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<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      The contents of this file are subject to the Erlang Public License,
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    <title>Definition of Agent Net if</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev></rev>
    <file>snmp_agent_netif.xml</file>
  </header>
  <p></p>
  <image file="snmp_agent_netif_1.gif">
    <icaption>The Purpose of Agent Net if</icaption>
  </image>
  <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 <c>snmpa_net_if</c> and
    it uses UDP as the transport protocol.
    </p>
  <p>This section describes how to write a Net if process.
    </p>

  <section>
    <marker id="mandatory_functions"></marker>
    <title>Mandatory Functions</title>
    <p>A Net if process must implement the SNMP agent 
      <seealso marker="snmpa_network_interface">network interface behaviour</seealso>.
      </p>
  </section>

  <section>
    <title>Messages</title>
    <p>The section <em>Messages</em> describes mandatory messages, which
      Net if must send and be able to receive.     
      </p>

    <section>
      <title>Outgoing Messages</title>
      <p>Net if must send the following message when it receives an
        SNMP PDU from the network that is aimed for the MasterAgent:
        </p>
      <pre>
MasterAgent ! {snmp_pdu, Vsn, Pdu, PduMS, ACMData, From, Extra}
      </pre>
      <list type="bulleted">
        <item><c>Vsn</c> is either <c>'version-1'</c>,
        <c>'version-2'</c>, or <c>'version-3'</c>.
        </item>
        <item><c>Pdu</c> is an SNMP PDU record, as defined in
        <c>snmp_types.hrl</c>, with the SNMP request.
        </item>
        <item><c>PduMS</c> is the Maximum Size of the response Pdu
         allowed.  Normally this is returned from
        <c>snmpa_mpd:process_packet</c> (see Reference Manual).
        </item>
        <item><c>ACMData</c> is data used by the Access Control Module
         in use.  Normally this is returned from
        <c>snmpa_mpd:process_packet</c> (see Reference Manual).
        </item>
        <item><c>From</c> is the source address. If UDP over IP is
         used, this should be a 2-tuple <c>{IP, UDPport}</c>, where
        <c>IP</c> is a 4-tuple with the IP address, and <c>UDPport</c>
         is an integer.
        </item>
        <item><c>Extra</c> is any term the Net if process wishes to
         send to the agent. This term can be retrieved by the
         instrumentation functions by calling
        <c>snmp:current_net_if_data()</c>. This data is also sent back
         to the Net if process when the agent generates a response to
         the request.</item>
      </list>
      <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>
      <pre>
Pid ! {snmp_response_received, Vsn, Pdu, From}
      </pre>
      <list type="bulleted">
        <item><c>Pid</c> is the Process that waits for the response
         for the request.  The Pid was specified in the
        <c>send_pdu_req</c> message <seealso marker="#message">(see below)</seealso>.
        </item>
        <item><c>Vsn</c> is either <c>'version-1'</c>, <c>'version-2'</c>, or
        <c>'version-3'</c>.
        </item>
        <item><c>Pdu</c> is the SNMP Pdu received
        </item>
        <item><c>From</c> is the source address. If UDP over IP is
         used, this should be a 2-tuple <c>{IP, UDPport}</c>, where
        <c>IP</c> is a 4-tuple with the IP address, and <c>UDPport</c>
         is an integer.
        </item>
      </list>
    </section>

    <section>
      <title>Incoming Messages</title>
      <p>This section describes the incoming messages which a Net if
        process must be able to receive.
        </p>
      <list type="bulleted">
        <item>
          <p><c>{snmp_response, Vsn, Pdu, Type, ACMData, To, Extra}</c></p>
          <p>This message is sent to the Net if process from a master
            agent as a response to a previously received request.
            </p>
          <list type="bulleted">
            <item><c>Vsn</c> is either <c>'version-1'</c>,
            <c>'version-2'</c>, or <c>'version-3'</c>.
            </item>
            <item><c>Pdu</c> is an SNMP PDU record (as defined in
             snmp_types.hrl) with the SNMP response.
            </item>
            <item><c>Type</c> is the <c>#pdu.type</c> of the original
             request.
            </item>
            <item><c>ACMData</c> is data used by the Access Control
             Module in use.  Normally this is just sent to
            <c>snmpa_mpd:generate_response_message</c> (see Reference Manual).
            </item>
            <item><c>To</c> is the destination address. If UDP over IP
             is used, this should be a 2-tuple <c>{IP, UDPport}</c>,
             where <c>IP</c> is a 4-tuple with the IP address, and
            <c>UDPport</c> is an integer.
            </item>
            <item><c>Extra</c> is the term that the Net if process
             sent to the agent when the request was sent to the agent.
            </item>
          </list>
        </item>
        <item>
          <p><c>{discarded_pdu, Vsn, ReqId, ACMData, Variable, Extra}</c></p>
          <p>This message is sent from a master agent if it for some
            reason decided to discard the pdu.
            </p>
          <list type="bulleted">
            <item><c>Vsn</c> is either <c>'version-1'</c>,
            <c>'version-2'</c>, or <c>'version-3'</c>.
            </item>
            <item><c>ReqId</c> is the request id of the original
             request.
            </item>
            <item><c>ACMData</c> is data used by the Access Control
             Module in use.  Normally this is just sent to
            <c>snmpa_mpd:generate_response_message</c> (see Reference Manual).
            </item>
            <item><c>Variable</c> is the name of an snmp counter that
             represents the error, e.g. <c>snmpInBadCommunityUses</c>.
            </item>
            <item><c>Extra</c> is the term that the Net if process
             sent to the agent when the request was sent to the agent.
            </item>
          </list>
        </item>
        <item>
          <p><c>{send_pdu, Vsn, Pdu, MsgData, To}</c></p>
          <p>This message is sent from a master agent when a trap is
            to be sent.
            </p>
          <list type="bulleted">
            <item><c>Vsn</c> is either <c>'version-1'</c>,
            <c>'version-2'</c>, or <c>'version-3'</c>.
            </item>
            <item><c>Pdu</c> is an SNMP PDU record (as defined in
             snmp_types.hrl) with the SNMP response.
            </item>
            <item><c>MsgData</c> is the message specific data used in
             the SNMP message.  This value is normally sent to
            <c>snmpa_mpd:generate_message/4</c>.  In SNMPv1 and
             SNMPv2c, this message data is the community string.  In
             SNMPv3, it is the context information.
            </item>
            <item><c>To</c> is a list of the destination addresses and
             their corresponding security parameters.  This value is
             normally sent to <c>snmpa_mpd:generate_message/4</c>.  
            </item>
          </list>
        </item>
        <item>
          <p><c>{send_pdu_req, Vsn, Pdu, MsgData, To, Pid}</c></p>
          <p>This            <marker id="message"></marker>
 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
            <c>snmp_response_received</c> message.
            </p>
          <list type="bulleted">
            <item><c>Vsn</c> is either <c>'version-1'</c>,
            <c>'version-2'</c>,  or <c>'version-3'</c>.
            </item>
            <item><c>Pdu</c> is an SNMP PDU record (as defined in
             snmp_types.hrl) with the SNMP response.
            </item>
            <item><c>MsgData</c> is the message specific data used in
             the SNMP message.  This value is normally sent to
            <c>snmpa_mpd:generate_message/4</c>.  In SNMPv1 and
             SNMPv2c, this message data is the community string.  In
             SNMPv3, it is the context information.
            </item>
            <item><c>To</c> is a list of the destination addresses and
             their corresponding security parameters.  This value is
             normally sent to <c>snmpa_mpd:generate_message/4</c>.  
            </item>
            <item><c>Pid</c> is a process identifier.
            </item>
          </list>
        </item>
      </list>
    </section>

    <section>
      <title>Notes</title>
      <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 <c>snmpa_mpd</c> for this purpose (refer to the 
        Reference Manual, section <c>snmp</c>, 
        module <seealso marker="snmp_pdus">snmpa_mpd</seealso>
        for more details.)
        </p>
      <p>There are also some useful functions for encoding and
        decoding of SNMP messages in the module 
        <seealso marker="snmp_pdus">snmp_pdus</seealso>.
        </p>
    </section>
  </section>
</chapter>

