Transport Layer Security J. Preuß Mattsson
Internet-Draft Ericsson
Intended status: Standards Track H. Tschofenig
Expires: 25 April 2024 23 October 2023
Super Jumbo Record Size Limit Extension for TLS 1.3
draft-mattsson-tls-super-jumbo-record-limit-00
Abstract
An extension "super_jumbo_record_size_limit" to (Datagram) Transport
Layer Security (TLS) is defined that allows endpoints to negotiate a
2^16 bytes maximum size of protected records. This is larger than
the default limit of around 2^14 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/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/draft-mattsson-tls-compact-ecc.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The "super_jumbo_record_size_limit" Extension . . . . . . . . 3
4. AEAD Limits . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The records in all version of TLS records has an uint16 length field
that could theoretically allow records 65536 octets in size. TLS
does however have a lower protocol-defined limit for maximum
plaintext record size. For TLS 1.2 [RFC5246], that limit is 2^14 =
16384 octets. TLS 1.3 [RFC8446] uses a limit of 2^14 + 1 = 16385
octets. In addition, TLS 1.2 allow expansion from compression and
protection up to 2048 octets (though typically this expansion is only
16 octets). TLS 1.3 reduces the allowance for expansion to 256
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], TLS 1.2 [RFC5246], DTLS 1.3 [RFC9147], and DTLS 1.2
[RFC6347].
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In some use cases such as DTLS over SCTP [RFC6083] the 2^14 bytes
limit is a severe limitation.
This document defines a "super_jumbo_record_size_limit" extension
(Section 3). The extension allows endpoints to negotiate a 2^16
bytes maximum size of protected records, which is larger than the
default limit of 2^14 bytes. This 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 "super_jumbo_record_size_limit" Extension
The "super_jumbo_record_size_limit" extension does not have any
ExtensionData. When the "super_jumbo_record_size_limit" extension is
negotiated, an endpoint MUST be prepared to accept protected records
with ciphertexts of length 2^16 bytes and protected record with
plaintext of length 2^16 - the allowed expansion. The maximum length
of a protected record plaintext is therefore 2^16 - 2^11 = 63488
octets in TLS 1.2 and 2^16 - 2^8 = 65280 octets in TLS 1.3.
Unprotected messages are still subject to the lower default limits in
TLS/DTLS 1.3.
The "super_jumbo_record_size_limit" extension MUST NOT be negotiated
together with the "record_size_limit" extension or the
"max_fragment_length" extension. A client MUST treat receipt of
"super_jumbo_record_size_limit" together with "record_size_limit" or
"max_fragment_length" as a fatal error, and it SHOULD generate an
"illegal_parameter" alert.
In TLS 1.3, the server sends the "super_jumbo_record_size_limit"
extension in the EncryptedExtensions message.
During renegotiation or 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 renegotiated or resumed.
For DTLS 1.3 [RFC9147] over UDP or DCCP, the Path Maximum
Transmission Unit (PMTU) also limits the size of records. The record
size limit does not affect PMTU discovery and SHOULD be set
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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.
See PMTU [RFC8201] and Section 4.1 of DTLS 1.3 [RFC9147] for more
detail on PMTU discovery. For DTLS over TCP or SCTP, which
automatically fragments and reassembles datagrams, there is no PMTU
limitation.
4. AEAD Limits
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 2^16 bytes while keeping the same
confidentiality and integrity advantage therefore requires lower AEAD
limits. When the "super_jumbo_record_size_limit" has been
negotiated, 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 "super_jumbo_record_size_limit" extension
in the "TLS ExtensionType Values" registry established in [RFC5246].
The "super_jumbo_record_size_limit" extension has been assigned a
code point of TBD. The IANA registry [RFC8447] [[will list|lists]]
this extension as "Recommended" (i.e., "Y") and indicates that it may
appear in the ClientHello (CH) or EncryptedExtensions (EE) messages
in TLS 1.3 [RFC8446].
7. References
7.1. Normative References
[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>.
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[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>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/rfc/rfc8447>.
[RFC8449] Thomson, M., "Record Size Limit Extension for TLS",
RFC 8449, DOI 10.17487/RFC8449, August 2018,
<https://www.rfc-editor.org/rfc/rfc8449>.
7.2. Informative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/rfc/rfc5246>.
[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>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/rfc/rfc8201>.
[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>.
Acknowledgments
Add your name here.
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Authors' Addresses
John Preuß Mattsson
Ericsson
Email: john.mattsson@ericsson.com
Hannes Tschofenig
Email: Hannes.Tschofenig@gmx.net
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