Re: [Gen-art] Gen-ART review of draft-daboo-webdav-sync-07

Russ Housley <> Fri, 20 January 2012 15:36 UTC

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From: Russ Housley <>
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Date: Fri, 20 Jan 2012 10:36:32 -0500
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To: Peter Saint-Andre <>
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Cc: Cyrus Daboo <>, IETF Gen-ART <>,
Subject: Re: [Gen-art] Gen-ART review of draft-daboo-webdav-sync-07
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I cleared.

On Jan 19, 2012, at 5:54 PM, <> <> wrote:

> Hi Peter,
> Yes, the new -07 of this draft is fine, as it resolves issues [1] and [2] as agreed
> with the authors, and also resolves issue [3] by changing RFC 5842 from a normative
> reference to an informative reference. That change is fine with me, as the references
> to RFC 5842 are all for examples that explain some of the synchronization functionality
> - it is not necessary to implement any of the requests listed as being specified in
> RFC 5842 in order to implement the synchronization functionality.
> Thanks,
> --David
>> -----Original Message-----
>> From: Peter Saint-Andre []
>> Sent: Thursday, January 19, 2012 11:53 AM
>> To: Black, David
>> Cc:;;;
>> Subject: Re: Gen-ART review of draft-daboo-webdav-sync-06
>> Hi David,
>> The text changes that Cyrus proposed have been incorporated into a
>> revised I-D:
>> Please let us know if the new version accurately reflects your
>> discussion with the authors.
>> Thanks!
>> Peter
>> On 1/2/12 12:55 PM, wrote:
>>> Hi Cyrus,
>>> The proposed changes for the two major issues look good to me:
>>> [1] I'm pleased that the concern about adding elements turned out to be a wording issue.
>>> [2] Your proposed new text is fine - it provides adequate notice/warning about possible
>>> collection inconsistency, so I'm ok with not providing pseudo-code.
>>> I'll leave the Downref issue ([3]) for you and Peter to work out with the IESG, and I'm
>>> fine with continued use of your name in the examples if that's common practice.
>>> Thanks,
>>> --David
>>>> -----Original Message-----
>>>> From: Cyrus Daboo []
>>>> Sent: Monday, January 02, 2012 2:44 PM
>>>> To: Black, David;;;
>>>> Cc:
>>>> Subject: Re: Gen-ART review of draft-daboo-webdav-sync-06
>>>> Hi David,
>>>> Thank you for your review. Comments inline:
>>>> --On December 27, 2011 11:07:49 PM -0500 wrote:
>>>>> [1] -Major- Section 3.5 does not appear to cover the case reporting added
>>>>> elements on a subsequent synchronization.  The problem may be that the
>>>>> word "changed" as used in Section 3.5.1 is assumed to cover adding an
>>>>> element - if so, that's not a good assumption, and the addition case
>>>>> should be explicitly called out in the title and body of Section 3.5.1.
>>>> The first sentence of 3.5.1 is:
>>>>    A member URL MUST be reported as changed if it has been mapped as a
>>>>    member of the target collection since the request sync-token was
>>>>    generated.
>>>> The term "mapped" implies creation/addition of a new resource in this case.
>>>> That may not be obvious to anyone who is not intimately familiar with
>>>> WebDAV terminology here, so I propose changing that to:
>>>>    A member URL MUST be reported as changed if it has been newly mapped as
>>>>    a member of the target collection since the request sync-token was
>>>>    generated (e.g., when a new resource has been created as a child of the
>>>>    collection).
>>>>> [2] -Major- The operations to retrieve changed members of a collection
>>>>> are not atomic wrt the operation that obtains a report on what has
>>>>> changed; collection changes can occur between retrieving the report and
>>>>> retrieving the changed elements or while retrieving the changed elements.
>>>>> For this reason, simply obtaining a change report and then retrieving the
>>>>> elements that have changed according to the report may not result in a
>>>>> consistent (e.g., as of a point in time) copy of a collection.  I believe
>>>>> that this absence of atomicity is a WebDAV "feature", as opposed to a
>>>>> "bug", but I believe that this behavior and what to do about it should be
>>>>> discussed in the draft.  I suggest the following, possibly to the end of
>>>>> section 3.1
>>>>> i) Add a sentence or two to warn that obtaining a change report and then
>>>>> retrieving the changed elements may not result in a consistent local
>>>>> version of the collection if nothing else is done because changes may
>>>>> have occurred in the interim.
>>>>> ii Add a discussion of how to ensure that a local copy of the collection
>>>>> is consistent. The basic idea is to re-presented the sync token for that
>>>>> copy to the server after the changed elements have been retrieved; the
>>>>> local copy is consistent if the server reports that there have been no
>>>>> changes.  Some pseudo-code may help, e.g.:
>>>>> 	GetSyncCollectionReport(in token, out newtoken, out report);
>>>>> 	while (ReportHasChangedItems(report) {
>>>>> 		GetChangedItems(report)
>>>>> 		token = newtoken;
>>>>> 		GetSyncCollectionReport(in token, out newtoken, out report);
>>>>> 	}
>>>>> Actual code should include a counter that counts the number of iterations
>>>>> of the while loop and exits with an error if the number of iterations
>>>>> exceeds some limit; that error exit implies that the collection is
>>>>> (currently) changing too rapidly to obtain a consistent local version.
>>>> Good point. I agree that this deserves some additional text to clarify this
>>>> situation. However, I would rather not go into too much detail of how
>>>> clients "re-sync" in cases like this as there are a bunch of different ways
>>>> that could happen each of which depends on exactly what the client is
>>>> trying to do (e.g., in a lot of cases clients will be doing two-way syncs
>>>> so will need to reconcile server and local changes within the loop you
>>>> propose above - the details of that are not in scope for this
>>>> specification). What I propose is the addition of the following paragraph
>>>> to the end of Section 3.1:
>>>>    Typically, a client will use the synchronization report to retrieve the
>>>>    list of changes, and will follow that with requests to retrieve the
>>>>    content of changed resources. It is possible that additional changes to
>>>>    the collection could occur between the time of the synchronization
>>>>    report and resource content retrieval, which could result in an
>>>>    inconsistent view of the collection. When clients use this method of
>>>>    synchronization, they need to be aware that such additional changes
>>>>    could occur, and track them through normal means (e.g., differences
>>>>    between the ETag values returned in the synchronization report and
>>>>    those returned when actually fetching resource content), conditional
>>>>    requests as described in Section 5, or repeating the synchronization
>>>>    process until no changes are returned.
>>>>> [3] -Minor- idnits 2.12.12  reports a Downref to RFC 5842.  Please
>>>>> consult your Area Director (Peter Saint-Andre) to determine what to do
>>>>> about this Downref (it requires attention, but may not require changes to
>>>>> the draft).
>>>> Working with IESG on this one.
>>>>> Nit: I suggest not using the author's own name (cyrusdaboo) in the
>>>>> examples.  Someone may copy the code from the resulting RFC.
>>>> This has been common practice in most of the other CalDAV/CardDAV RFCs I
>>>> have worked on and has not been the source of any problems, so I would
>>>> rather leave this unchanged. If there is an official IETF policy on using
>>>> "real names" in examples, then I would be happy to change to follow that,
>>>> but I am not aware of anything like that.
>>>>> Nit: idnits 2.12.12 reports that draft-ietf-vcarddav-carddav has been
>>>>> published as RFC 6352; the RFC Editor will correct this if a new version
>>>>> of the draft is not required for other reasons.
>>>> Fixed in my working copy.
>>>> --
>>>> Cyrus Daboo