Network+ Chapter 2 Notes

7/28/2019 Network+ Chapter 2 Notes

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CHAPTER 2 OPEN SYSTEMS INTERCONNECTION SPECIFICATION (OSI MODEL)

Seven hierarchal layers developed to enable different networks to communicate reliably betweendisparate systems

Encapsulation the process of encoding data as it goes down the OSI stack OSI (open systems interconnection) reference model was created by ISO to break barrier of

computers only being able to communicate within same manufacturer

Created interoperable network devices and software in the form of PROTOCOLS OSI model is the primary architectural model for networks Describes how data and network information are communicated from computer application

through network media to an application on another computer

The Layered Approach

Reference model conceptual blueprint of how communications should take place Addresses all processes required for effective communication, divides processes into logical

groups (Layers)

A communication system designed as such is called Layered Architecture What tasks must be done? Who will do them? What order will they be done? How will

they relate to one another?

Say you divide tasks into departments (a metaphor for the Layers) in which thedepartment members focus on their duties and no one elses

Each department will need practical methods to complete these duties (methodsbeing a metaphor for Protocols)

Software devs use reference model to understand computer communication processes andthe functions that need to be accomplished on each layer

When developing a protocol for a layer, all they worry about is THAT LAYERs functions Another layer and protocol will handle other functions Binding communication processes are bound to each other at a particular layer

Advantages of Reference Model Allows different vendors networks to interoperate through network component standardization

Also allows various types of hardware/software to communication OSI model divides network communication process into smaller and simpler components

Aids development, design, and troubleshooting Encourages industry standardization by defining functions occurring at each layer of OSI model Prevents changes in one layer from affecting other layers, doesnt hamper development, making

application programming easier

OSI Reference Model

Aiding data transfer between disparate hosts A set of guidelines that developers use to create applications which run on a network Acts as a framework for creating/implementing networking standards, devices, and internetworking

schemes


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7 LAYERS (and their functions) 7: APPLICATION (File, print, message, database, and application services)

Provides a user interface 6: PRESENTATION (Data encryption, compression, and translation services)

Presents Data Handles processing, such as encryption

5: SESSION (Dialog control) Keeps different applications data separate

4: TRANSPORT (End-to-end connection) Provides reliable/unreliable delivery Performs ERROR CORRECTION before retransmission

3: NETWORK (Routing) Provides logical addressing which routers use for path determination

2: DATA LINK (Framing) Combines packets into bytes and bytes into frames Provides access to media devices using MAC addresses Performs error detection, NOT CORRECTION

1: PHYSICAL (Physical topology) Moves bits between devices Specifies voltage, wire speed, and pin-out of cables

Pneumonic Device: Please Do Not Throw Sausage Pizza Away

Seven layers are divided into two groups Top 3 define how the applications within the end stations communicate with each other/users

These 3 know nothing about networking or network addresses Bottom 4 define how data is transmitted end to end through physical media, switches, & routers

Determine how to rebuild a data stream from transmitting host to destination hosts app Application layer provides a User Interface

User interfaces with computer Presentation layer presents data and handles processing (such as encryption) Session layer keeps different applications data separateThe Application Layer

Where users actually communicate with the computer Technically, users communicate with the network stack through application process interfaces

(aka APIs) connecting application to the OS

Application layer chooses and determines availability of communicating partners and resourcesneeded to connect

Coordinates partnering applications, forms a consensus on procedures for controlling data integrityand error recovery


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Acts only when its apparent that network access will be needed Example: If you uninstalled every networking component on a PC, you could still open a locally

saved HTML documents

BUT, if you tried to view this same HTML document retrieved using HTTP, FTP, or TFTP, itwouldnt work because Internet Explorer responds to those requests via Application Layer

Application Layer acts as interface between application program (NOT A PART OF THE LAYEREDSTRUCTURE) and the next layer down

Application layer provides a way for the program to send info down the protocol stack IE doesnt exist in Application Layer; IE interfaces with App Layer protocols when it needs

remote resources

Application layer identifies/establishes availability of intended communication partner andsufficiency of resources for the communication

Applications often require more than just desktop resources Unite communicating components from multiple network applications, such as file transfer,

email, remote access, network-management, client-server processes (ie- printing)APPLICATION LAYER ACTS AS INTERFACE BETWEEN APPLICATION PROGRAMS. THESE PROGRAMS DO

NOT RESIDE AT THE APPLICATION LAYER, BUT RATHER INTERFACE WITH IT VIA APPLICATION

LAYER PROTOCOLS WRITTEN IN THE PROGRAMS SOFTWARE, ENABLING NETWORK ACCESS

The Presentation Layer

Presentation Layer presents data to the Application Layer and handles data translation and codeformatting

It is a translator, providing coding and conversion functions Adapt data into a standard format before transmission Computers receive this generically formatted data then convert it back into its native format for

reading (ex. ASCII)

Presentation Layer ensures that data transferred from one systems Application Layer can beread by another systems Application Layer

Data compression, decompression, encryption, and decryption are handled by Presentation Layer,some multimedia operations

The Session Layer

Session Layer sets up, manages, and tears down sessions between Presentation Layer entries Provides dialog control between devices (nodes) Coordinates communication, offering Simplex, Half Duplex, and Full Duplex Session Layer keeps applications data separate from other applications data The Transport Layer

Transport Layer segments and reassembles data from upper-layers and u nites it onto the same datastream

Provides end-to-end data transport services, establishing logical connection btwn send/receive host


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Provides mechanisms for multiplexing upper-layer applications Establishes virtual connections and tears down virtual circuits afterwards

Hides details of network-dependent information from higher layers Provides transparent data transfer

Uses TCP (transmission control protocol) and UDP (user datagram protocol) TCP = RELIABLE UDP = UNRELIABLE

Basically means, TCP has means to check on successfulness of data transfer, where as UDPjust sends the data without worrying about whether it arrived or not

Reliable Networking: acknowledgments, sequencing, and flow control are used Transport Layer can be Connectionless (unreliable) or Connection-Oriented (reliable)Connection-Oriented Communication (Transport Layer continued)

Before transmitting host sends segments down the OSI model (when you send, you start at AppLayer and work down to Physical. When receiving, vice versa), the senders TCP process contacts

the destination TCP process to establish connection Creates a Virtual Circuit with Connection-Oriented Communication The initial Handshake has the two TCP processes agree on:

Amount of info to be sent in either direction before recipient TCP sends back anacknowledgement

Virtual Circuit Setup (known as Overhead)

Hosts application programs notify their Operating Systems that a connection is initiated OSes send messages over network, confirming the approved transfer After synchronization occurs, connection is established and data transfer begins This is all called a THREE-WAY HANDSHAKE

1st: Request for synchronization 2nd: Acknowledgement of request and establishment of connection parameter s(rules) between

hostsrequesting that the receivers sequencing is synced so bidirectional connection is formed

3rd: Final acknowledgement, notifying destination that connection agreement is accepted,connection is established, and data transfer can start

SYN, SYN/ACK, ACKSYNCHRONIZE, SYNCHRONIZE AND ACKNOWLEDGE, ACKNOWLEDGE

[Sending PC ] [Receiving PC]

Syn ----------Syn, Ack

Ack -----Connection Established

Data Transfer of Bytes of Segments Sometimes, congestion can occur because a high-speed PC generates data traffic faster than the

network can handle transferring it

Several computers simultaneously sending datagrams through single gateway or to single PC cancause slowness

Gateway or destination congested though no single source caused it bottlenecked


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Flow Control (Transport Layer continued)

Transport Layer also handles Data Integrity by maintaining Flow Control Allows users to request reliable data transport Flow Control provides means for receiver to govern amount of data send by sender

Prevents sending host from overflowing buffers of receiving host, resulting in loss of data Reliable data transport employs connection-oriented communication sessions, protocols ensure:

Segments delivered are acknowledged back to sender upon reception Segments not acknowledged are retransmitted Segments are sequenced back into proper order upon arrival Manageable data flow maintained to avoid congestion, overload, and data loss

A machine that receives datagrams too quickly stores them in memory called BUFFER Only solves issue if datagrams arrive in small bursts. If the flood capacity is exceeded, it discards

additional datagrams arriving

This is solved by the Transport Layer issuing not ready indicator to the sender, signaling thesending device to stop transmitting segment traffic Receiver processes segments in the buffer, then sends a ready transport indicator

In a fundamental, reliable, connection-oriented data transfer, datagrams are delivered in samesequence theyre transmitted

If the order is breached, transport fails Data segments lost, duplicated, or damaged in transmission cause a failure notice Receiving host acknowledges the reception of each data segment in correct order

SERVICE IS CONNECTION-ORIENTED IF IT HAS:

VIRTUAL CIRCUIT SET UP (SUCH AS 3-WAY HANDSHAKE)

USES SEQUENCING

USES ACKNOWLEDGMENTSUSES FLOW CONTROL

Windowing

Would be slow transmission if sender had to wait for acknowledgement after sending each segment With time available AFTER sender transmits data segment and BEFORE it processes

acknowledgments received from receiving machine, sender uses the break to transmit more data

Window: The quantity of data segments (in bytes) a transmitter is allowed to send withoutreceiving an acknowledgment

Windows are used to control number of outstanding, unacknowledged data segments Size of window = how much info is transferred from one end to the other

Some protocols quantify info by # of packets, but TCP/IP measures # of bytes A window size of 1 has the sending machine wait for acknowledgment for each data segment

transmitted before transmitting another

A window size of 3 allows the sending machine to transmit 3 data segments before anacknowledgment is received


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A receiving host that fails to receive all segments can improve communication by decreasingthe window size

Acknowledgments

Reliable data delivery ensures integrity of data stream Guarantees that data wont be duplicated or lost This is achieved via Positive Acknowledgment with Retransmission

Requires receiving machine to communicate with transmitting source, sending anacknowledgment back to sender when the receiving machine gets the data

Sender documents each segment and waits for acknowledgment before sending next segment After sending a segment, the transmitting machine starts a timer and retransmits if the timer

expires before an acknowledgment is returned from the receiver

Transport Layer doesnt need to use connection-oriented service (whether thats included in theway the software interfaces with the OSI model is up to the application developer)

If youre connection-oriented (have set up a virtual circuit) then youre using TCP If you havent set up a virtual circuit, then youre using UDP and are connectionless


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Certain devices operate at all seven layers of the OSI model:

Network Management Stations (NMSs)

Web & application servers

Gateways (not default gateways)

Network hosts

Some devices operate primarily at the Physical Layer of the OSI model, manipulating the physical

aspects of the data stream (voltages, signal direction, signal strength, etc)

NIC cards

Transceivers

Repeaters

Hubs (glorified repeaters)

Chapter 2 OSI Model (continued)

The Network Layer

Network layer manages device addressing, tracks location of devices, & determines best way tomove data

Transports traffic between devices that arent locally attached as wellRouters (layer 3) specified at Network layer provide data routing in internetwork

Process Packet received on router interface Destination IP address is checked If packet isnt destined for router, router looks up destination network address in routing table

If it cant find an entry for the destination network in routing table, drops packet Router chooses exit interface and sends packet to that interface to be framed/sent on network Two types of packets used at Network Layer Data packets

Transport user data through internetwork Uses routed protocols to support data traffic

IPv4 and IPv6 Route-Update packets

Used to update neighboring routers about networks connected to routers withininternetwork

Help build and maintain routing tables Protocols that send these route-update packets are called routing protocols

RIP: Routing Information Protocol and RIPv2 EIGRP: Enhanced Interior Gateway Routing Protocol OSPF: Open Shortest Path First

Routing table includes following info (demonstrated in the figure below):


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Network Address Protocol-specific (ie: IPv4 or IPv6) addresses Router maintains routing table for individual routing protocols

Each routing protocol keeps track of a network with a different addressing scheme Interface

This is the exit interfacethat a packet takes when destined for another network Metric

Metric value = distance to the remote network Different routing protocols compute this distance differently

Ex. RIP uses hop counts the number of routers a packet passes through en route tothe remote network

Others use bandwidth, delay of line, or tick count (1/18of a second) Routers break up broadcast domains, so broadcasts arent forwarded through routers Routers also break up collision domains, but this can also be accomplished with a Layer 2 (data linklayer) switch

Switches are also known as Layer 2 Routers Switches break up collision domains

Each interface in a router represents a separate network, so it must have a unique networkidentification number

Each host connected to that router must use that same network number


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As you can see, the Routing Table on the router connected to the network with Network Address 1 (NET

1) shows that data leaves the network on Exit Interface E0 (INT S0).

This router on NET ID 1 is 0 hops away from the Internet (a WAN service) which has Network Address 2

(NET ID 2) and Exit Interface S0 (INT S0).

NOTE: The router for NET ID 1 is 1 hop away from the router connected to the network with Network

Address 3 (NET ID 3), which has Exit Interface E0 (INT E0).

Notes about routers to memorize

By default, routers will not forward broadcastor multicastpackets Routers use logical address (ex. 192.168.1.5) in Network Layer header to determine next-hop router

to forward the packet to

Routers can use access lists to control security on types of packets entering/exiting an interface Routers can provide Layer 2 bridging functions and route through the same interface

Remember, switches are Layer 2, but when someone says Layer 3 Switch they mean Router Layer 3 Routers provide connections between VLANs (virtual local area networks) Routers provide QoS (quality of service) for network trafficThe Data Link Layer

Provides physical transmission of data and handles error notification (not correction), networktopology, and flow control

Ensures messages are delivered to proper device on LAN using hardware addresses (aka MACAddress or Ethernet Address)

Translates message from Network Layer into bits for Physical Layer to transmit The Data Link Layer formats messages into pieces called data frames and adds a header

(information encapsulating the message) containing: Destination hardware address (aka MAC address or Ethernet address) Source hardware address (aka MAC address or Ethernet address)

Routers at Network Layer dont care where host is located, only concerned with where networks arelocated and how to reach them

Data Link Layer is responsible for the identification of each device on the local network Router operates w/ Network Layer and finds the network (even remote ones) using LOGICAL

ADDRESSING (192.168.1.5), then Data Link Layer finds the appropriate host using HARDWARE

ADDRESSING (aka MAC or Ethernet) (00:1A:2B:3C:4D:5E)

For hosts to send packets to each other, as well as to send them to routers, the Data Link Layer usesHardware Addressing

Packet is framed with control info at Data Link Layer Control info is stripped off at the receiving router and original packet is left intact Framing of packet continues for each hop until its delivered to the correct host

Packet is never altered, only encapsulated with control info needed to be passed todifferent media devices


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Data Link Layer has two sublayers: Media Access Control (MAC) and Logical Link Control (LLC)

MAC defines how packets are placed on media Contention media access is first come/first served where everyone shares same bandwidth Physical addressing is defined here, so are Logical Topologies

Logical Topology: signal path through a physical topology MAC sublayer of Data Link Layer can also use

Line discipline Error Notification (NOT CORRECTION) Ordered delivery of frames Optional Flow Control

LLC identifies Network Layer protocols and encapsulates them LLC header tells Data Link Layer what to do with packet once frame is received

Host receives frame, looks at LLC to find where its destined for (IP address at Network Layer) LLC provides flow control and sequencing of control bits

802.X Standard

802.1 LAN/MAN (metropolitan) Management (and MAC bridges) 802.2 LLC 802.3 CSMA/CD (Carrier Sense Multiple Access with Collision Detection) 802.4 Token Passing Bus 802.5 Token Passing Ring 802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN) DQDB MAN 802.7 Broadband LAN 802.8 Fiber-Optic LAN/MAN 802.9 Isochronous LAN 802.10 LAN/MAN Security 802.11 Wireless LAN 802.12 Demand Priority Access Method 802.15 Wireless Personal Area Network 802.16 Wireless MAN (aka: WiMAX) 802.17 Resilient Packet Ring 802.18 LAN/MAN Standards Committee

The Physical Layer Does two important things: sends and receives bits

Bits only come in values of 1 or 0 Physical Layer communicates directly with various types of communication media

Different media devices represent the bit values in different ways Some use audio tones, some employ state transitions (changes in voltage from high to low)


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Protocols are needed for each type of media to describe proper bit patterns to be used, how data isencoded into signals, and qualities of the medias attachment interface

Physical Layer specifies electrical, mechanical, procedural, and functional requirements foractivating, maintaining, and deactivating physical link between systems

Physical Layer identifies interace between the DTE (data terminal equipment) and the DCE (datacommunication equipment)

DCE is usually located at customer, where DTE is attached device (a modem or CSU/DSUchannel service unit/data service unit

Physical layers connectors & topologies defined by standards, allowing disparate systems to talk Physical layer specifies layout of transmission media

Physical topology the way cabling is physically laid out Star, Bus, Ring, Mesh (partial & full mesh)

Encapsulation

Host transmitting data through network to another device has data go through encapsulation Data is wrapped with protocol info at each layer of OSI model

Each layer communicates only with its peer layer on other device To communicate/exchange data, each layer uses PDU(protocol data units)

The Protocol Data Units are as follows: Segments Packets Frames Bits That is how the units change as the data progresses from the top of the OSI model down to

the Physical Layer.

PDUs hold control info in which the data is encapsulated at each layer Usually attached to header, but can also be in the trailer

Data Encapsulation Method User info converted to data for transmission on network Data converted to segments and reliable connection set up between sender/receiver Segments converted to packets or datagrams, logical address placed in header so packets can

be routed through internetwork

Packets or datagrams converted to frames for transmission. Hardware Address (MAC/Ethernet)added to uniquely identify hosts on network

Frames converted to bits and a digital encoding/clocking scheme is used


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Things To Remember

Remember each of the 7 OSI Layers: Application, Presentation, Session, Transport, Network, DataLink, and Physical

Application, Presentation, and Session are Upper Layers responsible for communicating fromthe User Interface to the Application itself

Transport Layer handles segmentation of data, sequencing of the data into proper order, andestablishing virtual circuits (reliable connection = TCP, unreliable = UDP)

Network Layer converts segments to packets, and provides logical network addressing (IPaddress, v4 or 6) and routing through an internetwork

Data Link Layer provides framing of the packets before placing data on the network medium Physical Layer converts packets to bits (1s and 0s) and encodes them into digital signal to send

Sublayers of Data Link Layer Only layer that has sublayers: LLC and MAC

LLC: responsible for multiplexing of Network Layer protocols MAC: responsible for physical (Hardware/Ethernet) addressing and determining when to

place data on the network

Know devices that operate at each layer of OSI Model Hubs and Repaters see only bits, so theyre Layer 1 (Physical)

All networking devices have physical connectivity to network, so they all operate at Layer 1(Physical), but only Hubs/Repeaters operate only at the Physical Layer

Always consider that a device operates at the highest layer that it supports. That layersfunctionality is the main reason we implement the device


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Switches and Bridges are Layer 2 (Data Link) because they understand/make decisions based onLayer 2 (data link) addressing (MAC Addresses)

Routers are Layer 3 (Network Layer) because they handle logical (IPv4/IPv6) addressing Networking devices (such as workstations) run applications so theyre said to operate at Application

Layer

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