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The_IPv4_Header.md

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10. THE IPv4 HEADER

INTERNET PROTOCOL version 4 HEADER or IPv4 HEADER

HEADER is used at LAYER 3 to help send data between devices on separate networks, even on other sides of the world over the Internet.

This is known as ROUTING.

THE IPv4 HEADER is used to ENCAPSULATE a TCP or UDP Segment.

To Review:

image

FIELDS OF THE IPv4 HEADER

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FIELD # OF BITS
VERSION 4
IHL 4
DSCP 6
ECN 2
TOTAL LENGTH 16
IDENTIFICATION 16
FLAGS 3
FRAGMENT OFFSET 13
TIME TO LIVE 8
PROTOCOL 8
HEADER CHECKSUM 16
SOURCE ADDRESS 32
DESTINATION ADDRESS 32
OPTIONS 320 Max

VERSION:

  • LENGTH is 4 bits.
  • IDs version of IP used (IPv4 or IPv6)
    • IPv4 = 0100 in Binary (Decimal 4)
    • IPv6 = 0110 in Binary (Decimal 6)

INTERNET HEADER LENGTH (IHL):

  • LENGTH is 4 bits.
  • Final field of IPv4 Header (Options) is variable in length so this field is necessary to indicate the total length of the header.
  • IDs the length of the header in 4-BYTE INCREMENTS.
  • The MINIMUM value is 5 (5 * 4-bytes = 20 bytes) - Empty OPTIONS Field
  • The MAXIMUM value is 15 (15 * 4-bytes = 60 bytes)

MINIMUM IPv4 HEADER LENGTH = 20 Bytes! MAXIMUM IPv4 HEADER LENGTH = 60 Bytes!

DSCP (Differentiated Services Code Point):

  • LENGTH is 6 bits.
  • Used for QoS (Quality of Service)
  • Used to prioritize delay-sensitive data (streaming voice, video, etc.)

ECN (Explicit Congestion Notification):

  • LENGTH is 2 bits.
  • Provides end-to-end (between two endpoints) notification of network congestion WITHOUT dropping packets.
  • Optional feature that requires both endpoints, as well as the underlying network infrastructure to support it.

TOTAL LENGTH:

  • LENGTH is 16 bits.
  • Indicates the TOTAL length of the packet (L3 Header + L4 Segment)
  • Measured in bytes (not 4-byte increments like IHL)
  • Minimum value of 20 Bytes (IPv4 Header with NO encapsulated data)
  • Maximum value of 65,535 (MAXIMUM 16-bit value) = 2^16

IDENTIFICATION:

  • LENGTH is 16 bits.
  • If a packet is fragmented due to being too large, this field is used to identify which packet the fragment belongs to.
  • All fragments of the same packet will have their own IPv4 header with the same value in this field.
  • Packets are fragmented, if larger than the MTU (Maximum Transmission Unit)
  • The MTU is usually 1500 bytes (Max size of an Ethernet frame)
  • Fragments are reassembled by the receiving host.

FLAGS:

  • LENGTH is 3 bits
  • Used to control/identify fragments.
  • Bit 0: Reserved, always set to 0.
  • Bit 1: Don't Fragment (DF bit), used to indicate a packet that should not be fragmented.
  • Bit 2: More Fragments (MF bit), set to 1 if there are more fragments in the packet, set to 0 for the last fragment or NO fragments.

FRAGMENT OFFSET:

  • LENGTH is 13 bits
  • Used to indicated the position of the fragment within the original, unfragmented IP Packet.
  • Allows fragmented packets to be reassembled even if the fragments arrive out of order.

TIME TO LIVE (TTL):

  • LENGTH is 8 bits
  • A router will drop a packet with a TTL of 0
  • Used to prevent infinite loops
  • Originally designed to indicated a packets maximum lifetime in seconds.
  • In practice, indicates a 'hop count': each time the packet arrives at a router, the router decreases the TTL by 1.
  • Recommended default TTL is 64.

PROTOCOL:

  • LENGTH is 8 bits
  • Indicates the protocol of the encapsulated Layer 4 PDU
  • Value of 1 : ICMP
  • Value of 6 : TCP
  • Value of 17 : UDP
  • Value of 89 : OSPF (Dynamic Routing Protocol)
  • List of protocol numbers on Wikipedia : List of IP Protocol Numbers

HEADER CHECKSUM:

  • LENGTH is 16 bits
  • A calculated checksum used to check for errors in the IPv4 header.
  • When a router receives a packet, it calculates the checksum of the header and compares it to the one in this field of a header.
  • If they do not match, the router drops the packet.
  • Used to check for ERRORS only in the IPv4 Header.
  • IP relies on the encapsulated protocol to detect errors in the encapsulated data.
  • Both TCP and UDP have their own checksum fields to detect errors in the encapsulated data.

SOURCE and DESTINATION:

  • LENGTH is 32 bits each
  • SOURCE IP = IPv4 ADDRESS of the Sender of the Packet.
  • DESTINATION IP = IPv4 ADDRESS of the intended Receiver of the Packet.

OPTIONS:

  • LENGTH is 0-320 bits
  • Optional / Rarely Used
  • If the IHL field is greater than 5, it means that Options are present.