- ...number.
- The task of allocating numbers to sites
in the Internet has now become so vast, that it is delegated to a
number of organisations around the world - ask your Internet provider
where they get the numbers from if you are interested.
- ...annum.
- This is a
cautious estimate based on the host count made by Mark Lotter of DNS
registered hosts. The true number is much higher than this, but by how
much, no-one can accurately say.
- ...IP6
- It will have version number 6, as opposed to 5 which is one more than
the current IP version, as an experimental realtime protocol called ST is using
5.
- ...agencies.
- There is still a fiery debate raging about charging
policies and mechanisms. The prevailing technical wind is behind the
idea of keeping useage based charging (e.g. connect time or number
of packets sent) to a minimum; perhaps only charging for a premium
service when the network is overloaded/congested. However, the old Public
Network Operator
view that more income is made by charging all the time, is hovering
in the wings, despite the evidence that it costs a huge percentage of
profit to collect such charges, and it discourages new users. We
believe that charging in proportion to the value of data delivered
from Information services is a far more valid approach, and that the
networks really will become much like the roads in terms of cost
recovery.
- ...RFC
- An RFC is a Request For Comments, and is part of the standardisation process of the Internet Engineering Task Force
- ...this!
- Try `` telnet machine smtp''. In fact, for
many information services, the application search or browse protocol
is text based and can be driven by a human typing, albeit rather
obscure commands, rather than a client GUI program! Try this with WWW:
`` telnet www.cs.ucl.ac.uk 80. See later chapters for what to type next
and what comes back though!
- ...RFC822
- RFC822 is the basic standard for internet electronic mail
- ...protocol.
- You can tell it is
third generation as it talks about ``objects'' rather than files.
- ...IP.
- remember: ``on top of'' means ``encapsulated within'' as in
1.3
- ...CRLF
- CRLF is "Carriage Return" (ASCII character 13) followed by "Line Feed" (ASCII character 10)
- ...people.
- This bug is long since fixed. Basically, the finger daemon
had storage for receiving a limited request/command, but could
actually be handed a larger amount of information from the transport
protocol. The extra information would overwrite the stack of the
executing finger server program. An ingenious hacker could exploit
this by sending a finger command carefully constructed with executable
code that carried out his desired misdemeanour. The problem was
exacerbated on many systems where the finger server ran as a special
privileged process (root!), for no particular reason other than
laziness of the designers of the default configuration. Thus the wily
hacker gained access to arbitrary rights on the system.
- ...ITU
- The ITU is International
Telecommunications Union, the body that oversees all national
telecom companies
- ...hyper!
-
hyper comes from the Greek prefix meaning above or
over, and generally means some additional functionality is
present compared with simple text. In this case, that additional
functionality is in two forms: graphics or other media, and
links or references to other pieces of (hyper)-text.
These links are another component of the WWW, called Uniform
Resource Locators.
- ...caching.
- Cache is usually, but not always
pronounced the same way as cash. It is nothing to do with money, or
even ATM, whether ATM stands for Automatic Teller Machine, or
Asynchronous Transfer Mode, or even Another Terrible Mistake.
- ...time
- or a long time if your link is slow and the file is large
- ...TCP
- Transmission Control Protocol - the internet protocol that attempts to achieve a reliable connection over the unreliable internet - see chapters 1 and 2
- ...www.ncsa.uiuc.edu
- www.ncsa.uiuc.edu is the Domain Name System (DNS) name of a computer called www at the ncsa (National Center for Supercomputing Applications) group of uiuc (the University of Illinois at Urbana-Champagne), which is denoted an educational institution. See chapter 1 for more details of DNS
- ...hyper-links
- a hyper-link is a way of linking pieces of information together, so that when a user follows the link, she is taken from one piece of information to another related piece of information
- ...behind
- we tend to think of what we see on screen as being a front. The hard work goes on behind the scenes!
- ...GIF
- GIF stands for Graphical Interchange Format, and is one form a still image can take. GIF images are relatively compact as the data is compressed, so they're quite a good format to use in the Web
- ...from
- URLs used in links can be either absolute - that is they specify the protocol, the machine, the directory and the filename, or they can be relative, and the unspecified parts are assumed to be the same as they are for the page containing the link
- ...SGML
- SGML is an ISO standard, for what that is worth.
- ...fonts.
- Mosaic actually lets you configure which font you want to see for each heading from your Xresources, but this is independent from the actual markup specification - see chapter 4
- ...automatically
- most browsers let you delay image loading if you're working over a slow network
- ...authors.
- Fortunately this book isn't yet equipped with a mouse or a radio-modem, so this is unlikely to work!
- ...typewriter
- typewriter style fonts are fixed width - ie all
that characters are the same width. Book fonts and the default fonts
used by WWW clients such as Mosaic are variable width fonts, so
letters like ``l'' are narrower than letters like ``m''. Generally
variable width fonts are more pleasant to read than fixed width
fonts.
- ...parent
- The parent directory of a directory is the directory above it in the filesystem tree. On a Mac, you may think of the parent folder on a particular folder as being the folder that contains it
- ...browser
- We use the terms
client and browser interchangeably
- ...server
- Embedded images need not necessarily be stored
on the same server as the document they are embedded in
- ...data
- The image will be returned with
an `` image'' content type
- ...audio
- We still speak of a viewer program
even when we're ``viewing'' audio!
- ...#mime#1284>).
- note if you change your
mailcap file, you may need to restart your client program before
it
will recognise the changes.
- ...high).
- Remember: If you want to put
information in
the Web in an immutable way, then using some image or
postscript or similar
approach may be more appropriate.
- ...networking.
- In fact, without Windows, the best
option
for an MS DOS user is to buy dial up Internet access, and use a
line
mode client like lynx on the dial up Internet host. However,
windows tasking and networking are rapidly becoming available making
full access from GUI clients a lot easier
- ...port
- usually port 80
- ...client
- subject to the client passing any
security restrictions the server may have
- ...yourself
- This can often by used to attempt to debug a
misbehaving server. However, some servers don't check their input too
carefully, and it may be possible to crash them by typing incorrect
HTTP commands.
- ...system
- a
proxy server need not have a file system, but most do
- ...links
- or Aliases as they're called
on the Mac
- ...server
- For instance NCSA's HTTPD allows the directory cgi-bin to be defined as a ScriptAlias. MacHTTP allows you to configure filename extensions such as .script to denote executable scripts
- ...encoded
- URL encoding
replaces spaces with ``+'' and encodes other special characters as
``XX'' where XX is the ASCII code for the character in octal
- ...department's
-
This is the Department of Computer Science, University College London,
or UCL CS
- ...provided.
- All together now - there ain't
no Sanity Clause
- ...servers''
- available with
the URL
http://info.cern.ch/hypertext/WWW/Daemon/Overview.html
- ...system
- The Mac File Type could be edited using ResEdit, but this would have to be done manually for every file.
- ...a_user''
- The notation ``~ a_user'' is
often used to denote the home filestore of the user with username
`` a_user'' on Unix systems
- ...MIME
- MIME is the
multipurpose internet mail extensions, and MIME types are how a WWW
server indicated to a WWW client what type of data is being returned
- ...inetd
- inetd is the Intenet Daemon, which has a configuration file that specifies that when an incoming connection arrives on a particular port, the corresponding program will be started up to handle that connection
- ...a_user''
- The notation ``~ a_user'' is used to
denote the home filestore of the user with username `` a_user'' on
Unix systems
- ...CGI
- Common Gateway
Interface
- ...file
- This can be useful in an environment like a university where you don't necessarily trust your own system's users!
- ...before
- like most
programming languages, AppleScript is very unforgiving of typing
mistakes, so if you've not programmed before, but very careful to
ensure you don't miss any characters
- ...cgi-bin
- the
name cgi-bin is actually configurable as a ScriptAlias from the
server's configuration file srm.conf. CGI is the Common Gateway Interface - a standard way of passing
parameters into server scripts so that commands are portable across
different server types
- ...conf''
- The location of the configuration directory is set using the ServerRoot command in the httpd.conf file
- ...htimage
- assuming the configuration file contains an Exec rule specifying that
cgi-bin contains scripts to be executed
- ...do.
- We could have written this in any number of languages, but perl allows us to illustrate the main points with a relatively short program! Perl is freely available on Unix, DOS, and Macintosh systems
- ...shell
- the Bourne shell is the standard command
interpreter on Unix systems
- ...etc
- Very
very paranoid sites are concerned with two more security facets:
Traffic pattern analysis and covert signaling. Traffic pattern
analysis might concern finance houses who would be worried if it was
known which information providers they were gathering information from
in combination, for example, since it might permit others to
trade on this knowledge (impending mergers etc). Covert signaling is a
way of carrying information piggy-backed on legitimate
information. This is very handy for spies getting infomration out of
secure sites.
- ...legally.
- There are some who believe that every
type of Internet access should be billed for on a usage basis. This is
problematic, and in fact, it has been shown that does not maximise
profit. Only the user herself knows how "urgent" a file transfer is.
With very many types of data around, the net can only charge for the
ones it really knows about, like long-distance voice or high quality
video say.
- ...name
- whatever generic means
Jon Crowcroft
Wed May 10 11:46:29 BST 1995