[PLUG] UDP/ICMP packet loss question on RH
John Todd
jtodd at loligo.com
Thu Jul 17 01:55:02 UTC 2003
Forgive my ignorance if this is an obvious question, but I am not a
hardcore Linux person, and so my experiences with the Linux TCP/IP
stack are limited. I am, however, quite fluent with UNIX network
protocols and this is an issue that I have not been able to
adequately Google-search due to the number of hits I get with the
keywords.
I am trying to track down a problem with some VoIP RTP stream jitter
and (perhaps) loss, and the first test I did was to see if a crude
UDP packet loss test would pass. My results were inconclusive.
My test was to send 100 UDP packets via the 'traceroute' program, and
see how many came back. Yes, I know that only half the trip is UDP
and the other is ICMP, but I figured it would be a clue that I should
dig more deeply in that direction.
Here is what I got (IP address and hostname changed to protect the guilty):
[root at gw3 asterisk]# traceroute -f 4 -q 100 10.0.1.33
traceroute to 10.0.1.33 (10.0.1.33), 30 hops max, 38 byte packets
4 something.foo.com (10.0.1.33) 98.391 ms 93.140 ms 92.515 ms
94.367 ms 94.283 ms 110.650 ms * 93.908 ms 94.880 ms 101.979 ms
94.369 ms 98.621 ms 94.046 ms * 95.153 ms 93.934 ms 94.388 ms
94.367 ms 92.354 ms * 95.690 ms 95.359 ms 92.536 ms 96.805 ms
94.040 ms 94.309 ms * 99.286 ms 94.270 ms 94.923 ms 96.644 ms
99.903 ms * 93.415 ms 92.577 ms 98.413 ms 94.253 ms 93.105 ms
93.920 ms * 98.113 ms 102.201 ms 94.287 ms 94.510 ms 98.340 ms *
94.589 ms 93.834 ms 94.358 ms 94.460 ms 94.369 ms 92.612 ms *
96.740 ms 95.016 ms 95.308 ms 94.643 ms 92.585 ms * 94.607 ms
96.653 ms 95.599 ms 95.170 ms 94.210 ms 94.662 ms * 95.179 ms
94.476 ms 92.771 ms 93.929 ms 92.455 ms * 93.142 ms 98.432 ms
92.609 ms 95.418 ms 95.303 ms 100.843 ms * 92.146 ms 92.145 ms
93.523 ms 93.335 ms 92.679 ms * 94.248 ms 94.181 ms 94.578 ms
94.489 ms 92.427 ms 94.810 ms * 93.616 ms 93.377 ms 93.489 ms
125.445 ms 100.531 ms * 92.651 ms 96.571 ms
[root at gw3 asterisk]#
[root at gw3 asterisk]# uname -a
Linux gw3 2.4.7-10 #1 Thu Sep 6 17:27:27 EDT 2001 i686 unknown
[root at gw3 asterisk]#
Note that roughly every sixth packet has been dropped. "Very
strange," I said, "but let's try that on some other hosts to see if
it's consistent." Indeed, every Linux Redhat 7.2 and Redhat 8.0 host
I tested came back with the same symptoms, even between hosts on the
same LAN (where, by the way, I am experiencing no problems with any
VoIP RTP strangeness, even with the same codecs, so it's not a
"problem".)
Now, I have been unable to answer my first question about VoIP
badness (which I think may be software related elsewhere) but my
investigation makes me wonder why something is dropping these
packets, either inbound UDP or outbound ICMP. No other OS I tested
against showed the same symptoms. Is this some kind of rate-limiting
feature in Linux? I know that IOS will not process more than a
certain %age of packets that are ICMP outbound from the router
itself; perhaps someone thought that was a good idea to include in
the Linux TCP/IP stack.
(Don't harass me about the version of Redhat I'm running, or that I'm
running Redhat. I don't own these systems, I'm just the consultant.)
JT
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