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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 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 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).