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<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>1997</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<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>
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