One document matched: draft-ietf-sip-acr-code-00.txt
SIP J. Rosenberg
Internet-Draft Cisco Systems
Expires: July 20, 2006 January 16, 2006
Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)
draft-ietf-sip-acr-code-00
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Copyright (C) The Internet Society (2006).
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. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 3
3. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. 433 (Anonymity Disallowed) Definition . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1 Normative References . . . . . . . . . . . . . . . . . . . 5
8.2 Informative References . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 7
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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-ID
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-ID 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 to 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. An
indication that can be understood by an automata 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. Server Behavior
A server acting on behalf of the called party, such as the UAS or a
proxy in their domain, MAY generate a 433 (Anonymity Disallowed)
response when it receives an anonymous request, and the called party
refuses to fulfill the request because the requestor is anonymous. A
request is considered anonymous when the identity of the originator
of the request has been explicitly witheld 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
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even misspelling can cause an automata 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'. This explicitly excludes the 'header' and
'session' privacy services, since those do not directly convey the
identity of the requestor.
o The From or P-Asserted-ID 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 [4].
It is important to note that lack of a P-Asserted-ID header field, in
and of itself, is not an indication of anonymity. Even though a
Privacy header field value of 'id' will cause the removal of the
P-Asserted-ID header field, there is no way to differentiate this
case from one in which P-Asserted-ID was not supported by the
originating domain. As a consequence, a request without a
P-Asserted-ID is considered anonymous only when there is some other
indication of this, such as a From header field with a display name
of 'Anonymous'.
3. UAC Behavior
A UAC receiving a 433 (Anonymity Disallowed) response MAY retry the
request without requesting anonymity. It SHOULD only do so if 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. The UAC SHOULD NOT retry the request
if user continues to request anonymity.
A UAC the does not understand or care about the specific semantics of
the 433 response will treat it as a 400 response.
4. 433 (Anonymity Disallowed) Definition
This response indicates that the server refused the fulfill the
request because the requestor was anonymous. Its default reason
phrase is "Anonymity Disallowed".
5. IANA Considerations
This section registers a new SIP response code according to the
procedures of RFC 3261.
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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
6. Security Considerations
The fact that an request was rejected because it was anonymous does
reveal information about the called party - that they do 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
Rejection (ACR) feature is commonly used to prevent unwanted calls
from telemarketers (also known as spammers). Since telemarketers
frequently withhold their identity, this 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 [7] for a discussion of
the problem.
7. Acknowledgements
This draft was motivated based on the requirements in [6], and has
benefitted from the concepts in [5].
8. References
8.1 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.
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8.2 Informative References
[4] Rosenberg, J., "Identity Privacy in the Session Initiation
Protocol (SIP)", draft-rosenberg-sip-identity-privacy-00 (work
in progress), July 2005.
[5] 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.
[6] Jesske, R., "Input Requirements for the Session Initiation
Protocol (SIP) in support for the European Telecommunications
Standards Institute",
draft-jesske-sipping-tispan-requirements-02 (work in progress),
October 2005.
[7] Rosenberg, J., "The Session Initiation Protocol (SIP) and Spam",
draft-ietf-sipping-spam-01 (work in progress), July 2005.
[8] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-05 (work in progress), May 2005.
Author's Address
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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