Internet Protocol

OSI Model

• Layer 7: Application

• Layer 6: Presentation

• Layer 5: Session

• Layer 4: Transport Layer

  • TCP / UDP / DCCP / SCTP / RSVP / QUIC

• Layer 3: Network - IP / Internet Layer -

  • IP, ICMP, NDP, ECN, IGMP,

  • IP Packet - (Source IP + Destination IP + IP Data)

• Layer 2: Data Link

• Layer 1: Physical Layer

Network & Host

• a.b.c.d/x - x is the network bits and remains are host

• Example 192.168.254.0/24

  • The first 24 bits (3 byte) are network, the rest 8 are hosts

  • So - 2^24 - networks

  • each network has 2^8 hosts (255)

• 192.168.254.0/24 is also called a subnet

  • Subnet Mask: The subnet has a mask - 255.255.255.0

• Default Gateway

  • Most networks consists of hosts and default gateway

  • Two hosts can communicate directly, if they are in teh same subnet.

  • Each gateway has an IP address and host know its gateway. They communicate to other hosts which are not in the same subnet via the gateway.

  • Gateway is connected to multiple network and has multiple IP address - one for each network.

IP Packet

• Has headers and data sections

  • Headers - 20byte, can be upto 60 bytes if options are enabled

  • Data section can go upto 65536 (2^16) - 65KBs

  • average MTU (Maximum Transmission Unit) is 1500, so generally

• Headers

  • Version (4)

  • IHL (4) - Internet Header Length

  • DSCP (6) -

  • ECN (2)-

    • When a routers' buffer is about to fill, it will set ECN

    • The receiver receives it and in the next pcaket it will send with ECN set to to true, so client and server eventually both knows, they are experience congestion

  • Total length (16 bit) - hence max can be 2^16

  • Identification

  • Flags

  • Fragment Offset

  • Time to live

    • Sender sends a counter (no. of hops) in which it can reach

    • All intermediate routers, would decrement by one and send it forward.

    • When its zero, the router will stop forwarding it and send an ICMP message

  • Protocol

  • Header Checksum

  • Source IP Address

  • Destination IP Address

  • Options (if HL > 5)

  • Data

ICMP - Internet Control Message Protocol

• Layer 3 protocol

• Designed for informational messages

  • Host unreachable, port unreachable, fragmentation needed

  • Packet expired (infinite loop in routers) - TTL

• Uses IP directly

• PING and traceroute uses it.

• Doesn't require listeners or ports to be opened.

• TCP Blackhole - when ICMP is blocked but your packet needed fragemenation. The router

UDP - User Datagram Protocol

• Layer 4 protocol

• Ability to address processes in a host using ports.

• 8 byte header Datagram

• UDP is stateless and no prior communication is required.

• Usecases

  • Video streaming

  • VPN (Generally

  • DNS

  • WebRTC

• Headers

  • Source port

  • Destination port

  • Length

  • Checksum

• Pros

  • as

• Cons

Multiplexing & Demultiplexing

• Host runs many apps on different ports

• Sender multiplexes all its apps into UDP

• Receiver demultiplex UDP datagram to each apps.