Can we create a "secure Caller ID" for IP-based communications, a.k.a. voice-over-IP (VoIP)? And specifically for VoIP based on the Session Initiation Protocol (SIP)? Can we create a way to securely identify the origin of a call that can be used to combat robocalling, phishing and telephony denial-of-service (TDOS) attacks?
That is the challenge to be undertaken by the "Secure Telephone Identity Revisited (STIR)" group meeting tomorrow morning, July 30, 2013, at 9:00 am in Berlin, Germany, as part of the 87th meeting of the Internet Engineering Task Force (IETF). The meeting tomorrow is a "Birds Of a Feather (BOF)", which in IETF language is a meeting to determine whether there is sufficient interest to create a formal "working group" to take on a new body of work within the IETF. The proposed "charter" for this new work begins:
Over the last decade, a growing set of problems have resulted from the lack of security mechanisms for attesting the origins of real-time communications. As with email, the claimed source identity of a SIP request is not verified, and this permits unauthorized use of source identities as part of deceptive and coercive activities, such as robocalling (bulk unsolicited commercial communications), vishing (voicemail hacking, and impersonating banks) and swatting (impersonating callers to emergency services to stimulate unwarranted large scale law enforcement deployments). This working group will define a deployable mechanism that verifies the authorization of the calling party to use a particular telephone number.
The agenda for tomorrow's STIR meeting begins with a presentation by Henning Schulzrinne, now CTO of the US Federal Communications Commission (FCC) but also a long-time IETF participant and one of the co-authors of the original RFC 3261 specification for SIP. Henning will be laying out the problem statement and there will be a discussion of the proposed scope of the IETF work. He'll be followed by presentations of potential solutions by Jon Peterson, Eric Rescorla and Hadriel Kaplan and then a discussion of the proposed charter and the work to be done. Given the intense debate that has occurred on the STIR mailing list over the past weeks I expect tomorrow's session to be one where some points will receive a great amount of passionate debate and discussion. (If you are interested in listening in or participating remotely in tomorrow's STIR meeting, see the information later in this article.)
Revisiting Previous SIP Identity Work
As some background, the Internet Architecture Board (IAB) laid out some of the challenges to "secure origin identification" in IP-based communication last November and took a very high-level look at the overall issue. Next, in preparation for what became this STIR effort, Jon Peterson, Henning Schulzrinne and Hannes Tschofenig authored a draft problem statement and requirements document.
The "Revisited" part of the group name is a nod to the fact that this whole issue of asserting "identity" has been explored within the SIP community in the past. Way back in 2006, RFC 4474 defined what has been called "SIP Identity" and provided a method for cryptographically signing certain SIP headers to identify the origin of a call. Unfortunately, RFC 4474 turned out not to work well with the way SIP was actually deployed and so usage has been virtually non-existent. An effort to update that document, what is called "RFC4474bis", has also been proposed and some of those ideas may be incorporated into the new proposed work for the STIR group.
There have also been other efforts such as the "P-Asserted-Identity (P-A-I)" defined in RFC 3325. The challenge here, though is that theoretically P-A-I is supposed to be limited to usage within a trusted network, although in practice it may be seen by other networks. There have also been several efforts to define or document identifiers for billing purposes (including my own P-Charge-Info) although these efforts are trying to solve a slightly different problem.
The point here really is that the STIR effort is drawing upon a rich body of "SIP identity" work that dates all the way back to some early drafts in 2002. Much thought has been given to this issue and many of the people involved with STIR have also been involved with earlier efforts and understand well some of the challenges faced by that past work.
An Important Difference
One important difference between STIR and earlier "SIP identity" efforts is that initially the STIR effort is only focused on telephone numbers. The draft charter explicitly states this:
As its first work item, the working group will specify a SIP header-based authorization mechanism to verify the originator of a SIP session is authorized to use the claimed source telephone number, where the session is established with SIP end to end. This is called an in-band mechanism. The mechanism will use a canonical telephone number representation specified by the working group, including any mappings that might be needed between the SIP header fields and the canonical telephone number representation.
Expansion of the authorization mechanism to identities using the user@domain form deferred since the main focus of the working group is to develop a solution for telephone numbers.
Previous "identity" work was also undertaken to include a "SIP URI" or "SIP address" and while the ultimate STIR mechanism (or a variant thereof) might also work for SIP URIs, the focus in this initial work is all around securing the origin identification of telephone numbers.
This initial focus makes a great amount of sense given that so much of the SIP traffic today is a result of telecom service providers moving their regular calls to telephone numbers off of the legacy PSTN networks and over to IP networks where they use SIP. Additionally, a great amount of the "problem" traffic seen in VoIP today can be created by attackers who use simple VoIP software to generate their calls to regular telephone numbers.
Remotely Participating In Tomorrow's STIR BOF
If you are interested in participating in the meeting (or at least listening in) on Tuesday, July 30, the meeting will go from 9:00 - 11:30 local time in Berlin, Germany. Berlin is in Central European Summer Time (CEST) which is UTC+2 (and 3:00 am US EDT / midnight US PDT for my friends back in the USA).
You can hear the audio stream at:
You can also join the Jabber chat room at:
The slides and other meeting materials can be found at (and note that materials may not be uploaded until shortly before the session and so you may need to refresh your browser):
Alternatively you can use the "MeetEcho" conferencing system that integrates the audio, the slides and the Jabber chat room at:
More information about participately remotely can be found on the IETF 87 Remote Participation page.
To get the most out of the meeting, you'll also want to read these three Internet Drafts that will be part of the solutions being discussed:
.... and be prepared for what should be a LIVELY discussion!
If you are unable to participate remotely, the session will be recorded and you will be able to listen to the archived audio stream, view the Jabber chat logs and also playback the MeetEcho recording.
Getting More Involved
Beyond listening to tomorrow's BOF session, the best way to get involved - either to actively participate or to at least monitor the effort - is to join the STIR mailing list at:
The list is open to anyone to join. There are no membership or corporate requirements or fees - anyone with an email address may participate.
WARNING! - As can be seen in the list archive, there is currently a large volume of discussion and it will probably continue for some time. If you do join the mailing list you may want to consider setting up rules to sort the STIR email into a folder - or just prepare for the volume to be added to your inbox.
The other way to be involved is to monitor and read the documents that are created for the STIR effort. Newer documents are being created with "
stir" in the document name and so they can be easily found at:
Other documents that are useful to understand this effort are linked to earlier in this article and can also be found in the text of the proposed STIR charter. After tomorrow's STIR BOF session there will be more information about how the effort will proceed within the IETF. The meeting tomorrow should result, I expect, in the recommendation to go ahead with formally creating a working group and undertaking this work, but we'll see what outcome occurs.
Can a method of secure origin identification for SIP-based VoIP calls be created? Given that basically all telecom traffic is in the process of moving to be based on IP, the need for a secure origin identifier is very clearly here - and many of us do believe we can develop a system that will work in today's environment.
What do you think? Are you ready to join in and help?
Update: Added the additional charter text about "Expansion of the authorization mechanism to identities..."
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