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For the authoritative TLS extension codepoint list, use IANA’s Transport Layer Security (TLS) Extensions registry. Its ExtensionType Values table shows registered names, numeric values, TLS 1.3 message contexts, DTLS-only and recommendation designations, references, and comments. The registry page reports a last-updated date of August 11, 2026; check the live entry when you need current status.
What the TLS Extensions registry contains
The ExtensionType Values table is the place to look up a TLS extension number and name, check whether a value is assigned, and find the document governing an entry. IANA is the authority for registered TLS extension codepoints and names. The registry is an index and allocation record, not a complete description of how an extension behaves on the wire.
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The same IANA page groups several related registries together. In addition to TLS ExtensionType Values, it includes TLS Certificate Types, TLS Certificate Status Types, TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs, TLS CachedInformationType Values, and TLS Certificate Compression Algorithm IDs. These are separate namespaces: a number listed in one is not automatically a TLS ExtensionType codepoint.
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How to look up a TLS extension codepoint
- Open the ExtensionType Values table. Use the section titled TLS ExtensionType Values on IANA’s TLS Extensions registry page, rather than assuming that a similarly named entry in a neighboring table is an extension codepoint.
- Search by number or name. Find the numeric value or registered Extension Name. If searching for a number, check whether its row is assigned, Reserved, or Unassigned; do not treat those labels as synonyms.
- Read the context and status columns together. Check TLS 1.3 message contexts, the DTLS-Only designation, recommendation status, and any comment. They are qualifications on the registry entry, not full protocol instructions.
- Follow the reference. Open the cited RFC or other specification for normative details, including payload format, when the extension may appear, and version-specific behavior.
- Verify the live record before relying on it. Assignments and annotations can change. Record the registry status and the source date when the lookup informs an implementation or protocol document.
How to interpret the registry columns
Value and Extension Name
Value is the numeric codepoint. Extension Name is IANA’s registered name for the entry. The table also includes ranges or values marked Reserved or Unassigned. A number alone is not evidence that an active extension exists. If an entry’s name has changed and the registry notes a rename, retain that context when it matters to the question or implementation.
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TLS 1.3 message contexts
The TLS 1.3 column identifies handshake message contexts in which an extension is used. IANA’s abbreviations are CH for ClientHello, SH for ServerHello, EE for EncryptedExtensions, CT for Certificate, CR for CertificateRequest, NST for NewSessionTicket, and HRR for HelloRetryRequest. These labels tell you where the registry associates an extension with TLS 1.3 messages; they do not explain the extension’s full wire behavior or replace its specification.
DTLS-Only and Recommended
DTLS-Only indicates whether the entry is specific to DTLS in the registry. Read it alongside the cited reference; the field alone should not be used to infer broader transport support.
Recommended can show values such as Y, N, or D. IANA’s registration procedures depend in part on recommendation status, and the registry says D means discouraged. Do not interpret N as “broken,” or D as a general security judgment. Consult the relevant specification for the precise implications of a designation.
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Reference and Comment
Reference points to the specification or other source governing the entry. Use it for normative protocol details; the IANA table is not a substitute. Comment records additional qualifications that may affect how an entry should be read.
The registry notes that entries added after IESG approval of RFC 9851 are intended for TLS 1.3 or later and do not thereby impose a similar requirement on DTLS. That statement is conditional: it concerns entries added after that approval, not every historical entry. Read the applicable comment and the reference for the particular row rather than generalizing the note.
Assigned, reserved, and unassigned are different states
- Assigned: IANA lists a registered entry with a name and associated record. Its reference and annotations still matter for understanding permitted use.
- Reserved: The registry explicitly marks a value or range as reserved. Do not present it as an active named extension.
- Unassigned: The registry identifies a value or range as unassigned. It is not a registered extension just because the number is within the codepoint space.
For the question “is TLS extension value [number] reserved or unassigned?”, use the status shown for that exact value in the live ExtensionType Values table. Do not infer status from an old copy, a neighboring registry, or an application’s private label.
Finding the RFC for an extension
To answer “what RFC defines TLS extension [name]?”, first locate the exact name in the ExtensionType Values table and follow the Reference field. The registry’s label identifies the entry, while its reference points to the governing document. Read that document for normative requirements such as encoding, valid handshake locations, version constraints, and interactions with other protocol features.
If a registry comment and a cited specification appear to differ, keep their scopes distinct: the registry records allocation and current annotations, while the specification defines protocol behavior. Check the date and subject of each source instead of silently combining them into a broader claim.
Comparing two registry entries without conflating them
Entries appearing in the same table are not necessarily alternatives. Compare their recorded attributes and consult their own references:
| Comparison point | What to record |
|---|---|
| Codepoint and name | The exact numeric value and registered Extension Name for each entry. |
| Assignment state | Whether each value is assigned, reserved, or unassigned in the current registry. |
| TLS 1.3 context | The listed handshake-message abbreviations, if present, without treating them as a full behavior description. |
| DTLS-Only | The registry’s designation, interpreted alongside the reference. |
| Recommendation | The displayed status, with its meaning checked against the relevant specification. |
| Governing reference | The RFC or other cited document that supplies normative details. |
State the transport and protocol-version scope of a comparison. Similar placement in the registry does not make two extensions interchangeable.
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There is no single allocation path to assume for every entry. The registry’s procedure notes distinguish registration procedures, including procedures that depend on recommendation status. IANA’s Guidance for RFC Authors: Protocol Registration advises authors to use the exact registry name and follow the procedure specified for that registry.
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Preserving a visual copy of the registry page
If a team needs a rendered snapshot of IANA’s registry page for a report or record, a screenshot can preserve what the page looked like at capture time. It does not establish that the entry remains current or replace the live registry and referenced RFC.
For that narrow visual-capture task, ScreenshotNeo is a website screenshot API and MCP server. A single GET request can return an image or PDF; its consent-banner, popup, and chat-widget cleanup is configurable. Keep the IANA registry URL as the capture target, and retain the capture date with the artifact.
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Use this cURL request to capture the registry page; see the ScreenshotNeo documentation for API options:
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Common lookup mistakes
- Looking in the wrong namespace: Confirm the table heading says TLS ExtensionType Values, not ALPN Protocol IDs or another adjacent registry.
- Treating every number as an extension: Check whether the exact value is assigned, reserved, or unassigned.
- Reading a context abbreviation as a protocol specification: Use CH, SH, EE, CT, CR, NST, or HRR to identify a TLS 1.3 message context, then read the cited specification for wire behavior.
- Calling N “broken” or D “insecure”: These are registry recommendation labels. Check the referenced document for their precise implications.
- Assuming DTLS behavior from a single field: Read the DTLS-Only designation with the reference and applicable comments.
- Using stale status for an allocation decision: Check the current record and its procedure notes; the registry can change.
- Applying the RFC 9851 note to all entries: Its scope is entries added after IESG approval of RFC 9851, not the entire historical registry.
Frequently Asked Questions
What does CH mean in the TLS 1.3 column?
It means ClientHello, one of IANA’s abbreviated handshake-message context labels.
Does an IANA registry entry fully define an extension?
No. The entry is the allocation record and index; use its cited specification for normative protocol behavior.
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