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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<TITLE>Definition of Agent Net if</TITLE>
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<A NAME="12"><!-- Empty --></A>
<H2>12 Definition of Agent Net if</H2>
<P>
<P>
<CENTER>
<IMG ALT="snmp_agent_netif_1" SRC="snmp_agent_netif_1.gif"><BR>
<EM>The Purpose of Agent Net if</EM>
</CENTER>
<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>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>It is also possible to write your own Net if process. The default
Net if process is implemented in the module <CODE>snmpa_net_if</CODE> and
it uses UDP as the transport protocol.
<P>This section describes how to write a Net if process.
<A NAME="mandatory_functions"><!-- Empty --></A><A NAME="12.1"><!-- Empty --></A>
<H3>12.1 Mandatory Functions</H3>
<P>A Net if process must implement the SNMP agent
<A HREF="snmpa_network_interface.html">network interface behaviour</A>.
<A NAME="12.2"><!-- Empty --></A>
<H3>12.2 Messages</H3>
<P>The section <STRONG>Messages</STRONG> describes mandatory messages, which
Net if must send and be able to receive.
<A NAME="12.2.1"><!-- Empty --></A>
<H4>12.2.1 Outgoing Messages</H4>
<P>Net if must send the following message when it receives an
SNMP PDU from the network that is aimed for the MasterAgent:
<PRE>
MasterAgent ! {snmp_pdu, Vsn, Pdu, PduMS, ACMData, From, Extra}
</PRE>
<P>
<UL>
<LI>
<CODE>Vsn</CODE> is either <CODE>'version-1'</CODE>,
<CODE>'version-2'</CODE>, or <CODE>'version-3'</CODE>.
</LI>
<LI>
<CODE>Pdu</CODE> is an SNMP PDU record, as defined in
<CODE>snmp_types.hrl</CODE>, with the SNMP request.
</LI>
<LI>
<CODE>PduMS</CODE> is the Maximum Size of the response Pdu
allowed. Normally this is returned from
<CODE>snmpa_mpd:process_packet</CODE> (see Reference Manual).
</LI>
<LI>
<CODE>ACMData</CODE> is data used by the Access Control Module
in use. Normally this is returned from
<CODE>snmpa_mpd:process_packet</CODE> (see Reference Manual).
</LI>
<LI>
<CODE>From</CODE> is the source address. If UDP over IP is
used, this should be a 2-tuple <CODE>{IP, UDPport}</CODE>, where
<CODE>IP</CODE> is a 4-tuple with the IP address, and <CODE>UDPport</CODE>
is an integer.
</LI>
<LI>
<CODE>Extra</CODE> is any term the Net if process wishes to
send to the agent. This term can be retrieved by the
instrumentation functions by calling
<CODE>snmp:current_net_if_data()</CODE>. 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.
<PRE>
Pid ! {snmp_response_received, Vsn, Pdu, From}
</PRE>
<P>
<UL>
<LI>
<CODE>Pid</CODE> is the Process that waits for the response
for the request. The Pid was specified in the
<CODE>send_pdu_req</CODE> message <A HREF="#message">(see below)</A>.
</LI>
<LI>
<CODE>Vsn</CODE> is either <CODE>'version-1'</CODE>, <CODE>'version-2'</CODE>, or
<CODE>'version-3'</CODE>.
</LI>
<LI>
<CODE>Pdu</CODE> is the SNMP Pdu received
</LI>
<LI>
<CODE>From</CODE> is the source address. If UDP over IP is
used, this should be a 2-tuple <CODE>{IP, UDPport}</CODE>, where
<CODE>IP</CODE> is a 4-tuple with the IP address, and <CODE>UDPport</CODE>
is an integer.
</LI>
</UL>
<A NAME="12.2.2"><!-- Empty --></A>
<H4>12.2.2 Incoming Messages</H4>
<P>This section describes the incoming messages which a Net if
process must be able to receive.
<P>
<UL>
<LI>
<CODE>{snmp_response, Vsn, Pdu, Type, ACMData, To, Extra}</CODE>
This message is sent to the Net if process from a master
agent as a response to a previously received request.
<BR>
<UL>
<LI>
<CODE>Vsn</CODE> is either <CODE>'version-1'</CODE>,
<CODE>'version-2'</CODE>, or <CODE>'version-3'</CODE>.
</LI>
<LI>
<CODE>Pdu</CODE> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response.
</LI>
<LI>
<CODE>Type</CODE> is the <CODE>#pdu.type</CODE> of the original
request.
</LI>
<LI>
<CODE>ACMData</CODE> is data used by the Access Control
Module in use. Normally this is just sent to
<CODE>snmpa_mpd:generate_response_message</CODE> (see Reference Manual).
</LI>
<LI>
<CODE>To</CODE> is the destination address. If UDP over IP
is used, this should be a 2-tuple <CODE>{IP, UDPport}</CODE>,
where <CODE>IP</CODE> is a 4-tuple with the IP address, and
<CODE>UDPport</CODE> is an integer.
</LI>
<LI>
<CODE>Extra</CODE> is the term that the Net if process
sent to the agent when the request was sent to the agent.
</LI>
</UL>
</LI>
<LI>
<CODE>{discarded_pdu, Vsn, ReqId, ACMData, Variable,
Extra}</CODE>
This message is sent from a master agent if it for some
reason decided to discard the pdu.
<BR>
<UL>
<LI>
<CODE>Vsn</CODE> is either <CODE>'version-1'</CODE>,
<CODE>'version-2'</CODE>, or <CODE>'version-3'</CODE>.
</LI>
<LI>
<CODE>ReqId</CODE> is the request id of the original
request.
</LI>
<LI>
<CODE>ACMData</CODE> is data used by the Access Control
Module in use. Normally this is just sent to
<CODE>snmpa_mpd:generate_response_message</CODE> (see Reference Manual).
</LI>
<LI>
<CODE>Variable</CODE> is the name of an snmp counter that
represents the error, e.g. <CODE>snmpInBadCommunityUses</CODE>.
</LI>
<LI>
<CODE>Extra</CODE> is the term that the Net if process
sent to the agent when the request was sent to the agent.
</LI>
</UL>
</LI>
<LI>
<CODE>{send_pdu, Vsn, Pdu, MsgData, To}</CODE>
This message is sent from a master agent when a trap is
to be sent.
<BR>
<UL>
<LI>
<CODE>Vsn</CODE> is either <CODE>'version-1'</CODE>,
<CODE>'version-2'</CODE>, or <CODE>'version-3'</CODE>.
</LI>
<LI>
<CODE>Pdu</CODE> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response.
</LI>
<LI>
<CODE>MsgData</CODE> is the message specific data used in
the SNMP message. This value is normally sent to
<CODE>snmpa_mpd:generate_message/4</CODE>. In SNMPv1 and
SNMPv2c, this message data is the community string. In
SNMPv3, it is the context information.
</LI>
<LI>
<CODE>To</CODE> is a list of the destination addresses and
their corresponding security parameters. This value is
normally sent to <CODE>snmpa_mpd:generate_message/4</CODE>.
</LI>
</UL>
</LI>
<LI>
<CODE>{send_pdu_req, Vsn, Pdu, MsgData, To, Pid}</CODE>
This<A NAME="message"><!-- Empty --></A> 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
<CODE>snmp_response_received</CODE> message.
<BR>
<UL>
<LI>
<CODE>Vsn</CODE> is either <CODE>'version-1'</CODE>,
<CODE>'version-2'</CODE>, or <CODE>'version-3'</CODE>.
</LI>
<LI>
<CODE>Pdu</CODE> is an SNMP PDU record (as defined in
snmp_types.hrl) with the SNMP response.
</LI>
<LI>
<CODE>MsgData</CODE> is the message specific data used in
the SNMP message. This value is normally sent to
<CODE>snmpa_mpd:generate_message/4</CODE>. In SNMPv1 and
SNMPv2c, this message data is the community string. In
SNMPv3, it is the context information.
</LI>
<LI>
<CODE>To</CODE> is a list of the destination addresses and
their corresponding security parameters. This value is
normally sent to <CODE>snmpa_mpd:generate_message/4</CODE>.
</LI>
<LI>
<CODE>Pid</CODE> is a process identifier.
</LI>
</UL>
</LI>
</UL>
<A NAME="12.2.3"><!-- Empty --></A>
<H4>12.2.3 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 <CODE>snmpa_mpd</CODE> for this purpose (refer to the
Reference Manual, section <CODE>snmp</CODE>,
module <A HREF="snmp_pdus.html">snmpa_mpd</A>
for more details.)
<P>There are also some useful functions for encoding and
decoding of SNMP messages in the module
<A HREF="snmp_pdus.html">snmp_pdus</A>.
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