Fields
Version
Length: 4 bits
Identifies the version of IP used.
IPv4 = 4 (0100)
IPv6 = 6 (0110)
Internet Header Length (IHL)
Length: 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.
Identifies the length of the header in 4-byte incremets.
Value of 5 = 5 * 4-bytes = 20 bytes.
Minimum value is 5 (60 bytes).
DSCP
Differentiated Services Code Point
Length: 6 bits
Used for QoS (Quality of Service).
Used to prioritize delay-sensitive data (streaming voice, video, etc).
ECN
Explicit Congestion Notification
Length: 2 bits
Provides end-to-end 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: 16 bits
Indicates the toal length of the packet (L3 header + L4 segment).
Measured in bytes (not 4-bytes increments like IHL).
Minimum value of 20 (meaning a IPv4 header with no encapsulated data).
Maximum value of 65,535 (maximum of 16-bit value).
Identification
Length: 16 bits
If a packet is fragmented ue to being too large, this field is used to identify which packet the fragment belong to.
All fragments of the same packet will have their own IPv4 header with the same alue in this field.
Packets are fragmented if larger than the MTU.
The MTU is usually 1500 bytes
Flags
Length: 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.
Offset
Length: 13 bits
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.
Time To Live
Length: 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, indicates a hop count: each time the packet arrives at a router, the router decreases the TTL by 1.
Protocol
Length: 8 bits
Indicates the protocol of the encapsulated L4PDU.
Value of 6: TCP
Value of 17: UDP
Value of 1: ICMP
Value of 89: OSPD (dynamic routing protocol)
Heaer Checksum
Length: 16 bits
A calculated checksum used to check for erros in the IPv4 header.
When a router receives a packet, it calculates the checksum of the heaer and compares it to the one this field of the header.
If they do not match, the router drops the packet.
Used to check for erros only in the UPv4 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/Destination IP Address
Length: 32 bits (each)
Source IP Address
IPv4 address of the sender of the packet.
Destination IP Address
IPv4 address of the intended receiver of the packet.
Options
Length: 0 to 320 bits
Rarely used.
If the IHL field is greater than 5, it means that Options are present.
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