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List:       net-snmp-coders
Subject:    Re: ASN.1 and Request ID field
From:       Magnus Fromreide <magfr () lysator ! liu ! se>
Date:       2014-06-03 23:19:33
Message-ID: 20140603231933.GA18512 () sara ! home
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On Tue, Jun 03, 2014 at 12:50:28PM -0700, Juges, Didier wrote:
> Snmp newbie here, so bear WITH ME PLEASE...
> 
> I am confused and I hope someone can help.
> 
> I am writing an SNMP agent for a Cortex M4 application.
> The SNMP books I have bought and what I have read on the net indicate that all data \
> fields should be ASN.1 encoded However, looking at snmp commands sent by net-snmp, \
> it appears that the Request ID field is a simple (4 byte) 32 bit integer. Here is a \
> screen shot showing an snmpget transaction monitored through Wireshark: 
> http://www.ko4bb.com/net-snmp/RequestID.png
> 
> It shows the RequestID to be 1750020546 (decimal) and 0x684F31C2 in hex. The data \
> field in Wireshark also shows it to be "68 4f 31 c2" This is not ASN.1 encoded, \
> otherwise the first 3 bytes would have their bit 7 set to 1 and the last byte would \
> have bit 7 set to 0, meaning the  first 3 values would be >0x7F and the last value \
> should be < 0x80 
> So is ASN.1 not used for the RequestID field?
> 
> I know OIDs *are* ASN.1 encoded, but how about the other fields?
> How about the data field in a varbind?

You have misunderstood BER encoding.

The rules you quote above describe how the length part is encoded but the data
is encoded as a signed integer value just like you have observed.

/MF

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