Re: [TICTOC] Enterprise Profile

[John Fletcher <[e-mail]>]

Hi Kevin,

Well, I think it already accepted that there are applications where it is useful to know the local time.  Generating SMPTE Timecode is one that you know about.

>From Doug's comment, I'm not clear if he wanted to prohibit something different, i.e. using an ARB timescale to represent local time.  I'm happy to prohibit that but I would like to see the Alternate Timescales TLV of section 16.3 allowed - not required, just allowed.

How can the endstation know which is the correct local time?  Most likely use case is that the PTP deployment is designed so that masters do not serve more than one timezone.  Failing that, it could be user selection based on the name of the alternate timescale.

Yes, you can route multicast messages beyond a subnet but many network administrators, e.g. in my own company's enterprise network, choose not to allow that, especially for "Any Source Multicast" as used by 1588.

John

From: Kevin Gross [mailto:[e-mail]]
Sent: 29 October 2013 15:45
To: John Fletcher
Cc: Douglas Arnold; [e-mail]
Subject: Re: [TICTOC] Enterprise Profile

Multicast IP routing allows multicast messaging beyond a subnet. Multicast IP routing is not available over the internet but it is now fairly common on enterprise networks.

John, can you describe a use case for alternate timescales and explain how the endstation can know that the timescale is correct local time or which alternate timescale to use if there are multiple?

Kevin Gross
[phone]
Media Network Consultant
AVA Networks - www.AVAnw.com<http://www.avanw.com/>

On Tue, Oct 29, 2013 at 4:18 AM, John Fletcher <[e-mail]<mailto:[e-mail]>> wrote:
Just to be clear, I am not suggesting use of a different epoch in timestamp fields.  I am talking about the optional TLV described in section 16.3 which can give you the offset(s) between PTP time and one or more timescales of your choice.

Regarding spanning timezones, there is a difference between the extent of the enterprise network and the extent of the part of network served by a PTP master.  The latter is probably a subnet since multicast messages (e.g. Announce) are usually confined to a subnet.  A subnet spanning multiple timezones would be less common.  However you can have more than one Alternate Timescale if necessary.

This feature is just an option; I'm not suggesting it be required, just that it not be forbidden.

John

From: Douglas Arnold [mailto:[e-mail]<mailto:[e-mail]>]
Sent: 28 October 2013 20:06
To: John Fletcher

Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [TICTOC] Enterprise Profile

Hello John,

I agree that local time is useful.  I would not be averse to creating a TLV to distribute local time offset, so that end node can convert to local time and present it to users.

However, I do not support the idea of using a local time epoch in the PTP timestamp fields. This is an allowed option in IEEE 1588-2008, but I believe that this is likely to cause interoperability problems, when some vendors support it and others do not.  Also some system architechs have indicated that they deploy PTP across large networks, which span time zones. This is the feature that should be forbidden.

In the NTP standard (RFC5905) the timestamps in the packet all use a specific time epoch, and generate all other desired time epoch as corrections performed at the end points.  Note also that in most computer operating systems a single standard time epoch is used in and local time is generated at the presentation layer for human readable interfaces.

Doug

On Mon, Oct 28, 2013 at 8:10 AM, John Fletcher <[e-mail]<mailto:[e-mail]>> wrote:
I'd also like to ask that you don't prohibit the Alternate Timescales option.  This is potentially useful to distribute "local time" (your time-zone and dst) and there doesn't seem to be any need to prohibit it.

John
________________________________
From: [e-mail]<mailto:[e-mail]> [[e-mail]<mailto:[e-mail]>] on behalf of John Fletcher [[e-mail]<mailto:[e-mail]>]
Sent: 25 October 2013 11:01
To: Douglas Arnold
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [TICTOC] Enterprise Profile
Doug,

I would be interested to know the circumstances when you see the maximum phase adjustment information being used.

Some comments:

In section 6, you say, "In all three of the delay measurement modes...".  At that point in the text you have not yet mentioned multicast, hybrid and unicast modes so it's not clear which modes you are referring to.

In section 9, you say, "Slaves SHOULD NOT Synchronize to a Rogue Master."  It's not clear how a slave would know that a master is rogue; presumably it would need to examine the announce messages of all masters and do its own BMCA evaluation.  A more realistic "rogue" master would probably meet the BMCA criteria for selection.

In section 12, "Alternative Time Scales" should be "Alternate Timescales".


Regards,
John Fletcher

From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]<mailto:[e-mail]>] On Behalf Of Douglas Arnold
Sent: 25 October 2013 01:35
To: [e-mail]<mailto:[e-mail]>
Subject: [TICTOC] Enterprise Profile

Hello Everyone,

Attached is an updated version of the Enterprise profile.  Sorry I missed the upload deadline for  the Vancouver meeting.  If you get a chance to look at it before then I will be there and would love to discuss it.

Summary of changes:

I also changed the TLV format to fit the latest proposal in the 1588 revision.

I added a statement about not going into the master state unless one had a current UTC offset.

I added to the TLV fields to indicate maximum phase correction over a sync cycle.

--
Doug Arnold
Principal Technologist
JTime! Meinberg USA
[phone]<tel:%2B1-707-303-5559>





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--
Doug