[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

Versions: (draft-willis-sip-answeralert) 00 01 02 03 04 05 06 07 RFC 5373

SIP Working Group                                         D. Willis, Ed.
Internet-Draft                                             Cisco Systems
Expires: October 3, 2006                                        A. Allen
                                                Research in Motion (RIM)
                                                              April 2006


  Requesting Answering Modes for the Session Initiation Protocol (SIP)
                      draft-ietf-sip-answermode-01

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 3, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines two SIP extension 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-



Willis & Allen           Expires October 3, 2006                [Page 1]

Internet-Draft             SIP Answering Modes                April 2006


   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.

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 [1].







































Willis & Allen           Expires October 3, 2006                [Page 2]

Internet-Draft             SIP Answering Modes                April 2006


Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  5

   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Requirements for Requesting an Answering Mode  . . . . . .  7
     2.2.  Requirements for Indicating the Applied Answer  Mode
           in a Response  . . . . . . . . . . . . . . . . . . . . . .  9

   3.  Syntax of Header Field and Tags  . . . . . . . . . . . . . . .  9

   4.  Usage of the Answer-Mode and Priv-Answer-Mode Header
       Fields in Requests . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Procedures at User Agent Clients (UAC) . . . . . . . . . . 11
       4.1.1.  All Requests . . . . . . . . . . . . . . . . . . . . . 11
       4.1.2.  REGISTER Transactions  . . . . . . . . . . . . . . . . 11
       4.1.3.  INVITE Transactions  . . . . . . . . . . . . . . . . . 11
     4.2.  Procedures at Intermediate Proxies . . . . . . . . . . . . 13
       4.2.1.  General Proxy Behavior . . . . . . . . . . . . . . . . 13
       4.2.2.  Issues with Automatic Answering and Forking  . . . . . 14
     4.3.  Procedures at User Agent Servers (UAS) . . . . . . . . . . 14
       4.3.1.  INVITE Transactions  . . . . . . . . . . . . . . . . . 14
       4.3.2.  Special Considerations for Priv-Answer-Mode  . . . . . 15

   5.  Usage of the Answer-Mode and Priv-Answer-Mode Header
       Fields in a Response . . . . . . . . . . . . . . . . . . . . . 16
     5.1.  Procedures at the UAS  . . . . . . . . . . . . . . . . . . 17
     5.2.  Procedures at the UAC  . . . . . . . . . . . . . . . . . . 17

   6.  Examples of Usage  . . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  REGISTER Request . . . . . . . . . . . . . . . . . . . . . 18
     6.2.  INVITE Request . . . . . . . . . . . . . . . . . . . . . . 18
     6.3.  200 OK response  . . . . . . . . . . . . . . . . . . . . . 18

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     7.1.  Attack Sensitivity Depends on Media Characteristics  . . . 20
     7.2.  Application Design Affects Attack Opportunity  . . . . . . 21
     7.3.  Applying the Analysis  . . . . . . . . . . . . . . . . . . 21
     7.4.  Minimal Policy Requirement . . . . . . . . . . . . . . . . 23

   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  Registration of Header Fields  . . . . . . . . . . . . . . 23
     8.2.  Registration of Header Field Parameters  . . . . . . . . . 24
     8.3.  Registration of SIP Option Tags  . . . . . . . . . . . . . 24

   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24

   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25



Willis & Allen           Expires October 3, 2006                [Page 3]

Internet-Draft             SIP Answering Modes                April 2006


     10.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     10.2. Informative References . . . . . . . . . . . . . . . . . . 25

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28














































Willis & Allen           Expires October 3, 2006                [Page 4]

Internet-Draft             SIP Answering Modes                April 2006


1.  Background

   There has been discussion of how to deal with "auto-answer" and
   related issues in the SIP community for several years.  Discussion in
   the SIPPING working group, augmented by input from other
   organizations such as the Open Mobile Alliance, resulted in a
   consensus observed in the SIPPING meeting at IETF 62 to extend SIP,
   which is defined in [2].  Further discussion of the topic on the SIP
   mailing list after IETF 62 led to a consensus to pursue this work in
   the SIP working group as a standards-track effort.

   Two different use cases converged to create the consensus for the
   development of this specification.  Other use cases presumably exist,
   but two is enough to establish the level of reusability required to
   justify a standards-track extension as opposed to a "P-header" under
   [3].

   The first key use case was 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.
   SDP usage for this sort of flow is described in [9].  In this sort of
   application, it may not be necessary that the human using the node
   under test 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 use case is that of "Push to Talk" applications as
   described in [10] and relates to a service being 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, giving 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.

   Another representative use case was introduced during discussion of
   this topic on the mailing list of the SIP working group.  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 receptionist or
   operator, and it is not expected that these callers will always want
   "intercom" functionality -- they may instead want to make an ordinary



Willis & Allen           Expires October 3, 2006                [Page 5]

Internet-Draft             SIP Answering Modes                April 2006


   call.

   It should be noted that the above list of use cases is not
   exhaustive.  There are presumably many more use cases for the
   extensions defined in this specification.

   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 should 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 disambiguating "answering mode" from
   "alerting mode".  Some early work, such as [10], 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 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.


2.  Requirements

   Requirements in the following are expressed relative to the node
   initiating an INVITE request (UAC), the node receiving and
   potentially responding to that request (UAS), and the users of those
   nodes (UAC-user and UAS-user).












Willis & Allen           Expires October 3, 2006                [Page 6]

Internet-Draft             SIP Answering Modes                April 2006


   The relationship between the entities used in the following
   requirements discussion is shown in the following relationship
   diagram:


         UAC-User           UAC             UAS          UAS-User
             |               |               |               |
             |               |               |               |
             |               |               |               |
             |(1) Uses       |               |               |
             |-------------->|               |               |
             |               |               |(2) Uses       |
             |               |               |<--------------|
             |               |(3) REQUEST    |               |
             |               |-------------->|               |
             |               |(4) RESPONSE   |               |
             |               |<--------------|               |
             |               |               |               |
             |               |               |               |


   Figure 1

   Note that "UAC-User", "UAC", "UAS", and "UAS-User" are referenced as
   actors in the following requirements discussion.

2.1.  Requirements for Requesting an Answering Mode

   The requirements relating to requesting a specific answering mode
   include:

   Req-1: It MUST be possible for UAC to ask that the UAS answer the
      request without requiring active interaction between UAS-user and
      the user interface (UI) of the UAS.  We refer to this as
      "automatic answer mode".  This mode is useful for diagnostic
      loopback procedures and critical for "two-way radio" or "push to
      talk" applications.

   Req-2: It MUST be possible for UAC to ask that UAS answer the request
      only after UAS-user has actively directed UAS to answer this
      specific request.  We refer to this as "manual answer mode".  This
      mode is useful in "push to talk" applications where the sender
      requires a reassurance that somebody is listening.

   Req-3: It MUST be possible for UAS to apply local policy to each
      request and determine whether or not to provide the requested
      answer mode for this request.  This policy determination MAY
      include authentication checks, authorization against "buddy lists"



Willis & Allen           Expires October 3, 2006                [Page 7]

Internet-Draft             SIP Answering Modes                April 2006


      as used in some presence systems, or other mechanisms outside the
      scope of this specification.  This behavior is critical in
      avoiding major security pitfalls, such as turning the victim's
      phone into a "bug" or eavesdropping device.

   Req-4: It MUST be possible for UAC to indicate in the request that
      this extension for selecting answering mode is required, such that
      UAS MUST reject the request if it does not support this extension.
      This can be used to prevent automated diagnostic loopback requests
      from annoying the users of nodes not supporting this extension.

   Req-5: It MUST be possible for UAC to indicate at least two different
      priority levels for the desired answer mode.  We refer to these as
      "normal" and "override" priorities.  Policy at the user agent
      might be set differently for each priority level.  For example,
      policy might block automatic acceptance for "normal" priority
      requests, but allow it for "override" priority requests, if the
      authenticated requesting party is authorized for "override"
      priority.  In normal usage, we expect that "normal" priority would
      be used in a user-to-user fashion, whereas "override" priorities
      would be used for diagnostic procedures or some sorts of emergency
      session establishment.  This behavior allows a device to be set up
      such that it might not auto-answer routine calls, but could be
      convinced to auto-answer an emergency or other high-priority call.

   Req-6: It MUST be possible for UAS or proxies acting on behalf of UAS
      to apply policy relative to the indicated priority level.  This
      MAY include having different authentication and/or authorization
      procedures for each priority level.  This capability allows
      functions like time-of-day call screening, so that routine calls
      that would normally be rejected locally by the device would be
      blocked by a proxy without access network costs, but override-
      priority calls that would override routine call screening could be
      passed to the device.

   Req-7: It MUST be possible for UAS to indicate its support for the
      selection of answer modes in a REGISTER request so that the
      routing proxy can selectively route requests requiring the
      selection of answer mode to UAS.  This requirement enables the
      functions described in the next requirement.

   Req-8: It MUST be possible for the UAC to construct the request in
      such a way that the routing proxy infrastructure, if present, will
      select only contacts supporting the selection of answer modes.
      This can efficiently (minimal access network traffic and minimal
      forking load) prevent devices that do not support this extension
      from being reached by requests that require this extension.  This
      requirement does not include selection of a singular contact from



Willis & Allen           Expires October 3, 2006                [Page 8]

Internet-Draft             SIP Answering Modes                April 2006


      a set to which the request might be forked, i.e. the request would
      be forked to all contacts that support this extension.

   Req-9: It MUST be possible for UAC to discover whether UAS supports
      the selection of answer modes via a SIP OPTIONS request.

   Req-10: It MUST be possible for an intermediate proxy (none of which
      are depicted in the reference diagram above) acting on behalf of
      UAC or UAS to apply policy relative to the answer mode indicated
      in a request.  For example, a proxy may require special
      authentication and authorization for a request that places a high
      priority on auto-answer capabilities.  Application of policy here
      means altering the requested answer mode and/or inserting or
      deleting a request for a specific answer mode.

2.2.  Requirements for Indicating the Applied Answer  Mode in a Response

   The requirements relating to indicating which answering mode applied
   to the request include:


   Req-11: It MUST be possible for UAS when sending a positive response
      to a request to indicate the answering mode that applied to the
      request.  This allows UAC to inform UAC-user as to whether the
      request was answered automatically or as a result of user
      interaction.  This knowledge may be important to UAC-user's
      decision-making process about how to use the session.  For
      example, UAC-user may wish to ask "Are you there?" if the request
      was answered automatically.

   Req-12: It MUST be possible for UAS to maintain the privacy
      expectations of UAS-User.  According to local policy, UAS MAY
      either 1) not include information about which answering mode was
      applied in a response or 2) include misleading information about
      which answering mode was applied.  Consequently, applications MUST
      NOT rely on the veracity of this information in the response.


3.  Syntax of Header Field and Tags

   The following syntax uses ABNF as defined in [4].

   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



Willis & Allen           Expires October 3, 2006                [Page 9]

Internet-Draft             SIP Answering Modes                April 2006


      *(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".

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

   Note also that this syntax includes extension hooks, "token" for
   answer-modes values, and "generic-param" for optional parameters,
   that may be defined in future specifications extending this one.
   This specification defines only the behavior for the values given
   explicitly above.  In order to provide forward compatibility,
   implementations MUST ignore unknown values.


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 to
   request specific handling by the responding UAS related to "automatic
   answering" functionality for any dialog resulting from that 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 not accept the request (send a 200 OK)
      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 may
   include an optional parameter, "require".  If present, this parameter
   indicates that the UAS would prefer that the UAC reject the request
   if the UAC is unwilling (perhaps due to policy) to answer in the mode



Willis & Allen           Expires October 3, 2006               [Page 10]

Internet-Draft             SIP Answering Modes                April 2006


   requested, rather than answering in another mode.  For example, this
   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 [2], the SIP Identity
   mechanism [5], or (within the restricted networks for which it is
   suitable) the SIP mechanism for Asserted Identity Within Private
   Networks [6] and using 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.  Specific discussion of
   media sessions and appropriate policy is discussed under "Security
   Considerations", below.

4.1.  Procedures at User Agent Clients (UAC)

4.1.1.  All Requests

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

4.1.2.  REGISTER Transactions

   To indicate that it supports the answer-mode negotiation feature, a
   UA includes a SIP extension feature tag of:

       extensions="answermode"

   in the Contact: header field of its REGISTER requests.  This usage of
   feature tags is described in [7].

4.1.3.  INVITE Transactions

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

   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".




Willis & Allen           Expires October 3, 2006               [Page 11]

Internet-Draft             SIP Answering Modes                April 2006


       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".

       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".

       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".

       Answer-Mode: Auto;require

   To require that the UAS either support this extension or reject the
   request, the UAC includes a Require: header field having the value
   "answermode".  Note that 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.

       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 having a
   value of "extensions="answermode"".  This usage of Accept-Contact is
   described in [8].  Note that this would normally be used in
   conjunction with the "Require: answermode" header field as described
   above.

       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 having a
   value of "extensions="answermode"" and an option field of "require".
   This usage of Accept-Contact is described in [8].  Note that this



Willis & Allen           Expires October 3, 2006               [Page 12]

Internet-Draft             SIP Answering Modes                April 2006


   would normally be used in conjunction with the "Require: answermode"
   header field as described above.

      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 having a
   value of "extensions="answermode"" and option fields of "require" and
   "explicit".  This usage of Accept-Contact is described in [8].  Note
   that this would normally be used in conjunction with the "Require:
   answermode" header field as described above.

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

   The distinction between Answer-Mode and Priv-Answer-Mode relates to
   the level of authorization being claimed by the UAC and verified and
   policed by by the 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
   under the heading "Special Considerations for Priv-Answer-Mode".
   Note that 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 the "Priv-" part is equivalent to prefixing a UNIX command
   with "sudo".

4.2.  Procedures at Intermediate Proxies

4.2.1.  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 into a
   registered contact) MAY exercise control over the requested answer
   mode, either inserting or deleting an Answer-Mode header field or
   altering the value of an existing header field in accord with local
   policy.  Note that this may 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 must be
   done very carefully and only in the context of 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



Willis & Allen           Expires October 3, 2006               [Page 13]

Internet-Draft             SIP Answering Modes                April 2006


   to local policy, and SHOULD use the rejection codes as specified
   below for the UAS if they do so.

4.2.2.  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 UASes, and
   more than one of those responds with a 200 OK, the conventional
   approach is to continue the dialog with the first respondent, and
   tear down the dialog (via BYE) 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 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.

   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 UAs invited into that
   conference.  The mixer attached to the conference would then mediate
   traffic flows appropriately.

4.3.  Procedures at User Agent Servers (UAS)

4.3.1.  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 UASes or other local characteristics or policy.  For
   example, an auto-attendant or PSTN gateway system that always answers
   automatically would go ahead and answer, despite the presence of the
   "Manual" Answer-Mode header field value.




Willis & Allen           Expires October 3, 2006               [Page 14]

Internet-Draft             SIP Answering Modes                April 2006


   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
   and SHOULD do so with a "403 Forbidden" response and MAY include a
   reason string 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 string of
   "automatic answer forbidden".

   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 and
   SHOULD do so with a "403 Forbidden" response, and MAY include a
   reason string of "automatic answer forbidden"

4.3.2.  Special Considerations for Priv-Answer-Mode

   The Answer-Mode and Priv-Answer-Mode header fields have equivalent
   functions, except that Priv-Answer-Mode requests a higher level of
   privilege in granting the answering mode specified by the request.
   The model for this is that an "administrative level of privilege" is
   requested -- where "Answer-Mode" says "Please answer in the following
   mode, if your user preferences allow it", the Priv-Answer-Mode says
   "I command you to answer in the following mode, even if your user
   preferences would ordinarily disallow it".  The UAS MUST NOT grant
   this override capability to an unauthenticated UAC, and SHOULD apply
   a stricter authorization policy to a request with Priv-Answer-Mode
   header fields than it does to requests with Answer-Mode header
   fields.  The default policy SHOULD be to refuse requests containing
   "Priv-Answer-Mode" header fields.

   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



Willis & Allen           Expires October 3, 2006               [Page 15]

Internet-Draft             SIP Answering Modes                April 2006


   to provide a quiet environment for his car, so his communicator is
   configured for manual answer mode for push-to-talk calls.  Each time
   he gets a push-to-talk 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 (sending a 200 OK), 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 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 accepting the call.
   The operator speaks, and the resulting talk-burst is summarily played
   out the loudspeaker on Larry's communicator, waking him up.

   Note that 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.

   In normal operations, only one of "Answer-Mode" and "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.


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




Willis & Allen           Expires October 3, 2006               [Page 16]

Internet-Draft             SIP Answering Modes                April 2006


   The Answer-Mode header field or Priv-Answer-Mode may 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 may 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-class 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 that 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
   conversation would also reveal this same information, there might be
   cases where this information might need to be protected.
   Consequently, UAS supporting this specification SHOULD include
   appropriately configurable policy mechanisms for making this
   determination, and the default configuration SHOULD be to not include
   this header field in 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



Willis & Allen           Expires October 3, 2006               [Page 17]

Internet-Draft             SIP Answering Modes                April 2006


   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 responds with a 200 OK indicating that the call was
   answered automatically.

6.1.  REGISTER Request


       REGISTER sip:example.com SIP/2.0
       From: Bob <sip:bob@example.com>
       To: Bob <sip:bob@example.com>
       Contact: sip:cell-phone@example.com;\
          extensions="answermode";\
          methods="INVITE,BYE,OPTIONS,CANCEL,ACK"

6.2.  INVITE Request


       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








Willis & Allen           Expires October 3, 2006               [Page 18]

Internet-Draft             SIP Answering Modes                April 2006


   &#8776;
       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, with the most obvious being
   the ability to turn a phone into a remote listening device without
   its user being aware of it.  Additional potential attacks include
   reverse charge fraud, unsolicited "push to talk" communications (spam
   over push-to-talk or SPPTT), playout of obnoxious noises (the
   "whoopee cushion" attack), battery-rundown denial-of-service, "forced
   busy" denial of service, and phishing via session insertion (where an
   ongoing session is replaced by another without the victim's
   awareness).

   The existing body of SIP work provides strong capabilities for
   authentication of requests, prevention of man-in-the-middle attacks,
   protecting the privacy and integrity of media flows, and so on.  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.






Willis & Allen           Expires October 3, 2006               [Page 19]

Internet-Draft             SIP Answering Modes                April 2006


   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
   unbounded, we may 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 joke 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.


   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 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 may replicate
      both of the "whoopee-cushion" or the "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



Willis & Allen           Expires October 3, 2006               [Page 20]

Internet-Draft             SIP Answering Modes                April 2006


   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.

   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"), but the policy for the baby monitor would need to be very
   narrow (perhaps only "the baby's mother) or even completely closed.

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 may 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 may request that a UAS not deploy two of the five defensive
   mechanisms -- user alerting and user acceptance.  In order for this



Willis & Allen           Expires October 3, 2006               [Page 21]

Internet-Draft             SIP Answering Modes                April 2006


   to not produce undue risk of insertion attacks or any increased risk
   of interception attacks, the remaining defensive mechanisms must be
   carefully deployed.  In many cases, this comes down to effecting a
   constraint at the "if it hurts, don't do it" level, in other words,
   establishing a required (MUST-level) minimum policy threshold.

   To recap, we have five defense mechanisms available at the
   application level:


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

   2.  Policy -- Define rules about other factors.

   3.  Alerting -- Let the called user know what's happening.  Note that
       some applications can use inbound media as an alert.

   4.  Acceptance -- Require called user to make run-time decision.
       Note that asking the user to make a run-time decision without
       alerting the user to the need to make a decision is generally
       infeasible.

   5.  Limit the I/O -- Turn off loudspeakers or microphone.  Note that
       this may be used to convert a bidirectional media session into a
       unidirectional session while waiting for user acceptance.

   Since SIP and related work already provide several mechanisms
   (including SIP Digest Authentication [2], the SIP Identity mechanism
   [5], and the SIP mechanism for Asserted Identity Within Private
   Networks [6], 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.

   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.




Willis & Allen           Expires October 3, 2006               [Page 22]

Internet-Draft             SIP Answering Modes                April 2006


   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.

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.  Note that "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.


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 shall be added to the "Header Fields" section of
   the SIP parameter registry:

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




Willis & Allen           Expires October 3, 2006               [Page 23]

Internet-Draft             SIP Answering Modes                April 2006


   Editor Note: [RFCXXXX] should be replaced with the designation of
   this document.

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" may take the values "Manual" or "Auto".

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

   +------------------+----------------+-------------------+-----------+
   | Header Field     | Parameter Name | Predefined Values | Reference |
   +------------------+----------------+-------------------+-----------+
   | Answer-Mode      | Manual         | No                | [RFCXXXX] |
   | Answer-Mode      | Auto           | No                | [RFCXXXX] |
   | Priv-Answer-Mode | Manual         | No                | [RFCXXXX] |
   | Priv-Answer-Mode | Auto           | No                | [RFCXXXX] |
   +------------------+----------------+-------------------+-----------+

   Editor Note: [RFCXXXX] should be replaced with the designation of
   this document.

8.3.  Registration of SIP Option Tags

   This document defines the SIP option tag "answermode".

   The following row shall be added to the "Option Tags" section of the
   SIP Parameter Registry:

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

   Editor Note: [RFCXXXX] should be replaced with the designation of
   this document.


9.  Acknowledgements

   This document draws requirements and a large part of its methodology
   from the work of the Open Mobile Alliance, and specifically from the



Willis & Allen           Expires October 3, 2006               [Page 24]

Internet-Draft             SIP Answering Modes                April 2006


   internet draft [10] 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
   draft were broadly applicable to the SIP community, and that the
   concepts of alerting and answering should be clearly delineated.


10.  References

10.1.  Normative References

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

   [2]  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.

   [3]  Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B.
        Rosen, "Change Process for the Session Initiation Protocol
        (SIP)", BCP 67, RFC 3427, December 2002.

   [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

   [5]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
        Identity Management in the Session Initiation  Protocol (SIP)",
        draft-ietf-sip-identity-06 (work in progress), October 2005.

   [6]  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.

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

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

10.2.  Informative References

   [9]   Hedayat, K., "An Extension to the Session Initiation Protocol
         (SIP) for Media Loopback", draft-hedayat-media-loopback-01
         (work in progress), October 2004.



Willis & Allen           Expires October 3, 2006               [Page 25]

Internet-Draft             SIP Answering Modes                April 2006


   [10]  Allen, A., "Private Header (P-Header) Extensions to the Session
         Initiation Protocol  (SIP) for the Open Mobile Alliance (OMA)
         Push to talk over Cellular (PoC)",
         draft-allen-sipping-poc-p-headers-01 (work in progress),
         February 2005.














































Willis & Allen           Expires October 3, 2006               [Page 26]

Internet-Draft             SIP Answering Modes                April 2006


Authors' Addresses

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

   Phone: unlisted
   Fax:   unlisted
   Email: dean.willis@softarmor.com


   Andrew Allen
   Research in Motion (RIM)
   122 West John Carpenter Parkway, Suite 430
   Irving, Texas  75039
   USA

   Phone: unlisted
   Fax:   unlisted
   Email: aallen@rim.com





























Willis & Allen           Expires October 3, 2006               [Page 27]

Internet-Draft             SIP Answering Modes                April 2006


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




Willis & Allen           Expires October 3, 2006               [Page 28]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/