glossary.html

SDK FAQ集

<p>An example might help to show why this is important. First, the
most basic TCP/IP protocol header is 40 bytes, and it can be larger if
optional fields are used. Now, consider a program that sends many small,
two-byte packets. Without the Nagle algorithm, each of these packets
would be transmitted separately, with at least 20 bytes of overhead
for every byte of data. Obviously, this is quite wasteful. The Nagle
algorithm counteracts this effect by delaying the transmission of small
packets for some short time, such as 200 milliseconds. A packet is sent
when this Nagle timer times out, or when a full <a href="#MTU">MTU</a>
is accumulated.</p>

<p>This option is on by default in Winsock, but it can be turned off with
the TCP_NODELAY option of <code>setsockopt()</code>. This option should
<a href="intermediate.html#nagle">not be turned off</a> except in a
very few situations.</p>

<a name="non-blocking">
<p><b>non-blocking</b> - This mode of operation is a sort
of hybrid between <a href="#blocking">blocking</a> and
<a href="#asynchronous">asynchronous</a> modes. Like calls on an
asynchronous socket, non-blocking calls return immediately. Unlike
asynchronous sockets, Winsock does not tell you when that operation
completes, or when it is safe to try another similar operation. Instead,
you have to either use a "spin loop" to repeatedly re-try a function until
it succeeds (a Very Bad Thing), or use <code>select()</code> which will block
until the socket(s) you pass to it are ready to do something. (You make
a socket non-blocking in Winsock by calling the <code>ioctlsocket()</code>
function with the <code>FIONBIO</code> option.)</p>

<a name="osi-model"> 
<p><b>OSI model</b> - Of all the standards to come out of the Open
Systems Interconnect specifications, the OSI network model is the most
common and enduring. The OSI model is a seven-layer view of looking at
the network. It is presented below, with rough analogues from TCP/IP
and Winsock.</p>

<ul>
<table border=1 cellspacing=0 cellpadding=5>
<tr bgcolor="#FBFDB0">
<td><b>Layer 7</b></td>
<td align=center>Application (e.g. an FTP or email client)</td>
</tr>

<tr bgcolor="#F4D7AA">
<td><b>Layer 6</b></td>
<td align=center>Presentation (formats data, such as encrypting or compressing it)</td>
</tr>

<tr bgcolor="#FBFDB0">
<td><b>Layer 5</b></td>
<td align=center>Session (manages the logical client/server connection; e.g. the FTP protocol)</td>
</tr>

<tr bgcolor="#F4D7AA">
<td><b>Layer 4</b></td>
<td align=center>Transport (understands low-level client/server association; e.g. UDP or TCP)</td>
</tr>

<tr bgcolor="#FBFDB0">
<td><b>Layer 3</b></td>
<td align=center>Network (routable packet level of the protocol; e.g. IP)</td>
</tr>

<tr bgcolor="#F4D7AA">
<td><b>Layer 2</b></td>
<td align=center>Data Link (e.g. packet driver)</td>
</tr>

<tr bgcolor="#FBFDB0">
<td><b>Layer 1</b></td>
<td align=center>Physical (e.g. Ethernet hardware)</td>
</tr>
</table>
</ul>

<p>Layers 1 through 4 are the hardware and network <a
href="#stack">stack</a> (including the Winsock layer). Layers 5 through
7 are all under the application's control.</p>

<a name="rfc"> 
<p><b>RFC</b> - Request for Comments. RFCs are the Internet's standards
mechanism; they document most of the "open" aspects of the Internet. See
also the entry in <a href="http://www.netmeg.net/jargon/terms/r/RFC.html">TNHD</a>.</p>

<a name="router">
<p><b>router</b> - A router is a <a href="#multi-homed">multi-homed</a>
host that operates at OSI layer 3. A router moves packets between two
or more networks, usually based on configurable rules, based on the
contents of layer 3 (e.g. the IP addresses in a packet). Most modern
network hosts have at least some rudimentary routing capability. A
simplistic example: Consider a PC on a LAN that also has a modem for
connecting to the Internet. It might have a "LAN route" so that all
traffic destined for the LAN goes out the LAN adapter, and a "default
route" so that all other traffic goes out the modem. Try the "route"
and "netstat -r" commands on a Windows 95 or Windows NT machine.</p>

<a name="server">
<p><b>server</b> - A program that passively waits for network connections
on a well-known port. A typical server program has no user interface, and
it usually can handle multiple network connections at once. In a typical
client/server protocol, the server is passive: it usually only sends
data as a result of data sent by the <a href="#client">client</a>.</p>

<a name="stack">
<p><b>stack</b> - In network parlance, a stack is a set of layered
programs, each of which talks to the ones above and below it. Below is
an illustration of the most common kind of network stack, showing how an
application program talks through the stack to the low-level network:</p>

<ul>
<table border=1 cellspacing=0 cellpadding=5>
<tr bgcolor="#FBFDB0"><td align=center><b>Winsock application</b></td></tr>
<tr bgcolor="#F4D7AA"><td align=center>Winsock API</td></tr>
<tr bgcolor="#FBFDB0"><td align=center>Proprietary Winsock DLL (WINSOCK.DLL, WSOCK32.DLL, WS2-32.DLL)</td></tr>
<tr bgcolor="#F4D7AA"><td align=center>Protocol stack API</td></tr>
<tr bgcolor="#FBFDB0"><td align=center>Protocol stack (TCP/IP, IPX/SPX, DECnet, etc.)</td></tr>
<tr bgcolor="#F4D7AA"><td align=center>Hardware driver API (packet driver, NDIS, ODI, etc.)</td></tr>
<tr bgcolor="#FBFDB0"><td align=center>Hardware driver</td></tr>
<tr bgcolor="#F4D7AA"><td align=center>Hardware API (specific to the networking hardware)</td></tr>
<tr bgcolor="#FBFDB0"><td align=center>Networking hardware (network interface card, modem, CSU/DSU)</td></tr>
<tr bgcolor="#F4D7AA"><td align=center><b>The network (Ethernet, DEC LAT, serial line)</b></td></tr>
</table>
</ul>

<p>You might have noticed that there is a stack within the stack: the
protocol stack. Just like the network stack, the protocol stack is a
layered architecture, where higher-level components talk to lower-level
components, and vice versa. In TCP/IP, for example, TCP is the high-level
network transport that most applications use, while IP is the lower-level
packet protocol that TCP uses.</p>

<a name="stream">
<p><b>stream protocol</b> - A stream protocol, such as TCP, delivers data
in variable-length packets whose size have no necessary relationship
with the size of the packets that were sent. For example, if one host
sends two 50-byte packets to the other host, that host may receive them
as 100 single-byte packets, as a single 100-byte packet, or as a handful
of smaller packets. Compare <a href="#datagram">datagram protocol</a>.</p>

		</td>
	</tr>
</table>


<!--  ---- Document Footer ----  -->

<hr noshade size=1 color=#404040>

<table cellpadding=5 cellspacing=0 border=0 width=95% align=center> 
	<tr>
		<td align=left>
		    <a href="reviews/unp-v1.html">&lt;&lt; Unix Network Programming</a>
		</td>

		<td align=right>
		</td>
	</tr>

	<tr>
		<td align=left>
			<i>Last modified on 29 April 2000 at 15:52 UTC-7</i>
		</td>

		<td align=right>
			<font size=-1>Please send corrections to <a href="mailto:[e-mail]">[e-mail]</a>.</font>
		</td>
	</tr>	
</table>	

<table cellpadding=5 cellspacing=0 border=0 width=95% align=center> 
	<tr>
		<td align=left width=33%>
			<font size=-1>
				<a href="index.html"><b>&lt;</b> Go to the main FAQ page</a>
			</font>
		</td>

		<td width=33%>
			<font size=-1>
			<center>
				<a href="http://www.cyberport.com/~tangent/programming"><b>&lt;&lt;</b> Go to my Programming pages</a>
			</center>
			</font>
		</td>

		<td align=right width=33%>
			<font size=-1>
				<a href="http://www.cyberport.com/~tangent/"><b>&lt;&lt;&lt;</b> Go to my Home Page</a>
			</font>
		</td>
	</tr>	
</table>	


</body>
</html>