Re: [clue] AD Review: draft-ietf-clue-signaling-11

[Robert Hansen <[e-mail]>]

Hi Adam,

Thank you for the very detailed review - I'll find the time to review 
it, respond and post a revised draft ASAP.

Rob

On 03/06/17 01:14, Adam Roach wrote:
> CLUE working group --
>
> I have completed my AD review for the CLUE signaling document. The 
> document is generally in good shape, but I think we'll need another 
> revision before putting it in front of the IETF for last call 
> (especially due to the apparent incomplete removal of support for 
> specifying multiple CLUE groups per session).
>
> Most of my comments below are feedback that the document authors 
> should treat as normal last call comments. The feedback that I 
> consider to block progressing the document, in my role as AD, is 
> explicitly marked with the prefix "BLOCKER", and these will need to be 
> resolved in a new version of the document before progressing it 
> further. Note that it is entirely possible that something I have 
> marked "BLOCKER" may stem from an error on my part; so recognize that 
> these are not demands for change, as much as a need to have things 
> either fixed in the document or explained to me.
>
> Title: The rule of thumb is that all but a small handful of well-known 
> acronyms need to be expanded in titles and abstracts. I recognize that 
> "CLUE" is a bit tortured, as acronyms go, but the title of this 
> document is, broadly speaking, opaque. Please change it to something 
> meaningful, such as "Session Signaling for Controlling Multiple 
> Streams for Telepresence (CLUE)"
>
> BLOCKER: General: It is clear from reading this version of the 
> document that earlier versions contained the notion of multiple 
> "a=group:CLUE" lines in a single SDP description. This version appears 
> to have tried to remove all related text, but there are still enough 
> mentions that talk about CLUE groups in a way that implies that there 
> can be multiples so as to cause confusion. These need to be cleaned 
> up. I call the specific instances out on a section-by-section basis 
> below. I'm mentioning it up here since it's really only one blocker 
> issue with a bunch of instances.
>
> General: The _protocol_ document has a host of different terms for 
> each kind of CLUE message (e.g., "ADVERTISEMENT," "ADV," and 
> "advertisement" for the same operation). This document exacerbates the 
> situation by introducing yet more variations, such as "Advertisement" 
> and "Configure". Please coordinate with draft-ietf-clue-protocol to 
> use a consistent set of names for these operations between the two 
> documents.
>
> Introduction: The convention I see in this document is to write 
> defined terms with initial caps; please replace "encoding group" with 
> "Encoding Group."
>
> Section 3: Please add a reference to RFC3840 (e.g.: 'The "sip.clue" 
> media feature tag [RFC3840] indicates...")
>
> Section 4.2: "Presence of the data channel in a CLUE group..." implies 
> there can be more than one group. Replace "a" with "the."
>
> Section 4.3: ...its "mid" value MUST be included in a CLUE group' 
> implies there can be more than one group. Replace "a" with "the."
>
> Section 4.4.1: "...in a CLUE group as defined above." implies there 
> can be more than one group. Replace "a" with "the."
>
> Section 4.4.1: '..."m=" lines in the same CLUE group in the SDP 
> message...' very strongly implies there can be more than one group. 
> Rephrase, perhaps along the lines of "...CLUE-controlled "m=" lines in 
> the SDP message..."
>
> 4.4.2: 'These "m=" lines are CLUE-controlled and hence MUST include 
> their "mid" in the CLUE group corresponding to the CLUE group of the 
> Encoding they wish to receive.' is getting pretty explicit about the 
> presence of multiple CLUE groups. Fix.
>
> 4.5.2.1, first sentence: Replace "If the recipient is a 
> CLUE-capable..." with "If the recipient of an offer is a CLUE-capable..."
>
> BLOCKER: Section 4.5.2.2: For avoidance of doubt, this section should 
> clearly indicate what the answer should do with CLUE-controlled lines 
> that it has no intention of receiving (for sendonly) or sending (for 
> recvonly). I believe the expectation here is to set the port to zero 
> (rather than, e.g., setting the direction to inactive). The document 
> should explicitly state this behavior: if implementations make 
> different choices between port-zero and inactive and don't expect the 
> other behavior, you can end up with incompatibilities.
>
> Section 4.5.3.1, paragraph 2: My recollection is that telling 
> implementors not to send media is frequently misinterpreted to mean 
> that they don't have to send/receive RTCP either. This causes all 
> kinds of grief. It will probably head off issues if this section is 
> phrased more like "...MAY choose not to send RTP on the 
> non-CLUE-controlled channels (although RTCP is still sent and received 
> as normal) during the period..."
>
> Section 4.5.4.1: 'Subsequent offer/answer exchanges MAY add additional 
> "m=" lines...' -- this should probably also mention "and activate 
> inactive ones."
>
> Section 4.5.4.1: 'Subsequent offer/answer exchanges MAY also 
> deactivate "m=" lines for CLUE-controlled media.' -- again, the 
> interpretation of "deactivate" may be different between implementors. 
> Please be clear about whether this means "a=inactive", port=0, or both.
>
> Section 4.5.4.1: The final paragraph talks about "deactivating" 
> non-CLUE media. Again, this should be explicit about what is meant.
>
> Section 4.5.4.2: 'If, in an ongoing non-CLUE call, an SDP offer/answer 
> exchange completes with both sides having included a data channel "m=" 
> line in their SDP and with the "mid" for that channel in corresponding 
> CLUE groups..." implies that there can be more than one CLUE group. Fix.
>
> Section 4.5.4.3: "...include the data channel in a matching CLUE 
> group..." implies there can be more than one group. Replace "a 
> matching" with "the."
>
> Section 4.5.4.3: "Any active "m=" lines still included in a CLUE 
> group..." implies there can be more than one group. Replace "a" with 
> "the."
>
> Section 4.5.4.3: "Note that this is distinct from cases where the CLUE 
> protocol negotiation fails, or an error occurs in the CLUE protocol; 
> see [I-D.ietf-clue-protocol] for details of media and state 
> preservation in this circumstance." -- I carefully scrubbed the CLUE 
> protocol document to try to determine what this is referring to. 
> Please change it to "see [I-D.ietf-clue-protocol] section X.Y.Z", but 
> replacing "X.Y.Z" with the section that provides the details you 
> allude to.
>
>
> BLOCKER: Compare the normative statements in paragraph 2 of Section 5.3:
>
>    Generally, implementations that receive messages for which they have
>    incomplete information SHOULD wait until they have the corresponding
>    information they lack before sending messages to make changes related
>    to that information.  For example, an answerer that receives a new
>    SDP offer with three new "a=sendonly" CLUE "m=" lines for which it
>    has received no CLUE Advertisement providing the corresponding
>    capture information SHOULD include corresponding "a=inactive" lines
>    in its answer, and SHOULD make a new SDP offer with "a=recvonly" when
>    and if a new Advertisement arrives with Captures relevant to those
>    Encodings.
>
> With the normative statements in section 4.5.2.2:
>
>    If the initial offer contained "a=recvonly" CLUE-controlled media
>    lines the recipient SHOULD include corresponding "a=sendonly" CLUE-
>    controlled media lines for accepted Encodings
>    ...
>    If the initial offer contained "a=sendonly" CLUE-controlled media
>    lines the recipient MAY include corresponding "a=recvonly" CLUE-
>    controlled media lines
>
> 5.3 says "SHOULD set a=inactive" in the exact same circumstances 
> 4.5.2.2 says "SHOULD set a=sendonly". Please pick one expected 
> behavior and make sure both sections agree. Ideally, you would 
> refactor this so that the normative statement is made in only one 
> location.
>
>
> Section 7 appears to be oddly silent on the use of RTCP MUX. I suspect 
> that the intention is that CLUE will generally multiplex RTCP when 
> using BUNDLE? I would expect this to be called out.
>
> Section 7 offers that the use of BUNDLE has the advantage of reducing 
> the number of ICE candidates that need to be collected. This is true 
> only in the case that some m= section are marked as bundle-only; this, 
> in turn, implies that a bundled offer is going to contain one or more 
> m= sections that will fail if the remote endpoint does not implement 
> BUNDLE. This interacts particularly badly with the text in section 
> 4.5.1 that allows, under certain circumstances, that 'the initial 
> offer MAY contain "m=" lines for CLUE-controlled media.', since you 
> can end up with the whole CLUE session falling apart if the 
> bundle-only lines are not chosen carefully. This is a relatively 
> complicated issue that I wouldn't expect impementors to get correct 
> without some guidance on the use of bundle-only, and in particular its 
> interaction with CLUE-controlled lines. Please add text to Section 7 
> that discusses these issues.
>
> Section 8: Please insert a page break before the ladder diagram so 
> that the entity names don't appear on a page by themselves.
>
> Section 8: "In this case Bob is the Channel Initiator..." this isn't 
> clear (and, in fact, it's counterintuitive to me) -- perhaps there 
> should be some text indicating *why* Bob is the Channel Initiator.
>
> General, but surfaced in section 8: The procedures described in this 
> document virtually guarantee that every CLUE call that is established 
> will result in glare (response code 491) behavior. This might cause 
> the operations folks some heartburn, as it means that their error 
> counts will spike once CLUE is deployed. Further, without fairly 
> advanced analysis of the callflow, this will make it impossible to 
> distinguish "expected" CLUE-induced 491s from the oddball actual glare 
> conditions usually signaled by 491. Has any consideration been given 
> to avoiding this situation (e.g., by having the called party wait on 
> the order of one second before attempting to negotiate its encodings)?
>
>
> Section 8 contains the following text:
>
>    Bob also sends his SDP answer as part of SIP 200 OK 2. Alongside his
>    original audio, video and CLUE "m=" lines he includes two active
>    recvonly "m= "lines and a zeroed "m=" line for the third.
>
> This should probably be qualified to indicate that these are the same 
> m-sections as were present in the offer (as opposed to creating new 
> sections).
>
>
> Section 8: Replace "Having received this Alice..." with "Having 
> received this offer, Alice..."
>
> Section 9: "From the lack of the data channel and grouping 
> framework..." -- it should be noted that implementations that end up 
> in communication with normal non-CLUE WebRTC implementations might get 
> a datachannel but no CLUE group. The quoted text implies that the 
> presence or absence of a datachannel can be used to determine CLUE 
> support, which might cause implementors to rely on that exclusively. 
> Please change the text to indicate that the CLUE group should be used 
> as the sole determinant of CLUE support (at least, as far as SDP 
> signaling is concerned).
>
> Section 10: It is rather unusual to include authors in the 
> acknowledgements section. For each of Rob Hansen, Paul Kyzivat, and 
> Christian Groves, I suggest removing the individual's name from either 
> the Acknowledgements section or from the authors list.
>
> Section 11.2: "This specification registers a new media feature tag in 
> the SIP [RFC3264] tree..." This should cite RFC3261 for SIP rather 
> than RFC3264.
>
> Section 11.2: Please indicate "[e-mail]" as the contact for this 
> registration.
>
>
> BLOCKING: Section 12 indicates that no security mechanisms are 
> mandatory to use "due to the issues addressed in [RFC7202]." This 
> vastly misconstrues the intent and text of RFC7202, which (roughly 
> speaking) says "we don't mandate security for RTP in general because 
> it is used in a vast and varied array of applications." CLUE is a very 
> specific, very narrow application of RTP that does not suffer from the 
> generality that makes a one-size-fits-all solution for RTP infeasible. 
> In fact, RFC7202 is quite clear that its guidance applies exclusively 
> to RTP, and not to protocols that *use* RTP: "Documents that define an 
> interoperable class of applications using RTP are subject to 
> [RFC3365], and thus need to specify MTI security mechanisms." If there 
> are specific arguments put forth in RFC7202 that the working group 
> believes also apply to this document, please reiterate them here. The 
> current citation to RFC7202 is problematic, though, since it says the 
> opposite of what this security section implies.
>
> If I understand it correctly, though, the argument to put forth here 
> is that implementations MAY choose not to secure legacy 
> (non-CLUE-controlled) media lines because SIP was specified prior to 
> RFC3365, and was therefore not bound by its requirements; and, as a 
> consequence, many existing SIP endpoints are incapable of using 
> DTLS-SRTP. This rationale is orthogonal to the guidance in RFC7202, 
> and should not cite it.
>
> The use of "DTLS" to refer to "DTLS-SRTP" is also confusing, as they 
> are different things.
>
> I propose changing the paragraph as follows:
>
>    This attack can be prevented by ensuring that the media recipient 
> intends
>    to receive the media packets.  As such, all CLUE-capable devices MUST
>    support key negotiation and receiver intent assurance via DTLS-SRTP
>    [RFC5763] on CLUE-controlled RTP "m=" lines.  As specified in
>    [I-D.ietf-clue-framework], all CLUE-controlled RTP streams must be
>    secured and implemented using mechanisms such as SRTP [RFC3711].  CLUE
>    implementations MAY choose not to require the use of SRTP to secure 
> legacy
>    (non-CLUE-controlled) media for backwards compatibility with older
>    SIP clients that are incapable of supporting SRTP.
>
> Section 12: "To prevent this, SIP signaling SHOULD always be encrypted 
> using TLS..." While I agree with the statement, I think it's a bit 
> outside the purview of this document to specify this. Suggest: "...SIP 
> signaling used to set up CLUE sessions SHOULD..."
>
> /a
>