Re: Issues with LastModified

Alexey Melnikov wrote:
> Hi,
> 
> I would like to reopen the discussion about LastModified field.
> 
> Firstly, in draft-ietf-sacred-protocol-bss-01.txt UploadRequest doesn't
> contain LastModified anymore. I suspect it was removed by mistake from
> the XML schema. Without LastModified in UploadRequest there are no
> conditional uploads.
> 
> Secondly, it wouldn't be uncommon for a machine running a Sacred server
> also to run a NTP client or similar.
> Now imagine the following sequence of events:
> 1). Client A uploads a request and server assigns a LastModified value
> to it.
> 2). NTP client running on the same machine adjust current time to be 5
> minutes to the past from LastModified in 1).
> 3). Another upload request happens in these 5 minutes, assigned
> LastModified will be less than in 1).
> 
> Thus I suggest you change LastModified to be a sequence counter,
> incremented every time the credential is modified.

Those are two completely different things. A counter could be a nice addition, 
but cannot replace a timestamp. A timestamp shows _when_ a modification took 
place, a counter only _how many_ times.

The problem above can also occur if time is maintained 'manually'. NTP is not 
the problem, it could be the solution. If NTP is running on a machine, it's 
internal clock must me pretty bad, or the machine has been off-line for quite a 
while if it is adjusted by 5 five minutes at once. By the way, if the time on a 
machine is not correct, an NTP daemon usually adjusts the time gradually in 
stead of one big step.

One way to prevent suchs problems might be to compare system times of the server 
and client (based on UTC/GMT). If these differ, say, more than one minute, the 
systems could refuse to communicate any further.

Regards,

Menno Pieters
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