[e2e] Opportunistic Scheduling.

Caitlin Bestler caitlinb at broadcom.com
Tue Jul 10 09:26:58 PDT 2007

detlef.bosau at web.de wrote:
> 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 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.

I'd phrase that as knowing the earliest time when sending an IP
packet that would not be recklessly congesting the network. And
having an understanding of varying L2 delivery capacity, even without
understanding the cause, would be valuable for that at least when
the variation is on the first segment.

Ultimtately heroic efforts to deliver packets at L2 that stay\
confined to L2 have the potential to be counter-productive. If
you can deliver X frames in time Y then L4 will presume that the
network is capable of doing that and adjust accordingly. Heroic 
efforsts that succeed without an asterisk lead to heroic expectations,
eventually packets get dropped anyway -- possibly more.

Without some sort of feedback, L4 simply has no concept of an
opportunistically delivered packet. Congestion notification probably
plugs a major portion of that hole, but there is probably more required.

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