7. IANA Considerations

This section describes IANA (Internet Assigned Numbers Authority) registries and management related to TCP.

7.1. TCP Header Flags Registry

IANA maintains the "TCP Header Flags" registry for managing control bit assignments in the TCP header.

Currently Assigned Flags:

Bit	Name	Reference	Description
0	FIN	RFC 9293	Finish - No more data from sender
1	SYN	RFC 9293	Synchronize sequence numbers
2	RST	RFC 9293	Reset the connection
3	PSH	RFC 9293	Push function
4	ACK	RFC 9293	Acknowledgment field significant
5	URG	RFC 9293	Urgent pointer field significant
6	ECE	RFC 3168	ECN-Echo
7	CWR	RFC 3168	Congestion Window Reduced

Reserved Bits:

• Bits 8-11: Reserved for future use, must currently be set to zero

Registration Procedure:

• Assignment of new TCP header flags requires Standards Action
• Assignments are approved by the IESG

7.2. TCP Option Kind Numbers Registry

IANA maintains the "TCP Option Kind Numbers" registry.

Important Assigned Options:

Kind	Length	Name	Reference
0	-	End of Option List	RFC 9293
1	-	No-Operation	RFC 9293
2	4	Maximum Segment Size	RFC 9293
3	3	Window Scale	RFC 7323
4	2	SACK Permitted	RFC 2018
5	N	SACK	RFC 2018
8	10	Timestamps	RFC 7323
19	18	TCP-MD5 Signature	RFC 2385
28	4	User Timeout Option	RFC 5482
29	N	TCP-AO	RFC 5925
34	N	TCP Fast Open	RFC 7413
253	N	RFC3692-style Experiment 1	RFC 4727
254	N	RFC3692-style Experiment 2	RFC 4727

Registration Procedure:

• TCP option numbers 0-255 are managed by IANA
• Assignment of new options typically requires IETF Review or RFC publication

7.3. TCP Port Number Registry

IANA maintains the "Service Name and Transport Protocol Port Number Registry".

Port Ranges:

• 0-1023: System Ports / Well-Known Ports
  • Requires IETF Review or IESG approval
  • Used for standard services (e.g., HTTP:80, HTTPS:443, SSH:22)
• 1024-49151: User Ports / Registered Ports
  • Assigned through Expert Review
  • Used for specific applications and services
• 49152-65535: Dynamic Ports / Private Ports
  • Not assigned, used for ephemeral ports
  • Also called Ephemeral Ports

7.4. Updates and Modifications

RFC 9293 Updates to IANA Registries:

1. TCP Header Flags Registry Updates:

   • Clarified status of reserved bits
   • Confirmed CWR and ECE flags (from RFC 3168)
   • Updated registration procedure descriptions

2. TCP Option Registry Maintenance:

   • Confirmed mandatory options (0, 1, 2)
   • Referenced experimental options usage (253, 254)

3. Document Reference Updates:

   • Updated references from RFC 793 to RFC 9293

7.5. Experimental Values

RFC 4727 Experimental Options:

• Option kinds 253 and 254 are reserved for experimental use
• RFC 6994 describes mechanisms for shared use of experimental options
• Experimental options should not be used in production environments

7.6. Related Registries

Other Related IANA Registries:

• TCP Parameters - Top-level registry
• TCP Alternate Checksum Numbers - Alternate checksum algorithms
• TCP Urgent Pointer - Urgent pointer semantics
• TCP Timestamp Option - Timestamp option parameters

7.7. Registration Requests

How to Request New TCP Parameters:

1. Port Number Requests:

   • Visit IANA port number assignment page
   • Fill in service name, port number, transport protocol, description
   • Provide contact information and reference documentation

2. Option Kind Requests:

   • Typically through RFC publication process
   • Requires IETF working group discussion and consensus
   • IANA Considerations section must be included in RFC draft

3. Header Flag Requests:

   • Requires Standards Track RFC
   • Approved by IESG
   • Requires sufficient technical justification and community support

7.8. Registry Access

IANA Registry Locations:

Homepage: https://www.iana.org/
TCP Parameters: https://www.iana.org/assignments/tcp-parameters/
Port Numbers: https://www.iana.org/assignments/service-names-port-numbers/

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Note: This section is based on content from Section 7 of RFC 9293. For the latest registry information, visit the IANA official website.