Hi Drew,
Thanks for the detailed explanation. Do you know any reason that iperf doesn’t
or shouldn’t set IP_RECVERR on the sending socket(s)?
Thanks,
Bob
From: Andrew Gallatin [mailto:galla...@gmail.com]
Sent: Monday, September 08, 2014 6:53 AM
To: Martin T
Cc: iperf-users
Subject: Re: [Iperf-users] Iperf client sends out less UDP traffic than
determined with "-b" flag
Please, there are no "dropping" or "non dropping" NIC drivers, unless some
driver is so broken that
it silently returns NETDEV_TX_OK & does not stop its txqueue, but a driver that
broken would
not be accepted into the kernel.
What is really happening is that the linux kernel is silently dropping packets
when the device's
queue is full *AND* when the device queue is too small to hold the entire
socket buffer's worth
of traffic. If you want blocking behavior, you need to ensure you have the
driver's queue
large enough (or the socket buffer small enough) so that the driver's queue can
hold an
entire socket buffer worth of traffic (* the number of active sockets per
queue). You can change
the driver's queue size via ifconfig txqueuelen. It might be better to reduce
the socketbuffer
size (and remember that linux multiplies whatever value you give it by 2, to
account for
slop...).
Note that any silent drops are done by the kernel, not the NIC and that this is
(bizarrely) the expected behavior on linux. If you don't believe me, read the
send(2) man page on linux:
ENOBUFS
The output queue for a network interface was full. This
generally indicates that the
interface has stopped sending, but may be caused by transient
congestion. (Normally,
this does not occur in Linux. Packets are just silently dropped
when a device queue
overflows.)
If iperf wants a correct accounting of the packets sent by the stack vs
dropped, iperf must set
the IP_RECVERR sockopt on the sending socket. This stops the kernel from
silently
dropping packets, and causes send() to return -ENOBUFS rather than 0 when
datagram is dropped due to lack of resources (eg, like BSD does).
Drew
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