Minimum and Maximum Length of the Header in the TCP Segment and the IP Datagram

Last Updated : 02 May, 2024

The minimum and maximum length of the header in the TCP segment is 20 Bytes and 60 bytes respectively. The minimum and maximum length of the header in the IP datagram are 20 Bytes and 60 Bytes respectively.

What is a TCP Header?

The below diagram represents the TCP header.

Necessary fields for TCP header	4B	Source Port					Destination Port
	4B	Sequence Number
	4B	Acknowledgment Number
	4B	Header Length	Reserved	URG	ACK	PSH	RST	SYN	FIN	Window Size
	4B	Checksum					Urgent Pointer
Optional Field for TCP header	0 Bytes to 40 Bytes	Options

Explanation for the Minimum and Maximum Length of Header in TCP Segment

In the header of the TCP segment, we may include or exclude the optional field as it is not necessary to include the optional field in every TCP segment. Other fields in the TCP header are always included in the TCP segment. To find the minimum length of the header in the TCP segment we take the Options field size as zero bytes and to find the maximum length of header in TCP segment we take Option field maximum size i.e., 40 Bytes. The size of the header is the sum of the size of other fields and size of option field.

Length of Header	Other Fields in TCP header	Option field in TCP header	Size of the Header in TCP Segment
Minimum	20 Bytes	0 Bytes	20B + 0B = 20 B
Maximum	20 Bytes	40 Bytes	20B + 40B = 60 B

Therefore,

Minimum Length of Header in TCP Segment = 20 Bytes or 160 bits

Maximum Length of Header in TCP Segment = 40 Bytes 0r 360 bits

What is IP Header?

The below diagram represents the IP header.

Necessary fields for IP header	4B	Version	Header Length	Type of Service	Total Length
	4B	Identification	Reserve Bit	DF	MF	Fragment offset
	4B	TTL	Protocol	Header Checksum
	4B	Source IP
	4B	Destination IP
Optional field for IP header	0B – 40B	Options

Explanation for the Minimum and Maximum Length of Header in IP Datagram

In the header of the IP segment, we may include or exclude the optional field as it is not necessary to include the optional field in every IP datagram. Other fields in the IP header are always included in the IP datagram. To find the minimum length of the header in IP datagram we take Option field size as zero bytes and to find the maximum length of header in IP datagram we take Option field maximum size i.e., 40 Bytes. The size of the header is the sum of the size of other fields and size of option field.

Length of Header	Other Fields in IP header	Option field in IP header	Size of the Header in IP Datagram
Minimum	20 Bytes	0 Bytes	20B + 0B = 20 B
Maximum	20 Bytes	40 Bytes	20B + 40B = 60 B

Therefore,

Minimum Length of Header in IP Datagram= 20 Bytes or 160 bits

Maximum Length of Header in IP Datagram= 40 Bytes 0r 360 bits

Conclusion

From the above discussion we can conclude that the maximum and minimum length of header in the TCP segment and IP datagram is 20 Bytes and 40 Bytes respectively. Both the headers have option field which may and may not be included in the header. If we do not include the option field, then we get the minimum length of header and when we include maximum option field then we get the maximum header length.