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5.6 SNMPv2 Transport Mappings

SNMP version 1 was originally defined for transmission over UDP and IP. Subsequent research explored the use of SNMP with other transport protocols, including OSI transport (RFC 1418), AppleTalk’s Datagram Delivery Protocol (DDP) (RFC 1419), and Novell’s Internetwork Packet Exchange (IPX) (RFC 1420). SNMPv2 formally defines implementations over these other transports in the Transport Mapping document, RFC 1906 [5-7] (see Figure 5-3). The Transport Mapping document also includes instructions to provide proxy to SNMPv1 and for the use of the Basic Encoding Rules (BER).


Figure 5-3.  Transport mappings for SNMPv2

5.6.1 SNMPv2 over UDP

SNMPv2 over UDP is the preferred transport mapping. UDP provides compatibility with SNMPv1 at both the Transport and Network layers, although other higher-layer issues, such as SNMPv2 PDU structures, remain. RFC 1906 also suggests that SNMPv2 agents continue the practice of listening on UDP port 161, and that notifications listen on UDP port 162. (UDP port 162 was previously defined for SNMP traps.) Figure 5-4 illustrates the details of the UDP header, which precedes the SNMP message within the transmitted frame.


Figure 5-4.  SNMPv2 over UDP

5.6.2 SNMPv2 over OSI

RFC 1449 defines two options for transmitting SNMPv2 messages over OSI protocols. Both send the SNMPv2 message in a single transport service data unit (TSDU) using the provisions of the OSI Connectionless-mode Transport Service (CLTS). Then at the Network layer, either a Connectionless-mode Network Service (CLNS) or a Connection-oriented Network Service (CONS) may be used.

When you use Connectionless-Mode Network Service, you would use the OSI Connectionless Network Protocol (CLNP) as the Network layer protocol. Figure 5-5 shows details of the CLNP header, the position of the CLTS header, and the SNMPv2 message within the transmission frame.


Figure 5-5.  SNMPv2 over ISO CLNP

5.6.3 SNMPv2 over AppleTalk DDP

Apple Computer’s AppleTalk protocol suite is another option available for SNMPv2 transport. The SNMPv2 message is sent in a single Datagram Delivery Protocol (DDP) datagram, which operates at the OSI Network layer. Figure 5-6 shows the details of the DDP header and the position of the SNMPv2 message within the transmission frame. The final octet of the DDP header specifies the DDP Type, indicating the protocol in use. SNMPv2 messages use DDP Type = 8, since Apple has previously defined types 1 through 7. Other DDP parameters, such as socket numbers, are also defined for SNMPv2 use. SNMPv2 entities acting in the agent role use DDP socket number 8; notification sinks, which are entities receiving a notification, use DDP socket number 9.


Figure 5-6.  SNMPv2 over the AppleTalk DDP

5.6.4. SNMPv2 over Novell IPX

Novell Inc.’s NetWare protocol suite defines the Internetwork Packet Exchange (IPX) protocol at the Network layer. SNMPv2 messages are serialized into a single IPX datagram, as shown in Figure 5-7. Within the IPX header is a Packet Type parameter that specifies the protocol in use. SNMPv2 messages use Packet Type = 4, which is defined as a Packet Exchange Protocol packet. SNMPv2 entities acting in the agent role listen on IPX socket number 36879 (900FH), while notification sinks listen on socket 36880 (9010H).


Figure 5-7.  SNMPv2 over Novell IPX

5.6.5 SNMPv2 transport mapping definitions

For your reference, this section reproduces the following transport mapping definitions from RFC 1906 [5-7]. Note the use of the TEXTUAL-CONVENTIONS macro, and the DISPLAY-HINT within each type, which provides information about how the different types of addresses are to be displayed. Recall the following abbreviations:

Abbreviation Meaning

x hexadecimal
d decimal
o octal
b binary


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