Hi,
Martin T je dne 21/08/14 15:12 napisal-a:
if I execute "iperf -c 10.10.10.1 -fm -t 600 -i 60 -u -b 500m" and
10.10.10.1 is behind the firewall so that Iperf client is not able to
reach it, then I will see following results printed by Iperf client:
[ ID] Interval Transfer Bandwidth
[ 3] 0.0 - 60.0 sec 422744 MBytes 59104 Mbits/sec
[ 3] 60.0 - 120.0 sec 435030 MBytes 60822 Mbits/sec
etc
Why does Iperf client behave like that? Is this a know bug?
That's not a bug in iperf, it's how UDP is working. The main difference
between TCP and UDP is that with TCP, IP stack itself takes care of all the
details (such as in-order delivery, retransmissions, rate adaption, ...),
while with UDP stack that's responsibility of application. The only
functionality that iperf application does when using UDP is to fetch the
server (receiving side) report at the end of transmission. Even this function
is not performed in perfect way ... sending side only waits for server report
for short time and if it filled network buffers, this waiting time can be too
short.
The same phenomenon can be seen if there's a bottleneck somewhere between the
nodes and you try to push datarate too high ... routers at either side of the
bottle will discard packets when their TX buffers get filled up. If TCP was
used, this would trigger retransmission in IP stack and all of TCP-slow-start
would kick in and sending application would notice drop in throughput. If UDP
was used, IP stack would not react in any way and application would dump data
at top speed.
--
Peace!
Mkx
-- perl -e 'print $i=pack(c5,(41*2),sqrt(7056),(unpack(c,H)-2),oct(115),10);'
-- echo 16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D4D465452snlb xq | dc
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BOFH excuse #252:
Our ISP is having {switching,routing,SMDS,frame relay} problems
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