Section 6. IANA Considerations

In the "Transmission Control Protocol (TCP) Header Flags" registry,
   IANA has made several changes as described in this section.
   RFC 3168 originally created this registry but only populated it with
   the new bits defined in RFC 3168, neglecting the other bits that had
   previously been described in RFC 793 and other documents.  Bit 7 has
   since also been updated by RFC 8311 [54].
   The "Bit" column has been renamed below as the "Bit Offset" column
   because it references each header flag's offset within the 16-bit
   aligned view of the TCP header in Figure 1.  The bits in offsets 0
   through 3 are the TCP segment Data Offset field, and not header
   flags.
   IANA has added a column for "Assignment Notes".
   IANA has assigned values as indicated below.
      +========+===================+===========+====================+
      | Bit    | Name              | Reference | Assignment Notes   |
      | Offset |                   |           |                    |
      +========+===================+===========+====================+
      | 4      | Reserved for      | RFC 9293  |                    |
      |        | future use        |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 5      | Reserved for      | RFC 9293  |                    |
      |        | future use        |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 6      | Reserved for      | RFC 9293  |                    |
      |        | future use        |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 7      | Reserved for      | RFC 8311  | Previously used by |
      |        | future use        |           | Historic RFC 3540  |
      |        |                   |           | as NS (Nonce Sum). |
      +--------+-------------------+-----------+--------------------+
      | 8      | CWR (Congestion   | RFC 3168  |                    |
      |        | Window Reduced)   |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 9      | ECE (ECN-Echo)    | RFC 3168  |                    |
      +--------+-------------------+-----------+--------------------+
      | 10     | Urgent pointer    | RFC 9293  |                    |
      |        | field is          |           |                    |
      |        | significant (URG) |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 11     | Acknowledgment    | RFC 9293  |                    |
      |        | field is          |           |                    |
      |        | significant (ACK) |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 12     | Push function     | RFC 9293  |                    |
      |        | (PSH)             |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 13     | Reset the         | RFC 9293  |                    |
      |        | connection (RST)  |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 14     | Synchronize       | RFC 9293  |                    |
      |        | sequence numbers  |           |                    |
      |        | (SYN)             |           |                    |
      +--------+-------------------+-----------+--------------------+
      | 15     | No more data from | RFC 9293  |                    |
      |        | sender (FIN)      |           |                    |
      +--------+-------------------+-----------+--------------------+
                         Table 7: TCP Header Flags
   The "TCP Header Flags" registry has also been moved to a subregistry
   under the global "Transmission Control Protocol (TCP) Parameters"
   registry <https://www.iana.org/assignments/tcp-parameters/>.
   The registry's Registration Procedure remains Standards Action, but
   the Reference has been updated to this document, and the Note has
   been removed.