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Transmission Control Protocol (TCP) Parameters

Last Updated: 2025-10-08
Available Formats: XML
HTML
Plain text

Registries Included Below

TCP Option Kind Numbers
TCP Alternate Checksum Numbers
Cryptographic Algorithms for TCP-AO Registration
MPTCP Option Subtypes
MPTCP Handshake Algorithms
TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)
TCP Encryption Protocol Identifiers
tcpcrypt AEAD Algorithms
MPTCP MP_TCPRST Reason Codes
TCP Header Flags

TCP Option Kind Numbers

Registration Procedure(s)

Standards Action or IESG Approval

Reference

[RFC2780]

Note

The Transmission Control Protocol (TCP) has provision for optional
header fields identified by an option kind field.  Options 0 and 1 are
exactly one octet which is their kind field.  All other options have
their one octet kind field, followed by a one octet length field,
followed by length-2 octets of option data.
[*] It is only appropriate to use these values in explicitly-
    configured experiments; they MUST NOT be shipped as defaults in
    implementations.  See [RFC3692] for details.
    
[**] These values have been deployed in the Internet without proper
    IANA assignment. At some point, these numbers may be officially
    allocated for some new purpose. It is therefore strongly recommended
    that implementers migrate to mechanisms with IANA-assigned values to
    avoid potential issues with their systems or products at a later point.
[***] This value has been deployed on the Internet in ways that are 
    not compatible with the documented use in this IANA allocation.

Note

Values 16-18 were assigned before October 1994 (see [RFC1700]), and 
values 20-26 were assigned between Feb 9, 1999 and May 10, 2000. No 
further documentation is available.

Available Formats

CSV

Kind	Length	Meaning	Reference
0	-	End of Option List	[RFC9293]
1	-	No-Operation	[RFC9293]
2	4	Maximum Segment Size	[RFC9293]
3	3	Window Scale	[RFC7323]
4	2	SACK Permitted	[RFC2018]
5	N	SACK	[RFC2018]
6	6	Echo (obsoleted by option 8)	[RFC1072][RFC6247]
7	6	Echo Reply (obsoleted by option 8)	[RFC1072][RFC6247]
8	10	Timestamps	[RFC7323]
9	2	Partial Order Connection Permitted (obsolete)	[RFC1693][RFC6247]
10	3	Partial Order Service Profile (obsolete)	[RFC1693][RFC6247]
11		CC (obsolete)	[RFC1644][RFC6247]
12		CC.NEW (obsolete)	[RFC1644][RFC6247]
13		CC.ECHO (obsolete)	[RFC1644][RFC6247]
14	3	TCP Alternate Checksum Request (obsolete)	[RFC1146][RFC6247]
15	N	TCP Alternate Checksum Data (obsolete)	[RFC1146][RFC6247]
16		Skeeter	[Stev_Knowles]
17		Bubba	[Stev_Knowles]
18	3	Trailer Checksum Option	[Subbu_Subramaniam][Monroe_Bridges]
19	18	MD5 Signature Option (obsoleted by option 29)	[RFC2385]
20		SCPS Capabilities	[Keith_Scott]
21		Selective Negative Acknowledgements	[Keith_Scott]
22		Record Boundaries	[Keith_Scott]
23		Corruption experienced	[Keith_Scott]
24		SNAP	[Vladimir_Sukonnik]
25		Unassigned (released 2000-12-18)
26		TCP Compression Filter	[Steve_Bellovin]
27	8	Quick-Start Response	[RFC4782]
28	4	User Timeout Option (also, other known unauthorized use) [***][1]	[RFC5482]
29		TCP Authentication Option (TCP-AO)	[RFC5925]
30	N	Multipath TCP (MPTCP)	[RFC8684]
31		Reserved (known unauthorized use without proper IANA assignment) [**]
32		Reserved (known unauthorized use without proper IANA assignment) [**]
33		Reserved (known unauthorized use without proper IANA assignment) [**]
34	variable	TCP Fast Open Cookie	[RFC7413]
35-68		Reserved
69	N	Encryption Negotiation (TCP-ENO)	[RFC8547]
70		Reserved (known unauthorized use without proper IANA assignment) [**]
71-75		Reserved
76		Reserved (known unauthorized use without proper IANA assignment) [**]
77		Reserved (known unauthorized use without proper IANA assignment) [**]
78		Reserved (known unauthorized use without proper IANA assignment) [**]
79-171		Reserved
172	N	Accurate ECN Order 0 (AccECN0)	[RFC-ietf-tcpm-accurate-ecn-34]
173		Reserved
174	N	Accurate ECN Order 1 (AccECN1)	[RFC-ietf-tcpm-accurate-ecn-34]
175-252		Reserved
253	N	RFC3692-style Experiment 1 (also improperly used for shipping products) [*]	[RFC4727]
254	N	RFC3692-style Experiment 2 (also improperly used for shipping products) [*]	[RFC4727]

TCP Alternate Checksum Numbers

Registration Procedure(s)

Not defined

Reference

[RFC1146]

Available Formats

CSV

Number	Description	Reference
0	TCP Checksum	[RFC1146]
1	8-bit Fletchers's algorithm	[RFC1146]
2	16-bit Fletchers's algorithm	[RFC1146]
3	Redundant Checksum Avoidance	[J_Kay]

Cryptographic Algorithms for TCP-AO Registration

Registration Procedure(s)

RFC Publication After Expert Review

Reference

[RFC5926]

Available Formats

CSV

Algorithm	Reference
SHA1	[RFC5926]
AES128	[RFC5926]

MPTCP Option Subtypes

Registration Procedure(s)

Standards Action

Reference

[RFC8684]

Note

This registry specifies the MPTCP Option Subtypes for MPTCP v1,
which obsoletes the Experimental MPTCP v0.  For the MPTCP v0
subtypes, please refer to [RFC6824].

Available Formats

CSV

Value	Symbol	Name	Reference
0x0	MP_CAPABLE	Multipath Capable	[RFC8684, Section 3.1]
0x1	MP_JOIN	Join Connection	[RFC8684, Section 3.2]
0x2	DSS	Data Sequence Signal (Data ACK and Data Sequence Mapping)	[RFC8684, Section 3.3]
0x3	ADD_ADDR	Add Address	[RFC8684, Section 3.4.1]
0x4	REMOVE_ADDR	Remove Address	[RFC8684, Section 3.4.2]
0x5	MP_PRIO	Change Subflow Priority	[RFC8684, Section 3.3.8]
0x6	MP_FAIL	Fallback	[RFC8684, Section 3.7]
0x7	MP_FASTCLOSE	Fast Close	[RFC8684, Section 3.5]
0x8	MP_TCPRST	Subflow Reset	[RFC8684, Section 3.6]
0x9-0xe	Unassigned
0xf	Reserved for Private Use		[RFC8684]

MPTCP Handshake Algorithms

Registration Procedure(s)

Standards Action

Reference

[RFC8684]

Note

This registry specifies the MPTCP Handshake Algorithms for
MPTCP v1, which obsoletes the Experimental MPTCP v0.  For the MPTCP
v0 subtypes, please refer to [RFC6824].

Available Formats

CSV

Flag Bit	Meaning	Reference
A	Checksum required	[RFC8684, Section 3.1]
B	Extensibility	[RFC8684, Section 3.1]
C	Do not attempt to establish new subflows to the source address.	[RFC8684, Section 3.1]
D-G	Unassigned
H	HMAC-SHA256	[RFC8684, Section 3.2]

TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)

Registration Procedure(s)

First Come First Served

Reference

[RFC6994][RFC9868]

Note

16-bit ExIDs can be used with either TCP or UDP; 32-bit ExIDs can be
used with TCP or their first 16 bits can be used with UDP. Use with
each transport (TCP, UDP) is indicated in the protocol column, as
defined in [RFC9868].

Note

IANA will record known duplicate uses.

Available Formats

CSV

Value	Protocol	Description	Reference
0x00AC	TCP	TCP ACK Rate Request	[draft-gomez-tcpm-ack-rate-request-02]
0x00BD	TCP	Bidirectional Traceroute	[Kevin_Vermeulen]
0x0348	TCP	HOST_ID	[RFC7974]
0x0A0D	TCP	Autonomous System Compensation	[draft-donnerhacke-linktax-02]
0x0CA0	TCP	TCP Capability Option	[draft-boucadair-tcpm-capability-option-00]
0x0ED0	TCP	Extended Data Offset	[draft-ietf-tcpm-tcp-edo-00]
0x454E	TCP	TCP-ENO [2]	[RFC8547]
0x5323	TCP	Service Number	[draft-touch-tcpm-sno-00]
0x75ECFFEE	TCP	Timestamp Interval	[draft-trammell-tcpm-timestamp-interval-00]
0xACC0	TCP	AccECN Experimental Option - Order 0 (current and new implementations SHOULD use option kind 172)	[RFC-ietf-tcpm-accurate-ecn-34]
0xACC1	TCP	AccECN Experimental Option - Order 1 (current and new implementations SHOULD use option kind 174)	[RFC-ietf-tcpm-accurate-ecn-34]
0xACCE	TCP	AccECN Experimental Option (current and new implementations SHOULD use option kinds 172 or 174)	[RFC-ietf-tcpm-accurate-ecn-34]
0xE2D4C3D9	TCP	Shared Memory Communications over RMDA protocol	[RFC7609]
0xF989	TCP	Fast Open (current and new implementations SHOULD use option 34)	[RFC7413]
0xF990	TCP	Low Latency	[draft-wang-tcpm-low-latency-opt-00]

TCP Encryption Protocol Identifiers

Registration Procedure(s)

IETF Review or Expert Review with RFC Required

Expert(s)

Unassigned

Reference

[RFC8547]

Available Formats

CSV

Value	Meaning	Reference
0x20	Experimental Use	[RFC8547]
0x21	TCPCRYPT_ECDHE_P256	[RFC8548]
0x22	TCPCRYPT_ECDHE_P521	[RFC8548]
0x23	TCPCRYPT_ECDHE_Curve25519	[RFC8548]
0x24	TCPCRYPT_ECDHE_Curve448	[RFC8548]
0x25-0x6f	Unassigned
0x70-0x7f	Reserved for extended values	[RFC8547]

tcpcrypt AEAD Algorithms

Registration Procedure(s)

IETF Review or Expert Review with RFC Required

Expert(s)

Unassigned

Reference

[RFC8548]

Available Formats

CSV

Value	AEAD Algorithm	Reference
0x0000	Reserved
0x0001	AEAD_AES_128_GCM	[RFC8548]
0x0002	AEAD_AES_256_GCM	[RFC8548]
0x0003-0x000f	Unassigned
0x0010	AEAD_CHACHA20_POLY1305	[RFC8548]
0x0011-0xffff	Unassigned

MPTCP MP_TCPRST Reason Codes

Registration Procedure(s)

Specification Required

Expert(s)

Unassigned

Reference

[RFC8684]

Available Formats

CSV

Code	Meaning	Reference
0x00	Unspecified error	[RFC8684, Section 3.6]
0x01	MPTCP-specific error	[RFC8684, Section 3.6]
0x02	Lack of resources	[RFC8684, Section 3.6]
0x03	Administratively prohibited	[RFC8684, Section 3.6]
0x04	Too much outstanding data	[RFC8684, Section 3.6]
0x05	Unacceptable performance	[RFC8684, Section 3.6]
0x06	Middlebox interference	[RFC8684, Section 3.6]
0x07-0xff	Unassigned

TCP Header Flags

Registration Procedure(s)

Standards Action

Reference

[RFC9293]

Available Formats

CSV

Bit Offset	Name	Reference	Assignment Notes
4	Reserved for future use	[RFC9293]
5	Reserved for future use	[RFC9293]
6	Reserved for future use	[RFC9293]
7	AE (Accurate ECN)	[RFC-ietf-tcpm-accurate-ecn-34]	Previously used as NS (Nonce Sum) by [RFC3540], which is now historic [RFC8311].
8	CWR (Congestion Window Reduced)	[RFC3168]
9	ECE (ECN-Echo)	[RFC3168]
10	Urgent Pointer field is significant (URG)	[RFC9293]
11	Acknowledgment field is significant (ACK)	[RFC9293]
12	Push Function (PSH)	[RFC9293]
13	Reset the connection (RST)	[RFC9293]
14	Synchronize sequence numbers (SYN)	[RFC9293]
15	No more data from sender (FIN)	[RFC9293]

Contact Information

ID	Name	Contact URI	Last Updated
[J_Kay]	J. Kay	mailto:jkay&ucsd.edu	1994-09
[Keith_Scott]	Keith Scott	mailto:kscott&mitre.org	1999-02
[Kevin_Vermeulen]	Kevin Vermeulen	mailto:kevin.vermeulen&polytechnique.edu	2025-08-19
[Monroe_Bridges]	Monroe Bridges	mailto:monroe&cup.hp.com	1994-09
[Stev_Knowles]	Stev Knowles	mailto:stev&ftp.com	1995-03
[Steve_Bellovin]	Steve Bellovin	mailto:smb&research.att.com	2000-03
[Subbu_Subramaniam]	Subbu Subramaniam	mailto:subbu&cup.hp.com	1994-09
[Vladimir_Sukonnik]	Vladimir Sukonnik	mailto:vladimir&sitaranetworks.com	1999-02

Footnotes

[1]	[https://devcentral.f5.com/tech-tips/articles/accessing-tcp-options-from-irules]
[2]	current and new TCP-ENO implementations MUST use TCP option kind 69.

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