One document matched: draft-ietf-sip-acr-code-04.txt
Differences from draft-ietf-sip-acr-code-03.txt
SIP J. Rosenberg
Internet-Draft Cisco Systems
Expires: September 1, 2007 February 28, 2007
Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)
draft-ietf-sip-acr-code-04
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The Session Initiation Protocol (SIP) allows for users to make
anonymous calls. However, users receiving such calls have the right
to reject them because they are anonymous. SIP has no way to
indicate to the caller that the reason for call rejection was that
the call was anonymous. Such an indication is useful to allow the
call to be retried without anonymity. This specification defines a
new SIP response code for this purpose.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . . 3
4. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. 433 (Anonymity Disallowed) Definition . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
The Session Initiation Protocol (SIP) [1] allows for users to make
anonymous calls. In RFC 3261, this is done by including a From
header field whose display name has the value of "Anonymous".
Greater levels of anonymity were subsequently defined in RFC 3323
[2], which introduces the Privacy header field. The Privacy header
field allows a requesting UA to ask for various levels of anonymity,
including user level anonymity, header level anonymity, and session
level anonymity. RFC 3325 [3] additionally defined the P-Asserted-
Identity header field, used to contain an asserted identity. RFC
3325 also defined the 'id' value for the Privacy header field, which
is used to request the network to remove the P-Asserted-Identity
header field.
Though users need to be able to make anonymous calls, users that
receive such calls retain the right to reject the call because it is
anonymous. SIP does not provide a response code that allows the UAS,
or a proxy acting on its behalf, to explicitly indicate that the
request was rejected because it was anonymous. The closest response
code is 403 (Forbidden), which doesn't convey a specific reason.
While it is possible to include a reason phrase in a 403 response
that indicates to the human user that the call was rejected because
it was anonymous, that reason phrase is not useful for automata and
cannot be interpreted by callers that speak a different language. An
indication that can be understood by an automaton would allow for
programmatic handling, including user interface prompts, automatic
retries, or conversion to equivalent error codes in the Public
Switched Telephone Network (PSTN) when the client is a gateway.
To remedy this, this specification defines the 433 (Anonymity
Disallowed) response code.
2. Terminology
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 RFC 2119 [4].
3. Server Behavior
A server (generally acting on behalf of the called party, though this
need not be the case) MAY generate a 433 (Anonymity Disallowed)
response when it receives an anonymous request, and the server
refuses to fulfill the request because the requestor is anonymous. A
request SHOULD be considered anonymous when the identity of the
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originator of the request has been explicitly withheld by the
originator. This occurs in any one of the following cases:
o The From header field contains a URI within the anonymous.invalid
domain.
o The From header field contains a display name whose value is
either 'Anonymous' or 'anonymous'. Note that display names make a
poor choice for indicating anonymity, since they are meant to be
consumed by humans, not automata. Thus, language variations and
even misspelling can cause an automaton to miss a hint in the
display name. Despite these problems, a check on the display name
is included here because RFC 3261 explicitly calls out the usage
of the display name as a way to declare anonymity.
o The request contained a Privacy header field whose value was 'id'
[3] or 'user'.
o The From or P-Asserted-Identity header field contains a URI which
has an explicit indication that it is anonymous. One such example
of a mechanism that would meet this criteria is [7]. This
criteria is true even if the request has a validated Identity
header field [5], which can be used in concert with anonymized
From header fields.
Lack of a P-Asserted-Identity header field, in and of itself, SHOULD
NOT be considered an indication of anonymity. Even though a Privacy
header field value of 'id' will cause the removal of the P-Asserted-
Identity header field, there is no way to differentiate this case
from one in which P-Asserted-Identity was not supported by the
originating domain. As a consequence, a request without a
P-Asserted-Identity is considered anonymous only when there is some
other indication of this, such as a From header field with a display
name of 'Anonymous'.
In addition, requests where the identity of the requestor cannot be
determined or validated, but it is not a consequence of an explicit
action on the part of the requestor, are not consider anonymous. For
example, if a request contains a non-anonymous From header field,
along with the Identity and Identity-Info header fields [5], but the
certificate could not be obtained from the reference in the Identity-
Info header field, it is not considered an anonymous request, and the
433 response code SHOULD NOT be used.
4. UAC Behavior
A UAC receiving a 433 (Anonymity Disallowed) MUST NOT retry the
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request without anonymity unless it obtains confirmation from the
user that this is desirable. Such confirmation could be obtained
through the user interface, or by accessing user defined policy. If
the user has indicated that this is desirable, the UAC MAY retry the
request without requesting anonymity. Note that if the UAC were to
automatically retry the request without anonymity in the absence of
an indication from the user that this treatment is desirable, then
the user's expectations would not be met. Consequently, a user might
think they had completed a call anonymously when they are not
actually anonymous.
Receipt of a 433 response to a mid-dialog request SHOULD NOT cause
the dialog to terminate, and SHOULD NOT cause the specific usage of
that dialog to terminate [6].
A UAC that does not understand or care about the specific semantics
of the 433 response will treat it as a 400 response.
5. 433 (Anonymity Disallowed) Definition
This response indicates that the server refused to fulfill the
request because the requestor was anonymous. Its default reason
phrase is "Anonymity Disallowed".
6. IANA Considerations
This section registers a new SIP response code according to the
procedures of RFC 3261.
RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification]]
Response Code Number: 433
Default Reason Phrase: Anonymity Disallowed
7. Security Considerations
The fact that a request was rejected because it was anonymous does
reveal information about the called party - that the called party
does not accept anonymous calls. This information may or may not be
sensitive. If it is, a UAS SHOULD reject the request with a 403
instead.
In the Public Switched Telephone Network (PSTN), the Anonymous Call
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Rejection (ACR) feature is commonly used to prevent unwanted calls
from telemarketers (also known as spammers). Since telemarketers
frequently withhold their identity, anonymous call rejection has the
desired effect in many (but not all) cases. It is important to note
that the response code described here is likely to be ineffective in
blocking SIP-based spam. The reason is that a malicious caller can
include a From header field and display name that is not anonymous,
but is meaningless and invalid. Without a Privacy header field, such
a request will not appear anonymous and thus not be blocked by an
anonymity screening service. Dealing with SIP-based spam is not a
simple problem. The reader is referred to [10] for a discussion of
the problem.
When anonymity services are being provided as a consequence of an
anonymizer function acting as a back-to-back user agent (B2BUA) [2],
and the anonymizer receives a 433 response, the anonymizer MUST NOT
retry the request without anonymization unless it has been explicitly
configured by the user to do so. In essence, the same rules that
apply to a UA in processing of a 433 response apply to a network-
based anonymization function, and for the same reasons.
8. Acknowledgements
This draft was motivated based on the requirements in [9], and has
benefited from the concepts in [8]. Thanks to Keith Drage, Paul
Kyzivat and John Elwell for their reviews of this document.
9. References
9.1. Normative References
[1] 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.
[2] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
[3] 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.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Peterson, J. and C. Jennings, "Enhancements for Authenticated
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Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.
9.2. Informative References
[6] Sparks, R., "Multiple Dialog Usages in the Session Initiation
Protocol", draft-ietf-sipping-dialogusage-06 (work in
progress), February 2007.
[7] Rosenberg, J., "Coexistence of P-Asserted-ID and SIP Identity",
draft-rosenberg-sip-identity-coexistence-00 (work in progress),
June 2006.
[8] Hautakorpi, J. and G. Camarillo, "Extending the Session
Initiation Protocol Reason Header with Warning Codes",
draft-hautakorpi-reason-header-for-warnings-00 (work in
progress), October 2005.
[9] Jesske, R., "Input Requirements for the Session Initiation
Protocol (SIP) in support for the European Telecommunications
Standards Institute",
draft-jesske-sipping-tispan-requirements-03 (work in progress),
June 2006.
[10] Jennings, C. and J. Rosenberg, "The Session Initiation Protocol
(SIP) and Spam", draft-ietf-sipping-spam-03 (work in progress),
October 2006.
Author's Address
Jonathan Rosenberg
Cisco Systems
Edison, NJ
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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