Re: [Perc] Last Call: <draft-ietf-perc-private-media-framework-08.txt> (A Solution Framework for Private Media in Privacy Enhanced RTP Conferencing) to Proposed Standard

[Ben Campbell <[e-mail]>]

Hi Bernard,

Full disclosure: I asked the chairs to send that email. I gather from your response that you saw it as an attempt to shut down discussion. I don’t think that is the case at all.

I interpreted Nils’s email to be providing useful context about the discussions that occurred in the working group. I hope that context will be useful to people evaluating the current discussion. Several of the points currently under discussion were discussed in the WG, and they made tradeoffs. Not everyone was happy about each tradeoffs; that’s the “rough” in rough consensus.

It’s perfectly reasonable to open those issues again if new information has come up since then, or if people who were not participants in the WG have opinions to offer.  But we should also be careful about simply replaying the same discussions over again. I don’t mean to say that the entire discussion falls into the second category; I think we have some of both.

I would encourage participants in this discussion to focus on what we might have learned since the WG discussions, whether that means new implementation experience, new voices, new analyses, etc.

Thanks!

Ben.



> On Feb 12, 2019, at 9:10 PM, Bernard Aboba <[e-mail]> wrote:
> 
> Nils --
> 
> This is IETF Last Call.  Its purpose is to determine if there is IETF consensus to publish the PERC documents (and the framework, in particular) as proposed standards.
> 
> As described on this list, a number of potential implementers have seriously considered the PERC framework, but for reasons described in the IETF last call comments, they have encountered problems that have either prevented them from implementing the framework at all, or have caused them to choose a modified approach to one or more parts of it.
> 
> The overall picture is therefore not one of "rough consensus" in the IETF community - but rather of problems with the work that have caused implementers to seek alternatives either within the IETF or outside of it. In at least one case, such an implementation has achieved considerable usage, and other alternative implementations appear inevitable. Overall, this level of dissent is relatively rare within an IETF last call - and so is something for the IESG to take note of.
> 
> I will let the disputants speak for themselves, but the statements made in this IETF last call appear to indicate at least the following points of contention:
> 
> 1.  Problems with implementation of "Double" to the issues encountered with attempted support for FEC/RTX/RED, and the potential need for "triple protection".  These problems appear to have been fixed by implementers who have taken an alternative approach (e.g. "PERC-LITE").  The perceived excessive overhead of "Triple" has also lead to consideration of alternatives with respect to the scope of encryption (e.g. frame encryption versus payload encryption).
> 
> 2. Problems with SSRC-based key identification (leading to the requirement for immutability).  The effects of SSRC immutability have been pointed out, namely the inability to  support some scenarios (such as MOOCs) and also a requirement for support of simulcast reception that is perceived of as difficult to meet (e.g. multiple implementations that are less than robust). I'll let the implementers describe how their alternatives to dealing with this problem, but some of these have been transport independent so as to not allow for dependencies on RTP-specific constructs such as the SSRC.
> 
> 3. Problems with the DTLS-EKT Diet key management scheme.  I believe that this issue may be the impetus for the use of alternative approaches taken with respect to key management (e.g. consideration of EME).
> 
> Given this level of dissent, there would seem to be little chance that the PERC framework as currently described in the WG documents will ever be widely implemented. But despite that, it does seem possible for the WG to make a meaningful contribution - by laying out the problem and solution requirements well enough to guide future discussions.  The current PERC Framework document does not accomplish this goal.
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> In trying to understand the issues, it is
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>  PERC framework document were to at least layout the basic principles that such experiments need to follow.
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> As an example, it would be helpful to understand the implications of utilizing alternative key management schemes.
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> On Tue, Feb 12, 2019 at 5:34 PM Nils Ohlmeier <[e-mail] <mailto:[e-mail]>> wrote:
> Thank you for the input on the framework and the double documents from everyone.
> 
> The points raised by the individuals here are not new and have been discussed before by the WG at several occasions. And for these issues there has be no WG consensus.
> 
> 
> Specifically on the points regarding double encryption:
> At IETF 95 double had consensus and got adopted (after 4 design team meetings and 3 IETF meetings).
>   https://www.ietf.org/proceedings/95/minutes/minutes-95-perc <https://www.ietf.org/proceedings/95/minutes/minutes-95-perc>
> 
> At IETF 96 the WG chairs re-opened the discussions around SSRC mutability and Emil got asked to submit a draft on the security impact of SSRC mutability
>   https://www.ietf.org/proceedings/96/minutes/minutes-96-perc <https://www.ietf.org/proceedings/96/minutes/minutes-96-perc>
> At IETF 98 SSRC immutability and RTX considerations were discussed but no proposal made on security implications
>   https://www.ietf.org/proceedings/98/minutes/minutes-98-perc-00 <https://www.ietf.org/proceedings/98/minutes/minutes-98-perc-00>
> At IETF 99 the double authors and Sergio presented a joint proposal. The WG chairs called for consensus in the room and on the list and concluded that with rough consensus, the proposal got adopted.
>   https://datatracker.ietf.org/meeting/99/materials/minutes-99-perc-01 <https://datatracker.ietf.org/meeting/99/materials/minutes-99-perc-01>
>   https://mailarchive.ietf.org/arch/msg/perc/3fPqYTE2qhT351nvp1b7wkf4-bc <https://mailarchive.ietf.org/arch/msg/perc/3fPqYTE2qhT351nvp1b7wkf4-bc>
> Best regards
>   Nils & Suhas
>   PERC WG chairs
> 
>> On 2Feb, 2019, at 13:37, Sergio Garcia Murillo <[e-mail] <mailto:[e-mail]>> wrote:
>> 
>> PERC may be a valid solution for some scenarios, maybe SIP.
>> 
>> But in regards of WebRTC, my personal opinion is that it is not well suited and that we should do a fresh start, with an analysis of the requirements and specifics of webrtc, define trust models, role of the js apps, UI/UX, IdP and isolated media streams, key management within browser, compatibility with webrtc 1.0, if we need to support it in SDP or not, QUIC, WASM, etc.. and then check if PERC is able to fulfill them or what parts can be reused, if any.
>> 
>> Best regards
>> 
>> Sergio
>> 
>>> 
>>> On 02/02/2019 21:42, Bernard Aboba wrote:
>>>> Sergio -
>>>> 
>>>> In your opinion, what portions of PERC are salvageable, if any? Is this a situation where we need to start over or could some aspect of PERC (e.g. Double if the triple encryption problem were fixed) be suitably modified and then implemented?
>>>> 
>>>> On Feb 2, 2019, at 3:31 PM, Sergio Garcia Murillo <[e-mail] <mailto:[e-mail]>> wrote:
>>>> 
>>>>> I think Emil and Bernard have described quite precisely where we are and how we managed to get here.
>>>>> 
>>>>> In my opinion it would be a big mistake to consider PERC as *THE* solution for end to end encryption for multiconferencing, as if there was a one size fits all solution for the problem.
>>>>> 
>>>>> Speaking from a WebRTC perspective, PERC, apart of have taken some controversial technical decisions (OHB as header, the ssrc rewriting issue and reverse the the order of FEC/RTX and SRTP), does not take into consideration the specifics of WebRTC (it could be argued that that was not in the scope of this group), like the role of the js app, the possibility of               allowing key management in js, or the interaction with Idp and isolated media streams. Not to speak about the recent discussions about full frame vs per packet encryption or QUIC.
>>>>> 
>>>>> Best regards
>>>>> Sergio
>>>>> 
>>>>> 
>>>>> On 02/02/2019 18:42, Emil Ivov wrote:
>>>>>> 
>>>>>> 
>>>>>> On Sat 2 Feb 2019 at 16:50, Bernard Aboba <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>> Emil said:
>>>>>> 
>>>>>> "The need to do a triple encryption for example is a particularly egregious consequence of the order problem. That’s a problem specific to the “double” documents."
>>>>>> 
>>>>>> [BA] Can you describe how the need for "triple encryption" arises?  The framework document doesn't even mention the issues with ordering of FEC/RTX/RED and encryption, let alone the need for "triple encryption".
>>>>>> 
>>>>>> One of the goals that some members of the group seemed to have was to allow specific applications to become PERC-compliant without changing the app code itself and by simply replacing the libsrtp library with a PERC-enabled one.
>>>>>> 
>>>>>> I don’t know that this goal is a direct consequence of the framework’s conceptual approach (contrary to the imposition of key distribution and negotiation). I think it simply carries a promise for some minimal pragmatic value to some implementers.
>>>>>> 
>>>>>> The issue with this approach is that it leaves hop-by-hop protection mechanisms such FEC and RTC unavailable to the SFU as they are usually performed before SRTP, which would make them e2e encrypted.
>>>>>> 
>>>>>> The solution to that is simple. One merely needs to perform e2e encryption first, then apply FEC and/or RTX and only then apply the second (hop-by-hop) layer of SRTP.
>>>>>> 
>>>>>> This approach was referred to as “wedging RTX and FEC” as it places them in between the two encryption operations.
>>>>>> 
>>>>>> While wedging appeared to have overall support in hallway discussions by all SFU implementors except potentially one, it was mysteriously rejected by a subset of the WG and replaced with the following:
>>>>>> 
>>>>>> Applications will apply SRTP-double first and, those that need to use FEC and RTX would have to apply them only after.
>>>>>> 
>>>>>> It was quickly pointed out that this not only destroys the stated “don’t-change-the-app” goal, but also leaves RTX and mostly FEC unprotected and FEC receivers vulnerable to DoS. To this the proponents of this approach simply replied with: “well, those of you who use FEC/RTX will simply do a third round of SRTP”, thus arriving at a total of three encryptions for every packet..
>>>>>> 
>>>>>> The discussions around this topic were highly contentious.
>>>>>> 
>>>>>> Hope this makes things clear,
>>>>>> Emil
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> On Sat, Feb 2, 2019 at 11:46 AM Emil Ivov <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>> Yes pretty much those.
>>>>>> 
>>>>>> The need to do a triple encryption for example is a particularly egregious consequence of the order problem. That’s a problem specific to the “double” documents.
>>>>>> 
>>>>>> I would however also say that the decision to bake one specific way of performing key negotiation into the framework rather than leaving it open was both unnecessary and quite problematic.
>>>>>> 
>>>>>> Emil
>>>>>> 
>>>>>> On Sat 2 Feb 2019 at 12:23, Bernard Aboba <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>> "on the consensus not reached on this and other topics."
>>>>>> 
>>>>>> [BA] Out of curiosity, what other topics do you consider to be problematic within the framework?  I am aware of at least two others where implementers have chosen different paths than in the PERC framework:
>>>>>> 
>>>>>> * Order of application of encryption versus FEC/RTX/RED
>>>>>> * Whole frame encryption versus payload encryption
>>>>>> 
>>>>>> With respect to consensus, this is IETF last call, one of whose purposes is to determine whether there is IETF consensus to publish this document as a Proposed Standard.  Are you saying that you do not agree that there is an IETF consensus to publish this document as a Proposed Standard?  Or that there is no IETF consensus to publish *any* of the PERC WG output as a Proposed Standard?
>>>>>> 
>>>>>> On Sat, Feb 2, 2019 at 5:40 AM Alexandre GOUAILLARD <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>> +1 on ssrc rewriting, and on the consensus not reached on this and other topics.
>>>>>> 
>>>>>> Sent from my iPhone
>>>>>> 
>>>>>> On 2 Feb 2019, at 17:18, Lorenzo Miniero <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>> 
>>>>>>> +1, SSRC rewriting is pretty much fundamental to all SFUs out there.
>>>>>>> 
>>>>>>> Lorenzo
>>>>>>> 
>>>>>>> Il 2 febbraio 2019 10:19:06 CET, Emil Ivov <[e-mail] <mailto:[e-mail]>> ha scritto:
>>>>>>> I want to second that as it is a particularly major problem: not allowing SSRC rewriting makes the entire framework unusable with SFU implementation I represent as well as every other SFU I am familiar with.
>>>>>>> 
>>>>>>> I am also not sure that I agree with “SSRC rewriting could not be allowed” is a conclusion that ever had any consensus in PERC, regardless of what WG leadership is trying to make everyone believe.
>>>>>>> 
>>>>>>> On Sat 2 Feb 2019 at 06:21, Bernard Aboba <bernard.aboba@gmail..com <mailto:[e-mail]>> wrote:
>>>>>>> Richard said:
>>>>>>> 
>>>>>>> "Again, the answer is clear here, but the document should be clearer.  The working group discussed SSRC rewriting several times, and concluded that SSRC rewriting could not be allowed in this system.  This decision is reflected, e.g., in the fact that the Double transform does not allow modification of SSRCs."
>>>>>>> 
>>>>>>> [BA]  Not being able to rewrite SSRCs has some major implications with respect to requirements on PERC endpoints.  Typically today's MDD will switch between the simulcast streams provided by an endpoint, forwarding only a single stream to other participants, based on the bandwidth, resolution and framerates.  If rewriting of SSRCs is not possible, do PERC endpoints need to be able to receive simulcast? If PERC endpoints do need to be able to receive simulcast, what are the requirements for endpoints?  For example, should endpoints expect the MDD to use RID header extensions to identify the incoming simulcast streams?
>>>>>>> 
>>>>>>> Receiving of simulcast is tricky because the endpoint is receiving multiple streams with different sequence number spaces which may contain holes because of reordering or loss. This not only complicates the application of RTX, RED and FEC, but also the operation of the endpoint.  As a result, as noted in the WEBRTC specification Section 5.4.1, support for reception of simulcast is optional. I am aware of two ORTC implementations that have attempted to support simulcast reception, neither of which is robust in scenarios with considerable loss and/or reordering.  And neither implementation supports the RID header extension on received simulcast streams.
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> On Fri, Feb 1, 2019 at 12:23 PM Sergio Garcia Murillo <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>>> On 01/02/2019 17:18, Richard Barnes wrote:
>>>>>>>> So I would propose we add something like the following to this definition:
>>>>>>>> 
>>>>>>>> "In the context of WebRTC, where control of a session is divided between a JavaScript application and a browser, the browser acts as the Trusted Endpoint for purposes of this framework (just as it acts as the endpoint for DTLS-SRTP in one-to-one calls).
>>>>>>> 
>>>>>>> If we decide to adopt perc (big if) in webrtc, shouldn't this be defined within the https://tools.ietf.org/html/draft-ietf-rtcweb-security-arch-17 <https://tools.ietf.org/html/draft-ietf-rtcweb-security-arch-17> doc ?
>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>>    Optimally, we would not rely on trust in any entities other than the
>>>>>>>    browser.  However, this is unfortunately not possible if we wish to
>>>>>>>    have a functional system.  Other network elements fall into two
>>>>>>>    categories: those which can be authenticated by the browser and thus
>>>>>>>    can be granted permissions to access sensitive resources, and those
>>>>>>>    which cannot be authenticated and thus are untrusted.
>>>>>>> 
>>>>>>> 
>>>>>>> WebRTC already IdP as trusted for identity purposes, so it should be up to the RTCWEB group to decide what is a trusted endpoint and what is not in webrtc. As Bernard is stating, we could decide that there are other key management solutions trusted (even in JS or WASM), as for for example is being done in EME:
>>>>>>> 
>>>>>>> https://github.com/WICG/media-capabilities/blob/master/explainer.md#encryption <https://github.com/WICG/media-capabilities/blob/master/explainer..md#encryption>
>>>>>>> Best regards
>>>>>>> 
>>>>>>> Sergio
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> Perc mailing list
>>>>>>> [e-mail] <mailto:[e-mail]>
>>>>>>> https://www.ietf.org/mailman/listinfo/perc <https://www.ietf.org/mailman/listinfo/perc>
>>>>>>> --
>>>>>>> sent from my mobile
>>>>>>> 
>>>>>>> --
>>>>>>> Inviato dal mio dispositivo Android con K-9 Mail. Perdonate la brevità.
>>>>>> --
>>>>>> sent from my mobile
>>>>>> --
>>>>>> sent from my mobile
>>>>>> 
>>>>>> 
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>>>>> 
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