TLS                                                           M. Thomson
Internet-Draft                                                   Mozilla
Updates: 6066 (if approved)                              August 28, 2017
Intended status: Standards Track
Expires: March 1, 2018

     Record Size Limit Extension for Transport Layer Security (TLS)


   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

Status of This Memo

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   This Internet-Draft will expire on March 1, 2018.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

1.  Introduction

   Implementing Transport Layer Security (TLS) [I-D.ietf-tls-tls13] 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

   Receiving large protected records can be particularly difficult for a
   device with limited operating memory.  TLS versions 1.2 and earlier
   [RFC5246] 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.

   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.

2.  Conventions and Definitions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

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3.  Limitations of the "max_fragment_length" Extension

   The "max_fragment_length" extension has several limitations that make
   it unsuitable for use.

   A client that has no constraints preventing it from accepting a large
   record cannot use "max_fragment_length" without risking a reduction
   in the size of records.  The maximum value that the extension permits
   is 2^12, much smaller than the maximum record size of 2^14 that the
   protocol permits.

   For large data transfers, small record sizes can materially affect
   performance [TARREAU].  Consequently, clients that are capable of
   receiving large records could be unwilling to risk reducing
   performance by offering the extension, especially if the extension is
   rarely needed.

   This would not be an issue if a codepoint were available or could be
   added for fragments of 2^14 octets.  However, RFC 6066 requires that
   servers abort the handshake with an "illegal_parameter" alert if they
   receive the extension with a value they don't understand.  This makes
   it impossible to add new values to the extension without risking
   connection attempts failing.

   A server that negotiates "max_fragment_length" is required to echo
   the value selected by the client.  The server cannot request a lower
   limit than the one the client offered.  This is a significant problem
   if a server is more constrained than the clients it serves.

   The "max_fragment_length" extension is also ill-suited to cases where
   the capabilities of client and server are asymmetric.  Constraints on
   record size are often receiver constraints.  In particular, an
   Authentication Encryption with Additional Data (AEAD) ciphers (see
   [RFC5116]) API requires that an entire record be present to decrypt
   and authenticate it.  Some implementations choose not to implement an
   AEAD interface in this way to avoid this problem, but that exposes
   them to risks that an AEAD is intended to protect against.

   In comparison, an implementation might be able to send data
   incrementally.  Encryption does not have the same atomicity
   requirement.  Some ciphers can be encrypted and sent progressively.
   Thus, an endpoint might be willing to send more than its receive

   If these disincentives are sufficient to discourage clients from
   deploying the "max_fragment_length" extension, then constrained
   servers are unable to limit record sizes.

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4.  The "record_size_limit" Extension

   The ExtensionData of the "record_size_limit" extension is

      uint16 RecordSizeLimit;

   The value of RecordSizeLimit is the maximum size of record that the
   endpoint is willing to receive.  When the "record_size_limit"
   extension is negotiated, an endpoint MUST NOT generate a protected
   record with plaintext that is larger than the RecordSizeLimit value
   it receives from its peer.  Unprotected messages - handshake messages
   in particular - are not subject to this limit.

   This value is the size of the plaintext of a protected record.  The
   value includes the content type and padding added in TLS 1.3 (that
   is, the complete length of TLSInnerPlaintext).  Padding added as part
   of encryption, such as that added by a block cipher, is not included
   in this count.

   An endpoint that supports all record sizes can include any limit up
   to the protocol-defined limit for maximum record size.  For TLS 1.3
   and earlier, that limit is 2^14 octets.  Higher values are currently
   reserved for future versions of the protocol that may allow larger
   records; an endpoint MUST NOT send a value higher than the protocol-
   defined maximum record size unless explicitly allowed by such a
   future version or extension.

   Even if a larger record size limit is provided by a peer, an endpoint
   MUST NOT send records larger than the protocol-defined limit, unless
   explicitly allowed by a future TLS version or extension.

   The size limit expressed in the "record_size_limit" extension doesn't
   account for expansion due to compression or record protection.  It is
   expected that a constrained device will disable compression and know
   - and account for - the maximum expansion possible due to record
   protection based on the cipher suites it offers or selects.  Note
   that up to 256 octets of padding and padding length can be added to
   block ciphers.

   The record size limit only applies to records sent toward the
   endpoint that advertises the limit.  An endpoint MAY send records
   that are larger than the limit it advertises as its own limit.

   Clients SHOULD advertise the "record_size_limit" extension, even if
   they have no need to limit the size of records.  This allows servers
   to apply a limit at their discretion.  If this extension is not

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   negotiated, endpoints can send records of any size permitted by the
   protocol or other negotiated extensions.

   Endpoints MUST NOT send a "record_size_limit" extension with a value
   smaller than 64.  An endpoint MUST treat receipt of a smaller value
   as a fatal error and generate an "illegal_parameter" alert.

   In TLS 1.3, the server sends the "record_size_limit" extension in the
   EncryptedExtensions message.

5.  Deprecating "max_fragment_length"

   The "record_size_limit" extension replaces the "max_fragment_length"
   extension [RFC6066].  A server that supports the "record_size_limit"
   extension MUST ignore and "max_fragment_length" that appears in a
   ClientHello if both extensions appear.  A client MUST treat receipt
   of both "max_fragment_length" and "record_size_limit" as a fatal
   error, and SHOULD generate an "illegal_parameter" alert.

   Clients that depend on having a small record size MAY continue to
   advertise the "max_fragment_length".

6.  Security Considerations

   Very small record sizes might generate additional work for senders
   and receivers, limiting throughput and increasing exposure to denial
   of service.

7.  IANA Considerations

   This document registers the "record_size_limit" extension in the TLS
   "ExtensionType Values" registry established in [RFC5246].  The
   "record_size_limit" extension has been assigned a code point of TBD;
   it is recommended and marked as "Encrypted" in TLS 1.3.

8.  References

8.1.  Normative References

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

   [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-

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   [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-

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011, <https://www.rfc-

8.2.  Informative References

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016, <https://www.rfc-

   [TARREAU]  Tarreau, W., "Re: Stuck in a train -- reading HTTP/2
              draft.", n.d., <

Author's Address

   Martin Thomson


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