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Network Working Group D. Willis, Ed.
Request for Comments: 5373 Softarmor Systems
Category: Standards Track A. Allen
Research in Motion (RIM)
November 2008
Requesting Answering Modes for the Session Initiation Protocol (SIP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD1) for the standardization state and
status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2008 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This document extends SIP with two header fields and associated
option tags that can be used in INVITE requests to convey the
requester's preference for user-interface handling related to
answering of that request. The first header, "Answer-Mode",
expresses a preference as to whether the target node's user interface
waits for user input before accepting the request or, instead,
accepts the request without waiting on user input. The second
header, "Priv-Answer-Mode", is similar to the first, except that it
requests administrative-level access and has consequent additional
authentication and authorization requirements. These behaviors have
applicability to applications such as push-to-talk and to diagnostics
like loop-back. Usage of each header field in a response to indicate
how the request was handled is also defined.
Willis & Allen Standards Track [Page 1]
RFC 5373 SIP Answering Modes November 2008
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Syntax of Header Fields and Option Tags . . . . . . . . . . . 5
3. Usage of the Answer-Mode and Priv-Answer-Mode Header Fields . 6
4. Usage of the Answer-Mode and Priv-Answer-Mode Header
Fields in Requests . . . . . . . . . . . . . . . . . . . . . . 6
4.1. The Difference Between Answer-Mode and Priv-Answer-Mode . 7
4.2. The "require" Modifier . . . . . . . . . . . . . . . . . . 9
4.3. Procedures at User Agent Clients (UAC) . . . . . . . . . . 9
4.3.1. All Requests . . . . . . . . . . . . . . . . . . . . . 9
4.3.2. REGISTER Transactions . . . . . . . . . . . . . . . . 9
4.3.3. INVITE Transactions . . . . . . . . . . . . . . . . . 10
4.4. Procedures at Intermediate Proxies . . . . . . . . . . . . 12
4.4.1. General Proxy Behavior . . . . . . . . . . . . . . . . 12
4.4.2. Issues with Automatic Answering and Forking . . . . . 12
4.5. Procedures at User Agent Servers (UAS) . . . . . . . . . . 13
4.5.1. INVITE Transactions . . . . . . . . . . . . . . . . . 13
5. Usage of the Answer-Mode and Priv-Answer-Mode Header
Fields in Responses . . . . . . . . . . . . . . . . . . . . . 14
5.1. Procedures at the UAS . . . . . . . . . . . . . . . . . . 14
5.2. Procedures at the UAC . . . . . . . . . . . . . . . . . . 15
6. Examples of Usage . . . . . . . . . . . . . . . . . . . . . . 15
6.1. REGISTER Request . . . . . . . . . . . . . . . . . . . . . 15
6.2. INVITE Request . . . . . . . . . . . . . . . . . . . . . . 16
6.3. 200 (OK) Response . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Attack Sensitivity Depends on Media Characteristics . . . 17
7.2. Application Design Affects Attack Opportunity . . . . . . 19
7.3. Applying the Analysis . . . . . . . . . . . . . . . . . . 19
7.4. Minimal Policy Requirement . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8.1. Registration of Header Fields . . . . . . . . . . . . . . 22
8.2. Registration of Header Field Parameters . . . . . . . . . 22
8.3. Registration of SIP Option Tags . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 24
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RFC 5373 SIP Answering Modes November 2008
1. Background
The conventional model for session establishment using the Session
Initiation Protocol (SIP, [RFC3261]) involves 1) sending a request
for a session (a SIP INVITE) and notifying the user receiving the
request, 2) acceptance of the request and of the session by that
user, and 3) the sending of a response (SIP 200 OK) back to the
requester before the session is established. Some usage scenarios
deviate from this model, specifically with respect to the
notification and acceptance phase. While it has always been possible
for the node receiving the request to skip the notification and
acceptance phases, there has been no standard mechanism for the party
sending the request to specifically indicate a desire (or
requirement) for this sort of treatment. This document defines a SIP
extension header field that can be used to request specific treatment
related to the notification and acceptance phase.
The first usage scenario is the requirement for diagnostic loopback
calls. In this sort of scenario, a testing service sends an INVITE
to a node being tested. The tested node accepts and a dialog is
established. But rather than establishing a two-way media flow, the
tested node loops back or "echoes" media received from the testing
service back toward the testing service. The testing service can
then analyze the media flow for quality and timing characteristics.
Session Description Protocol (SDP) usage for this sort of flow is
described in [LOOPBACK]. In this sort of application, it might not
be necessary that the human using the tested node interact with the
node in any way for the test to be satisfactorily executed. In some
cases, it might be appropriate to alert the user to the ongoing test,
and in other cases it might not be.
The second scenario is that of push-to-talk applications, which have
been specified by the Open Mobile Alliance. In this sort of
environment, SIP is used to establish a dialog supporting
asynchronous delivery of unidirectional media flow, providing a user
experience like that of a traditional two-way radio. It is
conventional for the INVITES used to be automatically accepted by the
called UA (User Agent), and the media is commonly played out on a
loudspeaker. The called party's UA's microphone is not engaged until
the user presses the local "talk" button to respond.
A third scenario is the Private Branch Exchange (PBX) attendant.
Traditional office PBX systems often include intercom functionality.
A typical use for the intercom function is to allow a receptionist to
activate a loudspeaker on a desk telephone in order to announce a
visitor. Not every caller can access the loudspeaker, only the
Willis & Allen Standards Track [Page 3]
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receptionist or operator, and it is not expected that these callers
will always want "intercom" functionality -- they might instead want
to make an ordinary call.
There are presumably many more use cases for the extensions defined
in this specification, but this document was developed to
specifically meet the requirements of these scenarios, or others with
essentially similar properties.
These sorts of mechanisms are not required to provide the
functionality of an "answering machine" or "voice mail recorder".
Such a device knows that it is expected to answer and does not
require a SIP extension to support its behavior.
Much of the discussion of this topic in working group meetings and on
the mailing list dealt with differentiating "answering mode" from
"alerting mode". Some early work did not make this distinction. We
therefore proceed with the following definitions:
o Answering Mode includes behaviors in a SIP UA relating to
acceptance or rejection of a request that are contingent on
interaction between the UA and the user of that UA after the UA
has received the request. We are principally concerned with the
user interaction involved in accepting the request and initiating
an active session. An example of this might be pressing the "yes"
button on a mobile phone.
o Alerting Mode includes behaviors in a SIP UA relating to informing
the user of the UA that a request to initiate a session has been
received. An example of this might be activating the ring tone of
a mobile phone.
This document deals only with "Answering Mode". Issues relating to
"Alerting Mode" are outside its scope.
This document defines two SIP extension header fields: "Answer-Mode"
and "Priv-Answer-Mode". These two extensions take the same
parameters and operate in the same general way.
The distinction between Answer-Mode and Priv-Answer-Mode relates to
the level of authorization claimed by the User Agent Client (UAC) and
verified and policed by the User Agent Server (UAS). Requests are
usually made using Answer-Mode. Requests made using Priv-Answer-Mode
request "privileged" treatment from the UAS. This mechanism is
discussed in greater detail below, in Section 4.1.
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Priv-Answer-Mode is not an assertion of privilege. Instead, it is a
request for privileged treatment. This is similar to the UNIX model,
where a user might run a command normally or use "sudo" to request
administrative privilege for the command. Including "Priv-" is
equivalent to prefixing a UNIX command with "sudo". In other words,
a separate policy table (like "/etc/sudoers") is consulted to
determine whether the user may receive the requested treatment.
This distinction is discussed in greater detail in Section 4.1.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Syntax of Header Fields and Option Tags
The following syntax uses ABNF as defined in [RFC5234]. Further, it
relies on the syntax for SIP defined in [RFC3261].
The syntax for the header fields defined in this document is:
Answer-Mode = "Answer-Mode" HCOLON answer-mode-value
*(SEMI answer-mode-param)
Priv-Answer-Mode = "Priv-Answer-Mode" HCOLON answer-mode-value
*(SEMI answer-mode-param)
answer-mode-value = "Manual" / "Auto" / token
answer-mode-param= "require" / generic-param
The SIP option tag indicating support for this extension is
"answermode".
For implementors: SIP header field names and values are always
compared in a case-insensitive manner. The pretty capitalization
is just for readability.
This syntax includes extension hooks ("token" for answer-mode values
and "generic-param" for optional parameters) that could be defined in
future. This specification defines only the behavior for the values
given explicitly above. In order to provide forward compatibility,
implementations MUST ignore unknown values.
Willis & Allen Standards Track [Page 5]
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3. Usage of the Answer-Mode and Priv-Answer-Mode Header Fields
This document defines usage of the Answer-Mode and Priv-Answer-Mode
header fields in initial (dialog-forming) SIP INVITE requests and in
200 (OK) responses to those requests. This document specifically
does not define usage in any other sort of request or response,
including but not limited to ACK, CANCEL, or any mid-dialog usage.
This limitation stems from the intended usage of this extension,
which is to affect the way that users interact with communications
devices when requesting new communications sessions and when
responding to such requests. This sort of interaction occurs only
during the formation of a dialog and its initial usage, not during
subsequent operations such as re-INVITE. However, the security
aspects of the session initiation must be applied to changes in media
description introduced by re-INVITES or similar requests. See
Section 7.1 for further discussion of this issue.
4. Usage of the Answer-Mode and Priv-Answer-Mode Header Fields in
Requests
The Answer-Mode or Priv-Answer-Mode header field is used by a UAC in
an INVITE request to invoke specific handling by the responding UAS;
this handling is related to "automatic answering" functionality for
any dialog resulting from that INVITE request. If no Answer-Mode or
Priv-Answer-Mode header field is included in the request, answering
behavior is at the discretion of the UAS, as it would be in the
absence of this specification. The desired handling is indicated by
the value of the Answer-Mode or Priv-Answer-Mode header field, as
follows:
Manual: The UAS is asked to defer accepting the request until the
user of the UAS has interacted with the user interface (UI) of the
UAS in such a way as to indicate that the user desires the UAS to
accept the request.
Auto: The UAS is asked to accept the request automatically, without
waiting for the user of the UAS to interact with the UI of the UAS
in such a way as to indicate that the user desires the UAS to
accept the request.
Each value of the Answer-Mode or Priv-Answer-Mode header field can
include an optional parameter, "require". If present, this parameter
indicates that the UAC would prefer that the UAS reject the request
if the UAS is unwilling (perhaps due to policy) to answer in the mode
requested, rather than answering in another mode. For example, this
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parameter could be used to make sure that a test "loopback" call
doesn't disturb a user who has configured her phone to manually
answer even if the caller requests an automatic answer.
The UAS is responsible for deciding how to honor this preference. In
general, the UAS makes an authorization decision based on the
authenticated identity presented in the request using authentication
mechanisms such as SIP Digest Authentication [RFC3261], the SIP
Identity mechanism [RFC4474], or (within the restricted networks for
which it is suitable) the SIP mechanism for asserted identity within
trusted networks [RFC3325]. When making an authorization decision,
the UAS should also use authorization information or policy available
to the UAS. This decision-making MUST consider the risk model of the
media session corresponding to the request, and the UAS MUST NOT
answer without user input in cases where the privacy or security of
the user would be compromised as a result. Making this determination
is a matter of system or application design, and cannot in general be
addressed by having a set of functions that are configurable on or
off. Specific discussion of media sessions and appropriate policy is
discussed in Section 7.
4.1. The Difference Between Answer-Mode and Priv-Answer-Mode
The functions of the Answer-Mode and Priv-Answer-Mode header fields
are similar; they both ask that the UAS handle the request as
specified by the header field's value (automatic or manual). The
difference is in the way the requests interact with the UAS's policy.
A typical UAS will have different policies for handling each header
field. For example, assume that the user of a UAS has placed that
UAS into "meeting mode", indicating that she is engaged in an
important activity and does not wish to be spuriously interrupted.
The UAS might disallow automatic answering for Answer-Mode requests
while in "meeting mode". However, that UAS might allow automatic
answering for requests made with Priv-Answer-Mode. There will
probably be differences in authorization policy. For example, a UAS
might be configured such that callers on the "friends" list are
allowed to make requests using Answer-Mode but not Priv-Answer-Mode.
That same UAS might be configured to only allow callers on the
"administrators" list to use Priv-Answer-Mode. This is different
from always basing the behavior on the identity of the calling party.
For example, assume caller "Bob" is on both the "friends" list and
"administrators" list. If Bob wants his request to be processed
according to the regular policy, he uses Answer-Mode. If Bob wants
his request to be processed under the more restrictive "privileged"
policy, he uses Priv-Answer-Mode.
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A UAS SHOULD apply a stricter authorization policy to a request with
Priv-Answer-Mode than it does to requests with Answer-Mode. The
default policy SHOULD be to refuse requests containing Priv-Answer-
Mode header fields unless the requester is authenticated and
specifically authorized to make Priv-Answer-Mode requests. Failure
to enforce such a policy leaves the user potentially vulnerable to
abuses, as discussed in Section 7.
The use case envisioned for Priv-Answer-Mode relates to handling
urgent requests from authorized callers. For example, assume Larry
is a limousine driver working with a fleet dispatcher. Larry likes
to provide a quiet environment for his car, so his communicator is
configured for manual answer mode for all non-privileged calls,
including push-to-talk (Answer-Mode: Auto) calls. Each time he gets
a call, Larry's communicator chimes softly to alert him to the call.
If the circumstances permit it, Larry presses the communicator in
order to accept the call, the communicator sends a 200 (OK) response,
and the calling party's talk-burst is played out through the
communicator's loudspeaker. This treatment is delivered to incoming
requests that have an Answer-Mode header field having values of
"Manual" or "Auto" (or no Answer-Mode header field at all), no matter
who the caller is.
Larry's fleet dispatch operator is familiar with this policy, and
needs to inform Larry about a critical matter. The dispatch operator
tries several times to push-to-talk call Larry (including Answer-
Mode: Auto in the requests), but the calls aren't accepted because
Larry has fallen asleep, and therefore isn't pressing his
communicator to accept the call.
The operator then presses his "urgent" button and calls Larry again.
This time, the INVITE request carries a "Priv-Answer-Mode: Auto"
header field. Larry's communicator checks the identity of the caller
(using a SIP Identity assertion or functionally equivalent
mechanism), and matches the operator's identity against the list of
users allowed to do Priv-Answer-Mode. Since the operator is listed,
the communicator immediately returns a 200 (OK) response accepting
the call. The operator speaks, and the resulting talk-burst is
summarily played out the loudspeaker on Larry's communicator, waking
him up.
The effect of requesting Priv-Answer-Mode is different than the
effect of simply granting higher privilege to an Answer-Mode request
based on the requester's identity and corresponding authorization
level. This distinction is what allows the fleet operator to make
polite (Answer-Mode: Auto) requests to Larry under normal conditions,
and receive different handling (Priv-Answer-Mode: Auto) for a request
having greater urgency.
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In normal operations, only one of either Answer-Mode or Priv-Answer-
Mode would be used in an INVITE request. If both are present, the
UAS will first test the authorization of the requester for Priv-
Answer-Mode and, if authorized, process the request as if only Priv-
Answer-Mode had been included. If the requester is not authorized
for Priv-Answer-Mode, then the UAS will process the request as if
only Answer-Mode had been included.
4.2. The "require" Modifier
Both Answer-Mode and Priv-Answer-Mode allow a modifier of "require"
(example: "Priv-Answer-Mode: Auto;require"). This modifier does not
influence the UAS's policy in choosing whether to answer manually or
automatically. The UAS decides whether or not to answer
automatically based on other aspects of the request. The "require"
modifier is only evaluated after the UAS has selected an answering
mode. If the UAS's policy has resulted in an answering mode that is
different from that specified in the request, the presence of the
"require" modifier asks the UAS to reject the call. In the given
example, the UAS is being asked to answer automatically if the caller
is authorized for automatic answering under the "privileged" policy,
and to reject the call (rather than answering manually) if the caller
is not authorized for this mode. This is discussed in more depth in
Section 4.5.
4.3. Procedures at User Agent Clients (UAC)
4.3.1. All Requests
A UA supporting the Answer-Mode and Priv-Answer-Mode header fields
SHOULD indicate its support by including an option tag of
"answermode" in the Supported header field of all requests it sends.
4.3.2. REGISTER Transactions
To indicate that it supports the answer-mode negotiation feature, a
UA MAY include an extensions parameter with a value that includes
"answermode". Example:
;extensions="answermode,100rel,gruu"
in the Contact header field of its REGISTER requests. This usage of
feature tags is described in [RFC3840].
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If a UA is dependent on support for callee capabilities in the
registrar, it MAY include a Require header field with the value
"pref" in its REGISTER request. This will cause the registrar to
reject the request if the registrar does not support callee
capabilities and caller preferences. Example:
Require: pref
4.3.3. INVITE Transactions
A UAC supporting this specification MAY include an Answer-Mode or
Priv-Answer-Mode header field in an INVITE where it wishes to
influence the answering mode of the responding UAS.
Note: This is meaningful only in initial or dialog-forming INVITE
requests. Answer-Mode and Priv-Answer-Mode header fields
appearing in other requests are ignored. In general, if the
request would not normally result in a notification to the user
and acceptance by that user (for example, "ringing" and
"answering"), then these extensions are not applicable.
To request that the UAS answer only after having interacted with its
user and receiving an affirmative instruction from that user, the UAC
includes an Answer-Mode or Priv-Answer-Mode header field having a
value of "Manual". Example:
Answer-Mode: Manual
To request that the UAS answer manually, and ask that it reject the
INVITE request if unable or unwilling to answer manually, the UAC
includes an Answer-Mode or Priv-Answer-Mode header field having a
value of "Manual" and a parameter of "require". Example:
Answer-Mode: Manual;require
To request that the UAS answer automatically without waiting for
input from the user, the UAC includes an Answer-Mode or Priv-Answer-
Mode header field having a value of "Auto". Example:
Answer-Mode: Auto
To request that the UAS answer automatically, and ask that it reject
the INVITE request if unable or unwilling to answer automatically,
the UAC includes an Answer-Mode or Priv-Answer-Mode header field
having a value of "Auto" and a parameter of "require". Example:
Answer-Mode: Auto;require
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To require that the UAS either support this extension or reject the
request, the UAC includes a Require header field having the value
"answermode". This does not actually force the UAS to automatically
answer, it just requires that the UAS either understand this
extension or reject the request. We do not have a SIP negotiation
technique to force specific behavior. Rather, the desired behavior
is indicated in the SIP extension itself. Example:
Require: answermode
To request that retargeting proxies in the path preferentially select
targets that have indicated support for this extension in their
registration, a UAC includes an Accept-Contact header field with an
extensions parameter having a value of "answermode". This usage of
Accept-Contact is described in [RFC3841]. This would normally be
used in conjunction with the "Require: answermode" header field as
described above. Example:
Require: answermode Accept-Contact:
*;extensions="answermode";methods="INVITE"
To request that retargeting proxies in the path do not select targets
that have indicated non-support for this extension in their
registration, a UAC includes an Accept-Contact header field with an
extensions parameter having a value of "answermode" and an option
field of "require". This usage of Accept-Contact is described in
[RFC3841]. This would normally be used in conjunction with the
"Require: answermode" header field as described above. Example:
Require: answermode Accept-Contact:
*;extensions="answermode"; methods="INVITE";require
To request that retargeting proxies in the path exclusively select
targets that have indicated support for this extension in their
registration, a UAC includes an Accept-Contact header field
extensions parameter having a value of "answermode" and options of
"require" and "explicit". This usage of Accept-Contact is described
in [RFC3841]. This would normally be used in conjunction with the
"Require: answermode" header field as described above. Example:
Require: answermode Accept-Contact:
*;extensions="answermode";
methods="INVITE";require;explicit
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4.4. Procedures at Intermediate Proxies
4.4.1. General Proxy Behavior
The general procedure at all intermediate proxies, including the
UAC's serving proxy or proxies and the UAS's serving proxy or
proxies, is to ignore the Answer-Mode header field. However, the
serving proxies (proxies responsible for resolving an address-of-
record (AOR) into a registered contact) MAY exercise control over the
requested answer mode, either inserting or deleting an Answer-Mode or
Priv-Answer-Mode header field or altering the value of an existing
header field, in accord with local policy. This could result in
behavior that is inconsistent with user expectations (such as having
a call that was intended to be a diagnostic loopback answered by a
human) and consequently proxies MUST NOT insert, delete, or alter
Answer-Mode or Priv-Answer-Mode header fields unless explicitly
authorized to do so by an external agreement between the proxy
operator and the user of the UA that the proxy is serving. These
serving proxies MAY also reject a request according to local policy
and, if they do so, SHOULD use the rejection codes as specified below
for the UAS.
4.4.2. Issues with Automatic Answering and Forking
One of the well-known issues with forking is the problem of multiple
acceptance. If an INVITE request is forked to several UASs and more
than one replies with a 200 (OK) response, the conventional approach
is to continue the dialog with the first respondent and tear down the
dialog (using BYE requests) with all other respondents.
While this problem exists without an auto-answer negotiation
capability, it is apparent that widespread adoption of UAs that
engage in auto-answer behavior will exacerbate the multiple
acceptance problem. Consequently, systems designers need to take
this aspect into consideration. In general, auto-answer is NOT
RECOMMENDED in environments that include parallel forking.
As an alternative, it might be reasonable to use a variation on
manual-answer combined with no alerting and early media. In this
approach, the initial message or talk-burst is transmitted as early
media to all recipients, where it is displayed or played out. Any
response utterance (pushing the transmit key and talking) from the
user of a UAS following this would serve as an "acceptance",
resulting in a 200 (OK) response being transmitted by their UAS.
Consequently, the race-condition for acceptance would be limited to
the subset of UAs actually responding under user control, rather than
the full set of UAs to which the request was forked.
Willis & Allen Standards Track [Page 12]
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Another alternative would be to use dynamic conferencing instead of
forking. In this approach, instead of forking the request, a
conference would be initiated and all registered UAs invited into
that conference. The mixer attached to the conference would then
mediate traffic flows appropriately.
4.5. Procedures at User Agent Servers (UAS)
4.5.1. INVITE Transactions
For a request having an Answer-Mode value of "Manual" and not having
an Answer-Mode parameter of "require", the UAS SHOULD defer accepting
the request until the user of the UAS has confirmed willingness to
accept the request. This behavior MAY be altered as needed for
unattended UASs or other local characteristics or policy. For
example, an auto-attendant or Public Switched Telephone Network
(PSTN) gateway system that always answers automatically would go
ahead and answer, despite the presence of the "Manual" Answer-Mode
header field value.
For a request having an Answer-Mode value of "Manual" and an Answer-
Mode parameter of "require", the UAS MUST defer accepting the request
until the user of the UAS has confirmed willingness to accept the
request. If the UAS is not capable of answering the request in this
"Manual" mode or is unwilling to do so, it MUST reject the request,
SHOULD do so with a "403 (Forbidden)" response, and MAY include a
reason phrase of "manual answer forbidden".
For a request having an Answer-Mode value of "Auto", the UAS SHOULD,
if the calling party is authenticated and authorized for automatic
answering, accept the request without further user input. The UAS
MAY, according to local policy or user preferences, treat this
request as it would treat a request having an Answer-Mode with a
value of "Manual" or having no Answer-Mode header field. If the
calling party is not authenticated and authorized for automatic
answer, the UAS MAY either handle the request as per "manual", or
reject the request. If the UAS rejects the request, it SHOULD do so
with a "403 (Forbidden)" response, and MAY include a reason phrase of
"automatic answer forbidden". There may be an interaction with
[RFC3261] section 23.2, which in some cases requires user validation
of certificates used for S/MIME. Since this places the same
interrupt burden on the user as would manually answering the request,
a UAS experiencing this requirement for user validation of a request
that requires automatic answering SHOULD reject the request with a
"403 (Forbidden)" response and MAY include a reason phrase of
"certificate validation requires user input not compatible with
automatic answer."
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For a request having an Answer-Mode value of "Auto" and an Answer-
Mode parameter of "require", the UAS SHOULD, if the calling party is
authenticated and authorized for automatic answering, accept the
request. The UAS MUST NOT allow "manual" answer of this request, but
MAY reject it. If, for whatever reason, the UAS chooses not to
accept the request automatically, the UAS MUST reject the request,
SHOULD do so with a "403 (Forbidden)" response, and MAY include a
reason phrase of "automatic answer forbidden".
Similar behavior applies for Priv-Answer-Mode, except that the policy
for authorization may be different (and generally more stringent).
5. Usage of the Answer-Mode and Priv-Answer-Mode Header Fields in
Responses
The Answer-Mode or Priv-Answer-Mode header field can be inserted by a
UAS into a response in order to indicate how it handled the
associated request with respect to automatic answering functionality.
The UAC might use this information to inform the user or otherwise
adapt the behavior of the user interface. The handling is indicated
by the value of the header field, as follows:
Manual: The UAS responded after the user of the UAS interacted with
the user interface (UI) of the UAS in such a way as to indicate
that the user desires the UAS to accept the request.
Auto: The UAS responded automatically, without waiting for the user
of the UAS to interact with the UI of the UAS in such a way as to
indicate that the user desires the UAS to accept the request.
The Answer-Mode and Priv-Answer-Mode header fields, when used in
responses, are only valid in a 200 (OK) response to an INVITE
request.
5.1. Procedures at the UAS
A UAS supporting this specification inserts an Answer-Mode or Priv-
Answer-Mode header field into the 200 (OK) response to an INVITE
request when it wishes to inform the UAC as to whether the request
was answered manually or automatically. It is reasonable for a UAS
to assume that if the UAC included an Answer-Mode header field in the
request, it would probably like to see an Answer-Mode header field in
the response. The full rationale for including or not including this
header field in a response is outside of the scope of this
specification, and is sensitive to the privacy concerns of the user
of the UAS. For example, informing the calling party that a call was
answered manually might reveal the presence of an "actual human" at
the responding UAS. While in the general case the ensuing
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conversation would also reveal this same information, there might be
cases where this information might need to be protected.
Consequently, UASs supporting this specification SHOULD include
appropriately configurable policy mechanisms for making this
determination, and the default configuration SHOULD be to exclude
this header field from responses.
5.2. Procedures at the UAC
A UAC MAY use the value of the Answer-Mode or Priv-Answer-Mode header
field, if present, to adapt the user interface and/or inform the user
about the handling of the request. For example, the user of a push-
to-talk system might speak differently if she knows that the called
party answered "in person" vs. having the call blare out of an
unattended speaker phone.
6. Examples of Usage
The following examples show Bob registering a contact that supports
the negotiation of answering mode. Alice then calls Bob with an
INVITE request, asking for automatic answering and explicitly asking
that the request not be routed to contacts that have not indicated
support for this extension. Further, Alice requires that the request
be rejected if Bob's UA does not support negotiation of answering
mode. Bob replies with a 200 (OK) response indicating that the call
was answered automatically.
The Content-Length header field shown in the examples contains a
placeholder "..." instead of a valid Content-Length. Furthermore,
the SDP bodies that would be expected in the INVITE requests and
200 (OK) responses are not shown.
6.1. REGISTER Request
In the following example, Bob's UA is registering and indicating that
it supports the answermode extension.
REGISTER sip:example.com SIP/2.0
From: Bob<sip:bob@example.com>
To: Bob <sip:bob@example.com>
CallID: hh89as0d-asd88jkk@cell-phone.example.com
CSeq: 1 REGISTER
Contact: sip:cell-phone.example.com;
;audio
;+sip.extensions="answermode"
;methods="INVITE,BYE,OPTIONS,CANCEL,ACK"
;schemes="sip"
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6.2. INVITE Request
In this example, Alice is calling Bob and asking Bob's UA to answer
automatically. However, Alice is willing for Bob to answer manually
if Bob's policy is to prefer manual answer, so Alice does not include
a ";require" modifier on "Answer-Mode: Auto".
INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/TCP client-alice.example.com:5060; branch=z9hG4bK74b43
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Bob <sip:bob@example.com>
Call-ID:3848276298220188511@client-alice.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Require: answermode
Accept-contact:*;require;explicit;extensions="answermode"
Answer-Mode: Auto
Content-Type: application/sdp
Content-Length: ...
6.3. 200 (OK) Response
Here, Bob has accepted the call and his UA has answered
automatically, which it indicates in the 200 (OK) response.
SIP/2.0 200 OK
Via: SIP/2.0/TCP client-alice.example.com:5060; branch=z9hG4bK74b43
From: Alice <sip:alice@example.com>;tag=9fxced76sl
To: Bob <sip:bob@example.com>;tag=8321234356
Call-ID: 3848276298220188511@client-alice.example.com
CSeq: 1 INVITE
Contact: <sip:bob@client.biloxi.example.com;transport=tcp>
Answer-Mode: Auto
Content-Type: application/sdp
Content-Length: ...
7. Security Considerations
This specification adds the ability for a UAC to request potentially
risky user interface behavior relating to the acceptance of an INVITE
request by the UAS receiving the request. Specifically, the UAC can
request that the UAS accept the request without input to the UAS by
the user of the UAS (Answer-Mode: Auto).
There are several attacks possible here -- the most obvious being the
ability to turn a phone into a remote listening device without its
user being aware. Additional potential attacks include reverse
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charge fraud, unsolicited push-to-talk communications (spam over
push-to-talk (SPTT)), playout of obnoxious noises (the "whoopee
cushion" attack), battery-rundown denial of service, "forced busy"
denial of service, running up the victim's data transport bill, and
phishing via session insertion (where an ongoing session is replaced
by another without the victim's awareness).
Since SIP implementations do not commonly implement end-to-end
message protections, this specification is completely dependent on
transitive security across SIP proxies. Any misbehaving proxy can
insert, delete, and/or alter the contents of the Answer-Mode and
Priv-Answer-Mode header fields, and in general can do so without
being noticed by either the UAC or UAS. Consequently, it is critical
that any proxies in the path be not only trusted, but worthy of that
trust. While proxies do not generally intentionally insert, delete,
or alter the Answer-Mode and Priv-Answer-Mode header fields, this
specification does note a use case for such manipulation by proxies
acting on behalf of the user of a UAC or UAS that has limited support
for the authentication or policy enforcement needed to securely
exercise these extensions. Proxies that perform such extension-
sensitive manipulation MUST therefore provide complete policy
enforcement, as per the minimal policy discussed in Section 7.4.
The existing body of SIP work provides strong capabilities for
authentication of requests, prevention of man-in-the-middle attacks,
protection of the privacy and integrity of media flows, and so on
(although as noted above, these capabilities usually rely on
transitive trust across proxies). The behaviors added by the
extensions in this document raise additional possibilities for
attacks against media flows not completely addressed by existing SIP
work, and therefore require analysis in this document.
Media attacks can be loosely categorized as:
Insertion: Media is inserted into and played out by the victim UA
without consent of the UA's user.
Interception: The victim UA's media acquisition facility (such as a
microphone or camera) is activated, producing a media stream,
without the consent of the UA's user.
7.1. Attack Sensitivity Depends on Media Characteristics
The danger of abuse varies greatly depending on the media
characteristics of the session being established. Since the
expressive range of media sessions that can be established by SIP is
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unbounded, we might find it more effective to model loose categories
of media modality rather than explicitly describing every possible
scenario. Security analysis can then be applied per modality.
The media modalities of interest appear to be:
UAC-sourced (Inbound) Unidirectional Media Insertion: Sensitive
media flows from the UAC and is rendered by the UAS, annoying the
user of the UAS or disrupting the function of the UAS. We refer
to this as the "whoopee-cushion" attack because of its utility in
replicating the rude-noise-making seat cushion. The danger of
this attack is quite literally amplified by a loudspeaker
apparatus attached to the victim UAS. Media that has minimal
secondary implication (such as sending a move in a chess game to a
computer that isn't running a chess game) is related, but of far
less significance. This sort of attack can also have other
consequences, such as discharging the victim's battery or
increasing charges for data transport to be paid by the victim.
UAS-sourced (Outbound) Unidirectional Media Interception: Sensitive
media flows from the UAS and is rendered by the UAC, violating the
privacy of the user of the UAS. We refer to this as the "bug-my-
phone" attack because that would appear to be the primary attack
motivator.
Bidirectional Media Insertion or Interception: Bidirectional media
is the common case when SIP is used in a voice-over-IP scenario or
"traditional phone call". Once a media flow is established, both
ends send and receive media without further engagement. The media
information is presumed to be sensitive -- that is, if intercepted
it damages the victim's privacy, and if inserted, it annoys or
interferes with the recipient. Attacks of this sort might produce
either the "whoopee-cushion" or "bug-my-phone" scenarios,
potentially even simultaneously.
It seems reasonable to consider the "bug-my-phone" attack as being in
a different class (potentially far more severe) than the "whoopee-
cushion" attack. This distinction suggests that security policy
could be established in different and presumably less restrictive
fashion for inbound media flows than for outbound media flows. The
set of callers from which a user would be willing to automatically
accept inbound media is reasonably much broader than the set of
callers to which a user would be willing to automatically grant
outbound media access, although this may not be true in all
environments, especially those where reception of unwanted media has
unwanted financial consequences.
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For example, assume a UA is designed such that it can be used to
receive push-to-talk calls to a loudspeaker, and it can be used as a
"baby monitor" (has an open mic and streams received audio to
listeners). The policy for activating the push-to-talk loudspeaker
would probably need to be reasonably broad (perhaps "all the user's
buddies"). However, the policy for the baby monitor would need to be
very narrow (perhaps "only the baby's mother") or even completely
closed. The minimal policy defined in Section 7.4 explicitly forbids
the "baby monitor" functionality.
7.2. Application Design Affects Attack Opportunity
In the most common use cases, the security aspects are somewhat
mitigated by design aspects of the application. For example, in
traditional telephony, the called party is alerted to the request
(the phone rings), no media session is established without the
acceptance of the called party (picking up the phone), and the media
path is most commonly delivered to a single-user handset.
Consequently, this application (although bidirectional) is relatively
secure against both media insertion and media interception attacks of
the sort enabled by the extensions in this document. The use of
policy-free automatic-answering devices (like answering machines) and
amplifiers (speakerphones and call-screening devices) weakens this
defense.
In push-to-talk applications, media can be sent from UAC to UAS
without user oversight, but no media is sent from the called UAS
without user input (the "push" of "push-to-talk"). Consequently,
there is no "bug-my-phone" attack opportunity. Further, screening of
the UAC by eliminating UAC identities not on some sort of "white
list" (often, a buddy list) reduces the threat of "whoopee cushion"
attacks (except from one's buddies, of course).
Similar approaches apply to most applications. Insertion can be
controlled (but not eliminated) by combining identity mechanisms with
simple authorization policy, and interception can be effectively
eliminated by combining strong identity mechanisms with aggressive
authorization policy and/or user interaction.
7.3. Applying the Analysis
The extensions described in this document provide mechanisms by which
a UAC can request that a UAS not deploy two of the five defensive
mechanisms listed below -- user alerting and user acceptance. In
order for this not to produce undue risk of insertion attacks or
increased risk of interception attacks, we are therefore forced to
rely on the remaining defensive mechanisms. This document defines a
minimum threshold for satisfactory security. Certainly more
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restrictive policies might reasonably be used, but any policy less
restrictive than the approach described below is very likely to
result in significant security issues.
From the previous discussion of risks, attacks, and vulnerabilities,
we can derive five defensive mechanisms available at the application
level:
1. Identity -- Know who the request came from.
2. Alerting -- Let the called user know what's happening. Some
applications might use inbound media as an alert.
3. Acceptance -- Require called user to make run-time decision.
Asking the user to make a run-time decision without alerting the
user to the need to make a decision is generally infeasible.
This will have implications for possible alerting options that
are outside the scope of this document.
4. Limit the Input/Output (I/O) -- Turn off loudspeakers or
microphone. This could be used to convert a bidirectional media
session (very risky, possible "bug my phone") into a
unidirectional, inbound-only (less risky, possible "spam" or
"rundown", etc.) session while waiting for user acceptance.
5. Policy -- Rules about other factors, such as black- and
whitelisting based on identity, disallowing acceptance without
alerting, etc.
Since SIP and related work already provide several mechanisms
(including SIP Digest Authentication [RFC3261], the SIP Identity
mechanism [RFC4474], and the SIP mechanism for asserted identity
within private networks [RFC3325], in networks for which it is
suitable) for establishing the identity of the originator of a
request, we presume that an appropriately selected mechanism is
available for UAs implementing the extensions described in this
document. In short, UAs implementing these extensions MUST be
equipped with and MUST exercise a request-identity mechanism. The
analysis below proceeds from an assumption that the identity of the
sender of each request is either known or is known to be unknown, and
can therefore be considered in related policy considerations.
Failure to meet this identity requirement either opens the door to a
wide range of attacks or requires operational policy so tight as to
make these extensions useless.
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We previously established a class distinction between inbound and
outbound media flows, and can model bidirectional flows as "worst
case" sums of the risks of the other two classes. Given this
distinction, it seems reasonable to provide separate directionality
policy classes for:
1. Inbound media flows.
2. Outbound media flows.
For each directionality policy class, we can divide the set of
request identities into three classes:
1. Identities explicitly authorized for the class.
2. Identities explicitly denied for the class.
3. Identities for which we have no explicit policy and need the user
to make a decision.
Note that not all combinations of policies possible in this
decomposition are generally useful. Specifically, a policy of
"inbound media denied, outbound media allowed" equates to a "bug my
phone" attack, and is disallowed by the minimal policy of
Section 7.4, which as written excludes all cases of "Outbound media
explicitly authorized".
7.4. Minimal Policy Requirement
User agents implementing this specification SHOULD NOT establish a
session providing inbound media without explicit user acceptance
where the requesting user is unknown, or is known and has not been
granted authorization for such a session. This requirement is
intended to prevent "SPAM broadcast" attacks where unexpected and
unwanted media is played out at a UAS .
User agents implementing this specification MUST NOT establish a
session providing outbound or bidirectional media sourced from the
user agent without explicit user acceptance. Loopback media used for
connectivity testing is not constrained by this requirement. This
requirement is intended to assure that this extension can not be used
to turn a UAS into a remote-controlled microphone (or "bug") without
the knowledge of its user. Since SIP allows for a session to be
initially established with inbound-only media and then transitioned
(via re-INVITE or UPDATE) to an outbound or bidirectional session,
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enforcing this policy requires dialog-stateful inspection in the SIP
UAS. In other words, if a session was initiated with automatic
answering, the UAS MUST NOT transition to a mode that sends outbound
media without explicit acceptance by the user of the UAS.
8. IANA Considerations
8.1. Registration of Header Fields
This document defines new SIP header fields named "Answer-Mode" and
"Priv-Answer-Mode".
The following rows have been added to the "Header Fields" section of
the SIP parameter registry:
+------------------+--------------+-----------+
| Header Name | Compact Form | Reference |
+------------------+--------------+-----------+
| Answer-Mode | | [RFC5373] |
| Priv-Answer-Mode | | [RFC5373] |
+------------------+--------------+-----------+
8.2. Registration of Header Field Parameters
This document defines parameters for the header fields defined in the
preceding section. The header fields "Answer-Mode" and "Priv-Answer-
Mode" can take the values "Manual" or "Auto".
The following rows have been added to the "Header Field Parameters
and Parameter Values" section of the SIP parameter registry:
+------------------+----------------+-------------------+-----------+
| Header Field | Parameter Name | Predefined Values | Reference |
+------------------+----------------+-------------------+-----------+
| Answer-Mode | require | No | [RFC5373] |
| Priv-Answer-Mode | require | No | [RFC5373] |
+------------------+----------------+-------------------+-----------+
8.3. Registration of SIP Option Tags
This document defines the SIP option tag "answermode".
The following row has been added to the "Option Tags" section of the
SIP Parameter Registry:
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+------------+------------------------------------------+-----------+
| Name | Description | Reference |
+------------+------------------------------------------+-----------+
| answermode | This option tag is for support of the | [RFC5373] |
| | Answer-Mode and Priv-Answer-Mode | |
| | extensions used to negotiate automatic | |
| | or manual answering of a request. | |
+------------+------------------------------------------+-----------+
9. Acknowledgements
This document draws requirements and a large part of its methodology
from the work of the Open Mobile Alliance, and specifically from a
document by Andrew Allen, Jan Holm, and Tom Hallin.
The editor would also like to recognize the contributions of David
Oran and others who argued on the SIPPING mailing list and at the OMA
ad-hoc meeting at IETF 62 that the underlying ideas of the above
document were broadly applicable to the SIP community, and that the
concepts of alerting and answering should be clearly delineated.
Further, the security review provided by Sandy Murphy and the gen-art
review by Suresh Krishnan were very helpful in improving the quality
of this document.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
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[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
10.2. Informative References
[LOOPBACK] Hedayat, K., "An Extension to the Session Description
Protocol (SDP) for Media Loopback", Work in Progress,
August 2008.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
Authors' Addresses
Dean Willis (editor)
Softarmor Systems
3100 Independence Pkwy #311-164
Plano, Texas 75075
USA
EMail: dean.willis@softarmor.com
Andrew Allen
Research in Motion (RIM)
300 Knightsbridge Parkway, Suite 360
Lincolnshire, Illinois 60069
USA
EMail: aallen@rim.com
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