TLS                                                          E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes: 6347 (if approved)                              H. Tschofenig
Intended status: Standards Track                             Arm Limited
Expires: September 6, 2018                                   N. Modadugu
                                                            Google, Inc.
                                                          March 05, 2018

   The Datagram Transport Layer Security (DTLS) Protocol Version 1.3


   This document specifies Version 1.3 of the Datagram Transport Layer
   Security (DTLS) protocol.  DTLS 1.3 allows client/server applications
   to communicate over the Internet in a way that is designed to prevent
   eavesdropping, tampering, and message forgery.

   The DTLS 1.3 protocol is intentionally based on the Transport Layer
   Security (TLS) 1.3 protocol and provides equivalent security
   guarantees.  Datagram semantics of the underlying transport are
   preserved by the DTLS protocol.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2018.

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

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   ( 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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  DTLS Design Rationale and Overview  . . . . . . . . . . . . .   5
     3.1.  Packet Loss . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  Reordering  . . . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  Message Size  . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Replay Detection  . . . . . . . . . . . . . . . . . . . .   7
   4.  The DTLS Record Layer . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Sequence Number Handling  . . . . . . . . . . . . . . . .   9
       4.1.1.  Determining the Header Format . . . . . . . . . . . .  10
       4.1.2.  Reconstructing the Sequence Number and Epoch  . . . .  11
     4.2.  Transport Layer Mapping . . . . . . . . . . . . . . . . .  11
     4.3.  PMTU Issues . . . . . . . . . . . . . . . . . . . . . . .  12
     4.4.  Record Payload Protection . . . . . . . . . . . . . . . .  14
       4.4.1.  Anti-Replay . . . . . . . . . . . . . . . . . . . . .  14
       4.4.2.  Handling Invalid Records  . . . . . . . . . . . . . .  14
   5.  The DTLS Handshake Protocol . . . . . . . . . . . . . . . . .  15
     5.1.  Denial-of-Service Countermeasures . . . . . . . . . . . .  16
     5.2.  DTLS Handshake Message Format . . . . . . . . . . . . . .  18
     5.3.  ClientHello Message . . . . . . . . . . . . . . . . . . .  20
     5.4.  Handshake Message Fragmentation and Reassembly  . . . . .  21
     5.5.  DTLS Handshake Flights  . . . . . . . . . . . . . . . . .  22
     5.6.  Timeout and Retransmission  . . . . . . . . . . . . . . .  25
       5.6.1.  State Machine . . . . . . . . . . . . . . . . . . . .  25
       5.6.2.  Timer Values  . . . . . . . . . . . . . . . . . . . .  27
     5.7.  CertificateVerify and Finished Messages . . . . . . . . .  28

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     5.8.  Alert Messages  . . . . . . . . . . . . . . . . . . . . .  28
     5.9.  Establishing New Associations with Existing Parameters  .  28
   6.  Example of Handshake with Timeout and Retransmission  . . . .  29
     6.1.  Epoch Values and Rekeying . . . . . . . . . . . . . . . .  31
   7.  ACK Message . . . . . . . . . . . . . . . . . . . . . . . . .  33
     7.1.  Sending ACKs  . . . . . . . . . . . . . . . . . . . . . .  34
     7.2.  Receiving ACKs  . . . . . . . . . . . . . . . . . . . . .  35
   8.  Key Updates . . . . . . . . . . . . . . . . . . . . . . . . .  35
   9.  Application Data Protocol . . . . . . . . . . . . . . . . . .  35
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  36
   11. Changes to DTLS 1.2 . . . . . . . . . . . . . . . . . . . . .  36
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     13.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  40
   Appendix B.  Working Group Information  . . . . . . . . . . . . .  40
   Appendix C.  Contributors . . . . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction


   The source for this draft is maintained in GitHub.  Suggested changes
   should be submitted as pull requests at
   dtls13-spec.  Instructions are on that page as well.  Editorial
   changes can be managed in GitHub, but any substantive change should
   be discussed on the TLS mailing list.

   The primary goal of the TLS protocol is to provide privacy and data
   integrity between two communicating peers.  The TLS protocol is
   composed of two layers: the TLS Record Protocol and the TLS Handshake
   Protocol.  However, TLS must run over a reliable transport channel -
   typically TCP [RFC0793].

   There are applications that utilize UDP [RFC0768] as a transport and
   to offer communication security protection for those applications the
   Datagram Transport Layer Security (DTLS) protocol has been designed.
   DTLS is deliberately designed to be as similar to TLS as possible,
   both to minimize new security invention and to maximize the amount of
   code and infrastructure reuse.

   DTLS 1.0 [RFC4347] was originally defined as a delta from TLS 1.1
   [RFC4346] and DTLS 1.2 [RFC6347] was defined as a series of deltas to
   TLS 1.2 [RFC5246].  There is no DTLS 1.1; that version number was
   skipped in order to harmonize version numbers with TLS.  This

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   specification describes the most current version of the DTLS protocol
   aligning with the efforts around TLS 1.3 [I-D.ietf-tls-tls13].

   Implementations that speak both DTLS 1.2 and DTLS 1.3 can
   interoperate with those that speak only DTLS 1.2 (using DTLS 1.2 of
   course), just as TLS 1.3 implementations can interoperate with TLS
   1.2 (see Appendix D of [I-D.ietf-tls-tls13] for details).  While
   backwards compatibility with DTLS 1.0 is possible the use of DTLS 1.0
   is not recommended as explained in Section 3.1.2 of RFC 7525

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

   The following terms are used:

   -  client: The endpoint initiating the DTLS connection.

   -  connection: A transport-layer connection between two endpoints.

   -  endpoint: Either the client or server of the connection.

   -  handshake: An initial negotiation between client and server that
      establishes the parameters of their transactions.

   -  peer: An endpoint.  When discussing a particular endpoint, "peer"
      refers to the endpoint that is remote to the primary subject of

   -  receiver: An endpoint that is receiving records.

   -  sender: An endpoint that is transmitting records.

   -  session: An association between a client and a server resulting
      from a handshake.

   -  server: The endpoint which did not initiate the DTLS connection.

   The reader is assumed to be familiar with the TLS 1.3 specification
   since this document is defined as a delta from TLS 1.3.  As in TLS
   1.3 the HelloRetryRequest has the same format as a ServerHello
   message but for convenience we use the term HelloRetryRequest
   throughout this document as if it were a distinct message.

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   Figures in this document illustrate various combinations of the DTLS
   protocol exchanges and the symbols have the following meaning:

   -  '+' indicates noteworthy extensions sent in the previously noted

   -  '*' indicates optional or situation-dependent messages/extensions
      that are not always sent.

   -  '{}' indicates messages protected using keys derived from a

   -  '[]' indicates messages protected using keys derived from

3.  DTLS Design Rationale and Overview

   The basic design philosophy of DTLS is to construct "TLS over
   datagram transport".  Datagram transport does not require nor provide
   reliable or in-order delivery of data.  The DTLS protocol preserves
   this property for application data.  Applications such as media
   streaming, Internet telephony, and online gaming use datagram
   transport for communication due to the delay-sensitive nature of
   transported data.  The behavior of such applications is unchanged
   when the DTLS protocol is used to secure communication, since the
   DTLS protocol does not compensate for lost or re-ordered data

   TLS cannot be used directly in datagram environments for the
   following five reasons:

   1.  TLS does not allow independent decryption of individual records.
       Because the integrity check indirectly depends on a sequence
       number, if record N is not received, then the integrity check on
       record N+1 will be based on the wrong sequence number and thus
       will fail.  DTLS solves this problem by adding explicit sequence

   2.  The TLS handshake is a lock-step cryptographic handshake.
       Messages must be transmitted and received in a defined order; any
       other order is an error.  This is incompatible with reordering
       and message loss.

   3.  Not all TLS 1.3 handshake messages (such as the NewSessionTicket
       message) are acknowledged.  Hence, a new acknowledgement message
       has to be added to detect message loss.

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   4.  Handshake messages are potentially larger than any given
       datagram, thus creating the problem of IP fragmentation.

   5.  Datagram transport protocols, like UDP, are susceptible to
       abusive behavior effecting denial of service attacks against
       nonparticipants, and require a return-routability check with the
       help of cookies to be integrated into the handshake.  A detailed
       discussion of countermeasures can be found in Section 5.1.

3.1.  Packet Loss

   DTLS uses a simple retransmission timer to handle packet loss.
   Figure 1 demonstrates the basic concept, using the first phase of the
   DTLS handshake:

            Client                                   Server
            ------                                   ------
            ClientHello           ------>

                                    X<-- HelloRetryRequest

            [Timer Expires]

            ClientHello           ------>

                   Figure 1: DTLS retransmission example

   Once the client has transmitted the ClientHello message, it expects
   to see a HelloRetryRequest or a ServerHello from the server.
   However, if the server's message is lost, the client knows that
   either the ClientHello or the response from the server has been lost
   and retransmits.  When the server receives the retransmission, it
   knows to retransmit.

   The server also maintains a retransmission timer and retransmits when
   that timer expires.

   Note that timeout and retransmission do not apply to the
   HelloRetryRequest since this would require creating state on the
   server.  The HelloRetryRequest is designed to be small enough that it
   will not itself be fragmented, thus avoiding concerns about
   interleaving multiple HelloRetryRequests.

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3.1.1.  Reordering

   In DTLS, each handshake message is assigned a specific sequence
   number within that handshake.  When a peer receives a handshake
   message, it can quickly determine whether that message is the next
   message it expects.  If it is, then it processes it.  If not, it
   queues it for future handling once all previous messages have been

3.1.2.  Message Size

   TLS and DTLS handshake messages can be quite large (in theory up to
   2^24-1 bytes, in practice many kilobytes).  By contrast, UDP
   datagrams are often limited to less than 1500 bytes if IP
   fragmentation is not desired.  In order to compensate for this
   limitation, each DTLS handshake message may be fragmented over
   several DTLS records, each of which is intended to fit in a single IP
   datagram.  Each DTLS handshake message contains both a fragment
   offset and a fragment length.  Thus, a recipient in possession of all
   bytes of a handshake message can reassemble the original unfragmented

3.2.  Replay Detection

   DTLS optionally supports record replay detection.  The technique used
   is the same as in IPsec AH/ESP, by maintaining a bitmap window of
   received records.  Records that are too old to fit in the window and
   records that have previously been received are silently discarded.
   The replay detection feature is optional, since packet duplication is
   not always malicious, but can also occur due to routing errors.
   Applications may conceivably detect duplicate packets and accordingly
   modify their data transmission strategy.

4.  The DTLS Record Layer

   The DTLS record layer is similar to that of TLS 1.3.  There are three
   major changes:

   1.  The DTLSCiphertext structure omits the superfluous version number

   2.  DTLS adds an explicit epoch and sequence number in the record
       header.  This sequence number allows the recipient to correctly
       verify the DTLS MAC.

   3.  DTLS adds a short header format (DTLSShortCiphertext) that can be
       used to reduce overhead once the handshake is complete.

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   The DTLS record formats are shown below.  DTLSPlaintext records are
   used to send unprotected records and DTLSCiphertext or
   DTLSShortCiphertext are used to send protected records.

     struct {
         ContentType type;
         ProtocolVersion legacy_record_version;
         uint16 epoch = 0                                 // DTLS field
         uint48 sequence_number;                          // DTLS field
         uint16 length;
         opaque fragment[DTLSPlaintext.length];
     } DTLSPlaintext;

     struct {
          opaque content[DTLSPlaintext.length];
          ContentType type;
          uint8 zeros[length_of_padding];
     } DTLSInnerPlaintext;

     struct {
         ContentType opaque_type = 23; /* application_data */
         uint32 epoch_and_sequence;
         uint16 length;
         opaque encrypted_record[length];
     } DTLSCiphertext;

   opaque_type:  Identical to the opaque_type field in a TLS 1.3 record.

   epoch_and_sequence:  The low order two bits of the epoch and the low
      order 30 bits of the sequence number, laid out as a 32 bit
      integer.  The first 2 bits hold the low order bits from the epoch
      and the remaining 30 bits hold the low order bits from the
      sequence number (see Section 4.1.2 for how to use this value).

   length:  Identical to the length field in a TLS 1.3 record.

   encrypted_record:  Identical to the encrypted_record field in a TLS
      1.3 record.

   As with previous versions of DTLS, multiple DTLSPlaintext and
   DTLSCiphertext records can be included in the same underlying
   transport datagram.

   The short DTLS header format is:

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       struct {
         uint16 short_epoch_and_sequence;  // 001ESSSS SSSSSSSS
         opaque encrypted_record[remainder_of_datagram];
       } DTLSShortCiphertext;

   The short_epoch_and_sequence document contains the epoch and sequence
   packed into a 16 bit integer as follows:

   -  The first three bits are set to 001 in order to allow multiplexing
      between DTLS and VoIP protocols (STUN, RTP/RTCP, etc.)  [RFC7983]
      and distinguish the short from long header formats.

   -  The fourth bit is the low order bit of the epoch value.

   -  The remaining bits contain the low order 12 bits of the sequence

   In this format, the length field is omitted and therefore the record
   consumes the entire rest of the datagram in the lower level
   transport.  It is not possible to have multiple DTLSShortCiphertext
   format records in the same datagram.

   DTLSShortCiphertext MUST only be used for data which is protected
   with one of the application_traffic_secret values, and not for
   messages protected with either [sender]_handshake_traffic_sercret or
   [sender]_early_traffic_secret values.  When using an
   [sender]_application_traffic_secret for message protection,
   Implementations MAY use either DTLSCiphertext or DTLSShortCiphertext
   at their discretion.

4.1.  Sequence Number Handling

   DTLS uses an explicit sequence number, rather than an implicit one,
   carried in the sequence_number field of the record.  Sequence numbers
   are maintained separately for each epoch, with each sequence_number
   initially being 0 for each epoch.  For instance, if a handshake
   message from epoch 0 is retransmitted, it might have a sequence
   number after a message from epoch 1, even if the message from epoch 1
   was transmitted first.  Note that some care needs to be taken during
   the handshake to ensure that retransmitted messages use the right
   epoch and keying material.

   The epoch number is initially zero and is incremented each time
   keying material changes and a sender aims to rekey.  More details are
   provided in Section 6.1.

   Note that because DTLS records may be reordered, a record from epoch
   1 may be received after epoch 2 has begun.  In general,

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   implementations SHOULD discard packets from earlier epochs, but if
   packet loss causes noticeable problems implementations MAY choose to
   retain keying material from previous epochs for up to the default MSL
   specified for TCP [RFC0793] to allow for packet reordering.  (Note
   that the intention here is that implementers use the current guidance
   from the IETF for MSL, as specified in [RFC0793] or successors not
   that they attempt to interrogate the MSL that the system TCP stack is
   using.)  Until the handshake has completed, implementations MUST
   accept packets from the old epoch.

   Conversely, it is possible for records that are protected with the
   new epoch to be received prior to the completion of a handshake.  For
   instance, the server may send its Finished message and then start
   transmitting data.  Implementations MAY either buffer or discard such
   packets, though when DTLS is used over reliable transports (e.g.,
   SCTP [RFC4960]), they SHOULD be buffered and processed once the
   handshake completes.  Note that TLS's restrictions on when packets
   may be sent still apply, and the receiver treats the packets as if
   they were sent in the right order.  In particular, it is still
   impermissible to send data prior to completion of the first

   Implementations MUST either abandon an association or re-key prior to
   allowing the sequence number to wrap.

   Implementations MUST NOT allow the epoch to wrap, but instead MUST
   establish a new association, terminating the old association.

4.1.1.  Determining the Header Format

   Implementations can distinguish the three header formats by examining
   the first byte, which in the DTLSPlaintext and DTLSCiphertext header
   represents the content type.  If the first byte is alert(21),
   handshake(22), or ack(proposed, 25), the record MUST be interpreted
   as a DTLSPlaintext record.  If the first byte is application_data(23)
   then the record MUST be interpreted handled as DTLSCiphertext; the
   true content type will be inside the protected portion.

   If the first byte is any other other value, then receivers MUST check
   to see if the leading bits of the first byte are 001.  If so, they
   MUST process the record as DTLSShortCiphertext.  Otherwise, the
   record MUST be rejected as if it had failed deprotection, as
   described in Section 4.4.2.

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4.1.2.  Reconstructing the Sequence Number and Epoch

   When receiving protected DTLS records message, the recipient does not
   have a full epoch or sequence number value and so there is some
   opportunity for ambiguity.  Because the full epoch and sequence
   number are used to compute the per-record nonce, failure to
   reconstruct these values leads to failure to deprotect the record,
   and so implementations MAY use a mechanism of their choice to
   determine the full values.  This section provides an algorithm which
   is comparatively simple and which implementations are RECOMMENDED to

   If the epoch bits match those of the current epoch, then
   implementations SHOULD reconstruct the sequence number by computing
   the full sequence number which is numerically closest to one plus the
   sequence number of the highest successfully deprotected record.

   During the handshake phase, the epoch bits unambiguously indicate the
   correct key to use.  After the handshake is complete, if the epoch
   bits do not match those from the current epoch implementations SHOULD
   use the most recent past epoch which has matching bits, and then
   reconstruct the sequence number as described above.

   Note: the DTLSShortCiphertext format does not allow for easy
   reconstruction of sequence numbers if ~2000 datagrams in sequence are
   lost.  Implementations which may encounter this situation SHOULD use
   the DTLSCiphertext format.

4.2.  Transport Layer Mapping

   DTLS messages MAY be fragmentmented into multiple DTLS records.  Each
   DTLS record MUST fit within a single datagram.  In order to avoid IP
   fragmentation, clients of the DTLS record layer SHOULD attempt to
   size records so that they fit within any PMTU estimates obtained from
   the record layer.

   Multiple DTLS records MAY be placed in a single datagram.  They are
   simply encoded consecutively.  The DTLS record framing is sufficient
   to determine the boundaries.  Note, however, that the first byte of
   the datagram payload MUST be the beginning of a record.  Records MUST
   NOT span datagrams.

   DTLS records, as defined in this document, do not contain any
   association identifiers and applications must arrange to multiplex
   between associations.  With UDP, the host/port number is used to look
   up the appropriate security association for incoming records.
   However, the Connection ID extension defined in

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   [I-D.ietf-tls-dtls-connection-id] adds an association identifier to
   DTLS records.

   Some transports, such as DCCP [RFC4340], provide their own sequence
   numbers.  When carried over those transports, both the DTLS and the
   transport sequence numbers will be present.  Although this introduces
   a small amount of inefficiency, the transport layer and DTLS sequence
   numbers serve different purposes; therefore, for conceptual
   simplicity, it is superior to use both sequence numbers.

   Some transports provide congestion control for traffic carried over
   them.  If the congestion window is sufficiently narrow, DTLS
   handshake retransmissions may be held rather than transmitted
   immediately, potentially leading to timeouts and spurious
   retransmission.  When DTLS is used over such transports, care should
   be taken not to overrun the likely congestion window.  [RFC5238]
   defines a mapping of DTLS to DCCP that takes these issues into

4.3.  PMTU Issues

   In general, DTLS's philosophy is to leave PMTU discovery to the
   application.  However, DTLS cannot completely ignore PMTU for three

   -  The DTLS record framing expands the datagram size, thus lowering
      the effective PMTU from the application's perspective.

   -  In some implementations, the application may not directly talk to
      the network, in which case the DTLS stack may absorb ICMP
      [RFC1191] "Datagram Too Big" indications or ICMPv6 [RFC4443]
      "Packet Too Big" indications.

   -  The DTLS handshake messages can exceed the PMTU.

   In order to deal with the first two issues, the DTLS record layer
   SHOULD behave as described below.

   If PMTU estimates are available from the underlying transport
   protocol, they should be made available to upper layer protocols.  In

   -  For DTLS over UDP, the upper layer protocol SHOULD be allowed to
      obtain the PMTU estimate maintained in the IP layer.

   -  For DTLS over DCCP, the upper layer protocol SHOULD be allowed to
      obtain the current estimate of the PMTU.

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   -  For DTLS over TCP or SCTP, which automatically fragment and
      reassemble datagrams, there is no PMTU limitation.  However, the
      upper layer protocol MUST NOT write any record that exceeds the
      maximum record size of 2^14 bytes.

   The DTLS record layer SHOULD allow the upper layer protocol to
   discover the amount of record expansion expected by the DTLS

   If there is a transport protocol indication (either via ICMP or via a
   refusal to send the datagram as in Section 14 of [RFC4340]), then the
   DTLS record layer MUST inform the upper layer protocol of the error.

   The DTLS record layer SHOULD NOT interfere with upper layer protocols
   performing PMTU discovery, whether via [RFC1191] or [RFC4821]
   mechanisms.  In particular:

   -  Where allowed by the underlying transport protocol, the upper
      layer protocol SHOULD be allowed to set the state of the DF bit
      (in IPv4) or prohibit local fragmentation (in IPv6).

   -  If the underlying transport protocol allows the application to
      request PMTU probing (e.g., DCCP), the DTLS record layer SHOULD
      honor this request.

   The final issue is the DTLS handshake protocol.  From the perspective
   of the DTLS record layer, this is merely another upper layer
   protocol.  However, DTLS handshakes occur infrequently and involve
   only a few round trips; therefore, the handshake protocol PMTU
   handling places a premium on rapid completion over accurate PMTU
   discovery.  In order to allow connections under these circumstances,
   DTLS implementations SHOULD follow the following rules:

   -  If the DTLS record layer informs the DTLS handshake layer that a
      message is too big, it SHOULD immediately attempt to fragment it,
      using any existing information about the PMTU.

   -  If repeated retransmissions do not result in a response, and the
      PMTU is unknown, subsequent retransmissions SHOULD back off to a
      smaller record size, fragmenting the handshake message as
      appropriate.  This standard does not specify an exact number of
      retransmits to attempt before backing off, but 2-3 seems

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4.4.  Record Payload Protection

   Like TLS, DTLS transmits data as a series of protected records.  The
   rest of this section describes the details of that format.

4.4.1.  Anti-Replay

   Each DTLS record contains a sequence number to provide replay
   protection.  Sequence number verification SHOULD be performed using
   the following sliding window procedure, borrowed from Section 3.4.3
   of [RFC4303].

   The received packet counter for a session MUST be initialized to zero
   when that session is established.  For each received record, the
   receiver MUST verify that the record contains a sequence number that
   does not duplicate the sequence number of any other record received
   during the lifetime of the session.  This SHOULD be the first check
   applied to a packet after it has been matched to a session, to speed
   up the rejection of duplicate records.

   Duplicates are rejected through the use of a sliding receive window.
   (How the window is implemented is a local matter, but the following
   text describes the functionality that the implementation must
   exhibit.)  A minimum window size of 32 MUST be supported, but a
   window size of 64 is preferred and SHOULD be employed as the default.
   Another window size (larger than the minimum) MAY be chosen by the
   receiver.  (The receiver does not notify the sender of the window

   The "right" edge of the window represents the highest validated
   sequence number value received on the session.  Records that contain
   sequence numbers lower than the "left" edge of the window are
   rejected.  Packets falling within the window are checked against a
   list of received packets within the window.  An efficient means for
   performing this check, based on the use of a bit mask, is described
   in Section 3.4.3 of [RFC4303].

   If the received record falls within the window and is new, or if the
   packet is to the right of the window, then the receiver proceeds to
   MAC verification.  If the MAC validation fails, the receiver MUST
   discard the received record as invalid.  The receive window is
   updated only if the MAC verification succeeds.

4.4.2.  Handling Invalid Records

   Unlike TLS, DTLS is resilient in the face of invalid records (e.g.,
   invalid formatting, length, MAC, etc.).  In general, invalid records
   SHOULD be silently discarded, thus preserving the association;

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   however, an error MAY be logged for diagnostic purposes.
   Implementations which choose to generate an alert instead, MUST
   generate error alerts to avoid attacks where the attacker repeatedly
   probes the implementation to see how it responds to various types of
   error.  Note that if DTLS is run over UDP, then any implementation
   which does this will be extremely susceptible to denial-of-service
   (DoS) attacks because UDP forgery is so easy.  Thus, this practice is
   NOT RECOMMENDED for such transports, both to increase the reliability
   of DTLS service and to avoid the risk of spoofing attacks sending
   traffic to unrelated third parties.

   If DTLS is being carried over a transport that is resistant to
   forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts
   because an attacker will have difficulty forging a datagram that will
   not be rejected by the transport layer.

5.  The DTLS Handshake Protocol

   DTLS 1.3 re-uses the TLS 1.3 handshake messages and flows, with the
   following changes:

   1.  To handle message loss, reordering, and fragmentation
       modifications to the handshake header are necessary.

   2.  Retransmission timers are introduced to handle message loss.

   3.  A new ACK content type has been added for reliable message
       delivery of handshake messages.

   Note that TLS 1.3 already supports a cookie extension, which is used
   to prevent denial-of-service attacks.  This DoS prevention mechanism
   is described in more detail below since UDP-based protocols are more
   vulnerable to amplification attacks than a connection-oriented
   transport like TCP that performs return-routability checks as part of
   the connection establishment.

   With these exceptions, the DTLS message formats, flows, and logic are
   the same as those of TLS 1.3.  DTLS implementations SHOULD not use
   the TLS 1.3 "compatibility mode" described in [I-D.ietf-tls-tls13],
   Section D.4 and DTLS servers SHOULD ignore the "session_id" value
   generated by the client rather than sending ChangeCipherSpec
   messages.  Implementations MUST still ignore ChangeCipherSpec
   messages received during the handshake and at all other times SHOULD
   treat them as if they had failed to deprotect.

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5.1.  Denial-of-Service Countermeasures

   Datagram security protocols are extremely susceptible to a variety of
   DoS attacks.  Two attacks are of particular concern:

   1.  An attacker can consume excessive resources on the server by
       transmitting a series of handshake initiation requests, causing
       the server to allocate state and potentially to perform expensive
       cryptographic operations.

   2.  An attacker can use the server as an amplifier by sending
       connection initiation messages with a forged source of the
       victim.  The server then sends its response to the victim
       machine, thus flooding it.  Depending on the selected ciphersuite
       this response message can be quite large, as it is the case for a
       Certificate message.

   In order to counter both of these attacks, DTLS borrows the stateless
   cookie technique used by Photuris [RFC2522] and IKE [RFC7296].  When
   the client sends its ClientHello message to the server, the server
   MAY respond with a HelloRetryRequest message.  The HelloRetryRequest
   message, as well as the cookie extension, is defined in TLS 1.3.  The
   HelloRetryRequest message contains a stateless cookie generated using
   the technique of [RFC2522].  The client MUST retransmit the
   ClientHello with the cookie added as an extension.  The server then
   verifies the cookie and proceeds with the handshake only if it is
   valid.  This mechanism forces the attacker/client to be able to
   receive the cookie, which makes DoS attacks with spoofed IP addresses
   difficult.  This mechanism does not provide any defence against DoS
   attacks mounted from valid IP addresses.

   The DTLS 1.3 specification changes the way how cookies are exchanged
   compared to DTLS 1.2.  DTLS 1.3 re-uses the HelloRetryRequest message
   and conveys the cookie to the client via an extension.  The client
   receiving the cookie uses the same extension to place the cookie
   subsequently into a ClientHello message.  DTLS 1.2 on the other hand
   used a separate message, namely the HelloVerifyRequest, to pass a
   cookie to the client and did not utilize the extension mechanism.
   For backwards compatibility reason the cookie field in the
   ClientHello is present in DTLS 1.3 but is ignored by a DTLS 1.3
   compliant server implementation.

   The exchange is shown in Figure 2.  Note that the figure focuses on
   the cookie exchange; all other extensions are omitted.

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         Client                                   Server
         ------                                   ------
         ClientHello           ------>

                               <----- HelloRetryRequest
                                       + cookie

         ClientHello           ------>
          + cookie

         [Rest of handshake]

       Figure 2: DTLS exchange with HelloRetryRequest containing the
                            "cookie" extension

   The cookie extension is defined in Section 4.2.2 of
   [I-D.ietf-tls-tls13].  When sending the initial ClientHello, the
   client does not have a cookie yet.  In this case, the cookie
   extension is omitted and the legacy_cookie field in the ClientHello
   message SHOULD be set to a zero length vector (i.e., a single zero
   byte length field) and MUST be ignored by a server negotiating DTLS

   When responding to a HelloRetryRequest, the client MUST create a new
   ClientHello message following the description in Section 4.1.2 of

   If the HelloRetryRequest message is used, the initial ClientHello and
   the HelloRetryRequest are included in the calculation of the
   Transcript-Hash.  The computation of the message hash for the
   HelloRetryRequest is done according to the description in
   Section 4.4.1 of [I-D.ietf-tls-tls13].

   The handshake transcript is not reset with the second ClientHello and
   a stateless server-cookie implementation requires the transcript of
   the HelloRetryRequest to be stored in the cookie or the internal
   state of the hash algorithm, since only the hash of the transcript is
   required for the handshake to complete.

   When the second ClientHello is received, the server can verify that
   the cookie is valid and that the client can receive packets at the
   given IP address.  If the client's apparent IP address is embedded in
   the cookie, this prevents an attacker from generating an acceptable
   ClientHello apparently from another user.

   One potential attack on this scheme is for the attacker to collect a
   number of cookies from different addresses where it controls
   endpoints and then reuse them to attack the server.  The server can

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   defend against this attack by changing the secret value frequently,
   thus invalidating those cookies.  If the server wishes that
   legitimate clients be able to handshake through the transition (e.g.,
   they received a cookie with Secret 1 and then sent the second
   ClientHello after the server has changed to Secret 2), the server can
   have a limited window during which it accepts both secrets.
   [RFC7296] suggests adding a key identifier to cookies to detect this
   case.  An alternative approach is simply to try verifying with both
   secrets.  It is RECOMMENDED that servers implement a key rotation
   scheme that allows the server to manage keys with overlapping

   Alternatively, the server can store timestamps in the cookie and
   reject those cookies that were not generated within a certain amount
   of time.

   DTLS servers SHOULD perform a cookie exchange whenever a new
   handshake is being performed.  If the server is being operated in an
   environment where amplification is not a problem, the server MAY be
   configured not to perform a cookie exchange.  The default SHOULD be
   that the exchange is performed, however.  In addition, the server MAY
   choose not to do a cookie exchange when a session is resumed.
   Clients MUST be prepared to do a cookie exchange with every

   If a server receives a ClientHello with an invalid cookie, it MUST
   NOT respond with a HelloRetryRequest.  Restarting the handshake from
   scratch, without a cookie, allows the client to recover from a
   situation where it obtained a cookie that cannot be verified by the
   server.  As described in Section 4.1.4 of [I-D.ietf-tls-tls13],
   clients SHOULD also abort the handshake with an "unexpected_message"
   alert in response to any second HelloRetryRequest which was sent in
   the same connection (i.e., where the ClientHello was itself in
   response to a HelloRetryRequest).

5.2.  DTLS Handshake Message Format

   In order to support message loss, reordering, and message
   fragmentation, DTLS modifies the TLS 1.3 handshake header:

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     enum {
     } HandshakeType;

     struct {
         HandshakeType msg_type;    /* handshake type */
         uint24 length;             /* bytes in message */
         uint16 message_seq;        /* DTLS-required field */
         uint24 fragment_offset;    /* DTLS-required field */
         uint24 fragment_length;    /* DTLS-required field */
         select (HandshakeType) {
             case client_hello:          ClientHello;
             case server_hello:          ServerHello;
             case end_of_early_data:     EndOfEarlyData;
             case encrypted_extensions:  EncryptedExtensions;
             case certificate_request:   CertificateRequest;
             case certificate:           Certificate;
             case certificate_verify:    CertificateVerify;
             case finished:              Finished;
             case new_session_ticket:    NewSessionTicket;
             case key_update:            KeyUpdate;
         } body;
     } Handshake;

   The first message each side transmits in each association always has
   message_seq = 0.  Whenever a new message is generated, the
   message_seq value is incremented by one.  When a message is
   retransmitted, the old message_seq value is re-used, i.e., not
   incremented.  From the perspective of the DTLS record layer, the
   retransmission is a new record.  This record will have a new
   DTLSPlaintext.sequence_number value.

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   DTLS implementations maintain (at least notionally) a
   next_receive_seq counter.  This counter is initially set to zero.
   When a handshake message is received, if its message_seq value
   matches next_receive_seq, next_receive_seq is incremented and the
   message is processed.  If the sequence number is less than
   next_receive_seq, the message MUST be discarded.  If the sequence
   number is greater than next_receive_seq, the implementation SHOULD
   queue the message but MAY discard it.  (This is a simple space/
   bandwidth tradeoff).

   In addition to the handshake messages that are deprecated by the TLS
   1.3 specification DTLS 1.3 furthermore deprecates the
   HelloVerifyRequest message originally defined in DTLS 1.0.  DTLS
   1.3-compliant implements MUST NOT use the HelloVerifyRequest to
   execute a return-routability check.  A dual-stack DTLS 1.2/DTLS 1.3
   client MUST, however, be prepared to interact with a DTLS 1.2 server.

5.3.  ClientHello Message

   The format of the ClientHello used by a DTLS 1.3 client differs from
   the TLS 1.3 ClientHello format as shown below.

      uint16 ProtocolVersion;
      opaque Random[32];

      uint8 CipherSuite[2];    /* Cryptographic suite selector */

      struct {
          ProtocolVersion legacy_version = { 254,253 }; // DTLSv1.2
          Random random;
          opaque legacy_session_id<0..32>;
          opaque legacy_cookie<0..2^8-1>;                  // DTLS
          CipherSuite cipher_suites<2..2^16-2>;
          opaque legacy_compression_methods<1..2^8-1>;
          Extension extensions<8..2^16-1>;
      } ClientHello;

   legacy_version:  In previous versions of DTLS, this field was used
      for version negotiation and represented the highest version number
      supported by the client.  Experience has shown that many servers
      do not properly implement version negotiation, leading to "version
      intolerance" in which the server rejects an otherwise acceptable
      ClientHello with a version number higher than it supports.  In
      DTLS 1.3, the client indicates its version preferences in the
      "supported_versions" extension (see Section 4.2.1 of
      [I-D.ietf-tls-tls13]) and the legacy_version field MUST be set to
      {254, 253}, which was the version number for DTLS 1.2.

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   random:  Same as for TLS 1.3.

   legacy_session_id:  Same as for TLS 1.3.

   legacy_cookie:  A DTLS 1.3-only client MUST set the legacy_cookie
      field to zero length.

   cipher_suites:  Same as for TLS 1.3.

   legacy_compression_methods:  Same as for TLS 1.3.

   extensions:  Same as for TLS 1.3.

5.4.  Handshake Message Fragmentation and Reassembly

   Each DTLS message MUST fit within a single transport layer datagram.
   However, handshake messages are potentially bigger than the maximum
   record size.  Therefore, DTLS provides a mechanism for fragmenting a
   handshake message over a number of records, each of which can be
   transmitted separately, thus avoiding IP fragmentation.

   When transmitting the handshake message, the sender divides the
   message into a series of N contiguous data ranges.  These ranges MUST
   NOT be larger than the maximum handshake fragment size and MUST
   jointly contain the entire handshake message.  The ranges MUST NOT
   overlap.  The sender then creates N handshake messages, all with the
   same message_seq value as the original handshake message.  Each new
   message is labeled with the fragment_offset (the number of bytes
   contained in previous fragments) and the fragment_length (the length
   of this fragment).  The length field in all messages is the same as
   the length field of the original message.  An unfragmented message is
   a degenerate case with fragment_offset=0 and fragment_length=length.

   When a DTLS implementation receives a handshake message fragment, it
   MUST buffer it until it has the entire handshake message.  DTLS
   implementations MUST be able to handle overlapping fragment ranges.
   This allows senders to retransmit handshake messages with smaller
   fragment sizes if the PMTU estimate changes.

   Note that as with TLS, multiple handshake messages may be placed in
   the same DTLS record, provided that there is room and that they are
   part of the same flight.  Thus, there are two acceptable ways to pack
   two DTLS messages into the same datagram: in the same record or in
   separate records.

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5.5.  DTLS Handshake Flights

   DTLS messages are grouped into a series of message flights, according
   to the diagrams below.

Client                                             Server

ClientHello                                                 +----------+
 + key_share*                                               | Flight 1 |
 + pre_shared_key*      -------->                           +----------+

                        <--------        HelloRetryRequest  | Flight 2 |
                                          + cookie          +----------+

ClientHello                                                 +----------+
 + key_share*                                               | Flight 3 |
 + pre_shared_key*      -------->                           +----------+
 + cookie

                                              + key_share*
                                         + pre_shared_key*  +----------+
                                     {EncryptedExtensions}  | Flight 4 |
                                     {CertificateRequest*}  +----------+
                        <--------               {Finished}
                                       [Application Data*]

 {Certificate*}                                             +----------+
 {CertificateVerify*}                                       | Flight 5 |
 {Finished}             -------->                           +----------+
 [Application Data]

                        <--------                    [ACK]  | Flight 6 |
                                       [Application Data*]  +----------+

 [Application Data]     <------->      [Application Data]

     Figure 3: Message flights for a full DTLS Handshake (with cookie

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   ClientHello                                              +----------+
    + pre_shared_key                                        | Flight 1 |
    + key_share*         -------->                          +----------+

                                          + pre_shared_key  +----------+
                                              + key_share*  | Flight 2 |
                                     {EncryptedExtensions}  +----------+
                         <--------              {Finished}
                                       [Application Data*]
   {Finished}            -------->                          | Flight 3 |
   [Application Data*]                                      +----------+

                         <--------                   [ACK]  | Flight 4 |
                                       [Application Data*]  +----------+

   [Application Data]    <------->      [Application Data]

    Figure 4: Message flights for resumption and PSK handshake (without
                             cookie exchange)

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  Client                                            Server

    + early_data
    + psk_key_exchange_modes                                +----------+
    + key_share*                                            | Flight 1 |
    + pre_shared_key                                        +----------+
   (Application Data*)     -------->

                                          + pre_shared_key
                                              + key_share*  +----------+
                                     {EncryptedExtensions}  | Flight 2 |
                                                {Finished}  +----------+
                         <--------     [Application Data*]

   (EndOfEarlyData)                                         | Flight 3 |
   {Finished}            -------->                          +----------+
   [Application Data*]
                         <--------                   [ACK]  | Flight 4 |
                                       [Application Data*]  +----------+

   [Application Data]    <------->      [Application Data]

           Figure 5: Message flights for the Zero-RTT handshake

  Client                                            Server

                         <--------       [NewSessionTicket] | Flight 1 |

  [ACK]                  -------->                          | Flight 2 |

       Figure 6: Message flights for the new session ticket message

   Note: The application data sent by the client is not included in the
   timeout and retransmission calculation.

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5.6.  Timeout and Retransmission

5.6.1.  State Machine

   DTLS uses a simple timeout and retransmission scheme with the state
   machine shown in Figure 7.  Because DTLS clients send the first
   message (ClientHello), they start in the PREPARING state.  DTLS
   servers start in the WAITING state, but with empty buffers and no
   retransmit timer.

                                | PREPARING |
                   +----------> |           |
                   |            |           |
                   |            +-----------+
                   |                  |
                   |                  | Buffer next flight
                   |                  |
                   |                 \|/
                   |            +-----------+
                   |            |           |
                   |            |  SENDING  |<------------------+
                   |            |           |                   |
                   |            +-----------+                   |
           Receive |                  |                         |
              next |                  | Send flight or partial  |
            flight |                  | flight                  |
                   |  +---------------+                         |
                   |  |               | Set retransmit timer    |
                   |  |              \|/                        |
                   |  |         +-----------+                   |
                   |  |         |           |                   |
                   +--)---------|  WAITING  |-------------------+
                   |  |  +----->|           |   Timer expires   |
                   |  |  |      +-----------+                   |
                   |  |  |          |  |   |                    |
                   |  |  |          |  |   |                    |
                   |  |  +----------+  |   +--------------------+
                   |  | Receive record |   Read retransmit or ACK
           Receive |  |  Send ACK      |
              last |  |                |
            flight |  |                | Receive ACK
                   |  |                | for last flight
                  \|/\|/               |
               +-----------+           |
               |           | <---------+
               | FINISHED  |

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               |           |
                   |  /|\
                   |   |
                   |   |

             Server read retransmit
                 Retransmit ACK

          Figure 7: DTLS timeout and retransmission state machine

   The state machine has four basic states: PREPARING, SENDING, WAITING,
   and FINISHED.

   In the PREPARING state, the implementation does whatever computations
   are necessary to prepare the next flight of messages.  It then
   buffers them up for transmission (emptying the buffer first) and
   enters the SENDING state.

   In the SENDING state, the implementation transmits the buffered
   flight of messages.  If the implementation has received one or more
   ACKs (see Section 7) from the peer, then it SHOULD omit any messages
   or message fragments which have already been ACKed.  Once the
   messages have been sent, the implementation then enters the FINISHED
   state if this is the last flight in the handshake.  Or, if the
   implementation expects to receive more messages, it sets a retransmit
   timer and then enters the WAITING state.

   There are four ways to exit the WAITING state:

   1.  The retransmit timer expires: the implementation transitions to
       the SENDING state, where it retransmits the flight, resets the
       retransmit timer, and returns to the WAITING state.

   2.  The implementation reads a ACK from the peer: upon receiving an
       ACK for a partial flight (as mentioned in Section 7.1), the
       implementation transitions to the SENDING state, where it
       retransmits the unacked portion of the flight, resets the
       retransmit timer, and returns to the WAITING state.  Upon
       receiving an ACK for a complete flight, the implementation
       cancels all retransmissions and either remains in WAITING, or, if
       the ACK was for the final flight, transitions to FINISHED.

   3.  The implementation reads a retransmitted flight from the peer:
       the implementation transitions to the SENDING state, where it
       retransmits the flight, resets the retransmit timer, and returns
       to the WAITING state.  The rationale here is that the receipt of

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       a duplicate message is the likely result of timer expiry on the
       peer and therefore suggests that part of one's previous flight
       was lost.

   4.  The implementation receives some or all next flight of messages:
       if this is the final flight of messages, the implementation
       transitions to FINISHED.  If the implementation needs to send a
       new flight, it transitions to the PREPARING state.  Partial reads
       (whether partial messages or only some of the messages in the
       flight) may also trigger the implementation to send an ACK, as
       described in Section 7.1.

   Because DTLS clients send the first message (ClientHello), they start
   in the PREPARING state.  DTLS servers start in the WAITING state, but
   with empty buffers and no retransmit timer.

   In addition, for at least twice the default Maximum Segment Lifetime
   (MSL) defined for [RFC0793], when in the FINISHED state, the server
   MUST respond to retransmission of the client's second flight with a
   retransmit of its ACK.

   Note that because of packet loss, it is possible for one side to be
   sending application data even though the other side has not received
   the first side's Finished message.  Implementations MUST either
   discard or buffer all application data packets for the new epoch
   until they have received the Finished message for that epoch.
   Implementations MAY treat receipt of application data with a new
   epoch prior to receipt of the corresponding Finished message as
   evidence of reordering or packet loss and retransmit their final
   flight immediately, shortcutting the retransmission timer.

5.6.2.  Timer Values

   Though timer values are the choice of the implementation, mishandling
   of the timer can lead to serious congestion problems; for example, if
   many instances of a DTLS time out early and retransmit too quickly on
   a congested link.  Implementations SHOULD use an initial timer value
   of 100 msec (the minimum defined in RFC 6298 [RFC6298]) and double
   the value at each retransmission, up to no less than the RFC 6298
   maximum of 60 seconds.  Application specific profiles, such as those
   used for the Internet of Things environment, may recommend longer
   timer values.  Note that a 100 msec timer is recommend rather than
   the 3-second RFC 6298 default in order to improve latency for time-
   sensitive applications.  Because DTLS only uses retransmission for
   handshake and not dataflow, the effect on congestion should be

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   Implementations SHOULD retain the current timer value until a
   transmission without loss occurs, at which time the value may be
   reset to the initial value.  After a long period of idleness, no less
   than 10 times the current timer value, implementations may reset the
   timer to the initial value.  One situation where this might occur is
   when a rehandshake is used after substantial data transfer.

5.7.  CertificateVerify and Finished Messages

   CertificateVerify and Finished messages have the same format as in
   TLS 1.3.  Hash calculations include entire handshake messages,
   including DTLS-specific fields: message_seq, fragment_offset, and
   fragment_length.  However, in order to remove sensitivity to
   handshake message fragmentation, the CertificateVerify and the
   Finished messages MUST be computed as if each handshake message had
   been sent as a single fragment following the algorithm described in
   Section 4.4.3 and Section 4.4.4 of [I-D.ietf-tls-tls13],

5.8.  Alert Messages

   Note that Alert messages are not retransmitted at all, even when they
   occur in the context of a handshake.  However, a DTLS implementation
   which would ordinarily issue an alert SHOULD generate a new alert
   message if the offending record is received again (e.g., as a
   retransmitted handshake message).  Implementations SHOULD detect when
   a peer is persistently sending bad messages and terminate the local
   connection state after such misbehavior is detected.

5.9.  Establishing New Associations with Existing Parameters

   If a DTLS client-server pair is configured in such a way that
   repeated connections happen on the same host/port quartet, then it is
   possible that a client will silently abandon one connection and then
   initiate another with the same parameters (e.g., after a reboot).
   This will appear to the server as a new handshake with epoch=0.  In
   cases where a server believes it has an existing association on a
   given host/port quartet and it receives an epoch=0 ClientHello, it
   SHOULD proceed with a new handshake but MUST NOT destroy the existing
   association until the client has demonstrated reachability either by
   completing a cookie exchange or by completing a complete handshake
   including delivering a verifiable Finished message.  After a correct
   Finished message is received, the server MUST abandon the previous
   association to avoid confusion between two valid associations with
   overlapping epochs.  The reachability requirement prevents off-path/
   blind attackers from destroying associations merely by sending forged

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   Note: it is not always possible to distinguish which association a
   given packet is from.  For instance, if the client performs a
   handshake, abandons the connection, and then immediately starts a new
   handshake, it may not be possible to tell which connection a given
   protected record is for.  In these cases, trial decryption MAY be
   necessary, though implementations could also use some sort of
   connection identifier, such as the one specified in

6.  Example of Handshake with Timeout and Retransmission

   The following is an example of a handshake with lost packets and

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   Client                                                Server
   ------                                                ------

    Record 0                  -------->

                                X<-----                 Record 0
                                (lost)               ServerHello

                              <--------                 Record 1

    Record 1                  -------->
    ACK [1]

                              <--------                 Record 2

    Record 2                  -------->

                              <--------               Record 3
                                                       ACK [2]

         Figure 8: Example DTLS exchange illustrating message loss

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6.1.  Epoch Values and Rekeying

   A recipient of a DTLS message needs to select the correct keying
   material in order to process an incoming message.  With the
   possibility of message loss and re-order an identifier is needed to
   determine which cipher state has been used to protect the record
   payload.  The epoch value fulfills this role in DTLS.  In addition to
   the key derivation steps described in Section 7 of
   [I-D.ietf-tls-tls13] triggered by the states during the handshake a
   sender may want to rekey at any time during the lifetime of the
   connection and has to have a way to indicate that it is updating its
   sending cryptographic keys.

   This version of DTLS assigns dedicated epoch values to messages in
   the protocol exchange to allow identification of the correct cipher

   -  epoch value (0) is used with unencrypted messages.  There are
      three unencrypted messages in DTLS, namely ClientHello,
      ServerHello, and HelloRetryRequest.

   -  epoch value (1) is used for messages protected using keys derived
      from client_early_traffic_secret.  This includes early data sent
      by the client and the EndOfEarlyData message.

   -  epoch value (2) is used for messages protected using keys derived
      from [sender]_handshake_traffic_secret.  Messages transmitted
      during the initial handshake, such as EncryptedExtensions,
      CertificateRequest, Certificate, CertificateVerify, and Finished
      belong to this category.  Note, however, post-handshake are
      protected under the appropriate application traffic key and are
      not included in this category.

   -  epoch value (3) is used for payloads protected using keys derived
      from the initial traffic_secret_0.  This may include handshake
      messages, such as post-handshake messages (e.g., a
      NewSessionTicket message).

   -  epoch value (4 to 2^16-1) is used for payloads protected using
      keys from the traffic_secret_N (N>0).

   Using these reserved epoch values a receiver knows what cipher state
   has been used to encrypt and integrity protect a message.
   Implementations that receive a payload with an epoch value for which
   no corresponding cipher state can be determined MUST generate a
   "unexpected_message" alert.  For example, client incorrectly uses
   epoch value 5 when sending early application data in a 0-RTT

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   exchange.  A server will not be able to compute the appropriate keys
   and will therefore have to respond with an alert.

   Note that epoch values do not wrap.  If a DTLS implementation would
   need to wrap the epoch value, it MUST terminate the connection.

   The traffic key calculation is described in Section 7.3 of

   Figure 9 illustrates the epoch values in an example DTLS handshake.

   Client                                             Server
   ------                                             ------


                               <--------       HelloRetryRequest

    ClientHello                -------->

                               <--------             ServerHello

    {Certificate}              -------->

                               <--------                   [ACK]

    [Application Data]         -------->

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                               <--------      [Application Data]

                            Some time later ...
                    (Post-Handshake Message Exchange)

                               <--------      [NewSessionTicket]

    [ACK]                      -------->

                            Some time later ...

                               <--------      [Application Data]
    [Application Data]         -------->

          Figure 9: Example DTLS exchange with epoch information

7.  ACK Message

   The ACK message is used by an endpoint to indicate handshake-
   containing the TLS records it has received from the other side.  ACK
   is not a handshake message but is rather a separate content type,
   with code point TBD (proposed, 25).  This avoids it consuming space
   in the handshake message sequence.  Note that ACKs can still be
   piggybacked on the same UDP datagram as handshake records.

   struct {
          uint64 record_numbers<0..2^16-1>;
   } ACK;

   record_numbers:  a list of the records containing handshake messages
      in the current flight which the endpoint has received, in
      numerically increasing order.  ACKs only cover the current
      outstanding flight (this is possible because DTLS is generally a
      lockstep protocol).  Thus, an ACK from the server would not cover
      both the ClientHello and the client's Certificate.
      Implementations can accomplish this by clearing their ACK list
      upon receiving the start of the next flight.

   ACK records MUST be sent with an epoch that is equal to or higher
   than the record which is being acknowledged.  Implementations SHOULD
   simply use the current key.

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7.1.  Sending ACKs

   When an implementation receives a partial flight, it SHOULD generate
   an ACK that covers the messages from that flight which it has
   received so far.  Implementations have some discretion about when to
   generate ACKs, but it is RECOMMENDED that they do so under two

   -  When they receive a message or fragment which is out of order,
      either because it is not the next expected message or because it
      is not the next piece of the current message.  Implementations
      MUST NOT send ACKs for handshake messages which they discard as
      out-of-order, because otherwise those messages will not be

   -  When they have received part of a flight and do not immediately
      receive the rest of the flight (which may be in the same UDP
      datagram).  A reasonable approach here is to set a timer for 1/4
      the current retransmit timer value when the first record in the
      flight is received and then send an ACK when that timer expires.

   In addition, implementations MUST send ACKs upon receiving all of any
   flight which they do not respond to with their own messages.
   Specifically, this means the client's final flight of the main
   handshake, the server's transmission of the NewSessionTicket, and
   KeyUpdate messages.  ACKs SHOULD NOT be sent for other complete
   flights because they are implicitly acknowledged by the receipt of
   the next flight, which generally immediately follows the flight.
   Each NewSessionTicket or KeyUpdate is an individual flight; in
   particular, a KeyUpdate sent in response to a KeyUpdate with
   update_requested does not implicitly acknowledge that message.
   Implementations MAY ACK the records corresponding to each
   transmission of that flight or simply ACK the most recent one.

   ACKs MUST NOT be sent for other records of any content type other
   than handshake or for records which cannot be unprotected.

   Note that in some cases it may be necessary to send an ACK which does
   not contain any record numbers.  For instance, a client might receive
   an EncryptedExtensions message prior to receiving a ServerHello.
   Because it cannot decrypt the EncryptedExtensions, it cannot safely
   ACK it (as it might be damaged).  If the client does not send an ACK,
   the server will eventually retransmit its first flight, but this
   might take far longer than the actual round trip time between client
   and server.  Having the client send an empty ACK shortcuts this

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7.2.  Receiving ACKs

   When an implementation receives an ACK, it SHOULD record that the
   messages or message fragments sent in the records being ACKed were
   received and omit them from any future retransmissions.  Upon receipt
   of an ACK for only some messages from a flight, an implementation
   SHOULD retransmit the remaining messages or fragments.  Note that
   this requires implementations to track which messages appear in which
   records.  Once all the messages in a flight have been acknowledged,
   the implementation MUST cancel all retransmissions of that flight.
   As noted above, the receipt of any packet responding to a given
   flight MUST be taken as an implicit ACK for the entire flight.

8.  Key Updates

   As with TLS 1.3, DTLS 1.3 implementations send a KeyUpdate message to
   indicate that they are updating their sending keys.  As with other
   handshake messages with no built-in response, KeyUpdates MUST be
   acknowledged.  In order to facilitate epoch reconstruction
   Section 4.1.2 implementations MUST NOT send with the new keys or send
   a new KeyUpdate until the previous KeyUpdate has been acknowledged
   (this avoids having too many epochs in active use).

   Due to loss and/or re-ordering, DTLS 1.3 implementations may receive
   a record with an older epoch than the current one (the requirements
   above preclude receiving a newer record).  They SHOULD attempt to
   process those records with that epoch (see Section 4.1.2 for
   information on determining the correct epoch), but MAY opt to discard
   such out-of-epoch records.

   Although KeyUpdate MUST be ACKed, it is possible for the ACK to be
   lost, in which case the sender of the KeyUpdate will retransmit it.
   Implementations MUST retain the ability to ACK the KeyUpdate for up
   to 2MSL.  It is RECOMMENDED that they do so by retaining the pre-
   update keying material, but they MAY do so by responding to messages
   which appear to be out-of-epoch with a canned ACK message; in this
   case, implementations SHOULD rate limit how often they send such

9.  Application Data Protocol

   Application data messages are carried by the record layer and are
   fragmented and encrypted based on the current connection state.  The
   messages are treated as transparent data to the record layer.

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10.  Security Considerations

   Security issues are discussed primarily in [I-D.ietf-tls-tls13].

   The primary additional security consideration raised by DTLS is that
   of denial of service.  DTLS includes a cookie exchange designed to
   protect against denial of service.  However, implementations that do
   not use this cookie exchange are still vulnerable to DoS.  In
   particular, DTLS servers that do not use the cookie exchange may be
   used as attack amplifiers even if they themselves are not
   experiencing DoS.  Therefore, DTLS servers SHOULD use the cookie
   exchange unless there is good reason to believe that amplification is
   not a threat in their environment.  Clients MUST be prepared to do a
   cookie exchange with every handshake.

   Unlike TLS implementations, DTLS implementations SHOULD NOT respond
   to invalid records by terminating the connection.

   If implementations process out-of-epoch records as recommended in
   Section 8, then this creates a denial of service risk since an
   adversary could inject packets with fake epoch values, forcing the
   recipient to compute the next-generation application_traffic_secret
   using the HKDF-Expand-Label construct to only find out that the
   message was does not pass the AEAD cipher processing.  The impact of
   this attack is small since the HKDF-Expand-Label only performs
   symmetric key hashing operations.  Implementations which are
   concerned about this form of attack can discard out-of-epoch records.

11.  Changes to DTLS 1.2

   Since TLS 1.3 introduces a large number of changes to TLS 1.2, the
   list of changes from DTLS 1.2 to DTLS 1.3 is equally large.  For this
   reason this section focuses on the most important changes only.

   -  New handshake pattern, which leads to a shorter message exchange

   -  Support for AEAD-only ciphers

   -  HelloRetryRequest of TLS 1.3 used instead of HelloVerifyRequest

   -  More flexible ciphersuite negotiation

   -  New session resumption mechanism

   -  PSK authentication redefined

   -  New key derivation hierarchy utilizing a new key derivation

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   -  Removed support for weaker and older cryptographic algorithms

   -  Improved version negotation

12.  IANA Considerations

   IANA is requested to allocate a new value in the TLS ContentType
   Registry for the ACK message defined in Section 7, with content type

13.  References

13.1.  Normative References

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

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

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   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

13.2.  Informative References

              Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
              "The Datagram Transport Layer Security (DTLS) Connection
              Identifier", draft-ietf-tls-dtls-connection-id-00 (work in
              progress), December 2017.

              Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
              "The Datagram Transport Layer Security (DTLS) Connection
              Identifier", draft-rescorla-tls-dtls-connection-id-02
              (work in progress), November 2017.

   [RFC2522]  Karn, P. and W. Simpson, "Photuris: Session-Key Management
              Protocol", RFC 2522, DOI 10.17487/RFC2522, March 1999,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,

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   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
              the Datagram Congestion Control Protocol (DCCP)",
              RFC 5238, DOI 10.17487/RFC5238, May 2008,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <>.

   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,

13.3.  URIs




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Appendix A.  History


   IETF Drafts draft-03 - Only update keys after KeyUpdate is ACKed.

   draft-02 - Shorten the protected record header and introduce an
   ultra-short version of the record header.  - Reintroduce KeyUpdate,
   which works properly now that we have ACK.  - Clarify the ACK rules.

   draft-01 - Restructured the ACK to contain a list of packets and also
   be a record rather than a handshake message.

   draft-00 - First IETF Draft

   Personal Drafts draft-01 - Alignment with version -19 of the TLS 1.3


   -  Initial version using TLS 1.3 as a baseline.

   -  Use of epoch values instead of KeyUpdate message

   -  Use of cookie extension instead of cookie field in ClientHello and
      HelloVerifyRequest messages

   -  Added ACK message

   -  Text about sequence number handling

Appendix B.  Working Group Information

   The discussion list for the IETF TLS working group is located at the
   e-mail address [1].  Information on the group and
   information on how to subscribe to the list is at [2]

   Archives of the list can be found at:
   archive/web/tls/current/index.html [3]

Appendix C.  Contributors

   Many people have contributed to previous DTLS versions and they are
   acknowledged in prior versions of DTLS specifications.

   For this version of the document we would like to thank:

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   * Ilari Liusvaara

   * Martin Thomson

Authors' Addresses

   Eric Rescorla
   RTFM, Inc.


   Hannes Tschofenig
   Arm Limited


   Nagendra Modadugu
   Google, Inc.


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