rfc2886.txt

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Network Working Group                                          T. Taylor
Request for Comments: 2886                               Nortel Networks
Category: Standards Track                                    August 2000


                             Megaco Errata

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

1. Abstract

   This document records the errors found in the Megaco/H.248 protocol
   document [2], along with the changes proposed in the text of that
   document to resolve them.

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [1].

3. Errata

   All section numbers are those of the relevant text of [2].

   Section: 2 "References"
   ----------
   Editorial: Add missing references to I.363.5 (AAL5) and RFC 1661
   (PPP).

   Editorial: delete unused reference to Q.724.

   Editorial: Leave Recommendations referred to in Annex C undated,
   implying latest issue.

   Editorial: add reference to RFC 2805, the Megaco requirements
   document.




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   Section: 4 "Abbreviations"
   ----------
   Editorial: delete unused abbreviation BRI.

   Editorial: add explanations for GSM and IVR.

   Section: 6.2.2 "TerminationIDs"
   ----------
   Issue: Not clear what wildcard union responses look like.

   Resolution: Add the following text:
    "ie. Given termination Ta with properties p1=a, p2=b, and
   termination Tb with properties p2=c, p3=d a UNION
   response would be T* p1=a, p2=b,c, p3=d."

   Section: 6.2.4 "Termination Properties and descriptors"
   ----------
   Issue: Says that property values set in Null context are remembered
   and reinstated when termination returns to Null.  Contradicts latest
   text in 7.2.3.

   Resolution: Rewrite the first paragraph from the third sentence
   onward, to read:  "Most properties have default values, which are
   explicitly defined in this standard  or in a package (see Section 12)
   or set by provisioning.  If not provisioned otherwise, all
   descriptors except for TerminationState and LocalControl default to
   empty/"no value" when a termination is first created or is returned
   to the null context.  The default contents of the two exceptions are
   described in sections 7.1.5 and 7.1.7."

   Issue: DigitMap entry in table makes explicit reference to DTMF
   tones, but is also intended to apply to other in-band signalling
   systems.

   Resolution: Change existing text to new one as follows: "Defines
   patterns against which sequences of a specified set of events are to
   be matched so they can be reported as a group rather than singly."

   Section: 6.2.5 "Root Termination"
   ----------
   Issue: Root can have statistics as well as properties and events.

   Resolution: add "statistics" to the fourth sentence of the first
   paragraph, listing what is valid for Root.  Add "statistics" to what
   AuditValue can return for Root.






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   Section: 7.1 "Descriptors"
   ----------
   Issue: it is unclear how to express empty descriptors in responses to
   the MGC.

   Proposed resolution: In section 7.1, add the following text after the
   sentence: "Descriptors may be returned as output from a command.":
   "In any such return of descriptor contents, an empty descriptor is
   represented by its name unaccompanied by any list."

   In the ASN.1, add the following choice to the AuditReturnParameter
   production:

     "emptyDescriptors               AuditDescriptor,"

   In the ABNF, add the choice auditItem to the production
   auditReturnParameter.

   Section: 7.1.1 "Specifying Parameters"
   ----------
   Issue: Third paragraph (discussing unspecified parameters): not clear
   which parameters are mandatory.

   Resolution: Replace the first two sentences (each beginning with the
   word "unspecified") with the following text:  "If a required
   descriptor other than the Audit descriptor is unspecified (i.e.
   entirely absent) from a command, the previous values set in that
   descriptor for that termination, if any, are retained.  A missing
   Audit descriptor is equivalent to an empty Audit descriptor.  The
   behaviour of the MG with respect to unspecified parameters within a
   descriptor varies with the descriptor concerned, as indicated in
   succeeding sections."

   Section: 7.1.6 "Stream Descriptor", ABNF (Annex B.2)
   ----------
   Issue: Text indicates use of "true" and "false" values while ABNF
   uses "on" and "off"

   Resolution: Add comment in B.2 that "true" in text corresponds to
   "on" in ABNF, and "false" corresponds to "off".











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   Section: 7.1.8 "Local and Remote Descriptors"
   ----------
   Issue: case where reserveGroup or reserveValue is true, last bulleted
   item beginning: "If the Mode property of the LocalControl descriptor
   is RecvOnly or SendRecv,...", could also have DSP involvement in the
   case of loopback mode.

   Resolution: Add "loopback".

   Section: 7.1.9 "Events Descriptor"
   ----------
   Issue: EventDM is part of syntax but not mentioned in section.

   Resolution: Add text: "If a digit map completion event is present or
   implied in the EventsDescriptor, EventDM is used to carry either the
   name or the value of the associated digit map. See section 7.1.14."

   Issue: Not clear whether events trigger side effects (e.g. signal
   termination) at time of detection, at time of processing (whether or
   not in active Events Descriptor) or at time of notification.  Also,
   the third paragraph of the section fails to mention these side
   effects and limits MG action to notification.

   Resolution: Modify the third paragraph of the section, so that it
   reads:  "When an event (either newly detected or buffered) is
   processed against the contents of an active Events descriptor and
   found to be present in that descriptor ("recognized"), the default
   action of the MG is to send a Notify command to the MG.  Notification
   may be deferred if the event is absorbed into the current dial string
   of an active digit map (see section 7.1.14).  Any other action is for
   further study.  Moreover, event recognition may cause currently
   active signals to stop, or may cause the current Events and/or
   Signals descriptor to be replaced, as described at the end of this
   section."

   Replace "detection" with "recognition" in the fourth paragraph from
   the end, so that it reads:  "Normally, recognition of an event shall
   cause any active signals to stop.  When KeepActive is specified in
   the event, the MG shall not interrupt any signals active on the
   Termination on which the event is detected."

   In the third-last paragraph, replace all three instances of
   "detected" with "recognized".

   Leave the word "detected" as is in the final paragraph.






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   Issue: Case 1, step 2(a) is incorrectly formulated.  Existing logic
   may result in an infinite loop.  Also, the step excludes side effect
   actions such as signal termination.

   Resolution: Modify step 2(a) to read:  "If the event in the queue is
   in the events listed in the new EventsDescriptor, the MG acts on the
   event and removes it from the EventBuffer.  The time stamp of the
   Notify shall be the time the event was actually detected.  The MG
   then waits for a new EventsDescriptor.  While waiting for a new
   EventsDescriptor, any events detected which appear in the current
   EventsBufferDescriptor will be placed in the EventBuffer.  When a new
   EventDescriptor is received, event processing repeats from step 1."

   Section: 7.1.11 "Signals Descriptor"
   ----------
   Issue: There is a need for the MGC to be aware of signalling
   failures.  It is not possible to use the signal completion event to
   monitor for signalling failures only, without also having to process
   successes.

   Proposed Resolution: In section 7.1.11, replace the current text on
   completion notification, beginning: "Finally, the optional parameter
   "notifyCompletion" ..." and continuing to the end of the paragraph
   with the following text:

   "Finally, the optional parameter "notifyCompletion" allows a MGC to
   indicate the circumstances under which it wishes to be notified when
   the signal finishes playout.  The possible cases are that the signal
   timed out, that it was interrupted by an event, that it was halted
   when a Signals descriptor was replaced, or that it stopped or never
   started for other reasons.  If "notifyCompletion" is absent,
   notification is generated in the "other reasons" case only.  For
   reporting to occur the signal completion event (see section E.1.2)
   must be enabled in the currently active Events descriptor."

   In Annex A.2, make notifyCompletion a BIT STRING with the four cases.
   In B.2, make it a parameter consisting of a name and a list of values
   representing the possible completion methods.

   Section: 7.1.13 "ServiceChange Descriptor"
   ----------
   Issue: Missing "extension" component in the API.

   Resolution: Add missing component.







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   Section: 7.1.14 "DigitMap Descriptor"
   ----------
   Editorial: add the following subsections.

   7.1.14.1 DigitMap definition, creation, modification and termination
   (before first paragraph)

   7.1.14.2 DigitMap Timers (after the first bullet list)

   7.1.14.3 DigitMap syntax (before the long paragraph starting "The
   formal syntax of the digit map [...]")

   7.1.14.4 DigitMap completion event (before the paragraph starting "A
   digit map is active while the events descriptor [...]")

   7.1.14.5 DigitMap procedures (before the sentence "Pending
   completion, successive events shall be processed [...]", preceding
   the numbered list)

   7.1.14.6 DigitMap activation (after the numbered list)

   7.1.14.7 Interaction of DigitMap and event processing (before the
   next paragraph, starting "While the digit map is activated, [...]")

   7.1.14.8 Wildcards (before the the next paragraph, starting "Note
   that if a package contains [...]")

   7.1.14.9 Example (before the example)

   Issue: incorrect statements regarding the interaction of the dot
   symbol with default timing rules.

   Resolution: last sentence of the paragraph beginning "The letter "x"
   is used as a wildcard, ..." should be modified to read:  "As a
   consequence of the third timing rule above, inter-event timing while
   matching a terminal dot symbol uses the short timer by default."  In
   the next paragraph, the third sentence, which begins:  "A timer
   specifier following a dot ...", should be deleted.

   Issue: Step 5 in second numbered list  understates conditions for
   digit map completion when single candidate remains.

   Resolution: Add the words "and it has been fully matched" to the
   existing sentence.  Also remove the parenthetical remark:  "...
   (because the candidate set still contains more than one alternative
   event sequence)" from step 6.





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   Issue: Not clear when unmatched events are recognized and trigger
   side effects (e.g. signal termination).

   Resolution:  Modify the final sentence of the paragraph following
   procedural step 6 to read as follows:  "Normal event behaviour (e.g.
   stopping of signals unless the digit completion event has the
   KeepActive flag enabled) continues to apply for each such event
   detected, except that

     . the events in the package containing the specified digit map
       completion event other than the completion event itself are not
       individually notified, and
     . an event which triggers a "partial match" completion is not
       recognized and therefore has no side effects until reprocessed
       following the recognition of the digit map completion event."

   Issue: the last paragraph before the example is misleading if the
   package containing the digit map completion event is different from
   the package containing the actual digits.

   Resolution: qualify the second sentence of the paragraph as follows:
   "Regardless of whether a digit map is activated, if the package also
   contains the digit events themselves, this form of event
   specification ...".

   Section: 7.1.18 "Topology Descriptor"
   ----------
   Issue: Third bullet attached to third para: says loopback achieved by
   manipulating TerminationMode, which no longer has that name.

   Resolution: Delete the clause "; loopbacks are created by setting the
   TerminationMode"

   Section: 7.2
   ----------
   Issue: The order in which descriptors are processed within a command
   matters, but is not clear.  Moreover, it is fixed in the ASN.1 but
   variable in the ABNF.

   Resolution: Add text stating that descriptors are processed in the
   order in which they are provided.  Change ASN.1 in Annex A.2 to allow
   the MGC to specify ordering:









Taylor                      Standards Track                     [Page 7]