SIP Working Group D. Willis, Ed.
Internet-Draft Cisco Systems
Expires: June 17, 2006 A. Allen
Research in Motion (RIM)
December 14, 2005
Requesting Answering Modes for the Session Initiation Protocol (SIP)
draft-ietf-sip-answermode-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
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-
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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].
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Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Requirements for Requesting an Answering Mode . . . . . . 6
2.2. Requirements for Indicating the Applied Answer Mode
in a Response . . . . . . . . . . . . . . . . . . . . . . 8
3. Syntax of Header Field and Tags . . . . . . . . . . . . . . . 8
4. Usage of the Answer-Mode and Priv-Answer-Mode Header
Fields in Requests . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Procedures at the UAC . . . . . . . . . . . . . . . . . . 10
4.1.1. All Requests . . . . . . . . . . . . . . . . . . . . . 10
4.1.2. REGISTER Transactions . . . . . . . . . . . . . . . . 10
4.1.3. INVITE Transactions . . . . . . . . . . . . . . . . . 10
4.2. Procedures at Intermediate Proxies . . . . . . . . . . . . 12
4.2.1. General Proxy Behavior . . . . . . . . . . . . . . . . 12
4.2.2. Issues with Automatic Answering and Forking . . . . . 12
4.3. Procedures at the UAS . . . . . . . . . . . . . . . . . . 13
4.3.1. INVITE Transactions . . . . . . . . . . . . . . . . . 13
4.3.2. Special Considerations for Priv-Answer-Mode . . . . . 14
5. Usage of the Answer-Mode and Priv-Answer-Mode Header
Fields in a Response . . . . . . . . . . . . . . . . . . . . . 15
5.1. Procedures at the UAS . . . . . . . . . . . . . . . . . . 15
5.2. Procedures at the UAC . . . . . . . . . . . . . . . . . . 16
6. Examples of Usage . . . . . . . . . . . . . . . . . . . . . . 16
6.1. REGISTER Request . . . . . . . . . . . . . . . . . . . . . 16
6.2. INVITE Request . . . . . . . . . . . . . . . . . . . . . . 16
6.3. 200 OK response . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Attack Sensitivity Depends on Media Characteristics . . . 18
7.2. Application Design Affects Attack Opportunity . . . . . . 19
7.3. Applying the Analysis . . . . . . . . . . . . . . . . . . 20
7.4. Minimal Policy Requirement . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8.1. Registration of Header Fields . . . . . . . . . . . . . . 21
8.2. Registration of Header Field Parameters . . . . . . . . . 21
8.3. Regsitration of SIP Option Tags . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
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10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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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 intercomm 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
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call.
It should be noted that the above list of use caes is not exhaustive.
Ther are presumably many more use caes 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". Isues 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).
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.
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Req-2: It MUST be possible for UAC to ask that the 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"
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 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.
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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. Note
that this requirement does NOT include selection of a singular UAS
from a set to which the request might be forked.
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 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, knowledge that may be important in informing UAC-
user's usage of the session.
Req-12: It MUST be possible for UAS supporting this specification to
maintain the privacy expectations of their users. According to
local policy, they 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-param *(SEMI
answer-mode-option)
Priv-Answer-Mode = "Priv-Answer-Mode" HCOLON answer-mode-param
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*(SEMI answer-mode-option)
answer-mode-param = "Manual" / "Auto" / generic-param
answer-mode-option = "require" / token
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, "generic-param"
for parameters, and "token" for options, that may be defined in
future specifications extending this one. This specification defines
only the behavior for the values given explicitly above.
Consequentially, implementations 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 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
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
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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 the 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 "extension="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 and appropriate parameter 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 a Answer-Mode or Priv-Answer-Mode header field having a
parameter of "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 a Answer-Mode or Priv-Answer-Mode header field having a
parameter of "Manual" and an option of "require".
To request that the UAS answer automatically without waiting for
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input from the user, the UAC includes a Answer-Mode or Priv-Answer-
Mode header field having a parameter of "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 a Answer-Mode or Priv-Answer-Mode header field
having a parameter of "Auto" and an option of "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.
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
parameter of "extension="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.
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
parameter of "extension="answermode"" and an option field of
"require". 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.
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
parameter of "extension=""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.
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
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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 prefxing 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 a 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. These serving
proxies MAY also reject a request according 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 UAS, and more
than one of those UAS respond 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
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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 the UAS
4.3.1. INVITE Transactions
For a request having an Answer-Mode parameter of "Manual" and not
having an Answer-Mode option 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 UAS 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.
For a request having an Answer-Mode parameter of "Manual" and an
Answer-Mode option 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 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 a 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 parameter of "Auto" and an
Answer-Mode option 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
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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
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
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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 requestor for Priv-
Answer-Mode, and if authorized, process the request as if only Priv-
Answer-Mode had been included. If the requestor 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
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 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 a 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 a Answer-Mode header field in the
request that it would probably like to see a Answer-Mode header field
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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
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 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 asnwering
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
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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>
Requires: answermode
Accept-contact:*;require;explicit;extensions="answermode"
Answer-Mode: Auto
Content-Type: application/sdp
Content-Length: ...
6.3. 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, 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
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addressed by existing SIP work, and therefore require analysis in
this document.
Media attacks can be loosely categorized as:
Insertion: Media is inserted into and played out by the victim UA
without consent of the UA's user.
Interception: The victim UA's media acquisition facility (such as a
microphone or camera) is activated, producing a media stream,
without the consent of the UA's user.
7.1. Attack Sensitivity Depends on Media Characteristics
The danger of abuse varies greatly depending on the media
characteristics of the session being established. Since the
expressive range of media sessions that can be established by SIP is
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
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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 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.
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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
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 nee o 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
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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.
User agents implementing this specification MUST NOT establish a
session providing outbound or bidirectional media without explicit
user acceptance.
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] |
+------------------+--------------+-----------+
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".
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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. Regsitration 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
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.
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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.
[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.
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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
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