Editing Transmission Control Protocol (section)

===TCP checksum for IPv6===
When TCP runs over [[IPv6]], the method used to compute the checksum is changed:{{sfn|RFC 8200}}
<blockquote>
''Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses.''
</blockquote>
A pseudo-header that mimics the IPv6 header for computation of the checksum is shown below.
{{APHD|start|title=TCP pseudo-header for checksum computation (IPv6)}}
{{APHD|0|bits1=128|background1=mistyrose|field1=Source address}}
{{APHD|16|bits1=128|background1=mistyrose|field1=Destination address}}
{{APHD|32|bits1=32|background1=mistyrose|field1=TCP length}}
{{APHD|36|bits1=24|bits2=8|background1=mistyrose|background2=mistyrose|background3=mistyrose|field1=Zeroes|field2=Next header|value2=6}}
{{APHD|40|bits1=16|bits2=16|field1=Source port|field2=Destination port}}
{{APHD|44|bits1=32|field1=Sequence number}}
{{APHD|48|bits1=32|field1=Acknowledgement number}}
{{APHD|52|bits1=4|bits2=4|bits3=8|bits4=16|field1=Data offset|field2=Reserved|field3=Flags|field4=Window}}
{{APHD|56|bits1=16|background1=lightpink|field1=Checksum|bits2=16|field2=Urgent pointer}}
{{APHD|60|bits1=32|background1=linen|field1=(Options)}}
{{APHD|64|bits1=0|field1=Data}}
{{APHD|end}}
The checksum is computed over the following fields:
;{{APHD|def|name=Source address|length=128 bits|text=The address in the IPv6 header.}}
;{{APHD|def|name=Destination address|length=128 bits|text= The final destination; if the IPv6 packet doesn't contain a Routing header, TCP uses the destination address in the IPv6 header, otherwise, at the originating node, it uses the address in the last element of the Routing header, and, at the receiving node, it uses the destination address in the IPv6 header.}}
;{{APHD|def|name=TCP length|length=32 bits|text=The length of the TCP header and data (measured in octets).}}
;{{APHD|def|name=Zeroes|length=24 bits|constraint=<code>Zeroes == 0</code>|text=All zeroes.}}
;{{APHD|def|name=Next header|length=8 bits|text=The protocol value for TCP: {{val|6}}.}}