Networking and the Internet 3

1
Networking and the Internet (3)

• Last Week
• Computer Architecture
• Operating System concepts and examples
• Week 3 Focus
• Views of a network (Coope, chapters 12 14)
• Electrical transmission management addressing
  contents
• Data formats (the codes that flow) (Coope,
  chapter 2)
• More on systems for Online Computing
• e-commerce issues what do we need for B2C?
• Introduction to Writing HTML

2
Revision of Task Dispatching

• Remember that every action involves an interrupt
  that returns control to the Operating System
• The queues are OS control blocks

3
Dispatching Tasks

• Lets assume a Ready Queue of tasks waiting to
  run
• Add tasks to queue according to a priority
  pattern (cheapest is FIFO, but we may want to
  improve on that)
• Dispatch the task at the head of the queue
• When it gives up control, dispatch new head of
  queue
• May want to maintain queue of blocked tasks too
• What if task doesnt give up control?
• Need to interrupt it at end of time slice to let
  others run(Windows 3.x didnt do this except for
  DOS tasks)
• Can return it directly to Ready Queue (with risk
  that itll consume too much CPU time)
• Maybe we should favour short interactions over
  long

4
Life Cycle of an Interaction
But how do we know when its finished?
5
Working On-line

• On a PC, you start a program
• Feed it with data e.g. edit your document
• Produce some results saved or printed document
• Quit the program
• While youre editing, you expect the computer..
• To keep the program running, even if you walk
  away
• and Maintain all the information youve entered
• This involves keeping program and data in memory
• Very hard for an Internet Server to match this
• How long should it hold stuff in memory for you?
• What if there are 5,000 users like you?
• Chosen solution is to treat interactions as
  atomic

6
Keeping the Process Alive
User hits ENTER
Maintain page tables to access processs memory,
though itll get paged out
On idle queue
Task Created
User hits ENTER
Nothing to do
Executable Task
Dispatched
Running
Finishes
On Ready Queue
Terminated
Time expires
Enters WAIT
Requeued on Event
Blocked
7
Transactional Interactive Computing

• Lynn Brooks (IBM Poughkeepsie) recognized and
  described two distinct kinds of on-line
  computing
• Transactional
• Composed of separate, atomic actions
• Take an order, handle a billing query
• Very little state to carry between transactions
• Each transaction may use a different program
• Interactive
• Usually lots of interactions with a single
  program, as in editing a document
• Copious state information processs whole
  memory
• Typically heavy on memory, light on processing

8
The Internet is Transactional

• In general
• Every time you interact, browser sends message to
  server
• Server responds by sending you something
• Then it forgets you
• Presents a problem if you want a session
• For example, you sign on to authenticate yourself
• Server then treats you as a privileged user,for
  a whole series of interactions
• Server usually maintains minimum status
  information
• Table of whos signed on, with IP address and
  privilege
• On each request, you send a token for server to
  look up
• How long must it keep your entry in the table?

9
Impact on B2C e-commerce

• Successful Business to Consumer e-commerce needs
  friendly service to the end-user
• Server should recognize the user
• Authenticate when needed, but do it only once per
  session
• Not grind to a halt because servers run out of
  memory
• Example amazon.co.uk
• Stores cookie on your disk when you sign up
• Retrieves it each time you connect in future
• Asks you to confirm that its really you
• Requires user name and password for secure area
  (anything to do with account status or payment)
• Times out if you dont sign out of the secure area

10
Other e-commerce Considerations

• Server performance
• Users need to be able to rely on availability
• With adequate response time
• Not usually a problem for delivering HTML pages
• http daemons in server are tuned to be
  efficient
• Much more serious for transactional work
• Server may have to start a new process for each
  message
• Heavy-duty activity involving address-space
  creationunless it can exploit a pool of
  processes
• Database activity may involve multiple accesses
• User may interact multiple times per business
  activity
• Unless we can send a Java applet for the user to
  run

11
Networking and Communications

• Resolving confusion with concept of layers

12
Networking and Communications

• An area made confusing by range of different
  activities that fit under this heading
• Electrical (or equivalent) signals actually
  transmitted(down twisted-pair, coaxial cable, or
  by radio)
• How the transmission medium is managed(modem,
  network card, hub)
• How we address data to make sure it gets to
  destination
• How we code the information so receiver
  understands it
• How we package information to send it to other
  application(email, screen-shots, file transfer)
• Can resolve confusion by the concept of layers
• Convention is to show electrical signals at bottom

13
Communication Layers

• Seven layer model of SNA and OSI aims to let us
  think about one element of the network at a time
  
• Application layer Services for applications
• Presentation layer Maps data to what is
  displayed
• Session layer Ties multiple flows into logical
  call
• Transport layer Hides network details from above
• Network layer Routing and sequencing of packets
• Data Link layer Flow control and error handling
• Physical layer Point-to-point flows of bits
• Interfaces between layers are well-defined
  protocols

14
Seven Layer Model

• Simplify programming by encapsulating lower levels

Purpose/Examples File transfer show screen
image manage sign-on manage connections route
over several links V.34 , HDLC LAN, digital-,
analogue-circuit
Application Presentation Session Transport Network
Data Link Physical
OSI terms
Internet terms
15
Data Formats

• Computers work on numbers, but human
  communications involve more than numbers
• Need letters, signals, pictures, sounds
• Computer must be able to handle these formats
• Popular character format is ASCII
• 7-bit code representing characters (now 8-bit)
• Each letter or number or punctuation mark is
  coded
• to represent a character, need to know the right
  code
• Network needs to perform conversational actions
  
• Acknowledgement, slow down, what was that?
• Extra ASCII codes were added to handle these
• Also needs to check integrity of data

16
ASCII (examples)

• 32 ltspacegt 48 0 64 _at_
• 33 ! 49 1 65 A
• 34 50 2 66 B
• 35 51 3 67 C
• 36 52 4 68 D
• 37 53 5 69 E
• 38 54 6 70 F

17
Other forms of coding

• Can store integers in two or four bytes
• 2 (half-word) can go from -215 to 215-1 (-32,768
  to 32,767)
• Full-word from -2,147,483,648 to 2,147,483,647
• How about floating point?
• Can also code pure numeric decimal data with
  half a byte per decimal digit
• Main implementation is Packed decimal, for
  example597 would pack as 59 7CX -12345 as
  12 34 5DX
• zSeries (System/390) still uses EBCDIC for
  characters
• Conversion overhead on Mainframe/PC transmissions
• Different collating sequences can cause problems
• Double-byte character sets such as Unicode

18
Double-byte character sets

• Some orthographies use more than 256 characters,
  usually ideograms
• Examples Japanese Kanji, Korean Hangul, Chinese
• May also need room for syllabic characters (e.g.
  Kana)
• and for a range of alphabets
• Problem solved by coding each character in
  2-bytes
• 64K values are enough
• Fortunately, you need fewer characters per
  message
• DBCS can be pure or SISO
• Pure DBCS uses two bytes even for Latin letters
• An example is Unicode
• Alternatively you could Switch In and Switch Out
  of DBCS

19
Other SBCS Encodings

• Even Single-byte coding isnt trivial
• Currency symbols
• Latin alphabet extensions
• National characters (æ ð þ ø )
• diacritical marks (Turkish beats even Czech)
• Greek
• Cyrillic
• Thai
• Some run Right to Left (with L-gtR insertions!)
• Hebrew
• Arabic

20
Non-conforming Data

• What if the data were sending isnt just
  characters?
• Pictures and sounds are a stream of bytes that
  can be close to a random selection of binary
  values
• Binary data might well contain bytes that match
  the ASCII code to terminate the transmission
• One way is to take seven bytes in and write eight
  out
• That way you can avoid the values that give
  trouble
• done in MIME encoding, and in 3270 emulator
  downloads
• This is why e-mail attachments are sometimes
  gibberish
• We may also be sending data coded in a different
  way
• Such as EBCDIC, used on mainframes

21
World Wide Web Publishing

• How to write HTML
• (see references for more detail)

22
Publishing to a Diverse Audience

• Some users have 14 VGA screens
• Others have 21 high-resolution displays
• Users run many different browsers
• Multiple versions of Internet Explorer, FireFox,
  Netscape
• or something youve never heard of!
• Some users have character-based terminals, or
  dont have bandwidth to spare for graphics
• Others have high bandwidth via LAN to server
• How can we make content attractive and yet still
  comprehensible to all these users?
• The trick is to describe the content, not the
  intended layout

23
Resources on the Internet

• Internet is a network of networks, sharing a
  common 4-byte addressing scheme (e.g.
  194.66.202.100)
• Computer resources are collected into Domains,
  which are usually named, e.g. www.winchester.ac.u
  k
• Domain Name Servers map domain to 4-byte address
• Uniform Resource locator (URL) made up of three
  parts
• Service ID (such as http, ftp, mailto, gopher)
  terminated with
• Domain nameport, terminated with /
• Resource within domain
• Most URLs written service//domain/resource(mail
  to does not need the //)

24
The http Service

• HyperText Transfer Protocol is the keystone of
  the World Wide Web.
• Browser transmits request to target domain, where
  http daemon the server responds by sending
  back the requested resource (usually a file)
• Browser then reads file and formats it for
  display
• While doing so, browser will find mark-up
  requesting other resources, and forward them to
  the appropriate server usual example is for
  embedded graphics
• If browser cant handle the resource alone, it
  can
• Use a helper application such as Excel or media
  player
• Offer to save the file to disk for future use

25
Overall Document Structure

• ltHTMLgt
• ltHEADgt
• ltTITLEgtSimple HTML documentlt/TITLEgt
• lt/HEADgt
• ltBODY text"000060"gt (heres hexadecimal again)
• ltH1gtChapter 1lt/H1gt
• ltPgtThere isnt much in this document.
• This sentence will flow after the previous one.
• ltPgtBut this will be a new paragraph.
• lt/BODYgt
• lt/HTMLgt

26
Other Common Tags

• ltHR /gt Produces a horizontal rule on the column
• ltBR /gt Break a empty text element that is
  at the start of a line (you can misuse it to
  be like a paragraph, but more compact )
• ltULgt Starts an unordered list (with bullets)
• ltOLgt Starts an ordered list (usually numbered)
• ltLIgt List item in either sort of list
• (Lists can be nested, so its important to close
  them in reverse sequence)
• ltEMgt Emphasised text (usually italic)
• ltSTRONGgt Strong text (usually bold)

27
Advanced Text-descriptor Tags

• ltDIRgt Starts a Directory list (multi-column)
• ltMENUgt Starts an unordered list without bullets
• ltDLgt Starts a Definition (or Glossary) List This
  contains two kinds of list item
• ltDTgt The Term being defined
• ltDDgt The Description defining that term
• ltPREgt Pre-formatted text (as in an example)
• ltTABLEgt Well build tables later
• Exercise
• Experiment with the various list types. Try
  nesting them.

28
Creating Hyperlinks

• Any hyperlink reference needs to say what it is
  linking to
• In HTML, we use an Anchor tag with the HREF
  attribute
• ltA href"http//www.pdq.edu/freds.htm"gtQuick
  Univlt/Agt
• ltA href./freds.htm"gtLocal referencelt/Agt
• You can reference any resource on the Internet
  Just give its URL
• Any lack of specificity means look locally, so
  missing out the domain means it is in the local
  file system
• Always leave out as much as you can
• Makes it much easier to move the site
• Can also use relative references, e.g. to a
  sub-folder ltA href./subdir/fred2.htm"gtrelative
  referencelt/Agt

29
Internal Bookmarks

• You can also name a target inside a resource
• Like a bookmark in Word
• Also coded on Anchor tag, this time as NAME
  attribute
• ltA name"phred"gttarget of the linklt/Agt
• ltA href"phred"gtClick here to get to targetlt/Agt
• or if the link may be from a different
  document...ltA href"http//www.pdq.edu/freds.htm
  phred"gtClicklt/Agt
• So the full form of the HREF attribute is a URL
  followed by and a named anchor inside the URL
• Missing out the resource means its in this
  document
• If the name is omitted, it refers to the top of a
  resource

30
Graphical Images

• Easy to imbed with the IMG tag
• Need to define the source of the image
• Example ltimg src"fred.jpeg" alt"fred/gt
• Text of alt attribute appears on non-graphical
  displays You can also use attributes to modify
  the image
• ltimg src"/mh.jpg" width"80" alt"Picture of
  Mike"gt
• Image will be 80 dots wide, in original aspect
  ratio(dont set width and height, it can distort
  the image)
• Use align attribute to place it right left or
  center(HTML uses US spelling even though it
  was originally written by an Englishman working
  in Switzerland!)
• Use images in JPEG or GIF format (never BMP!)
• GIF better for line drawings, JPEG better for
  pictures

31
Stick to Relative Addressing

• At some point, well want to move pages and
  images on to a real Web Server
• Can test most things on local disks, providing
  we address directories relative to the current
  page, for example
• lta href"next.htmlgtforward to next pagelt/agt
• meaning switch to next.html in the current
  directory
• ltimg src"../graphics/freddo.jpeg" alt"fred"gt
• meaning go to next directory up, then look in its
  graphics subdirectory
• CGI scripts and Image maps do require a server,
  so cant usually be tested purely from disk

32
HTML Covered So Far

• Overall Document Structure
• ltHTMLgt, ltHEADgt, ltBODYgt tags
• Headings and other text tags
• ltH1gt, ltH2gt..ltH6gt tags ltPgt, ltBRgt, ltEMgt, ltSTRONGgt
• Lists and the items within them
• ltOLgt, ltULgt with ltLIgt
• ltDLgt with ltDTgt and ltDDgt
• Anchors and Hyperlinks
• ltA name"target"gt Text labelled (if any)lt/Agt
• ltA href"where_to_link_to"gt Explanatory textlt/Agt
• In-line graphics
• ltIMG src"path to image" alt"dont forget
  non-graphic user"gt

33
HTML Reference Materials

• Library has some OReilly pocket reference books
  about HTML and the Web (see module handbook for
  references)
• Or you can use one of the primers on the WWW
• Tom Drewrys Guide to HTML tags at
  http//www.cems.uwe.ac.uk/tdrewry/html/index.htm
• Marc Andreessens athttp//www.web-nation.com/les
  sons/html-pri.htm
• http//www.mcli.dist.maricopa.edu/tut/index.html
• http//www.telacommunications.com/nutshell/primer.
  htm
• Web design guidance http//www.winchester.ac.uk/de
  signproject/
• HTMLcard.html local copy (archive of NCSA primer)

34
Writing your own HTML

• See separate document for simple exercises (Week
  3 on the Learning Network)
• Exercise 1 gets you in there looking at the HTML
  source
• Pretty easy to do, providing you think about what
  each element of the document is, rather than what
  it should look like on the screen
• Normal process is to have a Notepad window for
  sourceand a browser window to view the results
• Update source in Notepad and Save
• Flip to browser and Reload to see the effect
• Exercise 2 has you writing your own HTML
• You can re-use stuff from the other files if you
  wish

35
Backing Up

• Reasons for Back-up
• Protect against loss of medium containing main
  data
• Protect against destruction of data itself (e.g.
  Erase file)
• Disaster recovery reconstruction of entire
  system
• Full, Incremental or Journal Strategy
• Full backup is greedy on backup medium (and time)
• Incremental backups have to be restored in order
• Data since last backup will be lost
• Journal approach saves missing any change
• RAID can minimize risk from hardware failure
• Doesnt help you recover a deleted file
• Can avoid need to journal

36
RAID

• Redundant Array of Inexpensive Disks
• Principle is to store the data more than
  once,then if you lose a bit in one place, you
  can use the copy
• Mirroring is a simple approach
• Two copies of data, each with checking
• When one copy fails its check, refresh from other
• If the checks are more complex, you can fix a
  lost bit, even without full duplication (32
  example last week)
• RAID-5 is the most common implementation
• 5 disks holding data that would fit on four of
  them
• RAID controller rebuilds data when you hot-swap a
  disk
• Still wise to back up to tape

37
Tape Back-up

• Head gap limits how small magnetic domain can be
• Small gaps are sensitive to dust and give low
  output
• Fast movement increases output as well as data
  rate
• Two tape types in common use
• Linear (like cassette, DCC) QIC or Travan
  technology
• Helical scan (like video tape) 4mm DAT and 8mm
• Linear machines are cheap, but need expensive
  tape to cope with the low speed of the tape
• DAT tapes are cheap (lt5 for 2GB) but drives are
  pricey