrinne, H., and R. Mahy, “An INVITE-Initiated Dialog Event Package for the Session Initiation Protocol (SIP),” RFC 4235, November 2005. |
| [RFC4458] | Jennings, C., Audet, F., and J. Elwell, “Session Initiation Protocol (SIP) URIs for Applications such as Voicemail and Interactive Voice Response (IVR),” RFC 4458, April 2006. |
| [draft-dnd] | Elwell, J. and M. Vakil, “SIP: Session Initiation Protocol,” draft-elwell-bliss-dnd-01 (work in progress), November 2007 (TXT). |
| [draft-config] | Petrie, D. and S. Channabasappa, “A Framework for Session Initiation Protocol User Agent Profile Delivery,” draft-ietf-sipping-config-framework-14 (work in progress), November 2007 (TXT). |
| [CSTA] | “International Standard ISO/IEC 18051 "Information Technology - Telecommunications and information exchange between systems - Services for Computer Supported Telecommunications Applications (CSTA) Phase III".” |
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| John Elwell | |
| Siemens Enterprise Communications GmbH & Co KG | |
| Hofmannstrasse 51 | |
| D-81379 Munich | |
| Germany | |
| Phone: | +44 115 943 4989 |
| Email: | john.elwell@siemens.com |
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