BLISS WG                                                       J. Elwell
Internet-Draft                         Siemens Enterprise Communications
Intended status:  Informational                         November 3, 2008
Expires:  May 7, 2009


  An Analysis of Automatic Call Handling Implementation Issues in the
                   Session Initiation Protocol (SIP)
                  draft-ietf-bliss-ach-analysis-03.txt

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Abstract

   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.










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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Examples of ACH  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Known problem areas with ACH . . . . . . . . . . . . . . . . .  5
     3.1.  Conflict between proxy and UA  . . . . . . . . . . . . . .  5
     3.2.  Conflict between UAs . . . . . . . . . . . . . . . . . . .  6
     3.3.  Obtaining information from UA for ACH at proxy . . . . . .  6
     3.4.  Informing the calling UA . . . . . . . . . . . . . . . . .  6
     3.5.  Scope of conditions  . . . . . . . . . . . . . . . . . . .  7
     3.6.  Configuring the proxy  . . . . . . . . . . . . . . . . . .  8
   4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Proxy versus UA  . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Avoiding inconsistent configurations . . . . . . . . . . . 10
     4.3.  Enterprise and carrier environments  . . . . . . . . . . . 10
   5.  Potential measures that could be taken . . . . . . . . . . . . 10
     5.1.  Conflict between proxy and UA  . . . . . . . . . . . . . . 11
     5.2.  Conflict between UAs . . . . . . . . . . . . . . . . . . . 12
     5.3.  Obtaining information from UA for ACH at proxy . . . . . . 13
       5.3.1.  Reason for rejection . . . . . . . . . . . . . . . . . 13
       5.3.2.  Desired scope of rejection . . . . . . . . . . . . . . 14
       5.3.3.  Response codes . . . . . . . . . . . . . . . . . . . . 14
     5.4.  Informing the calling UA . . . . . . . . . . . . . . . . . 15
     5.5.  Scope of conditions  . . . . . . . . . . . . . . . . . . . 15
     5.6.  Configuring the proxy  . . . . . . . . . . . . . . . . . . 16
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 18
   8.  Security considerations  . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   10. Informative References . . . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20



















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1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] 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.

   A number of other Standards Development Organisations (SDO) (e.g.,
   3GPP, ETSI) have specified specific features or "services" that
   involve specific forms of ACH.

   A survey was conducted prior to IETF70 to get a feeling for what are
   common practices in this area.  Although the number of responses was
   disappointingly small (see results [1]), in some cases it did give a
   clue as to the most common practices.  In the remainder of this
   document, this is referred to as "the survey".


2.  Examples of ACH

   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:

   o  Whether there are other registered contacts that can handle the
      call (e.g., a registered audio UA for an audio call).

   o  Whether the user's UA (or UAs) is known to be busy on another
      call.

   o  Whether the user has failed to answer the call within a given
      number of seconds.



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   o  Whether the user is known to be unwilling to receive calls at the
      present time (a condition often known as Do Not Disturb, DND).

   o  Whether the user is known not to be available (e.g., on vacation).

   o  Whether an alternative user (e.g., a colleague, an assistant,
      another family member) is known to be available.

   o  Whether the AoR at which the call is targeted represents a single
      user or a team or group of users.

   o  Time of day, day of the week, date, etc..

   o  The type of call (e.g., audio, audio plus video, messaging, etc.).

   o  The source of the call (e.g., whether the caller is anonymous,
      whether the caller is blacklisted or whitelisted, which
      organization the caller belongs to, etc.).

   The conditions above are detected though local information at the
   entity performing ACH, by access to presence information or through
   information received via SIP signalling (e.g., a UA's response to an
   INVITE request).

   Examples of particular treatment to be applied to a call if
   appropriate conditions are met are as follows:

   o  Reject the call, with an appropriate indication to the caller.
      This indication may or may not reveal the actual condition that
      led to rejection.

   o  Forward the call to another UA serving that user (e.g., voicemail,
      a mobile UA, a UA at another location).

   o  Forward the call to another user, e.g., the next member of a team,
      an assistant.

   o  Modify the nature of the call (e.g., downgrade from audio to
      messaging).

   o  Any of the above, but impacting presentation of the call at a
      given UA, without impacting presentation at other UAs serving the
      user.

   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



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   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".


3.  Known problem areas with ACH

3.1.  Conflict between proxy and UA

   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.

   If a UA attempts ACH, it may be possible for proxy to override it,
   e.g., by taking account of the response code returned or, in the case
   of a 3xx response code, whether the UA has provided further
   information concerning the reason for redirection in accordance with
   RFC 4458 [RFC4458].

   The survey showed that the majority of proxies perform ACH without
   first presenting the call to the UA, at least for certain types of
   ACH.

   The survey also showed that the majority of UAs implement some form
   of UA.  This does seem to confirm the potential for conflict between
   proxy and UA.  If, as a result of ACH, the call required redirection,
   302 was the response code used in the majority of situations.  Only a
   minority of such implementations used RFC 4458 to provide more
   information about the reason for redirection.



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3.2.  Conflict between UAs

   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 might 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.

   When a proxy forks a call, it can invoke ACH based on the first UA to
   respond, can wait for all UAs to respond or behave in some other way
   (e.g., act immediately on certain response codes).  The survey did
   not show a bias towards any one behavior.

3.3.  Obtaining information from UA for ACH at proxy

   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 [I-D.elwell-bliss-dnd]), this can be a problem.  Furthermore,
   there might be more than one flavour of DND (e.g., with/without
   forwarding to voicemail), requiring different responses.

   The survey showed that 486 (Busy Here) is the response code most
   commonly expected by proxies for indicating DND, but that accounted
   for less than half of the responses.  Other known response codes in
   use include 406 (Not Accepetable), 480 (Temporarily Unavailable), 600
   (Busy Everywhere) and 603 (Decline).

   The survey also showed that even for the busy condition, proxies
   expected different response codes.  Although a small majority
   expected 486 (Busy Here), other expectations included 480
   (Temporarily Unavailable) and 600 (Busy Everywhere).

   The survey did not yield significant information concerning response
   codes issued by UAs.

3.4.  Informing the calling UA

   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



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   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, 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
   there is forking proxy between the entity performing ACH and the
   calling UA, information may be lost because of the Heterogeneous
   Error Response Forking Problem (HERFP).

   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 [RFC4458].

3.5.  Scope of conditions

   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



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   response from a UA.

   Another example is the 603 Decline response code.  This is often is
   intended to be applied everywhere.

   There is also a question of whether a proxy should trust a UA to
   decide that all other branches need to be abandoned, particularly in
   applications like call centres, where the different branches might be
   different agents, rather than leading to different devices belonging
   to the same user.  It might wise to consider this a policy matter.

   The survey gave only a very small number of answers on the issue of
   handling 6xx responses, with no conclusions to be drawn other than
   that forwarding to voice mail is sometimes allowed following a 6xx
   response.

3.6.  Configuring the proxy

   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.:

   o  Via a web page.

   o  By uploading a CPL [RFC3880] script.

   o  Via a web services interface based on SOAP.

   o  Via Computer Supported Telecommunication Applications (CSTA)
      [CSTA].

   o  Via XCAP [RFC4825]

   o  Using standard HTTP primitives, a technique commonly known as
      REpresentational State Transfer (REST).

   The survey showed that web pages and SOAP-based web services were the
   most common mechanisms supported by proxies, but the sample was very
   small.  The majority of UA implementations provided a web user
   interface.

   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.



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   Related to this is the means by which a UA (and hence the user) can
   discover how the proxy is configured.  Most of the mechanisms listed
   above are applicable, and also a SIP SUBSCRIBE/NOTIFY mechanism could
   be used.  The survey indicated that only a minority of proxies
   provided support in this respect.


4.  Discussion

4.1.  Proxy versus UA

   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 [RFC3680] in combination with the dialog
   event package [RFC4235] 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
   [I-D.ietf-sipping-config-framework] 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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4.2.  Avoiding inconsistent configurations

   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.

4.3.  Enterprise and carrier environments

   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.


5.  Potential measures that could be taken

   In this section we explore potential measures that can be taken to
   some of the problems identified above







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5.1.  Conflict between proxy and UA

   This appears to be an important problem to solve, in order to have
   proxies and UAs from mixed vendors.

   One approach is to specify that particular features that must or must
   not be implemented in a proxy and particular features that must or
   must not be implemented in UA.  This is likely to fail for a number
   of reasons:

   o  There are far too many possible features, and enumerating and
      standardizing individual features is contrary to the philosophy of
      SIP and likely to inhibit innovation.

   o  For a given feature, there will be some deployments where it makes
      sense to do it at the proxy and other deployments where it makes
      sense to do it at the UA.  It will often be impracticable to
      choose one.

   o  Proxy vendors and UA vendors will want to provide as many features
      as possible on their products and are likely to ignore any
      recommendation not to implement a particular feature.

   Stipulating that ACH as a whole must always be done at the proxy or
   must always be done at the UA is clearly out of the question, because
   each has some advantages, depending on circumstances, and also
   because vendors of one or the other will not be prepared to give up
   producing features that play an important part in differentiating
   their products.

   Therefore it has to be accepted that ACH will be implemented on
   proxies and UAs, with feature overlap between the two.  The challenge
   then is to ensure that, when deployed, the two can co-exist in a
   sensible way.

   It should be possible to control whether a proxy defers to a UA or
   vice versa.  For a proxy to defer to a UA, it requires the proxy to
   deliver an INVITE request to a UA before taking any ACH action.
   Depending on the response of the UA, the proxy may then perform its
   own ACH action.  For a UA to defer to a proxy, it should report any
   conditions back to the proxy (e.g., by means of a suitable response
   to the INVITE request) rather than taking unilateral action such as
   redirecting or placing the call in a waiting state.  In other words,
   it should be possible to turn off ACH at a UA.

   There are several ways to achieve this control:





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   o  Configure the UA and proxy independently.  The SIP configuration
      framework [I-D.ietf-sipping-config-framework] is one possible
      means of configuring the UA.

   o  Configure the UA (e.g., by means of the SIP configuration
      framework [I-D.ietf-sipping-config-framework]) and use SIP to
      instruct the proxy (e.g., by means of an indicator in REGISTER
      requests).

   o  Configure the proxy and use SIP to instruct the UA (e.g., by means
      of an indicator in inbound INVITE requests or in the 200 response
      to a REGISTER request).

   The configuration approach is the simplest and does not require any
   enhancements to the SIP protocol.  In general, positive action has to
   be taken to configure any sort of ACH, i.e., ACH is turned off by
   default.  Therefore by default there should not be a problem, because
   ACH will be turned off in both places.  If the user is in control of
   ACH at the UA and ACH at the proxy, it is the user's responsibility
   not to configure conflicting behaviours.

   The situation is slightly different where there is more than one
   authority involved, e.g., if the user is able to configure the UA but
   some other authority is responsible for configuring the proxy.  This
   might arise in an enterprise environment, where the enterprise
   administration might configure the proxy.  In this case, the UA user
   could potentially configure the UA in a conflicting way.  In such
   cases it would be useful if the administration could prevent the user
   configuring ACH at the UA, i.e., place ACH configuration under
   administration control.  Many UAs aimed at the enterprise market have
   this form of control already.  In future there might be a
   standardised way of doing this based on the SIP configuration
   framework [I-D.ietf-sipping-config-framework] .  There does not seem
   to be a need for BLISS to specify anything further to address this
   issue at present.

5.2.  Conflict between UAs

   This can really only be addressed by configuration.  The SIP
   configuration framework can help here.  In fact, that would normally
   configure all UAs having the same AoR with the same information.
   Configuration outside this framework (e.g., local actions at the
   device) might introduce differences (intentional or otherwise).
   There seems little action that BLISS can take to address this issue.







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5.3.  Obtaining information from UA for ACH at proxy

   There seems to be a case for more precisely defining or at least
   recommending information given to the proxy when rejecting an inbound
   call, in order to assist the proxy in providing the most relevant
   ACH, if any.  Ideally this information needs to be given as a SIP
   response code, although potentially a response code could be
   supplemented by a header field.  In choosing a particular response
   code, two factors need to be taken into account:

   o  the reason for rejection;

   o  the desired scope of rejection.

5.3.1.  Reason for rejection

   The most relevant reasons for rejection (from an ACH perspective) are
   as follows:

   o  Busy.  UA resources are busy as a result of another call and
      further calls cannot be accepted until resources become free.  The
      condition may also be visible via a presence system.

   o  Explicit rejection.  The user has indicated an unwillingness to
      accept the call at the present time.  The user may have indicated
      this in advance, so that any incoming calls (or those fulfilling
      certain conditions) would be rejected in this way (a feature often
      known as "do not disturb").  The user may impose such a condition
      during a meeting or while working on a critical task.  The user
      may indicate in advance a time at which she expects the condition
      to be lifted.  The condition may also be visible via a presence
      system.  Alternatively the user may indicate an unwillingness to
      accept a particular call in response to being alerted.

   o  Silent rejection.  The user has responded to this call by
      rejecting it, but does not wish the reason to be revealed to the
      caller.  A user would use this facility when not currently in a
      position to answer a particular call or when she feels that it
      would be better handled elsewhere (e.g., by voicemail, by an
      assistant).

   Note that silent rejection could also be achieved by returning a 180
   response to the INVITE request, and then waiting (without alerting
   the user) until the call is cleared or times out.  In the meantime,
   the proxy would be unaware of what is happening and would be unable
   to take other action, such as cancelling other branches.  On the
   other hand, indicating silent rejection relies on the proxy to take
   alternative ACH action (e.g., waiting for a certain time before



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   forwarding to voice mail or reporting to the caller that the call has
   timed out), rather than revealing silent rejection to the caller.
   Therefore silent rejection is suitable for use only when it is known
   that the proxy will take appropriate action.

5.3.2.  Desired scope of rejection

   When rejecting a call the user (or the UA on the user's behalf) may
   desire the rejection to have one of the following scopes:

   o  Local.  Rejection impacts only the branch concerned or any
      branches to the user's other devices.  Forwarding to voicemail or
      to an assistant, for example, is not prevented and may be
      instigated by the proxy as a result of receiving a local
      rejection.

   o  Global.  Rejection impacts all branches, including voicemail.

   It is a matter for the proxy to determine what ACH to perform for a
   local rejection or a global rejection (although this may be based on
   per-user settings, which the user may have some control over, e.g.,
   via a web page, see Section 5.6).  For local rejection the proxy
   might, for example, cancel other branches (to the user's other
   devices) and forward immediately to voicemail or to an assistant.  On
   the other hand it may leave other branches unaffected.  For global
   rejection the proxy might reject the call outright, cancelling all
   other branches, although policy might require a less severe action to
   be taken.  For example, in a call centre there might be a requirement
   that all calls be answered.

5.3.3.  Response codes

   4xx response codes are appropriate for local rejection and 6xx
   response codes are appropriate for global rejection.

   The following response codes are recommended for local rejection:

   o  Busy/local. 486 should be used.  As indicated in RFC 3261, this
      can be accompanied by a Retry-After header field to indicate when
      the user expects to be available again.

   o  Explicit rejection/local. 480 could be used.  As indicated in RFC
      3261, this can be accompanied by a Retry-After header field to
      indicate when the user expects to be available again.  OPEN ISSUE.
      There is some concern about the overloaded use of this response
      code.  For example, if a domain proxy receives a 480, is it from
      the UAS (indicating DND) or is it from an edge proxy (indicating
      temporary lack of resources.



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   o  Silent rejection/local. 487 could be used.  This is the same
      response code that would be used if a proxy were to issue a CANCEL
      request.

   The following response codes are recommended for global rejection:

   o  Busy/global. 600 should be used.  As indicated in RFC 3261, this
      can be accompanied by a Retry-After header field to indicate when
      the user expects to be available again.

   o  Explicit rejection/global. 603 should be used.  As indicated in
      RFC 3261, this can be accompanied by a Retry-After header field to
      indicate when the user expects to be available again.

   o  Silent rejection/global.  No code recommended.

   No code is recommended at present for silent rejection/global.  It is
   not clear what is the use case for this.  One possible use case,
   however, is for dealing with SPIT, where the user can tell (e.g.,
   from caller ID) that the call is unwanted and does not wish the call
   to go to voicemail even.  In this case, other issues arise, such as
   indicating the SPIT status to an entity responsible for handling
   SPIT.  This should be pursued as part of anti-SPIT measures and is
   outside the scope of BLISS.

   OPEN ISSUE.  Do we need to write normative statements regarding
   response code usage?

5.4.  Informing the calling UA

   Whilst this might be interesting, it is unlikely to impact
   interoperability and is not seen as a priority issue for BLISS.

5.5.  Scope of conditions

   There seems to be a case for specifying the intended scope of 6xx
   response codes.  Ideally this should be a single specification for
   the entire class, although we may find that individual members of the
   class require special consideration.  Probably a 6xx response code
   should apply only to other contacts registered for that same AoR, and
   should probably also exclude special contacts such as voice mail
   systems.  At present a [RFC3840] defines a feature tag to indicate
   that a UA is an automaton, and another to indicate that it supports
   audio.  Consideration should be given as to whether these are
   sufficient to determine that a UA is a voicemail server.

   In view of existing practice, where 6xx response codes are generally
   meant to apply to all branches and to override forwarding, it is



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   seems safest to stick with this meaning.  If this is not the desired
   behaviour when a particular condition arises at the UA (e.g., if
   forwarding to voicemail is desired), a 4xx response code should be
   used instead.  The proposals in Section 5.3 take this into account.

5.6.  Configuring the proxy

   General methods for configuring proxies (including synchronization of
   multiple proxies serving a domain) are considered outside the scope
   of BLISS work.

   There may be justification for studying methods by which a user or UA
   can discover and possibly change the configuration of the proxy as
   far as ACH is concerned.  Often a user can do this by means such as
   web pages.  Such methods are not suitable for use when the UA needs
   to play a functional role.  For example, many UAs have a button that
   can be pressed to set forwarding and an associated indicator to show
   whether forwarding is currently active.  Such UAs cannot easily
   extract information from a web page to determine whether forwarding
   is active in order to adjust the indicator.  Similarly, such UAs
   cannot know what to send to the web server to change settings.

   Although several methods are used in practice, some of these being
   mentioned in Section 3.6, the one that is perhaps simplest and in
   line with industry trends is HTTP using a REST architecture.  With
   this approach, different URIs represent different resources, and
   standard HTTP methods such as GET, PUT, POST and DELETE are used to
   manipulate those resources.  More information is available in
   [I-D.griffin-bliss-rest]

   An example of a request to set forwarding would be:

      PUT
      http://server.example.com/joe/profile/callforwarding?value=true

   The resource is the call forwarding setting within the profile for
   user joe at server.example.com.  The corresponding request to
   retrieve the setting would be:

      GET http://server.example.com/joe/profile/callforwarding

   The ability for the UA to be able to create and delete this resource
   probably does not make sense, so POST and DELETE methods would not be
   used.

   OPEN ISSUE.  What exactly do we need to standardize here?  Clearly
   some elements of the resource identifier need to be configurable.
   The host name "server.example.com" probably needs to be configurable,



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   and element "joe" or "joe/profile" could either be configurable
   (e.g., by making "server.example.com/joe/profile" configurable") or
   by standardizing a method of deriving it from the user part of the
   AoR.  The final element "callforwarding" (and equivalent for other
   ACH features) probably needs to be standardised.

   OPEN ISSUE.  The examples above show managing the true/false value of
   call forwarding.  This does not provide a means of managing the URI
   to forward to (it assumes that this is already set).  Also it does
   not provide a means of managing conditions under which forwarding is
   to take place (e.g., based on state of user, time of day, caller ID).
   These are all different resources, requiring different or additional
   elements at the end of the identifier.  How far should we go in
   standardizing these resources?  Presumably there should be an
   extensibility mechanism to allow vendors to define their own
   elements.

   Under some circumstances a setting can change other than by action of
   the UA, and therefore if the UA relies on knowing the value of a
   setting (e.g., to provide a continuous indication to the user), the
   UA needs to be informed if the setting changes.  One example is where
   there are two or more UAs registered as contacts for an AoR.  If one
   of the UAs changes a setting, the other UAs might wish to know.
   Another example is where a user or administrator changes a setting
   from a web page.  For such situations it would be useful to have an
   event package whereby the UA can subscribe to receive notifications
   of changes to ACH settings.  The notification would only need to
   indicate that a change has occurred, and the UA could then issue GET
   requests to pull down the new settings.

   OPEN ISSUE.  Is an event package for this needed, or are there
   alternative solutions?  Is there any existing event package that
   could be re-used, e.g., the configuration framework?

   OPEN ISSUE.  Standardising a RESTful or other solution to configuring
   the proxy, along with some support for notifications, requires
   further work, starting with identification of requirements, an
   initial attempt at which is given in
   [I-D.zourzouvillys-bliss-ach-config-requirements]


6.  Conclusions

   TODO:  populate this section







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7.  IANA considerations

   None.


8.  Security considerations

   This document just discusses interoperability issues relating to ACH.
   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 ACH actions to callers.


9.  Acknowledgements

   The author would like to acknowledge the assistance of Francois
   Audet, Martin Dolly, Jason Fischl, Jonathan Rosenberg, Shida
   Schubert, Srivatsa Srinivasan and Theo Zourzouvillys in writing this
   draft, and also input on specific implementations from various
   members of the BLISS WG.


10.  Informative 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.

   [RFC3680]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
              Package for Registrations", RFC 3680, March 2004.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 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.



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   [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
              Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

   [I-D.elwell-bliss-dnd]
              Elwell, J. and S. Srinivasan, "An Analysis of Do Not
              Disturb (DND) Implementations in the Session  Initiation
              Protocol (SIP)", draft-elwell-bliss-dnd-01 (work in
              progress), November 2007.

   [I-D.ietf-sipping-config-framework]
              Channabasappa, S., "A Framework for Session Initiation
              Protocol User Agent Profile Delivery",
              draft-ietf-sipping-config-framework-15 (work in progress),
              February 2008.

   [I-D.griffin-bliss-rest]
              Griffin, K. and J. Rosenberg, "Representational State
              Transfer (REST) for Feature Configuration in Session
              Initiation Protocol (SIP)", draft-griffin-bliss-rest-00
              (work in progress), October 2008.

   [I-D.zourzouvillys-bliss-ach-config-requirements]
              Zourzouvillys, T., "Automatic Call Handling (ACH)
              Configuration Requirements",
              draft-zourzouvillys-bliss-ach-config-requirements-00 (work
              in progress), October 2008.

   [CSTA]     "International Standard ISO/IEC 18051 "Information
              Technology - Telecommunications and information exchange
              between systems - Services for Computer Supported
              Telecommunications Applications (CSTA) Phase III"".

   [1]  <http://www.bliss-ietf.org/ach_survey.html>


Author's Address

   John Elwell
   Siemens Enterprise Communications

   Phone:  +44 115 943 4989
   Email:  john.elwell@siemens.com









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