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SIP lacks a mechanism to determine which domain can claim ownership of a certain telephone number. Due to this, it is impossible to establish meaningful identity or to authenticate endpoints that use telephone number URIs and domain names in their From address. This document proposes a solution to this problem using a return routability test.
1.
Introduction
2.
Notational Conventions
3.
Operation
3.1.
Verifier Operation
3.2.
Authentication Service or Calling Endpoint Operation
4.
Performance Considerations
5.
Deployment Considerations
6.
Security Considerations
7.
Examples
8.
IANA Considerations
8.1.
Reverse-Route Event Package
8.2.
The "application/return-routability-nonce" Media Type
9.
Acknowledgements
10.
References
10.1.
Normative References
10.2.
Informational References
Appendix A.
Changes
A.1.
Changes from -00 to -01
§
Author's Address
§
Intellectual Property and Copyright Statements
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SIP (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) [RFC3261] allows using both email-style addresses (user@domain) and telephone-style addresses (1234@domain). The latter is most often used with E.164 (International Telecommunications Union, “The International Public Telecommunication Numbering Plan,” 1991.) [ITU.E164.1991] numbers (designated with ";user=phone") especially between different administrative domains.
SIP's use of E.164 numbers poses several problems. This draft provides a solution to one of the problems: determining if a domain name rightfully 'owns' an E.164 phone number. In order to do this, a new SIP request is routed towards that E.164 and, if it is received by the same domain, that domain is deemed to 'own' that E.164 number. This is termed a 'return routability check' (RRC).
The return routability check relies on SIP routing to ascertain which domain 'owns' a certain E.164 number.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
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In order to check if a domain actually 'owns' the E.164 number it claims to own, a new SIP request is sent towards that E.164. Upon receipt of a SIP request conaining an E.164 number in the From address, the verifying agent in the receiving domain sends a new, out-of-dialog request (a SUBSCRIBE) towards that E.164 using the verifiering domain's normal routing rules. This SUBSCRIBE contains a body (with a nonce) that the verifying domains wants the owner of the E.164 to sign. That SIP SUBSCRIBE request is routed to the 'owner' of that E.164. Upon receipt of the SUBSCRIBE, the owner of that E.164 number generates a NOTIFY request. This is signed, using RFC4474, and sent to the verifying domain. The verifing domain verifies the signature on the NOTIFY. If it verified, and if the same public key was used in the original SIP request and in this NOTIFY, the verifying domain has now verified the remote domain 'owns' that E.164 number. If a different public key was used in the original SIP request and in this NOTIFY, the verifying domain has no verified the remote domain does not own that E.164 number.
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Upon receipt of an INVITE where the From: address contains a SIP URI with an E.164, a Verifier (as defined in [RFC4474] (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.)) needs to verify if the signer is authorized to sign for that domain. To do this, the Verifier has an additional task: it sends an out of dialog SIP SUBSCRIBE request containing a random nonce to that E.164, using the Verifier's default SIP routing rules for routing an E.164 address. The domain that owns the E.164 will sign the nonce and send a NOTIFY request back.
The steps the Verifier uses to perform this operation are:
Upon receipt of the NOTIFY message, the Verifier performs the following steps:
- A.
- If they match, the E.164 return routability test has succeeded.
- B.
- If they do not match, the E.164 return routability test has failed.
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The steps described in this section can be performed by the authentication service or by the calling endpoint.
The authentication service or the calling endpoint, upon receiving a SUBSCRIBE for the return-routability event package, performs the following steps:
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To reduce SUBSCRIBE/NOTIFY traffic, a verifier SHOULD cache successful and failed return routability checks. Successful checks will only become unsuccessful if SIP E.164 routing is changed to a different terminating domain. This only occurs when a domain relinquishes an E.164, so it is RECOMMENDED that the result of ssuccessful tests be cached for 24 hours. However, unsuccessful tests could be a result of misconfiguration and it is useful to re-verify such failures in the event the misconfiguration is fixed. Thus, it is RECOMMENDED that the result of unsuccessful tests be cached for 1 hour.
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Intermediate SIP elements (proxies, SBCs, B2BUAs) MUST all forward the application/return-routability-nonce Content-Type.
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An attacker could cause a domain to perform public key operations by sending a bunch of bogus SUBSCRIBE messages. This can be thwarted by only responding with NOTIFYs if there is an active INVITE dialog. However, that could disclose information about active calls, and also restricts the usefulness of this feature to INVITEs. More evaluation of countermeasures against such an attack is needed.
[[This section will be completed in a later version of this document.]]
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Example message flow for a successful return routability check.
Calling Auth. Called UA Service proxies Verifier UA ------- ------- -------- -------- ------ -------- | | | | | ^ | INVITE | | | | | |--------->| | | | | | | | | | | | (signs request) | | | | | | | | | | |100 | INVITE | | | | steps which |<---------|--------->| | | | are part of | | | | | | normal RFC4474 | |100 | INVITE | | | | |<---------|--------->| | | | | | | | | | | |100 | | | | | |<---------| | | | | | | | | | | | (validates | | | | | signature) | V | | | | | ---------- | | |SUBSCRIBE | | | | |(containing nonce) | | |SUBSCRIBE |<---------| | | |<---------| | | | | | | | | |200 Ok | | | | |--------->|200 Ok | | | | |--------->| | | NOTIFY | | | | | (containing nonce) | | | | +----| | | | | | | | | | | +--->| | | | | | | | | | (signs NOTIFY) | | | | | | | | | |NOTIFY | | | | |--------->|NOTIFY | | | | |--------->| | | | | | | | | |200 Ok | | | |200 Ok |<---------| | | |<---------| | | | | | | | | | | (validates NOTIFY | | | | signature, and | | | | checks that same | | | | cert. signed both | | | | the INVITE and | | | | the NOTIFY) | | | | | | | | | | INVITE | | | | |--------->| | | | | | | | | | |display e.164 | | | | |------------> | | | | |
| Figure 1: Message Flow -- Return Routability Check Success |
Example message flow for an unsuccessful return routability check, where the NOTIFY is signed by a different RFC4474 authentication service:
Calling E.164
User's Owner's
Calling Auth. Auth. Called
UA Service Service proxies Verifier UA
------- ------- ------- ------- -------- ------
| | | | | |
| INVITE | | | | |
|--------->| | | | |
| | | | | |
| (signs request) | | | |
| | | | | |
|100 | INVITE | | | |
|<---------|-------------------->| | |
| | | | | |
| |100 | | INVITE | |
| |<--------------------|--------->| |
| | | | | |
| | | |100 | |
| | | |<---------| |
| | | | | |
| | | | (validates |
| | | | signature) |
| | | | | |
| | | |SUBSCRIBE | |
| | |SUBSCRIBE |(containing nonce) |
| | |<---------| | |
| | |200 Ok | | |
| | |--------->|200 Ok | |
| | NOTIFY | |--------->| |
| | (containing nonce) | | |
| | +----| | | |
| | | | | | |
| | +--->| | | |
| | | | | |
| | (signs NOTIFY) | | |
| | | | | |
| | |NOTIFY | | |
| | |--------->|NOTIFY | |
| | | |--------->| |
| | | | | |
| | | |200 Ok | |
| | |200 Ok |<---------| |
| | |<---------| | |
| | | | | |
| | | | (NOTIFY validates; |
| | | | however, different |
| | | | cert. was used for |
| | | | INVITE) |
| | | | | |
| | | | | |
| | | | | INVITE |
| | | | |--------->|
| | | | | |
| | | | | |display
| | | | | |e.164
| | | | | |------>
| | | | | |
| Figure 2: Message Flow -- Return Routability Check Failure |
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IANA is requested to register the following new event package and one new media type.
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This specification registers an event package, based on the registration procedures defined in [RFC3265] (Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” June 2002.). The following is the information required for such a registration:
Package Name: reverse-route
Package or Package-Template: This is a package.
Published Specification: <this document>.
Person to Contact: Dan Wing, <dwing@cisco.com>
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Type name: application
Subtype name: return-routability-nonce
Required parameters: None.
Optional parameters: None.
Encoding considerations: The nonce is primarily binary content.
Security considerations: See Section 6 (Security Considerations) of <this document>.
Interoperability considerations: See <this document>.
Published specification: <this document>
Applications which use this media type: SIP.
Additional information:
Magic number(s): None.
File extension(s): None.
Macintosh File Type Code(s): none.
Person & email address to contact for further information: Dan Wing <dwing@cisco.com>
Intended Usage: COMMON
Author/Change Controller: Dan Wing <dwing@cisco.com>
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Thanks to Paul Kyzivat and Hannes Tschofenig for their review and comments on this document. Thanks to Joel Halpern for pointing out the missing ";user=phone" parameter.
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| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC4474] | Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” RFC 4474, August 2006 (TXT). |
| [RFC3265] | Roach, A., “Session Initiation Protocol (SIP)-Specific Event Notification,” RFC 3265, June 2002 (TXT). |
| [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 (TXT). |
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| [ITU.E164.1991] | International Telecommunications Union, “The International Public Telecommunication Numbering Plan,” ITU-T Recommendation E.164, 1991. |
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[[RFC Editor: Please remove this section prior to publication.]]
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| Dan Wing | |
| Cisco Systems, Inc. | |
| 170 West Tasman Drive | |
| San Jose, CA 95134 | |
| USA | |
| Email: | dwing@cisco.com |
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