Record Size Limit Extension for TLS
RFC 8449

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Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8449                                       Mozilla
Updates: 6066                                                August 2018
Category: Standards Track
ISSN: 2070-1721

                  Record Size Limit Extension for TLS

Abstract

   An extension to Transport Layer Security (TLS) is defined that allows
   endpoints to negotiate the maximum size of protected records that
   each will send the other.

   This replaces the maximum fragment length extension defined in
   RFC 6066.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8449.

Copyright Notice

   Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Thomson                      Standards Track                    [Page 1]
RFC 8449                    TLS Record Limit                 August 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Limitations of the "max_fragment_length" Extension  . . . . .   3
   4.  The "record_size_limit" Extension . . . . . . . . . . . . . .   4
     4.1.  Record Expansion Limits . . . . . . . . . . . . . . . . .   6
   5.  Deprecating "max_fragment_length" . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Implementing Transport Layer Security (TLS) [TLS] or Datagram TLS
   (DTLS) [DTLS] for constrained devices can be challenging.  However,
   recent improvements to the design and implementation of cryptographic
   algorithms have made TLS accessible to some highly limited devices
   (see, for example, [RFC7925]).

   Receiving large protected records can be particularly difficult for a
   device with limited operating memory.  TLS versions 1.2 [RFC5246] and
   earlier permit senders to generate records 16384 octets in size, plus
   any 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.  Allocating up to 18K of
   memory for ciphertext is beyond the capacity of some implementations.

   An Authentication Encryption with Additional Data (AEAD) cipher (see
   [RFC5116]) API requires that an entire record be present to decrypt
   and authenticate it.  Similarly, other ciphers cannot produce
   authenticated data until the entire record is present.  Incremental
   processing of records exposes endpoints to the risk of forged data.

   The "max_fragment_length" extension [RFC6066] was designed to enable
   constrained clients to negotiate a lower record size.  However,
   "max_fragment_length" suffers from several design problems (see
   Section 3).

   This document defines a "record_size_limit" extension (Section 4).
   This extension replaces "max_fragment_length" [RFC6066], which this
   document deprecates.  This extension is valid in all versions of TLS.

Thomson                      Standards Track                    [Page 2]
RFC 8449                    TLS Record Limit                 August 2018

   A smaller protected record size is just one of many problems that a
   constrained implementation might need to address.  The
   "record_size_limit" extension only addresses the memory allocation
   problem; it does not address limits of code size, processing