2013-06-14 IESG Approval or Standards Action 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. 0 - End of Option List 1 - No-Operation 2 4 Maximum Segment Size 3 3 WSOPT - 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 TSOPT - Time Stamp Option 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) [**] 34-75 Reserved 69 Reserved (known unauthorized use without proper IANA assignment) [**] 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) [**] 79-252 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 0x0 MP_CAPABLE Multipath Capable , Section 3.1 0x1 MP_JOIN Join Connection , Section 3.2 0x2 DSS Data Sequence Signal (Data ACK and Data Sequence Mapping) , Section 3.3 0x3 ADD_ADDR Add Address , Section 3.4.1 0x4 REMOVE_ADDR Remove Address , Section 3.4.2 0x5 MP_PRIO Change Subflow Priority , Section 3.3.8 0x6 MP_FAIL Fallback , Section 3.6 0x7 MP_FASTCLOSE Fast Close , Section 3.5 0x8-0xe Unassigned 0xf Reserved for Private Use Standards Action A Checksum required , Section 3.1 B Extensibility , Section 3.2 C-G Unassigned H HMAC-SHA1 , Section 3.2 First Come First Served IANA will record known duplicate uses. J. Kay mailto:jkay&ucsd.edu 1994-09 Keith Scott mailto:kscott&mitre.org 1999-02 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