[MEDIACTRL] Review of draft-ietf-mediactrl-call-flows-10

worley@ariadne.com (Dale R. Worley) Wed, 19 December 2012 20:10 UTC

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From: worley@ariadne.com (Dale R. Worley)
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Subject: [MEDIACTRL] Review of draft-ietf-mediactrl-call-flows-10
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Review of draft-ietf-mediactrl-call-flows-10

The first thing that comes to mind is that this is a massive body of
work, and tremendously useful for the implementer!

I've listed the items in the order they appear in the draft.  Almost
all of them are editorial rather than technical.  The technical
questions are:

item 15 (is the listing of control packages in the SDP important or not)
item 17 (the negotiation of the timeout value is vague)
item 35 (mixing of playback of two recordings)
item 38 (semantics of setgain)
item 53 (how does AS send media INVITE to the correct MS)
item 55 (how does the MS know which AS sent each request)

item 1) various

The indentation of the header lines identifying various messages is
inconsistent.  I can find the following lines in various places:

1. AS -> MS (SIP INVITE)
 A1. AS -> MS (CFW CONTROL, createconference)
  A1. AS1 -> MS (CFW CONTROL, audit IVR)
   1. AS -> MS (CFW SYNC)

However, it may be easiest to have the RFC Editor fix that by giving
them an appropriate instruction, as they are likely to manually adjust
the indentation of the examples anyway.

item 2) various

There are a lot of places where an arrow seems to go from a network
entity to itself, such as "create conf and its ID" below:

                    AS                                 MS
                    |                                  |
                    | 1. CONTROL (create conference)   |
                    |++++++++++++++++++++++++++++++++>>|
                    |                                  |--+ create
                    |                                  |  | conf and
                    |       2. 200 OK (conferenceid=Y) |<-+ its ID
                    |<<++++++++++++++++++++++++++++++++|

However, the event described on the label, "create conf and its ID",
is not the transmission of a message, and so it doesn't seem like it
should be represented by an arrow.  A more accurate alternative would
be:

                    AS                                 MS
                    |                                  |
                    | 1. CONTROL (create conference)   |
                    |++++++++++++++++++++++++++++++++>>|
                    |                                  | create
                    |                                  | conf and
                    |       2. 200 OK (conferenceid=Y) | its ID
                    |<<++++++++++++++++++++++++++++++++|

item 3) section 1

"self-sustained" should be "self-contained".

item 4) section 2

   Note that due to RFC formatting conventions, this document often
   splits SIP/SDP and CFW across lines whose content would exceed 72
   characters.

"whose content ..." is applied to the wrong referent, a "lines", but
those lines are the post-split lines, not the pre-split lines.
Perhaps:

   Note that due to RFC formatting conventions, SIP/SDP and CFW lines
   whose content exceeds 72 characters are split across lines.  This
   line folding is marked by a backslash at the end of the first line.
   This backslash, the preceding whitespace, the following CRLF, and
   the whitespace beginning the next line would not appear in the
   actual protocol contents.

item 5) section 2

   Due to the same 72 characters limitation, this document also
   sometimes splits the content of XML elements across lines: please
   beware that, when this happens, no whitespace is actually meant to
   be neither at the beginning nor at the end of the element content.

This seems to be incorrect, as in many places, an XML *element*, that
is, everything from the start tag to the end tag (see XML spec, rule
"element"), is quite long and is necessarily split across lines, but
is also valid.  (However, that interpretation depends on entities
respecting the common convention that whitespace character content is
to be ignored if it appears immediately within an element that does
not admit immediate character content.)

I believe you meant "start tag" instead of "element".  But in that
case, line breaks between the attributes of a start tag are valid in
XML (see XML spec, rule "STag").  If this is correct, the above
sentence can be omitted.

However, the message B1 in section 7.1 may be an exception.  See the
appropriate item.

item 6) section 3

   This document makes use of the same terminology as the one that can
   be found in the referenced documents.

This is awkward.  Perhaps:

   This document makes use of the same terminology as the referenced
   documents [RFC6230] [RFC6231] [RFC6505] [I-D.ietf-mediactrl-mrb].

item 7) section 4

This section starts with a paragraph over one page long.  Could it be
broken into multiple paragraphs?

item 8) section 4

It seems overly complicated to explicitly describe "categories" to
contain only four things.  Also, the text needs to mention that Figure
3 contains *two* diagrams, otherwise the accounting seems to be
incorrect.  Perhaps:

   Four diagrams are provided.  Two of them (Figure 1 and Figure 2)
   describe normal operation of the framework.  Figure 3 contains two
   diagrams describing asynchronous event notifications.  Figure 1
   embraces the MS perspective, whereas Figure 2 is on on the AS side.
   The upper part of Figure 3 shows how events are generated, on the
   MS side, by issuing a CONTROL message addressed to the AS; events
   are acknowledged by the AS through standard 200 responses.  Hence,
   the behavior of the AS, which mirrors that of the MS, is depicted
   in the lower part of the figure.

item 9) section 4 figure 1

I think the state machine could be clarified:

- Replace the event "API 200" in "API 200/200" with "API terminate",
  which is how the API termination is described on other arcs.

- Replace the event "API 202" in "API 202/202" with "API update",
  which is how the API non-termination is described on other arcs.

- Further, change "API update" to "timer" or something like that, to
  indicate that the MS CFW state machine must generate a 202 *based on
  the passage of time*, not based on the API providing some sort of
  "update".  If these time intervals are globally fixed for the CFW
  protocol, their fixed values could be written in.

- The states "'202' sent" and "'update' sent" have the same outgoing
  transitions, so they should probably be merged.

- Is there truly an "Error" message that is comparable to the "200"
  and "202" messages?  Or is sending "Error" a shorthand for a cluster
  of numeric response codes?  If the latter, it would help to make
  that clearer here in the introductory material.

- What happens if the API detects an error after 202 has been sent?
  There is no "API Error" arc from "'202' sent"/"'update' confirmed".

item 10) section 4 figure 2

Clarifications of the state machine:

- There is no "Error" arc from the "202 received" and "'update'"
  states.  Are errors impossible here?

- The states "202 received" and "'update'" have the same outward arcs
  and so could be merged.

- The two figures are inconsistent regarding whether response numbers
  (200, 202) are within quotes '...'.

- The two figures are inconsistent regarding whether report categories
  (update, terminate) are within quotes '...'.

- The two figures are inconsistent regarding whether the sending of a
  request or response in an arc's label is indicated using "send" or
  not.

item 11) section 4 figure 3

There are similar inconsistencies between figure 3 and figures 1 and 2.

item 12) section 5

The term "COMEDIA" is used, and a reference is given to RFC 4145.  But
that RFC does not use the term "COMEDIA" at all.  The framework I-D
also uses "COMEDIA" without explicitly defining it.  It would be
helpful to insert here a definition of "COMEDIA", which seems to be
synonymous with "media of type application/cfw".

item 13) section 5.1

   If the request is fine, the MS answers with its own offer,

shouldn't this be:

   If the request is fine, the MS answers with its answer,

item 14) section 5.1

   The MS replies with the list of packages it supports, by
   adding a dummy example package to the list provided by the AS.

The "by" should be removed, as the replying is not done "by adding";
instead, the MS replies and also adds.

item 15) section 5.1

   It is worth noting that this exchange of information is not meant
   as a strict or formal negotiation of packages: in case the AS
   realizes that one or more packages it needs are not supported
   according to the indications sent by the MS, it may, or may not,
   choose not to open a control channel with the MS at all, if its
   application logic leads to such a decision.  The actual negotiation
   of control packages is done subsequently through the use of the
   framework SYNC transaction.

This confuses me:  Either the list of packages in the SDP offer/answer
is significant or it is not.  If it is not, why is it provided?

In particular, if the MS may support packages that it does not admit
to in the answer, then when the AS sees an answer that does not
contain the needed packages, it will have to continue with the control
channel connection in order to determine if the MS *actually* supports
the needed packages or not.  Thus, the MS could omit all ctrl-package
attributes in its answers all the time.

Also, it's not clear whether the AS is required to provide the list of
packages it will need.  If it is required, then the SDP can be used to
choose the correct MS based on MS capabilities.  If it is not
required, a different architectural mechanism is needed.

item 16) section 5.3

Whether "keep alive" is hyphenated is not consistent.

item 17) section 5.3

   In the previous section it has been described how a timeout value for
   the keep alive mechanism is negotiated.  Specifically, in the example
   the AS and the MS agreed on a value of 100 seconds.  

In fact, that section does not describe how the value is negotiated
(since it is not clear what happens if the MS dislikes the value
proposed by the AS).  However, this is correct:

   In example in the previous section, the AS and the MS agreed on a
   value of 100 seconds.


item 18) section 6

   Besides, unless stated otherwise, the same UAC session is referenced
   in all the above mentioned examples.

Is "above mentioned" the correct modifier?  The previous paragraph
refers to "the following scenarios".

item 19) section 6 figure 10

It would be useful at this point to make it clear that the two INVITEs
are different SIP dialogs (as these scenarios are constructed).  (Or,
if the AS may be inline and acting as a proxy, describe that there may
be two dialogs or only one.)  Including, replace the second "INVITE
(X)" with "INVITE (Y)" to show that the URIs need not be the same.

Also, why not include the establishment of the Control Channel in the
figure, as that will not need to be duplicated in any further figure.

item 20) section 6 figure 11

Figure 11 appears to be only a caption.  Or is it the SIP signaling?
Expositions of protocol messages are rarely considered to be figures.

item 21) section 6

The phrase "These three identifiers" is confusing because the text has
just introduced four identifiers.

I assume that if the SDP from AS to MS also contains label attributes,
the Mediactrl framework specifies how they will be used (or ignored),
so that the "addressing" of media streams is not made ambiguous.

item 22) section 6.1.2

   The 202+REPORT(s) mechanism is used whenever the MS wants to tell
   the AS that the requested operation might take some more time than
   expected.

This is probably incorrect.  What you mean is, "the requested
operation might take more than *** seconds".  The fixed or variable
time limit should be specified here.

   In this example, the preparation ends before the
   timeout, and so the transaction is concluded with a 'REPORT
   terminate', which acts as the 200 message in the previous example.

The last clause is rather confusing, as 'REPORT terminate' is a
request, not a response, and thus doesn't "act like" a 200 message.
Perhaps: "which reports the end of the transaction (as did the 200
message in the previous example)".

item 23) section 6.2 figure 18

It might be useful to distinguish the four(?) SIP dialogs involved.

item 24) section 6.2.2

Three hours is 10,800 seconds, not 1,800 seconds.  I think the easiest
fix is to replace "3 hours" with "30 minutes".

RFC 6230 section 6.2

   An entity acting as a Control Server, on receiving a request, MUST
   generate a response [...].  The response MUST conform to the ABNF
   Responses MUST NOT include the 'Status' or 'Timeout' message
   headers, [...]

RFC 6230 section 6.3.2.1

   A Control Server issuing a 202 response MUST ensure the message
   contains a 'Timeout' message header.

item 25) section 6.2.3

This section is rather difficult to follow.  I'm not sure why, but
partly it's because the text doesn't rigorously make clear what events
are "before", that is, when the two audio channels of the original
conference are being recorded, and "after", when the two recordings
are being played back.  For instance, in the second paragraph, the last
sentence is "after", but the first two sentences are "before".
Instead, there should be a paragraph break there, and starting "Later,
when we desire to play the whole conversation to UAC, the AS may
take...".

It would help if the UAC which is listening to the playback was called
"UAC3" or something like that, as "UAC" is also a generic term, and
it's not immediately clear that it is unrelated to UAC1 and UAC2.

It would also help if the two *recordings* had names different from
the UACs during the recording.  In particular "play UAC1 on confY" is
hard for the naive reader to understand until he realizes that "UAC1"
means the recording file of the audio generated by UAC1.

item 26) section 6.3

   o  The AS invokes the creation of a new conference instance by means
      of a CONTROL request (1); this request is addressed to the
      conferencing package (msc-mixer/1.0) and contains in the body the
      directive (createconference) with all the desired settings for it;

The antecedent of "it" is not particularly clear.  The antecedent is
"a new conference", which is the 6th preceding noun phrase, which is
only determined because it is the first preceding noun phrase which
has settings.  Better to say "... the desired settings for the new
conference instance".

item 27) section 6.3.1 figure 30

This figure uses "UAC1" and "UAC2", but the other figures in 6.3 use
"UAC A", "UAC B", and "UAC C".  Ditto for figure 31 and some of the
text.

item 28) section 6.3.4 figure 38

The notation "D+[a+c]" is clever, but "D+(A+C)/2" would be more
accurate.  (And see the following item regarding the volume level.)

item 29) section 6.3.4

The volume level at which the main conference is heard in the sidebar
is not stated consistently:

   the main conference.  The mix of the conference, instead, must be
   attached to the sidebar, but with a lower volume (50%), being just a
   background to the actual conversation.  This would allow both B and D

   2.  then, the main conference mix is attached to it as 'sendonly' and
       with a -5dB gain to limit the volume of its own contribution to
       30% (so that B and D can hear each other louder, while still
       listening to what's happening in the main conference in
       background);

  |         |         | B1. CONTROL (join confX-->confY) |
  |         |         |++++++++++++++++++++++++++++++++>>|
  |         |         |                                  |--+ join confX
  |         |         |                                  |  | & confY
  |         |         |                       B2. 200 OK |<-+ sendonly
  |         |         |<<++++++++++++++++++++++++++++++++|    (50% vol)

      <mscmixer version="1.0" xmlns="urn:ietf:params:xml:ns:msc-mixer">
         <join id1="2f5ad43" id2="519c1b9">
            <stream media="audio" direction="sendonly">
               <volume controltype="setgain" value="-5"/>
            </stream>
         </join>
      </mscmixer>

Of these, two references say 50% (-3dB) and two references say 30% or
-5dB.

item 30) section 6.3.5 figure 43

Is it intended that UAC (FCC) does not have any RTP going to the MS?
(Indeed, is it necessarily a UAC?  But perhaps SIP is needed to set up
BFCP.)

item 31) section 6.3.5

The participant UAC is labeled inconsistently as "UAC", "UAC (FCP)",
and "UAC1".  ("UAC" may be correct, as the context for that term my be
describing a generic or abstract UAC.)  Similarly, "UAC (FCC)" and
"UAC2" are used for the chair UAC.

item 32) section 6.3.5

It might be desirable to mention that the previous speaker should be
muted before UAC1 can be un-muted.  Usually, this muting will happen
between the Accepted and Granted BFCP messages, so it probably should
be illustrated in the sequence of events.

item 33) section 6.4

It might be helpful to expand "VCR" the first time it is used.
(Apparently it does not mean "video cassette recorder", and I see no
suitable meaning on the Wikipedia "disambiguation" page for "VCR".)

item 34) section 6.4.2

   In fact,
   rather than having the AS build different framework messages
   according to the current time to build an announcement, 

This is difficult to read.  Perhaps

   In fact, rather than having the AS build an announcement according
   to the current time using different framework messages,

item 35) section 6.4

The discussion of mixing in the background music should be contrasted
with the mixing of the two recordings during playback in section
6.2.3.  Naively, it would seem that the same mixing techniques could
be used in both cases, but the two sections implement mixing in
entirely different ways.

item 36) section 6.4.3

An example contains XML elements that seem to be unnecessarily
complex:

       <rule id="action" scope="public">
         <item>
           *
           <item><ruleref uri="#digit"/></item>
         </item>
       </rule>

As far as I can tell from the SRGS specification, <item> functions
more-or-less as a type of parenthesis, and is needed only when (1) a
"weight" or "repeat" attribute needs to be applied to the group, (2)
delimiting one alternative of a <one-of> element, or (3) constructing
the "null" regular expression (that consumes no input and always
matches).  Thus, these examples can be simplified to:

       <rule id="action" scope="public">
         *
         <ruleref uri="#digit"/>
       </rule>

Then again, if the example is taken from a real interaction, perhaps
it is best to leave it as it really happened.

item 37) section 6.4.3

      to implement is a <collect>, and we are only interested in two-
      digits DTMF strings (maxdigits); the AS is not interested in a

"two-digits" should be "two-digit".  (Because this phrase is used as a
modifier of "DTMF strings", the "digit" is not plural.)

      instructs the MS into modifying the volume of the UAC-->conference

"into modifying" would better be "to modify".

      audio flow (setgain=-5dB sendonly); notice how the request also

This is probably intended to be "-3dB", as all other uses of setgain
in this section (including the displayed messages) use a 3 dB
increment.

item 38) section 6.4.3

This section leads to a question about the semantics of the <volume
controltype="setgain" ...> operation:  Clearly, initially the stream
is output at the same volume as the input media.  But when "<volume
controltype='setgain' value='+1'>" is applied, does that cause the
stream's output volume to be set to +1 dB *relative to the input
media* or to +1 dB *relative to the previous volume setting*?

E.g., if I apply that operation twice, does the second application
leave the amplification at +1 dB or raise it to +2 dB?

The examples in section 6.4.3 suggest the second interpretation.

The text in draft-ietf-mediactrl-mixer-control-package-14 is not clear
on this point: "setgain" is only discussed in section 4.2.2.5.1, and
it says "use the value of the "value" attribute as a specific gain
measured in dB to apply".  That would be most likely to mean "as a
specific gain measured in dB to apply to the input", which suggests
the first interpretation.

In any case, the semantics of this need to be clarified and the
example adjusted accordingly.

item 39) section 7.1

   Each MS makes use of the publishing interface to provide an MRB with
   relevant information.

In the relationship between the MRB and MS, the MRB initiates the
relationship.  Thus, it would be clearer to express this:

   An MRB makes use of the MS's publishing interface to acquire
   relevant information.

   It is
   enough to point out that, in this case, the MRB adds a new voice to
   the 'Packages' it needs support for (mrb-publish/1.0).

"voice" should be "item".

   All
   this messages flow through the control channel as all the messages in
---^^^^
should be "these"
----------------------------------------------------^
insert "do"
   this document.

item 40) section 7.1 message B1

This seems to be the only place in the draft where whitespace has been
introduced where character content is valid, and so there is a
question whether the draft needs to indicate that the whitespace would
not be present on the wire.  The relevant elements are:

        <mixing-modes>
            <audio-mixing-modes>
                <audio-mixing-mode package="msc-ivr/1.0">
                     nbest
                </audio-mixing-mode>
            </audio-mixing-modes>
            <video-mixing-modes activespeakermix="true" vas="true">
                <video-mixing-mode package="msc-mixer/1.0">
                     single-view
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     dual-view
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     dual-view-crop
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     dual-view-2x1
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     dual-view-2x1-crop
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     quad-view
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     multiple-5x1
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     multiple-3x3
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0">
                     multiple-4x4
                </video-mixing-mode>
            </video-mixing-modes>
        </mixing-modes>

I'm not sure if the schema defining urn:ietf:params:xml:ns:mrb-publish
indicates that the whitespace before and after the mixing mode names
is to be ignored.  But this might be a place to use the
backslash-line-folding convention specified in section 2:

        <mixing-modes>
            <audio-mixing-modes>
                <audio-mixing-mode package="msc-ivr/1.0"> \
                     nbest \
                </audio-mixing-mode>
            </audio-mixing-modes>
            <video-mixing-modes activespeakermix="true" vas="true">
                <video-mixing-mode package="msc-mixer/1.0"> \
                     single-view \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     dual-view \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     dual-view-crop \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     dual-view-2x1 \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     dual-view-2x1-crop \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     quad-view \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     multiple-5x1 \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     multiple-3x3 \
                </video-mixing-mode>
                <video-mixing-mode package="msc-mixer/1.0"> \
                     multiple-4x4 \
                </video-mixing-mode>
            </video-mixing-modes>
        </mixing-modes>

item 41) section 7.1, section 7.2, section 7.2.1

   In the Inline mode, instead, the MRB is in the
   path between the AS and the pool of MSs it is handling.

   Anyway, the MRB is also
   conceived to work with ASs that are unaware of its functionality,

   and the SIP addresses of the chosen MSs.

Generally, "AS" and "MS" are used as the plurals of "AS" and "MS", but
in these instances, "ASs" and "MSs" are used.

item 42) section 7.1

   request itself and an SDP related to related to either the creation
-----------------------------^^^^^^^^^^^^^^^^^^^^^
"related to" is duplicated.

item 43) section 7.2.1

   MS the MRB has deemed better capable to fulfill the AS requirements),
-------------------------^^^^^^
This should be "best".
-------------------------------------------------------^^
This should be "AS's" or "AS'" (depending on which spelling convention
we are using).

      targeting the request, and as a consequence picks up two MS which
--------------------------------------------------^^^^^^^^
I think you want "picks" here.

   after the Query the AS<->interaction becomes 1:1, UAC media dialogs
----------------------------^
Something appears to be missing here.

item 44) section 7.2.2.1, section 7.2.2.2, section 7.2.3

I think it would make the diagrams a little easier for the naive
reader to follow if the media types specified in the SDP of the
message bodies were given.  E.g., if an INVITE or 200 carries an
application/sdp body for audio media, that is shown on the diagram.
This would be particularly helpful in 7.2.3, where it takes the reader
more consideration to deduce what the media types must be.  Updated in
this way, the diagrams are as follows.  (But this has removed the
comments noting where the SDP has been copied from one message to
another.  That may need to be improved.)

7.2.2.1.  Inline-aware Mode: CFW-based approach

   AS                      MRB                          MS
    |                       |                           |
    | 1. INVITE             |                           |
    | (multipart/mixed:     |                           |
    |  audio,               |                           |
    |  application/mrb-consumer+xml)                    |
    |---------------------->|                           |
    |       2. 100 (Trying) |                           |
    |<----------------------|                           |
    |                       |--+ Extract SDP and        |
    |                       |  | MRB payloads; handle   |
    |                       |<-+ Consumer request to    |
    |                       |    pick MS                |
    |                       |                           |
    |                       | 3. INVITE                 |
    |                       | (audio)                   |
    |                       |-------------------------->|
    |                       |           4. 100 (Trying) |
    |                       |<--------------------------|
    |                       |                           |--+ Negotiate
    |                       |                           |  | CFW Control
    |                       |                           |<-+ Channel
    |                       |                 5. 200 OK |
    |                       |                   (audio) |
    |                       |<--------------------------|
    |                       | 6. ACK                    |
    |                       |-------------------------->|
    |        Prepare new +--|                           |
    |       payload with |  |                           |
    |    SDP from MS and +->|                           |
    |     Consumer reply    |                           |
    |                       |                           |
    |             7. 200 OK |                           |
    |     (multipart/mixed: |                           |
    |      audio,           |                           |
    |      application/mrb-consumer+xml)                |
    |<----------------------|                           |
    | 8. ACK                |                           |
    |---------------------->|                           |
    |                       |                           |
    |--+ Read Cons. reply   |                           |
    |  | and use SDP to     |                           |
    |<-+ create CFW Chn.    |                           |
    |                       |                           |
    |                                                   |
    |<<############## TCP CONNECTION #################>>|
    |                                                   |
    | CFW SYNC                                          |
    |++++++++++++++++++++++++++++++++++++++++++++++++++>|
    |                                                   |

7.2.2.2.  Inline-aware Mode: Media dialog-based approach

   UAC              AS                     MRB                        MS
    |               |                       |                          |
    | 1. INVITE     |                       |                          |
    | (audio)       |                       |                          |
    |-------------->|                       |                          |
    | 2. 100 Trying |                       |                          |
    |<--------------|                       |                          |
    |               | 3. INVITE             |                          |
    |               | (multipart/mixed:     |                          |
    |               |  audio,               |                          |
    |               |  application/mrb-consumer_xml)                   |
    |               |---------------------->|                          |
    |               |       4. 100 (Trying) |                          |
    |               |<----------------------|                          |
    |               |                       |--+ Extract SDP and       |
    |               |                       |  | MRB payloads; handle  |
    |               |                       |<-+ Consumer request to   |
    |               |                       |    pick Media Servers    |
    |               |                       |                          |
    |               |                       | 5. INVITE                |
    |               |                       | (audio)                  |
    |               |                       |------------------------->|
    |               |                       |          6. 100 (Trying) |
    |               |                       |<-------------------------|
    |               |                       |                       +--|
    |               |                       |   Handle media dialog |  |
    |               |                       |       (connection-id) +->|
    |               |                       |                          |
    |               |                       |                7. 200 OK |       
    |               |                       |                  (audio) |
    |               |                       |<-------------------------|
    |               |                       | 8. ACK                   |
    |               |                       |------------------------->|
    |               |        Prepare new +--|                          |
    |               |       payload with |  |                          |
    |               |    SDP from MS and +->|                          |
    |               |     Consumer reply    |                          |
    |               |                       |                          |
    |               |             9. 200 OK |                          |
    |               |     (multipart/mixed: |                          |
    |               |      audio,           |                          |
    |               |      application/mrb-consumer+xml)               |
    |               |<----------------------|                          |
    |               | 10. ACK               |                          |
    |               |---------------------->|                          |
    |               |                       |                          |
    |               |--+ Read Cons. reply   |                          |
    |               |  | and send SDP       |                          |
    |               |<-+ back to UAC        |                          |
    |    11. 200 OK |                       |                          |
    |       (audio) |                       |                          |
    |<--------------|                       |                          |
    | 12. ACK       |                       |                          |
    |-------------->|                       |                          |
    |               |                       |                          |
    |<<*************************** RTP ******************************>>|
    |               |                       |                          |
    |               |--+ Negotiate          |                          |
    |               |  | CFW channel        |                          |
    |               |<-+ towards MS         |                          |
    |               |    (if needed)        |                          |
    |               |                       |                          |
    |               |<<############## TCP CONNECTION ################>>|
    |               |                                                  |
    |               | CFW SYNC                                         |
    |               |+++++++++++++++++++++++++++++++++++++++++++++++++>|
    |               |                                                  |

7.2.3.  Inline-unaware Mode

   AS                      MRB                          MS
    |                       |                           |
    | 1. INVITE             |                           |
    | (application/cfw)     |                           |
    |---------------------->|                           |
    |       2. 100 (Trying) |                           |
    |<----------------------|                           |
    |                       |--+ Pick a MS              |
    |                       |  | and redirect           |
    |                       |<-+ INVITE there           |
    |                       |                           |
    |                       | 3. INVITE                 |
    |                       | (application/cfw)         |
    |                       |-------------------------->|
    |                       |           4. 100 (Trying) |
    |                       |<--------------------------|
    |                       |                           |--+ Negotiate
    |                       |                           |  | CFW Control
    |                       |                           |<-+ Channel
    |                       |                 5. 200 OK |
    |                       |         (application/cfw) |
    |                       |<--------------------------|
    |                       | 6. ACK                    |
    |                       |-------------------------->|
    |                       |                           |
    |             7. 200 OK |                           |
    |     (application/cfw) |                           |
    |<----------------------|                           |
    | 8. ACK                |                           |
    |---------------------->|                           |
    |                       |                           |
    |                                                   |
    |<<############## TCP CONNECTION #################>>|
    |                                                   |
    | CFW SYNC                                          |
    |++++++++++++++++++++++++++++++++++++++++++++++++++>|
    |                                                   |

item 45) section 7.2.2

   An AS willing to issue a Concumer request by means of SIP, has to do
----------------------------^
This should be "Consumer".

   envisages two kind of SIP dialogs a Consumer request may be sent
-----------------^
This should be "kinds".

   setup a control channel) or a UAC media dialog (a SIP dialog the AS
---^
This should be "set up".  ("set up" functions like a verb; "setup"
functions like a noun.)

   The behaviour in the two cases, which are called respectively CFW-
-------^
Probably better to say "behaviours" here.
   based and Media dialog-based approach, is only slightly different,
------------------------------------------^
Use "are", to go with "behaviors".

item 46) section 7.2.2.1, section 7.2.2.2

   Please note that, to ease the reading of the protocol contents, a
   simple '=_Part' is used whenever a boundary for a 'multipart/mixed'
   payload is provided, instead of the actual boundary that would be
   inserted in the SIP messages.

This seems like a poor strategy, as the resulting figures are not
strictly correct.  Perhaps use "Part" as the boundary value, which
would result in figures that look like the following, which strictly
correct and are just as easy to read:

   [..]
   Content-Type: multipart/mixed;boundary="Part"

   --Part
   Content-Type: application/sdp

   v=0
   o=- 2890844526 2890842807 IN IP4 as.example.com
   [...]

   --Part
   Content-Type: application/mrb-consumer+xml

   <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
   <mrbconsumer version="1.0"
                xmlns="urn:ietf:params:xml:ns:mrb-consumer">
      [...]
   </mrbconsumer>

   --Part--

In both of these sections, just before the message listings, there is
a line containing only:

   o

In the following message titles, "SDP" appears in lower case:

section 7.2.2.1:
    3. MRB -> MS (INVITE sdp only)
    5. MRB <- MS (200 OK sdp)
section 7.2.2.2:
    5. MRB -> MS (INVITE sdp only)
    7. MRB <- MS (200 OK sdp)
    11. UAC <- AS (200 OK sdp)

item 47) section 7.2.3

   pick a MS rather than another.
--------^
Probably should be "one", to contrast with "another".

item 48) section 7.2.3

This example does not give the content of the messages, unlike the
previous two examples.

item 49) section 7.3.1

This section is titled "Query/IAMM".  The call flow does mesh with the
call flow in section 7.2.1, "Query Mode", but it does not mesh with
the call flow in section 7.2.2.1, "Inline-aware Mode: CFW-based
approach" nor with the call flow in section 7.2.2.2, "Inline-aware
Mode: Media dialog-based approach".

It would probably be best to change the title of the section to
"Query".  If necessary, an additional section should be added giving
examples relevant to inline-aware mode.

item 50) section 7.3.1, section 7.3.2

The use of "IAMM" and "IUMM" is a bit opaque here, as those acronyms
are not explicitly expanded anywhere in this document.  They seem to
mean what is called "inline-aware mode" and "inline-unaware mode", but
the acronyms of those phrases aren't the same as the acronyms used.

Perhaps these section titles could be changed to resemble those of
7.2.*, "7.3.1. Query and Inline-aware Mode" and "7.3.2. Inline-unaware
Mode".  That doesn't fix the problem with the acronyms per se but does
make the titles clearer.

item 51) section 7.3.1

We might want to mention that in this scenario, the AS obtains
assignment of an MS *before* any UAC makes a request, in order to have
facilities to satisfy future SIP requests.  That is probably how AS's
operate in practice, but a naive reader might expect that the AS will
make the consumer request only after the AS receives a demand for its
services.

   Nevertheless, there are cases when having an MRB in the signalling
   path ...

You might want to say "the SIP signalling path" here.  By my count,
there are *three* different signalling paths:  the COMEDIA negotiation
(INVITE with application/cfw SDP), the CFW connection, and the SIP
signalling (INVITE with audio/etc. SDP).

item 52) section 7.3.2

   As mentioned previously, in such a case the AS would interact with
What case is "in such a case"?  (This is the first sentence of a
section.)  It seems that this paragraph is trying to contrast "such a
case" with "IUMM", but it's not clear what is going on.

   That said, the IUMM case is also very interesting with respect to the
   media dialogs management.  In fact, in the MRB-unaware mode there
---------^
This should be "dialog".
   would be no Consumer request and an AS would actually see the MRB as
   an MS.  This means that, unlike the previous scenarios, there is
   actually no choice, meaning the MRB would likely be in the signaling
------------^
What is there "no choice" of?  The obvious choice is that of a MS, but
the MRB clearly can choose which MS to sent the request to.
   path anyway.  The MRB could either redirect the AS to a MS directly
   or transparently act a proxy/B2BUA and contact a MS (according to
   implementation-specific policies) on behalf of the unaware AS.

   To overcome the scalability limitations of such an approach, the MR
--------------------------------------------------------------^
You should add here "at least in regard to the MRB being in the SIP
signalling path for all calls" -- as far as I can tell, this is the
only scaling problem this approach removes.  (The major one is that
all calls handled by the AS have to be handled by only one MS.)

   Rather than recurring to explicit redirection, or always
---------------^
This isn't the right word.  Perhaps "resorting" was intended?

item 53) section 7.3.2

   While apparently not a problem, this raises an issue when the same
   unaware AS has several sessions with different MS: the AS would only
   see one "virtual" MS (the MRB) and so it would relay all calls there,
   making it hard for the MRB to understand where these media dialogs
   should belong: specifically, whether the UAC calling belongs to the
   AS application logic leading to MS1 or MS2, or wherever else.

This seems to be a deficiency in the CFW protocol:  If the AS
establishes a CFW channel with the MS (via a SIP URI to an MRB), and
then later the AS wants to addresses a SIP INVITE to the MS, it has no
choice but to use the same SIP URI (which routes to the MRB).  This
makes it impossible for the MRB to associate the SIP INVITE with the
CFW connection, and thus it is difficult for the MRB to route the
INVITE to the same MS.

What seems to be needed is for either the COMEDIA response
(application/cfw) or CFW connection to provide a SIP URI that is the
"real" SIP URI of the far-end of the CFW channel.

Actually, this seems to be a problem even if an AS is addressing an MS
URI, but the URI resolves to a redundant set of MSs:  There seems to
be no way to ensure that when a CFW channel is established that a
subsequent SIP INVITE can be sent to the same MS.  This issue seems to
be discussed in RFC 6230 section 3, but I can't find any solution
there.

item 54) section 8

   receiving requests that may cross the bounds each AS is constrained
   into.
---^
Probably better as "within".

item 55) section 8

The detailed discussion of how the MS enforces security against the
ASs is helpful, but it does not mention how the MS discerns which
requests come from AS1 and which come from AS2.

Dale