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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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11.2.4 REXX sockets<br />

Note: SET requests do not conform to the request/response sequence used<br />

by GET or GETNEXT requests. Instead, SET requests occur in a sequence of<br />

SET/COMMIT, SET/UNDO, or SET/COMMIT/UNDO. We discuss this in<br />

greater detail in Chapter 17, “Network management” on page 623.<br />

The subagent sends responses back to the SNMP agent through a RESPONSE<br />

packet, which the SNMP agent then encodes into an SNMP packet sent back to<br />

the requesting remote management station.<br />

If the subagent wants to report an important state change, it can send a DPI<br />

TRAP packet to the SNMP agent, which the agent will encode into an SNMP trap<br />

packet <strong>and</strong> send to designated remote management stations.<br />

A subagent can send an ARE_YOU_THERE to verify that the connection is still<br />

open. If so, the agent sends a RESPONSE with no error; otherwise, it sends a<br />

RESPONSE with an error.<br />

If the subagent wants to stop operations, it sends a DPI UNREGISTER <strong>and</strong> a<br />

DPI CLOSE packet to the agent. The agent sends a response to an<br />

UNREGISTER request, but there is no RESPONSE to a CLOSE. A CLOSE<br />

implies an UNREGISTER for all registrations that exist for the DPI connection<br />

being CLOSED, even if no UNREGISTER has been sent.<br />

An agent can send a DPI UNREGISTER if a higher priority registration arrives (or<br />

for other reasons) to the subagent. The subagent then responds with a DPI<br />

RESPONSE packet. An agent can also send a DPI CLOSE to indicate that it is<br />

terminating the DPI connection. This might occur when the agent terminates, or if<br />

the agent has timed out while awaiting a response from a subagent.<br />

The Restructured Extended Executor (REXX) programming language was<br />

originally developed by <strong>IBM</strong> in 1982, but since then a wide variety of platforms<br />

have created implementations for their platforms. In order to enable REXX<br />

applications to communicate over the <strong>TCP</strong>/<strong>IP</strong> network, most platforms have<br />

created a REXX socket API to allow:<br />

► Socket initialization<br />

► Data exchange through sockets<br />

► Management activity through sockets<br />

► Socket termination<br />

422 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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