2025-12-16 Standards Action or IESG Approval 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 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. Values 16-18 were assigned before October 1994 (see ), and values 20-26 were assigned between Feb 9, 1999 and May 10, 2000. No further documentation is available. 0 - End of Option List 1 - No-Operation 2 4 Maximum Segment Size 3 3 Window Scale 4 2 SACK Permitted 5 N SACK 6 6 Echo (obsoleted by option 8) 7 6 Echo Reply (obsoleted by option 8) 8 10 Timestamps 9 2 Partial Order Connection Permitted (obsolete) 10 3 Partial Order Service Profile (obsolete) 11 CC (obsolete) 12 CC.NEW (obsolete) 13 CC.ECHO (obsolete) 14 3 TCP Alternate Checksum Request (obsolete) 15 N TCP Alternate Checksum Data (obsolete) 16 Skeeter 17 Bubba 18 3 Trailer Checksum Option 19 18 MD5 Signature Option (obsoleted by option 29) 20 SCPS Capabilities 21 Selective Negative Acknowledgements 22 Record Boundaries 23 Corruption experienced 24 SNAP 25 Unassigned (released 2000-12-18) 26 TCP Compression Filter 27 8 Quick-Start Response 28 4 User Timeout Option (also, other known unauthorized use) [***] 29 TCP Authentication Option (TCP-AO) 30 N Multipath TCP (MPTCP) 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 35-68 Reserved 69 N Encryption Negotiation (TCP-ENO) 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) 173 Reserved 174 N Accurate ECN Order 1 (AccECN1) 175-252 Reserved 253 N RFC3692-style Experiment 1 (also improperly used for shipping products) [*] 254 N RFC3692-style Experiment 2 (also improperly used for shipping products) [*] Not defined 0 TCP Checksum 1 8-bit Fletchers's algorithm 2 16-bit Fletchers's algorithm 3 Redundant Checksum Avoidance RFC Publication After Expert Review SHA1 AES128 Standards Action This registry specifies the MPTCP Option Subtypes for MPTCP v1, which obsoletes the Experimental MPTCP v0. For the MPTCP v0 subtypes, please refer to . 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 Standards Action This registry specifies the MPTCP Handshake Algorithms for MPTCP v1, which obsoletes the Experimental MPTCP v0. For the MPTCP v0 subtypes, please refer to . 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 First Come First Served 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 . IANA will record known duplicate uses. 0x00AC TCP TCP ACK Rate Request 0x00BD TCP Bidirectional Traceroute 0x0348 TCP HOST_ID 0x0A0D TCP Autonomous System Compensation 0x0CA0 TCP TCP Capability Option 0x0ED0 TCP Extended Data Offset 0x454E TCP TCP-ENO 0x6F7D TCP SCONE-TCP 0x75ECFFEE TCP Timestamp Interval 0xACC0 TCP AccECN Experimental Option - Order 0 (current and new implementations SHOULD use option kind 172) 0xACC1 TCP AccECN Experimental Option - Order 1 (current and new implementations SHOULD use option kind 174) 0xACCE TCP AccECN Experimental Option (current and new implementations SHOULD use option kinds 172 or 174) 0xE2D4C3D9 TCP Shared Memory Communications over RMDA protocol 0xF989 TCP Fast Open (current and new implementations SHOULD use option 34) 0xF990 TCP Low Latency IETF Review or Expert Review with RFC Required Unassigned 0x20 Experimental Use 0x21 TCPCRYPT_ECDHE_P256 0x22 TCPCRYPT_ECDHE_P521 0x23 TCPCRYPT_ECDHE_Curve25519 0x24 TCPCRYPT_ECDHE_Curve448 0x25-0x6f Unassigned 0x70-0x7f Reserved for extended values IETF Review or Expert Review with RFC Required Unassigned 0x0000 Reserved 0x0001 AEAD_AES_128_GCM 0x0002 AEAD_AES_256_GCM 0x0003-0x000f Unassigned 0x0010 AEAD_CHACHA20_POLY1305 0x0011-0xffff Unassigned Specification Required Unassigned 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 Standards Action 4 Reserved for future use 5 Reserved for future use 6 Reserved for future use 7 AE (Accurate ECN) Previously used as NS (Nonce Sum) by , which is now historic . 8 CWR (Congestion Window Reduced) 9 ECE (ECN-Echo) 10 Urgent Pointer field is significant (URG) 11 Acknowledgment field is significant (ACK) 12 Push Function (PSH) 13 Reset the connection (RST) 14 Synchronize sequence numbers (SYN) 15 No more data from sender (FIN) current and new TCP-ENO implementations MUST use TCP option kind 69. J. Kay mailto:jkay&ucsd.edu 1994-09 Keith Scott mailto:kscott&mitre.org 1999-02 Kevin Vermeulen mailto:kevin.vermeulen&polytechnique.edu 2025-08-19 Monroe Bridges mailto:monroe&cup.hp.com 1994-09 Stev Knowles mailto:stev&ftp.com 1995-03 Steve Bellovin mailto:smb&research.att.com 2000-03 Subbu Subramaniam mailto:subbu&cup.hp.com 1994-09 Vladimir Sukonnik mailto:vladimir&sitaranetworks.com 1999-02