Basic network Concepts

1
Basic network Concepts

• Instructors
• Fu-Chiung Cheng
• (???)
• Associate Professor
• Computer Science Engineering
• Tatung University

2
Contents

• Networks
• Layers
• TCP/IP
• Packet format

3
Networks

• Network a collection of computers and other
  devices that can send data to and receive data
  from each other.
• Each machine on a network is called a node
• Nodes that are fully functional computers are
  also called hosts
• Each network node has an address

4
Internet network of networks
net 3
G
net 1
G
G
G
net 5
net 2
net 4
G
G gateway
5
Layers of a Network

• Networking is complex
• Networking is divided into several layers
• Each layer represents a different level of
  abstraction between the physical hardware and the
  information to be transmitted
• Layering the grouping of the communication
  functions into related and manageable sets
• Network architecture a set of protocols that
  specify how every layer is to function

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Layers of a Network

• Benefits of layering
• Simplifying he design process
• Leading to flexibility in modifying and
  developing the network
• There are several different layer models.
• OSI seven-layer model
• TCP/IP
• We focus on standard TCP/IP four-layer model.

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The OSI reference model

• There was pressure in the 1970s for an open
  systems architecture.
• International Organization for Standardization
  (ISO) developed a reference model for open system
  interconnect (OSI) and later to develop
  associated standard protocols.
• The OSI reference model provided a framework for
  the overall communications process and was
  intended to facilitate the development of
  standards.

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The OSI reference model

• The OSI model partitions the overall
  communication process into functions that are
  carried out by various layers.
• In each layer a process on one machine carries
  out a conversation with a peer process on the
  other machine.
• The processes at layer n are referred to as layer
  n entities.

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The OSI reference model

• The layer n1 entities make use of the services
  provided by layer n.
• Layer n1 passes a block of information to layer
  n through a software port called the layer n
  service access point (SAP).
• This block of information consists of control
  information and a layer n SDU , which is the
  layer n1 PDU.
• The layer n entity uses the control information
  to form the header of the layer n PDU.
• protocol data units (PDUs) header service
  data unit (SDU).

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Layer Services
n1 entity
n1 entity
n-SDU
n-SDU
n-SAP
n-SAP
n-SDU
H
n entity
n entity
n-SDU
H
n-PDU
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Application A
Application B
Application Layer
Application Layer
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
Communication Network
Network Layer
Network Layer
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
Physical Layer
Physical Layer
Electrical and/or Optical Signals
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PDUs in different layers
13
TCP/IP network architecture
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TCP/IP network architecture

• The TCP/IP architecture consists of four layers.
• TCP/IP model does not require strict layering.
• The application layer may bypass intermediate
  layers.
• Two basic types of services in the transport
  layer
• TCP (Transmission Control Protocol) reliable
  connection-oriented transfer
• UDP (User Datagram Protocol) best-effort
  connectionless transfer

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TCP/IP Layers

• Application layer
• web application sends a request to a web server
• Transport Layer TCP/UPD
• Break up the request into TCP segments,
• Add sequence numbers, checksum (Pass to IP)
• Internet Layer IP
• Fragment the segments into IP datagrams of
  necessary size for the local network
• Pass them to host-to-network layer
• Host-to-Network Layer
• Encodes the digital data as analog signals
• Send the request out of wire

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The 4 layer of TCP/IP software
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Layering in a TCP/IP internet
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Application Layer

• From the users point of view, the Internet
  appears to consists of a set of application
  programs that carry out useful communication
  tasks.
• The most popular Internet application services
  include
• WWW
• E-mail
• File transfer
• Remote login

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Transport Layer TCP/UPD

• There is no guarantee that datagrams will be
  delivered based on TCP/IP.
• Even datagrams may be delivered, they may have
  been corrupted in transit.
• Even datagrams arrive uncorrupted, they do not
  necessarily arrive in the order in which they are
  sent.
• Transport layer is responsible for ensuring that
  packets are received in the order they were sent
  and making sure that no data is lost or
  corrupted.

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Transport Layer TCP/UPD

• There are two primary protocols
• TCP (transmission control protocol)
• UDP (User Datagram Protocol)
• TCP (reliable protocol)
• High-overhead protocol that allows for
  transmission of lost or corrupted data and
  delivery of bytes in the order they were sent
• UDP (unreliable protocol)
• Allows the receiver to detect corrupted packages
  but does not guarantee that packets are delivered
  in the correct order
• Much faster than TCP

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Internet Layer

• OSI model network layer
• Network layer
• Define how bits and bytes of data are organized
  into larger groups called packets
• Define addressing scheme by which different
  machines can find each other
• Internet protocol (IP protocol) is the most
  widely used network layer protocol in the world.
• Other protocols IPX (NetWare), AppleTalk (Mac.),
  NetBEUI (Windows)

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Internet Layer

• Datagrams packets sent across internet
• IP datagram
• header 2060 bytes
• Data up to 65515
• In practice a few dozen byte to 8K
• At the network level, an internet provides two
  broad types of services that all application
  program use.
• Connectionless packet delivery service
• Reliable stream transport service

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Host-to-Network Layer

• Hardware
• OSI model (physical link layers)
• Physical layer is alanlog
• Bits and bytes are digital
• Digital-to-analog conversion on senders
• Analog-to-digital conversion on receivers
• Link layer
• Error correction and redundancy
• Real analog systems have noise

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Protocols

• Protocols provide the syntactic and semantic
  rules for communications.
• the details of message formats
• how a computer responds when a message arrives
• how a computer handles errors or abnormal
  conditions.
• Protocols are to communication what algorithms
  are to computation.
• Protocols allows one to understand data
  communication without depending on detailed
  knowledge of a particular vendors network
  hardware.

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TCP/IP Protocols
HTTP
SMTP
RTP
DNS
TCP
UDP
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An example of an internet
(2,1)
(1,1)
(2,2)
router
s
PPP
(1,3) r
w
Ethernet
(1,2)
Server
PC
HTTP
HTTP
TCP
TCP
Router
IP
IP
IP
Net Interface
Net Interface
Net Interface
Ethernet
PPP
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Two important boundaries
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Internet Standard

• The IETF (Internet Engineering Task Force)
  concentrate on short-term or medium-term
  engineering problems.
• The IRTF (Internet Research Task Force)
  coordinates research activities related to TCP/IP
  protocols or internet architecture in general.
• IETF RFCs (Request for Comments)
• page 4245
• Other group W3C (http, HTML, XML)

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Internet Standard
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Requirements for Success of a Service
Will it inter-operate?
Can it be built?
Technology
standards
Market
Regulation
Will it sell?
Is it allowed?
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How are networks interconnected?

• To have a viable internet, we need special
  computers that are willing to transfer packets
  from one network to another.
• These computers are called internet gateways or
  internet routers.

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The users view
33
The physical structure
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IP address

• To provide universal communication service, it
  needs a globally accepted method of identifying
  each computer that attached to it.
• Host identifiers are classified as
• names what an object is
• addresses where it is
• routes how rot get there
• Compact, binary addresses are chosen as the
  TCP/IP universal host identifiers.
• This make computations such as the selection of a
  route efficient.

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IP address

• Each host on a TCP/IP internet is assigned a
  unique 32-bit internet address that is used in
  all communication with that host.
• Each address is a pair (netid, hostid).
• netid identifies a network
• hostid identifies a host on that network
• IP addresses do not specify an individual
  computer, but a connection to a network.

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Addressing Scheme
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IP address

• IP addresses are written as four decimal integer
  separated by decimal points, where each integer
  gives the value of one octet of the IP address.
• 10000000 00001010 00000010 00011110 is written
• 128.10.2.30
• Most TCP/IP software that displays or requires a
  human to enter an IP address uses dotted decimal
  notation.

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Addressing Scheme

• The network prefix 127.0.0.0 is reserved for
  loopback, and is intended for use in testing
  TCP/IP and for inter-process communication on the
  local computer.
• A host or router should never propagate routing
  or reachability for network number 127.

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Special address conventions
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Internet addressing authority

• Originally, the Internet Assigned Number
  Authority (IANA) had control over numbers
  assigned, and set the policy.
• In late 1998, the Internet Corporation For
  Assigned Names and Numbers (ICANN) sets policy
  and assigns values for name and other constants
  used in protocols as well as address.

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Internet addressing authority

• Only the largest ISPs need to contact ICANN.
• Once an organization obtains a prefix for a
  network, the organization can choose how to
  assign a unique suffix to each host on the
  network without contacting the central authority.

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Network byte order

• The internet standard specifies that integers are
  sent with the most significant byte first (i.e.,
  big endian).
• Computers using Intel microprocessors are based
  on little-endian system.

43
Connectionless delivery system

• The packet delivery service is an unreliable,
  best-effort, connectionless service.
• The protocol that defines the unreliable,
  connectionless delivery mechanism is called the
  Internet Protocol, or IP.

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Connectionless delivery system

• Unreliable
• Delivery is not guaranteed.
• The packet may be lost, duplicated, delayed, or
  delivered out of order.
• Best-effort
• The internet software makes an earnest attempt to
  deliver packets.
• Unreliability arises only when resources are
  exhausted or underlying networks fail.
• Connectionless
• Each packet is treated independently from all
  others.

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IP Protocol

• IP provides three definitions
• IP specifies the exact format of all data as it
  passes across the internet.
• IP software performs the routing function.
• IP includes a set of rules that embody the idea
  of unreliable packet delivery.
• A TCP/IP internet is sometimes called an IP-based
  technology.

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Internet Datagram

• The internet calls its basic transfer unit an
  Internet datagram, IP datagram, or datagram.

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Internet Datagram

• Page 26 IPv4 (32 bits for IP address)
• IPv6 128 bits for IP address

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Protocol version

• All IP software is required to check the 4-bit
  version field before processing a datagram to
  ensure it matches the format the software
  expects.
• If standards change, machines will reject
  datagrams with protocol versions that differ from
  theirs.
• The current IP protocol version is 4.
• IPv4 is often used to denote the current protocol.

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Header length

• The 4-bit header length field gives the datagram
  header length measured in 32-bit words.
• All fields in the header have fixed length except
  for IP OPIONS and corresponding PADDING fields.
• The most common header, which contains no options
  and no padding, measures 20 octets and has a
  header length field equal to 5.

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Total length

• The TOTAL LENGTH field gives the length of IP
  datagram, including header and data.
• The size of data area can be computed by
  subtracting the length of the header from the
  TOTAL LENGTH.
• Because the TOTAL LENGTH field is 16 bits long,
  the maximum possible size of an IP datagram is
  216 or 65,535 octets.
• This may become more important in the future if
  higher speed networks can carry data packets
  larger than 65,535 octets.

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Datagram type of service

• The 8-bit SERVICE TYPE field specifies how the
  datagram should be handled.
• The field was originally divided into five
  subfields
• PRECEDENCE specify datagram precedence, with
  values ranging from 0 through 7
• D bit requests low delay, the T bit requests high
  throughput, and the R bit requests high
  reliability.
• Transport request is a hint to the routing
  algorithms, not as a demand.

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Fragmentation control
53
An example of fragmentation
Frame 1
Frame 2
Frame 3
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Fragmentation control

• Three fields in the datagram header control
  fragmentation and reassembly of datagrams.
• IDENTIFCATION
• Computers sending IP datagrams must generate a
  unique value for the IDENTIFCATION field for each
  datagram.

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Fragmentation control

• FLAGS
• Setting the do not fragment bit to 1 specifies
  that the datagram should not be fragmented.
• The more fragment bit specifies whether the
  fragment contains data from the middle of the
  original datagram or from the end.
• We need this bit because the TOTAL LENGTH field
  refers to the size of the fragment.
• FRAGMENT OFFSET
• This field specifies the offset in the original
  datagram of the data being carried in the
  fragment, measured in units of 8 octets, starting
  at offset zero.

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Time to Live

• Routers and hosts must decrement the TIME TO LIVE
  field by one and remove the datagram from the
  internet when its time expires.
• In practice, the TTL acts a hop limit rather
  than an estimate of delays.
• Two uses
• It guarantees that datagrams cannot travel around
  an internet forever.
• Source might want to intentionally limit the
  journey of the packet.

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Other datagram header fields

• Field PROTOCOL specifies which high-level
  protocol was used to create the message carried
  in the DATA area of the datagram.
• Field HEADER CHECKSUM ensures integrity of header
  values.
• Field SOURCE IP ADDRESS and DESTINATION IP
  ADDRESS contains the 32-bit IP addresses.
• Field PADDING contains zeros that may be needed
  to ensure the datagram header extends to an exact
  multiple of 32 bits.

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IP checksum

• This is formed by treating the header as a
  sequence of 16-bit integers, adding them together
  using ones complement arithmetic, an then taking
  the ones complement of the result.

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Internet datagram options

• Options are included primarily for network
  testing or debugging.

When the COPY bit is set to 1, the option should
be copied into all fragments.