IPv4 Header
Day 10
IPv4 Header
Version
Version is 4 bits long and it identifies the version of IP used: IPv4 - 4 (0100) or IPv6 - 6 (0110). IPv6 has a different structure of the header.
IHL
IHL (Internet Header Length) is 4 bits long. 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 in 4-byte increments, e.g. if the value is 5, then the length is 20 bytes. The minimum value is 5, and the maximum is 15. So, the minimum IPv4 header length is 20 bytes, and the maximum is 60 bytes.
DSCP
DSCP (Differentiated Services Code Point) is 6 bits long. It is used for QoS (Quality of Service), to prioritize delay-sensitive data (video/audio streaming).
ECN
ECN (Explicit Congestion Notification) is 2 bits in length. It provides end-to-end notification of network congestion without dropping packets. It is an optional field which requires both endpoints and the underlying network infrastructure to support it.
Total Length
Total Length is 16 bits long and indicated the total length of the packet in bytes. The minimum value is 20, and the maximum is 65535 (maximum 16-bit value).
Identification
Identification field is 16 bits. If a packet is fragmented because of being too large, this field helps 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.
Flags
Flags field in 3 bits and used to control/identify fragments. The first bit is always set to 0 (zero). The second bit is DF (Don't Fragment) bit and is used to indicate that the packet should not be fragmented. The third bit is MF (More Fragments) bit and is set to 1 if there are more fragments in the packet or 0 (zero) for the last fragment or if a packet is not fragmented.
Fragment Offset
Fragment Offset is 13 bits long and is used to indicate the position of the fragment within the original packet. It allows fragmented packets to reassemble even if they arrive out of order.
Time To Live
TTL is 8 bits long field. It is used to prevent infinite loops when the packet is continuously sent around without reaching its destination. It indicates a hop count - each time router receives a packet, it decreases the TTL value by 1. And when it reaches 0 (zero), a router drops the packet. The recommended default value of TTL is 64.
Protocol
Protocol field length is 8 bits and it indicates the protocol of the encapsulated segment (Layer 4 PDU). Value 6 is used for TCP, 17 - for UDP, 1 - for ICMP, and 89 - for OSPF (Open Shortest Path First).
Header Checksum
Header Checksum is 16 bits long. A calculated checksum is used to check for errors in the IPv4 header. When a router receives a packet, it calculates the header checksum and compares it to the one in the header. If there is a mismatch, the router drops the packet.
Source/Destination IP Addresses
An IP address is 32 bits long. Source indicates the sender, and destination - the intended receiver.
Options
The length of the Options field is 0-320 bits. This field is optional and rarely used. If the IHL field is greater than 5, Options are present.
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