delayed ack timer, slow start and LRO

[jean-pascal billaud <billaud@vmware.com>]

Hi,

I have a question related to the interaction between the delayed ack
timer, slow start and LRO.

If the sender is doing a slow start, it is going to send one packet. The
receiver's delayed ack timer is going to
kick in and when it expires it will send a ack.

Then the sender is going to send 2 packets now. LRO will aggregates them
and the receiver's delayed ack timer is going
to kick in, hoping another packet will arrives which is not going to be
the case. When the timer expires it is going to
send a ack.

The sender is now going to send 4 packets. LRO will aggregate them and
the receiver's delayed ack timer is going to
kick in, hoping another packet will arrives which is not going to be the
case. When the timer expires it is going to
send a ack.

The sender is now going to send ... So I am under the impression that
due to the fact that LRO is aggregating packets,
the delayed ack timer will kick in every single time.

So how is this fixed in linux ? I believe that ABC implementation will
fix this issue even if I am not completely sure
about that ?

Also as LRO adds some latency, it seems possible to me that the sender
retransmission timer will expires before the
delayed ack timer expires. In this case, how is this gonna work ? Is it
possible that the sender will stay stuck in
its slow start trying to retransmit endlessly the same n packets ?

Can anyone help on that ?

thanks,

--jp