rfc1592.txt

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Wijnen, Carpenter, Curran, Sehgal & Waters                      [Page 5]

RFC 1592                        SNMP-DPI                      March 1994


   Please note that in the following portion of this text the SNMP-DPI
   agent is referred simply as the agent.

   Once the transport connection has been set up, the sub-agent must
   also initialize the logical connection with the agent.  To do so it
   issues an OPEN request to the agent in which the sub-agent uniquely
   identifies itself and passes some other parameters to the agent, such
   as, the maximum number of varBinds per interaction it is prepared to
   handle, and the timeout the agent should use when waiting for a
   response from the sub-agent.

   When the sub-agent prepares to stop or cease operations, it first
   issues a CLOSE to shut down the logical connection with the agent,
   and then closes the transport connection.

2.2  REGISTRATION

   A sub-agent supports a collection of MIB variables or object
   identifiers (object IDs) that constitute its MIB (sub)tree.  Each of
   these object IDs consists of a group ID and an instance ID.  The
   group ID is the root of the sub-agent's MIB tree that it supports and
   the point of registration to the agent's MIB tree.  The instance ID
   is the piece of the Object Identifier that follows the group ID
   (registration point), so it is not an instance in the terms of the
   SNMP definition of an instance.

   Regardless of the transport mechanism used, after establishing a
   connection to the agent, the sub-agent registers a branch (group ID)
   to the Agent's MIB tree.  With the registration request, the sub-
   agent passes some parameters, such as, requested priority and a
   timeout value for this specific sub-tree.

   The agent sends back a response to indicate success or failure of the
   registration request.

2.3  NORMAL OPERATION

   Once the sub-agent has set up both the physical and logical
   connection to the agent, and once it has successfully registered the
   sub-tree(s) of the MIB(s) that it supports, it waits for requests
   from the SNMP agent or generates traps as required.

2.4  DPI ARCHITECTURE

   These are the requests that can be initiated by the SNMP agent:

       GET, GETNEXT, GETBULK, SET, COMMIT, UNDO, UNREGISTER, and CLOSE.




Wijnen, Carpenter, Curran, Sehgal & Waters                      [Page 6]

RFC 1592                        SNMP-DPI                      March 1994


   The first four of these correspond directly to SNMP requests that a
   network management station can make (By default a GETBULK request
   will be translated into multiple GETNEXT requests by the agent, but a
   sub-agent may request that the GETBULK be passed to it).  The COMMIT,
   UNDO, UNREGISTER, ARE_YOU_THERE and CLOSE requests are specific
   SNMP-DPI requests.  The sub-agent normally responds to a request with
   a RESPONSE packet.  The CLOSE request is an exception for which the
   sub-agent only closes the physical connection.

   These are the requests that can be initiated by a sub-agent:

       OPEN, REGISTER, TRAP, UNREGISTER, ARE_YOU_THERE and CLOSE.

   The agent responds to OPEN, REGISTER, UNREGISTER and ARE_YOU_THERE
   with a RESPONSE packet.  The TRAP packet is just accepted and
   forwarded by the agent without returning any information to the sub-
   agent.  The CLOSE packet is also just accepted by the agent upon
   which it closes the physical connection.

   See Figure 1 for an overview of the DPI packet flow.































Wijnen, Carpenter, Curran, Sehgal & Waters                      [Page 7]

RFC 1592                        SNMP-DPI                      March 1994


     -------------------------------------------------------------------

     *---------------------------------*
     |                                 |
     |  SNMP Network                   |
     |  Management Station             |
     |                                 |
     |---------------------------------|
     |  SNMP Protocol                  |
     *---------------------------------*
           A      | Get         A
           |      | GetNext     |  GetResponse
      Trap |      | GetBulk     |
           |      | Set         |
           |      V             |
     *------------------------------*             *-------------------*
     |     SNMP Protocol            |             |   DPI Interface   |
     |------------------------------|  Response   |    *--------------|
     |                       |      |<----------->|    |              |
     |                       |      |             |    |              |
     |    SNMP Agent         |      |             |    |              |
     |                       |      | Get,GetNext |    |              |
     |                       |      | (GetBulk)   |    |   Client     |
     |                       |      | Set,Commit  |    |              |
     |     A     *-----------+->    |  Undo       |    |              |
     |     |     | Get/Set   |      |------------>|    |     or       |
     | Trap|     | info      |      |             |    |              |
     |     |     |           | SNMP |             |    |              |
     |-----+-----+-------*   |      |  trap       |    |   SNMP       |
     |     |     V       |   | DPI  |<------------|    |   Sub-Agent  |
     |                   |   |      |             |    |              |
     | Statically Linked |   |      |             |    |              |
     | Instrumentation   |   |      |             |    |              |
     |   (like MIB II)   |   |      |             |    |              |
     |                   |   |      |   close     |    |              |
     |       A           |   |      | unregister  |    |              |
     |-------+-----------|   |      |<----------->|    |              |
     |       V           |   |      |             |    |              |
     |                   |   |      |             |    |              |
     |                   |   |      | AreYouThere |    |              |
     | TCP/IP layers     |   |      |    open     |    |              |
     | Kernel            |   |      |  register   |    |              |
     |                   |   |      |<------------|    |              |
     *------------------------------*             *-------------------*

     -------------------------------------------------------------------
     Figure 1. SNMP DPI overview




Wijnen, Carpenter, Curran, Sehgal & Waters                      [Page 8]

RFC 1592                        SNMP-DPI                      March 1994


     Remarks for Figure 1:

     o   The SNMP agent communicates with the SNMP manager via the
         standard SNMP protocol.
     o   The SNMP agent communicates with some statically linked-in
         instrumentation (potentially for the MIB II), which in turn
         talks to the TCP/IP layers and kernel (operating system) in an
         implementation-dependent manner.
     o   An SNMP sub-agent, running as a separate process (potentially
         on another machine), can set up a connection with the agent.
         The sub-agent has an option to communicate with the SNMP agent
         through UDP or TCP sockets, or even through other mechanisms.
     o   Once the connection is established, the sub-agent issues a DPI
         OPEN and one or more REGISTER requests to register one or more
         MIB sub-trees with the SNMP agent.
     o   The SNMP agent responds to DPI OPEN and REGISTER requests with
         a RESPONSE packet, indicating success or failure.
     o   The SNMP agent will decode SNMP packets.
         If such a packet contains a Get or GetNext request for an
         object in a sub-tree registered by a sub-agent, it sends a
         corresponding DPI packet to the sub-agent.
         If the request is for a GetBulk, then the agent translates it
         into multiple DPI GETNEXT packets and sends those to the
         sub-agent.  However, the sub-agent can request (in the REGISTER
         packet) that a GETBULK be passed to the sub-agent.
         If the request is for a Set, then the agent uses a 2-phase
         commit scheme and sends the sub-agent a sequence of SET/COMMIT,
         SET/UNDO or SET/COMMIT/UNDO DPI packets.
     o   The SNMP sub-agent sends responses back via a RESPONSE packet.
     o   The SNMP agent then encodes the reply into an SNMP packet and
         sends it back to the requesting SNMP manager.
     o   If the sub-agent wants to report an important state change, it
         sends a DPI TRAP packet to the SNMP agent which will encode it
         into an SNMP trap packet and send it to the manager(s).
     o   If the sub-agent wants to stop operations, it sends a DPI
         UNREGISTER and a DPI CLOSE packet to the agent.  The agent
         sends a response to an UNREGISTER request.
     o   There is no RESPONSE to a CLOSE, the agent just closes the DPI
         connection.  A CLOSE implies an UNREGISTER for all
         registrations that exist for the DPI connection being CLOSED.
     o   An agent can send DPI UNREGISTER (if a higher priority
         registration comes in or for other reasons) to the sub-agent,
         the sub-agent then responds with a DPI RESPONSE packet.
     o   An agent can also (for whatever reason) send a DPI CLOSE to
         indicate it is terminating the DPI connection.
     o   A sub-agent can send an ARE_YOU_THERE to verify that the
         "connection" is still open. If so, the agent sends a RESPONSE
         with no error, otherwise, it may send a RESPONSE with an error



Wijnen, Carpenter, Curran, Sehgal & Waters                      [Page 9]

RFC 1592                        SNMP-DPI                      March 1994


         indication, or not react at all.

3.  SNMP DPI PROTOCOL

   This section describes the actual protocol used between the SNMP
   agent and sub-agents.

3.1  CONNECTION ESTABLISHMENT

   In a TCP/IP environment, the SNMP agent listens on an arbitrary
   TCP/UDP port for a connection request from a sub-agent.  It is
   important to realize that a well-known port is not used: every
   invocation of the SNMP agent will potentially result in a different
   TCP/UDP port being used.

   A sub-agent needs to determine this port number to establish a
   connection.  The sub-agent learns the port number from the agent by
   sending it one conventional SNMP get-request PDU.  The port numbers
   are maintained by the SNMP agent as the objects whose identifiers
   are:

        1.3.6.1.4.1.2.2.1.1.0    dpiPort.0       (old DPI 1.x form)
        1.3.6.1.4.1.2.2.1.1.1.0  dpiPortForTCP.0
        1.3.6.1.4.1.2.2.1.1.2.0  dpiPortForUDP.0

   These variables are registered under the IBM enterprise-specific
   tree.  See 4, "DPI 2.0 MIB definition" for more information.  The
   SNMP agent replies with a conventional SNMP response PDU that
   contains the port number to be used.  This response is examined by
   the sub-agent and the port number is extracted.  The sub-agent then
   establishes the connection to the specified port.

   On the surface, this procedure appears to mean that the sub-agent
   must be able to create and parse SNMP packets, but this is not the
   case.  A DPI Application Programming Interface (API) normally
   provides a library routine, query_DPI_port(), which can be used to
   generate and parse the required SNMP packets.  This very small
   routine (under 100 lines of C), does not greatly increase the size of
   any sub-agent.

   NOTE: Since this RFC does not define an API, the actual code of and
   interface to a query_DPI_port() type of function depends on the
   implementation.

   For completeness, byte-by-byte descriptions of the packets to be
   generated by an SNMP DPI API routine query_DPI_port() are provided
   below.  This is probably of little interest to most readers and
   reading the source of a query_DPI_port() function provides much of



Wijnen, Carpenter, Curran, Sehgal & Waters                     [Page 10]