Re: [Lsr] LSR WG Adoption Poll for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-hegde-lsr-flex-algo-bw-con-02

"Acee Lindem (acee)" <acee@cisco.com> Tue, 25 May 2021 23:00 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: Anoop Ghanwani <anoop@alumni.duke.edu>, Tony Li <tony.li@tony.li>
CC: lsr <lsr@ietf.org>, "gregory.mirsky@ztetx.com" <gregory.mirsky@ztetx.com>
Thread-Topic: [Lsr] LSR WG Adoption Poll for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-hegde-lsr-flex-algo-bw-con-02
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Archived-At: <https://mailarchive.ietf.org/arch/msg/lsr/yQTB_0yjXBe3L3A74SFPya00lQM>
Subject: Re: [Lsr] LSR WG Adoption Poll for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-hegde-lsr-flex-algo-bw-con-02
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Speaking as a WG member:

I think the argument for delays < 1 usec is very weak and haven’t heard any compelling arguments.

Thanks,
Acee

From: Lsr <lsr-bounces@ietf.org> on behalf of Anoop Ghanwani <anoop@alumni.duke.edu>
Date: Tuesday, May 25, 2021 at 6:08 PM
To: Tony Li <tony.li@tony.li>
Cc: lsr <lsr@ietf.org>rg>, "gregory.mirsky@ztetx.com" <gregory.mirsky@ztetx.com>
Subject: Re: [Lsr] LSR WG Adoption Poll for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-hegde-lsr-flex-algo-bw-con-02

>>>
That’s not a big deal, but when we make the base more precise, we lose range.  If we go with 32 bits of nanoseconds, we limit ourselves to a link delay of ~4 seconds. Tolerable, but it will certainly disappoint Vint and his inter-planetary Internet. :-)
>>>
The current 24 bits of usec delay isn't that much better at ~16 sec.  (Unless there is a proposal to change that to 32 bits?)

On Tue, May 25, 2021 at 9:53 AM Tony Li <tony.li@tony.li<mailto:tony.li@tony.li>> wrote:

Hi Greg,

Firstly, I am not suggesting it be changed to a nanosecond, but, perhaps, 10 nanoseconds or 100 nanoseconds.

Ok.  The specific precision isn’t particularly relevant to me.  The real questions are whether microseconds are the right base or not, and whether we should shift to floating point for additional range or add more bits.



To Tony's question, the delay is usually calculated from the timestamps collected at measurement points (MP). Several formats of a timestamp, but most protocols I'm familiar with, use 64 bit-long, e.g., NTP or PTP, where 32 bits represent seconds and 32 bits - a fraction of a second. As you can see, the nanosecond-level resolution is well within the capability of protocols like OWAMP/TWAMP/STAMP. As for use cases that may benefit from higher resolution of the packet delay metric, I can think of URLLC in the MEC environment. I was told that some applications have an RTT budget of in the tens microseconds range.

It’s very true that folks have carried around nanosecond timestamps for a long time now.  No question there. My question is whether it is actually useful. While NTP has that precision in its timestamps, the actual precision  of NTP’s synchronization algorithms aren’t quite that strong.  In effect, many of those low order bits are wasted.

That’s not a big deal, but when we make the base more precise, we lose range.  If we go with 32 bits of nanoseconds, we limit ourselves to a link delay of ~4 seconds. Tolerable, but it will certainly disappoint Vint and his inter-planetary Internet. :-)

We could go with 64 bits of nanoseconds, but then we’ll probably only rarely use the high order bits, so that seems wasteful of bandwidth.

Or we can go to floating point. This will greatly increase the range, at the expense of having fewer significant bits in the mantissa.

Personally, I would prefer to stay with 32 bits, but I’m flexible after that.

Tony

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