rfc1592.txt

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RFC 1592                        SNMP-DPI                      March 1994


   The next field, present in all packets, is the packet ID.  It
   contains packet identification that can help an agent or sub-agent
   match responses with request.  This is useful with UDP connections
   over which packets can be lost.  The packet ID is a monotonically
   increasing unsigned 16-bit integer which wraps at its maximum value.

   The next field, present in all packets, is the packet type.  It
   indicates what kind of packet we're dealing with (OPEN, REGISTER,
   GET, GETNEXT, GETBULK, SET, COMMIT, UNDO, TRAP, RESPONSE, UNREGISTER,
   or CLOSE).  The permitted values for this field are defined in Table
   16.

     +-----------------------------------------------------------------+
     | Table 3. SNMP DPI packet header.  Present in all packets.       |
     +------------+----------------------------------------------------+
     | OFFSET     | FIELD                                              |
     +------------+----------------------------------------------------+
     | 0          | packet length to follow (MSB to LSB)               |
     +------------+----------------------------------------------------+
     | 2          | protocol major version                             |
     +------------+----------------------------------------------------+
     | 3          | protocol minor version                             |
     +------------+----------------------------------------------------+
     | 4          | protocol release                                   |
     +------------+----------------------------------------------------+
     | 5          | packet id (MSB to LSB)                             |
     +------------+----------------------------------------------------+
     | 7          | packet type                                        |
     +------------+----------------------------------------------------+

   From this point onwards, the contents of the packet are defined by
   the protocol being used.  The remainder of this section describes:

     o   Layout of packets for the SNMP DPI protocol, version 2.0.

     o   Constants as defined with this version of the protocol.

3.2.2  OPEN

   In order for a sub-agent to communicate with a DPI capable SNMP
   agent, it must first send an SNMP DPI OPEN request to the agent to
   setup the "connection" with that agent.

   Such a packet contains the standard SNMP DPI header plus OPEN
   specific data.  This data consists of:






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RFC 1592                        SNMP-DPI                      March 1994


     o   a timeout value (in seconds).
         This is a requested timeout value to be used for all requests
         for objects for which there is no timeout value specified for
         the sub-tree under which the object is registered.  If you
         specify a zero timeout value, then the agent will use its own
         default timeout value.  If you want a larger value than the
         default value, then you can specify it here. However, the agent
         may have a maximum value that you can never exceed. If you do
         ask for a larger timeout than that maximum, the agent will set
         it at the maximum it accepts.
     o   the maximum number of varBinds per DPI packet that the
         sub-agent is prepared to handle.
     o   Selected character set to be used for the representation of the
         OBJECT ID strings and DisplayStrings.
         The choices are the native character set (0) or the ASCII
         character set (1).  See 3.3.5, "Character set selection"
         for more information in character set selection.
         An agent may choose to support only the native character set.
     o   null terminated sub-agent ID, which is a unique ASN.1 OBJECT
         identifier, so in dotted ASN.1 notation.  This string is
         represented in the selected character set.
     o   null terminated sub-agent description, which is a DisplayString
         describing the sub-agent.  This string is represented in the
         selected character set.  This may be the null-string if there
         is no description.
     o   optionally a password that the agent uses to validate the
         sub-agent.  It depends on the agent implementation if a
         password is required.  If no password is passed, the length
         must be specified as zero.

   The sub-agent must expect a response indicating success or failure.
   See Table 19 for the valid codes in a DPI RESPONSE to a DPI OPEN
   request.

   If the error_code in the RESPONSE is not SNMP_ERROR_DPI_noError, then
   the agent closes the connection.















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RFC 1592                        SNMP-DPI                      March 1994


     +-----------------------------------------------------------------+
     | Table 4. Layout SNMP DPI OPEN packet                            |
     +------------+----------------------------------------------------+
     | OFFSET     | FIELD                                              |
     +------------+----------------------------------------------------+
     | 0          | packet length to follow (MSB to LSB)               |
     +------------+----------------------------------------------------+
     | 2          | protocol major version                             |
     +------------+----------------------------------------------------+
     | 3          | protocol minor version                             |
     +------------+----------------------------------------------------+
     | 4          | protocol release                                   |
     +------------+----------------------------------------------------+
     | 5          | packet id (MSB to LSB)                             |
     +------------+----------------------------------------------------+
     | 7          | packet type = SNMP_DPI_OPEN                        |
     +------------+----------------------------------------------------+
     | 8          | requested overall timeout (seconds, MSB to LSB)    |
     +------------+----------------------------------------------------+
     | 10         | max varBinds per DPI packet (MSB to LSB)           |
     +------------+----------------------------------------------------+
     | 12         | Selected character set (0=Native, 1=ASCII)         |
     +------------+----------------------------------------------------+
     | 13         | null terminated sub-agent ID (OID)                 |
     +------------+----------------------------------------------------+
     | 13+L1      | null terminated sub-agent Description              |
     +------------+----------------------------------------------------+
     | 13+L2      | password length (zero if no password, MSB to LSB)  |
     +------------+----------------------------------------------------+
     | 15+L2      | password (if any)                                  |
     +------------+----------------------------------------------------+
     | NOTE:                                                           |
     |                                                                 |
     | o   L1 = strlen(sub-agent ID) + 1                               |
     | o   L2 = L1 + strlen(sub-agent Description) + 1                 |
     | o   OID and Description strings use selected character set      |
     +-----------------------------------------------------------------+

3.2.3  CLOSE

   In order for a sub-agent to close the "connection" with the DPI
   capable SNMP agent, it must send an SNMP DPI CLOSE request to the
   agent.  The agent will not send a response, but closes the physical
   connection and implicitly unregisters any sub-trees related to the
   connection.

   An agent may also send to the sub-agent an SNMP DPI CLOSE packet that
   contains the standard SNMP DPI header plus CLOSE specific data.  This



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RFC 1592                        SNMP-DPI                      March 1994


   data consists of:

     o   a reason code for closing.  See Table 21 for a list
         of valid reason codes.

     +-----------------------------------------------------------------+
     | Table 5. Layout SNMP DPI CLOSE packet                           |
     +------------+----------------------------------------------------+
     | OFFSET     | FIELD                                              |
     +------------+----------------------------------------------------+
     | 0          | packet length to follow (MSB to LSB)               |
     +------------+----------------------------------------------------+
     | 2          | protocol major version                             |
     +------------+----------------------------------------------------+
     | 3          | protocol minor version                             |
     +------------+----------------------------------------------------+
     | 4          | protocol release                                   |
     +------------+----------------------------------------------------+
     | 5          | packet id (MSB to LSB)                             |
     +------------+----------------------------------------------------+
     | 7          | packet type = SNMP_DPI_CLOSE                       |
     +------------+----------------------------------------------------+
     | 8          | reason code (1 octet)                              |
     +------------+----------------------------------------------------+

3.2.4  ARE_YOU_THERE

   An ARE_YOU_THERE packet allows a sub-agent to determine if it still
   has a DPI connection with the agent.  This packet is necessary
   because a sub-agent passively awaits requests from an agent and
   normally will not detect problems with an agent connection in a
   timely manner.  (In contrast, an agent becomes aware of any sub-agent
   connection problem in a timely manner because it sets a timeout when
   sending request).

   A sub-agent can send a SNMP DPI ARE_YOU_THERE packet to an agent
   which will then return a RESPONSE with a zero error code and a a zero
   error index if the connection is healthy.  Otherwise, the agent may
   return a RESPONSE with an error indication.  If the connection is
   broken, the sub-agent will see no response at all.

   An ARE_YOU_THERE packet contains the standard SNMP DPI header with no
   additional data.








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RFC 1592                        SNMP-DPI                      March 1994


     +-----------------------------------------------------------------+
     | Table 6. Layout SNMP DPI ARE_YOU_THERE packet                   |
     +------------+----------------------------------------------------+
     | OFFSET     | FIELD                                              |
     +------------+----------------------------------------------------+
     | 0          | packet length to follow (MSB to LSB)               |
     +------------+----------------------------------------------------+
     | 2          | protocol major version                             |
     +------------+----------------------------------------------------+
     | 3          | protocol minor version                             |
     +------------+----------------------------------------------------+
     | 4          | protocol release                                   |
     +------------+----------------------------------------------------+
     | 5          | packet id (MSB to LSB)                             |
     +------------+----------------------------------------------------+
     | 7          | packet type = SNMP_DPI_ARE_YOU_THERE               |
     +------------+----------------------------------------------------+

3.2.5  REGISTER

   In order to register a branch in the MIB tree, an SNMP sub-agent
   sends an SNMP DPI REGISTER packet to the agent.

   Such a packet contains the standard SNMP DPI header plus REGISTER
   specific data.  This data consists of:

     o   a requested priority.
         There are 2 special values, namely minus one (-1, requests best
         available priority) and zero (0, requests next better priority
         than the highest priority in use).  Any other value requests a
         specific priority or the next best priority if already in use).
         The lower the number, the better the priority.  An agent will
         send requests to only the one sub-agent that has registered
         with the best priority.  The agent returns the actual priority
         assigned in the RESPONSE packet in the error_index field.
     o   a requested timeout.
         If a zero value is specified, then the agent uses the timeout
         value specified in the DPI OPEN request.
         If you want a shorter or longer timeout value for this specific
         sub-tree, then you specify it here.  The agent has a maximum
         timeout it will allow in this field.  The agent will use this
         value (or its maximum) to await a response to requests for this
         sub-tree.
     o   an indication as to whether the sub-agent wishes to handle MIB
         view selection (SNMPv1 community string authentication)
         in subsequent GET, GETNEXT or SET, COMMIT, UNDO requests.  Not
         all DPI capable agents need to support this feature, but they
         must at least recognize this indication and give an appropriate



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RFC 1592                        SNMP-DPI                      March 1994