The following fields are defined for the TCP header, as described in [RFC793]:
All fields listed above are based on tcp_hdr, which in turn is defined as follows:
field tcp_hdr = ip_nexthdr if ip_proto == IPPROTO_TCP;
Therefore, accessing any TCP header field automatically adds the
precondition (see section 2.6.2)
ip_proto == IPPROTO_TCP.
fields.tc defines a few additional fields for TCP options. Because tcng has currently no mechanism for iterating over a list of options, these fields are of no practical use and are therefore not documented.