Re: [Detnet] Traffic description in DetNet flow info model

[John Grant <[e-mail]>]

TCP isn't really intended for use with the kind of flows that would use CBR or VBR.

The main facilities provided by TCP are rate control and retransmission of lost packets. When transferring a block of data (such as, say, an image or a pre-recorded video or a PDF) over a best-effort service, these allow the transfer to make best use of what the network provides. But if the network service guarantees a particular data rate you don't need the kind of rate control that TCP does, and if the network hardly ever loses a packet you don't need frequent acknowledgements.

For instance, if you are sending live audio with a sampling rate of 48k samples per second, and packing 12 samples in a packet, you need to send a packet every 250 microseconds, so you need CBR with a rate of 4000 packets (of a certain size) per second.

The VBR service that was defined for ATM was intended for sending compressed video at constant quality, so that "busy" content needs more bits per frame than content with less detail. The peak rate is the bit rate for the "busiest" content, and the average bit rate was also signalled. I think the idea was that you could "overbook" a link by routing flows whose peak rates add up to more than the link capacity, in the expectation that they wouldn't all be demanding the peak rate at the same time. I think something similar is done with digital television multiplexes.

Note that with CBR (and VBR) the rate is determined by the application's requirements, whereas with TCP it is determined by what the best-effort service delivers.

John Grant
Nine Tiles, Cambridge, England
[phone] and [phone]
http://www.ninetiles.com

On 21 Apr 2017, at 08:37, zhayiyong wrote:

> Hi John,
>  
> Thank you for the reply. I agree that make reservation on peak rate is a simple solution. But the “peak rate” here depends on the observation interval times max packets per interval, which means for same flow, different interval leads to different peak rate. Below is a little testbed we built to test the burstness feature of TCP flow.
> For the same 25Mbps TCP flow with no shaping:
>  
> <image007.jpg>
> 1s observation interval, peak rate 450Mbps.
>  
> <image008.jpg>
> 100ms observation interval, peak rate 900Mbps.
>  
> <image009.jpg>
> 10ms observation interval, peak rate 1Gbps, which is the link speed/physical port speed.
>  
> And further test shows that, 8 of these TCP flows to a 1Gbps port cause packet loss. So my question is how can we make reservation based on “peak rate”? E.g., for the same flow, if we take 1s observation interval, we can serve 2 flows. But for 10ms observation interval, only 1 flow, which does not make sense. And it is hard to say long observation interval can guarantee delay and loss, since buffer in router is only milliseconds level.
>  
> Another thing is how to guarantee the source with max packets within the interval. If shaping is necessary, what kind of parameters are needed.
>  
> Cheers,
> Yiyong
> From: detnet [mailto:[e-mail]] On Behalf Of John Grant
> Sent: Thursday, April 20, 2017 5:27 PM
> To: DetNet WG
> Subject: Re: [Detnet] Traffic description in DetNet flow info model
>  
> As pointed out in 4.1.2, for a VBR flow you have to make a reservation for the peak rate, and there isn't any reason to do anything other than use the service that is already defined for CBR. With circuit-switched systems any part of the reservation that wasn't used was wasted, but in packet-based systems it can be used for best-effort traffic, as stated in 4.3.2.
>  
> Regarding synchronous flows, the first paragraph of 4.3.2 explains that if the reservations on incoming and outgoing links are time-aligned then the latency, and hence the amount of buffer space required, can be minimised. The details mechanism for achieving that would, I think, be out of scope for an architecture document.
>  
> For a description of a system that implements synchronous flows, see clause 5 of ETSI GR NGP 003, available from the "specifications" tab at http://www.etsi.org/technologies-clusters/technologies/next-generation-protocols
>  
> John Grant
> Nine Tiles, Cambridge, England
> [phone] and [phone]
> http://www.ninetiles.com
> 
> On 20 Apr 2017, at 08:38, zhayiyong wrote:
> 
> 
> Hi All,
>  
> Recently we have some discussion among authors of the two flow info model draft. The DetNet flow info model should provide some common concepts and description of the flow. One part is traffic specification of the flow. Here is something not clear, is DetNet dealing with VBR flows and what attributes are needed? And how to deal with VBR flow, for source guarantee purpose, do we need to define shaping parameters?
>  
> In architecture draft, section 4.1.2, “The traffic characteristics of an App-flow can be CBR (constant bit rate) or VBR (variable bit rate)”. In section 4.3.2, mentions synchronous flow and asynchronous flow, but no details of that.
> In use case draft, for some cases such as industrial and BAS, it can assumes that the traffic is periodic with constant rate. For cases such as Cellular Radio and M2M, there is usually no assumption on the traffic. E.g., CoMP traffic is between two eNBs, it is hard to say the flow is constant rate.
>  
> Cheers,
> Yiyong