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Internet Engineering Task Force (IETF)                        M. ThomsonRequest for Comments: 8449                                       MozillaUpdates:6066                                                August 2018Category: Standards TrackISSN: 2070-1721Record Size Limit Extension for TLSAbstract   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 inRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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 2018Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions and Definitions . . . . . . . . . . . . . . . . .33.  Limitations of the "max_fragment_length" Extension  . . . . .34.  The "record_size_limit" Extension . . . . . . . . . . . . . .44.1.  Record Expansion Limits . . . . . . . . . . . . . . . . .65.  Deprecating "max_fragment_length" . . . . . . . . . . . . . .66.  Security Considerations . . . . . . . . . . . . . . . . . . .77.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .78.  References  . . . . . . . . . . . . . . . . . . . . . . . . .78.1.  Normative References  . . . . . . . . . . . . . . . . . .78.2.  Informative References  . . . . . . . . . . . . . . . . .8   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .8   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .81.  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 (seeSection 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   capability, or bandwidth capacity.2.  Conventions and Definitions   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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.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.  Every record incurs additional costs, both in the   additional octets for record headers and for expansion due to   encryption.  Processing more records also adds computational   overheads that can be amortized more effectively for larger record   sizes.  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 the risk of   failed connection attempts.   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.Thomson                      Standards Track                    [Page 3]

RFC 8449                    TLS Record Limit                 August 2018   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 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 records larger than the   limit it advertises for records that it receives.   If these disincentives are sufficient to discourage clients from   deploying the "max_fragment_length" extension, then constrained   servers are unable to limit record sizes.4.  The "record_size_limit" Extension   The ExtensionData of the "record_size_limit" extension is   RecordSizeLimit:      uint16 RecordSizeLimit;   The value of RecordSizeLimit is the maximum size of record in octets   that the endpoint is willing to receive.  This value is used to limit   the size of records that are created when encoding application data   and the protected handshake message into records.   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 are not subject to this limit.   This value is the length 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).  In TLS 1.2 and   earlier, the limit covers all input to compression and encryption   (that is, the data that ultimately produces TLSCiphertext.fragment).   Padding added as part of encryption, such as that added by a block   cipher, is not included in this count (seeSection 4.1).   An endpoint that supports all record sizes can include any limit up   to the protocol-defined limit for maximum record size.  For TLS 1.2   and earlier, that limit is 2^14 octets.  TLS 1.3 uses a limit of   2^14+1 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.   A server MUST NOT enforce this restriction; a client might advertise   a higher limit that is enabled by an extension or version the serverThomson                      Standards Track                    [Page 4]

RFC 8449                    TLS Record Limit                 August 2018   does not understand.  A client MAY abort the handshake with an   "illegal_parameter" alert if the record_size_limit extension includes   a value greater than the maximum record size permitted by the   negotiated protocol version and extensions.   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 record size limit only applies to records sent toward the   endpoint that advertises the limit.  An endpoint can send records   that are larger than the limit it advertises as its own limit.  A TLS   endpoint that receives a record larger than its advertised limit MUST   generate a fatal "record_overflow" alert; a DTLS endpoint that   receives a record larger than its advertised limit MAY either   generate a fatal "record_overflow" alert or discard the record.   Endpoints SHOULD advertise the "record_size_limit" extension, even if   they have no need to limit the size of records.  For clients, this   allows servers to advertise a limit at their discretion.  For   servers, this allows clients to know that their limit will be   respected.  If this extension is not 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.   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.   The Path Maximum Transmission Unit (PMTU) in DTLS also limits the   size of records.  The record size limit does not affect PMTU   discovery and SHOULD be set 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] and Section 4.1.1.1 of [DTLS] for   more detail on PMTU discovery.Thomson                      Standards Track                    [Page 5]

RFC 8449                    TLS Record Limit                 August 2018   PMTU governs the size of UDP datagrams, which limits the size of   records, but does not prevent records from being smaller.  An   endpoint that sends small records is still able to send multiple   records in a single UDP datagram.4.1.  Record Expansion Limits   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 to avoid   unpredictable increases in record size.  Stream ciphers and existing   AEAD ciphers don't permit variable amounts of expansion, but block   ciphers do permit variable expansion.   In TLS 1.2, block ciphers allow from 1 to 256 octets of padding.   When a limit lower than the protocol-defined limit is advertised, a   second limit applies to the length of records that use block ciphers.   An endpoint MUST NOT add padding to records that would cause the   protected record to exceed the size of a protected record that   contains the maximum amount of plaintext and the minimum permitted   amount of padding.   For example, TLS_RSA_WITH_AES_128_CBC_SHA has 16-octet blocks and a   20-octet MAC.  Given a record size limit of 256, a record of that   length would require a minimum of 11 octets of padding (for   [RFC5246], where the MAC is covered by encryption); or 15 octets if   the "encrypt_then_mac" extension [RFC7366] is negotiated.  With this   limit, a record with 250 octets of plaintext could be padded to the   same length by including at most 17 octets of padding, or 21 octets   with "encrypt_then_mac".   An implementation that always adds the minimum amount of padding will   always comply with this requirement.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 a "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 it SHOULD generate an "illegal_parameter" alert.   Clients that depend on having a small record size MAY continue to   advertise the "max_fragment_length".Thomson                      Standards Track                    [Page 6]

RFC 8449                    TLS Record Limit                 August 20186.  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 28.   The IANA registry [TLS-REGISTRY] 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   [TLS].   In the same registry, the "max_fragment_length" has been changed to   not recommended (i.e., "N").8.  References8.1.  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/info/rfc2119>.   [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/info/rfc5246>.   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)              Extensions: Extension Definitions",RFC 6066,              DOI 10.17487/RFC6066, January 2011,              <https://www.rfc-editor.org/info/rfc6066>.   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer              Security (TLS) and Datagram Transport Layer Security              (DTLS)",RFC 7366, DOI 10.17487/RFC7366, September 2014,              <https://www.rfc-editor.org/info/rfc7366>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.Thomson                      Standards Track                    [Page 7]

RFC 8449                    TLS Record Limit                 August 2018   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol              Version 1.3",RFC 8446, DOI 10.17487/RFC8446, August 2018,              <https://www.rfc-editor.org/info/rfc8446>.8.2.  Informative References   [DTLS]     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/info/rfc6347>.   [PMTU]     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/info/rfc8201>.   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated              Encryption",RFC 5116, DOI 10.17487/RFC5116, January 2008,              <https://www.rfc-editor.org/info/rfc5116>.   [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-editor.org/info/rfc7925>.   [TLS-REGISTRY]              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/info/rfc8447>.Acknowledgments   Thomas Pornin and Hannes Tschofenig provided significant input to   this document.  Alan DeKok identified an issue with the interaction   between record size limits and PMTU.Author's Address   Martin Thomson   Mozilla   Email: martin.thomson@gmail.comThomson                      Standards Track                    [Page 8]