Large Record Sizes for TLS and DTLS
draft-mattsson-tls-super-jumbo-record-limit-02

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.

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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

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   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.

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   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

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   [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, <https://datatracker.ietf.org/doc/html/draft-
              ietf-tls-tlsflags-12>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC8449]  Thomson, M., "Record Size Limit Extension for TLS",
              RFC 8449, DOI 10.17487/RFC8449, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8449>.

   [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,
              <https://www.rfc-editor.org/rfc/rfc9147>.

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,
              <https://www.rfc-editor.org/rfc/rfc6083>.

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

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   Hannes Tschofenig
   Siemens
   Email: hannes.tschofenig@gmx.net

   Michael Tüxen
   Münster Univ. of Applied Sciences
   Email: tuexen@fh-muenster.de

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