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Basic network Concepts

Instructors:

Fu-Chiung Cheng

(鄭福炯 )

Associate Professor

Computer Science & Engineering

Tatung University

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Contents

Networks Layers TCP/IP Packet format

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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

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G

G

G

G

G

net 1

net 2

net 3

net 4

net 5

G = gateway

G

Internet: network of networks

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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 n+1 entities make use of the services provided by layer n.

• Layer n+1 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 n+1 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

n+1entity

n-SAP

n+1entity

n-SAP

n entity n entity

n-SDU

n-SDU

n-SDU

H

H n-SDU

n-PDU

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ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

ApplicationLayer

PresentationLayer

SessionLayer

TransportLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Electrical and/or Optical Signals

Application A Application B

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

Communication Network

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PDUs in different layers

Layer PDU

Transport Segment

Network Packet

Data link control Frame

Physical Bit

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TCP/IP network architecture

ApplicationLayer

TransportLayer

InternetLayer

NetworkInterface

ApplicationLayer

TransportLayer

InternetLayer

NetworkInterface

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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 user’s 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: 20~60 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 vendor’s network hardware.

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TCP/IP Protocols

HTTP SMTP RTP

TCP UDP

IP

Network

Interface 1

Network

Interface 3

Network

Interface 2

DNS

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An example of an internet

(1,1)

Net Interface

IP

TCP

HTTP

Net Interface

IP

Net Interface

IP

TCP

HTTP

Ethernet PPP

Router

routers

(1,2)

w

(2,1)

(1,3) r(2,2)

PPP

Ethernet

Server PC

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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 42~45

• Other group: W3C (http, HTML, XML)

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Internet Standard

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Requirements for Success of a Service

standards

Regulation

Technology

Market

Will it inter-operate? Can it be

built?

Is it allowed?

Will it sell?

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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 user’s view

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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 written128.10.2.30

• 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.

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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

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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 one’s complement arithmetic, an then taking the one’s 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.