Re: [e2e] Opportunistic Scheduling.

Caitlin Bestler wrote:
> end2end-interest-bounces@postel.org wrote:
>
>> I wonder, why there is absolutely no comment on my post. I
>> expected at least some criticism or contradiction. Or does anybody
>> agree?
>>
>> Detlef
>>
>
> My hunch is that this type of problem needs to be generalized
> so that variable local conditions can be fed back to end-to-end
> congestion/flow control in a way that is both effective and does
> not require the end-to-end logic to understand exactly what the
> local issue is.
>
>

I´m not quite sure about this. Please keep in mind that wireless network conditions may change several times within the transport process of one single IP packet. From that perspective, it is simply not possible for an IP sender to "follow" the wireless channel dynamics because an IP sender can only decide when to send an IP packet or whether to send it at all. This is perhaps far to coarse for wireless networks.

On the other hand, the question is whether TCP/IP needs to follow wireless channel conditions at all. Although this is frequently claimed, I wonder why Ethernet works. Although TCP/IP simply doesn´t care for Ethernet dynamics ;-)

I´m not yet convinced, that wireless channel dynamics really affect flow control and congestion control. As I´m not convinced on the often claimed dreadful spurious timeouts. Regarding spurious timeouts, I frequently refer to Hasenleitner et al. Either you make careful measurements, then you will not find spurious timeouts (or sp. t. are not significant), or you find a significant number of spurious timeouts, then..... (left to the reader).

Back to the subject: I first want to _understand_ the effects of opportunistic scheduling, and therefore I first want to _understand_, how OS works and how the actually used algorithms are justified. And then, let´s see, whether this results in any ramifications to the upper layers. If so, and if there are problems, we can look how to solve them. However, if there are no problems, we must not invent solutions looking for a problem.

And I well keep in mind, what IIRC Joe Touch wrote some months ago: TCP is not supposed to work perfect under any circumstances.

So, if a wireless channel is bad, the TCP connection may be bad. Period. You cannot make a silk purse from a sows ear.

BTW: Does someone happen to know, where I can find mappings from the actual SNR / C/I ratio onto the actual BLER, given a known Coding / Modulation / Puncturing scheme? And is there anything available about the policies how the MCS/PS is chosen with respect to an actual SNR? There must be quite some literature available on this topic, however I frequently fail to find it :-}

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Regards

Detlef

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