SNMPv1 agent for lwIP [preliminairy]

Author: Christiaan Simons

This is a brief introduction how to use and configure the SNMP agent.
Note the agent uses the raw-API UDP interface so you may also want to
read rawapi.txt to gain a better understanding of the SNMP messages handling.

Agent capabilities
==================

SNMPv1 per RFC1157
  This is an old(er) standard but is still widely supported.
  For SNMPv2c and v3 have a greater complexity and need many
  more lines of code. IMHO this breaks the idea of "lightweight IP".
  Note the S in SNMP stands for "Simple" and is actually a bad joke.

MIB II per RFC1213
  The standard lwIP stack management information base.
  This is a required MIB, so this is always enabled.

Loading additional MIBs
  MIBs can only be added in compile-time, not in run-time.
  There is no MIB compiler thus additional MIBs must be hand coded.

Large SNMP message support
  The packet decoding and encoding routines are designed
  to use pbuf-chains. Larger payloads then the minimum
  SNMP requirement of 484 octets are supported if the 
  PBUF_POOL_SIZE and IP_REASS_BUFSIZE are set to match your
  local requirement.

Building the agent
==================

First of all you'll need to add the following define
to your local lwipopts.h:

#define LWIP_SNMP               1

and add the source files in lwip/src/core/snmp
and some snmp headers in lwip/src/include/lwip to your makefile.

Note you'll might need to adapt you network driver to update
the mib2 variables for your interface.

Running the agent
=================

The following function calls must be made in your program to
actually get the SNMP agent running.

In the lwIP initialisation sequence call snmp_init() just after
the call to udp_init().

Exactly every 10 msec the SNMP uptime timestamp must be updated with
snmp_inc_sysuptime(). You should call this from a timer interrupt
or a timer signal handler depending on your runtime environment.

You _must_ create the following support functions for non-volatile storage
since lwIP does not have notion of files or other non-volatile memories.

void snmp_store_syscontact(u8_t* ocstr, u8_t ocstrlen);
void snmp_store_sysname(u8_t* ocstr, u8_t ocstrlen);
void snmp_store_syslocation(u8_t* ocstr, u8_t ocstrlen);



Private MIBs
============

If want to extend the agent with your own private MIB you'll need to
add the following define to your local lwipopts.h:

#define SNMP_PRIVATE_MIB        1

You must provide the private_mib.h and associated files yourself.
Note we don't have a "MIB compiler" that generates C source from a MIB,
so you're required to do some serious coding if you enable this!

Note the lwIP enterprise ID (26381) is assigned to the lwIP project,
ALL OBJECT IDENTIFIERS LIVING UNDER THIS ID ARE ASSIGNED BY THE lwIP
MAINTAINERS!

If you need to create your own private MIB you'll need
to apply for your own enterprise ID with IANA: http://www.iana.org/numbers.html 

You can set it by passing a struct snmp_obj_id to the agent
using snmp_set_sysobjid(&my_object_id), just before snmp_init().

Agent internals [advanced use]
==============================

todo