Hi,
I found a bug in my SNMP message size calculation and to make things
transparent, I am posting the full details here - just in case someone
wants to check what I did. Note that the calculation assumes no
payload and messages are sent without security (noAuthNoPriv in SNMP
speak).
A very rough way to estimate the size needed by a varbind that is
essentially a number is the following formula:
varbind-size = 6 + number-of-OID-subidentifier
That is for sysUpTime (1.3.6.1.2.1.1.3), we get 14 bytes and thus the
SNMPv1 message is 34 bytes and the SNMPv3/USM message is 81 bytes.
And here are the promised details of my calculation.
* SNMPv3 minimum message size calculation
SNMPv3Message 2 byte
msgVersion 3 byte
msgGlobalData (HeaderData) 15 byte
msgSecurityParameters 23 byte
msgData (ScopedPDU) 24 byte
-------
67 byte
UsmSecurityParameters 2 byte
msgAuthoritativeEngineID 7 byte
msgAuthoritativeEngineBoots 3 byte
msgAuthoritativeEngineTime 3 byte
msgUserName 2 byte
msgAuthenticationParameters 2 byte
msgPrivacyParameters 2 byte
-------
21 byte
HeaderData 2 byte
msgID 3 byte
msgMaxSize 4 byte
msgFlags 3 byte
msgSecurityModel 3 byte
-------
15 byte
ScopedPDU 2 byte
contextEngineID 7 byte
contextName 2 byte
PDU 13 byte
-------
24 byte
PDU 2 byte
request-id 3 byte
error-status 3 byte
error-index 3 byte
variable-bindings 2 byte
-------
13 byte
* SNMPv1 / SNMPv2c minimum mesage size calculation
SNMPv1Message 2 byte
version 3 byte
community 2 byte
data (PDU) 13 byte
-------
20 byte
/js
--
Juergen Schoenwaelder Jacobs University Bremen gGmbH
Phone: +49 421 200 3587 Campus Ring 1, 28759 Bremen, Germany
Fax: +49 421 200 3103 <http://www.jacobs-university.de/>
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