TCP IP Illustrated

Unlike most texts that describe snmp we have

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Unformatted text preview: cond, minus 200 ms. file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/snmp_sim.htm (30 of 33) [12/09/2001 14.47.40] Chapter 25. SNMP: Simple Network Management Protocol Figure 25.2 indicates that a trap message can contain interesting variables that the agents wants to send to the manager, but there aren't any in our examples. 25.11 ASN.1 and BER The formal specification of SNMP uses Abstract Syntax Notation 1 (ASN.1) and the actual encoding of the bits in the SNMP messages (Figure 25.2) uses the corresponding Basic Encoding Rules (BER). Unlike most texts that describe SNMP, we have purposely left a discussion of ASN.1 and BER until the end. When they're discussed first, it can confuse the reader and obfuscate the real purpose of SNMP-network management. In this section we only give an overview of these two topics. Chapter 8 of [Rose 1990] covers ASN.1 and BER in detail. ASN.1 is a formal language for describing data and the properties of the data. It says nothing about how the data is stored or encoded. All the fields in the MIB and the SNMP messages are described using ASN.1. For example, the ASN.1 definition of the data type IpAddress from the SMI looks like: IpAddress ::= [APPLICATION 0] -- in network-byte order IMPLICIT OCTET STRING (SIZE (4)) Similarly, from the MIB we find the following definition of a simple variable: udpNoPorts OBJECT-TYPE SYNTAX Counter ACCESS read-only STATUS mandatory DESCRIPTION "The total number of received UDP datagrams for which there was no application at the destination port." :: = { udp 2 } The definition of tables using SEQUENCE and SEQUENCE OF is more complex. Given these ASN.1 definitions, there are many ways to encode the data into a stream of bits for transmission. SNMP uses BER. The representation of a small integer, such as 64, requires 3 bytes using BER. One byte says the value is an integer, the next byte says how many bytes are used to store the integer (1), and the final byte contains the binary value. Fortuna...
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