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Normally in TLS there is no way for the client to signal to the server that it has been configured with a certificate suitable for mTLS. This document defines a TLS Flag that enables clients to provide this hint. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction This document specifies a TLS Flag that indicates to the server that the client supports mTLS. Sometimes a server does not want to negotiate mTLS with every client, but might wish to authenticate a subset of them. In TLS 1.3 this may be done with post-handshake auth, however this adds an extra round-trip, and requires negotiation at the application layer. A client sending the request mTLS flag in the ClientHello allows the server to request authentication during the initial handshake only when it receives a hint the client supports it. This enables a number of use cases, for example allowing bots to authenticate themselves when mixed in with general traffic.

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Flag specification A server receiving this flag MAY send a CertificateRequest message.

Security Considerations This flag should have no effect on the security of TLS, as the server may always send a CertificateRequest message during the handshake. This flag merely provides a hint that the client will be able to fulfil the request. If the client sets this flag but then fails to provide a certificate the server MAY terminate the connection with a bad_certificate error.

IANA Considerations This document requests IANA to add an entry to the TLS Flags registry in the TLS namespace with the following values:

• Value shall be TBD
• Flag Name shall be request_mtls.
• Message shall be CH
• Recommended shall be set to no (N)
• The reference shall be this document {!draft-jhoyla-req-mtls}

Normative References Dell Technologies A number of extensions are proposed in the TLS working group that carry no interesting information except the 1-bit indication that a certain optional feature is supported. Such extensions take 4 octets each. This document defines a flags extension that can provide such indications at an average marginal cost of 1 bit each. More precisely, it provides as many flag extensions as needed at 4 + the order of the last set bit divided by 8. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments TODO acknowledge.