IETF RFC 2873 PDF

This memo describes a conflict between TCP [RFC793] and DiffServ [RFC2475] on the use of the three leftmost bits in the TOS octet of an IPv4 header [RFC791]. In a network that contains DiffServ-capable nodes, such a conflict can cause failures in establishing TCP connections or can cause some established TCP connections to be reset undesirably. This memo proposes a modification to TCP for resolving the conflict.

Because the IPv6 [RFC2460] traffic class octet does not have any defined meaning except what is defined in RFC 2474, and in particular does not define precedence or security parameter bits, there is no conflict between TCP and DiffServ on the use of any bits in the IPv6 traffic class octet.