Request for Comments: 1592 G. Carpenter
Obsoletes: 1228 T.J. Watson Research Center, IBM Corp.
Category: Experimental K. Curran
A. Sehgal
G. Waters
Bell Northern Research, Ltd.
March 1994
Simple Network Management Protocol
Distributed Protocol Interface
Version 2.0
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Table of Contents
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Summary of Changes . . . . . . . . . . . . . . . . . . . . 4
2. THEORY OF OPERATION . . . . . . . . . . . . . . . . . . . . . 5
2.1 Connection Establishment and Termination . . . . . . . . . 5
2.2 Registration . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Normal Operation . . . . . . . . . . . . . . . . . . . . . 6
2.4 DPI Architecture . . . . . . . . . . . . . . . . . . . . . 6
3. SNMP DPI PROTOCOL . . . . . . . . . . . . . . . . . . . . . 10
3.1 Connection Establishment . . . . . . . . . . . . . . . . 10
3.1.1 SNMP PDU to GET the Agent's DPI port . . . . . . . . . 11
3.1.2 SNMP PDU Containing the RESPONSE to the GET . . . . . 13
3.2 SNMP DPI Packet Formats . . . . . . . . . . . . . . . . 15
3.2.1 DPI Packet Header . . . . . . . . . . . . . . . . . . 15
3.2.2 OPEN . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.3 CLOSE . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.4 ARE_YOU_THERE . . . . . . . . . . . . . . . . . . . . 19
3.2.5 REGISTER . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.6 UNREGISTER . . . . . . . . . . . . . . . . . . . . . . 22
3.2.7 GET . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2.8 GETNEXT . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.9 GETBULK . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.10 SET, COMMIT and UNDO . . . . . . . . . . . . . . . . 26
3.2.11 RESPONSE . . . . . . . . . . . . . . . . . . . . . . 29
3.2.12 TRAP . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Constants and Values . . . . . . . . . . . . . . . . . . 33
3.3.1 Protocol Version and Release Values . . . . . . . . . 33
3.3.2 Packet Type Values . . . . . . . . . . . . . . . . . . 34
3.3.3 Variable Type Values . . . . . . . . . . . . . . . . . 35
3.3.4 Value Representation . . . . . . . . . . . . . . . . . 36
3.3.5 Character set selection . . . . . . . . . . . . . . . 36
3.3.6 Error Code Values for SNMP DPI RESPONSE packets . . . 37
3.3.7 UNREGISTER Reason Codes . . . . . . . . . . . . . . . 40
3.3.8 CLOSE Reason Codes . . . . . . . . . . . . . . . . . . 41
4. DPI 2.0 MIB DEFINITION . . . . . . . . . . . . . . . . . . 41
5. SUBAGENT CONSIDERATIONS . . . . . . . . . . . . . . . . . . 42
5.1 DPI API . . . . . . . . . . . . . . . . . . . . . . . . 43
5.2 Overview of Request Processing . . . . . . . . . . . . . 44
5.2.1 GET Processing . . . . . . . . . . . . . . . . . . . . 44
5.2.2 SET Processing . . . . . . . . . . . . . . . . . . . . 44
5.2.3 GETNEXT Processing . . . . . . . . . . . . . . . . . . 46
5.2.4 GETBULK Processing . . . . . . . . . . . . . . . . . . 47
5.2.5 OPEN Request . . . . . . . . . . . . . . . . . . . . . 48
5.2.6 CLOSE Request . . . . . . . . . . . . . . . . . . . . 49
5.2.7 REGISTER Request . . . . . . . . . . . . . . . . . . . 49
5.2.8 UNREGISTER Request . . . . . . . . . . . . . . . . . . 50
5.2.9 TRAP Request . . . . . . . . . . . . . . . . . . . . . 51
5.2.10 ARE_YOU_THERE request . . . . . . . . . . . . . . . . 51
5.2.11 How to query the DPI port. . . . . . . . . . . . . . 51
6. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . 51
7. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 52
8. AUTHORS' ADDRESSES . . . . . . . . . . . . . . . . . . . . 53
9. SAMPLE SOURCES FOR ANONYMOUS FTP . . . . . . . . . . . . . 54
1. INTRODUCTION
This RFCdescribes version 2.0 of a protocol that International
Business Machines Corporation (IBM) has been implementing in most of
its SNMP agents to allow dynamic extension of supported MIBs. Bell
Northern Research (BNR) has also implemented a version of this
protocol in some of its SNMP agents for the same reason.
The Simple Network Management Protocol (SNMP [1]) Distributed
Protocol Interface (DPI) is an extension to SNMP agents that permits
end-users to dynamically add, delete or replace management variables
in the local Management Information Base without requiring
recompilation of the SNMP agent. This is achieved by writing a so-
called sub-agent that communicates with the agent via the SNMP-DPI.
For the author of a sub-agent, the SNMP-DPI eliminates the need to
know the details of ASN.1 [2] or SNMP PDU (Protocol Data Unit)
encoding/decoding [1, 3].
Versions 1.0 and 1.1 of this protocol have been in use within IBM
since 1989 and is included in the SNMP agents for VM, MVS and OS/2.
Version 1.2 of this protocol has been in use within BNR since 1992.
1.1 MOTIVATION
The Simple Network Management Protocol [1] defines a protocol that
permits operations on a collection of variables. This set of
variables is called the Management Information Base (MIB) and a core
set of variables has previously been defined [4, 5]; however, the
design of the MIB makes provision for extension of this core set.
Thus, an enterprise or individual can define variables of their own
which represent information of use to them. An example of a
potentially interesting variable which is not in the core MIB would
be CPU utilization (percent busy). Unfortunately, conventional SNMP
agent implementations provide no means for an end-user to make
available new variables.
Besides this, today there are many MIBs that people want to implement
on a system. Without a capability for sub-agents, this requires all
the MIBs to be implemented in one big monolithic agent, which is in
many cases undesirable.
The SNMP DPI addresses these issues by providing a light-weight
mechanism by which a process can register the existence of a MIB
variable or a MIB sub-tree with the SNMP agent. Requests for the
variable(s) that are received by the SNMP agent are passed to the
process acting as a sub-agent. The sub-agent then returns an
appropriate answer to the SNMP agent. The SNMP agent eventually
packages an SNMP response packet and sends the answer back to the
remote network management station that initiated the request.
Remote network management stations have no knowledge that the SNMP
agent calls on other processes to obtain an answer. As far as they
can tell, there is only one network management application (agent)
running on the host.
At the San Diego IETF (March 1992) a BOF was held on multiplexing
SNMP agent's requirements. Both the SMUX [6] and DPI [7] protocols
were discussed, as well as other unpublished approaches. There was
also discussion regarding a need for a standard for multiplexing SNMP
agents or sub-agent support. At the end of the BOF, however, there
was not enough support for defining a standard. This was due, at
least partially, to a few well known SNMP authors who stated that the
proxy and party support for SNMPv2 (SMP at the time) would solve the
problem.
Nevertheless, questions continue to be raised about sub-agent support
(both in SNMP and SNMP2 mail lists) in spite of both SNMPv2 [8] being
on the standard's track and SMUX being changed to a historic RFC.
Furthermore, within IBM and BNR we continue to see a substantial and
expanding use of the DPI protocol. with positive results.
Therefore, we believe that there is a place for a sub-agent protocol
and we again offer this new version as an experimental protocol. We
encourage people to try it and send us feedback. Depending on that
feedback, we may decide to try to get onto the standards track at a
later time.
During discussions about sub-agent interfaces at the San Diego BOF it
also became clear that we should reduce the focus on the API for the
sub-agent programmers. This RFC, therefore, specifies only the
protocol to distribute SNMP requests from the main SNMP agent to the
sub-agents. Programmers can build one or more Programming APIs on
top of that protocol as needed, and sample API code is available from
the authors of this document.
1.2 SUMMARY OF CHANGES
The following changes have been made since the initial definition of
SNMP-DPI [7]. Some of these resulted from comparing the SMUX [6] and
DPI [7] protocols.
o Documentation changes to cleanup and be more specific in some
areas. Among other things, this includes:
- Defining that integers are in network byte order
- Defining the character set used for strings
- Defining how DisplayStrings are handled.
- Including DPI20 MIB definition.
o Removal of the Programming API from the document.
o Addition of new DPI packet types:
- SNMP_DPI_OPEN for a sub-agent to open a "connection" with
the DPI SNMP capable agent. The sub-agent must now
identify itself and optionally provide a "password" for the
connection.
- SNMP_DPI_CLOSE for the agent or sub-agent to close the
connection in a graceful way.
- SNMP_DPI_ARE_YOU_THERE for the sub-agent to verify that the
agent still knows about the sub-agent.
- SNMP_DPI_UNREGISTER for the agent or sub-agent to terminate
the registration of a MIB variable or MIB sub-tree.
- SNMP_DPI_COMMIT which instructs the sub-agent to actually
commit a previous SNMP_DPI_SET request. This, together
with the UNDO, allows DPI sub-agents to be compliant with
SNMP in the sense that we can now handle the "as if
simultaneous" requirement.
- SNMP_DPI_UNDO which instructs the sub-agent to UNDO a SET
or COMMIT if such is needed.
o Changes to DPI packets:
- Multiple varBinds can now be exchanged in one DPI packet
(for GET, GETNEXT, SET, TRAP). The sub-agent can specify
the maximum it wants to handle per packet.
- The packet headers now contain a packet-ID (similar to SNMP
request ID in SNMP PDU). This allows to match RESPONSE
packets to REQUESTS, which is important for UDP based
DPI-connections.
- The SNMP_DPI_REGISTER packet has new fields for time_out
and for requested priority.
- The SNMP_DPI_TRAP packet allows to specify an enterprise
OID. In addition, the generic and specific trap types are
now 4 octets, so that we can pass the types correctly.
- In general, the packets have a more consistent layout.
o The agent now sends a RESPONSE to a REGISTER request
o Addition of SNMPv2 error codes and value types.
2. THEORY OF OPERATION
2.1 CONNECTION ESTABLISHMENT AND TERMINATION
Communication between the SNMP Agent and its clients (sub-agents)
takes place via a communication mechanism. The communication type
can be either a logical stream connection (via TCP, for instance) or
an unreliable datagram connection (UDP, for instance). It should be
noted that other stream oriented transport communication mechanisms
can also be used. For example, the VM SNMP agent allows DPI
connections over IUCV (Inter-User Communications Vehicle) [9, 10].
Other than the connection establishment procedure, the protocol used
is identical in these environments.
In Unix the number of processes is limited by the number of file-
descriptors that can be opened. Since each TCP socket represents a
file-descriptor, restricting SNMP-DPI protocol to TCP only
connections would limit the number of sub-agents an agent could
support. As a result, the some SNMP-DPI agents support both TCP and
UDP socket type communication mechanisms for the SNMP-DPI protocol.
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.
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.
-------------------------------------------------------------------
*---------------------------------*
| |
| 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
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
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 RFCdoes 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
the same information.
3.1.1 SNMP PDU TO GET THE AGENT'S DPI PORT
As noted, before a TCP/UDP connection to the SNMP agent can be made,
the sub-agent must learn which port that the agent is listening on.
To do so, it can issue an SNMP GET for the variable dpiPortForTCP.0
(1.3.6.1.4.1.2.2.1.1.1.0) or variable dpiPortForUDP.0
(1.3.6.1.4.1.2.2.1.1.2.0).
The SNMP PDU can be constructed as shown below. This PDU must be
sent to UDP port 161 on the host where the agent runs (probably the
same host where the sub-agent runs).
The (SNMPv1) packet shown below is for the TCP port.
+-----------------------------------------------------------------+
| Table 1 (Page 1 of 2). SNMP GET PDU for dpiPortForTCP.0 |
+---------------+----------------+--------------------------------+
| OFFSET | VALUE | FIELD |
+---------------+----------------+--------------------------------+
| 0 | 0x30 | ASN.1 header |
+---------------+----------------+--------------------------------+
| 1 | 37 + len | PDU_length, see formula below |
+---------------+----------------+--------------------------------+
| 2 | 0x02 0x01 0x00 | SNMP version: |
| | | (integer,length=1,value=0) |
+---------------+----------------+--------------------------------+
| 5 | 0x04 | community name (string) |
+---------------+----------------+--------------------------------+
| 6 | len | length of community name |
+---------------+----------------+--------------------------------+
| 7 | community name | varies |
+---------------+----------------+--------------------------------+
| 7 + len | 0xa0 0x1c | SNMP GET request: |
| | | request_type=0xa0,length=0x1c |
+---------------+----------------+--------------------------------+
| 7 + len + 2 | 0x02 0x01 0x01 | SNMP request ID: |
| | | integer,length=1,ID=1 |
+---------------+----------------+--------------------------------+
| 7 + len + 5 | 0x02 0x01 0x00 | SNMP error status: |
| | | integer,length=1,error=0 |
+---------------+----------------+--------------------------------+
| 7 + len + 8 | 0x02 0x01 0x00 | SNMP index: |
| | | integer,length=1,index=0 |
+---------------+----------------+--------------------------------+
| 7 + len + 11 | 0x30 0x11 | varBind list, length=0x11 |
+---------------+----------------+--------------------------------+
| 7 + len + 13 | 0x30 0x0f | varBind, length=0x0f |
+---------------+----------------+--------------------------------+
| 7 + len + 15 | 0x06 0x0b | Object ID, length=0x0b |
+---------------+----------------+--------------------------------+
+-----------------------------------------------------------------+
| Table 1 (Page 2 of 2). SNMP GET PDU for dpiPortForTCP.0 |
+---------------+----------------+--------------------------------+
| OFFSET | VALUE | FIELD |
+---------------+----------------+--------------------------------+
| 7 + len + 17 | 0x2b 0x06 0x01 | Object-ID: |
| | 0x04 0x01 0x02 | 1.3.6.1.4.1.2.2.1.1.1 |
| | 0x02 0x01 0x01 | Object-instance: 0 |
| | 0x01 0x00 | |
+---------------+----------------+--------------------------------+
| 7 + len + 28 | 0x05 0x00 | null value, length=0 |
+---------------+----------------+--------------------------------+
| NOTE: Formula to calculate "PDU_length": |
| |
| PDU_length = length of version field and string tag (4 bytes)|
| + length of community length field (1 byte) |
| + length of community name (depends...) |
| + length of SNMP GET request (32 bytes) |
| |
| = 37 + length of community name |
+-----------------------------------------------------------------+
3.1.2 SNMP PDU CONTAINING THE RESPONSE TO THE GET
Assuming that no errors occurred, the port is returned in the last
few octets of the received packet. In the simple case, where the
port number will be between 1024 and 16,385, the format of the packet
is shown below.
Note: In practice, the port number can be any positive number in the
range from 1 through 65,535. A port number of 0 means that the agent
does not have a dpiPort defined for the requested protocol. So the
actual port value maybe in the last 1, 2 or 3 octets. The sample
implementation code shows how to handle the response to cover all
those cases, including error conditions.
Note: The (SNMPv1) packet shown below is for the TCP port.
+-----------------------------------------------------------------+
| Table 2 (Page 1 of 3). SNMP RESPONSE PDU for dpiPortForTCP.0 |
+---------------+----------------+--------------------------------+
| OFFSET | VALUE | FIELD |
+---------------+----------------+--------------------------------+
| 0 | 0x30 | ASN.1 header |
+---------------+----------------+--------------------------------+
| 1 | 39 + len | length, see formula below |
+---------------+----------------+--------------------------------+
+-----------------------------------------------------------------+
| Table 2 (Page 2 of 3). SNMP RESPONSE PDU for dpiPortForTCP.0 |
+---------------+----------------+--------------------------------+
| OFFSET | VALUE | FIELD |
+---------------+----------------+--------------------------------+
| 2 | 0x02 0x01 0x00 | version |
| | | (integer,length=1,value=0) |
+---------------+----------------+--------------------------------+
| 5 | 0x04 | community name (string) |
+---------------+----------------+--------------------------------+
| 6 | len | length of community name |
+---------------+----------------+--------------------------------+
| 7 | community name | |
+---------------+----------------+--------------------------------+
| 7 + len | 0xa2 0x1e | SNMP RESPONSE: |
| | | request_type=0xa2,length=0x1e |
+---------------+----------------+--------------------------------+
| 7 + len + 2 | 0x02 0x01 0x01 | SNMP request ID: |
| | | integer,length=1,ID=1 |
+---------------+----------------+--------------------------------+
| 7 + len + 5 | 0x02 0x01 0x00 | SNMP error status: |
| | | integer,length=1,error=0 |
+---------------+----------------+--------------------------------+
| 7 + len + 8 | 0x02 0x01 0x00 | SNMP index: |
| | | integer,length=1,index=0 |
+---------------+----------------+--------------------------------+
| 7 + len + 11 | 0x30 0x13 | varBind list, length=0x13 |
+---------------+----------------+--------------------------------+
| 7 + len + 13 | 0x30 0x11 | varBind, length=0x11 |
+---------------+----------------+--------------------------------+
| 7 + len + 15 | 0x06 0x0b | Object ID, length=0x0b |
+---------------+----------------+--------------------------------+
| 7 + len + 17 | 0x2b 0x06 0x01 | Object-ID: |
| | 0x04 0x01 0x02 | 1.3.6.1.4.1.2.2.1.1.1 |
| | 0x02 0x01 0x01 | Object-instance: 0 |
| | 0x01 0x00 | |
+---------------+----------------+--------------------------------+
| 7 + len + 28 | 0x02 0x02 | integer, length=2 |
+---------------+----------------+--------------------------------+
| 7 + len + 30 | MSB LSB | port number (MSB, LSB) |
+---------------+----------------+--------------------------------+
+-----------------------------------------------------------------+
| Table 2 (Page 3 of 3). SNMP RESPONSE PDU for dpiPortForTCP.0 |
+---------------+----------------+--------------------------------+
| NOTE: Formula to calculate "PDU_length": |
| |
| PDU_length = length of version field and string tag (4 bytes)|
| + length of community length field (1 byte) |
| + length of community name (depends...) |
| + length of SNMP RESPONSE (34 bytes) |
| |
| = 39 + length of community name |
+-----------------------------------------------------------------+
3.2 SNMP DPI PACKET FORMATS
Each request to, or response from, the agent or sub-agent is
constructed as a "packet" and is written to the stream.
Each packet is prefaced with the length of the data remaining in the
packet. The length is stored in network byte order, the most
significant byte (MSB) first, least significant byte (LSB) last. If
we consider a stream connection (like TCP), the receiving side will
read the packet by doing something similar to:
unsigned char len_bfr[2];
unsigned char *bfr;
int len;
read(fd,len_bfr,2);
len = len_bfr[0] * 256 + len_bfr[1];
bfr = malloc(len);
read(fd,bfr,len);
Note: The above example makes no provisions for error handling or a
read returning less than the requested amount of data,and it is not
intended to be used literally.
3.2.1 DPI PACKET HEADER
The first part of every packet identifies the application protocol
being used as well as some version information. The protocol major
version is intended to indicate, in broad terms, what version of the
protocol is used. The protocol minor version is intended to identify
major incompatible versions of the protocol. The protocol release is
intended to indicate incremental modifications to the protocol. The
constants that are valid for these fields are defined in Table 15.
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:
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.
+-----------------------------------------------------------------+
| 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
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.
+-----------------------------------------------------------------+
| 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
response if they do not support it.
o an indication as to whether the sub-agent wishes to handle the
GETBULK itself. If not, then the agent will translate a
GETBULK into multiple GETNEXT requests.
Not all DPI capable agents need to support this feature. They
may opt to always translate a GETBULK into multiple GETNEXT
requests. In this case the agent will send the appropriate
RESPONSE to indicate this.
o the group ID (sub-tree) to be registered (with trailing dot).
The group ID is represented in the selected character set as
specified in DPI OPEN packet.
The agent will respond with an SNMP DPI RESPONSE packet indicating
registration error or success. The packet ID of the response will be
the same as that for the REGISTER request to which this is a
response.
The group ID will be the same as that specified in the REGISTER
request. There will be no instance returned (e.g. NULL string for
instance ID). The value will be an SNMP_TYPE_NULL value with a zero
length.
+-----------------------------------------------------------------+
| Table 7. Layout SNMP DPI REGISTER 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_REGISTER |
+------------+----------------------------------------------------+
| 8 | requested priority (MSB to LSB) |
+------------+----------------------------------------------------+
| 12 | timeout in seconds (MSB to LSB) |
+------------+----------------------------------------------------+
| 14 | view selection (0 = you (agent) do, 1 = I do) |
+------------+----------------------------------------------------+
| 15 | getbulk selection (0=use GetNext, 1=use GetBulk) |
+------------+----------------------------------------------------+
| 16 | null terminated group ID (with trailing dot) |
+------------+----------------------------------------------------+
| NOTE: |
| |
| o group ID string uses selected character set |
+-----------------------------------------------------------------+
3.2.6 UNREGISTER
In order to unregister a branch in the MIB tree, an SNMP sub-agent
sends an SNMP DPI UNREGISTER packet to the agent.
Such a packet contains the standard SNMP DPI header plus UNREGISTER
specific data: a null terminated string (represented in the selected
character set) representing the group ID in ASN.1 dotted notation and
an indication as to the reason for the unregister (see table 14).
The agent will respond with an SNMP DPI RESPONSE packet indicating
error or success. The packet ID of the response will be the same as
that for the UNREGISTER request to which this is a response.
The group ID will be the same as that specified in the UNREGISTER
request. There will be no instance returned (e.g. NULL string for
instance ID). The value will be an SNMP_TYPE_NULL value with a zero
length.
+-----------------------------------------------------------------+
| Table 8. Layout SNMP DPI UNREGISTER 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_UNREGISTER |
+------------+----------------------------------------------------+
| 8 | reason code |
+------------+----------------------------------------------------+
| 9 | null terminated group ID (with trailing dot) |
+------------+----------------------------------------------------+
| NOTE: |
| |
| o group ID string uses selected character set |
+-----------------------------------------------------------------+
3.2.7 GET
When the SNMP agent receives a PDU containing an SNMP GET request for
a variable that resides in a sub-tree registered by a sub-agent, it
passes an SNMP DPI GET packet to the sub-agent.
Such a packet contains the standard SNMP DPI header plus GET specific
data:
o the community name used in the SNMP PDU. The length is zero
unless view handling was selected by the sub-agent. The length
is also zero if the SNMP PDU was not in SNMPv1 format.
o per varBind two null terminated strings (in the selected
character set) representing the group and instance ID in ASN.1
dotted notation.
+-----------------------------------------------------------------+
| Table 9. Layout SNMP DPI GET 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_GET |
+------------+----------------------------------------------------+
| 8 | community name length (MSB to LSB) |
+------------+----------------------------------------------------+
| 10 | community name (if any) |
+------------+----------------------------------------------------+
| 10+L1 | null terminated group ID (with trailing dot) |
+------------+----------------------------------------------------+
| 10+L2 | null terminated instance ID (no trailing dot) |
+------------+----------------------------------------------------+
| 10+L3 | optionally more varBinds (group/instance ID pairs) |
+------------+----------------------------------------------------+
| NOTE: |
| |
| o L1 = length of community name |
| o L2 = L1 + strlen(group ID) + 1 |
| o L3 = L2 + strlen(instance ID) + 1 |
| o group and instance ID strings use selected character set |
+-----------------------------------------------------------------+
3.2.8 GETNEXT
When the SNMP agent receives a PDU containing an SNMP GETNEXT request
for a variable for which a sub-agent may be authoritative, it passes
an SNMP DPI GETNEXT packet to the sub-agent.
Such a packet contains the standard SNMP DPI header plus GETNEXT
specific data:
o the community name used in the SNMP PDU. The length is zero
unless view handling was selected by the sub-agent. The length
is also zero if the SNMP PDU was not in SNMPv1 format.
o per varBind two null terminated strings (in the selected
character set) representing the group and instance ID in ASN.1