SIPCORE H. Schulzrinne
Internet-Draft FCC
Intended status: Standards Track December 12, 2016
Expires: June 15, 2017
A SIP Response Code for Unwanted Calls
draft-ietf-sipcore-status-unwanted-00
Abstract
This document defines the 666 (Unwanted) SIP response code, allowing
called parties to indicate that the call was unwanted. The
terminating SIP entity may use this information to adjust future call
handling behavior for this called party or more broadly.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Normative Language . . . . . . . . . . . . . . . . . . . . . 2
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Behavior of SIP Entities . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
5.1. SIP Response Code . . . . . . . . . . . . . . . . . . . . 3
5.2. SIP Global Feature-Capability Indicator . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
In many countries, an increasing number of calls are unwanted
[RFC5039], as they might be fraudulent, illegal telemarketing or the
receiving party does not want to be disturbed by, say, surveys or
solicitation by charities. Carriers and other service providers may
want to help their subscribers avoid receiving such calls, using a
variety of global or user-specific filtering algorithms. One input
into such algorithms is user feedback. User feedback may be offered
through smartphone apps, APIs or within the context of a SIP-
initiated call. This document addresses only the last mode, where
the called party either rejects the SIP request, typically INVITE or
MESSAGE, as unwanted or terminates the call with a BYE request after
answering the call. To allow the called party to express that the
call was unwanted, this document defines the 666 (Unwanted) response
code. The called party or a automata acting on her behalf uses this
to indicate that future calls from the same caller are also unwanted.
2. Normative 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 BCP 14, RFC 2119
[RFC2119].
3. Motivation
None of the existing 4xx, 5xx or 6xx response codes allow the called
party to convey that they not only reject this call, e.g., using 480
(Temporarily Unavailable), 486 (Busy Here), 600 (Busy Everywhere),
603 (Decline) or 606 (Not Acceptable), but that the caller is
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unwanted. The particular response code number was chosen to reflect
the distaste felt by many upon receiving such calls.
4. Behavior of SIP Entities
The response code MAY also be used in Reason header fields [RFC3326],
typically when the UAS issues a BYE request terminating an incoming
call.
The SIP entities receiving this response code are not obligated to
take any particular action. The service provider delivering calls to
the user issuing the response may, for example, add the calling party
to a personal blacklist specific to the called party, or may use the
information as input when computing the likelihood that the calling
party is placing unwanted calls ("crowd sourcing"), might initiate a
traceback request, or could report the calling number to government
authorities. Receiving systems could decide to treat pre-call and
mid-call responses differently, given that the called party has had
access to call content for mid-call rejections. In other words,
depending on the implementation, the response code does not
necessarily automatically block all calls from that number. The same
user interface action might also trigger addition of the number to a
local, on-device blacklist or graylist, e.g., causing such calls to
be flagged or alert with a different ring tone.
We define a SIP feature-capability [RFC6809], sip.666, that allows
the registrar to indicate that it supports this particular response
code. This allows the UA, for example, to provide a suitable user
interface element, such as a "spam" button, only if its service
provider actually supports the feature. The presence of the feature
capability does not imply that the provider will take any particular
action, such as blocking future calls. A UA may still decide to
render a "spam" button even without such as a capability if, for
example, it maintains a device-local blacklist or reports unwanted
calls to a third party.
5. IANA Considerations
5.1. SIP Response Code
This document register a new SIP response code. This response code
is defined by the following information, which is to be added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.
Response Code Number 666
Default Reason Phrase Unwanted
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Reference [this RFC]
5.2. SIP Global Feature-Capability Indicator
This document defines the feature capability sip.666 in the "SIP
Feature-Capability Indicator Registration Tree" registry defined in
[RFC6809].
Name sip.666
Description This feature-capability indicator when used in a
REGISTER response indicates that the server will process the 666
response code. This does not imply any specific action.
Reference [this RFC]
6. Security Considerations
If the calling party number is spoofed, users may report the number
as placing unwanted calls, possibly leading to the blocking of calls
from the legitimate user of the number in addition to the unwanted
caller, i.e., creating a form of denial-of-service attack. Thus, the
response code SHOULD NOT be used for creating global call filters
unless the calling party number has been authenticated using
[I-D.ietf-stir-rfc4474bis] as being assigned to the caller placing
the unwanted call. (The creation of call filters local to a user
agent is beyond the scope of this document.)
Even if the number is not spoofed, a call recipient might flag
legitimate numbers, e.g., to extract vengeance on a person or
business, or simply by mistake. Thus, any additions to a personal
list of blocked numbers should be observable by the subscriber, e.g.,
on a web page or by regular email notification, and reservible. Any
additions to a global or carrier-wide list of unwanted callers needs
to consider that any user-initiated mechanism will suffer from an
unavoidable rate of false positives and tailor their algorithms
accordingly, e.g., by comparing the fraction of delivered calls for a
particular caller that are flagged as unwanted rather than just the
absolute number, and considering time-weighted filters that give more
credence to recent feedback.
Since telephone numbers are routinely re-assigned to new subscribers,
algorithms are advised to consider whether the number has been re-
assigned to a new subscriber and possibly reset any related rating.
For both individually-authenticated and unauthenticated calls,
recipients may want to distinguish responses sent before and after
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the call has been answered, ascertaining whether either response
timing suffers from a lower false-positive rate.
7. Acknowledgements
Martin Dolly, Keith Drage, Paul Kyzivat, Brian Rosen and Chris Wendt
provided helpful comments.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, DOI 10.17487/RFC3326, December 2002,
<http://www.rfc-editor.org/info/rfc3326>.
[RFC6809] Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism to
Indicate Support of Features and Capabilities in the
Session Initiation Protocol (SIP)", RFC 6809,
DOI 10.17487/RFC6809, November 2012,
<http://www.rfc-editor.org/info/rfc6809>.
8.2. Informative References
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-15
(work in progress), October 2016.
[RFC5039] Rosenberg, J. and C. Jennings, "The Session Initiation
Protocol (SIP) and Spam", RFC 5039, DOI 10.17487/RFC5039,
January 2008, <http://www.rfc-editor.org/info/rfc5039>.
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Author's Address
Henning Schulzrinne
FCC
445 12th Street SW
Washington, DC 20554
US
Email: henning.schulzrinne@fcc.gov
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