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RFC Number : 1592 Title : Simple Network Management Protocol Distributed Protocol Interface Version 2. Network Working Group B. Wijnen 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 Wijnen, Carpenter, Curran, Sehgal & Waters [Page 1]  RFC 1592 SNMP-DPI March 1994 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 RFC describes 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 Wijnen, Carpenter, Curran, Sehgal & Waters [Page 2]  RFC 1592 SNMP-DPI March 1994 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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 3]  RFC 1592 SNMP-DPI March 1994 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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 4]  RFC 1592 SNMP-DPI March 1994 - 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. 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]  RFC 1592 SNMP-DPI March 1994 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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 11]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | 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 | +---------------+----------------+--------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 12]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | 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 | +---------------+----------------+--------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 13]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | 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) | +---------------+----------------+--------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 14]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | 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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 15]  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: Wijnen, Carpenter, Curran, Sehgal & Waters [Page 16]  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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 17]  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 Wijnen, Carpenter, Curran, Sehgal & Waters [Page 18]  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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 19]  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 Wijnen, Carpenter, Curran, Sehgal & Waters [Page 20]  RFC 1592 SNMP-DPI March 1994 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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 21]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | 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 Wijnen, Carpenter, Curran, Sehgal & Waters [Page 22]  RFC 1592 SNMP-DPI March 1994 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. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 23]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | 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 Wijnen, Carpenter, Curran, Sehgal & Waters [Page 24]  RFC 1592 SNMP-DPI March 1994 character set) representing the group and instance ID in ASN.1 dotted notation. +-----------------------------------------------------------------+ | Table 10. Layout SNMP DPI GETNEXT 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_GETNEXT | +------------+----------------------------------------------------+ | 8 | community name length (MSB to LSB) | +------------+----------------------------------------------------+ | 10 | community name | +------------+----------------------------------------------------+ | 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 | +-----------------------------------------------------------------+ 3.2.9 GETBULK When the SNMP agent receives a PDU containing an SNMP GETBULK request that includes variables for which a sub-agent may be authoritative, it checks if the sub-agent wants to handle the GETBULK itself (as specified at registration time). If so, it sends an SNMP DPI GETBULK packet to the sub-agent. Such a packet contains the standard SNMP DPI header plus GETBULK specific data: Wijnen, Carpenter, Curran, Sehgal & Waters [Page 25]  RFC 1592 SNMP-DPI March 1994 o non-repeaters o max repetitions o per varBind two null terminated strings (in the selected character set) representing the group and instance ID in ASN.1 dotted notation. +-----------------------------------------------------------------+ | Table 11. Layout SNMP DPI GETBULK 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_GETBULK | +------------+----------------------------------------------------+ | 8 | non-repeaters (4 octets, MSB to LSB) | +------------+----------------------------------------------------+ | 12 | max-repetitions (4 octets, MSB to LSB) | +------------+----------------------------------------------------+ | 16 | null terminated group ID (with trailing dot) | +------------+----------------------------------------------------+ | 16+L1 | null terminated instance ID (no trailing dot) | +------------+----------------------------------------------------+ | 16+L2 | optionally more varBinds (group/instance ID pairs) | +------------+----------------------------------------------------+ | NOTE: | | | | o L1 = strlen(group ID) + 1 | | o L2 = L1 + strlen(instance ID) + 1 | | o group and instance ID strings use selected character set | +-----------------------------------------------------------------+ 3.2.10 SET, COMMIT AND UNDO When the SNMP agent receives a PDU containing an SNMP SET request for a variable that is in a sub-tree registered by a sub-agent, it passes one of 3 sequences of SNMP DPI packets to the sub-agent: Wijnen, Carpenter, Curran, Sehgal & Waters [Page 26]  RFC 1592 SNMP-DPI March 1994 o SET, COMMIT This is the normal sequence. The SET request is the first phase. The sub-agent must verify that the SET request is valid and that the resources needed are available. The COMMIT request comes next. The sub-agent must now effectuate the SET request. o SET, UNDO If an SNMP packet has a SET request for multiple varBinds that reside in different sub-trees, then the agent first sends a SET to all sub-agents. If any sub-agent returns an error on the SET, then the agent sends UNDO to those sub-agents that returned no error on the SET, meaning the SET is being canceled. o SET, COMMIT, UNDO In the very rare circumstance where all sub-agents have responded error-free to a SET and where one of them fails to perform the COMMIT, then the agent sends an UNDO to all involved sub-agents (also those who completed COMMIT). Sub-agents should try, to the best of their ability, to never let a commit fail and to undo an already committed set if asked to do so. Such packets contain the standard SNMP DPI header plus SET 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. - the type, value length and value to be set. The permitted types for the type field are defined in Table 17. See 3.3.4, 'Value Representation' for information on how the value data is represented in the packet value field. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 27]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | Table 12. Layout SNMP DPI SET, COMMIT, UNDO 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_SET/COMMIT/UNDO | +------------+----------------------------------------------------+ | 8 | community name length (MSB to LSB) | +------------+----------------------------------------------------+ | 10 | community name | +------------+----------------------------------------------------+ | 10+L1 | null terminated group ID (with trailing dot) | +------------+----------------------------------------------------+ | 10+L2 | null terminated instance ID (no trailing dot) | +------------+----------------------------------------------------+ | 10+L3 | SNMP Variable Type Value | +------------+----------------------------------------------------+ | 10+L3+1 | Length of value (2 octets, MSB to LSB) | +------------+----------------------------------------------------+ | 10+L3+3 | Value | +------------+----------------------------------------------------+ | 10+L4 | optionally more varBinds (sequences of group ID, | | | instance ID, Type, Length and Value) | +------------+----------------------------------------------------+ | NOTE: | | | | o L1 = length of community name | | o L2 = L1 + strlen(group ID) + 1 | | o L3 = L2 + strlen(instance ID) + 1 | | o L4 = L3 + 3 + length of value | | o group and instance ID strings use selected character set | | o OID and DisplayString values use selected character set | | o Integer values are in network byte order | +-----------------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 28]  RFC 1592 SNMP-DPI March 1994 3.2.11 RESPONSE An SNMP sub-agent must respond to a GET, GETNEXT, GETBULK, SET, COMMIT, UNDO or UNREGISTER request that it has received from the agent (unless it fails or has a bug ;-)). To do so, it sends an SNMP DPI RESPONSE packet to the agent. Such a packet contains the standard SNMP DPI header plus RESPONSE specific data: o an error_code, o an error_index, o plus for a successful GET, GETNEXT, or GETBULK, the name/type/length/value tuple(s) representing the returned object(s). For each varBind this is described as: - two null terminated strings (in the selected character set) representing the group and instance ID in ASN.1 dotted notation. - the type, value length and value of the object that is returned. The permitted types for the type field are defined in Table 17. See 3.3.4, 'Value Representation' for information on how the value data is represented in the packet value field. For an unsuccessful GET, GETNEXT or GETBULK, the sub-agent does not need to return any name/type/length/value tuple(s), because by definition, the varBind information is the same as in the request to which this is a response, and the agent still has that information. The group ID and the packet ID must always be the same as the corresponding fields in request PDU which has prompted the RESPONSE. If the response is to a SET, COMMIT or UNDO request, there is no need to return any varBind information, because by definition, the varBind information is the same as in the request to which this is a response, and the agent still has that information. If the response is to a REGISTER or UNREGISTER, no variable (instance) is being returned, so the instance ID is the NULL string (one 0x00 byte). In the response to a REGISTER request indicating success, the error index contains the priority assigned by the agent. If the response is to an OPEN, ARE_YOU_THERE or CLOSE, no varBind data will be passed, so no group ID, instance ID or value data. The packet will only include the header, the error code and the error Wijnen, Carpenter, Curran, Sehgal & Waters [Page 29]  RFC 1592 SNMP-DPI March 1994 index. +-----------------------------------------------------------------+ | Table 13. Layout SNMP DPI RESPONSE 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_RESPONSE | +------------+----------------------------------------------------+ | 8 | error code (1 octet) | +------------+----------------------------------------------------+ | 9 | error index (4 octets, MSB to LSB) | +------------+----------------------------------------------------+ | 15 | null terminated group ID (with trailing dot) | +------------+----------------------------------------------------+ | 15+L1 | null terminated instance ID (no trailing dot) | +------------+----------------------------------------------------+ | 15+L2 | SNMP Variable Type Value | +------------+----------------------------------------------------+ | 15+L2+1 | Length of value (MSB to LSB) | +------------+----------------------------------------------------+ | 15+L2+3 | Value | +------------+----------------------------------------------------+ | 15+L3 | optionally more varBinds (sequences of group ID, | | | instance ID, Type, Length and Value) | +------------+----------------------------------------------------+ | NOTE: | | | | o L1 = strlen(group ID) + 1 | | o L2 = L1 + strlen(instance ID) + 1 | | o L3 = L2 + 3 + length of value | | o group and instance ID strings use selected character set | | o OID and DisplayString values use selected character set | | o Integer values are in network byte order | +-----------------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 30]  RFC 1592 SNMP-DPI March 1994 3.2.12 TRAP An SNMP sub-agent can request the agent to generate an SNMPv1 or SNMPv2 TRAP (depending on the trap destinations defined at the agent) by sending an SNMP DPI TRAP packet to the agent. Such a packet contains the standard SNMP DPI header plus TRAP specific data: o the generic and specific trap codes o optionally a null terminated string (in the selected character set) representing the enterprise ID in ASN.1 dotted notation. This enterprise ID will be sent with the TRAP. If the null string is passed, then the agent uses the sub-agent Identifier (OID as passed with the DPI OPEN packet) as the Enterprise ID. o optionally a set of one or more name/type/length/value tuples. representing varBinds to be sent with the trap. Each varBind consists of: - two null terminated strings (in the selected character set) representing the group and instance ID in ASN.1 dotted notation. - the type, value length and value of the object that is returned. The permitted types for the type field are defined in Table 17. See 3.3.4, 'Value Representation' for information on how the value data is represented in the packet value field. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 31]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | Table 14. Layout SNMP DPI TRAP 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_TRAP | +------------+----------------------------------------------------+ | 8 | SNMP generic trap code | +------------+----------------------------------------------------+ | 12 | SNMP specific trap code | +------------+----------------------------------------------------+ | 14 | null terminated enterprise ID (no trailing dot) | +------------+----------------------------------------------------+ | 14+L1 | null terminated group ID (with trailing dot) | +------------+----------------------------------------------------+ | 14+L2 | null terminated instance ID (no trailing dot) | +------------+----------------------------------------------------+ | 14+L3 | SNMP Variable Type Value | +------------+----------------------------------------------------+ | 14+L3+1 | Length of value (MSB to LSB) | +------------+----------------------------------------------------+ | 14+L3+3 | Value | +------------+----------------------------------------------------+ | 14+L4 | optionally more varBinds (sequences of group ID, | | | instance ID, Type, Length and Value) | +------------+----------------------------------------------------+ | NOTE: | | | | o L1 = strlen(enterprise ID) + 1 | | o L2 = L1 + strlen(group ID) + 1 | | o L3 = L1 + L2 + strlen(instance ID) + 1 | | o L4 = L1 + L2 + L3 + 3 + length of Value | | o enterprise, group and instance ID strings use selected | | character set | | o OID and DisplayString values use selected character set | | o Integer values are in network byte order | +-----------------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 32]  RFC 1592 SNMP-DPI March 1994 3.3 CONSTANTS AND VALUES This section describes the constants that have been defined for this version of the SNMP DPI Protocol. 3.3.1 PROTOCOL VERSION AND RELEASE VALUES +-----------------------------------------------------------------+ | Table 15. Protocol version and release values | +--------------------------------+--------------------------------+ | FIELD | VALUE | +--------------------------------+--------------------------------+ | protocol major version | 2 (SNMP DPI protocol) | +--------------------------------+--------------------------------+ | protocol minor version | 2 (version 2) | +--------------------------------+--------------------------------+ | protocol release | 0 (release 0) | +--------------------------------+--------------------------------+ Previous versions of this protocol exist and should preferably be supported by an agent: o version 1, release 0, described in [7] Previous internal versions of this protocol exist and may or may not be supported by an agent: o version 1, release 1, experimental within IBM. o version 1, release 2, experimental within BNR. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 33]  RFC 1592 SNMP-DPI March 1994 3.3.2 PACKET TYPE VALUES +-----------------------------------------------------------------+ | Table 16. Valid values for the packet type field | +-------+---------------------------------------------------------+ | VALUE | PACKET TYPE | +-------+---------------------------------------------------------+ | 1 | SNMP_DPI_GET | +-------+---------------------------------------------------------+ | 2 | SNMP_DPI_GETNEXT | +-------+---------------------------------------------------------+ | 3 | SNMP_DPI_SET | +-------+---------------------------------------------------------+ | 4 | SNMP_DPI_TRAP | +-------+---------------------------------------------------------+ | 5 | SNMP_DPI_RESPONSE | +-------+---------------------------------------------------------+ | 6 | SNMP_DPI_REGISTER | +-------+---------------------------------------------------------+ | 7 | SNMP_DPI_UNREGISTER | +-------+---------------------------------------------------------+ | 8 | SNMP_DPI_OPEN | +-------+---------------------------------------------------------+ | 9 | SNMP_DPI_CLOSE | +-------+---------------------------------------------------------+ | 10 | SNMP_DPI_COMMIT | +-------+---------------------------------------------------------+ | 11 | SNMP_DPI_UNDO | +-------+---------------------------------------------------------+ | 12 | SNMP_DPI_GETBULK | +-------+---------------------------------------------------------+ | 13 | SNMP_DPI_TRAPV2 (not yet used) | +-------+---------------------------------------------------------+ | 14 | SNMP_DPI_INFORM (not yet used) | +-------+---------------------------------------------------------+ | 15 | SNMP_DPI_ARE_YOU_THERE | +-------+---------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 34]  RFC 1592 SNMP-DPI March 1994 3.3.3 VARIABLE TYPE VALUES +-----------------------------------------------------------------+ | Table 17. Valid values for the Value Type field | +-------+---------------------------------------------------------+ | VALUE | VALUE TYPE | +-------+---------------------------------------------------------+ | 129 | SNMP_TYPE_Integer32 | +-------+---------------------------------------------------------+ | 2 | SNMP_TYPE_OCTET_STRING | +-------+---------------------------------------------------------+ | 3 | SNMP_TYPE_OBJECT_IDENTIFIER | +-------+---------------------------------------------------------+ | 4 | SNMP_TYPE_NULL (empty, no value) | +-------+---------------------------------------------------------+ | 5 | SNMP_TYPE_IpAddress | +-------+---------------------------------------------------------+ | 134 | SNMP_TYPE_Counter32 | +-------+---------------------------------------------------------+ | 135 | SNMP_TYPE_Gauge32 | +-------+---------------------------------------------------------+ | 136 | SNMP_TYPE_TimeTicks (1/100ths seconds) | +-------+---------------------------------------------------------+ | 9 | SNMP_TYPE_DisplayString | +-------+---------------------------------------------------------+ | 10 | SNMP_TYPE_BIT_STRING | +-------+---------------------------------------------------------+ | 11 | SNMP_TYPE_NsapAddress | +-------+---------------------------------------------------------+ | 140 | SNMP_TYPE_UInteger32 | +-------+---------------------------------------------------------+ | 13 | SNMP_TYPE_Counter64 | +-------+---------------------------------------------------------+ | 14 | SNMP_TYPE_Opaque | +-------+---------------------------------------------------------+ | 15 | SNMP_TYPE_noSuchObject | +-------+---------------------------------------------------------+ | 16 | SNMP_TYPE_noSuchInstance | +-------+---------------------------------------------------------+ | 17 | SNMP_TYPE_endOfMibView | +-------+---------------------------------------------------------+ Notes: 1. A 32-bit integer value has its base base type ORed with 128. 2. DisplayString is a textual convention. An SNMP PDU shows a type of OCTET_STRING for the value. An agent can handle such an object as DisplayString if the object is included in some Wijnen, Carpenter, Curran, Sehgal & Waters [Page 35]  RFC 1592 SNMP-DPI March 1994 form of a compiled MIB for the agent. If not, the agent passes the value as an OCTET_STRING. 3.3.4 VALUE REPRESENTATION Values in the DPI packets are represented as follows: o 32-bit integers are 4-byte elements in network byte order, MSB (most significant byte) first, LSB (least significant byte) last. Example: '00000001'h represents 1. o 64-bit integers are 8-byte elements in network byte order, MSB first, LSB last. Example: '0000000100000001'h represents 4,294,967,297. o Object Identifiers are NULL terminated strings in the selected character set, representing the OID in ASN.1 dotted notation. The length includes the terminating NULL. Example ASCII: '312e332e362e312e322e312e312e312e3000'h represents '1.3.6.1.2.1.1.1.0' which is sysDescr.0. Example EBCDIC: 'f14bf34bf64bf14bf24bf14bf14bf14bf000'h represents '1.3.6.1.2.1.1.1.0' which is sysDescr.0. o DisplayStrings are in the selected character set. The length specifies the length of the string. Example ASCII: '6162630d0a'h represents 'abc ', no NULL. Example EBCDIC: '8182830d25'h represents 'abc ', no NULL. o IpAddress, NsapAddress and Opaque are implicit OCTET_STRING, so they are octets (e.g. IpAddress in network byte order). o NULL has a zero length for the value, no value data. o noSuchObject, noSuchInstance and endOfMibView are implicit NULL and represented as such. o BIT_STRING is an OCTET_STRING of the form uubbbb...bb, where the first octet (uu) is 0x00-0x07 and indicates the number of unused bits in the last octet (bb). The bb octets represent the bit string itself, where bit zero (0) comes first and so on. 3.3.5 CHARACTER SET SELECTION In the DPI OPEN packet, the sub-agent can specify the character set to be used for the representation of: o group and instance ID in the DPI REGISTER, UNREGISTER, GET, GETNEXT, GETBULK, SET, UNDO, COMMIT, RESPONSE and TRAP packets. o sub-agent ID and sub-agent Description in DPI OPEN packet. o Object Identifiers in the value field for a value of type SNMP_TYPE_OBJECT_IDENTIFIER. o DisplayString in the value field for a value of type SNMP_TYPE_DPI_DisplayString. The choice is the native character set or the ASCII character set. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 36]  RFC 1592 SNMP-DPI March 1994 The native set is the set native to the platform where the agent runs. If the native set is ASCII, then character set selection is a moot point. On non-ASCII based platforms, the agent must convert between native and ASCII if the native character set is chosen. 3.3.6 ERROR CODE VALUES FOR SNMP DPI RESPONSE PACKETS When the RESPONSE packet is a response to a GET, GETNEXT, GETBULK, SET, COMMIT, or UNDO, then the error code can have one of the following values: Wijnen, Carpenter, Curran, Sehgal & Waters [Page 37]  RFC 1592 SNMP-DPI March 1994 +-----------------------------------------------------------------+ | Table 18. Valid SNMP_ERROR values for RESPONSE error code | +-------+---------------------------------------------------------+ | VALUE | ERROR CODE | +-------+---------------------------------------------------------+ | 0 | SNMP_ERROR_noError | +-------+---------------------------------------------------------+ | 1 | SNMP_ERROR_tooBig | +-------+---------------------------------------------------------+ | 2 | SNMP_ERROR_noSuchName (SNMPv1, do not use) | +-------+---------------------------------------------------------+ | 3 | SNMP_ERROR_badValue (SNMPv1, do not use) | +-------+---------------------------------------------------------+ | 4 | SNMP_ERROR_readOnly (SNMPv1 do not use) | +-------+---------------------------------------------------------+ | 5 | SNMP_ERROR_genErr | +-------+---------------------------------------------------------+ | 6 | SNMP_ERROR_noAccess | +-------+---------------------------------------------------------+ | 7 | SNMP_ERROR_wrongType | +-------+---------------------------------------------------------+ | 8 | SNMP_ERROR_wrongLength | +-------+---------------------------------------------------------+ | 9 | SNMP_ERROR_wrongEncoding | +-------+---------------------------------------------------------+ | 10 | SNMP_ERROR_wrongValue | +-------+---------------------------------------------------------+ | 11 | SNMP_ERROR_noCreation | +-------+---------------------------------------------------------+ | 12 | SNMP_ERROR_inconsistentValue | +-------+---------------------------------------------------------+ | 13 | SNMP_ERROR_resourceUnavailable | +-------+---------------------------------------------------------+ | 14 | SNMP_ERROR_commitFailed | +-------+---------------------------------------------------------+ | 15 | SNMP_ERROR_undoFailed | +-------+---------------------------------------------------------+ | 16 | SNMP_ERROR_authorizationError | +-------+---------------------------------------------------------+ | 17 | SNMP_ERROR_notWritable | +-------+---------------------------------------------------------+ | 18 | SNMP_ERROR_inconsistentName | +-------+---------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 38]  RFC 1592 SNMP-DPI March 1994 When the RESPONSE packet is a response to an OPEN, REGISTER or UNREGISTER, then the error code can have one of the following values: +-----------------------------------------------------------------+ | Table 19. Valid SNMP_ERROR_DPI values for RESPONSE error code | +-------+---------------------------------------------------------+ | VALUE | ERROR CODE | +-------+---------------------------------------------------------+ | 0 | SNMP_ERROR_DPI_noError | +-------+---------------------------------------------------------+ | 101 | SNMP_ERROR_DPI_otherError | +-------+---------------------------------------------------------+ | 102 | SNMP_ERROR_DPI_notFound | +-------+---------------------------------------------------------+ | 103 | SNMP_ERROR_DPI_alreadyRegistered | +-------+---------------------------------------------------------+ | 104 | SNMP_ERROR_DPI_higherPriorityRegistered | +-------+---------------------------------------------------------+ | 105 | SNMP_ERROR_DPI_mustOpenFirst | +-------+---------------------------------------------------------+ | 106 | SNMP_ERROR_DPI_notAuthorized | +-------+---------------------------------------------------------+ | 107 | SNMP_ERROR_DPI_viewSelectionNotSupported | +-------+---------------------------------------------------------+ | 108 | SNMP_ERROR_DPI_getBulkSelectionNotSupported | +-------+---------------------------------------------------------+ | 109 | SNMP_ERROR_DPI_duplicateSubAgentIdentifier | +-------+---------------------------------------------------------+ | 110 | SNMP_ERROR_DPI_invalidDisplayString | +-------+---------------------------------------------------------+ | 111 | SNMP_ERROR_DPI_characterSetSelectionNotSupported | +-------+---------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 39]  RFC 1592 SNMP-DPI March 1994 3.3.7 UNREGISTER REASON CODES The following are valid reason codes in an UNREGISTER packet. +-----------------------------------------------------------------+ | Table 20. Valid UNREGISTER reason codes | +-------+---------------------------------------------------------+ | VALUE | REASON CODE | +-------+---------------------------------------------------------+ | 1 | SNMP_UNREGISTER_otherReason | +-------+---------------------------------------------------------+ | 2 | SNMP_UNREGISTER_goingDown | +-------+---------------------------------------------------------+ | 3 | SNMP_UNREGISTER_justUnregister | +-------+---------------------------------------------------------+ | 4 | SNMP_UNREGISTER_newRegistration | +-------+---------------------------------------------------------+ | 5 | SNMP_UNREGISTER_higherPriorityRegistered | +-------+---------------------------------------------------------+ | 6 | SNMP_UNREGISTER_byManager | +-------+---------------------------------------------------------+ | 7 | SNMP_UNREGISTER_timeout | +-------+---------------------------------------------------------+ Wijnen, Carpenter, Curran, Sehgal & Waters [Page 40]  RFC 1592 SNMP-DPI March 1994 3.3.8 CLOSE REASON CODES The following are valid reason codes in a CLOSE packet. +-----------------------------------------------------------------+ | Table 21. Valid CLOSE reason codes | +-------+---------------------------------------------------------+ | VALUE | REASON CODE | +-------+---------------------------------------------------------+ | 1 | SNMP_CLOSE_otherReason | +-------+---------------------------------------------------------+ | 2 | SNMP_CLOSE_goingDown | +-------+---------------------------------------------------------+ | 3 | SNMP_CLOSE_unsupportedVersion | +-------+---------------------------------------------------------+ | 4 | SNMP_CLOSE_protocolError | +-------+---------------------------------------------------------+ | 5 | SNMP_CLOSE_authenticationFailure | +-------+---------------------------------------------------------+ | 6 | SNMP_CLOSE_byManager | +-------+---------------------------------------------------------+ | 7 | SNMP_CLOSE_timeout | +-------+---------------------------------------------------------+ | 8 | SNMP_CLOSE_openError | +-------+---------------------------------------------------------+ 4. DPI 2.0 MIB DEFINITION DPI20-MIB DEFINITIONS ::= BEGIN -- Objects in this MIB are implemented in the local SNMP agent. IMPORTS MODULE-IDENTITY, OBJECT-TYPE, snmpModules, enterprises FROM SNMPv2-SMI ibm OBJECT IDENTIFIER ::= { enterprises 2 } ibmDPI OBJECT IDENTIFIER ::= { ibm 2 } dpi20MIB OBJECT IDENTIFIER ::= { ibmDPI 1 } -- dpi20MIB MODULE-IDENTITY -- LAST-UPDATED '9401210000Z' -- ORGANIZATION 'IBM Research - T.J. Watson Research Center' -- CONTACT-INFO ' Bert Wijnen -- Postal: IBM International Operations -- Watsonweg 2 -- 1423 ND Uithoorn -- The Netherlands Wijnen, Carpenter, Curran, Sehgal & Waters [Page 41]  RFC 1592 SNMP-DPI March 1994 -- Tel: +31 2975 53316 -- Fax: +31 2975 62468 -- E-mail: wijnen@vnet.ibm.com' -- DESCRIPTION 'MIB module describing DPI objects.' -- ::= { snmpModules x } dpiPort OBJECT IDENTIFIER ::= { dpi20MIB 1 } dpiPortForTCP OBJECT-TYPE SYNTAX INTEGER (0..65535) ACCESS read-only STATUS mandatory DESCRIPTION 'The TCP port number on which the agent listens for DPI connections. A zero value means the agent has no DPI TCP port.' ::= { dpiPort 1 } dpiPortForUDP OBJECT-TYPE SYNTAX INTEGER (0..65535) ACCESS read-only STATUS mandatory DESCRIPTION 'The UDP port number on which the agent listens for DPI packets. A zero value means the agent has no DPI UDP port.' ::= { dpiPort 2 } END 5. SUBAGENT CONSIDERATIONS When implementing a sub-agent, it is strongly recommended to use the DPI version 2 approach (SNMPv2 based). This means: o Use SNMPv2 error codes only (even though we have definitions for the old SNMPv1 error codes). o Do implement SET, COMMIT, UNDO processing properly. o For GET requests, use the SNMPv2 approach and pass back noSuchInstance or noSuchObject value if such is the case. Continue to process all remaining varBinds in this case. o For GETNEXT, use the SNMPv2 approach and pass back endOfMibView value if such is the case. Continue to process all remaining varBinds in this case. o When you are processing a request from the agent (GET, GETNEXT, GETBULK, SET, COMMIT, UNDO) you are supposed to respond within the timeout period (which you can specify in the OPEN and REGISTER packets). If you fail to respond within that timeout period, the agent will most probably close your DPI connection and then discard your RESPONSE packet if it comes in later. If you can detect that the response is not going to make it in Wijnen, Carpenter, Curran, Sehgal & Waters [Page 42]  RFC 1592 SNMP-DPI March 1994 time, then you might decide to abort the request and return an SNMP_ERROR_genErr in the RESPONSE. o If you have a UDP 'connected' sub-agent, or one that uses another unreliable protocol, you may want to issue an SNMP DPI ARE_YOU_THERE request once in a while to ensure that the agent is still alive and still knows about you. o When you are running on an EBCDIC based machine, and you use the (default) native character set, then all OID strings (as used for things like group ID, instance ID, Enterprise ID, sub-agent ID) and also all variable values of type OBJECT_IDENTIFIER or DisplayString will be passed to you in EBCDIC format. When you return a response, you should then also use EBCDIC FORMAT. o When you are running on an EBCDIC based machine, and you use the ASCII character set (specified in DPI OPEN), then all OID strings (as used for things like group ID, instance ID, Enterprise ID, sub-agent ID) and also all variable values of type OBJECT_IDENTIFIER or DisplayString will be passed to you in ASCII format. When you return a response, you should then also use ASCII FORMAT. o When you are running on an ASCII machine, then the character set selection for you basically is moot. Except maybe when you connect to an EBCDIC based agent, in which case you may want to specify in the DPI OPEN packet that you want to use ASCII character set. After that, all this is transparent to you and the burden of conversion is on the EBCDIC based agent. o Please realize that DisplayString is only a textual convention. In the SNMP PDU (SNMP packet), the type is just an OCTET_STRING, and from that it is not clear if this is a DisplayString or any arbitrary data. This means that the agent can only know about an object being a DisplayString if the object is included in some sort of a compiled MIB. If it is, then the agent will use SNMP_TYPE_DisplayString in the type field of the varBind in a DPI SET packet. When you send a DisplayString in a RESPONSE packet, the agent will handle it as such (e.g. translate EBCDIC to ASCII if needed). 5.1 DPI API The primary goal of this document is to specify the SNMP DPI, a protocol by which sub-agents can exchange SNMP related information with an agent. On top of this protocol, one can imagine one or possibly many Application Programming Interfaces, but those are not addressed in this document. Wijnen, Carpenter, Curran, Sehgal & Waters [Page 43]  RFC 1592 SNMP-DPI March 1994 However, in order to provide an environment that is more or less platform independent, we strongly suggest to also define a DPI API. We have a sample DPI API available, see 9, 'Sample Sources for Anonymous FTP' for a place to obtain that sample DPI API. 5.2 OVERVIEW OF REQUEST PROCESSING 5.2.1 GET PROCESSING A GET request is the easiest to process. The DPI GET packet holds one or more varBinds that the sub-agent has taken responsibility for. If the sub-agent encounters an error while processing the request, it creates a DPI RESPONSE packet with an appropriate error indication in the error_code field and sets the error_index to the position of the varBind at which the error occurs (first varBind is index 1, second varBind is index 2, and so on). No name/type/length/value information needs to be provided in the packet, because by definition, the varBind information is the same as in the request to which this is a response, and the agent still has that information. If there are no errors, then the sub-agent creates a DPI RESPONSE packet in which the error_code is set to SNMP_ERROR_noError (zero) and error_index is set to zero. The packet must also include the name/type/length/value of each varBind requested. When you get a request for a non-existing object or a non-existing instance of an object, then you must return a NULL value with a type of SNMP_TYPE_noSuchObject or SNMP_TYPE_noSuchInstance respectively. These two values are not considered errors, so the error_code and error_index should be zero. The DPI RESPONSE packet is then sent back to the agent. 5.2.2 SET PROCESSING Processing a DPI SET request is