The `PDU` contains the SNMP request or response fields and a list of `VarBinds`.
function PDU() {
    this.type = asn1ber.pduTypes.GetRequestPDU;
    this.reqid = 1;
    this.error = 0;
    this.errorIndex = 0;
    this.varbinds = [ new VarBind() ];
}