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CS403: Online Network Exploration

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Title: CS403: Online Network Exploration


1
CS403 Online Network Exploration
  • The Internet
  • Spring, 2007
  • Modified by Linda Kenney
  • January 22, 2007

2
Defining the Internet
  • What is the Internet?

3
Defining the Internet (cont.)
  • There are two equally valid ways to answer the
    question What is the Internet?

4
Defining the Internet (cont.)
  • One perspective would be an answer based on what
    the Internet is.
  • This answer would describe the physical presence
    of the Internet.
  • This is the answer you might get from a computer
    professional.

5
Defining the Internet (cont.)
  • A more common perspective would be an answer
    focused on what the Internet does.
  • Or, more accurately, what the Internet allows us
    to do.
  • This is the answer youd most likely get from a
    knowledgeable person on the street.

6
Defining the Internet (cont.)
  • Well explore both perspectives, but lets start
    with a look at what the Internet allows us to do.

7
Internet services
  • What the Internet allows us to do is determined
    by the services that it provides.
  • When we are using the Internet, we are utilizing
    one or more of these Internet services.
  • World Wide Web allows self-publication in
    electronic formats.
  • E-mail allows directed communication with other
    Internet users.
  • Remote login allows computers to be controlled
    remotely by users on other computers.
  • File transfer allows files to be moved and copied
    between various computers.
  • Network news and mailing lists allow indirect
    communication with other Internet users.

8
Data communications
  • All Internet services involve the transfer of
    data between computers.
  • We call this process data communication.
  • Although we often think of the data being
    communicated between computers, computers are
    simply devices for running software programs.
  • Nearly everything a computer does is done under
    the specific direction of a software program.
  • Therefore, communications typically occur between
    software programs running on different computers.
  • As you are probably aware, there are several
    different groups, companies and individuals
    writing communications software and building
    communications hardware somehow the software and
    hardware from one manufacturer must interoperate
    with software and hardware from other
    manufacturers.

9
An example
  • Consider sending an e-mail as an example.
  • When you send an e-mail to a friend, all you need
    to know is your friends e-mail address.
  • You dont need to know what kind of computer your
    friend has, what operating system its running,
    or even what software program your friend uses
    for e-mail.
  • Somehow the e-mail you are sending will get
    delivered to your friend regardless of all these
    details.
  • And somehow your friends e-mail software will
    allow them to read your message.
  • And somehow any reply they send will make its
    way back to you in a readable format.
  • Interoperability makes this disregard for details
    possible.
  • And the basis of interoperability is having a
    common ground for communications.

10
Protocols
  • The languages that computers speak when
    communicating are called protocols.
  • They are far more specialized, and therefore
    limited, than the natural languages spoken by
    humans.
  • So, functionally, protocols serve the same
    purpose as natural language they establish a
    predetermined basis for communication.
  • But practically they are far more specialized,
    dealing only with a narrow range of carefully
    determined possibilities.
  • Protocols determine exactly what can be said
    and how to say it.

11
Protocols and services
  • All Internet services require communications and
    all computer communications require protocols.
  • Therefore, each Internet service is based upon
    one or more protocols.
  • World Wide Web Hypertext Transport Protocol
    (HTTP)
  • E-mail (sending) Simple Mail Transfer Protocol
    (SMTP )
  • Email (receiving) Post Office Protocol
    (POP3)and Internet Message Access Protocol (IMAP)
  • Remote login Telnet
  • File transfer File Transfer Protocol (FTP)
  • Network news Network News Transfer Protocol
    (NNTP)
  • As long as hardware or software is designed to
    follow a protocol exactly, it should interoperate
    with other hardware and software based on the
    same protocol.

12
WWW Popular Activities
  • What do you do on the web?

13
WWW popular activities
  • From Internet Effectively A Beginners Guide to
    the WWW
  • by Tyrone Adams and Sharon Scollard

14
Examples of Internet services
  • http//www.rootsweb.com/jfuller/gen_mail.html
  • http//www.freecycle.org/
  • http//www.quiltart.com/subscribe_info.html
  • http//www.ftpplanet.com/cgi/_listreview/listrevie
    w.pl?list_pathShareware_FTP

15
Future Internet Web Trends
  • Continued importance of E-Commerce
  • Wireless Web access
  • Blogs
  • Podcasting
  • Wikis
  • RSS (Really Simple Syndication or Rich Site
    Summary)
  • Constant Change!

16
The physical Internet
  • Having covered the basics of what the Internet
    allows us to do, lets turn now to an examination
    of what the Internet is.
  • Stated most simply, the Internet is a
    globe-spanning collection of interconnected
    networks.
  • Or stated in other words, the Internet is a
    network of networks.

17
Network basics
  • A computer network is two or more computers
    connected together so that they can communicate.

18
Network basics (cont.)
  • Computers connected to a network are commonly
    called hosts.
  • The connection that carries data among hosts is
    called the transmission medium.

19
Growth of Internet (an aside)
  • Hobbes Internet Timeline
  • http//www.zakon.org/robert/internet/timeline/

Year 1969 1989 1992 1995 2001 2002 2003 2005
Host Computers 4
100,000 1,000,000 8,000,000
109,000,000 147,000,000 171,600,000 353,000,000

20
Network basics (cont.)
  • All the hosts on the network must somehow share
    the transmission medium.
  • Only one data transfer can take place at a time
    on the transmission medium.
  • This means that if host A is transferring data to
    host B, and host C needs to transfer data to host
    D, host C must wait until host A is done before
    it begins its transfer.
  • If host A is transferring a lot of data to host
    B, host C could end up waiting a long time.
  • For this reason, its not desirable to give host
    A exclusive access to the transmission medium
    until its done with its transfer.
  • Instead networks use a system to ensure that
    access to the transmission medium is shared
    equally among all connected hosts.
  • The mechanism used is called packet switching.

21
Packet switching
  • Packet switching avoids network delays by
    ensuring that access to the transmission medium
    is shared equally by all hosts.
  • Each host that has data to transfer is given a
    turn.
  • During its turn, each host gets exclusive access
    to the transmission medium.
  • However, the total amount of data its allowed to
    transfer during its turn is strictly limited.
  • The largest amount of data a single host is
    allowed to transfer during one turn determines
    the size of a single packet.
  • Data that is bigger than the packet size for the
    network must be broken up into multiple packets.
  • If a host needs to send multiple packets, it must
    wait for later turns to send the others.
  • No host is allowed to send more than one packet
    per turn.

22
Packet switching (cont.)
  • Every packet is labeled with the identity of the
    host that sent it and the identity of the host
    for which its destined.
  • Every host on a given network must therefore have
    a unique identity.
  • Host identities are called addresses.
  • Every network must have rules to determine how
    these identities are assigned.
  • And every packet must be constructed and labeled
    according to a common set of rules as well.
  • When we need common rules for computers to
    follow, we define protocols.

23
Packet Example
  • As an example, let's look at how an e-mail
    message might get broken into packets. Let's say
    that you send an e-mail to a friend. The e-mail
    is about 3,500 bits (3.5 kilobits) in size. The
    network you send it over uses fixed-length
    packets of 1,024 bits (1 kilobit). The header of
    each packet is 96 bits long and the trailer is 32
    bits long, leaving 896 bits for the payload
    (actual message). To break the 3,500 bits of
    message into packets, you will need four packets
    (divide 3,500 by 896). Three packets will contain
    896 bits of payload and the fourth will have 812
    bits. Here is what one of the four packets would
    contain
  • From http//computer.howstuffworks.com/question525
    .htm

24
Packet Example (cont.)
  • From http//computer.howstuffworks.com/question525
    .htm

25
Data transfer protocols
  • Each host on a network must be able to
    communicate with all the others.
  • This means they must all follow a common protocol
    in order to transfer data across the network.
  • These data transfer protocols set rules for how
    hosts are assigned addresses, how packets are
    formed and labeled, and all other aspects of a
    networks operations.
  • The protocol is generally implemented in the
    hardware that connects each host to the
    transmission medium.
  • This hardware is commonly referred to as a
    Network Interface Card (NIC).
  • All hosts on a single network must therefore be
    using compatible NICs that implement a single
    networking protocol.
  • We will discuss some of the specific data
    transfer protocols in use today a little bit
    later.

26
Types of networks
  • There are several different types of networks.
  • Each has its own protocol and other
    distinguishing characteristics.
  • But all networks in widespread use today are
    fundamentally packet switching networks.
  • To understand the structure of the Internet, we
    need to understand the different types of
    networks that comprise it.
  • Several types of networks exist in order to meet
    a variety of needs and budgets.
  • At their simplest, networks can be divided into
    two broad categories.
  • Local area networks (LANs) are intended to
    connect many hosts over relatively short
    distances.
  • Wide area networks (WANs) are intended to connect
    relatively few hosts over very long distances.

27
Local area networks
  • The vast majority of the networks we encounter
    directly in offices, dorm rooms and computer
    clusters are LANs.
  • These networks support large numbers of connected
    hosts and emphasize performance.
  • But to do so, they enforce strict limits on the
    maximum length they are able to span.
  • Different types of LAN have different maximum
    lengths, but they are generally limited to less
    than 1 mile.
  • Consequently, LANs are in widespread use.
  • LANs have become very affordable and they are
    therefore used whenever practical.

28
Wide area networks
  • Most of us use WANs every day, but we seldom
    encounter them directly.
  • They move data behind the scenes among smaller
    numbers of hosts over virtually unlimited
    distances.
  • Since they connect fewer hosts and cover much
    longer distances, WANs generally are much more
    costly to establish and maintain than LANs.
  • WANs are used only when a LAN will not work due
    to distance limitations.

29
Wide area networks (cont.)
  • As with LANs, all hosts on a WAN must adhere to
    the same protocol.
  • And the protocols used by WANs are different from
    those used on LANs.
  • In fact, there are several mutually incompatible
    WAN protocols.
  • So hosts on one WAN cannot communicate directly
    with hosts on another WAN.
  • Nor can hosts on a WAN communicate directly with
    hosts on a LAN.
  • Each network is inherently self-contained and
    communication is limited to the hosts on that
    network.
  • Well soon see, however, that there are ways to
    overcome this limitation.

30
Data transfer protocols for LANs
  • There are also several different protocols to
    govern data transfer on LANs, designed to meet
    different needs and budgets.
  • Ethernet is by far the most popular protocol in
    use today.
  • TokenRing is an alternative protocol that has
    become less popular in recent years.
  • Fiber Distributed Data Interface (FDDI) is a
    high-performance protocol for special purpose
    LANs.
  • Asynchronous Transfer Mode (ATM) is a very
    flexible protocol that carries more than just
    computer data.
  • These are just a few of the many LAN protocols in
    existence.
  • And for the most part they are mutually
    incompatible.
  • Which means that TokenRing NICs will only work on
    a network with other TokenRing NICs.
  • They cannot be intermixed on the same network as
    Ethernet NICs.

31
Topology
  • Each LAN protocol is designed to operate on a
    network of a specific shape
  • This shape is called the networks topology
  • Different shapes require different considerations
  • This is one of many reasons why different LAN
    protocols are incompatible
  • The three most common topologies are bus, ring
    and star

32
Transmission media
  • Many LAN protocols are designed to operate over
    specific types of transmission media.
  • As with topologies, different media require
    different considerations.
  • And this, in turn, results in incompatibilities
    beyond the obvious physical mismatches.
  • In general, transmission media are divided into
    physical and wireless categories
  • Common physical transmission media used in LANs
    include twisted pair wire, coaxial cable and
    optical fibers.
  • WANs commonly use optical fiber or leased phone
    lines.
  • Common wireless transmission media used in LANs
    include radio and infrared waves.
  • WANs might use radio with satellites or
    microwaves with earth-based receptors.

33
Speed, bandwidth and throughput
  • The relative performance of different networks is
    a very important consideration.
  • Performance can be influenced by a number of
    factors, including the protocol, the topology,
    and the transmission media being used.
  • In addition, there are different types of
    performance to consider
  • Speed is analogous to the speed limit on a
    freeway.
  • Assuming all cars always travel at the speed
    limit, that limit determines the time it takes
    any given car to get from point A to point B.
  • Bandwidth is analogous to the number of lanes in
    a freeway.
  • The number of lanes in a freeway determines the
    maximum capacity of that freeway. The number of
    lanes determines how many cars can travel between
    point A and point B in a given time period.
  • Most commonly, network performance is discussed
    in terms of throughput
  • Throughput is closely related to speed and
    bandwidth, but is a more practical measure of how
    much data can be moved from point A to point B in
    a given amount of time.
  • All three are measured using the same units
    kbps, Mbps, Gbps

34
Network incompatibility
  • There are many sources of incompatibilities among
    different types of networks.
  • Different topologies and transmission media are
    just two examples.
  • As long as your needs remain modest enough for a
    single network, these incompatibilities do not
    present a problem.
  • However, if you have a growing company, at some
    point you are likely to exceed the maximum number
    of hosts a single network can support or the
    maximum distance one can span.
  • At this point, the incompatibilities will become
    an issue.
  • How do you expand your business with more hosts
    or greater distances?
  • Generally, by adding additional networks
  • When you have an organization with multiple
    networks, you need to devise some way to connect
    those networks together otherwise, some employees
    cannot communicate with some of their coworkers

35
Interconnected networks
  • When two or more networks are connected together
    they form what is called an interconnected
    network.
  • This phrase is commonly shortened to internet.
  • Note that the lowercase i is used because its
    a common noun.
  • To connect two networks together, all we need is
    a host with a separate NIC for each network.
  • And some software to tell that host how and when
    to transfer data from one network to the other.
  • If this host sits between two networks using the
    same protocol, it is considered a bridge.
  • If it sits between two networks using different
    protocols, it is considered a gateway.
  • For our purposes, bridges and gateways are
    important only insofar as they function as
    routers.
  • A router is a host connected to two or more
    networks that interconnects those networks and
    routes data between them.
  • Routers are the glue that holds the various
    networks in an internet together.

36
Advantages of internets
  • Using routers to interconnect various networks
    greatly expands the range of possibilities.
  • We can use a LAN when we have to connect several
    hosts in a concentrated area.
  • If we have more hosts than a single LAN can
    support, we can interconnect multiple LANs.
  • If we have different groups of hosts with
    different communications needs, we can
    interconnect LANs of various types.
  • If we have hosts that need to be connected over
    long distances we can use a WAN.
  • If we have clusters of hosts scattered over a
    wide area we can use one or more LANs at each
    clustered site and one or more WANs to connect
    the sites.
  • With internets at our disposal, our
    communications are no longer limited by distance
    or the number of hosts .

37
The Internet defined
  • So whats the difference between an internet
    and the Internet?
  • Really, the difference is nothing more than one
    of scale.
  • The Internet (we use an uppercase I to indicate
    that its a proper noun) is nothing more than a
    really, really big internet.
  • Like any other internet, the Internet relies on
    routers to connect thousands of individual LANs
    and WANs located all over the planet.
  • By connecting those LANs and WANs together, the
    Internet effectively connects the millions of
    hosts that are in turn connected to those
    individual networks.

38
Redundant connections
  • As internets grow in scale and complexity, its
    not unusual for redundant connections to be
    incorporated into them.
  • Redundant connections allow multiple paths
    between various networks within the internet
  • This increases flexibility and reliability
  • The net effect creates a series of paths similar
    to the roads on a map
  • Looking at most maps there are likely to be a
    variety of routes one might choose to get from
    point A to point B
  • Note that a given path may involve a variety of
    network types, each with its own protocol
  • Theres no way to predict which path your data
    will take
  • And even if there were, it would be impractical
    to require each host to understand the protocols
    of every conceivable type of network
  • In effect, data can get to any place on an
    internet, but theres no way to predict what it
    will encounter along the way

39
Combining packet switching networks
  • Recall that on a packet switching network, each
    host is limited to sending only one packet per
    turn.
  • And the data transfer protocol for that specific
    network determines the maximum size allowed for
    that packet.
  • If theres no way to predict what types of
    networks our data will need to cross as it
    traverses an internet, we should send that data
    in a form thats likely to work with the packet
    sizes of all the networks it may need to cross.
  • This works well, since each small packet travels
    independently across the internet.
  • Small, independent packets are more flexible in
    how they travel and interact.
  • Much like cars on highways are more flexible than
    trains on tracks.
  • Cars will generally make more efficient use of a
    roads overall capacity.
  • Trains spend a fair percentage of their time
    waiting for other trains to clear the track.
  • If a packet doesnt reach its intended
    destination, its easy to replace just that
    packet.
  • If we were trying to send all the data as a
    single large chunk, wed need to replace the
    whole thing every time something happened.
  • In packet switching internets, the routers that
    interconnect the networks act like switches,
    directing the packets onto the next leg of their
    journey.

40
The need for a virtual network
  • Another consequence of interconnecting networks
    of various types is that its impractical for
    each host to know the protocols used by every
    network involved.
  • Ideally, it should not matter what type of
    network each host is connected to on an internet.
  • What we want is for the hosts to behave as if
    theyre on a single network even though they are
    not.
  • In computer science, when we want to behave as if
    something exists even though it doesnt, we use
    the term virtual to describe it.
  • Therefore, in this case, what we want is for our
    internet to function as a virtual network even
    though in reality it is a collection of several
    different networks.
  • With a virtual network in place, all hosts
    connected to it should behave as if they are
    connected to the same network.

41
The virtual network illustrated
  • What we know is there

42
The virtual network illustrated
  • What we wish was there

43
Creating a virtual network
  • The essence of a virtual network is that all
    hosts connected to it can communicate equally,
    just as they would on an actual network.
  • We now know that when we want hosts to
    communicate, we need them all to play by the same
    rules.
  • And when we want a bunch of hosts to play by the
    same rules, we define those rules formally as a
    protocol.

44
Creating a virtual network (cont.)
  • Therefore, what we need to establish a virtual
    network on an internet is a protocol that can be
    followed by all the hosts on that internet
    without violating the protocol of their specific
    network.
  • In other words, our virtual network protocol
    should be applicable to a host on an Ethernet
    network in such a way that that host can still
    follow the Ethernet protocol (Since it will still
    need to use the Ethernet protocol to connect to
    its network).
  • Likewise, for FDDI, TokenRing, ATM and others.

45
Internet Protocol
  • The protocol that is used to turn the whole
    Internet into a single virtual network is called
    the Internet Protocol (IP).
  • This protocol performs three primary tasks to
    define the operation of the virtual network.
  • IP defines the rules for how datagrams are
    formed.
  • IP defines the rules for how routers function to
    move the datagrams toward their destinations.
  • IP defines the rules for how hosts on the network
    are uniquely identified.
  • In general, IP is not implemented in hardware
    like NICs instead it is implemented in software.
  • All hosts connected to the Internet must have IP
    software.
  • Thats what allows the Internet to function as a
    single, enormous virtual network.

46
IP datagrams
  • The virtual network created by IP is a packet
    switching network.
  • That simply means that IP requires all hosts to
    break up data into small packets before sending
    it.
  • When these IP packets travel over actual networks
    inside the virtual network, however, they need to
    travel inside the packets dictated by that
    network.
  • For this reason, IP calls its own packets
    datagrams to create a distinction.
  • IP carefully defines exactly how each host should
    build its datagrams.
  • This allows all other hosts on the Internet to
    recognize any and all datagrams they receive.
  • In addition to the data it carries, each packet
    is also labeled with the identity of its source
    host and the identity of its destination host .

47
IP routing
  • In order to travel over the actual networks
    within the virtual network, IP datagrams must be
    placed within packets appropriate to that actual
    network.
  • When they need to move between two networks, IP
    software on the router between those networks
    must extract the IP datagram from the incoming
    packet and insert it into an appropriate outgoing
    packet.
  • Neither of the routers NICs ever has to deal
    with an IP datagram.
  • Instead, each NIC only has to deal with the
    packets it was designed to handle.
  • The IP datagram is wrapped up inside the network
    packet as data.
  • And the NIC doesnt ever need to know anything
    about the data inside a packet.
  • IP defines specific rules for the routers to
    follow when transferring datagrams between
    networks.
  • Since all the routers follow the same rules, they
    are able to work cooperatively without
    communicating among themselves.

48
IP addresses
  • As with any network, hosts on the Internet must
    each have a unique identity.
  • Otherwise, thered be no reliable way to send
    packets to a specific host .
  • Because computers are inherently digital, IP
    defines a system of unique numeric identities for
    hosts .
  • These numeric identities are called IP addresses.
  • Every host on the Internet must have its own
    unique IP address.
  • In order to communicate with the IP software on a
    destination host , the IP software on a source
    host must know the numeric IP address of that
    destination.

49
IP addresses (cont.)
  • Because there are so many computers on the
    Internet, IP addresses must be very large
    numbers.
  • Since its hard to work with very large numbers
    accurately, IP addresses are commonly written
    using dotted quad notation.
  • Dotted quad notation consists of four numbers (in
    the range 0 to 255) separated by dots (or
    periods)
  • For example, 137.177.137.7 is the IP address for
    the cisunix computer named turing

50
IP versions
  • Currently there are two types of Internet
    Protocol (IP) addresses in active use IP version
    4 (IPv4) and IP version 6 (IPv6). IPv4 was
    initially deployed on 1 January 1983 and is still
    the most commonly used version. IPv4 addresses
    are 32-bit numbers often expressed as 4 octets in
    "dotted decimal" notation (for example,
    192.0.32.67). Deployment of the IPv6 protocol
    began in 1999. IPv6 addresses are 128-bit numbers
    and are conventionally expressed using
    hexadecimal strings (for example,
    10800008800200C417A).
  • http//www.iana.org/ipaddress/ip-addresses.htm

51
Assignment of IP addresses
  • Every single computer added to the Internet must
    be given an IP address that is not already in
    use.
  • With millions of computers already on the
    Internet and thousands more being added every
    day, thats a considerable challenge.
  • Its not practical for each individual computer
    user to come up with their own unique IP address.
  • Imagine how many youd need to try before you
    found an unused one!

52
Assignment of IP addresses (cont.)
  • Since virtually all connections to the Internet
    are managed by an organization of some sort
    (school, company, ISP, etc.), its much more
    practical to place each organization in charge of
    its own addresses.
  • When an organization first establishes a
    connection to the Internet, it is assigned an
    address space consisting of the first one, two or
    three values in a dotted quad address.
  • Once assigned such an address space, the
    organization is free to assign the remaining
    three, two or single values as it sees fit to
    each computer it connects to the Internet.
  • No other organization is entitled to create
    addresses within that assigned address space.
  • And the organization that owns a given address
    space is solely responsible for ensuring that no
    address within that space is ever in use by two
    computers at the same time.
  • At UNH, our address space consists of all
    addresses that begin with 132.177.

53
  • Both IPv4 and IPv6 addresses are assigned in a
    delegated manner. Users are assigned IP addresses
    by Internet service providers (ISPs). ISPs obtain
    allocations of IP addresses from a local Internet
    registry (LIR) or national Internet registry
    (NIR), or from their appropriate Regional
    Internet Registry (RIR)
  • http//www.iana.org/ipaddress/ip-addresses.htm

54
How datagrams travel
  • So how does all this fit together to get
    datagrams from their source to their destination?
  • The IP software on the source host creates a
    datagram labeled with the IP address of the
    destination host and gives it to the NIC as data.
  • The NIC on the source host creates a network
    packet that contains the datagram, labels it with
    the network address of the router on its network
    and sends that packet out across the network.
  • The NIC connecting the router to the source
    network receives the incoming network packet,
    extracts the datagram and passes it to the IP
    software on the router.
  • The IP software on the router examines the
    destination IP address in the datagram and passes
    it to whichever of the routers NICs will move it
    closer to that destination.
  • The outgoing NIC on the router wraps the datagram
    in a network packet suitable for the outgoing
    network, labels it either with the network
    address of the destination host or another router
    on that network (if the destination host is not
    on that network) and sends it out on that
    network.
  • If the network packet is sent to another router,
    that router repeats the above three steps, moving
    the datagram on the next hop of its journey
    until it eventually reaches the router that can
    deliver it to the destination host.
  • When the network packet eventually reaches the
    NIC on the destination host, that NIC extracts
    the datagram from the network packet and passes
    it to the IP software running on the destination
    host.

55
Datagrams are mindless
  • In other words, datagrams are mindless and
    passive.
  • They do not actively seek out their destination.
  • In fact, they have no abilities whatsoever.
  • Like letters in the mail, they rely on others to
    move them along on their journey to their
    destination.
  • Letters have an address on the envelope and
    postal employees use that address to move the
    letter progressively closer to its intended
    recipient.
  • Datagrams are labeled with the IP address of
    their destination host and the routers of the
    Internet move the datagram progressively closer
    to its intended destination.
  • And like the postal service, IP is what we call a
    best effort system.

56
IPs shortcomings
  • Like the U.S. Postal Service, IP is remarkably
    good at what it does, but things can go wrong.
  • As with letters sent by first class mail,
    datagrams are not guaranteed to reach their
    destination.
  • The overwhelming majority of them do, but for any
    single datagram theres always a small chance it
    will disappear in transit.

57
IPs shortcomings (cont.)
  • There are a variety of reasons why a datagram may
    disappear en route, but they generally all have
    to do with the fact that IP was designed (like
    the postal service) to favor efficiency over
    reliability.
  • It endeavors to deliver as many datagrams as
    possible as quickly as possible, but if some get
    lost in the process so be it.
  • The most notable obstacle to efficiency on the
    Internet is congestion.
  • If you view datagrams as analogous to cars,
    network congestion would be equivalent to a
    traffic jam on a freeway.
  • The problem with this analogy is that datagrams
    traveling on a network cant stop and sit their
    until the traffic jam clears.
  • Instead, when congestion occurs on an Internet,
    the routers surrounding the congestion begin
    discarding datagrams and/or directing incoming
    datagrams along an alternate route.
  • This can cause the occasional disappearance, or
    dropping, of datagrams.
  • It can also cause datagrams to arrive out of
    order or even in duplicate.
  • Fortunately, theres a protocol that addresses
    this issue.

58
Transmission Control Protocol
  • The protocol that is commonly used with IP to
    provide guaranteed delivery of datagrams is
    Transmission Control Protocol (TCP).
  • TCP was designed in concert with IP.
  • IPs shortcomings are not problems, per se
    they are unavoidable consequences of IPs
    priorities.
  • Sometimes the loss of some packets is not a
    problem.
  • But when it is, TCP can be layered above IP to
    ensure that datagrams arrive at their destination
    in order without loss or duplication.
  • Since TCP is layered over IP, a program wishing
    to send data across the network would pass that
    data to the TCP software and TCP would then pass
    it to IP which would then pass it on to the NIC
    for transmission.
  • TCP stamps each outgoing datagram with a
    sequential number so the TCP software on the
    receiving host can reassemble the data in the
    proper order and identify datagrams that are
    missing or have been duplicated.
  • Any missing datagrams can then be requested from
    and resent by the source host.
  • The TCP software on the source and destination
    host communicate with each other to ensure the
    transmission goes smoothly.
  • This allows the source and destination host to
    carry on a two-way communication quite easily.
  • Because of this communication back and forth, TCP
    is considered a connection-oriented system.

59
Hostnames
  • Having learned the importance of IP addresses,
    its natural to wonder why we dont see them more
    often.
  • Logically, one might think that an IP address
    would be necessary for any communications on an
    internet, be it e-mail, web browsing, etc.
  • We dont see IP addresses very often because
    theyre generally meant for use by software.
  • As humans, we generally dont like to deal with
    large numbers any more than we have to.
  • Instead, we prefer to give things names, and
    hosts are no exception.
  • So in addition to having a unique IP address,
    nearly all hosts on the Internet are also
    assigned a unique hostname.
  • IP addresses are necessary for software, but
    hostnames are more convenient for humans.

60
Assignment of hostnames
  • Since hostnames, like IP addresses, establish
    identities for hosts, they too must be unique
    across the entire Internet.
  • Recall that with IP addresses, an organization is
    assigned a unique address space when it connects
    to the Internet.
  • At the same time, an organization also typically
    obtains a unique domain name.

61
Assignment of hostnames (cont.)
  • Domain names are an organization-specific
    identifier combined with a high-level domain
    identifier indicating the type of organization or
    the country in which its registered.
  • Common high-level domains include .com, .edu,
    .net, .gov, .org, .mil, .int, .aero, .name, .biz,
    .museum, .info, .coop, .pro
  • Country-specific high-level domains include .ca,
    .uk, .de, .jp.
  • For example, the domain name for UNH is unh.edu.
  • UNH is free to name its hosts anything it wishes,
    provided the name is used for only a single
    computer at a time and ends in .unh.edu.
  • http//www.unh.edu/NIS/Docs/Intranet/index.html

62
  • Welcome to the InterNIC Website!
  • This website has been established to provide the
    public information regarding Internet domain name
    registration services and will be updated
    frequently.
  • http//www.internic.net/

63
URL Uniform Resource Locator
  • URL
  • Represents the address of a resource on the
    Internet.

64
The problem with hostnames
  • There are millions of hosts on the Internet, and
    humans prefer to refer to them using hostnames.
  • Yet the software those humans use must have an IP
    address in order to properly address its packets.
  • The dotted notation used for both might initially
    suggest that a simple translation is possible.
  • But this is just a coincidence there is no
    direct correspondence between the numbers in an
    IP address and the components of the associated
    hostname.
  • IP addresses always consist of four numeric
    parts, while hostnames might consist of two,
    three, four, five or more parts.
  • To better understand this problem, lets use
    phone numbers as an analogy.
  • To call someone, you must have their phone
    number.
  • Yet, most of us are far better at remembering
    names than numbers.
  • Fortunately, if we know someones full name, we
    can call directory assistance to get their phone
    number.
  • What we need on the Internet, then, is something
    like directory assistance that software can use.

65
Domain Name Service
  • Domain Name Service (DNS) is the Internets
    equivalent to directory assistance.
  • However, its generally used by software, not
    humans.
  • Humans give host identities to their software as
    hostnames.
  • The software then uses DNS in order to find the
    IP address that matches that hostname.
  • If the software needs to announce an incoming
    communication, it can also use DNS to find the
    hostname that matches the source IP address in
    those datagrams.
  • This keeps both the humans and their software
    happy.
  • When it is first connected to the Internet, a
    host is configured with the IP addresses of one
    or more hosts that will provide it with Domain
    Name Service.
  • For most of us, those are the only IP addresses
    we will ever need to enter.
  • From then on, we need only use hostnames.
  • Any time we do, the software can contact one of
    those DNS servers to find the matching IP
    address.
  • We often call this process resolving the
    address or DNS resolution.

66
The client/server model
  • Note that we used the term DNS server for the
    host that our software uses to resolve addresses.
  • More specifically, we use this term to refer the
    software running on that host that provides the
    services our software requires.
  • In general, we call any software that provides
    services to other software a server.
  • For example, a DNS server is software that
    resolves addresses as a service to other
    software.
  • Any software that utilizes the services of a
    server is generally referred to as a client.
  • For example, any software that uses a DNS server
    to resolve an address is acting as a client when
    it does so.
  • The division of labor between programs acting as
    servers and other programs acting as clients is
    collectively known as the client/server model.
  • Nearly all Internet services you use are services
    provided by servers somewhere on the Internet.
  • And nearly all the programs you use on your host
    to access those services qualify as clients.

67
Summary
  • The rest of this semester will involve learning
    more about various Internet services.
  • Knowing how the Internet works behind the scenes
    will help us better understand those services.
  • Nearly all of the services available on the
    Internet are based on the client/server model.
  • And nearly all of the communications that take
    place between those clients and servers occurs
    using TCP and IP.
  • In fact, the collection of protocols that any
    host connected to the Internet must implement in
    software is collectively known as the TCP/IP
    Protocol Suite.
  • UDP, DNS, HTTP, SMTP and nearly all of the other
    protocols well cover this semester join TCP and
    IP in this collection.
  • The first Internet service we will examine in
    depth is the World Wide Web.

68
Sample Review Questions
  1. Weve discussed two equally valid answers to the
    question What is the Internet? Can you explain
    them?
  2. What are Internet services? What makes them
    possible? Can you give some examples?
  3. What is a protocol and why do they exist? Compare
    and contrast them with natural languages and give
    some examples of each.
  4. What is packet switching and why is it useful?
  5. What is the underlying physical structure of the
    Internet? Why is the Internet structured that
    way?
  6. Can you define the terms data transfer
    protocol, topology, transmission media,
    speed, bandwidth and throughput? Can you
    give some examples of each?
  7. What is the difference between an internet and
    The Internet?
  8. What are the respective roles of the Internet
    Protocol (IP) and Transmission Control Protocol
    (TCP) and how do they fulfill those roles?
  9. Can you explain the concept of a virtual network
    and its importance in the context of the
    Internet?
  10. Explain the different roles played by IP
    addresses and hostnames. How are they assigned?
  11. What is the role of Domain Name Service (DNS)?
  12. Explain the meaning of the client/server model
    and discuss its importance to the Internet. Give
    two examples of its use.

69
Key terms
  • Address
  • Address space
  • ATM
  • Bandwidth
  • Best effort system
  • Bridge
  • Broadcast communications
  • Bus
  • Client
  • Client/server model
  • Coaxial cable
  • Computer network
  • Connection-oriented system
  • Data communication
  • Datagram
  • Data transfer protocol
  • Direct communications
  • DNS
  • DNS resolution

Physical transmission media Protocol Radio
waves Redundant connection Remote
login Ring Router Server SMTP Speed Star TCP Telne
t Throughput TokenRing Topology Transmission
medium Twisted-pair wire UDP Virtual
network WAN Wireless transmission media World
Wide Web
File transfer FTP Gateway High-level
domain Host Hostname HTTP Infrared
waves Interconnected network Internet Internet
service Interoperability IP IP address LAN Layerin
g Mailing list Natural language Network
news NIC NNTP Optical fiber Packet Packet
switching
70
  • Thanks to Mike Gildersleeve for sharing the
    information from his Summer, 2006 CS403
    PowerPoint.
  • Information also used from
  • Web Developer Design Foundations with XHTML by
    Terry Felke-Morris
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