112 lines
7.1 KiB
HTML
112 lines
7.1 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<head>
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Out-of-band data" />
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<meta name="abstract" content="Out-of-band (OOB) data is user-specific data that only has meaning for connection-oriented (stream) sockets." />
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<meta name="description" content="Out-of-band (OOB) data is user-specific data that only has meaning for connection-oriented (stream) sockets." />
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<meta name="DC.Relation" scheme="URI" content="aconcepts.htm" />
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<meta name="DC.Relation" scheme="URI" content="csignals.htm" />
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<meta name="DC.Relation" scheme="URI" content="../apis/sendms.htm" />
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<meta name="DC.Relation" scheme="URI" content="../cl/chgtcpa.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2001, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 2001, 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="coobd" />
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Out-of-band data</title>
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</head>
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<body id="coobd"><a name="coobd"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Out-of-band data</h1>
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<div><p>Out-of-band (OOB) data is user-specific data that only has meaning
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for connection-oriented (stream) sockets. </p>
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<p>Stream data is generally received in the same order it is sent. OOB data
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is received independent of its position in the stream (independent of the
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order in which it was sent). This is possible because the data is marked in
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such a way that, when it is sent from program A to program B, program B is
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notified of its arrival.</p>
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<p>OOB data is supported on AF_INET (SOCK_STREAM) and AF_INET6 (SOCK_STREAM)
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only.</p>
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<p>OOB data is sent by specifying the MSG_OOB flag on the <span class="apiname">send()</span>, <span class="apiname">sendto()</span>,
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and <span class="apiname">sendmsg()</span> functions.</p>
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<p>The transmission of OOB data is the same as the transmission of regular
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data. It is sent after any data that is buffered. In other words, OOB data
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does not take precedence over any data that may be buffered; data is transmitted
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in the order that it was sent.</p>
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<p>On the receiving side, things are a little more complex:</p>
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<ul><li>The sockets API keeps track of OOB data that is received on a system by
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using an OOB marker. The OOB marker points to the last byte in the OOB data
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that was sent. <div class="note"><span class="notetitle">Note:</span> The value that indicates which byte the OOB marker points
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to is set on a system basis (all applications use that value). This value
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must be consistent between the local and remote ends of a TCP connection.
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Socket applications that use this value must use it consistently between
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the client and server applications. </div>
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<p>The <span class="apiname">SIOCATMARK ioctl()</span> request determines if the read
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pointer is pointing to the last OOB byte.</p>
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<div class="note"><span class="notetitle">Note:</span> If multiple occurrences
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of OOB data are sent, the OOB marker points to the last OOB byte of the final
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OOB data occurrence.</div>
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</li>
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<li>Independent of whether OOB data is received inline, an
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input operation processes data up to the OOB marker, if OOB data was sent.</li>
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<li>A <span class="apiname">recv()</span>, <span class="apiname">recvmsg()</span>,
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or <span class="apiname">recvfrom()</span> function (with the MSG_OOB flag set) is used
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to receive OOB data. An error of [EINVAL] is returned if one of the receive
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functions has been completed and one of the following situations occurs: <ul><li>The socket option SO_OOBINLINE is not set and there is no OOB data to
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receive.</li>
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<li>The socket option SO_OOBINLINE is set.</li>
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</ul>
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<p>If the socket option SO_OOBINLINE is not set, and the sending program
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sent OOB data with a size greater than one byte, all the bytes
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but the last are considered normal data. (Normal data means that the receiving
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program can receive data without specifying the MSG_OOB flag.) The last byte
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of the OOB data that was sent is not stored in the normal data stream. This
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byte can only be retrieved by issuing a<span class="apiname"> recv()</span>, <span class="apiname">recvmsg()</span>,
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or <span class="apiname">recvfrom()</span> function with the MSG_OOB flag set. If a
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receive operation is issued with the MSG_OOB flag not set, and normal data
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is received, the OOB byte is deleted. Also, if multiple occurrences of OOB
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data are sent, the OOB data from the preceding occurrence is lost, and the
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position of the OOB data of the final OOB data occurrence is remembered.</p>
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<p>If
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the socket option SO_OOBINLINE is set, then all of the OOB data that was sent
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is stored in the normal data stream. Data can be retrieved by issuing one
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of the three receive functions without specifying the MSG_OOB flag (if it
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is specified, an error of [EINVAL] is returned). OOB data is not lost if multiple
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occurrences of OOB data are sent.</p>
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</li>
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<li>OOB data is not discarded if SO_OOBINLINE is not set, OOB data has been
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received, and the user then sets SO_OOBINLINE on. The initial OOB byte is
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considered normal data.</li>
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<li>If SO_OOBINLINE is not set, OOB data was sent, and the receiving program
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issued an input function to receive the OOB data, then the OOB marker is still
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valid. The receiving program can still check if the read pointer is at the
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OOB marker, even though the OOB byte was received.</li>
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</ul>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="aconcepts.htm" title="Advanced socket concepts go beyond a general discussion of what sockets are and how they work. They provide ways to design socket applications for larger and more complex networks.">Advanced socket concepts</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="csignals.htm" title="An application program can request to be notified asynchronously (request that the system send a signal) when a condition that the application is interested in occurs.">Signals</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../apis/sendms.htm">sendmsg()</a></div>
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<div><a href="../cl/chgtcpa.htm">Change TCP Attributes (CHGTCPA) command</a></div>
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</div>
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</div>
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</body>
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</html>
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