Large Record Sizes for TLS and DTLS

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Table of Contents

1. 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 [RFC8446], 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 [RFC8449] 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 [RFC8446], and DTLS 1.3 [RFC9147]. In some use cases such as DTLS over SCTP [RFC6083] the 2¹⁴ bytes limit is a severe limitation.¶

This document defines a "large_record_size" flag extension using the TLS flags extension mechanism [I-D.ietf-tls-tlsflags]. The record size limit extension for TLS as specified in [RFC8449] 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 size of protected records to 2¹⁶ - 1 bytes, which is larger than the default limit of 2¹⁴ bytes. This flag extension is defined for version 1.3 of TLS and DTLS.¶

2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. 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 ciphertexts of the negotiated length and protected record with plaintext of the negotiated length of minus the allowed expansion. 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.¶

4. Limits on Key Usage

The maximum record size limit is an input to the AEAD limits calculations in TLS 1.3 [RFC8446] and DTLS 1.3 [RFC9147]. 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 [RFC8446] with a 64 kB record limit, only 2^22.5 records (about 6 million) may be encrypted on a given connection.¶

5. 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.¶

6. IANA Considerations

This document registers the following entry to the "TLS Flags" registry defined in [I-D.ietf-tls-tlsflags]:¶

• Value: TBD1¶

• Flag Name: large_record_size¶

• Messages: CH, EE¶

• Recommended: Y¶

• Reference: [This document]¶

Acknowledgments

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