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+SNMPv1 agent for lwIP
+
+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 message handling.
+
+0 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". Note that "Simple" is
+  relative. SNMP is simple compared to the complex ISO network
+  management protocols CMIP (Common Management Information Protocol)
+  and CMOT (CMip Over Tcp).
+
+MIB II per RFC1213
+  The standard lwIP stack management information base.
+  This is a required MIB, so this is always enabled.
+  When builing lwIP without TCP, the mib-2.tcp group is omitted.
+  The groups EGP, CMOT and transmission are disabled by default.
+  
+  Most mib-2 objects are not writable except:
+  sysName, sysLocation, sysContact, snmpEnableAuthenTraps.
+  Writing to or changing the ARP and IP address and route
+  tables is not possible.
+ 
+  Note lwIP has a very limited notion of IP routing. It currently
+  doen't have a route table and doesn't have a notion of the U,G,H flags.
+  Instead lwIP uses the interface list with only one default interface
+  acting as a single gateway interface (G) for the default route.
+
+  The agent returns a "virtual table" with the default route 0.0.0.0
+  for the default interface and network routes (no H) for each
+  network interface in the netif_list.
+  All routes are considered to be up (U).
+
+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.
+
+1 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.
+
+2 Running the Agent
+===================
+
+The following function calls must be made in your program to
+actually get the SNMP agent running.
+
+Before starting the agent you should supply pointers
+to non-volatile memory for sysContact, sysLocation,
+and snmpEnableAuthenTraps. You can do this by calling
+
+snmp_set_syscontact()
+snmp_set_syslocation()
+snmp_set_snmpenableauthentraps()
+
+Additionally you may want to set
+
+snmp_set_sysdescr()
+snmp_set_sysobjid() (if you have a private MIB)
+snmp_set_sysname()
+
+Also before starting the agent you need to setup
+one or more trap destinations using these calls:
+
+snmp_trap_dst_enable();
+snmp_trap_dst_ip_set();
+
+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.
+
+An alternative way to update the SNMP uptime timestamp is to do a call like
+snmp_add_sysuptime(100) each 1000ms (which is bigger "step", but call to
+a lower frequency). Another one is to not call snmp_inc_sysuptime() or
+snmp_add_sysuptime(), and to define the SNMP_GET_SYSUPTIME(sysuptime) macro.
+This one is undefined by default in mib2.c. SNMP_GET_SYSUPTIME is called inside
+snmp_get_sysuptime(u32_t *value), and enable to change "sysuptime" value only
+when it's queried (any function which need "sysuptime" have to call
+snmp_get_sysuptime).
+
+
+3 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().
+
+Note the object identifiers for thes MIB-2 and your private MIB
+tree must be kept in sorted ascending (lexicographical) order.
+This to ensure correct getnext operation.
+
+An example for a private MIB is part of the "minimal Unix" project:
+contrib/ports/unix/proj/minimal/lwip_prvmib.c
+
+The next chapter gives a more detailed description of the
+MIB-2 tree and the optional private MIB.
+
+4 The Gory Details
+==================
+
+4.0 Object identifiers and the MIB tree.
+
+We have three distinct parts for all object identifiers:
+
+The prefix
+  .iso.org.dod.internet
+
+the middle part 
+  .mgmt.mib-2.ip.ipNetToMediaTable.ipNetToMediaEntry.ipNetToMediaPhysAddress
+
+and the index part
+  .1.192.168.0.1
+
+Objects located above the .internet hierarchy aren't supported.
+Currently only the .mgmt sub-tree is available and
+when the SNMP_PRIVATE_MIB is enabled the .private tree
+becomes available too.
+
+Object identifiers from incoming requests are checked
+for a matching prefix, middle part and index part
+or are expanded(*) for GetNext requests with short
+or inexisting names in the request.
+(* we call this "expansion" but this also
+resembles the "auto-completion" operation)
+
+The middle part is usually located in ROM (const)
+to preserve precious RAM on small microcontrollers.
+However RAM location is possible for an dynamically
+changing private tree.
+
+The index part is handled by functions which in
+turn use dynamically allocated index trees from RAM.
+These trees are updated by e.g. the etharp code
+when new entries are made or removed form the ARP cache.
+
+/** @todo more gory details */