Re: [BLISS] AD review: draft-ietf-bliss-call-completion-14

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Thread-Topic: [BLISS] AD review: draft-ietf-bliss-call-completion-14
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Subject: Re: [BLISS] AD review: draft-ietf-bliss-call-completion-14
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Hi Robert,

thank you very much for the detailed review.

We will provide a new version considering your comments.

Please find already some answers inside.

Best regards, Martin

> Von: []
> Im Auftrag von Robert Sparks
> Gesendet: Freitag, 13. Januar 2012 23:41
> An:;
> Betreff: [BLISS] AD review: draft-ietf-bliss-call-completion-14
> Summary: This document has issues that need to be addressed
>           before progressing to IETF Last Call.
> This document was very difficult to review. Please call out
> anyone who provided a substantive review in the document
> quality section of an updated shepherd writeup - they deserve
> the acknowledgement!
> I think there might be places where the structure of the
> document became stressed as the design changed over time.
> Please consider an editorial pass focusing on organizing the
> resulting implementation requirements in ways that are easier
> to reference.
> Major issues
>    - The document's proposed use of PUBLISH is not consistent with the
>      semantics of that method. It attempts to use PUBLISH to affect
>      the subscription state, not the state of the event being
>      subscribed to (it's telling that PUBLISHing to this event package
>      doesn't allow setting the state being subscribed to). Among other
>      things, this prevents separating the state agent from the state
>      authority.

The concept is the following: The caller sends it's own status information in the PUBLISH request. In this case there is no change in the CC monitor (the queue) state done by the PUBLISH request directly, but either an indirect change of the state based on the information about the caller status. The CC monitor acts as a state composer, composing the state of the queue from the states of the different callers.

>      The document modifies how PUBLISH identifies the
>      resource being manipulated by looking at the From URI and not
>      only at the Request-URI.  How the Callee's agent responds to the
>      request to change this subscription state is underspecified -
>      when can it reject a request? What is the caller supposed to do
>      if the request fails?
As mentioned the PUBLISH is not a RPC, the CC agent on behalf of the callee publishes information about the reachability of the callee for CC recalls. The monitor composes the state. This state can be used to evaluate if it is useful to start a CC recall or not.

>    - As written, the first paragraph of section 9.7 asks this package
>      to violate the basic mechanics of RFC3265. It is a violation of
>      the architecture to completely ignore the the expiration time
>      value requested in an initial or refresh SUBSCRIBE request. The
>      responder may choose an expiration time less than or equal to the
>      value there. It may not choose a longer expiration time for the
>      subscription.
Yes this is unclear, what is meant is that the duration in the exoires answer shall not exceed the remaining CC timer.

>    - There is some important conversation missing from the security
>      considerations section.
>        - The dialog event package requires authentication, and digest
>          authentication is mandatory to implement. This package
>          doesn't appear to require any authentication other than
>          presenting a (possibly well known) URI. More discussion of
>          the policy for accepting subscriptions is needed to allow
>          implementers to protect the privacy of the callee. Otherwise,
>          it becomes trivial to use this package to obtain, for
>          instance, information about the callee's phone usage.
>          Similarly, the presence event package has a rich
>          authorization model, and discusses the security (particularly
>          privacy) implications of having the authorization settings
>          too open.

>        - As written, there does not appear to be any protection
>          against an attacker causing everyone else that might be in a
>          queue to be marked not-available, ensuring his call moves to
>          the front of the queue. He only needs to know the AoRs of the
>          callers he might be competing with and send PUBLISH requests
>          with those AoRs in the From header field.

>        - What keeps a new caller from just adding the m= attribute to
>          a new INVITE in order to get the preferential treatment by
>          the network and the callee's UA described in several sections
>          of the document? Was an approach that used a temp-GRUU
>          considered instead? It would not have the property of being
>          as easy to guess as adding an m= URI parameter to an AoR.
As for many mechanisms in the CC draft also here we had to consider the interworking to the PSTN. There we have only a very basic CC prioritization indicator in the IAM, saying not much more than 'this IAM is prioritized for CC'. That's why the callee's monitor MUST record the From URI from the initial call, to have the chance to check it against incoming INVITEs and verify id this INV is in fact for CC. Clarifying text for this is needed in the draft.

>        - A malicious callee could return several (many) NOTIFYs with
>          different to-tags, each containing a different cc-URI,
>          leading the caller to parallel-fork a large number of
>          subscriptions to a victim.

> Some questions
>    - Section 9.10 calls out that subscribers need to be prepared to
>      get NOTIFYs from multiple places due to forks in the SUBSCRIBE,
>      but nothing in the document explores how this affects the
>      call-completion application. What keeps the following scenario
>      from occurring: Adam tries to call me, but I'm busy (on my desk
>      phone). He subscribes for call completion, and the subscribe gets
>      forked to both my desk and home phone. My home phone is not busy,
>      so it sends a NOTIFY with "ready" right away. Adam's phone calls
>      my home phone.
To avoid this situation Adam should add the 'm' (mode) URI parameter, in this case set to 'BS' (busy subscriber). In this case a CC recall is triggered twhen a busy condition at a callee UA has ended. Of course this situation also depends a littlebit on the forking proxy, does it for the initial INVITE send back the 180 from your home phone (indication CC possible 'NR') and also the 486 (indication CC possible 'BS')? Clarifying text is needed.

>    - What keeps this from happening? Adam calls and I reject his call
>      because I'm waiting for another  (I press X and the phone just
>      reports that I'm busy). Adam's phone subscribes for
>      call-completion and gets a NOTIFY of "ready" - his phone calls
>      mine again, forcing me to re-reject him. This repeats until I
>      take my phone off the hook (or engage a global DND) causing me to
>      not be able to receive the call I was waiting for.
If the callee's monitor does not want to enable the caller to make use of the CC service, it will not insert a Call-Info header field with "purpose=call-completion" in the final response message. Of course Adam's phone could try to sunscribe for CC at your phone, but in this case your phone simply rejects the subscription, which should not affect your reachability for the call you are waiting for.

>    - Would a callee ever want to subscribe to call-completion.winfo to
>      see who's in his queue? Will the current design prevent
>      implementing a server for call-completion.winfo?
Actually we never discussed this option. We will check it.

> Remaining issues (mostly in document order)
>    - application/call-completion needs to be sent to type review.

>    - It would be useful to more carefully describe exactly what the
>      resource being subscribed to.

>    - Please call out how this document updated 3261 in the
>      introduction.

>    - Section 4.2 paragraph 1: Is 100rel required? recommended?

>    - It's not easy to understand from the text why the subscribing UA
>      is attempting to subscribe to multiple URIs (the first occurrence
>      is in 4.2 paragraph 4). Some additional motivating text would
>      help.

>    - The document mischaracterizes 'merged' requests as being those
>      that share the same Call-ID. As Section of RFC3261
>      defines, it's more than that - the things that have to be the
>      same are the From tag, Call-Id, and CSeq. This occurs several
>      places in the document:  6.2 second paragraph, description of
>      example in section 8, 9.7 third paragraph. It's worth noting that
>      the UA core in does this merge detection - you are
>      restating a requirement, not adding one -  you should probably
>      just note that the UA will behave as required by that section of
>      RFC3261.

>    - It's worth explicitly calling out (at least in section 6.2) that
>      you are expecting the subscribing UA to fork its own requests (so
>      that the merge behavior you are describing can take place). This
>      means keeping more than the Call-Id constant. An implementer will
>      have to select or develop a SIP implementation that allows them
>      to do that.

>    - There needs to be additional clarity to the specification of the
>      use of the service-retention indication. What is the caller's
>      (the subscriber's) endpoint supposed to do differently when it
>      sees the service-retention option arrive in a NOTIFY? The
>      difference in the behavior of the callee's system is hard to
>      extract - the most salient description is the last paragraph of
>      4.2.

>    - Section 6.2 first paragraph: m parameter of a SUBSCRIBE SHOULD
>      match the m parameter passed through the Call-Info header. Why is
>      this not MUST?
Again because of the PSTN interworking, on TCAP there isn't an equivalent for all the m-parameter values.

I see there are more questions why there is SHOULD and not MUST. I still have to check them in particular. But as i said, most of the softening in the draft is due to enable an interworking with the PSTN CC service. For the latter more information can be found in ETSI ETS 300 356-18 and ITU-T Q.733.

>    - Why does the document specify a request-disposition of no-cancel
>      for SUBSCRIBE requests? An intermediary cannot send a CANCEL to
>      forked legs of a SUBSCRIBE request in the first place.

>    - In section 6.2 paragraph 4, you mean to say the caller's agent
>      must be prepared to receive multiple NOTIFYs establishing
>      different dialogs for each initial SUBSCRIBE request it sends. It
>      is not possible for the agent to receive multiple (final)
>      responses to the SUBSCRIBE request itself.

>    - The string 'cc-state' appears for the first time in section 6.3
>      with no context. The discussion of state before that in the
>      document is a superset of the states represented with cc-state.
>      Please at least provide a forward pointer. It would be better to
>      explicitly describe what cc-state is before you get to this
>      section.

>    - The first sentence in section 6.3 is hard to parse. Could it be
>      broken into more than one sentence? Why are the SHOULDs in this
>      section not MUSTs?

>    - In section 7.1, why is the callee's monitor required to send at
>      least one non-100 provisional (with a Call-Info in it)? Is it
>      because the final response might not be delivered to the calling
>      endpoint due to forking. If so, don't you need to require 100rel?

>    - Why is the SHOULD in 7.1 paragraph 3 not a MUST?

>    - Why does 7.1 paragraph 4 start "When applicable,"?
Means simply 'if CC is offered'.

>    - In this version of the document, the last paragraph of 7.1 is the
>      only definition of the possible values for the m= URI parameter.
>      It would help to list them with the definition of the parameter
>      itself.

>    - The requirements around forking in section 7.2 paragraph 2 belong
>      in section 9. Why is the requirement to respond with a 482 to all
>      but one fork a SHOULD and not a MUST?

>    - Why is the SHOULD in 7.3 paragraph 2 not a MUST?

>    - Subsections of Section 7 use SHALL instead of MUST - it would be
>      better to be consistent throughout the document.

>    - In 7.4 paragraph one, where you say "if the CC call fails", it
>      would be better to say "if the CC call is not accepted". The call
>      could fail without the callee's monitor seeing any of the
>      signalling.

>    - In 7.4 paragraph 1, last sentence, in what circumstance would the
>      callee's monitor NOT terminate the relevant subscription?
I think this SHOULD should or better must be a MUST.

>    - 7.4 paragraph 2 (which assumes the UA can only handle one call at
>      a time) should be made consistent with 7.3 paragraph 3 (which
>      allows UAs that can support multiple calls)

>    - 7.6 paragraph 1 says "SHALL process the queue as described in
>      subclause 7.3". But 7.3 does not talk about processing queues.

>    - In the example, you show a 487 to the invite and motivate it by
>      some proxy having generated a CANCEL. That proxy would have
>      received a 487, but assuming it got no better responses from any
>      other leg, it would most likely send a 480. If there weren't
>      intervening proxies, the response might be one of several
>      400-class responses (perhaps a 408). Please call out that there
>      may be many variations in this failure response.

>    - Proxies will not aggregate Call-Info header fields from multiple
>      final responses into the response they send upstream. In a
>      general deployment, the only time you will see that the callee
>      supports call-completion (at least given how the capability is
>      signaled in this document) is if it's final response is chosen as
>      "best" by every proxy in the chain. It's worth pointing out that
>      some 4xx responses from the callee's UA are more likely to be
>      chosen as "best" than others. It's also probably worth pointing
>      out that in in situations like you allude to in the example in
>      section 8, when proxies cancel legs, the 487 they stimulate from
>      the callee's UAs are not likely to be chosen as "best".
Text for forking proxies need, s.a.

>    - The third paragraph of section 9.4 is very unclear. I can't parse
>      the first sentence at all. In the second sentence, it might be
>      clearer to say "can never" instead of "cannot" (assuming my guess
>      at what the paragraph is trying to say is correct). The third
>      sentence doesn't make sense, and I wonder if the text matched a
>      previous design better? Moving between available and
>      not-available (using PUBLISH) doesn't affect the subscription
>      duration - what is the sentence trying to talk about when it
>      mentions granting a duration as part of resuming a subscription?
For example if it is clear that you leave your office latest at 8 in the evening, and a subscription for CC arrives at your desk phone at 7:45, expires set to 1 hour, expires in the response should be set to 15 minutes.

>    - Section 9.5 third paragraph points to a format described in
>      section 8. It means to point to section 10.
>    - The description of NOTIFY bodies in section 9.5 allows bodies of
>      type application/sdp to be sent in notifies as long as that type
>      occurs in the Accept header field of the most recent SUBSCRIBE
>      request on the dialog. Is that intentional?
>    - Section 9.6 is vague about a call-completion service specific
>      timer. It points into 9.4 claiming the timer is described there,
>      but 9.4 is talking about subscription duration, only noting that
>      the duration default value is chosen based on a timer value from
>      other specifications. Why is this MAY important? What are the
>      implementations supposed to do with this implication?
>    - In the second paragraph of section 9.7, should the 480 include a
>      retry-after? Why was 403 chosen for long-term-denial _error_
>      situations. Why isn't that a 500?
>    - The first sentence of section 9.8 would be much more effective if
>      it said (or pointed to text that describes) what the event
>      triggering conditions actually are.
>    - The third paragraph of section 9.8 has a MUST requirement that is
>      conditional on an agent initiating an INVITE "promptly", but
>      there's no characterization of "promptly" in the document. How
>      does it account for the time it takes to reconfirm the caller is
>      actually present and available before initiating the INVITE due
>      to a recall? (This should also be accounted for in the first part
>      of the security considerations).
>    - Section 9.9 (corresponding to section 4.4.8 of RFC3261) is not
>      adequate. It needs to actually describe the package specific
>      subscription processing (including how the state is built), or
>      provide a finer reference to where that specification lies than
>      "in this and possibly in other documents". Section 7 has most of
>      this information, but it's fairly widely scattered. Please
>      consider consolidating the normative behavior into one place.
>    - Section 9.11 claims the service typically involves a single
>      notification per notifier per subscription. This cannot be the
>      case. There will typically be three - the initial notify in
>      response to the subscribe request, the notify representing the
>      state transition from queued to ready, and the notify
>      corresponding to the termination of the subscription. (It is not
>      clear from the document when you expect the notification of
>      "ready" to immediately terminate the subscription, if ever.)
>    - The timing restrictions in section 9.11 seem artificial, and
>      interact badly with the application this package is intended to
>      support (the implication is that the server should delay  send a
>      "ready" for example). Can the document explain how these
>      restrictions were chosen?
>    - Why does the call-completion information format make a provision
>      for X- headers since you ignore lines with unknown names?
>    - Instead of saying "Two lines with the same name MUST NOT be
>      present, except where specifically permitted", consider saying
>      "The header lines defined in this document can occur at most once
>      in any given call-completion document. Extensions must define
>      whether defined lines may occur more than once. How likely is
>      this format to be extended? Do these need to be put in a
> registry?
>    - Why does the syntax for cc-URI allow cc-URI header line
>      parameters? You certainly want the URI to be able to contain URI
>      parameters, but when would you ever use the header line
>      parameters? What you have now allows
>      cc-URI: random display text
> <sip:name@domain;uri-param=uri-value>;cc-uri-header-param-name
> =cc-uri-header-param-value
>      How is having that display text ever useful? When would
> you every use
>      a cc-uri-header-param? In other words, why isn't this simply
>      cc-URI = "cc-URI" HCOLON addr-spec?
>    - Item 2 in the security considerations section is unclear. It
>      seems to be placing a requirement on the subscriber (the caller),
>      but it's not clear what that requirement is (don't suspend any
>      subscriptions longer than a typical call? than some duration a
>      user entered for _this_ call? or what?). What's the subscriber
>      supposed to do if it would have suspended a subscription that
>      long - terminate the subscription? How does this protect the
>      privacy of the callee?
>    - The media-type form sections should point to specific sections in
>      this document. Consider calling out the most important
>      interoperability and security considerations.
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