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Computer Networks
sjocorpe
Front Back
OSI Model - 7 Layers
� 1 - Physical� 2 - Data
� 3 - Network� 4 -Transport� 5 - Session
� 6 - Presentation� 7 - Application
OSI Model - Physical Layer
To transmit bits over a medium; toprovide mechanical and electrical
specificationserror control coding
Units is Bits
OSI Model - Data Layer
To organise bits into frames; to provide
hop-to-hop deliveryError control
Units is Frames
OSI Model - Network Layer
To organise bits into frames; to provide
hop-to-hop deliveryProvides connection or connectionless service
Units is Packets
OSI Model - Transport Layer
To provide reliable process to process
message delivery and recoveryUnit is Segment
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OSI Model - Session Layer
To establish, manage and terminatesessions Unit is Message
OSI Model - Presentation Layer
To translate, encrypt and compress data Unit is Message
OSI Model - Application Layer
To allow access to network resources Unit is Message
Connection-oriented service
Connection oriented service requires a pair of processes
in the two computers to establish a connection before
sending data.
Connectionless-oriented service
In connectionless service, data packets are independent
of each other and they may not arrive at the destination
in the proper order.
Connection-Orentated service - Mulitple links Connection-Orentated
Path from source to destination must be established before data
packets can be sent Called a Virtual Circuit (VC)
Connection-Orentated service - Mulitple links
Connectionless-Orentated
No advanced setup is needed Packets called datagrams and carry the full destination address and are
thus routed through the system independent of all the others.
Connection-Orentated service - Single links Connection-Orentated
A logical connection is established, with both sides initilize variables
and counters to track which frames have been recieved
Connection-Orentated service - Mulitple links
Connectionless-Orentated
Connection doesn't need to be established, data frames can be lost,
duplicated, and out of order.
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TCP/IP Networks 5 Layer model used
TCP NodeProvision of a reliable connection
oriented service
IP Node
Routing on IP AddressesProvision of an unreliable Datagram service over multiple networks for
the transport of UDP or TCP messages
Low Earth Orbit Satellites (LEOs) - Advantage
The transmission power and the propagation delay is much smaller
compared to geostationary satellites.
Low Earth Orbit Satellites (LEOs) - Disadvantage
Sophisticated tracking and acquisition systems are needed to
pick the wanted signal. One satellite may only be visible for a few miutes, thus suitable hand off
procedures must be organized to switch to the next satellite.
Connection Orentiated Vs Connectionless differences
Connection orentiated needs to make a connection between the two computers before sending the data
arrives in specific order Connectionless doesn't arrives in
any order
byte stuffing
A special ESC byte inserted before each accidental flag byte in the data.If an escape byte occurs naturally in
the middle of the data, it's stuffed with an escape byte as well.
Error Control
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Types of Error
� Single Bit
� Burst Error
Parity Check Code
Checks on the number of ones in a byte
Can only detect single bit errors
� Even Parity
� Odd Parity
Hamming Distance
This is the number of differences of two words between corresponding
bits. 000 XOR 011 (two 1's difference)
Detecting Errors
To guarantee detection of up to S errors, d
min = S+1. To guarantee
correction of up to T errors, dmin
=
2T+1.
Cyclic Redundancy Codes (CRC) - Remainder and
Divisor
The bit pattern used is equvalent to the remainder from the message and
generator.The Divisor is used to generate the codeword (data word + remainder)
CRC Code Word
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Generation
IEEE802 (Project 802)
Specifies functions of the physical and data link layers of major LAN protocols Data Link layer into 2
parts
� LLC Link Layer Control
� MAC (media access control) Layer
Logical Link Control (LLC)
Combines flow control, error control, and part of framing duties.
Single data link layer control protocol for all IEEE LANs makes MAC sub-layer transparent PDU
similar to HDLC
Data Link Layer - FramingLLC header contains a control fiels like HDLC used for flow and error
control.
Media Access Control -MAC
� 3 access protocols
Multiple access protocols
� Random access � Controlled access � Channelization
Frames data from upper layer and passes it to physical layer
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MAC Sub-layer frame format
� 7 Fields
� Preamble� SFD (Start Frame Delimiter)� DA (Desitination Address)
� SA (Source Address)� Length or type of PDU
(Protocol Data Unit)� Upper Layer data Unit
� CRC (Cyclic Redundancy Check)
Ethernet - reliabilityNo mechanism for frame acknowledgements so it is
"unreliable"
MAC frame - Minimum Length
Payload length = 46 bytesFrame length = 64 bytes
MAC frame - Maximum Length
Payload length = 1500 bytesFrame length = 1518 bytes
Types of error - Single Bit one bit in data changes
Types of error - Burstmultiple bits in data segment
changes
Even ParityIf even number of ones in byte,
binary 0 added to end
Odd ParityIf odd number of ones in byte ,
binary 1 added to end
Hamming Distance -Limitations
Can only detext (dmin-1) errors
dmin is the smallest hamming
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distance between all possible pairs in a set of words.
Data Link layer - framing
� Two access point fields to define upper
layer protocol
� Destination service, DSAP� Source Service, SSAP
These define the upper layer protocol at the source and
destinations that uses LLC.
802.11 Standard - 3 Frame Types
� Management Frames � Control Frames � Data Frames
Hidden Station Problem
Exposed Station Problem
Hub
� small, 2-8 computer networks � only 1 protocol
� speed not prime importance � Common subnet
� small to med, 5-x computers � 1 protocol
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Switch� Speed id of importance
� common subnet
Router
� Any number of computers on different subnets, or seperate
from Public IP Network � Number of protocols � Security is potentially
important � connects 2 networks
Bridge
� connect 2 LANs using similar LAN protocols
� Accept and forward frames to different networks
� minor protocol translation � OSI kayer 2 (data link)
Internet - 3 Key concepts
� Addressing � Routing
� Interent Protocol
WWW Documents - 3 Categories
Static, Dynamic and active
WWW Documents - Static-Explain
Document content that does not change with time
WWW Documents -Dynamic-Explain
� a request for running program from server,
� sends content at time of request
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WWW Documents -Active
-Explain
� Applet� runs at client end to generate
document requested at server� Sent to client pc to run.
Proxy Server
� Acts on behalf of other clients to present requests to a server
� Acts as a server interacting with clients, and as a client
when interacting with servers � Security intermediary,
firewall with proxy
HTTP request message contains
� Request type � URL
� HTTP version
HTTP request method
Digital Signiture - Provides
� Integrity � Non-reproduction
� Authorization
JPEG - 3 Phase Process
� Discrete Cosine Transformation
� Quantization � Data Compression
� Works in a zig-zag format
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JPEG - Data Compression Methods
� Run length encoding � Variable length encoding
JPEG - Discrete Cosine Function Codes the blocks
JPEG - Quantization Looses Data
Jitter - Introduced in realtime data by?
� delays between packets � caused by packets taking
different routes
Jutter - Prevention methods� Time stamp
� Playback buffer
ICMP - Located in which layer protocol
Network Layer
ICMP - Message composition
� header size of 64bits � 8 bits for type� 8 bits for Code
� 16 bits for Checksum
IPv4 - 1st Header
� 32 bit address � Version
� IHL � Type of service � Total Length
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Address Classes A-E
� A 0 7 bits net, 24 bits host � B 10 14 bits net, 16 bits host � C 110 21 bits net, 8 bits host
� D 1110 28 bits multicast group
� E 1111 experimental
IPv4 header - versionspecifies which version of the
internet protocol is used
IPv4 Header - IHLInternet header length, expressed in
32-bit words
IPv4 Header - Type of service
Packet’s precedence and desired routing
IPv4 Header - Total Length
� Number of octets contained in the packet
� 16 bits long, limits to 65.5K octetc
IPv6 Header - 1st 32 bits Contents
� Version � Priority
� Flow Label
IPv6 Header - 2nd 32 bits Contents
� Payload length � Next header � Hop limit
IPv6 Header - Version� 4 bits
� 6 for IPv6, 4 for IPv4
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IPv6 Header - PriorityEnables a source to identify the desired delivery priority of this
packet (4 bits).
IPv6 Header - Flow Label
Used by a source to identify associated packets
needing the same type of special handling, eg real-time service
between a pair of hosts (24 bits).
IPv6 Header - Payload Length
Length of the payload following the header
IPv6 Header - Next HeaderIdentifies the type of header
immediately following the IPv6 header
IPv6 Header - Hop LimitSpecifies the maximum number of hops that a packet may take before
it is discarded (8 bits).
Link state routing -Prefered over distance
distance metric focuses on how many hops it takes to get to the
destination linked state calculates the states of the outgoing links and chooses the fastest and most suited
link.
Distance Vector RoutingThis is where the router will find
the path with the shortest number of hops to reach it's destination.
RepeaterWhat comes in, it will rebroadcast
along its output lines.
POP3 Advantages Straight foward, Faster
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POP3 Disadvantages If you lose an email then it is gone forever
IMAP Advantages More Redundant, Keeps a copy of everything on the server.
IMAP Disadvantages Slower, takes up more space
POP3 Description
PassThrough tech, the server holds the mail until you request it and
then when you do it will send the mail to you.
IMAP Description
Syncing server holds the mail on the server and when you request
new mail it will send a copy of the mail while keeping a copy on the
server If you delete or compose on your client then these actions will be performed automatically on the
server copies.
Internet Assigned Number Authority - 3 groups
� System - IETF � User - IANA � Dynamic - NOT
IPV4 to IPV6
To ease the transition there are a few options for networks. There can
be Duel stacks or tunneling. Duel Stacks is where IPV4 and IPV6 can
coexist where tunneling is where the non supported addressing
version is encapsulated within the supported version.
Public Vs Private key
Private - Both parties have a copy of the encryption/decryption
algorithm and they use these to send messages. Public - There is only
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encryption one private key that will decrypt the message if it is encoded with the
persons public key. The user sends out the public key to others so they can use it to encrypt data for them.
Capabilities provided by multi-service Core in the Next Generation Network
� Allows a heighrachy of charging systems to support local, reginal international and interconnect acounts.
� Support for a layered security model with centeralised
administration with passive and active monitoring
capabilities. � Support for tighter QOS
specifications for performance improvements in
various data connections.
Differences between TCP and UDP
� TCP implements Flow control so the data is sent accuratly to the receiving computer. This method is reliable but slow. This is
generally used for webpages and databases.
� UDP is only concerned with speed. There is no flow
control and this is usually used for non essential
appliactions like streaming media across the internet/
equal cost multipath routing
if there are multiple best paths the tie for the top position in the routing
table, packets ay be sent via different paths to balance the loading in the network which
should improve the bandwidth per pipe.
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The two army problem
This is where two armies are in a war, one army splits their forces and
forms a pincer movement. If both sides of the pincer attack at the
same time then this army will win. They both set up camp and then the problem is how to communicate to
the other party when they will attack. If they send a messenger and
the messenger does not arrive in time then they will attack alone and lose. This is like the connectionless
service and hence we call it unreliable as we dont actually know
if the message was sent and received.
NAT
� Network Address Translation Converts an external IP to an
internal IP in a network. � This allows less IPV4 addresses to be assigned to
end users. � This meant that there would
be a great reduction in external assignmnets and thus
saving IPV4.
Packet Switching Network Four sources of delay
� Nodal processing � Queuing
� Transmission delay � Propagation delay
Packet switching network Virtual Circuit
This is where all the packets are sent along the same routing path
and this means that the packets cant arrive out of order.
Shortest Path AdvantagesThis algorithm allows the network
to use the shortest path between any two nodes minimising delay.
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Shortest Path Disadvantages
This algorithm is very resourse heavy to search
Security in the OSI layers
� Application- Firewall and Anti-virus
� Session - Authentication checks to make sure the
session is correst � Datalink - Encryption so when transmitted people can't
intercept it. � Physical - Gas Pressure in
lines
IPv6 - Factors influncing adoption
� IPv4 is running out of addresses to be used
� increased security � quality of service
Modulo-2 arithmeticProduces the same results with
addition and subtraction
Protocol Designed to handle realtime internet
trafficUser Datagram Protocol (UDP)
UDP header - First line
� Source Port (16 bits) � Destination Port (16 bits)
UDP header - Second line
� Length (16 bits) � Checksum (16 bits)
UDP header - Thrid line Data
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UDP Header - Source PortThe port number of the sender.
Cleared to zero if not used.
UDP Header - Destination Port
The port this packet is addressed to.
UDP Header - Length
The length in bytes of the UDP header and the encapsulated data.
The minimum value for this field is 8.
UDP Header - Checksum
Computed as the 16-bit one's complement of the one's
complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded as needed with zero
bytes at the end to make a multiple of two bytes. If the checksum is
cleared to zero, then checksuming is disabled. If the computed checksum is zero, then this field must be set to
0xFFFF.
Error control in Transport layer
Has ICMP where identification of the errors takes place
Session Initated Protocol
� Intitates, terminates or modifies a multimedia
session
Session Initated Protocol -Message types
� Requests � Response
� REGISTER: Used by a UA to
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Session Initated Protocol -Request Messages
indicate its current IP address and the URLs for which it would like to receive calls.
� INVITE: Used to establish a media session between user
agents. � ACK: Confirms reliable
message exchanges. � CANCEL: Terminates a
pending request. � BYE: Terminates a session
between two users in a conference.
� OPTIONS: Requests information about the capabilities of a caller,
without setting up a call. � PRACK(Provisional
Response Acknowledgement): PRACK improves network reliability
by adding an acknowledgement system to the provisional Responses (1xx). PRACK is sent in response to provisional
response (1xx).
Session Initated Protocol -Response Messages
� Provisional (1xx): Request received and being processed.� Success (2xx): The action
was successfully received, understood, and accepted.
� Redirection (3xx): Further action needs to be taken (typically by sender) to complete the request.
� Client Error (4xx): The request contains bad syntax or cannot be fulfilled at the
server.� Server Error (5xx): The
server failed to fulfill an apparently valid request.
� Global Failure (6xx): The request cannot be fulfilled at
any server.
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Flooding Algorithm
� Receive message. Look for record in database.
� If the record is not yet included, add it to the
database, broadcast the message.
� Else if the number in the base is lower than the number in
the message, replace � the value with the new one
and broadcast the message. � Else if the number in the base
is higher, transmit the current database value
� through the incoming interface.
� Else if both numbers were the same, do nothing.
IPv4 - 2nd Header
� Identification � Flags
� Fragment offset
IPv4 - 3rd Header
� Time to live � Protocol
� Header Checksum
IPv4 - 4th Header Source address
IPv4 - 5th Header Destination Address
IPv4 - 6th Header
� Options � Padding
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IPv4 Header - Protocolwhen packet reaches the
destination, this field determines what program uses it
IPv4 Header - header Checksum
Used to error check
IPv4 Header - Options e.g routing options (no satellites)
IPv4 Header - Padding Contains No Op fields
BPG - 4 basic message types
� Open � Keepative
� Update � Notification
BPG - Open messageopen and confirm the
connection parameters
BPG - Keepative messagesent periodically to ensure the live-
ness ofthe connection.
BPG - Update messageIncremental updates are sent as the
routing tables change.
BPG - Notification message
sent in response to errors or specialconditions.
IP, LAN, WAN ProtocolClassed as unreliable as it has no acknowledgement for messages
An IPv6 address can have up to __________ 32
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hexadecimal digits.
An ICMP message has _____ header and a variable-size data
section.
8 byte
“hidden station” problem can arise in an 802.11x
BasicService Set. Briefly explain
how this difficulty is overcome.
To overcome this problem, handshaking is implemented in
conjunction with the CSMA/CAscheme.
port addressport address is the address of a
session running on a host asused by the transport layer.
logical address
logical address is an address used in the network layer to identifya host that is connected to the
Internet.
physical address.
physical address (link address) is the address of a node as
defined by its LAN or WAN, it is the lowest level address and isassigned to a physical device.
An organisation is granted the block 125.238.0.0/16.
The administratorwants to create 512
subnets:- Find the subnet mask
required
This is a class A address range so that the default mask is /8.
We need sufficient bits to produce 512 subnets so 2x = 512
where x is the number of bits required. Hence x = 9, thus we
require the mask to be /8+9 = /17. OR 255.255.128.0
An organisation is granted the block 125.238.0.0/16.
The administratorwants to create 512
There will be 15 bits remaining so the number of hosts per
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subnets:
� Find the number of addresses in each
subnet
subnet is 32768 – 2 = 32766 since 2 addresses are not able to
be used for hosts (they are reserved addresses).
An organisation is granted the block 125.238.0.0/16.
The administratorwants to create 512
subnets:
� Find the first and last allocatable addresses
in subnet 1
01111101 11101110 00000000 00000001 (125.238.0.1)
Finally, we note that the broadcast address is the last one of
the 32768 addresses so the last allocatable one is the one
before that broadcast address so it must be
01111101 11101110 00000000 00000000 (125.0.127.254)
An organisation is granted the block 125.238.0.0/16.
The administratorwants to create 512
subnets:
� Find the first and last allocatable addresses
in subnet 14
The first and last allocatable addresses in subnet 14 are:
To find the 14th subnet we have to add 13 to the subnet number
of the first subnet. We can add this amount to the first
allocatable address of the first subnet as we know that the first
address in every block is not allocatable as it is a special
address.
BGP is regarded as a Path Vector Protocol, how does
it differ from LinkState and Distance Vector
Protocols?
The primary function of a BGP system is to exchange network
reachability information with other BGP systems. BGP is rule based
rather than metric based as distinct from the case of the other two
protocols. Link state protocols pass around topology and network
status information using a flooding protocol.
� Fast, loopless convergence. � Support of precise metrics
and multiple metrics - if
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List three reasons why most network specialists
prefer Link StateRouting instead of
Distance Vector Routing.
desired. � Support of multiple paths to a
destination. � Separate representation of
external routes.
methods for enabling a smooth transition from
IPv4 to IPv6.
� Dual-stack methods � Tunnelling
� Header translation
Dual-stack method
Dual stack means running IPv4 and IPv6 simultaneously until everyone
is ready for IPv6. (To figure out which version of IP to use when
sending a packet to the destination, the source must send a query to the
DNS server.If the DNS server returns an IPv4 address then the source sends an IPv4 packet, else it sends an IPv6
packet.)
Tunnelling
Tunnelling involves encapsulating an IPv4 packet inside an IPv6
packet or vice versa and setting the protocol indicator for the packet as
“IP”
Header translation
Header translation used if the receiver doesn’t understand IPv4and you are using IPv6.
Translation is required at the router that connects to the IPv4 destination
network.
� Version- IP version number (Version 4)
� IHL – Internet Header Length� Type of Service – Type of
service
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IPv4 header
� Total Length – Total length of the packet
� Identification -� Flags – Indications as to
whether a packet has been segmented
� Fragment offset – Offset if a packet must be segmented
� Time to live – number of hops before packet is
dicarded� Protocol – identifier for the
program to be run at the destination (TCP, UDP
OSPF, etc etc)� Header Checksum – A checksum CRC-32 for the
header only� Source address – a 32 bit address for the source address
� Destination address – a 32 bit address for the destination
address� Options – Extra options required to be passed to the
destination� Padding – padding to complete 32 bit fields if
options don’t use all 32 bits of a word.
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