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<year>1997</year><year>2016</year>
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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 i.e the transport domains
<c>transportDomainUdpIpv4</c> and/or <c>transportDomainUdpIpv6</c>.
</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>
<marker id="messages"></marker>
<title>Messages</title>
<p>The section <em>Messages</em> describes mandatory messages, which
Net if must send and be able to receive.
</p>
<p>In this section an <c>Address</c> field is a
<c>{Domain, Addr}</c> tuple where <c>Domain</c> is
<c>transportDomainUdpIpv4</c> or <c>transportDomainUdpIpv4</c>,
and <c>Addr</c> is an
<c>{</c><seealso marker="kernel:inet#type-ip_address"><c>IpAddr</c></seealso><c>,IpPort}</c> tuple.</p>
<section>
<marker id="outgoing_messages"></marker>
<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>
<marker id="om_snmp_pdu"></marker>
<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 <c>Address</c>.
</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>
<marker id="om_snmp_response_received"></marker>
<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="#im_send_pdu_req">(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 <c>Address</c>.
</item>
</list>
</section>
<section>
<marker id="incoming_messages"></marker>
<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>
<marker id="im_snmp_response"></marker>
<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>
<p><c>Vsn</c> is either <c>'version-1'</c>,
<c>'version-2'</c>, or <c>'version-3'</c>. </p>
</item>
<item>
<p><c>Pdu</c> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response. </p>
</item>
<item>
<p><c>Type</c> is the <c>#pdu.type</c>
of the original request. </p>
</item>
<item>
<p><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). </p>
</item>
<item>
<p><c>To</c> is the destination <c>Address</c> that comes
from the <c>From</c> field in the corresponding <c>snmp_pdu</c>
message previously sent to the MasterAgent.</p>
</item>
<item>
<p><c>Extra</c> is the term that the Net if process
sent to the agent when the request was sent to the agent. </p>
</item>
</list>
</item>
<item>
<marker id="im_discarded_pdu"></marker>
<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>
<p><c>Vsn</c> is either <c>'version-1'</c>,
<c>'version-2'</c>, or <c>'version-3'</c>. </p>
</item>
<item>
<p><c>ReqId</c> is the request id of the original request. </p>
</item>
<item>
<p><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). </p>
</item>
<item>
<p><c>Variable</c> is the name of an snmp counter that
represents the error, e.g. <c>snmpInBadCommunityUses</c>. </p>
</item>
<item>
<p><c>Extra</c> is the term that the Net if process
sent to the agent when the request was sent to the agent. </p>
</item>
</list>
</item>
<item>
<marker id="im_send_pdu"></marker>
<p><c>{send_pdu, Vsn, Pdu, MsgData, To, Extra}</c></p>
<p>This message is sent from a master agent when a trap is
to be sent. </p>
<list type="bulleted">
<item>
<p><c>Vsn</c> is either <c>'version-1'</c>,
<c>'version-2'</c>, or <c>'version-3'</c>.</p>
</item>
<item>
<p><c>Pdu</c> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response. </p>
</item>
<item>
<p><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. </p>
</item>
<item>
<p><c>To</c> is a list of <c>{Address, SecData}</c>
tuples i.e the destination addresses and
their corresponding security parameters. This value is
normally sent to <c>snmpa_mpd:generate_message/4</c>. </p>
</item>
<item>
<p><c>Extra</c> is any term that the notification sender
wishes to pass to the Net if process when sending a notification
(see
<seealso marker="snmpa#send_notification2">send notification
</seealso> for more info). </p>
</item>
</list>
</item>
<item>
<marker id="im_send_pdu_req"></marker>
<p><c>{send_pdu_req, Vsn, Pdu, MsgData, To, Pid, Extra}</c></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 <c>snmp_response_received</c> message. </p>
<list type="bulleted">
<item>
<p><c>Vsn</c> is either <c>'version-1'</c>,
<c>'version-2'</c>, or <c>'version-3'</c>.</p>
</item>
<item>
<p><c>Pdu</c> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response. </p>
</item>
<item>
<p><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. </p>
</item>
<item>
<p><c>To</c> is a list of <c>{Address, SecData}</c>
tuples i.e the destination addresses and
their corresponding security parameters. This value is
normally sent to <c>snmpa_mpd:generate_message/4</c>. </p>
</item>
<item>
<p><c>Pid</c> is a process identifier. </p>
</item>
<item>
<p><c>Extra</c> is any term that the notification sender
wishes to pass to the Net if process when sending a notification
(see
<seealso marker="snmpa#send_notification2">send notification
</seealso> for more info). </p>
</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>
