2009-03-17 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. 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 10 3 Partial Order Service Profile 11 CC 12 CC.NEW 13 CC.ECHO 14 3 TCP Alternate Checksum Request 15 N TCP Alternate Checksum Data 16 Skeeter 17 Bubba 18 3 Trailer Checksum Option 19 18 MD5 Signature Option 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 29-252 Unassigned 253 N RFC3692-style Experiment 1 (*) 254 N RFC3692-style Experiment 2 (*) Not defined 0 TCP Checksum 1 8-bit Fletchers's algorithm 2 16-bit Fletchers's algorithm 3 Redundant Checksum Avoidance 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