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<li title="Appendix B"><a href="snmp_app_b.html#id87320">Appendix B</a></li>
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<h1>12 Definition of Agent Net if</h1>
<p></p>
<img alt="IMAGE MISSING" src="snmp_agent_netif_1.gif"><br>
<em>Figure
12.1:
The Purpose of Agent Net if</em>
<p>The Network Interface (Net if) process delivers SNMP PDUs to a
master agent, and receives SNMP PDUs from the master agent. The most
common behaviour of a Net if process is that is receives bytes from
a network, decodes them into an SNMP PDU, which it sends to a master
agent. When the master agent has processed the PDU, it sends a
response PDU to the Net if process, which encodes the PDU into bytes
and transmits the bytes onto the network.
</p>
<p>However, that simple behaviour can be modified in numerous
ways. For example, the Net if process can apply some kind of
encrypting/decrypting scheme on the bytes or
act as a proxy filter, which sends some packets to a proxy agent and
some packets to the master agent.
</p>
<p>It is also possible to write your own Net if process. The default
Net if process is implemented in the module <span class="code">snmpa_net_if</span> and
it uses UDP as the transport protocol.
</p>
<p>This section describes how to write a Net if process.
</p>
<h3><a name="id84750">12.1
Mandatory Functions</a></h3>
<a name="mandatory_functions"></a>
<p>A Net if process must implement the SNMP agent
<span class="bold_code"><a href="snmpa_network_interface.html">network interface behaviour</a></span>.
</p>
<h3><a name="id84772">12.2
Messages</a></h3>
<a name="messages"></a>
<p>The section <strong>Messages</strong> describes mandatory messages, which
Net if must send and be able to receive.
</p>
<h4>Outgoing Messages</h4>
<a name="outgoing_messages"></a>
<p>Net if must send the following message when it receives an
SNMP PDU from the network that is aimed for the MasterAgent:
</p>
<a name="om_snmp_pdu"></a>
<div class="example"><pre>
MasterAgent ! {snmp_pdu, Vsn, Pdu, PduMS, ACMData, From, Extra}
</pre></div>
<ul>
<li>
<span class="code">Vsn</span> is either <span class="code">'version-1'</span>,
<span class="code">'version-2'</span>, or <span class="code">'version-3'</span>.
</li>
<li>
<span class="code">Pdu</span> is an SNMP PDU record, as defined in
<span class="code">snmp_types.hrl</span>, with the SNMP request.
</li>
<li>
<span class="code">PduMS</span> is the Maximum Size of the response Pdu
allowed. Normally this is returned from
<span class="code">snmpa_mpd:process_packet</span> (see Reference Manual).
</li>
<li>
<span class="code">ACMData</span> is data used by the Access Control Module
in use. Normally this is returned from
<span class="code">snmpa_mpd:process_packet</span> (see Reference Manual).
</li>
<li>
<span class="code">From</span> is the source address. If UDP over IP is
used, this should be a 2-tuple <span class="code">{IP, UDPport}</span>, where
<span class="code">IP</span> is a 4-tuple with the IP address, and <span class="code">UDPport</span>
is an integer.
</li>
<li>
<span class="code">Extra</span> is any term the Net if process wishes to
send to the agent. This term can be retrieved by the
instrumentation functions by calling
<span class="code">snmp:current_net_if_data()</span>. This data is also sent back
to the Net if process when the agent generates a response to
the request.</li>
</ul>
<p>The following message is used to report that a response to a
request has been received. The only request an agent can send
is an Inform-Request.
</p>
<a name="om_snmp_response_received"></a>
<div class="example"><pre>
Pid ! {snmp_response_received, Vsn, Pdu, From}
</pre></div>
<ul>
<li>
<span class="code">Pid</span> is the Process that waits for the response
for the request. The Pid was specified in the
<span class="code">send_pdu_req</span> message
<span class="bold_code"><a href="#im_send_pdu_req">(see below)</a></span>.
</li>
<li>
<span class="code">Vsn</span> is either <span class="code">'version-1'</span>, <span class="code">'version-2'</span>, or
<span class="code">'version-3'</span>.
</li>
<li>
<span class="code">Pdu</span> is the SNMP Pdu received
</li>
<li>
<span class="code">From</span> is the source address. If UDP over IP is
used, this should be a 2-tuple <span class="code">{IP, UDPport}</span>, where
<span class="code">IP</span> is a 4-tuple with the IP address, and <span class="code">UDPport</span>
is an integer.
</li>
</ul>
<h4>Incoming Messages</h4>
<a name="incoming_messages"></a>
<p>This section describes the incoming messages which a Net if
process must be able to receive.
</p>
<ul>
<li>
<a name="im_snmp_response"></a>
<p><span class="code">{snmp_response, Vsn, Pdu, Type, ACMData, To, Extra}</span></p>
<p>This message is sent to the Net if process from a master
agent as a response to a previously received request.
</p>
<ul>
<li>
<p><span class="code">Vsn</span> is either <span class="code">'version-1'</span>,
<span class="code">'version-2'</span>, or <span class="code">'version-3'</span>. </p>
</li>
<li>
<p><span class="code">Pdu</span> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response. </p>
</li>
<li>
<p><span class="code">Type</span> is the <span class="code">#pdu.type</span>
of the original request. </p>
</li>
<li>
<p><span class="code">ACMData</span> is data used by the Access Control
Module in use. Normally this is just sent to
<span class="code">snmpa_mpd:generate_response_message</span>
(see Reference Manual). </p>
</li>
<li>
<p><span class="code">To</span> is the destination address. If UDP over IP
is used, this should be a 2-tuple <span class="code">{IP, UDPport}</span>,
where <span class="code">IP</span> is a 4-tuple with the IP address, and
<span class="code">UDPport</span> is an integer. </p>
</li>
<li>
<p><span class="code">Extra</span> is the term that the Net if process
sent to the agent when the request was sent to the agent. </p>
</li>
</ul>
</li>
<li>
<a name="im_discarded_pdu"></a>
<p><span class="code">{discarded_pdu, Vsn, ReqId, ACMData, Variable, Extra}</span></p>
<p>This message is sent from a master agent if it for some
reason decided to discard the pdu. </p>
<ul>
<li>
<p><span class="code">Vsn</span> is either <span class="code">'version-1'</span>,
<span class="code">'version-2'</span>, or <span class="code">'version-3'</span>. </p>
</li>
<li>
<p><span class="code">ReqId</span> is the request id of the original request. </p>
</li>
<li>
<p><span class="code">ACMData</span> is data used by the Access Control
Module in use. Normally this is just sent to
<span class="code">snmpa_mpd:generate_response_message</span>
(see Reference Manual). </p>
</li>
<li>
<p><span class="code">Variable</span> is the name of an snmp counter that
represents the error, e.g. <span class="code">snmpInBadCommunityUses</span>. </p>
</li>
<li>
<p><span class="code">Extra</span> is the term that the Net if process
sent to the agent when the request was sent to the agent. </p>
</li>
</ul>
</li>
<li>
<a name="im_send_pdu"></a>
<p><span class="code">{send_pdu, Vsn, Pdu, MsgData, To, Extra}</span></p>
<p>This message is sent from a master agent when a trap is
to be sent. </p>
<ul>
<li>
<p><span class="code">Vsn</span> is either <span class="code">'version-1'</span>,
<span class="code">'version-2'</span>, or <span class="code">'version-3'</span>.</p>
</li>
<li>
<p><span class="code">Pdu</span> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response. </p>
</li>
<li>
<p><span class="code">MsgData</span> is the message specific data used in
the SNMP message. This value is normally sent to
<span class="code">snmpa_mpd:generate_message/4</span>. In SNMPv1 and
SNMPv2c, this message data is the community string. In
SNMPv3, it is the context information. </p>
</li>
<li>
<p><span class="code">To</span> is a list of the destination addresses and
their corresponding security parameters. This value is
normally sent to <span class="code">snmpa_mpd:generate_message/4</span>. </p>
</li>
<li>
<p><span class="code">Extra</span> is any term that the notification sender
wishes to pass to the Net if process when sending a notification
(see
<span class="bold_code"><a href="snmpa.html#send_notification2">send notification
</a></span> for more info). </p>
</li>
</ul>
</li>
<li>
<a name="im_send_pdu_req"></a>
<p><span class="code">{send_pdu_req, Vsn, Pdu, MsgData, To, Pid, Extra}</span></p>
<p>This message is sent from a master agent when a request is to
be sent. The only request an agent can send is Inform-Request.
The net if process needs to remember the request id and the Pid,
and when a response is received for the request id, send it to Pid,
using a <span class="code">snmp_response_received</span> message. </p>
<ul>
<li>
<p><span class="code">Vsn</span> is either <span class="code">'version-1'</span>,
<span class="code">'version-2'</span>, or <span class="code">'version-3'</span>.</p>
</li>
<li>
<p><span class="code">Pdu</span> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response. </p>
</li>
<li>
<p><span class="code">MsgData</span> is the message specific data used in
the SNMP message. This value is normally sent to
<span class="code">snmpa_mpd:generate_message/4</span>. In SNMPv1 and
SNMPv2c, this message data is the community string. In
SNMPv3, it is the context information. </p>
</li>
<li>
<p><span class="code">To</span> is a list of the destination addresses and
their corresponding security parameters. This value is
normally sent to <span class="code">snmpa_mpd:generate_message/4</span>. </p>
</li>
<li>
<p><span class="code">Pid</span> is a process identifier. </p>
</li>
<li>
<p><span class="code">Extra</span> is any term that the notification sender
wishes to pass to the Net if process when sending a notification
(see
<span class="bold_code"><a href="snmpa.html#send_notification2">send notification
</a></span> for more info). </p>
</li>
</ul>
</li>
</ul>
<h4>Notes</h4>
<p>Since the Net if process is responsible for encoding and
decoding of SNMP messages, it must also update the relevant
counters in the SNMP group in MIB-II. It can use the functions
in the module <span class="code">snmpa_mpd</span> for this purpose (refer to the
Reference Manual, section <span class="code">snmp</span>,
module <span class="bold_code"><a href="snmp_pdus.html">snmpa_mpd</a></span>
for more details.)
</p>
<p>There are also some useful functions for encoding and
decoding of SNMP messages in the module
<span class="bold_code"><a href="snmp_pdus.html">snmp_pdus</a></span>.
</p>
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