Messages in this thread |  | | Date | Thu, 24 Jan 2013 10:44:32 -0800 | From | Rick Jones <> | Subject | Re: Doubts about listen backlog and tcp_max_syn_backlog |
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On 01/24/2013 04:22 AM, Leandro Lucarella wrote: > On Wed, Jan 23, 2013 at 11:28:08AM -0800, Rick Jones wrote: >>> Then if syncookies are enabled, the time spent in connect() shouldn't be >>> bigger than 3 seconds even if SYNs are being "dropped" by listen, right? >> >> Do you mean if "ESTABLISHED" connections are dropped because the >> listen queue is full? I don't think I would put that as "SYNs being >> dropped by listen" - too easy to confuse that with an actual >> dropping of a SYN segment. > > I was just kind of quoting the name given by netstat: "SYNs to LISTEN > sockets dropped" (for kernel 3.0, I noticed newer kernels don't have > this stat anymore, or the name was changed). I still don't know if we > are talking about the same thing.
Are you sure those stats are not present in 3.X kernels? I just looked at /proc/net/netstat on a 3.7 system and noticed both the ListenMumble stats and the three cookie stats. And I see the code for them in the tree:
aj@tardy:~/net-next/net/ipv4$ grep MIB_LISTEN *.c proc.c: SNMP_MIB_ITEM("ListenOverflows", LINUX_MIB_LISTENOVERFLOWS), proc.c: SNMP_MIB_ITEM("ListenDrops", LINUX_MIB_LISTENDROPS), tcp_ipv4.c: NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS); tcp_ipv4.c: NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENDROPS);
raj@tardy:~/net-next/net/ipv4$ grep MIB_SYN *.c proc.c: SNMP_MIB_ITEM("SyncookiesSent", LINUX_MIB_SYNCOOKIESSENT), proc.c: SNMP_MIB_ITEM("SyncookiesRecv", LINUX_MIB_SYNCOOKIESRECV), proc.c: SNMP_MIB_ITEM("SyncookiesFailed", LINUX_MIB_SYNCOOKIESFAILED), syncookies.c: NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT); syncookies.c: NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); syncookies.c: NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
I will sometimes be tripped-up by netstat's not showing a statistic with a zero value...
>> But yes, I would not expect a connect() call to remain incomplete >> for any longer than it took to receive an SYN|ACK from the other >> end. > > So the only reason to experience these high times spent in connect() > should be because a SYN or SYN|ACK was actually loss in a lower layer, > like an error in the network device or a transmission error?
Modulo the/some other drop-without-stat point such as Vijay mentioned yesterday.
You might consider taking some packet traces. If you can I would start with a trace taken on the system(s) on which the long connect() calls are happening. I think the tcpdump manpage has an example of a tcpdump command with a filter expression that catches just SYNchronize and FINished segments which I suppose you could extend to include ReSeT segments. Such a filter expression would be missing the client's ACK of the SYN|ACK but unless you see incrementing stats relating to say checksum failures or other drops on the "client" side I suppose you could assume that the client ACKed the server's SYN|ACK.
>> That would be 3 (,9, 21, etc...) seconds on a kernel with 3 >> seconds as the initial retransmission timeout. > > Which can't be changed without recompiling, right?
To the best of my knowledge.
rick jones
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