Re: [PATCH net-next] netem: fix classful handling

* Eric Dumazet | 2011-12-29 10:12:02 [+0100]:

quoted

Also, the whole tfifo idea is only to support the wierd idea that
if doing random delay that packets should get reordered based on the
results of the random value; it was an behavior some users wanted
because that is what NISTnet did.

tfifo supports a time ordered queuing, wich mimics some jitter in the
network. This seems quite useful.

I see what you suggest : adding 'time_to_send' in the generic qdisc cb.

But it makes no sense if we attach a reordering qdisc, like SFQ :
A 'high prio' packet will block the whole netem because we'll have to
throttle since this packet time_to_send will be in the future, while
many other elligible packets are in queue.

In other words netem jitter and a qdisc !tfifo will not work. Correct? The
rate extension also peak the last packet to get the reference time (assuming a
strict ordering):

[...]
now = netem_skb_cb(skb_peek_tail(list))->time_to_send;
[...]


We should avoid a different (unseeable) behavior depending on the queue
(tfifo, SFQ).  Another point: operate netem and qdisc on the same computer can
lead to timing abnormalities. In our test setups we operate qdisc/tcp/whatever
setups and netem on more then on computer.

Hagen