Manav Goyal

Network Layer - Computer Networks

Theory

  • Categories of Network
    • Local Area Network (LAN)
      • Network in a single office, building, or campus
    • Metropolitan Area Network (MAN)
      • Network extended over an entire city
    • Wide Area Network (WAN)
      • Network providing long-distance communication over a country, a continent, or the whole world
  • Responsibilities
    • Host to Host (Source to Destination/Machine to Machine) Delivery
      • Delivery of Packets across multiple networks
      • Network Layer
    • Uses Logical Address (IP Address) to perform Packet delivery
      • IP gives information about message to be sent and to which Host in that network
    • Routing => Decides the path to be followed using Router
      • RIP
      • OSPF
    • Fragmentation
      • Diving a larger message into small Fragments
    • Congestion Control
  • Addressing (TCP/IP Protocol) => Each address is related to one layer in the TCP/IP architecture
    • Physical
      • IPV4 Header
      • IPV6 Header
    • Logical
    • Port => A port address is a 16-bit address represented by one decimal number
    • Application Specific
  • Address Resolution Protocol (ARP)
    • Converting Logical address to Physical address => Converts IP address into Mac address (48 bit)
    • Level 3 protocol
      • Source sends
        • Source IP | Mac address | Destination IP | Broadcast Message
      • Message is sent to all, then destination reply in unicast way giving its Mac address
    • Types
      • Host - Host
      • Host - Router
      • Router - Host
      • Router - Router
    • Error

Classful Addressing

  • Before 1980 => Total 32 bits in IPV4 => 8 bit Network number and 24 bit Host number
  • Classful Addressing => Dotted Decimal Representation
    • 32 bits are divided into 8 bits octet
    • NID → HID → PID
  • Broadcast Address
    • Limited
      • Address is 255.255.255.255
      • Broadcast to all hosts of the network from inside the network
    • Direct
      • Address is Last address of that network
      • Broadcast to all hosts of the network from different network
  • Problems of Classful Addressing
    • Wastage of IP Address
    • Maintenance is time consuming
    • More prone to errors
  • Network Address Translation (NAT)
    • To solve IPv4 low number problem
    • Translates IP address
      • Public IP to Private IP
      • Private IP to Public IP
    • NAT Translation Table
      • | Port | Private IP | Public IP |

Class A

  • First 1 bit of 1st octet = 0
    • Used as a Prefix representing Class A
  • First 1 octet represents Network ID
    • Number of IP address = 231
    • Number of Networks = 27 - 2 = 128 - 2 = 126
      • 00000000 & 01111111 are not given to anyone
  • Last 24 bits (3 octet) represents Host ID
    • Number of hosts/users possible in every Networks = 224 - 2
      • N.0.0.0 represents the whole network
      • N.255.255.255 is used as Directed Broadcast Address (DBA) to send packet to every host
  • Default Mask = 255.0.0.0
    • Doing & operation of IP address will give Network ID
    • It is number of continuous 1

Class B

  • First 2 bit of 1st octet = 10
    • Range = 128 to 191 = 64 values
  • First 2 octet represents Network ID
    • Number of IP address = 230 => About 25% of total
    • Number of Networks = 214 = 16384
  • Last 2 octet represents Host ID
    • Number of Host possible in every Network = 216 - 2 = 65536 - 2 = 65534
      • 10N.M.0.0 represents the whole network
      • 10N.M.255.255 is used as DBA
  • Default Mask = 255.255.0.0

Class C

  • First 3 bit of 1st octet = 110
    • Range = 192 to 223 = 32 values
  • First 3 octet represents Network ID
    • Number of IP address = 229 => About 12.5% of total
    • Number of Networks = 221 = About 20 Lakh
  • Last 1 octet represents Host ID
    • Number of Host possible in every Network = 28 - 2 = 256 - 2 = 254
      • 110N.M.P.0 represents the whole network
      • 110N.M.P.255 is used as DBA
  • Default Mask = 255.255.255.0

Class D

  • First 4 bit of 1st octet = 1110
    • Range = 224 to 239 = 15 values
  • All 4 octet represents Network ID
    • Number of IP address = 228 => About 6.25% of total => About 250 Million
    • Number of Networks = 0
    • Number of Host = 0
  • All address are reserved for multi-casting or Group email/broadcast

Class E

  • First 4 bit of 1st octet = 1111
    • Range = 240 to 255 = 15 values
  • All 4 octet represents Network ID
    • Number of IP address = 228 => About 6.25% of total => About 250 Million
  • All address are reserved for Military purpose

Classless Addressing (CIDR)

  • After 1993 => No classes => Only Blocks
  • Format
    • Block ID => Represents network
    • Host ID
  • Notation
    • x.y.z.w/n
      • n represents mask
    • Number of bits for Block ID = n
    • Number of bits for Host ID = 32 - n
      • Number of Host = 232 - n
    • Network ID = n & IP address
  • Rules
    • Address should be contiguous
    • Number of address in a block must be in power of 2
    • First address of every block must be evenly divisible with size of block
      • Last n bits of the first address should be 0, where n is power of 2

Subnetting

  • Dividing the big network into small network
  • Classful Addressing
    • Divide the hosts into 2 parts by fixing the 1st bit of the host
      • Subnet ID = 1st address
      • Subnet DBA = Last address
    • Number of hosts gets reduced
    • One router is placed inside the network to decide subnet for sending packets
      • Calculated by subnet mask => Number of continuous 1
    • Computation increased
      • NID → SID → HID → PID
    • Variable length Subnet Masking (VLSM)
      • Keep fixing the MSB bit to divide the remaining hosts into 2 parts
      • Available Hosts => 2n - 2n, where n is number of division
      • Router is set up inside the network
  • Classless Addressing (CIDR)
    • Fix the MSB bit of host ID to divide the remaining hosts into 2 parts
      • Mask will increase by 1, each time of a division
    • Router is set up inside the network
      • Do & operation of the packet received with the Default mask of all the interface, if you get that interface then it is the interface
      • If more than one answer than choose the interface will more number of 1 in prefix, as searching will become easier due to lesser number of 1
    • VLSM
      • Keep fixing the MSB bit of host ID to divide the remaining hosts into 2 parts
        • Mask will increase by 1, each time of a division

IPv4

  • Connectionless
  • Datagram Service => Can choose any path
  • Use a 32-bit (4-byte) physical address written as decimal digits, Every byte (2 hexadecimal digits) is separated by a dot
    • Payload => Data received from Transport Layer
      • Size from 0 to 65525 bytes
    • Total value is 216
  • Error
  • Version (VER) = 0100 = 4
  • Header Length (HLEN)
    • Header Size => 160 bits => 20 - 60 Bytes
      • Should be in scale of 4
    • Its actual value is written by multiplying by 4
      • Minimum value will be 5
  • Differentiated Services Code Point (DSCP)
    • Traffic Type
      • First 3 bits represents precedence => Like Priority defined, Routine packet, Immediate send
      • 4th Bit => Delayed => Can afford delay or not
        • Voice call cannot afford delay
      • 5th Bit => Throughput => Rate of transfer more or less
      • 6th Bit => Reliability => Packet reach reliably and does not drop in the way
        • File transfer needs to be reliable
      • 7th Bit => Cost => Less cost by following shortest path or Routine send
      • 8th Bit => Reserved for future like DSCP
    • DSCP
      • Changes made later
      • 0 to 5 bits are reserved for DSCP
      • 6 to 7 bits are reserved for Explicit Congestion Notification (ESN) => To reduce the speed if congestion increases
  • Total Length = 65535 = 216
  • Fragmentation
    • Identification Bits
      • Identify if the packet belongs to one Packet when they reach the destination from different routes
    • Flag
      • 1st Bit, MSB is reserved = 0
      • 2nd Bit, Do not Fragment (DF)
        • 0 means Fragmentation is done
        • 1 means Fragmentation is not done
      • 3nd Bit, More Fragment (MF)
        • 1 if there is more fragment behind it
    • Fragment Offset
      • Number of Payload Bytes ahead of it
      • Answer is written after diving by 8
  • Time To Live (TTL)
    • If Packet is struck on loop due to data changes or router does not contains proper information
    • Else will create congestion
  • Protocols => Which protocols are being used
  • Header Checksum => Used for error detection
  • Options & Padding => Some extra information, Not mandatory
    • If header is of 10 Byte than this is 0, If header is of 60 Byte than this is of 40 Byte
    • Record Route
      • Route of the node is recorded
      • Maximum possible number of router whose address can be stored is 40B/4B < 10
    • Source Routing => Source will give the information about which address it will pass through
      • Strict Source Routing => Exact route is defined
      • Loose Source Routing => Exact route is not defined
    • Padding
      • Adding extra bytes to the size of address to make it in multiple of 4

IPv6

  • Connectionless, Datagram Service
  • Low processing for routers as size of base header is reduced
  • Error
  • Version (VER) = 0110 = 6
  • Priority
    • Traffic Type
    • Used for Congestion control
  • Flow Label
    • Used for real time data processing
    • Datagram circuit is converted into virtual circuit
      • Makes sure that all packets follow same path, Reservation is used for less data loss
  • Payload Length = 65535
    • Jumbograms => Extra Packets up to 4 GB
      • Using Extension Headers
  • Next Header => Contains information about Extension header if added
    • Format => Base Header | Extension Headers | Data
    • Base Header => 40 bytes = 320 bits
      • Mandatory field, Fixed size
  • Hop Limit
    • Maximum hops it can go to
    • Values keeps on decreasing and Packet removed to avoid congestion
  • Extension Headers
    • Routing Header
      • Sender decides which router the Packet should go through
    • Hop by Hop Option
      • Information that you want to provide to the hops you go through
    • Fragment Header
      • Like IPv4 but Fragmentation cannot be done by router but the source only
    • Authentication Header
      • To send User password, Data integration maintain, Hash values
    • Destination options
      • Data that only destination should read
    • Encapsulating Security Payload
      • Uses encryption & decryption for security

Routing Protocols

  • Works on Uni-casting
  • Forwarding
    • Sending the Packet in the right path
  • Inter-networks
    • Two or more networks connected become an inter-network, or internet
    • Internet is divided into Autonomous systems
  • Opens the Packet and decides by the help of Routing table, where to send, Every router maintains its Routing table
    • Static Routing Table => Made Manually
    • Dynamic Routing Table => Made Automatically
  • Types
    • Intra Domain => Within same Autonomous Systems
      • Distance Vector Routing (DVR) => Routing Information Protocol (RIP)
      • Link State Routing (LSR) => Open Shortest Path First (OSPF)
    • Inter Domain => Within different Autonomous Systems
      • Path Vector Routing (PVR) => Border Gateway Protocol (BGP)
  • Distance Vector Routing (DVR) => Routing Information Protocol (RIP)
    • Routing Table (RT)
      • | Destination/Node/Router | Cost/Distance/Propagation Delay | Next |
    • Steps
      • Every Router in the network will know the total number of Router in network
      • First RT will only contain information about its neighbors, all others cost will be infinite, creating a local RT
      • Then information of Distance vector will be shared with all its neighbors
      • Now each will update and create its new Routing Table
      • All the route will go thorough its neighbor, If more than one neighbor than choose the minimum value
      • Similarly it goes on
    • Count to Infinity Problem
      • When link breaks and wrong interpretation is received as only Distance vector is being sent and not the actual path
  • Link State Routing (LSR) => Open Shortest Path First (OSPF)
    • First Link state table is created
      • Contains information about its neighbor and their distance
      • Also contains extra information about Sequence number and TTL
    • Flooding
      • Each router will sent its Link state table to every other Router
      • High Bandwidth & Congestion
      • Global Knowledge => Each node will know the entire connection diagram
    • Dijkstra's Algorithm => Single source shortest path
      • Each node will find its shortest path to every other node
    • Routing Table (RT)
      • | Node/Router | Cost/Distance | Via |
  • Path Vector Routing (PVR) => Border Gateway Protocol (BGP)
  • IP Security (IPSec)
    • Used both by IPv4 & IPv6
    • Uses
      • Confidentiality, Authentication/Integrity, Replay attack protection
    • Collection of protocols
      • Encapsulating security payload (ESP)
      • Authentication header (AH)
      • Internet key exchange (IKE)
    • Two modes of operation
      • Transport mode
        • Used for end to end communication
        • Encapsulates the message received from the transport layer with IPSec header & tail before adding IPv4 or IPv6 header
      • Tunnel mode
        • Generally used in VPN
        • Used for router to router, More secure
        • Encapsulates the message received from the transport layer with IPSec header & tail after adding IPv4 or IPv6 header
        • Again IPv4 or IPv6 header needs to be added as original header gets hidden
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