Internet DRAFT - draft-mattsson-tls-super-jumbo-record-limit
draft-mattsson-tls-super-jumbo-record-limit
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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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, <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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