Level Of Management Protocols - SNMP Tutorial

30.2 The Level Of Management Protocols

　　Originally, many wide area networks included management protocols as part of their link level protocols. If a packet switch began misbehaving, the network manager could instruct a neighboring packet switch to send it a special control packet. Control packets caused the receiver to suspend normal operation and respond to commands from the manager. The manager could interrogate the packet switch to identify problems, examine or change routes, test one of the communication interfaces, or reboot the switch. Once managers repaired the problem, they could instruct the switch to resume normal operations. Because management tools were part of the lowest level protocol, managers were often able to control switches even if higher level protocols failed.

　　Unlike a homogeneous wide area network, a TCPm intemet does not have a single link level protocol. Instead, the internet consists of multiple physical networks interconnected by IP routers. As a result, internet management differs from network management. First, a single manager can control heterogeneous devices, including IP routers, bridges, modems, workstations, and printers. Second, the controlled entities may not share a common link level protocol. Third, the set of machines a manager controls may lie at arbitrary points in an internet. In particular, a manager may need to control one or more machines that do not attach to the same physical network as the manager's computer. Thus, it may not be possible for a manager to communicate with machines being controlled unless the management software uses protocols that provide end-to-end connectivity across an internet. As a consequence, the internet management protocol used with TCP/IP operates above the transport level:

　　In a TCP/IP internet, a manager needs to examine and control routers and other network devices. Because such devices attach to arbitrary networks, protocols for internet management operate at the application level and communicate using TCP/IP transport-level protocols.

　　Designing internet management software to operate at the application level has several advantages. Because the protocols can be designed without regard to the underlying network hardware, one set of protocols can be used for all networks. Because the protocols can be designed without regard to the hardware on the managed machine, the same protocols can be used for all managed devices. From a manager's point of view, having a single set of management protocols means uniformity -- routers respond to exactly the same set of commands. Furthermore, because the management software uses IPfor communication, a manager can control the routers across an entire TCP/IP internet without having direct attachment to every physical network or router.

　　Of course, building management software at the application level also has disadvantages. Unless the operating system, IP software, and transport protocol software work correctly, the manager may not be able to contact a router that needs managing. For example, if a router's routing table becomes damaged, it may be impossible to correct the table or reboot the machine from a remote site. If the operating system on a router crashes, it will be impossible to reach the application program that implements the internet management protocols even if the router can still field hardware interrupts and route packets.

Abstract from Internetworking With TCP/IP Vol I: Principles, Protocols, and Architecture Fourth Edition,

DOUGLAS E. COMER,

Department of Computer Sciences Purdue University, West Lafayette, IN 47907,

PRENTICE HALL,

Upper Saddle River, New Jersey 07458