
The Unix and Internet Fundamentals HOWTO

Eric Raymond

              esr@thyrsus.com
   
   Revision History
   Revision 2.0 5 August 2000 Revised by: esr
   First DocBook version. Detailed description of memory hierarchy.
   Revision 1.7 6 March 2000 Revised by: esr
   Correct and expanded the section on file permissions.
   Revision 1.4 25 September 1999 Revised by: esr
   Be more precise about what kernel does vs. what init does.
   Revision 1.3 27 June 1999 Revised by: esr
   The sections `What happens when you log in?' and `File ownership,
   permissions and security'.
   Revision 1.2 26 December 1998 Revised by: esr
   The section `How does my computer store things in memory?'.
   Revision 1.0 29 October 1998 Revised by: esr
   Initial revision.
   
   This document describes the working basics of PC-class computers,
   Unix-like operating systems, and the Internet in non-technical
   language.
     _________________________________________________________________
   
   Table of Contents
   1. [1]Introduction
          
        1.1. [2]Purpose of this document
        1.2. [3]Related resources
        1.3. [4]New versions of this document
        1.4. [5]Feedback and corrections
                
   2. [6]Basic anatomy of your computer
   3. [7]What happens when you switch on a computer?
   4. [8]What happens when you log in?
   5. [9]What happens when you run programs from the shell?
   6. [10]How do input devices and interrupts work?
   7. [11]How does my computer do several things at once?
   8. [12]How does my computer keep processes from stepping on each
          other?
          
        8.1. [13]Virtual memory: the simple version
        8.2. [14]Virtual memory: the detailed version
        8.3. [15]The Memory Management Unit
                
   9. [16]How does my computer store things in memory?
          
        9.1. [17]Numbers
        9.2. [18]Characters
                
   10. [19]How does my computer store things on disk?
          
        10.1. [20]Low-level disk and file system structure
        10.2. [21]File names and directories
                
   11. [22]Mount points
   12. [23]How a file gets looked up
          
        12.1. [24]File ownership, permissions and security
        12.2. [25]How things can go wrong
                
   13. [26]How do computer languages work?
          
        13.1. [27]Compiled languages
        13.2. [28]Interpreted languages
        13.3. [29]P-code languages
                
   14. [30]How does the Internet work?
          
        14.1. [31]Names and locations
        14.2. [32]Packets and routers
        14.3. [33]TCP and IP
        14.4. [34]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 specific
   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 good 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 [35]How To Become A Hacker FAQ. 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 [36]comp.os.linux.help and
   [37]news:comp.os.linux.announce and [38]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
   [39]http://metalab.unc.edu/LDP/HOWTO/Unix-Internet-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 [40]esr@thyrsus.com. 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 Unix term is a folk
   memory from when RAM consisted of ferrite-core donuts). 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 (in memory).
   
   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 processor that it has read the data and put it in a
   certain location in memory. The processor can then use the bus to look
   at that data.
   
   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 memory 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 memory,
   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). After this first stage, the kernel hands control to a special
   process called `init' which spawns several housekeeping processes.
   
   The init process's first job is usually 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 [41]how file
   systems can go wrong.
   
   Init'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).
   
   The next step is to prepare for users. Init 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. Once all daemons and getty processes for each terminal
   are started, we're at run level 2. At this level, you can log in and
   run programs.
   
   But we're not done yet. The next step is to start up various daemons
   that support networking and other services. Once that's done, we're at
   run level 3 and the system is fully ready for use.
     _________________________________________________________________
   
4. What happens when you log in?

   When you log in (give a name to getty) you identify yourself to the
   computer. It then runs a program called (naturally enough) login,
   which takes your password and checks to see if you are authorized to
   be using the machine. If you aren't, your login attempt will be
   rejected. If you are, login does a few 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.)
   
   Here's a bit more about what the system does before giving you a shell
   (you'll need to know this later when we talk about file permissions).
   You identify yourself with a login name and password. That login name
   is looked up in a file called /etc/passwd, which is a sequence of
   lines each describing a user account.
   
   One of these fields is an encrypted version of the account password
   (sometimes the encrypted fields are actually kept in a second
   /etc/shadow file with tighter permissions; this makes password
   cracking harder). What you enter as an account password is encrypted
   in exactly the same way, and the login program checks to see if they
   match. The security of this method depends on the fact that, while
   it's easy to go from your clear password to the encrypted version, the
   reverse is very hard. Thus, even if someone can see the encrypted
   version of your password, they can't use your account. (It also means
   that if you forget your password, there's no way to recover it, only
   to change it to something else you choose.)
   
   Once you have successfully logged in, you get all the privileges
   associated with the individual account you are using. You may also be
   recognized as part of a group. A group is a named collection of users
   set up by the system administrator. Groups can have privileges
   independently of their members' privileges. A user can be a member of
   multiple groups. (For details about how Unix privileges work, see the
   section below on [42]permissions.)
   
   (Note that although you will normally refer to users and groups by
   name, they are actually stored internally as numeric IDs. The password
   file maps your account name to a user ID; the /etc/group file maps
   group names to numeric group IDs. Commands that deal with accounts and
   groups do the translation automatically.)
   
   Your account entry also contains your home directory, the place in the
   Unix file system where your personal files will live. Finally, your
   account entry also sets your shell, the command interpreter that login
   will start up to accept your commmands.
     _________________________________________________________________
   
5. What happens when you run programs from the shell?

   The shell is Unix's interpreter for the commands you type in; it's
   called a shell because it wraps around and hides the operating system
   kernel. 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 a 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.
     _________________________________________________________________
   
6. 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 memory. 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.
     _________________________________________________________________
   
7. 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 processes, 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.
     _________________________________________________________________
   
8. 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
   can't step on each others' working memory. Even if you assume that all
   programs are trying to be cooperative, you don't want a bug in one of
   them to be able to corrupt others. The things your operating system
   does to solve this problem are called memory management.
   
   Each process in your zoo needs its own area of 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.
     _________________________________________________________________
   
8.1. Virtual memory: the simple version

   Efficiency is important, because 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 technique called virtual
   memory. It doesn't try to hold all the code and data for a process in
   memory. 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.
   
   Note that in the past, that "Sometimes" last paragraph ago was "Almost
   always," -- the size of memory 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 memory and up, it's
   possible to run X and a typical mix of jobs without ever swapping
   after they're initially loded into core.
     _________________________________________________________________
   
8.2. Virtual memory: the detailed version

   Actually, the last section oversimplified things a bit. Yes, programs
   see most of your memory as one big flat bank of addresses bigger than
   physical memory, and disk swapping is used to maintain that illusion.
   But your hardware actually has no fewer than five different kinds of
   memory in it, and the differences between them can matter a good deal
   when programs have to be tuned for maximum speed. To really understand
   what goes on in your machine, you should learn how all of them work,
   
   The five kinds of memory are these: processor registers, internal (or
   on-chip) cache, external (or off-chip) cache, main memory, and disk.
   And the reason there are so many kinds is simple; speed costs money, I
   listed these kinds of memory in decreasing order of access time and
   cost; register memory is the fastest and most expensive and can be
   random-accessed about a billion times a second, while disk is the
   slowest and cheapest and can do about 100 random accesses a second.
   
   Here's a full list reflecting early-2000 speeds and prices for a
   typical desktop machine. While speed and capacity will go up and
   prices will drop, you can expect these ratios to remain fairly
   constant -- and it's those ratios that shape the memory hierarchy.
   
   Disk
          Size: 13000MB Accesses: 100/sec
          
   Main memory
          Size: 256MB Accesses: 100M/sec
          
   External cache
          Size: 512KB Accesses: 250M/sec
          
   Internal Cache
          Size: 32KB Accesses: 500M/sec
          
   Processor
          Size: 28 bytes Accesses: 1000M/sec
          
   We can't build everything out of the fastest kinds of memory. It would
   be way too expensive -- and even if it weren't, fast memory is
   volatile. That is, it loses its marbles when the power goes off. Thus,
   computers have to have hard disks or other kinds of non-volatile
   storage that retains data when the power goes off. And there's a huge
   mismatch between the speed of processors and the speed of disks. The
   middle three levels of the memory hierarchy (internal cache, external
   cache, and main memory) basically exist to bridge that gap.
   
   Linux and other Unixes have a feature called virtual memory. What this
   means is that the operating system behaves as though it has much more
   main memory than it actually does. Your actual physical main memory
   behaves like a set of windows or caches on a much larger "virtual"
   memory space, most of which at any given time is actually stored on
   disk in a special zone called the swap area. Out of sight of user
   programs, the OS is moving blocks of data (called "pages") between
   memory and disk to maintain this illusion. The end result is that your
   virtual memory is much larger but not too much slower than real
   memory.
   
   How much slower virtual memory is than physical depends on how well
   the operating system's swapping algorithms match the way your programs
   use virtual memory. Fortunately, memory reads and writes that are
   close together in time also tend to cluster in memory space. This
   tendency is called locality, or more formally locality of reference --
   and it's a good thing. If memory references jumped around virtual
   space at random, you'd typically have to do a disk read and write for
   each new reference and virtual memory would be as slow as a disk. But
   because programs do actually exhibit strong locality, your operating
   system can do relatively few swaps per reference.
   
   It's been found by experience that the most effective method for a
   broad class of memory-usage patterns is very simple; it's called LRU
   or the "least recently used" algorithm. The virtual-memory system
   grabs disk blocks into its working set as it needs them. When it runs
   out of physical memory for the working set, it dumps the
   least-recently-used block. All Unixes, and most other virtual-memory
   operating systems, use minor variations on LRU.
   
   Virtual memory is the first link in the bridge between disk and
   processor speeds. It's explicitly managed by the OS. But there is
   still a major gap between the speed of physical main memory and the
   speed at which a processor can access its register memory. The
   external and internal caches address this, using a technique similar
   to virtual memory as we've described it.
   
   Just as the physical main memory behaves like a set of windows or
   caches on the disk's swap area, the external cache acts as windows on
   main memory. External cache is faster (250M accesses per sec, rather
   than 100M) and smaller. The hardware (specifically, your computer's
   memory controller) does the LRU thing in the external cache on blocks
   of data fetched from the main memory. For historical regions, the unit
   of cache swapping is called a "line" rather than a page.
   
   But we're not done. The internal cache gives us the final step-up in
   effective speed by caching portions of the external cache. It is
   faster and smaller yet -- in fact, it lives right on the processor
   chip.
   
   If you want to make your programs really fast, it's useful to know
   these details. Your programs get faster when they have stronger
   locality, because that makes the caching work better. The easiest way
   to make programs fast is therefore to make them small. If a program
   isn't slowed down by lots of disk I/O or waits on network events, it
   will usually run at the speed of the largest cache that it will fit
   inside.
   
   If you can't make your whole program small, some effort to tune the
   speed-critical portions so they have stronger locality can pay off.
   Details on techniques for doing such tuning are beyond the scope of
   this tutorial; by the time you need them, you'll be intimate enough
   with some compiler to figure out many of them yourself.
     _________________________________________________________________
   
8.3. The Memory Management Unit

   Even when you have enough physical core to avoid swapping, 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 (core) 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.
   
   There is another aspect to protecting processes from each other
   besides segregating the memory they access. You also want to be able
   to control their file accesses so a buggy or malicious program can't
   corrupt critical pieces of the system. This is why Unix has [43]file
   permissions which we'll discuss later.
     _________________________________________________________________
   
9. How does my computer store things in memory?

   You probably know that everything on a computer is stored as strings
   of bits (binary digits; you can think of them as lots of little on-off
   switches). Here we'll explain how those bits are used to represent the
   letters and numbers that your computer is crunching.
   
   Before we can go into this, you need to understand about the the word
   size of your computer. The word size is the computer's preferred size
   for moving units of information around; technically it's the width of
   your processor's registers, which are the holding areas your processor
   uses to do arithmetic and logical calculations. When people write
   about computers having bit sizes (calling them, say, ``32-bit'' or
   ``64-bit'') computers, this is what they mean.
   
   Most computers (including 386, 486, and Pentium PCs) have a word size
   of 32 bits. The old 286 machines had a word size of 16. Old-style
   mainframes often had 36-bit words. A few processors (like the Alpha
   from what used to be DEC and is now Compaq) have 64-bit words. The
   64-bit word will become more common over the next five years; Intel is
   planning to replace the Pentium series with a 64-bit chip called the
   `Itanium'.
   
   The computer views your memory as a sequence of words numbered from
   zero up to some large value dependent on your memory size. That value
   is limited by your word size, which is why older machines like 286s
   had to go through painful contortions to address large amounts of
   memory. I won't describe them here; they still give older programmers
   nightmares.
     _________________________________________________________________
   
9.1. Numbers

   Numbers are represented as either words or pairs of words, depending
   on your processor's word size. One 32-bit machine word is the most
   common size.
   
   Integer arithmetic is close to but not actually mathematical base-two.
   The low-order bit is 1, next 2, then 4 and so forth as in pure binary.
   But signed numbers are represented in twos-complement notation. The
   highest-order bit is a sign bit which makes the quantity negative, and
   every negative number can be obtained from the corresponding positive
   value by inverting all the bits. This is why integers on a 32-bit
   machine have the range -2^31 + 1 to 2^31 - 1 (where ^ is the `power'
   operation, 2^3 = 8). That 32nd bit is being used for sign.
   
   Some computer languages give you access to unsigned arithmetic which
   is straight base 2 with zero and positive numbers only.
   
   Most processors and some languages can do in floating-point numbers
   (this capability is built into all recent processor chips).
   Floating-point numbers give you a much wider range of values than
   integers and let you express fractions. The ways this is done vary and
   are rather too complicated to discuss in detail here, but the general
   idea is much like so-called `scientific notation', where one might
   write (say) 1.234 * 10^23; the encoding of the number is split into a
   mantissa (1.234) and the exponent part (23) for the power-of-ten
   multiplier.
     _________________________________________________________________
   
9.2. Characters

   Characters are normally represented as strings of seven bits each in
   an encoding called ASCII (American Standard Code for Information
   Interchange). On modern machines, each of the 128 ASCII characters is
   the low seven bits of an 8-bit octet; octets are packed into memory
   words so that (for example) a six-character string only takes up two
   memory words. For an ASCII code chart, type `man 7 ascii' at your Unix
   prompt.
   
   The preceding paragraph was misleading in two ways. The minor one is
   that the term `octet' is formally correct but seldom actually used;
   most people refer to an octet as byte and expect bytes to be eight
   bits long. Strictly speaking, the term `byte' is more general; there
   used to be, for example, 36-bit machines with 9-bit bytes (though
   there probably never will be again).
   
   The major one is that not all the world uses ASCII. In fact, much of
   the world can't -- ASCII, while fine for American English, lacks many
   accented and other special characters needed by users of other
   languages. Even British English has trouble with the lack of a
   pound-currency sign.
   
   There have been several attempts to fix this problem. All use the
   extra high bit that ASCII doesn't, making it the low half of a
   256-character set. The most widely-used of these is the so-called
   `Latin-1' character set (more formally called ISO 8859-1). This is the
   default character set for Linux, HTML, and X. Microsoft Windows uses a
   mutant version of Latin-1 that adds a bunch of characters such as
   right and left double quotes in places proper Latin-1 leaves
   unassigned for historical reasons (for a scathing account of the
   trouble this causes, see the [44]demoroniser page).
   
   Latin-1 handles the major European languages, including English,
   French, German, Spanish, Italian, Dutch, Norwegian, Swedish, Danish.
   However, this isn't good enough either, and as a result there is a
   whole series of Latin-2 through -9 character sets to handle things
   like Greek, Arabic, Hebrew, Esperanto, and Serbo-Croatian. For
   details, see the [45]ISO alphabet soup page.
   
   The ultimate solution is a huge standard called Unicode (and its
   identical twin ISO/IEC 10646-1:1993). Unicode is identical to Latin-1
   in its lowest 256 slots. Above these in 16-bit space it includes
   Greek, Cyrillic, Armenian, Hebrew, Arabic, Devanagari, Bengali,
   Gurmukhi, Gujarati, Oriya, Tamil, Telugu, Kannada, Malayalam, Thai,
   Lao, Georgian, Tibetan, Japanese Kana, the complete set of modern
   Korean Hangul, and a unified set of Chinese/Japanese/Korean (CJK)
   ideographs. For details, see the [46]Unicode Home Page.
     _________________________________________________________________
   
10. 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.
     _________________________________________________________________
   
10.1. 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 original
   reasons for partitions had to do with crash recovery in a world of
   much slower and more error-prone disks; the boundaries between them
   reduce the fraction of your disk likely to become inaccessible or
   corrupted by a random bad spot on the disk. Nowadays, it's more
   important that partitions can be declared read-only (preventing an
   intruder from modifying critical system files) or shared over a
   network through various means we won't discuss here. The
   lowest-numbered partition on a disk 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 [47]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).
     _________________________________________________________________
   
10.2. 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 (in higher-numbered blocks).
   
   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.
     _________________________________________________________________
   
11. 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 have 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.
     _________________________________________________________________
   
12. 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.
     _________________________________________________________________
   
12.1. File ownership, permissions and security

   To keep programs from accidentally or maliciously stepping on data
   they shouldn't, Unix has permission features. These were originally
   designed to support timesharing by protecting multiple users on the
   same machine from each other, back in the days when Unix ran mainly on
   expensive shared minicomputers.
   
   In order to understand file permissions, you need to recall our
   description of of users and groups in the section [48]What happens
   when you log in?. Each file has an owning user and an owning group.
   These are initially those of the file's creator; they can be changed
   with the programs and .
   
   The basic permissions that can be associated with a file are `read'
   (permission to read data from it), `write' (permission to modify it)
   and `execute' (permission to run it as a program). Each file has three
   sets of permissions; one for its owning user, one for any user in its
   owning group, and one for everyone else. The `privileges' you get when
   you log in are just the ability to do read, write, and execute on
   those files for which the permission bits match your user ID or one of
   the groups you are in.
   
   To see how these may interact and how Unix displays them, let's look
   at some file listings on a hypothetical Unix system. Here's one:
snark:~$ ls -l notes
-rw-r--r--   1 esr      users         2993 Jun 17 11:00 notes

   This is an ordinary data file. The listing tells us that it's owned by
   the user `esr' and was created with the owning group `users'. Probably
   the machine we're on puts every ordinary user in this group by
   default; other groups you commonly see on timesharing machines are
   `staff', `admin', or `wheel' (for obvious reasons, groups are not very
   important on single-user workstations or PCs). Your Unix may use a
   different default group, perhaps one named after your user ID.
   
   The string `-rw-r--r--' represents the permission bits for the file.
   The very first dash is the position for the directory bit; it would
   show `d' if the file were a directory. After that, the first three
   places are user permissions, the second three group permissions, and
   the third are permissions for others (often called `world'
   permissions). On this file, the owning user `esr' may read or write
   the file, other people in the `users' group may read it, and everybody
   else in the world may read it. This is a pretty typical set of
   permissions for an ordinary data file.
   
   Now let's look at a file with very different permissions. This file is
   GCC, the GNU C compiler.
snark:~$ ls -l /usr/bin/gcc
-rwxr-xr-x   3 root     bin         64796 Mar 21 16:41 /usr/bin/gcc

   This file belongs to a user called `root' and a group called `bin'; it
   can be written (modified) only by root, but read or executed by
   anyone. This is a typical ownership and set of permissions for a
   pre-installed system command. The `bin' group exists on some Unixes to
   group together system commands (the name is a historical relic, short
   for `binary'). Your Unix might use a `root' group instead (not quite
   the same as the `root' user!).
   
   The `root' user is the conventional name for numeric user ID 0, a
   special, privileged account that can override all privileges. Root
   access is useful but dangerous; a typing mistake while you're logged
   in as root can clobber critical system files that the same command
   executed from an ordinary user account could not touch.
   
   Because the root account is so powerful, access to it should be
   guarded very carefully. Your root password is the single most critical
   piece of security information on your system, and it is what any
   crackers and intruders who ever come after you will be trying to get.
   
   About passwords: Don't write them down -- and don't pick a passwords
   that can easily be guessed, like the first name of your
   girlfriend/boyfriend/spouse. This is an astonishingly common bad
   practice that helps crackers no end. In general, don't pick any word
   in the dictionary; there are programs called `dictionary crackers'
   that look for likely passwords by running through word lists of common
   choices. A good technique is to pick a combination consisting of a
   word, a digit, and another word, such as `shark6cider' or `jump3joy';
   that will make the search space too large for a dictionary cracker.
   Don't use these examples, though -- crackers might expect that after
   reading this document and put them in their dictionaries.
   
   Now let's look at a third case:
snark:~$ ls -ld ~
drwxr-xr-x  89 esr      users          9216 Jun 27 11:29 /home2/esr
snark:~$

   This file is a directory (note the `d' in the first permissions slot).
   We see that it can be written only by esr, but read and executed by
   anybody else.
   
   Read permission gives you the ability to list the directory -- that
   is, to see the names of files and directories it contains. Write
   permission gives you the ability to create and delete files in the
   directory. If you remember that the directory imcludes a list of the
   names of the files and subdirectories it contains, these rules will
   make sense.
   
   Execute permission on a directory means you can get through the
   directory to open the files and directories below it. In effect, it
   gives you permission to access the inodes in thbe directory. A
   directory with execute completely turned off would be useless.
   
   Occasionally you'll see a directory that is world-executable but not
   world-readable; this means a random user can get to files and
   directories beneath it, but only by knowing their exact names (the
   directory cannot be listed).
   
   It's important to remember that read, write, or execute permission on
   a directory is independent of the permissions on the files and
   directories beneath. In particular, write access on a directory means
   you can create new files or delete existing files there, but ity does
   not automatically give you write access to existing files.
   
   Finally, let's look at the permissions of the login program itself.
snark:~$ ls -l /bin/login
-rwsr-xr-x   1 root     bin         20164 Apr 17 12:57 /bin/login

   This has the permissions we'd expect for a system command -- except
   for that 's' where the owner-execute bit ought to be. This is the
   visible manifestation of a special permission called the `set-user-id'
   or setuid bit.
   
   The setuid bit is normally attached to programs that need to give
   ordinary users the privileges of root, but in a controlled way. When
   it is set on an executable program, you get the privileges of the
   owner of that program file while the program is running on your
   behalf, whether or not they match your own.
   
   Like the root account itself, setuid programs are useful but
   dangerous. Anyone who can subvert or modify a setuid program owned by
   root can use it to spawn a shell with root privileges. For this
   reason, opening a file to write it automatically turns off its setuid
   bit on most Unixes. Many attacks on Unix security try to exploit bugs
   in setuid programs in order to subvert them. Security-conscious system
   administrators are therefore extra-careful about these programs and
   relucutant to install new ones.
   
   There are a couple of important details we glossed over when
   discussing permissions above; namely, how the owning group and
   permissions are assigned when a file or directory is first created.
   The group is an issue because users can be members of multiple groups,
   but one of them (specified in the user's /etc/passwd entry) is the
   user's default group and will normally own files created by the user.
   
   The story with initial permission bits is a little more complicated. A
   program that creates a file will normally specify the permissions it
   is to start with. But these will be modified by a variable in the
   user's environment called the umask. The umask specifies which
   permission bits to turn off when creating a file; the most common
   value, and the default on most systems, is -------w- or 002, which
   turns off the world-write bit. See the documentation of the umask
   command on your shell's manual page for details.
   
   Initial directory group is also a bit complicated. On some Unixes a
   new directory gets the default group of the creating user (this in the
   System V convention); on others, it gets the owning group of the
   parent directory in which it's created (this is the BSD convention).
   On some modern Unixes, including Linux, the latter behavior can be
   selected by setting the set-group-ID on the directory (chmod g+s).
   
   There is a useful discussion of file permissions in Eric
   Goebelbecker's article [49]Take Command.
     _________________________________________________________________
   
12.2. 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 not 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...).
     _________________________________________________________________
   
13. How do computer languages work?

   We've already discussed [50]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.
     _________________________________________________________________
   
13.1. 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++.
     _________________________________________________________________
   
13.2. 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.
     _________________________________________________________________
   
13.3. 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 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, Perl, and Java.
     _________________________________________________________________
   
14. 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://metalab.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 metalab.unc.edu.
     _________________________________________________________________
   
14.1. 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 metalab.unc.edu
   (`host' is short for `host machine' or `network host'; metalab.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
   metalab.unc.edu, but this usually happens very quickly (as in less
   than a second).
   
   The nameserver will tell your browser that Metalab's IP address is
   152.2.22.81; knowing this, your machine will be able to exchange bits
   with metalab directly.
     _________________________________________________________________
   
14.2. Packets and routers

   What the browser wants to do is send a command to the Web server on
   Metalab 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 an
   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.
     _________________________________________________________________
   
14.3. 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 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.
     _________________________________________________________________
   
14.4. 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 metalab.unc.edu's webserver with service
   number 80, it will be 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).

References

   1. Unix-and-Internet-Fundamentals-HOWTO.html#AEN45
   2. Unix-and-Internet-Fundamentals-HOWTO.html#AEN47
   3. Unix-and-Internet-Fundamentals-HOWTO.html#AEN54
   4. Unix-and-Internet-Fundamentals-HOWTO.html#AEN58
   5. Unix-and-Internet-Fundamentals-HOWTO.html#AEN66
   6. Unix-and-Internet-Fundamentals-HOWTO.html#AEN70
   7. Unix-and-Internet-Fundamentals-HOWTO.html#AEN87
   8. Unix-and-Internet-Fundamentals-HOWTO.html#LOGIN
   9. Unix-and-Internet-Fundamentals-HOWTO.html#RUN
  10. Unix-and-Internet-Fundamentals-HOWTO.html#AEN170
  11. Unix-and-Internet-Fundamentals-HOWTO.html#AEN191
  12. Unix-and-Internet-Fundamentals-HOWTO.html#AEN207
  13. Unix-and-Internet-Fundamentals-HOWTO.html#AEN220
  14. Unix-and-Internet-Fundamentals-HOWTO.html#AEN234
  15. Unix-and-Internet-Fundamentals-HOWTO.html#AEN290
  16. Unix-and-Internet-Fundamentals-HOWTO.html#AEN307
  17. Unix-and-Internet-Fundamentals-HOWTO.html#AEN319
  18. Unix-and-Internet-Fundamentals-HOWTO.html#AEN340
  19. Unix-and-Internet-Fundamentals-HOWTO.html#AEN357
  20. Unix-and-Internet-Fundamentals-HOWTO.html#AEN360
  21. Unix-and-Internet-Fundamentals-HOWTO.html#AEN385
  22. Unix-and-Internet-Fundamentals-HOWTO.html#AEN400
  23. Unix-and-Internet-Fundamentals-HOWTO.html#AEN412
  24. Unix-and-Internet-Fundamentals-HOWTO.html#AEN418
  25. Unix-and-Internet-Fundamentals-HOWTO.html#AEN470
  26. Unix-and-Internet-Fundamentals-HOWTO.html#AEN477
  27. Unix-and-Internet-Fundamentals-HOWTO.html#AEN495
  28. Unix-and-Internet-Fundamentals-HOWTO.html#AEN507
  29. Unix-and-Internet-Fundamentals-HOWTO.html#AEN515
  30. Unix-and-Internet-Fundamentals-HOWTO.html#AEN526
  31. Unix-and-Internet-Fundamentals-HOWTO.html#AEN531
  32. Unix-and-Internet-Fundamentals-HOWTO.html#AEN549
  33. Unix-and-Internet-Fundamentals-HOWTO.html#AEN576
  34. Unix-and-Internet-Fundamentals-HOWTO.html#AEN590
  35. http://www.tuxedo.org/~esr/faqs/hacker-howto.html
  36. news:comp.os.linux.help
  37. news:comp.os.linux.announce
  38. news:news.answers
  39. http://metalab.unc.edu/LDP/HOWTO/Unix-Internet-Fundamentals-HOWTO.html
  40. mailto:esr@thyrsus.com
  41. Unix-and-Internet-Fundamentals-HOWTO.html#AEN470
  42. Unix-and-Internet-Fundamentals-HOWTO.html#PERMISSIONS
  43. Unix-and-Internet-Fundamentals-HOWTO.html#PERMISSIONS
  44. http://www.fourmilab.ch/webtools/demoroniser/
  45. http://www.utia.cas.cz/user_data/vs/documents/ISO-8859-X-charsets.html
  46. http://www.unicode.org/
  47. Unix-and-Internet-Fundamentals-HOWTO.html#VM
  48. Unix-and-Internet-Fundamentals-HOWTO.html#LOGIN
  49. http://www2.linuxjournal.com/cgi-bin/frames.pl/lj-issues/issue21/tc21.html
  50. Unix-and-Internet-Fundamentals-HOWTO.html#RUN
