Interview Stuff

Front 1

OSI Layer Services: service user

Back 1

the OSI layer that requests services from an adjacent OSI layer

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OSI Layer Services: service provider

Back 2

the OSI layer that provides services to service users

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OSI Layer Services: service access point (SAP)

Back 3

a conceptual location at which one OSI layer can request the services of another OSI layer (kinda like the border between layers)

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Internetworking evolved as a solution to three key problems:

Back 4

isolated LANs, duplication of resources, and a lack of network management

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control information

Back 5

headers & trailers added to data at different layers of the osi model.

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OSI Model: Physical Layer

Back 6

the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications.

Front 7

OSI Model: Data Link Layer

Back 7

provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing (ie MAC addresses), network topology (bus, ring), error notification, sequencing of frames, and flow control. split into Logical Link Control (LLC) and Media Access Control (MAC)

Front 8

OSI Model: Data Link Layer: Logical Link Control Layer

Back 8

manages communications between devices over a single link of a network.IEEE 802.2. supports both connectionless and connection-oriented services

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OSI Model: Data Link Layer: Media Access Control Layer

Back 9

manages protocol access to the physical network medium. IEEE MAC specification defines MAC addresses

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OSI Model: Network Layer

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defines the network address. defines the logical network layout. IP lives here.

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OSI Model: Transport Layer

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TCP & UDP live here. segments the data for transport across the network. responsible for making sure that the data is delivered error-free and in the proper sequence. Flow control. Virtual circuits are established, maintained, and terminated by the transport layer. error checking, error recovery.

Front 12

OSI Model: Session Layer

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establishes, manages, and terminates communication sessions. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol ; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.

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OSI Model: Presentation Layer

Back 13

provides a variety of coding and conversion functions that are applied to application layer data. ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. Think ASCII, JPEG, 3DES, etc...

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OSI Model: Application Layer

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interacts directly with user. Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP), HTTP. functions typically include identifying communication partners, determining resource availability, and synchronizing communication

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frame, packet, datagram, segment, message, cell

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frame - data link layer
packet - network layer
datagram - network layer (connectionless)

segment - transport layer
message - application layer
cell - data link layer for ATM & SMDS (contains 5 byte header & 48 byte payload

Front 16

service data units (SDUs), protocol data units, bridge protocol data units (BPDUs)

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SDU - information units from upper-layer protocols that define a service request to a lower-layer protocol

PDU - OSI terminology for a packet

BPDU - used by the spanning-tree algorithm as hello messages

Front 17

end system (ES), intermediate system (IS), area, and autonomous system (AS)

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ES - no routing or traffic forwarding ie desktop
IS - performs routing and traffic forwarding (ie routers, switches, bridges.) Interdomain and Intradomain IS's are defined
Area - logical group of network segments and their attached devices. subdivisions of an AS
AS - collection of networks under a common administration that share a common routing strategy (aka domain).

Front 18

connectionless vs connection-oriented network services

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layer 4. TCP vs UDP. TCP = establish connection, transmit, dissolve connection.

Front 19

Data Link Layer Addresses

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physical or hardware address. flat address space. pre-established and typically fixed relationship to a specific device

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MAC Addresses

Back 20

type of data link layer address. used in LANs. Defined by IEEE MAC spec.48 bits as 12 hex digits. First 6 are Organizationally Unique Identifier (OUI) given by IEEE to vendor. Last 6 values administered by the specific vendor

Front 21

Address Resolution Protocol

Back 21

maps network addresses to MAC addresses. Hello protocol enables network devices to learn the MAC addresses of other network devices

Front 22

Network layer protocols that Imbed MAC addresses

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IPX, DECNET, Phase IV, XNS

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Network Layer Address

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hierarchical address space. called virtual or logical addresses. Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported

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Flow Control

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a function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data. L4

Front 25

three commonly used methods for handling network congestion

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buffering - temporarily store bursts of excess data in memory until they can be processed.

transmitting source-quench messages - receiving device sends source-quench messages to request that the source reduce its current rate of data transmission 1 message per dropped packet

windowing - a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted

Front 26

Error-checking schemes

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determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination.

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cyclic redundancy check (CRC)

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detects and discards corrupted data

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Multiplexing

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multiple data channels are combined into a single data or physical channel at the source (ie, different types of layer 7 data make it into 1 layer 3 packet.
OR
when data from different hosts is put on the same physical channel)

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multiplexer

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a physical layer device that combines multiple data streams into one or more output channels at the source and demultiplexes the channels into multiple data streams at the remote end.

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Types of Multiplexing

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time-division multiplexing (TDM), asynchronous time-division multiplexing (ATDM), frequency-division multiplexing (FDM), and statistical multiplexing.

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Time Division Multiplexing

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information from each data channel is allocated bandwidth based on preassigned time slots, regardless of whether there is data to transmit

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Asychronous Time Division Multiplexing

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ATDM, information from data channels is allocated bandwidth as needed by using dynamically assigned time slots. Basically, the same as TDM, but no slots wasted on hosts that have no data to send.

Front 33

Frequency Division Multiplexing

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information from each data channel is allocated bandwidth based on the signal frequency of the traffic

Front 34

Statistical Multiplexing

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bandwidth is dynamically allocated to any data channels that have information to transmit

Front 35

nternational Organization for Standardization (ISO)

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OSI reference model and the OSI protocol suite

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American National Standards Institute (ANSI)

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subsidiary of ISO, Fiber Distributed Data Interface (FDDI) and other communications standards

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Electronic Industries Association (EIA)

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electrical transmission standards. EIA/TIA-232 standard (formerly known as RS-232) which defines pinout for DCE/DTE

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Institute of Electrical and Electronic Engineers (IEEE)

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IEEE 802.3 and IEEE 802.5

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International Telecommunication Union Telecommunication Standardization Sector (ITU-T)

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developed X.25 and other communications standards.

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Internet Activities Board (IAB)

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set Internet policies through decisions and task forces. The IAB designates some Request For Comments (RFC) documents as Internet standards, including Transmission Control Protocol/Internet Protocol (TCP/IP) and the Simple Network Management Protocol (SNMP).

Front 41

Local Area Network (LAN)

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a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LAN protocols function at the lowest two layers of the OSI reference model

Front 42

LAN Media Access

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CSMA/CD & token passing

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CSMA/CD

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Carrier Sense Multiple Access/Collision Detection -- used in Ethernet networks
1. Listen, if no traffic
2. Transmit
3. Look for collision
4. If collision, random backoff
restart at 1.

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Token Passing

Back 44

Media Access used by Token Ring and Fiber Distributed Data Interface (FDDI). a special network frame called a token is passed around the network from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, the token is released so that other devices may use the network media. main advantage is it is easy to calculate max time until each device will get the token. Good for factories.

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LAN Transmission Methods

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unicast, multicast, and broadcast

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LAN Topologies

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bus, ring, star, and tree--logical architectures

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LAN Bus Topology

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a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations--used by Ethernet/IEEE 802.3 networks-including 100BaseT

Front 48

LAN Ring Topology

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a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop--used by Token Ring/IEEE 802.5 and FDDI

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LAN Tree Topology

Back 49

a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case

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LAN Star topology

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a LAN architecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology

Front 51

WAN

Back 51

a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. They generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer

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WAN L3 Protocols

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X.25 PLP

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WAN L2 Protocols

Back 53

Frame Relay, HDLC, PPP,

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Point-to-Point Links

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single pre-established connection between 2 remote networks leased from a carrier (ie phone company). hardware is dedicated. more expensive than shared tech like Frame Relay.

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Circuit Switching

Back 55

only connected when needed. same idea as POTS, wherein one switch circuit "calls" the other end, establishes connection, sends data, disconnects. ISDN is an example.

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Packet Switching

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users share common carrier resources.
the increased efficiency gained from shared resources translates to much lower costs.
-employs virtual circuits
-ie ATM, Frame Relay, Switched Multimegabit Data Services (SMDS), X.25

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WAN Virtual Circuit

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logical circuit between 2 networks.
2 types - Switched Virtual Circuit and Permanent Virtual Circuit

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Switched Virtual Circuit

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virtual circuits that are dynamically established on demand and terminated when transmission is complete

three phases: circuit establishment, data transfer, and circuit termination

used in situations in which data transmission between devices is sporadic

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Permanent Virtual Circuit

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a permanently established virtual circuit that consists of one mode: data transfer

used in situations in which data transfer between devices is constant

Front 60

WAN Switch

Back 60

a multiport internetworking device used in carrier networks

typically switch such traffic as Frame Relay, X.25, and SMDS, and operate at the data link layer of the OSI reference model

Front 61

Access Server

Back 61

acts as a concentration point for dial-in and dial-out connections

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Modem

Back 62

a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines

MODulator/DEModulator

Front 63

CSU/DSU

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channel service unit/digital service unit

a digital-interface device used to connect a router to a digital circuit like a T1

also provides signal timing for communication between these devices

Front 64

Switches & Bridges

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Layer 2 Devices that break up LANs into different network segments (1 collision domain per port on switches)

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store-and-forward switching

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an entire frame must be received before it is forwarded--can add latency in cases where frame is large

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Cut-through switching

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allows the switch to begin forwarding the frame when enough of the frame is received to make a forwarding decision. unlike store-and-forward, this type of switching can result in corrupted frames being forwarded.

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ATM Switch

Back 67

provide high-speed switching and scalable bandwidths in the workgroup, the enterprise network backbone, and the wide area.
switch fixed size data units called "cells"

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LAN Switch

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used to interconnect multiple LAN segments.
provides dedicated, collision-free communication between network devices
designed to switch data frames at high speeds.

Front 69

Packet Switching

Back 69

Routing. Moving packets across the network towards their destination

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Path Determination

Back 70

using metrics to evaluate what path will be the best for a packet to travel

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Routing Metric

Back 71

a standard of measurement used to determine the best path for data to take through the network. (ie bandwidth, hop count, etc...)

Front 72

Routing Tables

Back 72

Information on best paths to destinations around the network as decided by routing algorithms. Contain
1. next hop for each destination network in the table
2. desirability/cost of each path
3. Info on which routing protocol reported the route

Front 73

Switching algorithms

Back 73

a host determines that it must send a packet to another host. Having acquired a router's address by some means, the source host sends a packet addressed specifically to a router's physical (Media Access Control [MAC]-layer) address, this time with the protocol (network layer) address of the destination host

Front 74

Routing Algorithms:
Single Path vs Multipath

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multipath algorithms can load balance, or keep more than 1 path to a destination network in the routing table

Front 75

Routing Algorithms:
Flat vs hierarchical

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flat routing algorithms consider all Routers to be equal. Hierarchical routing algorithms dedicate certain routers as 'backbone' this is beneficial because it supports areas, domains, etc...

Front 76

Routing Algorithms:
Host-Intelligent vs Router-Intelligent

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Host Intelligent = source routing = host knows where its going so just forward packet to next hop.

Router Intelligent = routers decide best path based on calculations

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Routing Algorithms:
Link-State vs Distance Vector

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link state = spf. every router sends its full link information out to all and from others knows all paths in the area, calculates its own best paths
faster to converge, more resource intensive, more scalable, more expensive

distance vector =Bellman-Ford algorithms. each router sends routing table to neighbors. "routing by rumor"

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Network Management

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a service that employs a variety of tools, applications, and devices to assist human network managers in monitoring and maintaining networks

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Proxy ARP

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Used in a situation in which a host believes its destination is on the same network when it is not. If the router knows the destination IP is on another network, it will respond to an ARP request for a remote host with its own MAC address. and then pass the data on once its received by the router

Front 80

ARP

Back 80

Address Resolution Protocol. mechanism on Ethernet to allow host to obtain layer 2 MAC address of a host given the layer 3 IP address.
1. Host sends a broadcast addressed to an IP address.
2. If the destination host is on the same subnet, it responds with its MAC address
3. Source host puts ethernet address/IP address mapping in the ARP table

Front 81

Reverse ARP

Back 81

Used to help a host get an IP. In reverse ARP a host broadcasts its Ethernet address and the DHCP server responds with an IP.

Front 82

ISO Network Management Model

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Performance Management
Configuration Management
Accounting Management
Fault Management
Security Management

Front 83

ISO Network Management Model: Performance Management

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Gather Data about network performance, Define thresholds, Alert if thresholds exceeded. (optimize)

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ISO Network Management Model:
Configuration Management

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Maintain a database of version information and configuration information for all devices on the network to assist in troubleshooting.

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ISO Network Management Model: Accounting Management

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Measure network utilization by users or groups--especially important when the costs of the network must be billed. Also, it can be used to see who is overusing or misusing the network resources. establishing quotas falls under accounting mgt.

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ISO Network Management Model: Fault Management

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detect, log, notify (and if possible fix) faults on the network. most network mgt resources reside here.determine symptoms, identify problem, identify solution, test, (plan and) deploy

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ISO Network Management Model: Security Management

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identify sensitive assets, control access, monitor usage,

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Internet Protocol

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The Internet Protocol (IP) is a network-layer (Layer 3) protocol that contains addressing information and some control information that enables packets to be routed. RFC 791

provides connectionless, best-effort delivery of datagrams through an internetwork

provides fragmentation and reassembly of datagrams to support data links with different maximum-transmission unit (MTU) sizes

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IP Packet Format

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Version-Indicates the version of IP currently used.

IP Header Length (IHL)-Indicates the datagram header length in 32-bit words.

Type-of-Service-Specifies how an upper-layer protocol would like a current datagram to be handled, and assigns datagrams various levels of importance.

Total Length-Specifies the length, in bytes, of the entire IP packet, including the data and header.

Identification-Contains an integer that identifies the current datagram. This field is used to help piece together datagram fragments.

Flags-Consists of a 3-bit field of which the two low-order (least-significant) bits control fragmentation. The low-order bit specifies whether the packet can be fragmented. The middle bit specifies whether the packet is the last fragment in a series of fragmented packets. The third or high-order bit is not used.

Fragment Offset-Indicates the position of the fragment's data relative to the beginning of the data in the original datagram, which allows the destination IP process to properly reconstruct the original datagram.

Time-to-Live-Maintains a counter that gradually decrements down to zero, at which point the datagram is discarded. This keeps packets from looping endlessly.

Protocol-Indicates which upper-layer protocol receives incoming packets after IP processing is complete.

Header Checksum-Helps ensure IP header integrity.

Source Address-Specifies the sending node.

Destination Address-Specifies the receiving node.

Options-Allows IP to support various options, such as security.

Data-Contains upper-layer information.

Front 90

distance vector vs link state

Back 90

distance vector rp's broadcast their full routing table to their directly connected neighbors. in distance vector rp's.information on all known links are sent on adjacency formation and then only updates to links are sent. but updates are only sent to neighbors with full adjacency. link state routers have to do all of their own calculations, so more resource intensive.