[e2e] Opportunistic Scheduling.
detlef.bosau at web.de
Tue Jul 10 06:27:26 PDT 2007
Caitlin Bestler wrote:
> end2end-interest-bounces at 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
> 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 :-}
Detlef Bosau Mail: detlef.bosau at web.de
Galileistrasse 30 Web: http://www.detlef-bosau.de
70565 Stuttgart Skype: detlef.bosau
Mobile: +49 172 681 9937
More information about the end2end-interest