[snmp] SNMP Set Response is lost / packet dropped by Westhawk API

[Josh Bers]

Birgit,

In trying to understand why the stack might drop some responses, there are a
few questions:

1. Are set requests retried the same way as get requests? Namely if a
response is not received they are resent? (this might be risky for setting
things that should only be set once...)

2. If an observer for receiving PDU responses does a lot of processing, what
happens to the receiver thread that is listening on the inbound socket?

If answer to above is that the receiver thread and the thread that calls
update on the observer are the same then there may be a problem of dropping
UDP packets...
Since there are no guarantees on UDP and there is a finite queue that the OS
has for UDP sockets (I think 5 deep is the default), then the TCP/IP in the
OS may well just drop response PDU's when there are a lot of them and the
Observers are causing calls to receivePacket to be spaced out.

3. If the answer to the above is that there is a separate thread to listen
for response on the socket and a separate one that dispatches to observers,
then there may be a queuing problem. How does the stack queue up these
received packets?

Josh

-----Original Message-----
From: snmp-bounces at snmp.westhawk.co.uk
[mailto:snmp-bounces at snmp.westhawk.co.uk] On Behalf Of Birgit Arkesteijn
Sent: Tuesday, June 03, 2008 2:44 AM
To: List for discussion of the Westhawk SNMP stack
Subject: Re: [snmp] SNMP Set Response is lost / packet dropped by Westhawk
API

Hi,

This is not good news, it shouldn't happen.
I currently don't have much time to investigate, sorry, and problems 
like this are not easy to reproduce.

Which version of the stack are you using
and which SNMP version are you using?

Some background:
As soon as a SnmpContext sends a request, it's run() method will keep 
listening for incoming responses until destroy() is called. Dealing with 
the response is done on the receive thread.

It might be possible that the context misses an incoming request, 
however, the context retransmits the request a couple of times.
There could be a race condition.


Kevin,
The AsnObject class has a static method called setDebug.
If you call
   AsnObject.setDebug(11);
early on when using the stack, it will print out (amongst many other 
things) every message the SnmpContext receives.

It won't fix your problem, but it will tell you if the stack receives 
the response at the lower level, but somehow not deals with it higher up.

Could you look into it and let us know your findings?

Thanks, Birgit



On 30/05/08 22:07, Josh Bers wrote:
> I too have noticed problems with set requests on the stack when attempting
> to change snmpv3 passwords using the USM mib, RFC 3414...
> 
> -----Original Message-----
> From: snmp-bounces at snmp.westhawk.co.uk
> [mailto:snmp-bounces at snmp.westhawk.co.uk] On Behalf Of Kevin Bond
> Sent: Friday, May 30, 2008 5:37 AM
> To: snmp at snmp.westhawk.co.uk
> Subject: [snmp] SNMP Set Response is lost / packet dropped by Westhawk API
> 
> 
> Hi,
> 
>      I have been using WestHawk SNMP API for few years now.     Currently
I
> am facing a peculiar packet loss issue.
> 
>      I have a program that sends a series of set requests to an SNMP
> device. All the set-requests work perfectly in standalone. When executed
one
> after another, they execute successfully 5 out of six times in the other
> instance I get a Timeout exception in a Random place.
> 
> When I attempted to trace the packets with Wireshark (assuming that it is
a
> device problem) I noticed that the response for the set request was
properly
> received by the machine immediately, but It was not captured by the
> particular Westhawk session.
> 
> I am not sure how to fine tune the WestHawk API to prevent it from
dropping
> any packets. Thanks, Kevin 
 >

-- 
-- Birgit Arkesteijn, birgit at westhawk.co.uk,
-- Westhawk Ltd, Albion Wharf, 19 Albion Street, Manchester M1 5LN, UK
-- Company no: 1769350
-- Registered Office:
-- 15 London Road, Stockton Heath, Warrington WA4 6SJ. UK.
-- tel.: +44 (0)161 237 0660
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