Editing Transmission Control Protocol (section)

===TCP checksum for IPv4===
When TCP runs over [[IPv4]], the method used to compute the checksum is defined as follows:{{sfn|RFC 9293}}
<blockquote>
''The checksum field is the 16-bit ones' complement of the ones' complement sum of all 16-bit words in the header and text. The checksum computation needs to ensure the 16-bit alignment of the data being summed. If a segment contains an odd number of header and text octets, alignment can be achieved by padding the last octet with zeros on its right to form a 16-bit word for checksum purposes. The pad is not transmitted as part of the segment. While computing the checksum, the checksum field itself is replaced with zeros.''</blockquote>

In other words, after appropriate padding, all 16-bit words are added using [[end-around carry|ones' complement arithmetic]]. The sum is then bitwise complemented and inserted as the checksum field. A pseudo-header that mimics the IPv4 packet header used in the checksum computation is as follows:
{{APHD|start|title=TCP pseudo-header for checksum computation (IPv4)}}
{{APHD|0|bits1=32|background1=mistyrose|field1=Source address}}
{{APHD|4|bits1=32|background1=mistyrose|field1=Destination address}}
{{APHD|8|bits1=8|bits2=8|bits3=16|background1=mistyrose|background2=mistyrose|background3=mistyrose|field1=Zeroes|field2=Protocol|value2=6|field3=TCP length}}
{{APHD|12|bits1=16|bits2=16|field1=Source port|field2=Destination port}}
{{APHD|16|bits1=32|field1=Sequence number}}
{{APHD|20|bits1=32|field1=Acknowledgement number}}
{{APHD|24|bits1=4|bits2=4|bits3=8|bits4=16|field1=Data offset|field2=Reserved|field3=Flags|field4=Window}}
{{APHD|28|bits1=16|background1=lightpink|field1=Checksum|bits2=16|field2=Urgent pointer}}
{{APHD|32|bits1=32|background1=linen|field1=(Options)}}
{{APHD|36|bits1=0|field1=Data}}
{{APHD|end}}
The checksum is computed over the following fields:
;{{APHD|def|name=Source address|length=32 bits|text=The source address in the IPv4 header}}
;{{APHD|def|name=Destination address|length=32 bits|text=The destination address in the IPv4 header}}
;{{APHD|def|name=Zeroes|length=8 bits|text=All zeroes}}
;{{APHD|def|name=Protocol|length=8 bits|text=The protocol value for TCP: {{val|6}}}}
;{{APHD|def|name=TCP length|length=16 bits|text=The length of the TCP header and data (measured in octets). For example, let's say we have IPv4 packet with Total Length of {{val|200|u=bytes}} and IHL value of 5, which indicates a length of {{val|5|x|32|u=bits}} = {{val|160|u=bits}} = {{val|20|u=bytes}}. We can compute the TCP length as {{nowrap|(Total Length) &minus; (IPv4 Header Length)}} i.e. {{nowrap|200 &minus; 20}}, which results in {{val|180|u=bytes}}.}}