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+
+Ignored SNMP packets
+--------------------
+
+Q. Some network devices do not respond to PySNMP-based management 
+   requests for particular OIDs.
+
+.. code-block:: bash
+
+   $ pysnmpget -v2c -c public 10.0.0.33 1.3.6.1.2.1.2.2.1.10.3
+   SNMPv2-SMI::mib-2.2.2.1.10.3 = Counter32: 1519568842
+   $ snmpget.py -v2c -c public 10.0.0.33 1.3.6.1.2.1.2.2.1.10.4
+   requestTimedOut
+
+   Meanwhile, tcpcump shows request-response sequence:
+
+.. code-block:: bash
+
+   13:33:30.161843 IP 10.0.0.33.snmp > 10.0.0.1.51094: 
+   GetResponse(31)  interfaces.ifTable.ifEntry.ifInOctets.3=1532504859
+   13:33:30.161881 IP 10.0.0.33.snmp > 10.0.0.1.51094: 
+   GetResponse(31)  interfaces.ifTable.ifEntry.ifInOctets.3=1532504859
+
+   In some cases, particularily when running v1arch PySNMP code, the 
+   following exception may be thrown on response processing:
+
+.. code-block:: python
+
+   Traceback (most recent call last):
+   ....
+   File "build/bdist.linux-i686/egg/pyasn1/type/base.py", line 64, in
+  __init__
+   File "build/bdist.linux-i686/egg/pyasn1/type/base.py", line 32, in _verifySubtypeSpec
+   File "build/bdist.linux-i686/egg/pyasn1/type/constraint.py", line 33, in __call__
+   pyasn1.type.error.ValueConstraintError: ConstraintsIntersection(ConstraintsIntersection(), ValueRangeConstraint(0, 4294967295)) failed at: ValueRangeConstraint(0, 4294967295) failed at: -1413698940
+
+A. This appears to be a [widespread] bug in BER integer encoders. It usually 
+   gets noticed on Counter values as they are constrained to be positive while 
+   wrong encoding yelds them negative.
+
+   Here's broken encoding:
+
+.. code-block:: python
+
+   >>> decoder.decode('A\x04\xab\xbc\xaa\x84', asn1Spec=rfc1155.Counter())
+   Traceback (most recent call last):
+   ...
+   pyasn1.type.error.ValueConstraintError: ConstraintsIntersection(ConstraintsIntersection(), ValueRangeConstraint(0, 4294967295)) failed at: ValueRangeConstraint(0, 4294967295) failed at: -1413698940
+
+And here's a good one:
+
+.. code-block:: python
+
+   >>> decoder.decode('A\x05\x00\xab\xbc\xaa\x84',
+   >>> asn1Spec=rfc1155.Counter())
+   (Counter('2881268356'), '')
+
+   Notice the third octet -- positive values must have its highest bit set 
+   to zero.
+
+   Here's an example hack that converts negated values into their positive 
+   complimentaries for Counter type.
+
+.. code-block:: python
+
+   from pysnmp.proto import rfc1155, rfc1902, api
+   from pyasn1.codec.ber import encoder, decoder
+
+   # --- hack Counter type
+
+   def counterCloneHack(self, *args):
+       if args and args[0] < 0:
+           args = (0xffffffff+args[0]-1,) + args[1:]
+
+       return self.__class__(*args)
+
+   rfc1155.Counter.clone = counterCloneHack
+   rfc1902.Counter32.clone = counterCloneHack
+
+   Execute this hack before any SNMP message processing occures in your app.
+
+   The bad news is that if this BER encoding bug also affects Integer values, 
+   in that case it is theoretically impossible to fix because, unlike Counter,
+   Integer values may legally be negative so they could not unconditionally be 
+   converted into positives.
+
+   Therefore the best solutoin would be to get vendors fixing their 
+   BER encoders.
+