Transmission Control Protocol (TCP) Parameters

Last Updated
2026-04-24
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Registries Included Below

TCP Option Kind Numbers

Registration Procedure(s)
Standards Action or IESG Approval
Reference
[RFC 2780]
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 [RFC 3692] 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 [RFC 1700]), 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 [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]
6 6 Echo (obsoleted by option 8) [RFC 1072][RFC 6247]
7 6 Echo Reply (obsoleted by option 8) [RFC 1072][RFC 6247]
8 10 Timestamps [RFC 7323]
9 2 Partial Order Connection Permitted (obsolete) [RFC 1693][RFC 6247]
10 3 Partial Order Service Profile (obsolete) [RFC 1693][RFC 6247]
11 CC (obsolete) [RFC 1644][RFC 6247]
12 CC.NEW (obsolete) [RFC 1644][RFC 6247]
13 CC.ECHO (obsolete) [RFC 1644][RFC 6247]
14 3 TCP Alternate Checksum Request (obsolete) [RFC 1146][RFC 6247]
15 N TCP Alternate Checksum Data (obsolete) [RFC 1146][RFC 6247]
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) [RFC 2385]
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 [RFC 4782]
28 4 User Timeout Option (also, other known unauthorized use) [***][1] [RFC 5482]
29 TCP Authentication Option (TCP-AO) [RFC 5925]
30 N Multipath TCP (MPTCP) [RFC 8684]
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 [RFC 7413]
35-68 Reserved
69 N Encryption Negotiation (TCP-ENO) [RFC 8547]
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 9768]
173 Reserved
174 N Accurate ECN Order 1 (AccECN1) [RFC 9768]
175-252 Reserved
253 N RFC3692-style Experiment 1 (also improperly used for shipping products) [*] [RFC 4727]
254 N RFC3692-style Experiment 2 (also improperly used for shipping products) [*] [RFC 4727]

TCP Alternate Checksum Numbers

Registration Procedure(s)
Not defined
Reference
[RFC 1146]
Available Formats

CSV
Number Description Reference
0 TCP Checksum [RFC 1146]
1 8-bit Fletchers's algorithm [RFC 1146]
2 16-bit Fletchers's algorithm [RFC 1146]
3 Redundant Checksum Avoidance [J_Kay]

Cryptographic Algorithms for TCP-AO Registration

Registration Procedure(s)
RFC Publication After Expert Review
Reference
[RFC 5926]
Available Formats

CSV
Algorithm Reference
SHA1 [RFC 5926]
AES128 [RFC 5926]

MPTCP Option Subtypes

Registration Procedure(s)
Standards Action
Reference
[RFC 8684]
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 [RFC 6824].
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 [RFC 8684]

MPTCP Handshake Algorithms

Registration Procedure(s)
Standards Action
Reference
[RFC 8684]
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 [RFC 6824].
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
[RFC 6994][RFC 9868]
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 [RFC 9868].
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 [RFC 7974]
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] [RFC 8547]
0x6F7D TCP SCONE-TCP [draft-eddy-tcpm-scone-01]
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 9768]
0xACC1 TCP AccECN Experimental Option - Order 1 (current and new implementations SHOULD use option kind 174) [RFC 9768]
0xACCE TCP AccECN Experimental Option (current and new implementations SHOULD use option kinds 172 or 174) [RFC 9768]
0xE2D4C3D9 TCP Shared Memory Communications over RMDA protocol [RFC 7609]
0xF989 TCP Fast Open (current and new implementations SHOULD use option 34) [RFC 7413]
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
[RFC 8547]
Available Formats

CSV
Value Meaning Reference
0x20 Experimental Use [RFC 8547]
0x21 TCPCRYPT_ECDHE_P256 [RFC 8548]
0x22 TCPCRYPT_ECDHE_P521 [RFC 8548]
0x23 TCPCRYPT_ECDHE_Curve25519 [RFC 8548]
0x24 TCPCRYPT_ECDHE_Curve448 [RFC 8548]
0x25-0x6f Unassigned
0x70-0x7f Reserved for extended values [RFC 8547]

tcpcrypt AEAD Algorithms

Registration Procedure(s)
IETF Review or Expert Review with RFC Required
Expert(s)
Unassigned
Reference
[RFC 8548]
Available Formats

CSV
Value AEAD Algorithm Reference
0x0000 Reserved
0x0001 AEAD_AES_128_GCM [RFC 8548]
0x0002 AEAD_AES_256_GCM [RFC 8548]
0x0003-0x000f Unassigned
0x0010 AEAD_CHACHA20_POLY1305 [RFC 8548]
0x0011-0xffff Unassigned

MPTCP MP_TCPRST Reason Codes

Registration Procedure(s)
Specification Required
Expert(s)
Unassigned
Reference
[RFC 8684]
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
[RFC 9293]
Available Formats

CSV
Bit Offset Name Reference Assignment Notes
4 Reserved for future use [RFC 9293]
5 Reserved for future use [RFC 9293]
6 Reserved for future use [RFC 9293]
7 AE (Accurate ECN) [RFC 9768] Previously used as NS (Nonce Sum) by [RFC 3540], which is now Historic [RFC 8311].
8 CWR (Congestion Window Reduced) [RFC 3168]
9 ECE (ECN-Echo) [RFC 3168]
10 Urgent Pointer field is significant (URG) [RFC 9293]
11 Acknowledgment field is significant (ACK) [RFC 9293]
12 Push Function (PSH) [RFC 9293]
13 Reset the connection (RST) [RFC 9293]
14 Synchronize sequence numbers (SYN) [RFC 9293]
15 No more data from sender (FIN) [RFC 9293]

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.