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Background: Know: SNMPv3 message, request-ID, authentication, privacy, report, snmpEngineMaxMessageSize Recognize:

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Header DataEdit

msgVersion:Represents the SNMP version number and is used for ensuring compatibility between version.Eg : SNMPv3 (3)

msgID:It is a Unique identifier, that is used between two SNMP entities for cordination of request and response messages, msgID also assist message processor to cordinate the message by different subsystem models with in architecture.MsgID range is from 0 through231-1.

msgMaxSize:, msgMaxSize indicate the maximum size of a message in a octet supported by sender of the message, with a range of 484 through 231-1. This is the maximum segment size that the sender can accept from another SNMP engine (whether a response or some other message type).  Limited by local engine requirements, snmpEngineMaxMessageSize, and communication system, e.g. link, network and transport protocols.  

.msgFlags: It contains an octet string which consist 3 flags in least significant 3 bits, namely as authFlag, privFlag and reportableFlag

. reportableFlag: indicate the necessity of report feedback. If reportableFlag =1, then a Report of PDU must be returned to the sender under those conditions that can cause the generation of a report PDU, similarly when reportableFlag = 0, then a Report PDU may not be sent. The reportableFlag is set to 1 by the sender in all message that contains(Get,Set) or by an Inform, and set to 0 for messages containing a Response, a Trap, or a Report PDU. It is a secondary aid in determining when to send a Report, and only used in scanerios where PDU message cannot be decoded (e.g, when decryption fails because of incorrect key). The privFlag and authFlag are set by the sender to indicate the security level that was applied to the message.

authFlag: Message having authFlag indicate message contains authentication information.

privFlag: When privFlag=1, encryption was applied and for privFlag = 0 authentication was applied. All combinations of privFlag are allowed except (privFlag = 1 AND authFlag = 0); that is, encryption without authentication is not allowed.

Priv and Auth options
 authpriv
noAuthNoPriv00
authNoPriv10
authPriv11


Encryption without Authentication is not allowed. Hence {auth,priv} = {1,0} does not apply. Authentication is required to ensure message integrity for privacy settings.