]> 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 TLS 1.3 records limit the inner plaintext (TLSInnerPlaintext) size to 2¹⁴ + 1 bytes, which includes one byte for the content type. Records also have a 3-byte overhead due to the fixed opaque_type and legacy_record_version fields. This document defines a TLS extension that allows endpoints to negotiate a larger maximum inner plaintext size, up to 2³² - 256 bytes, while reducing overhead. 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 TLS 1.3 records limit the inner plaintext (TLSInnerPlaintext) size to 2¹⁴ + 1 bytes, which includes one byte for the content type. Records also have a 3-byte overhead due to the fixed opaque_type and legacy_record_version fields. TLS-based protocols are increasingly used to secure long-lived interfaces in critical infrastructure, such as telecommunication networks. In some infrastructure use cases, the upper layer of DTLS expects a message oriented service and uses message sizes much larger than 2¹⁴-bytes. In these cases, the 2¹⁴-byte limit in TLS necessitates an additional protocol layer for fragmentation, resulting in increased CPU and memory consumption and additional complexity. Allowing 2³²-byte records would eliminate additional fragmentation in almost all use cases. In (DTLS over SCTP), the 2¹⁴-byte limit is a severe restriction. This document defines a "large_record_size_limit" extension that allows endpoints to negotiate a larger maximum inner plaintext (TLSInnerPlaintext) size. This extension is valid in TLS 1.3 and DTLS 1.3. The extension works similarly to the "record_size_limit" extension defined in . Additionally, this document defines new TLS 1.3 TLSLargeCiphertext and DTLS 1.3 unified_hdr structures to enable inner plaintexts up to 2³² - 256 bytes with reduced overhead. For example, inner plaintexts up to 2¹⁶ - 256 bytes can be supported with 3 bytes less overhead, which is useful in constrained IoT environments. The "large_record_size_limit" extension is incompatible with middleboxes expecting TLS 1.2 records.

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_limit" Extension The ExtensionData of the "large_record_size_limit" extension is LargeRecordSizeLimit: LargeRecordSizeLimit denotes the maximum size, in bytes, of inner plaintexts that the endpoint is willing to receive. It includes the content type and padding (i.e., the complete length of TLSInnerPlaintext). AEAD expansion is not included. The large record size limit only applies to records sent toward the endpoint that advertises the limit. An endpoint can send records that are larger than the limit it advertises as its own limit. A TLS endpoint that receives a record larger than its advertised limit MUST generate a fatal "record_overflow" alert; a DTLS endpoint that receives a record larger than its advertised limit MAY either generate a fatal "record_overflow" alert or discard the record. An endpoint MUST NOT add padding to records that would cause the length of TLSInnerPlaintext to exceed the limit advertised by the other endpoint. Endpoints MUST NOT send a "large_record_size_limit" extension with a value smaller than 64 or larger than 2³² - 256. An endpoint MUST treat receipt of a smaller or larger value as a fatal error and generate an "illegal_parameter" alert. The server sends the "large_record_size_limit" extension in the EncryptedExtensions message. During resumption, the 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 during resumption. Unprotected messages and records protected with early_traffic_secret or handshake_traffic_secret are not subject to the large record size limit. When the "large_record_size_limit" extension is negotiated:

• All TLS 1.3 records protected with application_traffic_secret MUST use the TLSLargeCiphertext structure instead of the TLSCiphertext structure. The size of the length field depends on the limit advertised by the receiver. If the limit is less than 2¹⁶ - 255 an uint16 is used, if the limit is larger than 2²⁴ - 256 an uint32 is used, and otherwise an uint24 is used. The length is fixed for the connection. Different lengths might be used in different directions.

• All DTLS 1.3 records protected with application_traffic_secret and with length present MUST use a unified_hdr structure with a length equal to the TLS 1.3 length field defined above.

• An endpoint MAY generate records protected with application_traffic_secret with inner plaintext that is equal to or smaller than the LargeRecordSizeLimit value it receives from its peer. An endpoint MUST NOT generate a protected record with inner plaintext that is larger than the LargeRecordSizeLimit value it receives from its peer.

The "large_record_size_limit" extension is not compatible with middleboxes expecting TLS 1.2 records and SHOULD NOT be negotiated where such middleboxes are expected. A server MUST NOT send extension responses to more than one of "large_record_size_limit", "record_size_limit", and "max_fragment_length". A client MUST treat receipt of more than one of "large_record_size_limit", "record_size_limit", and "max_fragment_length" as a fatal error, and it SHOULD generate an "illegal_parameter" alert. The Path Maximum Transmission Unit (PMTU) in DTLS also limits the size of records. The record size limit does not affect PMTU discovery and SHOULD be set independently. The record size limit is fixed during the handshake and so should be set based on constraints at the endpoint and not based on the current network environment. In comparison, the PMTU is determined by the network path and can change dynamically over time.

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 2¹⁴ + 1 bytes, existing AEAD limits SHALL be decreased by a factor of (LargeRecordSizeLimit) / (2^14-256). For example, when AES-CGM is used in TLS 1.3 with a 64 kB record limit, only arounf 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. Additionally, larger record sizes also means that more processing is done before verification of non-authentic records fails. The use of larger record sizes can either simplify or complicate traffic analysis, depending on the application. The LargeRecordSizeLimit is just an upper limit and it is still the sender that decides the size of the inner plaintexts up to that limit.

IANA Considerations IANA is requested to assign a new value in the TLS ExtensionType Values registry defined by :

• The Extension Name should be large_record_size_limit
• The TLS 1.3 value should be CH, EE
• The DTLS-Only value should be N
• The Recommended value should be Y
• The Reference should be 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. This document describes a number of changes to TLS and DTLS IANA registries that range from adding notes to the registry all the way to changing the registration policy. These changes were mostly motivated by WG review of the TLS- and DTLS-related registries undertaken as part of the TLS 1.3 development process. This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, and 7301. 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. 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]

Change Log Changes from -04 to -05:

• Grammar and comprehension tweaks.
• Added change log

Changes from -03 to -04:

• Corrected uint24 to uint32.

Changes from -02 to -03:

• Major rewrite based on discussions at IETF 119
• New independant extension instead of flag extension used together with record_size_limit
• New record format without opaque_type and legacy_record_version fields. This reduces overhead
• Support inner plaintext size up to 2^32 - 256 bytes

Acknowledgments The authors would like to thank , , and for their valuable comments and feedback. Some of the text were inspired by and borrowed from .