clock reads impossible??

Vernon Schryver vjs@calcite.rhyolite.com
Fri Jan 4 15:55:49 UTC 2002


> From: me <lar@trib.com>

> I have an inbound sendmail-dccm machine handling about 6000 messages/hour

It would be swell if the bulky checksums among those 6000 could be 
exchanged with other DCC users.  The inter-server flooding mechanism
only sends the reports of checksums that look like spam, since the
default value of `dccd -t` is 20 and for only the body checksums.
Since almost all legitimate mail is sent to far fewer than 20 targets,
that radically reduces the cost of inter-server flooding.

I'd be quite happy to help set up an exchange of checksums.


> ...
> My problem is that as long as the CommuniGatePro machine isn't sending
> outbound email to the outbound machine everything works fine. (very low
> loads) When I activate the outbound messages from CommuniGate the outbound 
> logging services quickly shows the following message:
>
> Jan  3 19:14:59 mailhost0 dccm[1179]: clock reads impossible -12171512
> Jan  3 19:16:05 mailhost0 dccm[1229]: clock reads impossible -10038471
> Jan  3 19:16:16 mailhost0 dccm[1233]: clock reads impossible -1548276
> Jan  3 19:16:20 mailhost0 dccm[1247]: clock reads impossible -10274395
> Jan  3 19:16:48 mailhost0 dccm[1270]: clock reads impossible -5320351
> Jan  3 19:16:58 mailhost0 dccm[1291]: clock reads impossible -10567373
> Jan  3 19:18:03 mailhost0 dccm[1348]: clock reads impossible -3456553

Each of those messages say that the DCC client library thinks that
time has jumped backwards by 12, 10, 1.5, etc. seconds or forward by
more than an hour since the effort to ask a DCC server about a set fo
checksums was started.  Such time jumps would mess up the RTT
measurements.  Since the syslog timestamps don't look crazy, it seems
unlikely that time is really bouncing around.

If the system running dccm is SunOS or AIX, it is conceivable that a
bug I fixed in 10.0.38 might be the cause.  I wasn't building some of
the DCC client library to be thread-safe.  It looks like dcc.trib.com
is running 1.0.37.

I've never seen that impossible clock message actually generated.
I've tweaked it to be a little less obscure in 1.0.42, which I just
released.  1.0.42 includes a fix for a dccm crash seen on SunOS.


> cdcc info reports:

> ...
> dcc.trib.com,-             32XXX XXXXXXX
> # * 63.229.150.17,6277     DCC.TRIB.COM                 TRIB server-ID XXXXX
> #     100% of 32 requests ok   70.75 ms RTT              6 ms queue wait

70 ms is a very long RTT under the circumstances as I understand them. 
Either dcc.trib.com is far more distant from the client then sounds likely,
the client or the server is awfully busy, or something is broken.

> (numbers and password replaced with 'X's)

When cdcc is not run as root or a UID that can normally read the 
/var/dcc/map file, it does not blab the passwords.

> The process table shows dozens of dccm processes. The number of outgoing 
> messages should have been a few hundred/hour, nothing compared to the other
> machine.

Are there really dozens of dccm processes running?  What kind operating
systems are involved?  Systems without kernel thread support (and some
with) show a single process for all of the threads that are a dccm
process.  If there are dozens of separate dccm process, why?


Vernon Schryver    vjs@rhyolite.com



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