





Network Working Group                                     D. Willis, Ed.
Request for Comments: 5373                             Softarmor Systems
Category: Standards Track                                       A. Allen
                                                Research in Motion (RIM)
                                                           November 2008


  Requesting Answering Modes for the Session Initiation Protocol (SIP)

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" (STD1) for the standardization state and
   status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2008 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document extends SIP with two header fields and associated
   option tags that can be used in INVITE requests to convey the
   requester's preference for user-interface handling related to
   answering of that request.  The first header, "Answer-Mode",
   expresses a preference as to whether the target node's user interface
   waits for user input before accepting the request or, instead,
   accepts the request without waiting on user input.  The second
   header, "Priv-Answer-Mode", is similar to the first, except that it
   requests administrative-level access and has consequent additional
   authentication and authorization requirements.  These behaviors have
   applicability to applications such as push-to-talk and to diagnostics
   like loop-back.  Usage of each header field in a response to indicate
   how the request was handled is also defined.








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Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Syntax of Header Fields and Option Tags  . . . . . . . . . . .  5
   3.  Usage of the Answer-Mode and Priv-Answer-Mode Header Fields  .  6
   4.  Usage of the Answer-Mode and Priv-Answer-Mode Header
       Fields in Requests . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  The Difference Between Answer-Mode and Priv-Answer-Mode  .  7
     4.2.  The "require" Modifier . . . . . . . . . . . . . . . . . .  9
     4.3.  Procedures at User Agent Clients (UAC) . . . . . . . . . .  9
       4.3.1.  All Requests . . . . . . . . . . . . . . . . . . . . .  9
       4.3.2.  REGISTER Transactions  . . . . . . . . . . . . . . . .  9
       4.3.3.  INVITE Transactions  . . . . . . . . . . . . . . . . . 10
     4.4.  Procedures at Intermediate Proxies . . . . . . . . . . . . 12
       4.4.1.  General Proxy Behavior . . . . . . . . . . . . . . . . 12
       4.4.2.  Issues with Automatic Answering and Forking  . . . . . 12
     4.5.  Procedures at User Agent Servers (UAS) . . . . . . . . . . 13
       4.5.1.  INVITE Transactions  . . . . . . . . . . . . . . . . . 13
   5.  Usage of the Answer-Mode and Priv-Answer-Mode Header
       Fields in Responses  . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Procedures at the UAS  . . . . . . . . . . . . . . . . . . 14
     5.2.  Procedures at the UAC  . . . . . . . . . . . . . . . . . . 15
   6.  Examples of Usage  . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  REGISTER Request . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  INVITE Request . . . . . . . . . . . . . . . . . . . . . . 16
     6.3.  200 (OK) Response  . . . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     7.1.  Attack Sensitivity Depends on Media Characteristics  . . . 17
     7.2.  Application Design Affects Attack Opportunity  . . . . . . 19
     7.3.  Applying the Analysis  . . . . . . . . . . . . . . . . . . 19
     7.4.  Minimal Policy Requirement . . . . . . . . . . . . . . . . 21
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Registration of Header Fields  . . . . . . . . . . . . . . 22
     8.2.  Registration of Header Field Parameters  . . . . . . . . . 22
     8.3.  Registration of SIP Option Tags  . . . . . . . . . . . . . 22
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 24











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1.  Background

   The conventional model for session establishment using the Session
   Initiation Protocol (SIP, [RFC3261]) involves 1) sending a request
   for a session (a SIP INVITE) and notifying the user receiving the
   request, 2) acceptance of the request and of the session by that
   user, and 3) the sending of a response (SIP 200 OK) back to the
   requester before the session is established.  Some usage scenarios
   deviate from this model, specifically with respect to the
   notification and acceptance phase.  While it has always been possible
   for the node receiving the request to skip the notification and
   acceptance phases, there has been no standard mechanism for the party
   sending the request to specifically indicate a desire (or
   requirement) for this sort of treatment.  This document defines a SIP
   extension header field that can be used to request specific treatment
   related to the notification and acceptance phase.

   The first usage scenario is the requirement for diagnostic loopback
   calls.  In this sort of scenario, a testing service sends an INVITE
   to a node being tested.  The tested node accepts and a dialog is
   established.  But rather than establishing a two-way media flow, the
   tested node loops back or "echoes" media received from the testing
   service back toward the testing service.  The testing service can
   then analyze the media flow for quality and timing characteristics.
   Session Description Protocol (SDP) usage for this sort of flow is
   described in [LOOPBACK].  In this sort of application, it might not
   be necessary that the human using the tested node interact with the
   node in any way for the test to be satisfactorily executed.  In some
   cases, it might be appropriate to alert the user to the ongoing test,
   and in other cases it might not be.

   The second scenario is that of push-to-talk applications, which have
   been specified by the Open Mobile Alliance.  In this sort of
   environment, SIP is used to establish a dialog supporting
   asynchronous delivery of unidirectional media flow, providing a user
   experience like that of a traditional two-way radio.  It is
   conventional for the INVITES used to be automatically accepted by the
   called UA (User Agent), and the media is commonly played out on a
   loudspeaker.  The called party's UA's microphone is not engaged until
   the user presses the local "talk" button to respond.

   A third scenario is the Private Branch Exchange (PBX) attendant.
   Traditional office PBX systems often include intercom functionality.
   A typical use for the intercom function is to allow a receptionist to
   activate a loudspeaker on a desk telephone in order to announce a
   visitor.  Not every caller can access the loudspeaker, only the





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   receptionist or operator, and it is not expected that these callers
   will always want "intercom" functionality -- they might instead want
   to make an ordinary call.

   There are presumably many more use cases for the extensions defined
   in this specification, but this document was developed to
   specifically meet the requirements of these scenarios, or others with
   essentially similar properties.

   These sorts of mechanisms are not required to provide the
   functionality of an "answering machine" or "voice mail recorder".
   Such a device knows that it is expected to answer and does not
   require a SIP extension to support its behavior.

   Much of the discussion of this topic in working group meetings and on
   the mailing list dealt with differentiating "answering mode" from
   "alerting mode".  Some early work did not make this distinction.  We
   therefore proceed with the following definitions:

   o  Answering Mode includes behaviors in a SIP UA relating to
      acceptance or rejection of a request that are contingent on
      interaction between the UA and the user of that UA after the UA
      has received the request.  We are principally concerned with the
      user interaction involved in accepting the request and initiating
      an active session.  An example of this might be pressing the "yes"
      button on a mobile phone.

   o  Alerting Mode includes behaviors in a SIP UA relating to informing
      the user of the UA that a request to initiate a session has been
      received.  An example of this might be activating the ring tone of
      a mobile phone.

   This document deals only with "Answering Mode".  Issues relating to
   "Alerting Mode" are outside its scope.

   This document defines two SIP extension header fields: "Answer-Mode"
   and "Priv-Answer-Mode".  These two extensions take the same
   parameters and operate in the same general way.

   The distinction between Answer-Mode and Priv-Answer-Mode relates to
   the level of authorization claimed by the User Agent Client (UAC) and
   verified and policed by the User Agent Server (UAS).  Requests are
   usually made using Answer-Mode.  Requests made using Priv-Answer-Mode
   request "privileged" treatment from the UAS.  This mechanism is
   discussed in greater detail below, in Section 4.1.






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   Priv-Answer-Mode is not an assertion of privilege.  Instead, it is a
   request for privileged treatment.  This is similar to the UNIX model,
   where a user might run a command normally or use "sudo" to request
   administrative privilege for the command.  Including "Priv-" is
   equivalent to prefixing a UNIX command with "sudo".  In other words,
   a separate policy table (like "/etc/sudoers") is consulted to
   determine whether the user may receive the requested treatment.

   This distinction is discussed in greater detail in Section 4.1.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Syntax of Header Fields and Option Tags

   The following syntax uses ABNF as defined in [RFC5234].  Further, it
   relies on the syntax for SIP defined in [RFC3261].

   The syntax for the header fields defined in this document is:

     Answer-Mode = "Answer-Mode" HCOLON answer-mode-value
       *(SEMI answer-mode-param)

     Priv-Answer-Mode = "Priv-Answer-Mode" HCOLON answer-mode-value
       *(SEMI answer-mode-param)

     answer-mode-value = "Manual" / "Auto" / token

     answer-mode-param= "require" / generic-param

   The SIP option tag indicating support for this extension is
   "answermode".

      For implementors: SIP header field names and values are always
      compared in a case-insensitive manner.  The pretty capitalization
      is just for readability.

   This syntax includes extension hooks ("token" for answer-mode values
   and "generic-param" for optional parameters) that could be defined in
   future.  This specification defines only the behavior for the values
   given explicitly above.  In order to provide forward compatibility,
   implementations MUST ignore unknown values.






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3.  Usage of the Answer-Mode and Priv-Answer-Mode Header Fields

   This document defines usage of the Answer-Mode and Priv-Answer-Mode
   header fields in initial (dialog-forming) SIP INVITE requests and in
   200 (OK) responses to those requests.  This document specifically
   does not define usage in any other sort of request or response,
   including but not limited to ACK, CANCEL, or any mid-dialog usage.

   This limitation stems from the intended usage of this extension,
   which is to affect the way that users interact with communications
   devices when requesting new communications sessions and when
   responding to such requests.  This sort of interaction occurs only
   during the formation of a dialog and its initial usage, not during
   subsequent operations such as re-INVITE.  However, the security
   aspects of the session initiation must be applied to changes in media
   description introduced by re-INVITES or similar requests.  See
   Section 7.1 for further discussion of this issue.

4.  Usage of the Answer-Mode and Priv-Answer-Mode Header Fields in
    Requests

   The Answer-Mode or Priv-Answer-Mode header field is used by a UAC in
   an INVITE request to invoke specific handling by the responding UAS;
   this handling is related to "automatic answering" functionality for
   any dialog resulting from that INVITE request.  If no Answer-Mode or
   Priv-Answer-Mode header field is included in the request, answering
   behavior is at the discretion of the UAS, as it would be in the
   absence of this specification.  The desired handling is indicated by
   the value of the Answer-Mode or Priv-Answer-Mode header field, as
   follows:

   Manual:  The UAS is asked to defer accepting the request until the
      user of the UAS has interacted with the user interface (UI) of the
      UAS in such a way as to indicate that the user desires the UAS to
      accept the request.

   Auto:  The UAS is asked to accept the request automatically, without
      waiting for the user of the UAS to interact with the UI of the UAS
      in such a way as to indicate that the user desires the UAS to
      accept the request.

   Each value of the Answer-Mode or Priv-Answer-Mode header field can
   include an optional parameter, "require".  If present, this parameter
   indicates that the UAC would prefer that the UAS reject the request
   if the UAS is unwilling (perhaps due to policy) to answer in the mode
   requested, rather than answering in another mode.  For example, this





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   parameter could be used to make sure that a test "loopback" call
   doesn't disturb a user who has configured her phone to manually
   answer even if the caller requests an automatic answer.

   The UAS is responsible for deciding how to honor this preference.  In
   general, the UAS makes an authorization decision based on the
   authenticated identity presented in the request using authentication
   mechanisms such as SIP Digest Authentication [RFC3261], the SIP
   Identity mechanism [RFC4474], or (within the restricted networks for
   which it is suitable) the SIP mechanism for asserted identity within
   trusted networks [RFC3325].  When making an authorization decision,
   the UAS should also use authorization information or policy available
   to the UAS.  This decision-making MUST consider the risk model of the
   media session corresponding to the request, and the UAS MUST NOT
   answer without user input in cases where the privacy or security of
   the user would be compromised as a result.  Making this determination
   is a matter of system or application design, and cannot in general be
   addressed by having a set of functions that are configurable on or
   off.  Specific discussion of media sessions and appropriate policy is
   discussed in Section 7.

4.1.  The Difference Between Answer-Mode and Priv-Answer-Mode

   The functions of the Answer-Mode and Priv-Answer-Mode header fields
   are similar; they both ask that the UAS handle the request as
   specified by the header field's value (automatic or manual).  The
   difference is in the way the requests interact with the UAS's policy.
   A typical UAS will have different policies for handling each header
   field.  For example, assume that the user of a UAS has placed that
   UAS into "meeting mode", indicating that she is engaged in an
   important activity and does not wish to be spuriously interrupted.
   The UAS might disallow automatic answering for Answer-Mode requests
   while in "meeting mode".  However, that UAS might allow automatic
   answering for requests made with Priv-Answer-Mode.  There will
   probably be differences in authorization policy.  For example, a UAS
   might be configured such that callers on the "friends" list are
   allowed to make requests using Answer-Mode but not Priv-Answer-Mode.
   That same UAS might be configured to only allow callers on the
   "administrators" list to use Priv-Answer-Mode.  This is different
   from always basing the behavior on the identity of the calling party.
   For example, assume caller "Bob" is on both the "friends" list and
   "administrators" list.  If Bob wants his request to be processed
   according to the regular policy, he uses Answer-Mode.  If Bob wants
   his request to be processed under the more restrictive "privileged"
   policy, he uses Priv-Answer-Mode.






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   A UAS SHOULD apply a stricter authorization policy to a request with
   Priv-Answer-Mode than it does to requests with Answer-Mode.  The
   default policy SHOULD be to refuse requests containing Priv-Answer-
   Mode header fields unless the requester is authenticated and
   specifically authorized to make Priv-Answer-Mode requests.  Failure
   to enforce such a policy leaves the user potentially vulnerable to
   abuses, as discussed in Section 7.

   The use case envisioned for Priv-Answer-Mode relates to handling
   urgent requests from authorized callers.  For example, assume Larry
   is a limousine driver working with a fleet dispatcher.  Larry likes
   to provide a quiet environment for his car, so his communicator is
   configured for manual answer mode for all non-privileged calls,
   including push-to-talk (Answer-Mode: Auto) calls.  Each time he gets
   a call, Larry's communicator chimes softly to alert him to the call.
   If the circumstances permit it, Larry presses the communicator in
   order to accept the call, the communicator sends a 200 (OK) response,
   and the calling party's talk-burst is played out through the
   communicator's loudspeaker.  This treatment is delivered to incoming
   requests that have an Answer-Mode header field having values of
   "Manual" or "Auto" (or no Answer-Mode header field at all), no matter
   who the caller is.

   Larry's fleet dispatch operator is familiar with this policy, and
   needs to inform Larry about a critical matter.  The dispatch operator
   tries several times to push-to-talk call Larry (including Answer-
   Mode: Auto in the requests), but the calls aren't accepted because
   Larry has fallen asleep, and therefore isn't pressing his
   communicator to accept the call.

   The operator then presses his "urgent" button and calls Larry again.
   This time, the INVITE request carries a "Priv-Answer-Mode: Auto"
   header field.  Larry's communicator checks the identity of the caller
   (using a SIP Identity assertion or functionally equivalent
   mechanism), and matches the operator's identity against the list of
   users allowed to do Priv-Answer-Mode.  Since the operator is listed,
   the communicator immediately returns a 200 (OK) response accepting
   the call.  The operator speaks, and the resulting talk-burst is
   summarily played out the loudspeaker on Larry's communicator, waking
   him up.

   The effect of requesting Priv-Answer-Mode is different than the
   effect of simply granting higher privilege to an Answer-Mode request
   based on the requester's identity and corresponding authorization
   level.  This distinction is what allows the fleet operator to make
   polite (Answer-Mode: Auto) requests to Larry under normal conditions,
   and receive different handling (Priv-Answer-Mode: Auto) for a request
   having greater urgency.



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   In normal operations, only one of either Answer-Mode or Priv-Answer-
   Mode would be used in an INVITE request.  If both are present, the
   UAS will first test the authorization of the requester for Priv-
   Answer-Mode and, if authorized, process the request as if only Priv-
   Answer-Mode had been included.  If the requester is not authorized
   for Priv-Answer-Mode, then the UAS will process the request as if
   only Answer-Mode had been included.

4.2.  The "require" Modifier

   Both Answer-Mode and Priv-Answer-Mode allow a modifier of "require"
   (example: "Priv-Answer-Mode: Auto;require").  This modifier does not
   influence the UAS's policy in choosing whether to answer manually or
   automatically.  The UAS decides whether or not to answer
   automatically based on other aspects of the request.  The "require"
   modifier is only evaluated after the UAS has selected an answering
   mode.  If the UAS's policy has resulted in an answering mode that is
   different from that specified in the request, the presence of the
   "require" modifier asks the UAS to reject the call.  In the given
   example, the UAS is being asked to answer automatically if the caller
   is authorized for automatic answering under the "privileged" policy,
   and to reject the call (rather than answering manually) if the caller
   is not authorized for this mode.  This is discussed in more depth in
   Section 4.5.

4.3.  Procedures at User Agent Clients (UAC)

4.3.1.  All Requests

   A UA supporting the Answer-Mode and Priv-Answer-Mode header fields
   SHOULD indicate its support by including an option tag of
   "answermode" in the Supported header field of all requests it sends.

4.3.2.  REGISTER Transactions

   To indicate that it supports the answer-mode negotiation feature, a
   UA MAY include an extensions parameter with a value that includes
   "answermode".  Example:

     ;extensions="answermode,100rel,gruu"

   in the Contact header field of its REGISTER requests.  This usage of
   feature tags is described in [RFC3840].








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   If a UA is dependent on support for callee capabilities in the
   registrar, it MAY include a Require header field with the value
   "pref" in its REGISTER request.  This will cause the registrar to
   reject the request if the registrar does not support callee
   capabilities and caller preferences.  Example:

     Require: pref

4.3.3.  INVITE Transactions

   A UAC supporting this specification MAY include an Answer-Mode or
   Priv-Answer-Mode header field in an INVITE where it wishes to
   influence the answering mode of the responding UAS.

      Note: This is meaningful only in initial or dialog-forming INVITE
      requests.  Answer-Mode and Priv-Answer-Mode header fields
      appearing in other requests are ignored.  In general, if the
      request would not normally result in a notification to the user
      and acceptance by that user (for example, "ringing" and
      "answering"), then these extensions are not applicable.

   To request that the UAS answer only after having interacted with its
   user and receiving an affirmative instruction from that user, the UAC
   includes an Answer-Mode or Priv-Answer-Mode header field having a
   value of "Manual".  Example:

     Answer-Mode: Manual

   To request that the UAS answer manually, and ask that it reject the
   INVITE request if unable or unwilling to answer manually, the UAC
   includes an Answer-Mode or Priv-Answer-Mode header field having a
   value of "Manual" and a parameter of "require".  Example:

     Answer-Mode: Manual;require

   To request that the UAS answer automatically without waiting for
   input from the user, the UAC includes an Answer-Mode or Priv-Answer-
   Mode header field having a value of "Auto".  Example:

     Answer-Mode: Auto

   To request that the UAS answer automatically, and ask that it reject
   the INVITE request if unable or unwilling to answer automatically,
   the UAC includes an Answer-Mode or Priv-Answer-Mode header field
   having a value of "Auto" and a parameter of "require".  Example:

     Answer-Mode: Auto;require




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   To require that the UAS either support this extension or reject the
   request, the UAC includes a Require header field having the value
   "answermode".  This does not actually force the UAS to automatically
   answer, it just requires that the UAS either understand this
   extension or reject the request.  We do not have a SIP negotiation
   technique to force specific behavior.  Rather, the desired behavior
   is indicated in the SIP extension itself.  Example:

     Require: answermode

   To request that retargeting proxies in the path preferentially select
   targets that have indicated support for this extension in their
   registration, a UAC includes an Accept-Contact header field with an
   extensions parameter having a value of "answermode".  This usage of
   Accept-Contact is described in [RFC3841].  This would normally be
   used in conjunction with the "Require: answermode" header field as
   described above.  Example:

     Require: answermode Accept-Contact:
               *;extensions="answermode";methods="INVITE"

   To request that retargeting proxies in the path do not select targets
   that have indicated non-support for this extension in their
   registration, a UAC includes an Accept-Contact header field with an
   extensions parameter having a value of "answermode" and an option
   field of "require".  This usage of Accept-Contact is described in
   [RFC3841].  This would normally be used in conjunction with the
   "Require: answermode" header field as described above.  Example:

     Require: answermode Accept-Contact:
             *;extensions="answermode"; methods="INVITE";require

   To request that retargeting proxies in the path exclusively select
   targets that have indicated support for this extension in their
   registration, a UAC includes an Accept-Contact header field
   extensions parameter having a value of "answermode" and options of
   "require" and "explicit".  This usage of Accept-Contact is described
   in [RFC3841].  This would normally be used in conjunction with the
   "Require: answermode" header field as described above.  Example:

     Require: answermode Accept-Contact:
             *;extensions="answermode";
             methods="INVITE";require;explicit








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4.4.  Procedures at Intermediate Proxies

4.4.1.  General Proxy Behavior

   The general procedure at all intermediate proxies, including the
   UAC's serving proxy or proxies and the UAS's serving proxy or
   proxies, is to ignore the Answer-Mode header field.  However, the
   serving proxies (proxies responsible for resolving an address-of-
   record (AOR) into a registered contact) MAY exercise control over the
   requested answer mode, either inserting or deleting an Answer-Mode or
   Priv-Answer-Mode header field or altering the value of an existing
   header field, in accord with local policy.  This could result in
   behavior that is inconsistent with user expectations (such as having
   a call that was intended to be a diagnostic loopback answered by a
   human) and consequently proxies MUST NOT insert, delete, or alter
   Answer-Mode or Priv-Answer-Mode header fields unless explicitly
   authorized to do so by an external agreement between the proxy
   operator and the user of the UA that the proxy is serving.  These
   serving proxies MAY also reject a request according to local policy
   and, if they do so, SHOULD use the rejection codes as specified below
   for the UAS.

4.4.2.  Issues with Automatic Answering and Forking

   One of the well-known issues with forking is the problem of multiple
   acceptance.  If an INVITE request is forked to several UASs and more
   than one replies with a 200 (OK) response, the conventional approach
   is to continue the dialog with the first respondent and tear down the
   dialog (using BYE requests) with all other respondents.

   While this problem exists without an auto-answer negotiation
   capability, it is apparent that widespread adoption of UAs that
   engage in auto-answer behavior will exacerbate the multiple
   acceptance problem.  Consequently, systems designers need to take
   this aspect into consideration.  In general, auto-answer is NOT
   RECOMMENDED in environments that include parallel forking.

   As an alternative, it might be reasonable to use a variation on
   manual-answer combined with no alerting and early media.  In this
   approach, the initial message or talk-burst is transmitted as early
   media to all recipients, where it is displayed or played out.  Any
   response utterance (pushing the transmit key and talking) from the
   user of a UAS following this would serve as an "acceptance",
   resulting in a 200 (OK) response being transmitted by their UAS.
   Consequently, the race-condition for acceptance would be limited to
   the subset of UAs actually responding under user control, rather than
   the full set of UAs to which the request was forked.




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   Another alternative would be to use dynamic conferencing instead of
   forking.  In this approach, instead of forking the request, a
   conference would be initiated and all registered UAs invited into
   that conference.  The mixer attached to the conference would then
   mediate traffic flows appropriately.

4.5.  Procedures at User Agent Servers (UAS)

4.5.1.  INVITE Transactions

   For a request having an Answer-Mode value of "Manual" and not having
   an Answer-Mode parameter of "require", the UAS SHOULD defer accepting
   the request until the user of the UAS has confirmed willingness to
   accept the request.  This behavior MAY be altered as needed for
   unattended UASs or other local characteristics or policy.  For
   example, an auto-attendant or Public Switched Telephone Network
   (PSTN) gateway system that always answers automatically would go
   ahead and answer, despite the presence of the "Manual" Answer-Mode
   header field value.

   For a request having an Answer-Mode value of "Manual" and an Answer-
   Mode parameter of "require", the UAS MUST defer accepting the request
   until the user of the UAS has confirmed willingness to accept the
   request.  If the UAS is not capable of answering the request in this
   "Manual" mode or is unwilling to do so, it MUST reject the request,
   SHOULD do so with a "403 (Forbidden)" response, and MAY include a
   reason phrase of "manual answer forbidden".

   For a request having an Answer-Mode value of "Auto", the UAS SHOULD,
   if the calling party is authenticated and authorized for automatic
   answering, accept the request without further user input.  The UAS
   MAY, according to local policy or user preferences, treat this
   request as it would treat a request having an Answer-Mode with a
   value of "Manual" or having no Answer-Mode header field.  If the
   calling party is not authenticated and authorized for automatic
   answer, the UAS MAY either handle the request as per "manual", or
   reject the request.  If the UAS rejects the request, it SHOULD do so
   with a "403 (Forbidden)" response, and MAY include a reason phrase of
   "automatic answer forbidden".  There may be an interaction with
   [RFC3261] section 23.2, which in some cases requires user validation
   of certificates used for S/MIME.  Since this places the same
   interrupt burden on the user as would manually answering the request,
   a UAS experiencing this requirement for user validation of a request
   that requires automatic answering SHOULD reject the request with a
   "403 (Forbidden)" response and MAY include a reason phrase of
   "certificate validation requires user input not compatible with
   automatic answer."




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   For a request having an Answer-Mode value of "Auto" and an Answer-
   Mode parameter of "require", the UAS SHOULD, if the calling party is
   authenticated and authorized for automatic answering, accept the
   request.  The UAS MUST NOT allow "manual" answer of this request, but
   MAY reject it.  If, for whatever reason, the UAS chooses not to
   accept the request automatically, the UAS MUST reject the request,
   SHOULD do so with a "403 (Forbidden)" response, and MAY include a
   reason phrase of "automatic answer forbidden".

   Similar behavior applies for Priv-Answer-Mode, except that the policy
   for authorization may be different (and generally more stringent).

5.  Usage of the Answer-Mode and Priv-Answer-Mode Header Fields in
    Responses

   The Answer-Mode or Priv-Answer-Mode header field can be inserted by a
   UAS into a response in order to indicate how it handled the
   associated request with respect to automatic answering functionality.
   The UAC might use this information to inform the user or otherwise
   adapt the behavior of the user interface.  The handling is indicated
   by the value of the header field, as follows:

   Manual:  The UAS responded after the user of the UAS interacted with
      the user interface (UI) of the UAS in such a way as to indicate
      that the user desires the UAS to accept the request.

   Auto:  The UAS responded automatically, without waiting for the user
      of the UAS to interact with the UI of the UAS in such a way as to
      indicate that the user desires the UAS to accept the request.

   The Answer-Mode and Priv-Answer-Mode header fields, when used in
   responses, are only valid in a 200 (OK) response to an INVITE
   request.

5.1.  Procedures at the UAS

   A UAS supporting this specification inserts an Answer-Mode or Priv-
   Answer-Mode header field into the 200 (OK) response to an INVITE
   request when it wishes to inform the UAC as to whether the request
   was answered manually or automatically.  It is reasonable for a UAS
   to assume that if the UAC included an Answer-Mode header field in the
   request, it would probably like to see an Answer-Mode header field in
   the response.  The full rationale for including or not including this
   header field in a response is outside of the scope of this
   specification, and is sensitive to the privacy concerns of the user
   of the UAS.  For example, informing the calling party that a call was
   answered manually might reveal the presence of an "actual human" at
   the responding UAS.  While in the general case the ensuing



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   conversation would also reveal this same information, there might be
   cases where this information might need to be protected.
   Consequently, UASs supporting this specification SHOULD include
   appropriately configurable policy mechanisms for making this
   determination, and the default configuration SHOULD be to exclude
   this header field from responses.

5.2.  Procedures at the UAC

   A UAC MAY use the value of the Answer-Mode or Priv-Answer-Mode header
   field, if present, to adapt the user interface and/or inform the user
   about the handling of the request.  For example, the user of a push-
   to-talk system might speak differently if she knows that the called
   party answered "in person" vs. having the call blare out of an
   unattended speaker phone.

6.  Examples of Usage

   The following examples show Bob registering a contact that supports
   the negotiation of answering mode.  Alice then calls Bob with an
   INVITE request, asking for automatic answering and explicitly asking
   that the request not be routed to contacts that have not indicated
   support for this extension.  Further, Alice requires that the request
   be rejected if Bob's UA does not support negotiation of answering
   mode.  Bob replies with a 200 (OK) response indicating that the call
   was answered automatically.

      The Content-Length header field shown in the examples contains a
      placeholder "..." instead of a valid Content-Length.  Furthermore,
      the SDP bodies that would be expected in the INVITE requests and
      200 (OK) responses are not shown.

6.1.  REGISTER Request

   In the following example, Bob's UA is registering and indicating that
   it supports the answermode extension.

     REGISTER sip:example.com SIP/2.0
     From: Bob<sip:bob@example.com>
     To: Bob <sip:bob@example.com>
     CallID: hh89as0d-asd88jkk@cell-phone.example.com
     CSeq: 1 REGISTER
     Contact: sip:cell-phone.example.com;
       ;audio
       ;+sip.extensions="answermode"
       ;methods="INVITE,BYE,OPTIONS,CANCEL,ACK"
       ;schemes="sip"




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6.2.  INVITE Request

   In this example, Alice is calling Bob and asking Bob's UA to answer
   automatically.  However, Alice is willing for Bob to answer manually
   if Bob's policy is to prefer manual answer, so Alice does not include
   a ";require" modifier on "Answer-Mode: Auto".

     INVITE sip:bob@example.com SIP/2.0
     Via: SIP/2.0/TCP client-alice.example.com:5060; branch=z9hG4bK74b43
     Max-Forwards: 70
     From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
     To: Bob <sip:bob@example.com>
     Call-ID:3848276298220188511@client-alice.example.com
     CSeq: 1 INVITE
     Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
     Require: answermode
     Accept-contact:*;require;explicit;extensions="answermode"
     Answer-Mode: Auto
     Content-Type: application/sdp
     Content-Length: ...

6.3.  200 (OK) Response

   Here, Bob has accepted the call and his UA has answered
   automatically, which it indicates in the 200 (OK) response.

     SIP/2.0 200 OK
     Via: SIP/2.0/TCP client-alice.example.com:5060; branch=z9hG4bK74b43
     From: Alice <sip:alice@example.com>;tag=9fxced76sl
     To: Bob <sip:bob@example.com>;tag=8321234356
     Call-ID: 3848276298220188511@client-alice.example.com
     CSeq: 1 INVITE
     Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
     Answer-Mode: Auto
     Content-Type: application/sdp
     Content-Length: ...

7.  Security Considerations

   This specification adds the ability for a UAC to request potentially
   risky user interface behavior relating to the acceptance of an INVITE
   request by the UAS receiving the request.  Specifically, the UAC can
   request that the UAS accept the request without input to the UAS by
   the user of the UAS (Answer-Mode: Auto).

   There are several attacks possible here -- the most obvious being the
   ability to turn a phone into a remote listening device without its
   user being aware.  Additional potential attacks include reverse



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   charge fraud, unsolicited push-to-talk communications (spam over
   push-to-talk (SPTT)), playout of obnoxious noises (the "whoopee
   cushion" attack), battery-rundown denial of service, "forced busy"
   denial of service, running up the victim's data transport bill, and
   phishing via session insertion (where an ongoing session is replaced
   by another without the victim's awareness).

   Since SIP implementations do not commonly implement end-to-end
   message protections, this specification is completely dependent on
   transitive security across SIP proxies.  Any misbehaving proxy can
   insert, delete, and/or alter the contents of the Answer-Mode and
   Priv-Answer-Mode header fields, and in general can do so without
   being noticed by either the UAC or UAS.  Consequently, it is critical
   that any proxies in the path be not only trusted, but worthy of that
   trust.  While proxies do not generally intentionally insert, delete,
   or alter the Answer-Mode and Priv-Answer-Mode header fields, this
   specification does note a use case for such manipulation by proxies
   acting on behalf of the user of a UAC or UAS that has limited support
   for the authentication or policy enforcement needed to securely
   exercise these extensions.  Proxies that perform such extension-
   sensitive manipulation MUST therefore provide complete policy
   enforcement, as per the minimal policy discussed in Section 7.4.

   The existing body of SIP work provides strong capabilities for
   authentication of requests, prevention of man-in-the-middle attacks,
   protection of the privacy and integrity of media flows, and so on
   (although as noted above, these capabilities usually rely on
   transitive trust across proxies).  The behaviors added by the
   extensions in this document raise additional possibilities for
   attacks against media flows not completely addressed by existing SIP
   work, and therefore require analysis in this document.

   Media attacks can be loosely categorized as:

   Insertion:  Media is inserted into and played out by the victim UA
      without consent of the UA's user.

   Interception:  The victim UA's media acquisition facility (such as a
      microphone or camera) is activated, producing a media stream,
      without the consent of the UA's user.

7.1.  Attack Sensitivity Depends on Media Characteristics

   The danger of abuse varies greatly depending on the media
   characteristics of the session being established.  Since the
   expressive range of media sessions that can be established by SIP is





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   unbounded, we might find it more effective to model loose categories
   of media modality rather than explicitly describing every possible
   scenario.  Security analysis can then be applied per modality.

   The media modalities of interest appear to be:

   UAC-sourced (Inbound) Unidirectional Media Insertion:  Sensitive
      media flows from the UAC and is rendered by the UAS, annoying the
      user of the UAS or disrupting the function of the UAS.  We refer
      to this as the "whoopee-cushion" attack because of its utility in
      replicating the rude-noise-making seat cushion.  The danger of
      this attack is quite literally amplified by a loudspeaker
      apparatus attached to the victim UAS.  Media that has minimal
      secondary implication (such as sending a move in a chess game to a
      computer that isn't running a chess game) is related, but of far
      less significance.  This sort of attack can also have other
      consequences, such as discharging the victim's battery or
      increasing charges for data transport to be paid by the victim.

   UAS-sourced (Outbound) Unidirectional Media Interception:  Sensitive
      media flows from the UAS and is rendered by the UAC, violating the
      privacy of the user of the UAS.  We refer to this as the "bug-my-
      phone" attack because that would appear to be the primary attack
      motivator.

   Bidirectional Media Insertion or Interception:  Bidirectional media
      is the common case when SIP is used in a voice-over-IP scenario or
      "traditional phone call".  Once a media flow is established, both
      ends send and receive media without further engagement.  The media
      information is presumed to be sensitive -- that is, if intercepted
      it damages the victim's privacy, and if inserted, it annoys or
      interferes with the recipient.  Attacks of this sort might produce
      either the "whoopee-cushion" or "bug-my-phone" scenarios,
      potentially even simultaneously.

   It seems reasonable to consider the "bug-my-phone" attack as being in
   a different class (potentially far more severe) than the "whoopee-
   cushion" attack.  This distinction suggests that security policy
   could be established in different and presumably less restrictive
   fashion for inbound media flows than for outbound media flows.  The
   set of callers from which a user would be willing to automatically
   accept inbound media is reasonably much broader than the set of
   callers to which a user would be willing to automatically grant
   outbound media access, although this may not be true in all
   environments, especially those where reception of unwanted media has
   unwanted financial consequences.





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   For example, assume a UA is designed such that it can be used to
   receive push-to-talk calls to a loudspeaker, and it can be used as a
   "baby monitor" (has an open mic and streams received audio to
   listeners).  The policy for activating the push-to-talk loudspeaker
   would probably need to be reasonably broad (perhaps "all the user's
   buddies").  However, the policy for the baby monitor would need to be
   very narrow (perhaps "only the baby's mother") or even completely
   closed.  The minimal policy defined in Section 7.4 explicitly forbids
   the "baby monitor" functionality.

7.2.  Application Design Affects Attack Opportunity

   In the most common use cases, the security aspects are somewhat
   mitigated by design aspects of the application.  For example, in
   traditional telephony, the called party is alerted to the request
   (the phone rings), no media session is established without the
   acceptance of the called party (picking up the phone), and the media
   path is most commonly delivered to a single-user handset.
   Consequently, this application (although bidirectional) is relatively
   secure against both media insertion and media interception attacks of
   the sort enabled by the extensions in this document.  The use of
   policy-free automatic-answering devices (like answering machines) and
   amplifiers (speakerphones and call-screening devices) weakens this
   defense.

   In push-to-talk applications, media can be sent from UAC to UAS
   without user oversight, but no media is sent from the called UAS
   without user input (the "push" of "push-to-talk").  Consequently,
   there is no "bug-my-phone" attack opportunity.  Further, screening of
   the UAC by eliminating UAC identities not on some sort of "white
   list" (often, a buddy list) reduces the threat of "whoopee cushion"
   attacks (except from one's buddies, of course).

   Similar approaches apply to most applications.  Insertion can be
   controlled (but not eliminated) by combining identity mechanisms with
   simple authorization policy, and interception can be effectively
   eliminated by combining strong identity mechanisms with aggressive
   authorization policy and/or user interaction.

7.3.  Applying the Analysis

   The extensions described in this document provide mechanisms by which
   a UAC can request that a UAS not deploy two of the five defensive
   mechanisms listed below -- user alerting and user acceptance.  In
   order for this not to produce undue risk of insertion attacks or
   increased risk of interception attacks, we are therefore forced to
   rely on the remaining defensive mechanisms.  This document defines a
   minimum threshold for satisfactory security.  Certainly more



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   restrictive policies might reasonably be used, but any policy less
   restrictive than the approach described below is very likely to
   result in significant security issues.

   From the previous discussion of risks, attacks, and vulnerabilities,
   we can derive five defensive mechanisms available at the application
   level:

   1.  Identity -- Know who the request came from.

   2.  Alerting -- Let the called user know what's happening.  Some
       applications might use inbound media as an alert.

   3.  Acceptance -- Require called user to make run-time decision.
       Asking the user to make a run-time decision without alerting the
       user to the need to make a decision is generally infeasible.
       This will have implications for possible alerting options that
       are outside the scope of this document.

   4.  Limit the Input/Output (I/O) -- Turn off loudspeakers or
       microphone.  This could be used to convert a bidirectional media
       session (very risky, possible "bug my phone") into a
       unidirectional, inbound-only (less risky, possible "spam" or
       "rundown", etc.) session while waiting for user acceptance.

   5.  Policy -- Rules about other factors, such as black- and
       whitelisting based on identity, disallowing acceptance without
       alerting, etc.

   Since SIP and related work already provide several mechanisms
   (including SIP Digest Authentication [RFC3261], the SIP Identity
   mechanism [RFC4474], and the SIP mechanism for asserted identity
   within private networks [RFC3325], in networks for which it is
   suitable) for establishing the identity of the originator of a
   request, we presume that an appropriately selected mechanism is
   available for UAs implementing the extensions described in this
   document.  In short, UAs implementing these extensions MUST be
   equipped with and MUST exercise a request-identity mechanism.  The
   analysis below proceeds from an assumption that the identity of the
   sender of each request is either known or is known to be unknown, and
   can therefore be considered in related policy considerations.
   Failure to meet this identity requirement either opens the door to a
   wide range of attacks or requires operational policy so tight as to
   make these extensions useless.







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   We previously established a class distinction between inbound and
   outbound media flows, and can model bidirectional flows as "worst
   case" sums of the risks of the other two classes.  Given this
   distinction, it seems reasonable to provide separate directionality
   policy classes for:

   1.  Inbound media flows.

   2.  Outbound media flows.

   For each directionality policy class, we can divide the set of
   request identities into three classes:

   1.  Identities explicitly authorized for the class.

   2.  Identities explicitly denied for the class.

   3.  Identities for which we have no explicit policy and need the user
       to make a decision.

   Note that not all combinations of policies possible in this
   decomposition are generally useful.  Specifically, a policy of
   "inbound media denied, outbound media allowed" equates to a "bug my
   phone" attack, and is disallowed by the minimal policy of
   Section 7.4, which as written excludes all cases of "Outbound media
   explicitly authorized".

7.4.  Minimal Policy Requirement

   User agents implementing this specification SHOULD NOT establish a
   session providing inbound media without explicit user acceptance
   where the requesting user is unknown, or is known and has not been
   granted authorization for such a session.  This requirement is
   intended to prevent "SPAM broadcast" attacks where unexpected and
   unwanted media is played out at a UAS .

   User agents implementing this specification MUST NOT establish a
   session providing outbound or bidirectional media sourced from the
   user agent without explicit user acceptance.  Loopback media used for
   connectivity testing is not constrained by this requirement.  This
   requirement is intended to assure that this extension can not be used
   to turn a UAS into a remote-controlled microphone (or "bug") without
   the knowledge of its user.  Since SIP allows for a session to be
   initially established with inbound-only media and then transitioned
   (via re-INVITE or UPDATE) to an outbound or bidirectional session,






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   enforcing this policy requires dialog-stateful inspection in the SIP
   UAS.  In other words, if a session was initiated with automatic
   answering, the UAS MUST NOT transition to a mode that sends outbound
   media without explicit acceptance by the user of the UAS.

8.  IANA Considerations

8.1.  Registration of Header Fields

   This document defines new SIP header fields named "Answer-Mode" and
   "Priv-Answer-Mode".

   The following rows have been added to the "Header Fields" section of
   the SIP parameter registry:

              +------------------+--------------+-----------+
              | Header Name      | Compact Form | Reference |
              +------------------+--------------+-----------+
              | Answer-Mode      |              | [RFC5373] |
              | Priv-Answer-Mode |              | [RFC5373] |
              +------------------+--------------+-----------+

8.2.  Registration of Header Field Parameters

   This document defines parameters for the header fields defined in the
   preceding section.  The header fields "Answer-Mode" and "Priv-Answer-
   Mode" can take the values "Manual" or "Auto".

   The following rows have been added to the "Header Field Parameters
   and Parameter Values" section of the SIP parameter registry:

   +------------------+----------------+-------------------+-----------+
   | Header Field     | Parameter Name | Predefined Values | Reference |
   +------------------+----------------+-------------------+-----------+
   | Answer-Mode      | require        | No                | [RFC5373] |
   | Priv-Answer-Mode | require        | No                | [RFC5373] |
   +------------------+----------------+-------------------+-----------+

8.3.  Registration of SIP Option Tags

   This document defines the SIP option tag "answermode".

   The following row has been added to the "Option Tags" section of the
   SIP Parameter Registry:







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   +------------+------------------------------------------+-----------+
   | Name       | Description                              | Reference |
   +------------+------------------------------------------+-----------+
   | answermode | This option tag is for support of the    | [RFC5373] |
   |            | Answer-Mode and Priv-Answer-Mode         |           |
   |            | extensions used to negotiate automatic   |           |
   |            | or manual answering of a request.        |           |
   +------------+------------------------------------------+-----------+

9.  Acknowledgements

   This document draws requirements and a large part of its methodology
   from the work of the Open Mobile Alliance, and specifically from a
   document by Andrew Allen, Jan Holm, and Tom Hallin.

   The editor would also like to recognize the contributions of David
   Oran and others who argued on the SIPPING mailing list and at the OMA
   ad-hoc meeting at IETF 62 that the underlying ideas of the above
   document were broadly applicable to the SIP community, and that the
   concepts of alerting and answering should be clearly delineated.
   Further, the security review provided by Sandy Murphy and the gen-art
   review by Suresh Krishnan were very helpful in improving the quality
   of this document.

10.  References

10.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC3840]   Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
               "Indicating User Agent Capabilities in the Session
               Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC3841]   Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
               Preferences for the Session Initiation Protocol (SIP)",
               RFC 3841, August 2004.

   [RFC4474]   Peterson, J. and C. Jennings, "Enhancements for
               Authenticated Identity Management in the Session
               Initiation Protocol (SIP)", RFC 4474, August 2006.




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   [RFC5234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", STD 68, RFC 5234, January 2008.

10.2.  Informative References

   [LOOPBACK]  Hedayat, K., "An Extension to the Session Description
               Protocol (SDP) for Media Loopback", Work in Progress,
               August 2008.

   [RFC3325]   Jennings, C., Peterson, J., and M. Watson, "Private
               Extensions to the Session Initiation Protocol (SIP) for
               Asserted Identity within Trusted Networks", RFC 3325,
               November 2002.

Authors' Addresses

   Dean Willis (editor)
   Softarmor Systems
   3100 Independence Pkwy #311-164
   Plano, Texas  75075
   USA

   EMail: dean.willis@softarmor.com


   Andrew Allen
   Research in Motion (RIM)
   300 Knightsbridge Parkway, Suite 360
   Lincolnshire, Illinois  60069
   USA

   EMail: aallen@rim.com



















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