]> Ericsson

john.mattsson@ericsson.com

Siemens

hannes.tschofenig@gmx.net

Münster Univ. of Applied Sciences

tuexen@fh-muenster.de

Security Transport Layer Security next generation unicorn sparkling distributed ledger RFC 8449 defines a record size limit extension for TLS and DTLS allowing endpoints to negotiate a record size limit smaller than the protocol-defined maximum record size, which is around 2¹⁴ bytes. This document specifies a TLS flag extension to be used in combination with the record size limit extension allowing endpoints to use a record size limit larger than the protocol-defined maximum record size, but not more than about 2¹⁶ bytes. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Transport Layer Security Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The records in all versions of TLS have an uint16 length field that could theoretically allow records 65535 octets in size. TLS does however have a lower protocol-defined limit for maximum plaintext record size. For TLS 1.3 , that limit is 2¹⁴ = 16384 octets. In addition, TLS 1.3 expands the plaintext with 1 octet for content type and allow AEAD expansion up to 255 octets (though typically this expansion is only 16 octets). The "record_size_limit" extension enables endpoints to negotiate a lower limit for the maximum plaintext record size, but does not allow endpoints to increase the limits enforced by TLS 1.3 , and DTLS 1.3 . In some use cases such as DTLS over SCTP the 2¹⁴ bytes limit is a severe limitation. This document defines a "large_record_size" flag extension using the TLS flags extension mechanism . The record size limit extension for TLS as specified in used in combination with the flag extension defined in this document allow endpoints to negotiate a record size limit larger than the protocol-defined maximum record size. This can be used to bump up the maximum plaintext record size for protected records to 2¹⁶ - 257 bytes, which is larger than the default limit of 2¹⁴ bytes. This flag extension is defined for version 1.3 of TLS and DTLS.

Terminology 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.

The "large_record_size" Flag Extension When the "large_record_size" flag extension in addition to the "record_size_limit" extension is negotiated, an endpoint MUST be prepared to accept protected records with plaintext of the negotiated length. Since the 2¹⁶ - 1 limit also applies to the ciphertext length, the maximum length of a protected record plaintext that can be negotiated is therefore 2¹⁶ - 257 = 65279 octets. Unprotected messages are still subject to the lower default limits. The "large_record_size" flag extension MUST be negotiated together with the "record_size_limit" extension and MUST NOT be negotiated together with the "max_fragment_length" extension. A client MUST treat receipt of the "large_record_size" flags extension without the "record_size_limit" extension or together with the "max_fragment_length" extension as a fatal error, and it SHOULD generate an "illegal_parameter" alert. During resumption, the record size limit is renegotiated. Records are subject to the limits that were set in the handshake that produces the keys that are used to protect those records. This admits the possibility that the extension might not be negotiated when a connection is resumed.

Limits on Key Usage The maximum record size limit is an input to the AEAD limits calculations in TLS 1.3 and DTLS 1.3 . Increasing the maximum record size to more than 2¹⁴ + 256 bytes while keeping the same confidentiality and integrity advantage per write key therefore requires lower AEAD limits. When the "large_record_size" has been negotiated record size limit larger than the protocol-defined maximum record size, existing AEAD limits SHALL be decreased by a factor of 4. For example, when AES-CGM is used in TLS 1.3 with a 64 kB record limit, only 2^22.5 records (about 6 million) may be encrypted on a given connection.

Security Considerations Large record sizes might require more memory allocation for senders and receivers. Large record sizes also means that more processing is done before verification of non-authentic records fails.

IANA Considerations This document registers the following entry to the "TLS Flags" registry defined in :

• Value: TBD1
• Flag Name: large_record_size
• Messages: CH, EE
• Recommended: Y
• Reference: [This document]

References Normative References 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. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. An extension to Transport Layer Security (TLS) is defined that allows endpoints to negotiate the maximum size of protected records that each will send the other. This replaces the maximum fragment length extension defined in RFC 6066. This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. 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. Informative References This document describes the usage of the Datagram Transport Layer Security (DTLS) protocol over the Stream Control Transmission Protocol (SCTP). DTLS over SCTP provides communications privacy for applications that use SCTP as their transport protocol and allows client/server applications to communicate in a way that is designed to prevent eavesdropping and detect tampering or message forgery. Applications using DTLS over SCTP can use almost all transport features provided by SCTP and its extensions. [STANDARDS-TRACK]

Acknowledgments The authors would like to thank for his valuable comments and feedback.