Internet Assigned Numbers Authority
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Transmission Control Protocol (TCP) Parameters
Last Updated
2025-09-30
Available Formats
[IMG]
XML [IMG]
HTML [IMG]
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
[IMG]
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
[IMG]
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
[IMG]
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
[IMG]
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
[IMG]
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][RFC-ietf-tsvwg-udp-options-45]
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-ietf-tsvwg-udp-options-45].
Note
IANA will record known duplicate uses.
Available Formats
[IMG]
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 [RFC-ietf-tcpm-accurate-ecn-34]
option kind 172)
0xACC1 TCP AccECN Experimental Option - Order 1 (current and new implementations SHOULD use [RFC-ietf-tcpm-accurate-ecn-34]
option kind 174)
0xACCE TCP AccECN Experimental Option (current and new implementations SHOULD use option [RFC-ietf-tcpm-accurate-ecn-34]
kinds 172 or 174)
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
[IMG]
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
[IMG]
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
[IMG]
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
[IMG]
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.
Licensing Terms
Presently we were in a very dark road, and at a point where it dropped suddenly between steep sides we halted in black shadow. A gleam of pale sand, a whisper of deep flowing waters, and a farther glimmer of more sands beyond them challenged our advance. We had come to a "grapevine ferry." The scow was on the other side, the water too shoal for the horses to swim, and the bottom, most likely, quicksand. Out of the blackness of the opposite shore came a soft, high-pitched, quavering, long-drawn, smothered moan of woe, the call of that snivelling little sinner the screech-owl. Ferry murmured to me to answer it and I sent the same faint horror-stricken tremolo back. Again it came to us, from not farther than one might toss his cap, and I followed Ferry down to the water's edge. The grapevine guy swayed at our side, we heard the scow slide from the sands, and in a few moments, moved by two videttes, it touched our shore. Soon we were across, the two videttes riding with us, and beyond a sharp rise, in an old opening made by the swoop of a hurricane, we entered the silent unlighted bivouac of Ferry's scouts. Ferry got down and sat on the earth talking with Quinn, while the sergeants quietly roused the sleepers to horse. Plotinus is driven by this perplexity to reconsider the whole theory of Matter.477 He takes Aristotle¡¯s doctrine as the groundwork of his investigation. According to this, all existence is divided into Matter and Form. What we know of things¡ªin other words, the sum of their differential characteristics¡ªis their Form. Take away this, and the unknowable residuum is their Matter. Again, Matter is the vague indeterminate something out of which particular Forms are developed. The two are related as Possibility to Actuality, as the more generic to the more specific substance through every grade of classification and composition. Thus there are two Matters, the one sensible and the other intelligible. The former constitutes the common substratum of bodies, the other the common element of ideas.478 The general distinction between Matter and Form was originally suggested to Aristotle by Plato¡¯s remarks on the same subject; but he differs325 from his master in two important particulars. Plato, in his Timaeus, seems to identify Matter with space.479 So far, it is a much more positive conception than the ?λη of the Metaphysics. On the other hand, he constantly opposes it to reality as something non-existent; and he at least implies that it is opposed to absolute good as a principle of absolute evil.480 Thus while the Aristotelian world is formed by the development of Power into Actuality, the Platonic world is composed by the union of Being and not-Being, of the Same and the Different, of the One and the Many, of the Limit and the Unlimited, of Good and Evil, in varying proportions with each other. The Lawton woman had heard of an officer's family at Grant, which was in need of a cook, and had gone there. [See larger version] On the 8th of July an extraordinary Privy Council was summoned. All the members, of whatever party, were desired to attend, and many were the speculations as to the object of their meeting. The general notion was that it involved the continuing or the ending of the war. It turned out to be for the announcement of the king's intended marriage. The lady selected was Charlotte, the second sister of the Duke of Mecklenburg-Strelitz. Apart from the narrowness of her education, the young princess had a considerable amount of amiability, good sense, and domestic taste. These she shared with her intended husband, and whilst they made the royal couple always retiring, at the same time they caused them to give, during their lives, a moral air to their court. On the 8th of September Charlotte arrived at St. James's, and that afternoon the marriage took place, the ceremony being performed by the Archbishop of Canterbury. On the 22nd the coronation took place with the greatest splendour. Mother and girls were inconsolable, for each had something that they were sure "Si would like," and would "do him good," but they knew Josiah Klegg, Sr., well enough to understand what was the condition when he had once made up his mind. CHAPTER V. THE YOUNG RECRUITS Si proceeded to deftly construct a litter out of the two guns, with some sticks that he cut with a knife, and bound with pawpaw strips. His voice had sunk very low, almost to sweetness. A soft flurry of pink went over her face, and her eyelids drooped. Then suddenly she braced herself, pulled herself taut, grew combative again, though her voice shook. HoME²Ô¾®Ïè̫ʲôÐÇ×ù
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