On Thu, Mar 5, 2015 at 1:39 PM, Thomas Besemer <thomas.bese...@gmail.com>
wrote:
>
>
> On Thu, Mar 5, 2015 at 7:37 AM, Bill Fenner <fen...@gmail.com> wrote:
>
>> On Wed, Mar 4, 2015 at 11:51 AM, Thomas Besemer <thomas.bese...@gmail.com
>> > wrote:
>>
>>> I am working with net-snmp 5.7.3, and will be using several Sub-Agents
>>> in my project, with fairly large amounts of data. I have noted that
>>> issuing a 'snmpbulkwalk' to Master Agent results in this broken down into
>>> GETNEXT requests to the Sub-Agent. This results in a lot of network
>>> traffic, as well as a tremendous number of context switches (everything
>>> running on same host).
>>>
>>> Is there support in the Sub-Agent to accept GETBULK requests from the
>>> Master? I have looked at the code, their appears to be some infrastructure
>>> in place, but it is not clear to me if it is fully functional.
>>>
>>
>>> More so, is there a means to configure the Master to send GETBULK
>>> requests to the Sub-Agent?
>>>
>>
>> This is something that I've been working on a little. Both of these are
>> relatively small tweaks. However, the bigger problem is that while the
>> subagent is processing the request, it's basically lost the scoping info
>> that the master agent has sent it, and so the response from the subagent
>> can include info from subtrees that the master didn't ask for. There's
>> code in the subagent to handle this for getnext, and we could do the same
>> for getbulk, except I was also concerned about the overhead of fetching
>> this data within the subagent - I wanted to find a way to retain the
>> scoping info while processing the request so that request processing could
>> be stopped.
>>
>> I'd be happy to share what I have; if you're up for hacking too maybe we
>> should create a branch on github?
>>
>>
>>
>
> Hi Bill -
>
> I'd be more than happy to work with you on this, and perhaps do a github
> branch so that when we get it working, we can generate a patch set for
> review by community.
>
> My early investigation shows that Sub-Agent Doesn't handle anything but
> GETNEXT, per handle_subagent_response() - perhaps this can be confirmed?
> I have not looked at Master side, so it seems to me part of this could be
> updating handle_subagent_response() to at least return some data set, if
> this makes sense, and then have Master send OID down in PDU, and deal with
> return data. I am assuming that a return packet from a GETBULK should be a
> complete data set?
>
Yes, my changes include handle_subagent_response and code in the subagent
itself.
>
> From what I can tell, a GETBULK PDU only contains OID, so any filtering
> would have to be on Master side?
>
The GETNEXT and GETBULK PDUs contain SearchRangeList, which scopes the
request. The master sends SearchRangeList for each tree that the subagent
supports, e.g., if you have registrations of
x.1 - master agent
x.2 - subagent 1
x.3 - subagent 2
x.4 - subagent 1
the master agent will send subagent 1 a request for x.2, scoped to [ x.2,
x.3 ), then will ask subagent 2, then will ask subagent 1 again about [
x.4, x.5 ). The problem with the current code in my branch is that
subagent 1 can respond with x.2.1, x.2.2, x.4.1, x.4.2. There are two
problems with this:
1) it may be expensive for the subagent to fetch x.4.1 and x.4.2 and
they're not relevant
2) without filtering, the master agent will get confused by this response.
We can fix 2 by filtering it, just like we already filter getnext. I
haven't pursued this path yet because I wanted to also address 1.
See https://github.com/fenner/net-snmp/tree/agentx-getbulk for my code so
far.
Bill
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