Date: Thu, 15 Mar 2018 21:05:52 +0100 (CET) From: sthaug@nethelp.no To: freebsd-net@FreeBSD.org Subject: Does FreeBSD do proactive ARP refresh? Message-ID: <20180315.210552.74686746.sthaug@nethelp.no>
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I have a reproducible problem on 11.1-STABLE where, during a longterm iperf3 session, some packets are lost every time ARP is refreshed (every net.link.ether.inet.max_age seconds). Checking with tcpdump, I can indeed see that the packet loss is happening as the hosts are doing ARP request/reply. How to reproduce the problem: - pkg install iperf3-3.5 - Decrease ARP aging timer to 120 (net.link.ether.inet.max_age=120) - Run receiver as "iperf3 -s -i10" - Run sender as "iperf3 -c <receiver> -u -b4m -l100 -i10 -t3600" - Observe packet loss of a few packets every 120 seconds on the receiver I've tried this on several different hosts, with different Ethernet cards. Same result. Also, reducing net.link.ether.inet.max_age isn't *needed* for the problem to occur - it simply makes the problem visible faster. The loss interval is clearly correlated with the net.link.ether.inet.max_age value. Extract of typical iperf3 output on the receiver end showing loss: [ 5] 1810.00-1820.00 sec 4.77 MBytes 4.00 Mbits/sec 0.036 ms 0/50000 (0%) [ 5] 1820.00-1830.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/50000 (0%) [ 5] 1830.00-1840.00 sec 4.77 MBytes 4.00 Mbits/sec 0.006 ms 4/50000 (0.008%) [ 5] 1840.00-1850.00 sec 4.77 MBytes 4.00 Mbits/sec 0.029 ms 0/49997 (0%) [ 5] 1850.00-1860.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/49998 (0%) [ 5] 1860.00-1870.00 sec 4.77 MBytes 4.00 Mbits/sec 0.006 ms 0/50000 (0%) [ 5] 1870.00-1880.00 sec 4.77 MBytes 4.00 Mbits/sec 0.030 ms 0/50000 (0%) [ 5] 1880.00-1890.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/50000 (0%) [ 5] 1890.00-1900.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/50000 (0%) [ 5] 1900.00-1910.00 sec 4.77 MBytes 4.00 Mbits/sec 0.006 ms 0/49999 (0%) [ 5] 1910.00-1920.00 sec 4.77 MBytes 4.00 Mbits/sec 0.006 ms 0/49996 (0%) [ 5] 1920.00-1930.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/50000 (0%) [ 5] 1930.00-1940.00 sec 4.77 MBytes 4.00 Mbits/sec 0.013 ms 0/50000 (0%) [ 5] 1940.00-1950.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/50000 (0%) [ 5] 1950.00-1960.00 sec 4.77 MBytes 4.00 Mbits/sec 0.006 ms 4/50000 (0.008%) [ 5] 1960.00-1970.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/49999 (0%) [ 5] 1970.00-1980.00 sec 4.77 MBytes 4.00 Mbits/sec 0.007 ms 0/49996 (0%) I've tried to read some of the code in /sys/netinet/if_ether.c, and I get impression that FreeBSD is supposed to (proactively) refresh an ARP entry *before* it expires - specifically the arptimer() routine which has the comment * Expiration time is approaching. * Let's try to refresh entry if it is still * in use. However, I'm uncertain of whether my reading here is correct. Can somebody tell me if FreeBSD is supposed to proactively refresh an ARP entry? My idea here is that as long as you have a valid ARP entry, it should be possible to refresh the ARP entry *and* continue sending traffic (with no packet drops) - i.e. the goal is to completely avoid the drops I'm currently seeing on every ARP refresh. Steinar Haug, Nethelp consulting, sthaug@nethelp.no
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