rfc1157 https://datatracker.ietf.org/doc/html/rfc1157#section-3.2.2

.  Protocol Specification

   The network management protocol is an application protocol by which
   the variables of an agent's MIB may be inspected or altered.

   Communication among protocol entities is accomplished by the exchange
   of messages, each of which is entirely and independently represented
   within a single UDP datagram using the basic encoding rules of ASN.1
   (as discussed in 
 
Section 3.2.2).  A message consists of a version
   identifier, an SNMP community name, and a protocol data unit (PDU).
   A protocol entity receives messages at UDP port 161 on the host with
   which it is associated for all messages except for those which report
   traps (i.e., all messages except those which contain the Trap-PDU).
   Messages which report traps should be received on UDP port 162 for
   further processing.  An implementation of this protocol need not
   accept messages whose length exceeds 484 octets.  However, it is
   recommended that implementations support larger datagrams whenever
   feasible.

   It is mandatory that all implementations of the SNMP support the five
   PDUs:  GetRequest-PDU, GetNextRequest-PDU, GetResponse-PDU,
   SetRequest-PDU, and Trap-PDU.

    
 
RFC1157-SNMP DEFINITIONS ::= BEGIN

     IMPORTS
          ObjectName, ObjectSyntax, NetworkAddress, IpAddress, TimeTicks
                  FROM RFC1155-SMI;


     -- top-level message

             Message ::=
                     SEQUENCE {
                          version        -- version-1 for this RFC
                             INTEGER {
                                 version-1(0)
                             },

                         community      -- community name
                             OCTET STRING,

                         data           -- e.g., PDUs if trivial
                             ANY        -- authentication is being used
                     }






Case, Fedor, Schoffstall, & Davin                              [Page 16]
 

RFC 1157                          SNMP                          May 1990


     -- protocol data units

             PDUs ::=
                     CHOICE {
                         get-request
                             GetRequest-PDU,

                         get-next-request
                             GetNextRequest-PDU,

                         get-response
                             GetResponse-PDU,

                         set-request
                             SetRequest-PDU,

                         trap
                             Trap-PDU
                          }

     -- the individual PDUs and commonly used
     -- data types will be defined later

     END