PC World 1999 August/PCWorld_1999-08_cd.bin/doc/HOWTO/Unix-Internet-Fundamentals-HOWTO

Text File | 1999-02-09 | 41KB | 925 lines

  The Unix and Internet Fundamentals HOWTO
  by Eric S. Raymond
  v1.1, 3 December 1998

  This document describes the working basics of PC-class computers,
  Unix-like operating systems, and the Internet in non-technical lan¡
  guage.
  ______________________________________________________________________

  Table of Contents


  1. Introduction

     1.1 Purpose of this document
     1.2 Related resources
     1.3 New versions of this document
     1.4 Feedback and corrections

  2. Basic anatomy of your computer

  3. What happens when you switch on a computer?

  4. What happens when you run programs from the shell?

  5. How do input devices and interrupts work?

  6. How does my computer do several things at once?

  7. How does my computer keep processes from stepping on each other?

  8. How does my computer store things on disk?

     8.1 Low-level disk and file system structure
     8.2 File names and directories
     8.3 Mount points
     8.4 How a file gets looked up
     8.5 How things can go wrong

  9. How do computer languages work?

     9.1 Compiled languages
     9.2 Interpreted languages
     9.3 P-code languages

  10. How does the Internet work?

     10.1 Names and locations
     10.2 Packets and routers
     10.3 TCP and IP
     10.4 HTTP, an application protocol


  ______________________________________________________________________

  1.  Introduction



  1.1.  Purpose of this document

  This document is intended to help Linux and Internet users who are
  learning by doing.  While this is a great way to acquire skills,
  sometimes it leaves peculiar gaps in one's knowledge of the basics --
  gaps which can make it hard to think creatively or troubleshoot
  effectively, from lack of a clear mental model of what is really going
  on.

  I'll try to describe in clear, simple language how it all works.  The
  presentation will be tuned for people using Unix or Linux on PC-class
  hardware.  Nevertheless I'll usually refer simply to `Unix' here, as
  most of what I will describe is constant across platforms and across
  Unix variants.

  I'm going to assume you're using an Intel PC.  The details differ
  slightly if you're running an Alpha or PowerPC or some other Unix box,
  but the basic concepts are the same.

  I won't repeat things, so you'll have to pay attention, but that also
  means you'll learn from every word you read.  It's a good idea to just
  skim when you first read this; you should come back and reread it a
  few times after you've digested what you have learned.

  This is an evolving document.  I intend to keep adding sections in
  response to user feedback, so you should come back and review it
  periodically.


  1.2.  Related resources

  If you're reading this in order to learn how to hack, you should also
  read the How To Become A Hacker FAQ
  <http://www.tuxedo.org/~esr/faqs/hacker-howto.html>.  It has links to
  some other useful resources.


  1.3.  New versions of this document


  New versions of the Unix and Internet Fundamentals HOWTO will be
  periodically posted to comp.os.linux.help and and news.answers
  <news:answers>.  They will also be uploaded to various Linux WWW and
  FTP sites, including the LDP home page.

  You can view the latest version of this on the World Wide Web via the
  URL <http://sunsite.unc.edu/LDP/HOWTO/Fundamentals-HOWTO.html>.


  1.4.  Feedback and corrections


  If you have questions or comments about this document, please feel
  free to mail Eric S. Raymond, at [e-mail]. I welcome any
  suggestions or criticisms. I especially welcome hyperlinks to more
  detailed explanations of individual concepts.  If you find a mistake
  with this document, please let me know so I can correct it in the next
  version. Thanks.


  2.  Basic anatomy of your computer

  Your computer has a processor chip inside it that does the actual
  computing.  It has internal memory (what DOS/Windows people call
  ``RAM'' and Unix people often call ``core'').  The processor and
  memory live on the motherboard which is the heart of your computer.

  Your computer has a screen and keyboard.  It has hard drives and
  floppy disks.  The screen and your disks have controller cards that
  plug into the motherboard and help the computer drive these outboard
  devices.  (Your keyboard is too simple to need a separate card; the
  controller is built into the keyboard chassis itself.)

  We'll go into some of the details of how these devices work later.
  For now, here are a few basic things to keep in mind about how they
  work together:

  All the inboard parts of your computer are connected by a bus.
  Physically, the bus is what you plug your controller cards into (the
  video card, the disk controller, a sound card if you have one).  The
  bus is the data highway between your processor, your screen, your
  disk, and everything else.

  The processor, which makes everything else go, can't actually see any
  of the other pieces directly; it has to talk to them over the bus.
  The only other subsystem it has really fast, immediate access to is
  memory (the core).  In order for programs to run, then, they have to
  be in core.

  When your computer reads a program or data off the disk, what actually
  happens is that the processor uses the bus to send a disk read request
  to your disk controller.  Some time later the disk controller uses the
  bus to signal the computer that it has read the data and put it in a
  certain location in core.  The processor can then use the bus to look
  at that memory.

  Your keyboard and screen also communicate with the processor via the
  bus, but in simpler ways.  We'll discuss those later on.  For now, you
  know enough to understand what happens when you turn on your computer.


  3.  What happens when you switch on a computer?

  A computer without a program running is just an inert hunk of
  electronics.  The first thing a computer has to do when it is turned
  on is start up a special program called an operating system.  The
  operating system's job is to help other computer programs to work by
  handling the messy details of controlling the computer's hardware.

  The process of bringing up the operating system is called booting
  (originally this was bootstrapping and alluded to the difficulty of
  pulling yourself up ``by your bootstraps'').  Your computer knows how
  to boot because instructions for booting are built into one of its
  chips, the BIOS (or Basic Input/Output System) chip.

  The BIOS chip tells it to look in a fixed place on the lowest-numbered
  hard disk (the boot disk) for a special program called a boot loader
  (under Linux the boot loader is called LILO).  The boot loader is
  pulled into core and started.  The boot loader's job is to start the
  real operating system.

  The loader does this by looking for a kernel, loading it into core,
  and starting it.  When you boot Linux and see "LILO" on the screen
  followed by a bunch of dots, it is loading the kernel.  (Each dot
  means it has loaded another disk block of kernel code.)

  (You may wonder why the BIOS doesn't load the kernel directly -- why
  the two-step process with the boot loader?  Well, the BIOS isn't very
  smart.  In fact it's very stupid, and Linux doesn't use it at all
  after boot time.  It was originally written for primitive 8-bit PCs
  with tiny disks, and literally can't access enough of the disk to load
  the kernel directly.  The boot loader step also lets you start one of
  several operating systems off different places on your disk, in the
  unlikely event that Unix isn't good enough for you.)

  Once the kernel starts, it has to look around, find the rest of the
  hardware, and get ready to run programs.  It does this by poking not
  at ordinary memory locations but rather at I/O ports -- special bus
  addresses that are likely to have device controller cards listening at
  them for commands.  The kernel doesn't poke at random; it has a lot of
  built-in knowledge about what it's likely to find where, and how
  controllers will respond if they're present.  This process is called
  autoprobing.

  Most of the messages you see at boot time are the kernel autoprobing
  your hardware through the I/O ports, figuring out what it has
  available to it and adapting itself to your machine.  The Linux kernel
  is extremely good at this, better than most other Unixes and much
  better than DOS or Windows.  In fact, many Linux old-timers think the
  cleverness of Linux's boot-time probes (which made it relatively easy
  to install) was a major reason it broke out of the pack of free-Unix
  experiments to attract a critical mass of users.

  But getting the kernel fully loaded and running isn't the end of the
  boot process; it's just the first stage (sometimes called run level
  1).

  The kernel's next step is to check to make sure your disks are OK.
  Disk file systems are fragile things; if they've been damaged by a
  hardware failure or a sudden power outage, there are good reasons to
  take recovery steps before your Unix is all the way up.  We'll go into
  some of this later on when we talk about ``how file systems can go
  wrong''.

  The kernel's next step is to start several daemons.  A daemon is a
  program like a print spooler, a mail listener or a WWW server that
  lurks in the background, waiting for things to do.  These special
  programs often have to coordinate several requests that could
  conflict.  They are daemons because it's often easier to write one
  program that runs constantly and knows about all requests than it
  would be to try to make sure that a flock of copies (each processing
  one request and all running at the same time) don't step on each
  other.  The particular collection of daemons your system starts may
  vary, but will almost always include a print spooler (a gatekeeper
  daemon for your printer).

  Once all daemons are started, we're at run level 2.  The next step is
  to prepare for users.  The kernel starts a copy of a program called
  getty to watch your console (and maybe more copies to watch dial-in
  serial ports).  This program is what issues the login prompt to your
  console.  We're now at run level 3 and ready for you to log in and run
  programs.

  When you log in (give a name and password) you identify yourself to
  getty and the computer.  It then runs a program called (naturally
  enough) login, which does some housekeeping things and then starts up
  a command interpreter, the shell.  (Yes, getty and login could be one
  program.  They're separate for historical reasons not worth going into
  here.)

  In the next section, we'll talk about what happens when you run
  programs from the shell.


  4.  What happens when you run programs from the shell?

  The normal shell gives you the '$' prompt that you see after logging
  in (unless you've customized it to something else).  We won't talk
  about shell syntax and the easy things you can see on the screen here;
  instead we'll take a look behind the scenes at what's happening from
  the computer's point of view.

  After boot time and before you run a program, you can think of your
  computer of containing a zoo of processes that are all waiting for
  something to do.  They're all waiting on events. An event can be you
  pressing a key or moving a mouse.  Or, if your machine is hooked to a
  network, an event can be a data packet coming in over that network.

  The kernel is one of these processes.  It's s special one, because it
  controls when the other user processes can run, and it is normally the
  only process with direct access to the machine's hardware.  In fact,
  user processes have to make requests to the kernel when they want to
  get keyboard input, write to your screen, read from or write to disk,
  or do just about anything other than crunching bits in memory.  These
  requests are known as system calls.

  Normally all I/O goes through the kernel so it can schedule the
  operations and prevent processes from stepping on each other.  A few
  special user processes are allowed to slide around the kernel, usually
  by being given direct access to I/O ports.  X servers (the programs
  that handle other programs' requests to do screen graphics on most
  Unix boxes) are the most common example of this.  But we haven't
  gotten to an X server yet; you're looking at a shell prompt on a
  character console.

  The shell is just a user process, and not a particularly special one.
  It waits on your keystrokes, listening (through the kernel) to the
  keyboard I/O port.  As the kernel sees them, it echos them to your
  screen then passes them to the shell.  When the kernel sees an `Enter'
  it passes your line of text to the shell. The shell tries to interpret
  those keystrokes as commands.

  Let's say you type `ls' and Enter to invoke the Unix directory lister.
  The shell applies its built-in rules to figure out that you want to
  run the executable command in the file `/bin/ls'.  It makes a system
  call asking the kernel to start /bin/ls as a new child process and
  give it access to the screen and keyboard through the kernel.  Then
  the shell goes to sleep, waiting for ls to finish.

  When /bin/ls is done, it tells the kernel it's finished by issuing an
  exit system call.  The kernel then wakes up the shell and tells it it
  can continue running.  The shell issues another prompt and waits for
  another line of input.

  Other things may be going on while your `ls' is executing, however
  (we'll have to suppose that you're listing a very long directory).
  You might switch to another virtual console, log in there, and start a
  game of Quake, for example.  Or, suppose you're hooked up to the
  Internet.  Your machine might be sending or receiving mail while
  /bin/ls runs.


  5.  How do input devices and interrupts work?

  Your keyboard is a very simple input device; simple because it
  generates small amounts of data very slowly (by a computer's
  standards).  When you press or release a key, that event is signalled
  up the keyboard cable to raise a hardware interrupt.

  It's the operating system's job to watch for such interrupts.  For
  each possible kind of interrupt, there will be an interrupt handler, a
  part of the operating system that stashes away any data associated
  with them (like your keypress/keyrelease value) until it can be
  processed.

  What the interrupt handler for your keyboard actually does is post the
  key value into a system area near the bottom of core.  There, it will
  be available for inspection when the operating system passes control
  to whichever program is currently supposed to be reading from the
  keyboard.

  More complex input devices like disk or network cards work in a
  similar way.  Above, we referred to a disk controller using the bus to
  signal that a disk request has been fulfilled.  What actually happens
  is that the disk raises an interrupt.  The disk interrupt handler then
  copies the retrieved data into memory, for later use by the program
  that made the request.

  Every kind of interrupts has an associated priority level.  Lower-
  priority interrupts (like keyboard events) have to wait on higher-
  priority interrupts (like clock ticks or disk events).  Unix is
  designed to give high priority to the kinds of events that need to be
  processed rapidly in order to keep the machine's response smooth.

  In your OS's boot-time messages, you may see references to IRQ
  numbers.  You may be aware that one of the common ways to misconfigure
  hardware is to have two different devices try to use the same IRQ,
  without understanding exactly why.

  Here's the answer.  IRQ is short for "Interrupt Request".  The
  operating system needs to know at startup time which numbered
  interrupts each hardware device will use, so it can associate the
  proper handlers with each one.  If two different devices try use the
  same IRQ, interrupts will sometimes get dispatched to the wrong
  handler.  This will usually at least lock up the device, and can
  sometimes confuse the OS badly enough that it will flake out or crash.


  6.  How does my computer do several things at once?

  It doesn't, actually.  Computers can only do one task (or process) at
  a time.  But a computer can change tasks very rapidly, and fool slow
  human beings into thinking it's doing several things at once.  This is
  called timesharing.

  One of the kernel's jobs is to manage timesharing.  It has a part
  called the scheduler which keeps information inside itself about all
  the other (non-kernel) processes in your zoo.  Every 1/60th of a
  second, a timer goes off in the kernel, generating a clock interrupt.
  The scheduler stops whatever process is currently running, suspends it
  in place, and hands control to another process.

  1/60th of a second may not sound like a lot of time.  But on today's
  microprocessors it's enough to run tens of thousands of machine
  instructions, which can do a great deal of work.  So even if you have
  many proceses, each one can accomplish quite a bit in each of its
  timeslices.

  In practice, a program may not get its entire timeslice. If an
  interrupt comes in from an I/O device, the kernel effectively stops
  the current task, runs the interrupt handler, and then returns to the
  current task.  A storm of high-priority interrupts can squeeze out
  normal processing; this misbehavior is called thrashing and is
  fortunately very hard to induce under modern Unixes.

  In fact, the speed of programs is only very seldom limited by the
  amount of machine time they can get (there are a few exceptions to
  this rule, such as sound or 3-D graphics generation).  Much more
  often, delays are caused when the program has to wait on data from a
  disk drive or network connection.

  An operating system that can routinely support many simultaneous
  processes is called "multitasking".  The Unix family of operating
  systems was designed from the ground up for multitasking and is very
  good at it -- much more effective than Windows or the Mac OS, which
  have had multitasking bolted into it as an afterthought and do it
  rather poorly.  Efficient, reliable multitasking is a large part of
  what makes Linux superior for networking, communications, and Web
  service.


  7.  How does my computer keep processes from stepping on each other?

  The kernel's scheduler takes care of dividing processes in time.  Your
  operating system also has to divide them in space, so that processes
  don't step on each others' working memory.  The things your operating
  system does to solve this problem are called memory management.

  Each process in your zoo needs its own area of core memory, as a place
  to run its code from and keep variables and results in.  You can think
  of this set as consisting of a read-only code segment (containing the
  process's instructions) and a writeable data segment (containing all
  the process's variable storage).  The data segment is truly unique to
  each process, but if two processes are running the same code Unix
  automatically arranges for them to share a single code segment as an
  efficiency measure.

  Efficiency is important, because core memory is expensive.  Sometimes
  you don't have enough to hold the entirety of all the programs the
  machine is running, especially if you are using a large program like
  an X server.  To get around this, Unix uses a strategy called virtual
  memory.  It doesn't try to hold all the code and data for a process in
  core.  Instead, it keeps around only a relatively small working set;
  the rest of the process's state is left in a special swap space area
  on your hard disk.

  As the process runs, Unix tries to anticipate how the working set will
  change and have only the pieces that are needed in core.  Doing this
  effectively is both complicated and tricky, so I won't try and
  describe it all here -- but it depends on the fact that code and data
  references tend to happen in clusters, with each new one likely to
  refer to somewhere close to an old one.  So if Unix keeps around the
  code or data most frequently (or most recently) used, you will usually
  succeed in saving time.

  Note that in the past, that "Sometimes" two paragraphs ago was "Almost
  always," -- the size of core was typically small relative to the size
  of running programs, so swapping was frequent.  Memory is far less
  expensive nowadays and even low-end machines have quite a lot of it.
  On modern single-user machines with 64MB of core and up, it's possible
  to run X and a typical mix of jobs without ever swapping.

  Even in this happy situation, the part of the operating system called
  the memory manager still has important work to do.  It has to make
  sure that programs can only alter their own data segments -- that is,
  prevent erroneous or malicious code in one program from garbaging the
  data in another.  To do this, it keeps a table of data and code
  segments.  The table is updated whenever a process either requests
  more memory or releases memory (the latter usually when it exits).

  This table is used to pass commands to a specialized part of the
  underlying hardware called an MMU or memory management unit.  Modern
  processor chips have MMUs built right onto them.  The MMU has the
  special ability to put fences around areas of memory, so an out-of-
  bound reference will be refused and cause a special interrupt to be
  raised.

  If you ever see a Unix message that says "Segmentation fault", "core
  dumped" or something similar, this is exactly what has happened; an
  attempt by the running program to access memory outside its segment
  has raised a fatal interrupt.  This indicates a bug in the program
  code; the core dump it leaves behind is diagnostic information
  intended to help a programmer track it down.
  8.  How does my computer store things on disk?

  When you look at a hard disk under Unix, you see a tree of named
  directories and files.  Normally you won't need to look any deeper
  than that, but it does become useful to know what's going on
  underneath if you have a disk crash and need to try to salvage files.
  Unfortunately, there's no good way to describe disk organization from
  the file level downwards, so I'll have to describe it from the
  hardware up.


  8.1.  Low-level disk and file system structure

  The surface area of your disk, where it stores data, is divided up
  something like a dartboard -- into circular tracks which are then pie-
  sliced into sectors.  Because tracks near the outer edge have more
  area than those close to the spindle at the center of the disk, the
  outer tracks have more sector slices in them than the inner ones.
  Each sector (or disk block) has the same size, which under modern
  Unixes is generally 1 binary K (1024 8-bit words).  Each disk block
  has a unique address or disk block number.

  Unix divides the disk into disk partitions.  Each partition is a
  continuous span of blocks that's used separately from any other
  partition, either as a file system or as swap space.  The lowest-
  numbered partition is often treated specially, as a boot partition
  where you can put a kernel to be booted.

  Each partition is either swap space (used to implement ``virtual
  memory'' or a file system used to hold files.  Swap-space partitions
  are just treated as a linear sequence of blocks.  File systems, on the
  other hand, need a way to map file names to sequences of disk blocks.
  Because files grow, shrink, and change over time, a file's data blocks
  will not be a linear sequence but may be scattered all over its
  partition (from wherever the operating system can find a free block
  when it needs one).


  8.2.  File names and directories

  Within each file system, the mapping from names to blocks is handled
  through a structure called an i-node.  There's a pool of these things
  near the ``bottom'' (lowest-numbered blocks) of each file system (the
  very lowest ones are used for housekeeping and labeling purposes we
  won't describe here).  Each i-node describes one file.  File data
  blocks live above the inodes.

  Every i-node contains a list of the disk block numbers in the file it
  describes.  (Actually this is a half-truth, only correct for small
  files, but the rest of the details aren't important here.)  Note that
  the i-node does not contain the name of the file.

  Names of files live in directory structures.  A directory structure
  just maps names to i-node numbers.  This is why, in Unix, a file can
  have multiple true names (or hard links); they're just multiple
  directory entries that happen to point to the same inode.


  8.3.  Mount points

  In the simplest case, your entire Unix file system lives in just one
  disk partition.  While you'll see this arrangement on some small
  personal Unix systems, it's unusual.  More typical is for it to be
  spread across several disk partitions, possibly on different physical
  disks.   So, for example, your system may one small partition where
  the kernel lives, a slightly larger one where OS utilities live, and a
  much bigger one where user home directories live.

  The only partition you'll have access to immediately after system boot
  is your root partition, which is (almost always) the one you booted
  from.  It holds the root directory of the file system, the top node
  from which everything else hangs.

  The other partitions in the system have to be attached to this root in
  order for your entire, multiple-partition file system to be
  accessible.  About midway through the boot process, your Unix will
  make these non-root partitions accessible.  It will mount each one
  onto a directory on the root partition.

  For example, if you have a Unix directory called `/usr', it is
  probably a mount point to a partition that contains many programs
  installed with your Unix but not required during initial boot.


  8.4.  How a file gets looked up

  Now we can look at the file system from the top down.  When you open a
  file (such as, say, /home/esr/WWW/ldp/fundamentals.sgml) here is what
  happens:

  Your kernel starts at the root of your Unix file system (in the root
  partition).  It looks for a directory there called `home'.  Usually
  `home' is a mount point to a large user partition elsewhere, so it
  will go there.  In the top-level directory structure of that user
  partition, it will look for a entry called `esr' and extract an inode
  number.  It will go to that i-node, notice it is a directory
  structure, and look up `WWW'.  Extracting that i-node, it will go to
  the corresponding subdirectory and look up `ldp'.  That will take it
  to yet another directory inode.  Opening that one, it will find an i-
  node number for `fundamentals.sgml'.  That inode is not a directory,
  but instead holds the list of disk blocks associated with the file.


  8.5.  How things can go wrong

  Earlier we hinted that file systems can be fragile things.  Now we
  know that to get to file you have to hopscotch through what may be an
  arbitrarily long chain of directory and i-node references.  Now
  suppose your hard disk develops a bad spot?

  If you're lucky, it will only trash some file data.  If you're
  unlucky, it could corrupt a directory structure or i-node number and
  leave an entire subtree of your system hanging in limbo -- or, worse,
  result in a corrupted structure that points multiple ways at the same
  disk block or inode.  Such corruption can be spread by normal file
  operations, trashing data that was bot in the original bad spot.

  Fortunately, this kind of contingency has become quite uncommon as
  disk hardware has become more reliable.  Still, it means that your
  Unix will want to integrity-check the file system periodically to make
  sure nothing is amiss.  Modern Unixes do a fast integrity check on
  each partition at boot time, just before mounting it.  Every few
  reboots they'll do a much more thorough check that takes a few minutes
  longer.

  If all of this sounds like Unix is terribly complex and failure-prone,
  it may be reassuring to know that these boot-time checks typically
  catch and correct normal problems before they become really
  disasterous.  Other operating systems don't have these facilities,
  which speeds up booting a bit but can leave you much more seriously
  screwed when attempting to recover by hand (and that's assuming you
  have a copy of Norton Utilities or whatever in the first place...).
  9.  How do computer languages work?

  We've already discussed ``how programs are run''.  Every program
  ultimately has to execute as a stream of bytes that are instructions
  in your computer's machine language.  But human beings don't deal with
  machine language very well; doing so has become a rare, black art even
  among hackers.

  Almost all Unix code except a small amount of direct hardware-
  interface support in the kernel itself is nowadays written in a high-
  level language.  (The `high-level' in this term is a historical relic
  meant to distinguish these from `low-level' assembler languages, which
  are basically thin wrappers around machine code.)

  There are several different kinds of high-level languages.  In order
  to talk about these, you'll find it useful to bear in mind that the
  source code of a program (the human-created, editable version) has to
  go through some kind of translation into machine code that the machine
  can actually run.


  9.1.  Compiled languages

  The most conventional kind of language is a compiled language.
  Compiled languages get translated into runnable files of binary
  machine code by a special program called (logically enough) a
  compiler.  Once the binary has been generated, you can run it directly
  without looking at the source code again.  (Most software is delivered
  as compiled binaries made from code you don't see.)

  Compiled languages tend to give excellent performance and have the
  most complete access to the OS, but also to be difficult to program
  in.

  C, the language in which Unix itself is written, is by far the most
  important of these (with its variant C++).  FORTRAN is another
  compiled language still used among engineers and scientists but years
  older and much more primitive.  In the Unix world no other compiled
  languages are in mainstream use.  Outide it, COBOL is very widely used
  for financial and business software.

  There used to be many other compiler languages, but most of them have
  either gone extinct or are strictly research tools.  If you are a new
  Unix developer using a compiled language, it is overwhelmingly likely
  to be C or C++.


  9.2.  Interpreted languages

  An interpreted language depends on an interpreter program that reads
  the source code and translates it on the fly into computations and
  system calls.  The source has to be re-interpreted (and the
  interpreter present) each time the code is executed.

  Interpreted languages tend to be slower than compiled languages, and
  often have limited access to the underlying operating system and
  hardware.  On the other hand, they tend to be easier to program and
  more forgiving of coding errors than compiled languages.

  Many Unix utilities, including the shell and bc(1) and sed(1) and
  awk(1), are effectively small interpreted languages.  BASICs are
  usually interpreted.  So is Tcl.  Historically, the most important
  interpretive language has been LISP (a major improvement over most of
  its successors).  Today Perl is very widely used and steadily growing
  more popular.

  9.3.  P-code languages

  Since 1990 a kind of hybrid language that uses both compilation and
  interpretation has become increasingly important.  P-code languages
  are like compiled languages in that the source is translated to a
  compact binary form which is what you actually execute, but that form
  is not machine code.  Instead it's pseudocode (or p-code), which is
  usually a lot simpler but more powerful than a real machine language.
  When you run the program, you interpret the p-code.

  P-code can can run nearly as fast as a compiled binary (p-code
  interpreters can be made quite simple, small and speedy).  But p-code
  languages can keep the flexibility and power of a good interpreter.

  Important p-code languages include Python and Java.


  10.  How does the Internet work?

  To help you understand how the Internet works, we'll look at the
  things that happen when you do a typical Internet operation --
  pointing a browser at the front page of this document at its home on
  the Web at the Linux Documentation Project.  This document is


  http://sunsite.unc.edu/LDP/HOWTO/Fundamentals.html



  which means it lives in the file LDP/HOWTO/Fundamentals.html under the
  World Wide Web export directory of the host sunsite.unc.edu.


  10.1.  Names and locations


  The first thing your browser has to do is to establish a network
  connection to the machine where the document lives.  To do that, it
  first has to find the network location of the host sunsite.unc.edu
  (`host' is short for `host machine' or `network host'; sunsite.unc.edu
  is a typical hostname).  The corresponding location is actually a
  number called an IP address (we'll explain the `IP' part of this term
  later).

  To do this, your browser queries a program called a name server.  The
  name server may live on your machine, but it's more likely to run on a
  service machine that yours talks to.  When you sign up with an ISP,
  part of your setup procedure will almost certainly involve telling
  your Internet software the IP address of a nameserver on the ISP's
  network.

  The name servers on different machines talk to each other, exchanging
  and keeping up to date all the information needed to resolve hostnames
  (map them to IP addresses).  Your nameserver may query three or four
  different sites across the network in the process of resolving
  sunsite.unc.edu, but this usually happens very quickly (as in less
  than a second).

  The nameserver will tell your browser that Sunsite's IP address is
  152.2.22.81; knowing this, your machine will be able to exchange bits
  with sunsite directly.





  10.2.  Packets and routers


  What the browser wants to do is send a command to the Web server on
  Sunsite that looks like this:


  GET /LDP/HOWTO/Fundamentals.html HTTP/1.0



  Here's how that happens.  The command is made into a packet, a block
  of bits like a telegram that is wrapped with three important things;
  the source address (the IP address of your machine), the destination
  address (152.2.22.81), and a service number or port number (80, in
  this case) that indicates that it's a World Wide Web request.

  Your machine then ships the packet down the wire (modem connection to
  your ISP, or local network) until it gets to a specialized machine
  called a router.  The router has a map of the Internet in its memory
  -- not always a complete one, but one that completely describes your
  network neighborhood and knows how to get to the routers for other
  neighborhoods on the Internet.

  Your packet may pass through several routers on the way to its
  destination.  Routers are smart.  They watch how long it takes for
  other routers to acknowledge having received a packet.  They use that
  information to direct traffic over fast links.  They use it to notice
  when another routers (or a cable) have dropped off the network, and
  compensate if possible by finding another route.

  There's an urban legend that the Internet was designed to survive
  nuclear war.  This is not true, but the Internet's design is extremely
  good at getting reliable performance out of flaky hardware in am
  uncertain world..  This is directly due to the fact that its
  intelligence is distributed through thousands of routers rather than a
  few massive switches (like the phone network).  This means that
  failures tend to be well localized and the network can route around
  them.

  Once your packet gets to its destination machine, that machine uses
  the service number to feed the packet to the web server.  The web
  server can tell where to reply to by looking at the command packet's
  source IP address. When the web server returns this document, it will
  be broken up into a number of packets.  The size of the packets will
  vary according to the transmission media in the network and the type
  of service.


  10.3.  TCP and IP

  To understand how multiple-packet transmissions are handled, you need
  to know that the Internet actually uses two protocols, stacked one on
  top of the other.

  The lower level, IP (Internet Protocol), knows how to get individual
  packets from a source address to a destination address (this is why
  these are called IP addresses).  However, IP is not reliable; if a
  packet gets lost or dropped, the source and destination machines may
  never know it.  In network jargon, IP is a connectionless protocol;
  the sender just fires a packet at the receiver and doesn't expect an
  acknowledgement.

  IP is fast and cheap, though.  Sometimes fast, cheap and unreliable is
  OK.  When you play networked Doom or Quake, each bullet is represented
  by an IP packet.  If a few of those get lost, that's OK.
  The upper level, TCP (Transmission Control Protocol), gives you
  reliability.  When two machines negotiate a TCP connection (which they
  do using IP), the receiver knows to send acknowledgements of the
  packets it sees back to the sender.  If the sender doesn't see an
  acknowledgement for a packet within some timeout period, it resends
  that packet.  Furthermore, the sender gives each TCP packet has a
  sequence number, which the receiver can use you reassemble packets in
  case they show up out of order.  (This can happen if network links go
  up or down during a connection.)

  TCP/IP packets also contain a checksum to enable detection of data
  corrupted by bad links.  So, from the point of view of anyone using
  TCP/IP and nameservers, it looks like a reliable way to pass streams
  of bytes between hostname/service-number pairs.  People who write
  network protocols almost never have to think about all the
  packetizing, packet reassembly, error checking, checksumming, and
  retransmission that goes on below that level.


  10.4.  HTTP, an application protocol

  Now let's get back to our example.  Web browsers and servers speak an
  application protocol that runs on top of TCP/IP, using it simply as a
  way to pass strings of bytes back and forth.  This protocol is called
  HTTP (Hyper-Text Transfer Protocol) and we've already seen one command
  in it -- the GET shown above.

  When the GET command goes to sunsite.unc.edu's webserver with service
  number 80, it will dispatched to a server daemon listening on port 80.
  Most Internet services are implemented by server daemons that do
  nothing but wait on ports, watching for and executing incoming
  commands.

  If the design of the Internet has one overall rule, it's that all the
  parts should be as simple and human-accessible as possible.  HTTP, and
  its relatives (like the Simple Mail Transfer Protocol, SMTP, that is
  used to move electronic mail between hosts) tend to use simple
  printable-text commands that end with a carriage-return/line feed.

  This is marginally inefficient; in some circumstances you could get
  more speed by using a tightly-coded binary protocol.  But experience
  has shown that the benefits of having commands be easy for human
  beings to describe and understand outweigh any marginal gain in
  efficiency that you might get at the cost of making things tricky and
  opaque.

  Therefore, what the server daemon ships back to you via TCP/IP is also
  text.  The beginning of the response will look something like this (a
  few headers have been suppressed):


  HTTP/1.1 200 OK
  Date: Sat, 10 Oct 1998 18:43:35 GMT
  Server: Apache/1.2.6 Red Hat
  Last-Modified: Thu, 27 Aug 1998 17:55:15 GMT
  Content-Length: 2982
  Content-Type: text/html



  These headers will be followed by a blank line and the text of the web
  page (after which the connection is dropped).  Your browser just
  displays that page.  The headers tell it how (in particular, the
  Content-Type header tells it the returned data is really HTML).