Transport Layer Security J. Preuß Mattsson Internet-Draft Ericsson Intended status: Standards Track H. Tschofenig Expires: 5 September 2024 Siemens M. Tüxen Münster Univ. of Applied Sciences 4 March 2024 Large Record Sizes for TLS and DTLS draft-mattsson-tls-super-jumbo-record-limit-02 Abstract 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 2^14 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 2^16 bytes. About This Document This note is to be removed before publishing as an RFC. The latest revision of this draft can be found at https://emanjon.github.io/tls-super-jumbo-record-limit/draft- mattsson-tls-super-jumbo-record-limit.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft- mattsson-tls-super-jumbo-record-limit/. Discussion of this document takes place on the Transport Layer Security Working Group mailing list (mailto:tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/. Subscribe at https://www.ietf.org/mailman/listinfo/tls/. Source for this draft and an issue tracker can be found at https://github.com/emanjon/tls-super-jumbo-record-limit. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Preuß Mattsson, et al. Expires 5 September 2024 [Page 1] Internet-Draft Large Record Sizes for TLS and DTLS March 2024 Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 5 September 2024. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. The "large_record_size" Flag Extension . . . . . . . . . . . 3 4. Limits on Key Usage . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 7.1. Normative References . . . . . . . . . . . . . . . . . . 4 7.2. Informative References . . . . . . . . . . . . . . . . . 5 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 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 2^14 = 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 Preuß Mattsson, et al. Expires 5 September 2024 [Page 2] Internet-Draft Large Record Sizes for TLS and DTLS March 2024 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 2^14 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 plaintext record size for protected records to 2^16 - 257 bytes, which is larger than the default limit of 2^14 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 plaintext of the negotiated length. Since the 2^16 - 1 limit also applies to the ciphertext length, the maximum length of a protected record plaintext that can be negotiated is therefore 2^16 - 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. Preuß Mattsson, et al. Expires 5 September 2024 [Page 3] Internet-Draft Large Record Sizes for TLS and DTLS March 2024 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 2^14 + 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] 7. References 7.1. Normative References Preuß Mattsson, et al. Expires 5 September 2024 [Page 4] Internet-Draft Large Record Sizes for TLS and DTLS March 2024 [I-D.ietf-tls-tlsflags] Nir, Y., "A Flags Extension for TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-tlsflags-12, 23 July 2023, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [RFC8449] Thomson, M., "Record Size Limit Extension for TLS", RFC 8449, DOI 10.17487/RFC8449, August 2018, . [RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, . 7.2. Informative References [RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)", RFC 6083, DOI 10.17487/RFC6083, January 2011, . Acknowledgments The authors would like to thank Benjamin Kaduk for his valuable comments and feedback. Authors' Addresses John Preuß Mattsson Ericsson Email: john.mattsson@ericsson.com Preuß Mattsson, et al. Expires 5 September 2024 [Page 5] Internet-Draft Large Record Sizes for TLS and DTLS March 2024 Hannes Tschofenig Siemens Email: hannes.tschofenig@gmx.net Michael Tüxen Münster Univ. of Applied Sciences Email: tuexen@fh-muenster.de Preuß Mattsson, et al. Expires 5 September 2024 [Page 6]