to compute the checksum is changed: 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. A pseudo-header that mimics the IPv6 header for computation of the checksum is shown below. Source address: the one in the IPv6 header Destination address: 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. TCP length: the length of the TCP header and data Next Header: the protocol value for TCP Checksum offload Many TCP/IP software stack implementations provide options to use hardware assistance to automatically compute the checksum in the network adapter prior to transmission onto the network or upon reception from the network for validation. This may relieve the OS from using precious CPU cycles calculating the checksum. Hence, overall network performance is increased. This feature may cause packet analyzers that are unaware or uncertain about the use of checksum offload to report invalid checksums in outbound packets that have not yet reached the network adapter. This will only occur for packets that are intercepted before being transmitted by the network adapter; all packets transmitted by the network adaptor on the wire will have valid checksums. This issue can also occur when monitoring packets being transmitted between virtual machines on the same host, where a virtual device driver may omit the checksum calculation (as an optimization), knowing that the checksum will be calculated later by the VM host kernel or its physical hardware. RFC documents – Specification of Internet Transmission Control Program, December 1974 Version – TCP v4 – includes some error corrections for TCP – TCP Extensions for High Performance [Obsoleted by RFC 7323] – Extending TCP for Transactions—Concepts [Obsoleted by RFC 6247] – Defending Against Sequence Number Attacks – TCP Selective Acknowledgment Options – TCP Congestion Control – Moving the Undeployed TCP Extensions to Historic Status – Computing TCP's Retransmission Timer – TCP Extensions for Multipath Operation with Multiple Addresses – TCP Extensions for High Performance – A Roadmap for TCP Specification Documents – Transmission Control Protocol (TCP) See also Connection-oriented