Hi all-
I've written a tiny program using raw sockets to perform a reduced set of the
functions of tcpdump for our local network. We want it to be able to handle
collection of traffic at wire speed over a gigabit ethernet link. To test this
functionality, I extended the program to write ethernet packets over a raw
socket, which I figured would be able to transmit at high rates.
Unfortunately, I'm only getting a few megabits per second! I was hoping some
gurus out there could either point me to more documentation on raw sockets under
Linux, especially concerning their use at user-level with kernel 2.2.18.
The weird behavior I see is twofold; first, the initial run of the program will
achieve an order of magnitude lower bandwidth than future runs (some weird
caching?). Second, the bandwidth the program acquires varies depending on the
number of packets sent; performance increases up to about 15Mbps for counts up
to 45 packets, then suddenly the performance drops to about 2Mbps for most runs,
with about 1 in 10 randomly getting the larger values. The machines are
otherwise unloaded.
In more detail, my methodology is pretty simple:
open a raw socket
do ioctl to get interface index from name "eth0"
bind the socket to the interface
get timestamp (rdtsc)
loop {
send a maximum sized packet
}
get timestamp
calculate Mbps from timestamps.
*Any* insight would be greatly appreciated. I would hate to have to start
sending UDP packets to get better performance ;(
Thanks,
-Eric
--------------------------------------------
Eric H. Weigle CCS-1, RADIANT team
ehw@lanl.gov Los Alamos National Lab
(505) 665-4937 http://home.lanl.gov/ehw/
--------------------------------------------
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