IPv4 Header

Version field

Length of 4 bits

• It identifies the version of IP used, which are:

Version	IP	Decimal
0100	IPv4	4
0110	IPv6	6

IHL - Internet Header Length

Length of 4 bits

• The final field of the IPv4 header (Options) is variable in length, so this field is necessary to indicate the total length of the header.

• It identifies the length of the header in 4-byte increments.

e.g:

The IHL is 5 which means that the length of the header is 5 x 4 = 20 bytes

• The minimum value is 5 (=20 bytes). That is the length of the IPv4 header without any IP options at the end.

• The maximum value is 15 (15 x 4 = 60 bytes).

DSCP - Differentiated Services Code Point

Length of 6 bits

• It is used for QoS (Quality of Service)

• It is used to prioritize delay-sensitive data (streaming voice, video, etc.)

ECN - Explicit Congestion Notification

Length of 2 bits

• Provides end-to-end (between two endpoints) notification of network congestion without dropping packets.

• It's a 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 (=IPv4 header with no encapsulated data)

• Maximum value is 65,535 (maximum 16-bit value)

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

• Maximum payload size of a Ethernet frame is 1500 bytes (these are related).

• Fragments are reassembled by the receiving host.

Flags

Length of 3 bits

• Used to control/identify fragments.

  • The first bit is 0: Reserved, always set to 0.

  • The second bit is "Dont Fragment" (DF bit), used to indicate a packet that should not be fragmented.

  • The third bit is "More Fragments" (MF bit), set to 1 if there are more fragments in the packet, set to 0 for the last fragment.

Unfragmented packets will always have their MF bit set to 0.

Fragment Offset

13 bits in length

• Used to indicate 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.

TTL - Time To Live

Length is 8 bits

• A router will drop a packet with a TTL of 0.

• Used to prevent infinite loops.

• Originally designed to indicate the packet's maximum lifetime in seconds.

• In practice, it indicates a 'hop count':

  • each time the packet arrives at a router, the router decreases the TTL by 1.

The recommended default TTL is 64.

Protocol

Length of 8 bits

• It indicates the protocol of the encapsulated Layer 4 PDU.

  • 6 - for TCP

  • 17 - for UDP

  • 1 - for ICMP

  • 89 - for OSPF (dynamic routing protocol)

Header Checksum

Length of 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 the header.

• If they do not match, the router drops the packet.

• Only used to check for errors only in the IPv4 header.

  • IP relies on the encapsulated protocol to detect error in the encapsulated data.

  • Both TCP and UDP have their own checksum fields to detect errors in the encapsulated data.

Source/Destination IP Address

Length of 32 bits (each)

• The source IP address indicates the IPv4 address of the sender of the packet.

• The destination IP address indicated the IPv4 address of the intended receiver of the packet.

Options

Length varies from 0 to 320 bits.

• Rarely used.

• If the IHL field is greater than 5, it means that Options are present.