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jkois 2009-02-10 11:20:38 UTC
FreeBSD German Documentation Repository
Modified files:
books/developers-handbook/sockets chapter.sgml
Log:
Das noch nicht ersetzte Kapitel 7 - Sockets durch einen Hinweis auf das englische Original ersetzt. Damit steht einem MFde prinzipiell
nichts mehr im Wege.
Revision Changes Path
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Index: chapter.sgml
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<chapter id="sockets">
- <chapterinfo>
- <authorgroup>
- <author>
- <firstname>G. Adam</firstname>
- <surname>Stanislav</surname>
- <contrib>Contributed by </contrib>
- </author>
- </authorgroup>
- </chapterinfo>
-
<title>Sockets</title>
- <sect1 id="sockets-synopsis">
- <title>Synopsis</title>
-
- <para><acronym>BSD</acronym> sockets take interprocess
- communications to a new level. It is no longer necessary for the
- communicating processes to run on the same machine. They still
- <emphasis>can</emphasis>, but they do not have to.</para>
-
- <para>Not only do these processes not have to run on the same
- machine, they do not have to run under the same operating
- system. Thanks to <acronym>BSD</acronym> sockets, your FreeBSD
- software can smoothly cooperate with a program running on a
- &macintosh;, another one running on a &sun; workstation, yet another
- one running under &windows; 2000, all connected with an
- Ethernet-based local area network.</para>
-
- <para>But your software can equally well cooperate with processes
- running in another building, or on another continent, inside a
- submarine, or a space shuttle.</para>
-
- <para>It can also cooperate with processes that are not part of a
- computer (at least not in the strict sense of the word), but of
- such devices as printers, digital cameras, medical equipment.
- Just about anything capable of digital communications.</para>
-
- </sect1>
-
- <sect1 id="sockets-diversity">
- <title>Networking and Diversity</title>
-
- <para>We have already hinted on the <emphasis>diversity</emphasis>
- of networking. Many different systems have to talk to each
- other. And they have to speak the same language. They also have
- to <emphasis>understand</emphasis> the same language the same
- way.</para>
-
- <para>People often think that <emphasis>body language</emphasis>
- is universal. But it is not. Back in my early teens, my father
- took me to Bulgaria. We were sitting at a table in a park in
- Sofia, when a vendor approached us trying to sell us some
- roasted almonds.</para>
-
- <para>I had not learned much Bulgarian by then, so, instead of
- saying no, I shook my head from side to side, the
- <quote>universal</quote> body language for
- <emphasis>no</emphasis>. The vendor quickly started serving us
- some almonds.</para>
-
- <para>I then remembered I had been told that in Bulgaria shaking
- your head sideways meant <emphasis>yes</emphasis>. Quickly, I
- started nodding my head up and down. The vendor noticed, took
- his almonds, and walked away. To an uninformed observer, I did
- not change the body language: I continued using the language of
- shaking and nodding my head. What changed was the
- <emphasis>meaning</emphasis> of the body language. At first, the
- vendor and I interpreted the same language as having completely
- different meaning. I had to adjust my own interpretation of that
- language so the vendor would understand.</para>
-
- <para>It is the same with computers: The same symbols may have
- different, even outright opposite meaning. Therefore, for
- two computers to understand each other, they must not only
- agree on the same <emphasis>language</emphasis>, but on the
- same <emphasis>interpretation</emphasis> of the language.
- </para>
- </sect1>
-
- <sect1 id="sockets-protocols">
- <title>Protocols</title>
-
- <para>While various programming languages tend to have complex
- syntax and use a number of multi-letter reserved words (which
- makes them easy for the human programmer to understand), the
- languages of data communications tend to be very terse. Instead
- of multi-byte words, they often use individual
- <emphasis>bits</emphasis>. There is a very convincing reason
- for it: While data travels <emphasis>inside</emphasis> your
- computer at speeds approaching the speed of light, it often
- travels considerably slower between two computers.</para>
-
- <para>Because the languages used in data communications are so
- terse, we usually refer to them as
- <emphasis>protocols</emphasis> rather than languages.</para>
-
- <para>As data travels from one computer to another, it always uses
- more than one protocol. These protocols are
- <emphasis>layered</emphasis>. The data can be compared to the
- inside of an onion: You have to peel off several layers of
- <quote>skin</quote> to get to the data. This is best
- illustrated with a picture:</para>
-
- <mediaobject>
- <imageobject>
- <imagedata fileref="sockets/layers">
- </imageobject>
-
- <textobject>
- <literallayout class="monospaced">+----------------+
-| Ethernet |
-|+--------------+|
-|| IP ||
-||+------------+||
-||| TCP |||
-|||+----------+|||
-|||| HTTP ||||
-||||+--------+||||
-||||| PNG |||||
-|||||+------+|||||
-|||||| Data ||||||
-|||||+------+|||||
-||||+--------+||||
-|||+----------+|||
-||+------------+||
-|+--------------+|
-+----------------+</literallayout>
- </textobject>
-
- <textobject>
- <phrase>Protocol Layers</phrase>
- </textobject>
- </mediaobject>
-
- <para>In this example, we are trying to get an image from a web
- page we are connected to via an Ethernet.</para>
-
- <para>The image consists of raw data, which is simply a sequence
- of <acronym>RGB</acronym> values that our software can process,
- i.e., convert into an image and display on our monitor.</para>
-
- <para>Alas, our software has no way of knowing how the raw data is
- organized: Is it a sequence of <acronym>RGB</acronym> values, or
- a sequence of grayscale intensities, or perhaps of
- <acronym>CMYK</acronym> encoded colors? Is the data represented
- by 8-bit quanta, or are they 16 bits in size, or perhaps 4 bits?
- How many rows and columns does the image consist of? Should
- certain pixels be transparent?</para>
-
- <para>I think you get the picture...</para>
-
- <para>To inform our software how to handle the raw data, it is
- encoded as a <acronym>PNG</acronym> file. It could be a
- <acronym>GIF</acronym>, or a <acronym>JPEG</acronym>, but it is
- a <acronym>PNG</acronym>.</para>
-
- <para>And <acronym>PNG</acronym> is a protocol.</para>
-
- <para>At this point, I can hear some of you yelling,
- <emphasis><quote>No, it is not! It is a file
- format!</quote></emphasis></para>
-
- <para>Well, of course it is a file format. But from the
- perspective of data communications, a file format is a protocol:
- The file structure is a <emphasis>language</emphasis>, a terse
- one at that, communicating to our <emphasis>process</emphasis>
- how the data is organized. Ergo, it is a
- <emphasis>protocol</emphasis>.</para>
-
- <para>Alas, if all we received was the <acronym>PNG</acronym>
- file, our software would be facing a serious problem: How is it
- supposed to know the data is representing an image, as opposed
- to some text, or perhaps a sound, or what not? Secondly, how is
- it supposed to know the image is in the <acronym>PNG</acronym>
- format as opposed to <acronym>GIF</acronym>, or
- <acronym>JPEG</acronym>, or some other image format?</para>
-
- <para>To obtain that information, we are using another protocol:
- <acronym>HTTP</acronym>. This protocol can tell us exactly that
- the data represents an image, and that it uses the
- <acronym>PNG</acronym> protocol. It can also tell us some other
- things, but let us stay focused on protocol layers here.
- </para>
-
- <para>So, now we have some data wrapped in the <acronym>PNG</acronym>
- protocol, wrapped in the <acronym>HTTP</acronym> protocol.
- How did we get it from the server?</para>
-
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Received on Tue 10 Feb 2009 - 12:20:53 CET