Bruno Wolff III wrote:
On Thu, Apr 10, 2008 at 13:39:36 -0400,
Gautam Thaker <gthaker@xxxxxxxxxxxx> wrote:
From my tests it appears that it takes between 4 to 6 seconds after the
"up" command before my UDP packets actually start going out. The tests
are between two machines with addresses 192.168.0.2 <---> 192.168.0.3.
Any hints welcome. The "/sbin/ifconfig eth2 up 192.168.0.2" command
returns right away, but the interface is not ready to go for a few
seconds. What can this be due to?
It might be waiting for a test to see if any other machine is using that IP
address on that subnet. The time delay seems to be reasonable for a timeout
to a whohas arp request. Looking at the network traffic should allow you
to refute this or add some more suggestive evidence that this is the cause.
I have done some investigations as suggested by Bruno. I have two
machines, "a" (192.168.0.2) and "b" (192.168.0.3). These are the only
two machines on the subnet.
I do the "/sbin/ifconfig up/down" in "a", and I use tcpdump to monitor
what packets come thru at "b". I thought that by doing this I can answer
Bruno's question if the delay is due to some "whohas arp" packets and
related timeout. The trace that I capture at "b" shows this:
root@b> /usr/sbin/tcpdump -i eth4
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth4, link-type EN10MB (Ethernet), capture size 96 bytes
# node "a" does "ping b -c 1"
11:27:49.137672 IP a-link0 > b-link0: ICMP echo request, id 10290, seq
1, length 8
11:27:49.137695 IP b-link0 > a-link0: ICMP echo reply, id 10290, seq 1,
length 8
11:27:54.137816 arp who-has b-link0 tell a-link0
11:27:54.137822 arp reply b-link0 is-at 00:04:23:b7:22:a4 (oui Unknown)
node "a" does /sbin/ifconfig eth4 down and then waits for about 30 more
seconds.
node "a" does /sbin/ifconfig eth4 up 192.168.0.2 and waits...
11:28:35.045148 IP6 fe80::204:23ff:feb7:17be > ff02::2: ICMP6, router
solicitation, length 16
11:28:39.045283 IP6 fe80::204:23ff:feb7:17be > ff02::2: ICMP6, router
solicitation, length 16
11:28:43.045367 IP6 fe80::204:23ff:feb7:17be > ff02::2: ICMP6, router
solicitation, length 16
# NOTE: ABOVE SEEMS TO BE IPV6 RELATED, I WONDER IF I COULD DISABLE IPV6
# AND IF ANYTHING WOULD BE DIFFERENT. Comments welcome.
# node "a" does "ping b -c 1" and immediately on "b" one sees:
11:28:55.521795 arp who-has b-link0 tell a-link0
11:28:55.521805 arp reply b-link0 is-at 00:04:23:b7:22:a4 (oui Unknown)
11:28:55.521944 IP a-link0 > b-link0: ICMP echo request, id 11570, seq
1, length 8
11:28:55.521956 IP b-link0 > a-link0: ICMP echo reply, id 11570, seq 1,
length 8
11:29:00.521513 arp who-has a-link0 tell b-link0
11:29:00.521642 arp reply a-link0 is-at 00:04:23:b7:17:be (oui Unknown)
Bottom line, I don't know yet why after a
/sbin/ifconfig eth4 up 192.168.0.4
it takes 5 seconds before ping traffic can actually get thru from node a
to node b. Is there an RFC mandated delay someplace? (I understand
networks but am not a guru.)
Gautam
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