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Network Working Group                                     D. Willis, Ed.Request for Comments: 5373                             Softarmor SystemsCategory: Standards Track                                       A. Allen                                                Research in Motion (RIM)                                                           November 2008Requesting 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 toBCP 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.Willis & Allen              Standards Track                     [Page 1]

RFC 5373                  SIP Answering Modes              November 2008Table of Contents1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Requirements Language  . . . . . . . . . . . . . . . . . .52.  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  .  74.2.  The "require" Modifier . . . . . . . . . . . . . . . . . .94.3.  Procedures at User Agent Clients (UAC) . . . . . . . . . .94.3.1.  All Requests . . . . . . . . . . . . . . . . . . . . .94.3.2.  REGISTER Transactions  . . . . . . . . . . . . . . . .94.3.3.  INVITE Transactions  . . . . . . . . . . . . . . . . .104.4.  Procedures at Intermediate Proxies . . . . . . . . . . . .124.4.1.  General Proxy Behavior . . . . . . . . . . . . . . . .124.4.2.  Issues with Automatic Answering and Forking  . . . . .124.5.  Procedures at User Agent Servers (UAS) . . . . . . . . . .134.5.1.  INVITE Transactions  . . . . . . . . . . . . . . . . .13   5.  Usage of the Answer-Mode and Priv-Answer-Mode Header       Fields in Responses  . . . . . . . . . . . . . . . . . . . . .145.1.  Procedures at the UAS  . . . . . . . . . . . . . . . . . .145.2.  Procedures at the UAC  . . . . . . . . . . . . . . . . . .156.  Examples of Usage  . . . . . . . . . . . . . . . . . . . . . .156.1.  REGISTER Request . . . . . . . . . . . . . . . . . . . . .156.2.  INVITE Request . . . . . . . . . . . . . . . . . . . . . .166.3.  200 (OK) Response  . . . . . . . . . . . . . . . . . . . .167.  Security Considerations  . . . . . . . . . . . . . . . . . . .167.1.  Attack Sensitivity Depends on Media Characteristics  . . .177.2.  Application Design Affects Attack Opportunity  . . . . . .197.3.  Applying the Analysis  . . . . . . . . . . . . . . . . . .197.4.  Minimal Policy Requirement . . . . . . . . . . . . . . . .218.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .228.1.  Registration of Header Fields  . . . . . . . . . . . . . .228.2.  Registration of Header Field Parameters  . . . . . . . . .228.3.  Registration of SIP Option Tags  . . . . . . . . . . . . .229.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .2310. References . . . . . . . . . . . . . . . . . . . . . . . . . .2310.1. Normative References . . . . . . . . . . . . . . . . . . .2310.2. Informative References . . . . . . . . . . . . . . . . . .24Willis & Allen              Standards Track                     [Page 2]

RFC 5373                  SIP Answering Modes              November 20081.  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 theWillis & Allen              Standards Track                     [Page 3]

RFC 5373                  SIP Answering Modes              November 2008   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, inSection 4.1.Willis & Allen              Standards Track                     [Page 4]

RFC 5373                  SIP Answering Modes              November 2008   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 inSection 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.Willis & Allen              Standards Track                     [Page 5]

RFC 5373                  SIP Answering Modes              November 20083.  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.  SeeSection 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, thisWillis & Allen              Standards Track                     [Page 6]

RFC 5373                  SIP Answering Modes              November 2008   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 inSection 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.Willis & Allen              Standards Track                     [Page 7]

RFC 5373                  SIP Answering Modes              November 2008   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 inSection 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.Willis & Allen              Standards Track                     [Page 8]

RFC 5373                  SIP Answering Modes              November 2008   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 inSection 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].Willis & Allen              Standards Track                     [Page 9]

RFC 5373                  SIP Answering Modes              November 2008   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: pref4.3.3.  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;requireWillis & Allen              Standards Track                    [Page 10]

RFC 5373                  SIP Answering Modes              November 2008   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;explicitWillis & Allen              Standards Track                    [Page 11]

RFC 5373                  SIP Answering Modes              November 20084.4.  Procedures at Intermediate Proxies4.4.1.  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.Willis & Allen              Standards Track                    [Page 12]

RFC 5373                  SIP Answering Modes              November 2008   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."Willis & Allen              Standards Track                    [Page 13]

RFC 5373                  SIP Answering Modes              November 2008   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 ensuingWillis & Allen              Standards Track                    [Page 14]

RFC 5373                  SIP Answering Modes              November 2008   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"Willis & Allen              Standards Track                    [Page 15]

RFC 5373                  SIP Answering Modes              November 20086.2.  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 reverseWillis & Allen              Standards Track                    [Page 16]

RFC 5373                  SIP Answering Modes              November 2008   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 inSection 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 isWillis & Allen              Standards Track                    [Page 17]

RFC 5373                  SIP Answering Modes              November 2008   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.Willis & Allen              Standards Track                    [Page 18]

RFC 5373                  SIP Answering Modes              November 2008   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 inSection 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 moreWillis & Allen              Standards Track                    [Page 19]

RFC 5373                  SIP Answering Modes              November 2008   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.Willis & Allen              Standards Track                    [Page 20]

RFC 5373                  SIP Answering Modes              November 2008   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 ofSection 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,Willis & Allen              Standards Track                    [Page 21]

RFC 5373                  SIP Answering Modes              November 2008   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 Considerations8.1.  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:Willis & Allen              Standards Track                    [Page 22]

RFC 5373                  SIP Answering Modes              November 2008   +------------+------------------------------------------+-----------+   | 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.  References10.1.  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.Willis & Allen              Standards Track                    [Page 23]

RFC 5373                  SIP Answering Modes              November 2008   [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.comWillis & Allen              Standards Track                    [Page 24]