Ron,
Lots of thanks for the important input.
I am somewhat less worried about the forthcoming rollover of NTP timestamps in
25 years from now. (Reminds me of Bug 2000:-). But leap seconds represent a
serious technical point that cannot be ignored IMHO.
My 2c,
Sasha
________________________________
From: [email protected] [[email protected]] On Behalf Of Ron Cohen
[[email protected]]
Sent: Thursday, March 03, 2011 9:21 PM
To: [email protected]
Cc: [email protected]; [email protected]; [email protected]
Subject: Re: [TICTOC] draft-ietf-mpls-loss-delay Timestamp
Hi,
I apologize in advance if my comments below where already been discussed.
The draft specify using PTPv1 timestamp format and not PTPv2 timestamp format.
The seconds part of the PTPv2 timestamp is 48bits, and not 32bits as written in
the draft. NTP timestamps will roll over in 2036. PTPv2 timestamps do not
rollover.
I don't think its advisable to include timestamps that rolls over ~20 years
from now.
In addition, NTP timestamp, unlike PTP timestamp, 'jumps' in leap seconds
events. This makes calculation of time differences hard, and would render some
measurements ambiguous. It is much easier to use continuous timescale like PTP.
In my opinion changing the timestamp format to PTPv2 timestamp format (i.e.
48bit/32bit sec/ns) should be the right way to go. I would also remove the NTP
timestamp format option to avoid leap seconds confusion and 2036 rollover
events.
Best,
Ron
On Thu, Mar 3, 2011 at 8:26 PM,
<[email protected]<mailto:[email protected]>> wrote:
My feeling is 1588 is the right answer in the long run since it is already
coupled to the hardware and widely deployed in networks using the "packet
clock derived time" in precise time and frequency delivery.
Pat
"Shahram Davari"
<davari@broadcom.
com> To
Sent by:
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tictoc-bounces@ie
<[email protected]<mailto:[email protected]>>,
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cc
"[email protected]<mailto:[email protected]>"
<[email protected]<mailto:[email protected]>>
03/03/2011 10:05 Subject
AM Re: [TICTOC]
draft-ietf-mpls-loss-delay
Timestamp
Hi Stewart,
Accurate delay measurement requires the Timestamp to be done in HW. It is
true that most routers support NTP today, but the majority of those are
really maintained in Software and AFAIK most HW based timestamps
implementations are 1588 based and really the industry is shifting toward
1588 and SyncE for network synchronization. So I support 1588 as being the
default.
Thanks,
Shahram
From: [email protected]<mailto:[email protected]>
[mailto:[email protected]<mailto:[email protected]>] On Behalf Of
Stewart Bryant
Sent: Thursday, March 03, 2011 8:07 AM
To: [email protected]<mailto:[email protected]>
Cc: [email protected]<mailto:[email protected]>
Subject: [TICTOC] draft-ietf-mpls-loss-delay Timestamp
We have received a LC comment on draft-ietf-mpls-loss-delay concerning the
default timestamp.
In the first version of the draft we proposed NTP, but following initial
comments from the MPLS-TP community we changed to IEEE1588. This
requirement for IEEE1588 can be traced back to the choice of using IEEE1588
in Y.1731.
We have now received a LC request to change the default back to NTP.
NTP is the "natural" choice for an IETF protocol. NTP is specified in IETF
and is used in other MPLS protocols such as LSP ping. It is implemented on
almost every host and every router. However in general the implementations
provides a relatively low precision timestamp, and the NTP time
distribution infrastructure operates on a best effort basis. Thus even a
good client implementation would normally have a relatively low quality
path to the server, which would result in a low quality of timestamp. NTP
could be made to work to higher accuracy, but defacto upgrading NTP and
providing high quality NTP paths to the time servers is not getting much
attention. Computing timestamp differences is easier with NTP.
On the other hand IEEE1588 has defacto become the two way time tranfer
protocol for precision applications. IEEE1588 only provides high quality
time in well engineered networks with some form of hop by hop assistance.
It is not widely implemented other than in equipment targeted to specific
markets, although one of those markets is in mobile backhaul applications
where we expect to see significant initial deployment of MPLS-TP. Computing
timestamp differences is harder with IEEE1588
The loss-delay work is targeting to be able to do one way packet delay
measurement, so it does need a higher quality of timestamp than is required
in general purpose network instrumentation.
Converting from IEEE1588 to NTP is not trivial, since they use different
epochs, and different representations of sub-second time. In a
"traditional" NTP implementation the time error due to on the fly
conversion is likely to be small compared to the time error in the time
synchronization system, but an IEEE1588 system would need hardware
conversion to maintain the accuracy for an NTP timestamp.
So the question arises, should we make IEEE1588 or NTP the default for
draft-ietf-mpls-loss-delay. Much as I would like to suggest that we should
go back to NTP for consistency with other IETF protocols, I have difficulty
reconciling this with the situation in network deployments and thus suggest
that we continue to use IEEE1588.
What is the opinion of the working group?
Stewart (speaking as a draft editor)
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