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So far, this book has discussed the structure of management information (SMI) and management information bases (MIBs). This chapter completes the discussion of the Internet Network Management Framework by looking at SNMP, the protocol that communicates management information. This chapter will discuss SNMP version 1, and the next chapter will consider SNMP version 2. Note that we will adopt the convention of referring to SNMP version 1 as SNMP, and SNMP version 2 as SNMPv2.
RFC 1157 states that SNMP explicitly minimizes the number and complexity of management functions realized by the management agent itself [4-1]. In other words, SNMP is designed to be simple. SNMP does this in three ways. By reducing the development cost of the agent software, SNMP has decreased the burden on vendors who wish to support the protocol, thereby increasing the protocols acceptance. Second, SNMP is extensible, allowing vendors to add network management functions. Third, it separates the management architecture from the architecture of hardware devices, such as hosts and routers, widening the base of multivendor support. Network Management and the Design of SNMP discusses additional architectural issues relating to SNMP [4-2].
SNMP has a very straightforward architecture. Figure 4-1a compares the SNMP architecture to the ISO/OSI model and the Advanced Research Projects Agency (ARPA) model, around which the Internet protocols and TCP/IP were developed. Note that the four layers of the ARPA model do not map evenly to the seven layers of the OSI model.
Figure 4-1a. Comparing the SNMP architecture with the OSI and ARPA models
Lets use an example to see how the processes within the SNMP architecture interact. Suppose a management console requests information about one of the managed nodes. The SNMP processes in both the manager and the agent respond to the console. The ASN.1 encoding at the Application layer provides the proper syntax for the SNMP message. The remaining functions authenticate the data (attach the SNMP header) and communicate the information request.
Because most management information does not demand the reliable delivery that connection-oriented systems provide, the communication channel between the SNMP manager and the agent is connectionless. When you compare the SNMP model to the ISO/OSI model, SNMPs connectionless communication mechanism removes some of the need for a Session layer and reduces the responsibilities of the lower four layers. For most implementations, the User Datagram Protocol (UDP) performs the Transport layer functions, the Internet Protocol (IP) provides the Network layer functions, and LANs such as Ethernet or token ring or WANs such as a leased line or a frame relay connection provide the Data Link and Physical layer functions. (There are some exceptions to this rule. RFCs 1418, 1419, and 1420 describe implementations that use other transport mechanisms, such as OSI Apple Computers AppleTalk, or Novell Inc.s IPX protocols. However, RFC 1270, called SNMP Communication Services [4-3], states that UDP/IP are the protocols of choice for most circumstances.)
If you compare SNMP to the Internet (or ARPA) architectural model (see Figure 4-1b), youll notice that the ARPA model uses four layers to describe the entire communication function. In the ARPA model, SNMP would reside at the Process/Application layer. However, while the ARPA Host-to-Host layer provides end-to-end communication reliability, SNMPs use of UDP assures only proper port addressing and a checksum; it does not provide octet-by-octet error control. IP provides the Internet layer functions, such as addressing and fragmentation, that are necessary to deliver an SNMP message from the source to the destination. Finally, the Network Interface layer deals with the LAN or WAN hardware, such as an interface to an FDDI or Frame Relay network connection. Notice that Figure 4-1b also shows the relative complexities of the host and router functions. Hosts implement all four layers of the ARPA model, whereas routers implement only the lower two.
Figure 4-1b. Application-to-application connection
Comparing the SNMP architecture to the ISO/OSI and ARPA architectural models provides a theoretical basis for this discussion. But from a practical perspective, the SNMP model works as shown in Figure 4-2. This model contains several elements discussed in Chapter 1. It includes a management system that uses the SNMP manager, an SNMP agent, and managed resources, and the SNMP messages communicate management information via five SNMP protocol data units (PDUs). The management application issues the Get, GetNext, or Set PDUs. The managed system returns a GetResponse PDU. The agent may initiate a Trap (sometimes called an Event) PDU when predefined conditions are met. Section 4.3 discusses these five PDUs in detail.
Figure 4-2. SNMP architecture (c 1990, IEEE)
The SNMP processes described in the previous sections must occur in physical devices. For example, a router must have a physical processor that implements the software acting as an SNMP agent. Two sets of logical processes occur within those physical elements: the relationships that are specified between various network management entities, and the way network management information is communicated.
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