Snmp configuation db schema proposal

doc/snmp-schema-addition.md

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+# SONiC SNMP TABLE schema proposal #
+
+## Scope of the change ##
+
+Currently SNMP configuration is managed from a mix of yaml files and DB based ACLs, we propose to integrate the SMP configs into an SNMP table in the DB.  
+The following document proposes the database Schema in Json format and the list of required changes to the code.
+
+## Current configurtions for SNMP ##
+sonic-buidimage and sonic-snmpagent depend on the following files:
+1. */etc/sonic/snmp.yaml*
+    defines 2 keys:  
+        *snmp_rocommunity:*  
+        *snmp_location:*  
+   this file is consumed in the snmp docker container by */usr/share/sonic/templates/snmpd.conf.j2* and */usr/bin/snmpd-config-updater*.  
+2. */etc/sonic/config_db.json* or */etc/sonic/minigraph.xml* for ACL definitions 'SNMP_ACL'
+   this file is consumed by */usr/bin/snmpd-config-updater*, */usr/share/sonic/templates/alias_map.j2*, */usr/share/sonic/templates/sysDescription.j2*.
+
+In the end this is used to produce */etc/snmp/snmpd.conf*.
+
+## Limits and incentives to change ##
+- This goes against the principle of having the config DB be the central repository of configuration data.
+- Imposes the provisioning of multiple static files.
+- the snmpContact is hardcoded in the */usr/share/sonic/templates/snmpd.conf.j2* file.
+- pass-throughs are hard-coded.
+
+## Proposal: integrate the SNMP configuration into config DB ##
+### Proposed Schema ###
+
+```
+"SNMP": {
+    "location": LOCATION_STRING,
+    "contact": CONTACT_STRING,
+    "v2c": {
+        COMMUNITY_STRING: {
+            "type": "rw"|"ro",
+        }
+    },
+    "pass_trough": {
+        OID_STRING: COMMAND_STRING,
+    }
+}
+```
+
+Where:
+- LOCATION_STRING:  String, defines the snmpLocation, default: "".  
+- CONTACT_STRING:   String, defines the snmpContact, default: "" or the current hardcoded value for backwards compatibility.  
+- COMMUNITY_STRING: String, defines the community string.  
+- OID_STRING:       String. a dotted notation OID prefix.  
+- COMMAND_STRING:   String: command to be called when OID_STRING is requested.
+
+New keys:
+- "v2c": we define a "v2c" tree to allow for future expansion for other versions of the SNMP protocol, this spec only defines for SNMP v2  
+         we could imagine the implementation of "v3" with the inclusion of users or references to central PAM methods.  
+- "type":  Optional, if ommited defaults to 'ro', there are 2 possible values:  
+           "ro": read-only, the only implemented method at this time.  
+           "rw": well you never know - here for completeness but unused in the code.  
+- "pass_trough": used for pass-trough definitions - currently the config only has one hard-coded pass-through defined for SysDescription.
+
+### Files needing modification for implementation ###
+
+The changes we propose are only additive to remain compatible with the current install base and the current way of doing things.
+
+In repo *sonic-buidlimage*:
+
+*dockers/docker-snmp-v2/snmpd.conf.j2*:  
+    verify the existence of the SNMP table in the datatbase and fork behavior if present, if not continue using old method.
+
+*dockers/docker-snmp-v2/snmpd-config-updater*:  
+    this file will be deprecated soon by caclmgrd so no updates will be done
+
+
+In repo *sonic-swss-common*: 
+
+*common/schema.h*:  
+```
+#define CFG_SNMP_TABLE_NAME           "SNMP"
+```
+
+In repo *sonic-swss*:
+
+*doc/swss-schema.md*:
+    add the definition of this schema
+
+## Unsolved Issues ##
+- Uploading custom pass-through