TLS                                                          E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes: 6347 (if approved)                              H. Tschofenig
Intended status: Standards Track                             Arm Limited
Expires: May 6, 2021                                         N. Modadugu
                                                            Google, Inc.
                                                       November 02, 2020

   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 with the exception of order protection/non-replayability.
   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 May 6, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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   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
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   Without obtaining an adequate license from the person(s) controlling
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   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
     3.2.  Reordering  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Message Size  . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Replay Detection  . . . . . . . . . . . . . . . . . . . .   8
   4.  The DTLS Record Layer . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Determining the Header Format . . . . . . . . . . . . . .  12
     4.2.  Sequence Number and Epoch . . . . . . . . . . . . . . . .  12
       4.2.1.  Processing Guidelines . . . . . . . . . . . . . . . .  12
       4.2.2.  Reconstructing the Sequence Number and Epoch  . . . .  13
       4.2.3.  Sequence Number Encryption  . . . . . . . . . . . . .  14
     4.3.  Transport Layer Mapping . . . . . . . . . . . . . . . . .  15
     4.4.  PMTU Issues . . . . . . . . . . . . . . . . . . . . . . .  15
     4.5.  Record Payload Protection . . . . . . . . . . . . . . . .  17
       4.5.1.  Anti-Replay . . . . . . . . . . . . . . . . . . . . .  17
       4.5.2.  Handling Invalid Records  . . . . . . . . . . . . . .  18
       4.5.3.  AEAD Limits . . . . . . . . . . . . . . . . . . . . .  18
   5.  The DTLS Handshake Protocol . . . . . . . . . . . . . . . . .  20
     5.1.  Denial-of-Service Countermeasures . . . . . . . . . . . .  20
     5.2.  DTLS Handshake Message Format . . . . . . . . . . . . . .  23
     5.3.  ClientHello Message . . . . . . . . . . . . . . . . . . .  25
     5.4.  Handshake Message Fragmentation and Reassembly  . . . . .  26

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     5.5.  End Of Early Data . . . . . . . . . . . . . . . . . . . .  27
     5.6.  DTLS Handshake Flights  . . . . . . . . . . . . . . . . .  27
     5.7.  Timeout and Retransmission  . . . . . . . . . . . . . . .  31
       5.7.1.  State Machine . . . . . . . . . . . . . . . . . . . .  31
       5.7.2.  Timer Values  . . . . . . . . . . . . . . . . . . . .  33
       5.7.3.  State machine duplication for post-handshake messages  34
     5.8.  CertificateVerify and Finished Messages . . . . . . . . .  35
     5.9.  Cryptographic Label Prefix  . . . . . . . . . . . . . . .  35
     5.10. Alert Messages  . . . . . . . . . . . . . . . . . . . . .  35
     5.11. Establishing New Associations with Existing Parameters  .  35
   6.  Example of Handshake with Timeout and Retransmission  . . . .  36
     6.1.  Epoch Values and Rekeying . . . . . . . . . . . . . . . .  38
   7.  ACK Message . . . . . . . . . . . . . . . . . . . . . . . . .  40
     7.1.  Sending ACKs  . . . . . . . . . . . . . . . . . . . . . .  41
     7.2.  Receiving ACKs  . . . . . . . . . . . . . . . . . . . . .  42
     7.3.  Design Rational . . . . . . . . . . . . . . . . . . . . .  43
   8.  Key Updates . . . . . . . . . . . . . . . . . . . . . . . . .  43
   9.  Connection ID Updates . . . . . . . . . . . . . . . . . . . .  44
     9.1.  Connection ID Example . . . . . . . . . . . . . . . . . .  45
   10. Application Data Protocol . . . . . . . . . . . . . . . . . .  47
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  47
   12. Changes to DTLS 1.2 . . . . . . . . . . . . . . . . . . . . .  48
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     14.2.  Informative References . . . . . . . . . . . . . . . . .  50
   Appendix A.  Protocol Data Structures and Constant Values . . . .  53
   Appendix B.  Analysis of Limits on CCM Usage  . . . . . . . . . .  53
     B.1.  Confidentiality Limits  . . . . . . . . . . . . . . . . .  54
     B.2.  Integrity Limits  . . . . . . . . . . . . . . . . . . . .  54
     B.3.  Limits for AEAD_AES_128_CCM_8 . . . . . . . . . . . . . .  54
   Appendix C.  History  . . . . . . . . . . . . . . . . . . . . . .  55
   Appendix D.  Working Group Information  . . . . . . . . . . . . .  57
   Appendix E.  Contributors . . . . . . . . . . . . . . . . . . . .  57
   Appendix F.  Acknowledgements . . . . . . . . . . . . . . . . . .  58
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  58

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.

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   The primary goal of the TLS protocol is to establish an
   authenticated, confidentiality and integrity protected channel
   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 use UDP [RFC0768] as a transport and to
   offer communication security protection for those applications the
   Datagram Transport Layer Security (DTLS) protocol has been developed.
   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
   specification describes the most current version of the DTLS protocol
   based on TLS 1.3 [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 [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 [RFC7525].

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.

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   -  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.

   -  CID: Connection ID

   -  MSL: Maximum Segment Lifetime

   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.

   The reader is also as to be familiar with
   [I-D.ietf-tls-dtls-connection-id] as this document applies the CID
   functionality to DTLS 1.3.

   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

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   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 reordered data traffic.

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

   1.  TLS relies on an implicit sequence number on records.  If a
       record is not received, then the recipient will use the wrong
       sequence number when attempting to remove record protection from
       subsequent records.  DTLS solves this problem by adding 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.  DTLS handshake messages are also
       assigned sequence numbers to enable reassembly in the correct
       order in case datagrams are lost or reordered.

   3.  During the handshake, messages are implicitly acknowledged by
       other handshake messages.  Some handshake messages, such as the
       NewSessionTicket message, do not result in any direct response
       that would allow the sender to detect loss.  DTLS adds an
       acknowledgment message to enable better loss recovery.

   4.  Handshake messages are potentially larger than can be contained
       in a single datagram.  DTLS adds fields to handshake messages to
       support fragmentation and reassembly.

   5.  Datagram transport protocols, like UDP, are susceptible to
       abusive behavior effecting denial of service attacks against
       nonparticipants.  DTLS adds a return-routability check that uses
       the TLS HelloRetryRequest message (see Section 5.1 for details).

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:

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            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.

3.2.  Reordering

   In DTLS, each handshake message is assigned a specific sequence
   number.  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 received.

3.3.  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
   UDP datagram.  Each DTLS handshake message contains both a fragment
   offset and a fragment length.  Thus, a recipient in possession of all

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   bytes of a handshake message can reassemble the original unfragmented

3.4.  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 1.3 record layer is different from the TLS 1.3 record layer
   and also different from the DTLS 1.2 record layer.

   1.  The DTLSCiphertext structure omits the superfluous version number
       and type fields.

   2.  DTLS adds an epoch and sequence number to the TLS record header.
       This sequence number allows the recipient to correctly verify the
       DTLS MAC.  However, the number of bits used for the epoch and
       sequence number fields in the DTLSCiphertext structure have been
       reduced from those in previous versions.

   3.  The DTLSCiphertext structure has a variable length header.

   DTLSPlaintext records are used to send unprotected records and
   DTLSCiphertext records are used to send protected records.

   The DTLS record formats are shown below.  Unless explicitly stated
   the meaning of the fields is unchanged from previous TLS / DTLS

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

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

       struct {
           opaque unified_hdr[variable];
           opaque encrypted_record[length];
       } DTLSCiphertext;

                     Figure 2: DTLS 1.3 Record Format

   unified_hdr:  The unified_hdr is a field of variable length, as shown
      in Figure 3.

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

   The DTLSCiphertext header is tightly bit-packed, as shown below:

       0 1 2 3 4 5 6 7
       |0|0|1|C|S|L|E E|
       | Connection ID |   Legend:
       | (if any,      |
       /  length as    /   C   - Connection ID (CID) present
       |  negotiated)  |   S   - Sequence number length
       +-+-+-+-+-+-+-+-+   L   - Length present
       |  8 or 16 bit  |   E   - Epoch
       |Sequence Number|
       | 16 bit Length |
       | (if present)  |

                   Figure 3: DTLS 1.3 CipherText Header

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   Fixed Bits:  The three high bits of the first byte of the
      DTLSCiphertext header are set to 001.

   C: The C bit (0x10) is set if the Connection ID is present.

   S: The S bit (0x08) indicates the size of the sequence number.  0
      means an 8-bit sequence number, 1 means 16-bit.

   L: The L bit (0x04) is set if the length is present.

   E: The two low bits (0x03) include the low order two bits of the

   Connection ID:  Variable length CID.  The CID functionality is
      described in [I-D.ietf-tls-dtls-connection-id].  An example can be
      found in Section 9.1.

   Sequence Number:  The low order 8 or 16 bits of the record sequence
      number.  This value is 16 bits if the S bit is set to 1, and 8
      bits if the S bit is 0.

   Length:  Identical to the length 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.

   Figure 4 illustrates different record layer header types.

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    0 1 2 3 4 5 6 7       0 1 2 3 4 5 6 7       0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
   | Content Type  |     |0|0|1|1|1|1|E E|     |0|0|1|0|0|0|E E|
   +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
   |   16 bit      |     |               |     |8-bit Seq. No. |
   |   Version     |     / Connection ID /     +-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+     |               |     |               |
   |   16 bit      |     +-+-+-+-+-+-+-+-+     |   Encrypted   |
   |    Epoch      |     |    16 bit     |     /   Record      /
   +-+-+-+-+-+-+-+-+     |Sequence Number|     |               |
   |               |     +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
   |               |     |   16 bit      |
   |   48 bit      |     |   Length      |       DTLSCiphertext
   |Sequence Number|     +-+-+-+-+-+-+-+-+         Structure
   |               |     |               |         (minimal)
   |               |     |  Encrypted    |
   +-+-+-+-+-+-+-+-+     /  Record       /
   |    16 bit     |     |               |
   |    Length     |     +-+-+-+-+-+-+-+-+
   |               |      DTLSCiphertext
   |               |        Structure
   /   Fragment    /          (full)
   |               |


                         Figure 4: Header Examples

   The length field MAY be omitted by clearing the L bit, which means
   that the record consumes the entire rest of the datagram in the lower
   level transport.  In this case it is not possible to have multiple
   DTLSCiphertext format records without length fields in the same
   datagram.  Omitting the length field MUST only be used for the last
   record in a datagram.

   If a connection ID is negotiated, then it MUST be contained in all
   datagrams.  Sending implementations MUST NOT mix records from
   multiple DTLS associations in the same datagram.  If the second or
   later record has a connection ID which does not correspond to the
   same association used for previous records, the rest of the datagram
   MUST be discarded.

   When expanded, the epoch and sequence number can be combined into an
   unpacked RecordNumber structure, as shown below:

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       struct {
           uint16 epoch;
           uint48 sequence_number;
       } RecordNumber;

   This 64-bit value is used in the ACK message as well as in the
   "record_sequence_number" input to the AEAD function.

   The entire header value shown in Figure 4 (but prior to record number
   encryption) is used as as the additional data value for the AEAD
   function.  For instance, if the minimal variant is used, the AAD is 2
   octets long.  Note that this design is different from the additional
   data calculation for DTLS 1.2 and for DTLS 1.2 with Connection ID.

4.1.  Determining the Header Format

   Implementations can distinguish the two header formats by examining
   the first byte:

   -  If the first byte is alert(21), handshake(22), or ack(proposed,
      26), the record MUST be interpreted as a DTLSPlaintext record.

   -  If the first byte is any other value, then receivers MUST check to
      see if the leading bits of the first byte are 001.  If so, the
      implementation MUST process the record as DTLSCiphertext; the true
      content type will be inside the protected portion.

   -  Otherwise, the record MUST be rejected as if it had failed
      deprotection, as described in Section 4.5.2.

4.2.  Sequence Number and Epoch

   DTLS uses an explicit or partly 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.

   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.

4.2.1.  Processing Guidelines

   Because DTLS records could be reordered, a record from epoch M may be
   received after epoch N (where N > M) has begun.  In general,
   implementations SHOULD discard records 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

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   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

   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
   records, 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 records
   may be sent still apply, and the receiver treats the records as if
   they were sent in the right order.

   Implementations MUST send retransmissions of lost messages using the
   same epoch and keying material as the original transmission.

   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.

4.2.2.  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.

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4.2.3.  Sequence Number Encryption

   In DTLS 1.3, when records are encrypted, record sequence numbers are
   also encrypted.  The basic pattern is that the underlying encryption
   algorithm used with the AEAD algorithm is used to generate a mask
   which is then XORed with the sequence number.

   When the AEAD is based on AES, then the Mask is generated by
   computing AES-ECB on the first 16 bytes of the ciphertext:

     Mask = AES-ECB(sn_key, Ciphertext[0..15])

   When the AEAD is based on ChaCha20, then the mask is generated by
   treating the first 4 bytes of the ciphertext as the block counter and
   the next 12 bytes as the nonce, passing them to the ChaCha20 block
   function (Section 2.3 of [CHACHA]):

     Mask = ChaCha20(sn_key, Ciphertext[0..3], Ciphertext[4..15])

   The sn_key is computed as follows:

     [sender]_sn_key  = HKDF-Expand-Label(Secret, "sn" , "", key_length)

   [sender] denotes the sending side.  The Secret value to be used is
   described in Section 7.3 of [TLS13].

   The encrypted sequence number is computed by XORing the leading bytes
   of the Mask with the sequence number.  Decryption is accomplished by
   the same process.

   This procedure requires the ciphertext length be at least 16 bytes.
   Receivers MUST reject shorter records as if they had failed
   deprotection, as described in Section 4.5.2.  Senders MUST pad short
   plaintexts out (using the conventional record padding mechanism) in
   order to make a suitable-length ciphertext.  Note most of the DTLS
   AEAD algorithms have a 16-byte authentication tag and need no
   padding.  However, some algorithms such as TLS_AES_128_CCM_8_SHA256
   have a shorter authentication tag and may require padding for short

   Note that sequence number encryption is only applied to the
   DTLSCiphertext structure and not to the DTLSPlaintext structure,
   which also contains a sequence number.

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4.3.  Transport Layer Mapping

   DTLS messages MAY be fragmented 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.  Records
   are encoded consecutively.  The length field from DTLS records
   containing that field can be used to determine the boundaries between
   records.  The final record in a datagram can omit the length field.
   The first byte of the datagram payload MUST be the beginning of a
   record.  Records MUST NOT span datagrams.

   DTLS records without CIDs 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.

   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

4.4.  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

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      [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.

   -  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.

   Note that DTLS does not defend against spoofed ICMP messages;
   implementations SHOULD ignore any such messages that indicate PMTUs
   below the IPv4 and IPv6 minimums of 576 and 1280 bytes respectively

   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.

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   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

4.5.  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.

4.5.1.  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 record 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 check SHOULD happen after
   deprotecting the record; otherwise the record discard might itself
   serve as a timing channel for the record number.  Note that
   decompressing the records number is still a potential timing channel
   for the record number, though a less powerful one than whether it was

   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.)  The receiver SHOULD pick a window large enough to handle
   any plausible reordering, which depends on the data rate.  (The
   receiver does not notify the sender of the window size.)

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   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.  Records falling within the window are checked against a
   list of received records 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 record is to the right of the
   window, then the record is new.

   The window MUST NOT be updated until the record has been deprotected

4.5.2.  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;
   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.

4.5.3.  AEAD Limits

   Section 5.5 of TLS [TLS13] defines limits on the number of records
   that can be protected using the same keys.  These limits are specific
   to an AEAD algorithm, and apply equally to DTLS.  Implementations
   SHOULD NOT protect more records than allowed by the limit specified
   for the negotiated AEAD.  Implementations SHOULD initiate a key
   update before reaching this limit.

   [TLS13] does not specify a limit for AEAD_AES_128_CCM, but the
   analysis in Appendix B shows that a limit of 2^23 packets can be used
   to obtain the same confidentiality protection as the limits specified
   in TLS.

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   The usage limits defined in TLS 1.3 exist for protection against
   attacks on confidentiality and apply to successful applications of
   AEAD protection.  The integrity protections in authenticated
   encryption also depend on limiting the number of attempts to forge
   packets.  TLS achieves this by closing connections after any record
   fails an authentication check.  In comparison, DTLS ignores any
   packet that cannot be authenticated, allowing multiple forgery

   Implementations MUST count the number of received packets that fail
   authentication with each key.  If the number of packets that fail
   authentication exceed a limit that is specific to the AEAD in use, an
   implementation SHOULD immediately close the connection.
   Implementations SHOULD initiate a key update with update_requested
   before reaching this limit.  Once a key update has been initiated,
   the previous keys can be dropped when the limit is reached rather
   than closing the connection.  Applying a limit reduces the
   probability that an attacker is able to successfully forge a packet;
   see [AEBounds] and [ROBUST].

   For AEAD_AES_128_GCM, AEAD_AES_256_GCM, and AEAD_CHACHA20_POLY1305,
   the limit on the number of records that fail authentication is 2^36.
   Note that the analysis in [AEBounds] supports a higher limit for the
   AEAD_AES_128_GCM and AEAD_AES_256_GCM, but this specification
   recommends a lower limit.  For AEAD_AES_128_CCM, the limit on the
   number of records that fail authentication is 2^23.5; see Appendix B.

   The AEAD_AES_128_CCM_8 AEAD, as used in TLS_AES_128_CCM_SHA256, does
   not have a limit on the number of records that fail authentication
   that both limits the probability of forgery by the same amount and
   does not expose implementations to the risk of denial of service; see
   Appendix B.3.  Therefore, TLS_AES_128_CCM_SHA256 MUST NOT used in
   DTLS without additional safeguards against forgery.  Implementations
   MUST set usage limits for AEAD_AES_128_CCM_8 based on an
   understanding of any additional forgery protections that are used.

   Any TLS cipher suite that is specified for use with DTLS MUST define
   limits on the use of the associated AEAD function that preserves
   margins for both confidentiality and integrity.  That is, limits MUST
   be specified for the number of packets that can be authenticated and
   for the number packets that can fail authentication.  Providing a
   reference to any analysis upon which values are based - and any
   assumptions used in that analysis - allows limits to be adapted to
   varying usage conditions.

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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.

   DTLS implementations do not use the TLS 1.3 "compatibility mode"
   described in Section D.4 of [TLS13].  DTLS servers MUST NOT echo the
   "session_id" value from the client and endpoints MUST NOT send
   ChangeCipherSpec messages.

   With these exceptions, the DTLS message formats, flows, and logic are
   the same as those of TLS 1.3.

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 parameters
       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

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   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 defense against DoS
   attacks mounted from valid IP addresses.

   The DTLS 1.3 specification changes 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 reasons, 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 5.  Note that the figure focuses on
   the cookie exchange; all other extensions are omitted.

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

                               <----- HelloRetryRequest
                                       + cookie

         ClientHello           ------>
          + cookie

         [Rest of handshake]

       Figure 5: DTLS exchange with HelloRetryRequest containing the
                            "cookie" extension

   The cookie extension is defined in Section 4.2.2 of [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 MUST be set to a zero
   length vector (i.e., a single zero byte length field).

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   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 [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
   defend against this attack by changing the secret value frequently,
   thus invalidating those cookies.  If the server wishes to allow
   legitimate clients to handshake through the transition (e.g., a
   client 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 cookies that were generated outside a certain interval of

   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 or, more
   generically, when the DTLS handshake uses a PSK-based key exchange.

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   Clients MUST be prepared to do a cookie exchange with every

   If a server receives a ClientHello with an invalid cookie, it MUST
   NOT terminate the handshake with an "illegal_parameter" alert.  This
   allows the client to restart the connection from scratch without a

   As described in Section 4.1.4 of [TLS13], clients MUST 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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   Note: In DTLS 1.2 the message_seq was reset to zero in case of a
   rehandshake (i.e., renegotiation).  On the surface, a rehandshake in
   DTLS 1.2 shares similarities with a post-handshake message exchange
   in DTLS 1.3.  However, in DTLS 1.3 the message_seq is not reset to
   allow distinguishing a retransmission from a previously sent post-
   handshake message from a newly sent post-handshake message.

   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

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      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 [TLS13]) and
      the legacy_version field MUST be set to {254, 253}, which was the
      version number for DTLS 1.2.  The version fields for DTLS 1.0 and
      DTLS 1.2 are 0xfeff and 0xfefd (to match the wire versions) but
      the version field for DTLS 1.3 is 0x0304.

   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.  If a DTLS 1.3 ClientHello is received with
      any other value in this field, the server MUST abort the handshake
      with an "illegal_parameter" alert.

   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.  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.
   Each range MUST be delivered in a single UDP datagram.

   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.

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   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.

5.5.  End Of Early Data

   The DTLS 1.3 handshake has one important difference from the TLS 1.3
   handshake: the EndOfEarlyData message is omitted both from the wire
   and the handshake transcript: because DTLS records have epochs,
   EndOfEarlyData is not necessary to determine when the early data is
   complete, and because DTLS is lossy, attackers can trivially mount
   the deletion attacks that EndOfEarlyData prevents in TLS.  Servers
   SHOULD aggressively age out the epoch 1 keys upon receiving the first
   epoch 2 record and SHOULD NOT accept epoch 1 data after the first
   epoch 3 record is received.  (See Section 6.1 for the definitions of
   each epoch.)

5.6.  DTLS Handshake Flights

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

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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 6: 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 7: 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*]

   {Finished}            -------->                          | Flight 3 |
   [Application Data*]                                      +----------+

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

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

           Figure 8: Message flights for the Zero-RTT handshake

  Client                                            Server

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

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

       Figure 9: 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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5.7.  Timeout and Retransmission

5.7.1.  State Machine

   DTLS uses a simple timeout and retransmission scheme with the state
   machine shown in Figure 10.  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 10: 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 sets a retransmit
   timer and 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 an 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
       a duplicate message is the likely result of timer expiry on the

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       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 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

   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 records 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.

5.7.2.  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 recommended 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

   Implementations SHOULD retain the current timer value until a
   transmission without loss occurs, at which time the value may be

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   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.

5.7.3.  State machine duplication for post-handshake messages

   DTLS 1.3 makes use of the following categories of post-handshake

   1.  NewSessionTicket

   2.  KeyUpdate

   3.  NewConnectionId

   4.  RequestConnectionId

   5.  Post-handshake client authentication

   Messages of each category can be sent independently, and reliability
   is established via independent state machines each of which behaves
   as described in Section 5.7.1.  For example, if a server sends a
   NewSessionTicket and a CertificateRequest message, two independent
   state machines will be created.

   As explained in the corresponding sections, sending multiple
   instances of messages of a given category without having completed
   earlier transmissions is allowed for some categories, but not for
   others.  Specifically, a server MAY send multiple NewSessionTicket
   messages at once without awaiting ACKs for earlier NewSessionTicket
   first.  Likewise, a server MAY send multiple CertificateRequest
   messages at once without having completed earlier client
   authentication requests before.  In contrast, implementations MUST
   NOT have send KeyUpdate, NewConnectionId or RequestConnectionId
   message if an earlier message of the same type has not yet been

   Note: Except for post-handshake client authentication, which involves
   handshake messages in both directions, post-handshake messages are
   single-flight, and their respective state machines on the sender side
   reduce to waiting for an ACK and retransmitting the original message.
   In particular, note that a RequestConnectionId message does not force
   the receiver to send a NewConnectionId message in reply, and both
   messages are therefore treated independently.

   Creating and correctly updating multiple state machines requires
   feedback from the handshake logic to the state machine layer,
   indicating which message belongs to which state machine.  For

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   example, if a server sends multiple CertificateRequest messages and
   receives a Certificate message in response, the corresponding state
   machine can only be determined after inspecting the
   certificate_request_context field.  Similarly, a server sending a
   single CertificateRequest and receiving a NewConnectionId message in
   response can only decide that the NewConnectionId message should be
   treated through an independent state machine after inspecting the
   handshake message type.

5.8.  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 [TLS13], respectively.

5.9.  Cryptographic Label Prefix

   Section 7.1 of [TLS13] specifies that HKDF-Expand-Label uses a label
   prefix of "tls13 ".  For DTLS 1.3, that label SHALL be "dtls13".
   This ensures key separation between DTLS 1.3 and TLS 1.3.  Note that
   there is no trailing space; this is necessary in order to keep the
   overall label size inside of one hash iteration because "DTLS" is one
   letter longer than "TLS".

5.10.  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.

5.11.  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

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   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

   Note: it is not always possible to distinguish which association a
   given record 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 use CIDs.

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 []

                              <--------                 Record 2

    Record 2                  -------->

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

        Figure 11: 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-ordering, 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 TLS 1.3-defined key derivation steps, see Section 7 of [TLS13], a
   sender may want to rekey at any time during the lifetime of the
   connection.  It therefore needs 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.  Note this epoch is skipped if
      the client does not offer early data.

   -  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 [sender]_application_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 [sender]_application_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, if a client incorrectly
   uses epoch value 5 when sending early application data in a 0-RTT
   exchange.  A server will not be able to compute the appropriate keys
   and will therefore have to respond with an alert.

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   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 [TLS13].

   Figure 12 illustrates the epoch values in an example DTLS handshake.

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


                               <--------             ServerHello

    ClientHello                -------->

                               <--------             ServerHello

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

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

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

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

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

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

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

                            Some time later ...

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

          Figure 12: Example DTLS exchange with epoch information

7.  ACK Message

   The ACK message is used by an endpoint to indicate which handshake
   records it has received and processed 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 having ACK being added to
   the handshake transcript.  Note that ACKs can still be sent in the
   same UDP datagram as handshake records.

       struct {
           RecordNumber 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 and either
      processed or buffered, in numerically increasing order.

   Implementations MUST NOT acknowledge records containing handshake
   messages or fragments which have not been processed or buffered.
   Otherwise, deadlock can ensue.  As an example, implementations MUST
   NOT send ACKs for handshake messages which they discard because they
   are not the next expected message.

   During the handshake, 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.

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   After the handshake, ACKs SHOULD be sent once for each received and
   processed handshake record (potentially subject to some delay) and
   MAY cover more than one flight.  This includes messages which are
   discarded because a previous copy has been received.

   During the handshake, ACK records MUST be sent with an epoch that is
   equal to or higher than the record which is being acknowledged.  Note
   that some care is required when processing flights spanning multiple
   epochs.  For instance, if the client receives only the Server Hello
   and Certificate and wishes to ACK them in a single record, it must do
   so in epoch 2, as it is required to use an epoch greater than or
   equal to 2 and cannot yet send with any greater epoch.
   Implementations SHOULD simply use the highest current sending epoch,
   which will generally be the highest available.  After the handshake,
   implementations MUST use the highest available sending epoch.

7.1.  Sending ACKs

   When an implementation detects a disruption in the receipt of the
   current incoming flight, it SHOULD generate an ACK that covers the
   messages from that flight which it has received and processed so far.
   Implementations have some discretion about which events to treat as
   signs of disruption, but it is RECOMMENDED that they generate ACKs
   under two circumstances:

   -  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.

   -  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 general, flights MUST be ACKed unless they are implicitly
   acknowledged.  In the present specification the following flights are
   implicitly acknowledged by the receipt of the next flight, which
   generally immediately follows the flight,

   1.  Handshake flights other than the client's final flight

   2.  The server's post-handshake CertificateRequest.

   ACKs SHOULD NOT be sent for these flights unless generating the
   responding flight takes significant time.  In this case,
   implementations MAY send explicit ACKs for the complete received
   flight even though it will eventually also be implicitly acknowledged

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   through the responding flight.  A notable example for this is the
   case of post-handshake client authentication in constrained
   environments, where generating the CertificateVerify message can take
   considerable time on the client.  All other flights MUST be ACKed.
   Implementations MAY acknowledge the records corresponding to each
   transmission of each flight or simply acknowledge the most recent
   one.  In general, implementations SHOULD ACK as many received packets
   as can fit into the ACK record, as this provides the most complete
   information and thus reduces the chance of spurious retransmission;
   if space is limited, implementations SHOULD favor including records
   which have not yet been acknowledged.

   Note: While some post-handshake messages follow a request/response
   pattern, this does not necessarily imply receipt.  For example, a
   KeyUpdate sent in response to a KeyUpdate with update_requested does
   not implicitly acknowledge that message because the KeyUpdates might
   have crossed in flight.

   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
   acknowledge 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 process.

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 that leaves it with only some messages from a flight having
   been acknowledged an implementation SHOULD retransmit the
   unacknowledged 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.
   Implementations MUST treat a record as having been acknowledged if it
   appears in any ACK; this prevents spurious retransmission in cases
   where a flight is very large and the receiver is forced to elide
   acknowledgements for records which have already been ACKed.  As noted
   above, the receipt of any record responding to a given flight MUST be
   taken as an implicit acknowledgement for the entire flight.

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7.3.  Design Rational

   ACK messages are used in two circumstances, namely :

   -  on sign of disruption, or lack of progress, and

   -  to indicate complete receipt of the last flight in a handshake.

   In the first case the use of the ACK message is optional because the
   peer will retransmit in any case and therefore the ACK just allows
   for selective retransmission, as opposed to the whole flight
   retransmission in previous versions of DTLS.  For instance in the
   flow shown in Figure 11 if the client does not send the ACK message
   when it received and processed record 1 indicating loss of record 0,
   the entire flight would be retransmitted.  When DTLS 1.3 is used in
   deployments with loss networks, such as low-power, long range radio
   networks as well as low-power mesh networks, the use of ACKs is

   The use of the ACK for the second case is mandatory for the proper
   functioning of the protocol.  For instance, the ACK message sent by
   the client in Figure 12, acknowledges receipt and processing of
   record 2 (containing the NewSessionTicket message) and if it is not
   sent the server will continue retransmission of the NewSessionTicket

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 4.2.2 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 4.2.2 for
   information on determining the correct epoch), but MAY opt to discard
   such out-of-epoch records.

   Due to the possibility of an ACK message for a KeyUpdate being lost
   and thereby preventing the sender of the KeyUpdate from updating its
   keying material, receivers MUST retain the pre-update keying material
   until receipt and successful decryption of a message using the new

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9.  Connection ID Updates

   If the client and server have negotiated the "connection_id"
   extension [I-D.ietf-tls-dtls-connection-id], either side can send a
   new CID which it wishes the other side to use in a NewConnectionId

       enum {
           cid_immediate(0), cid_spare(1), (255)
       } ConnectionIdUsage;

       opaque ConnectionId<0..2^8-1>;

       struct {
           ConnectionIds cids<0..2^16-1>;
           ConnectionIdUsage usage;
       } NewConnectionId;

   cid  Indicates the set of CIDs which the sender wishes the peer to

   usage  Indicates whether the new CIDs should be used immediately or
      are spare.  If usage is set to "cid_immediate", then one of the
      new CID MUST be used immediately for all future records.  If it is
      set to "cid_spare", then either existing or new CID MAY be used.

   Endpoints SHOULD use receiver-provided CIDs in the order they were
   provided.  Endpoints MUST NOT have more than one NewConnectionId
   message outstanding.

   If the client and server have negotiated the "connection_id"
   extension, either side can request a new CID using the
   RequestConnectionId message.

       struct {
         uint8 num_cids;
       } RequestConnectionId;

   num_cids  The number of CIDs desired.

   Endpoints SHOULD respond to RequestConnectionId by sending a
   NewConnectionId with usage "cid_spare" containing num_cid CIDs soon
   as possible.  Endpoints MUST NOT send a RequestConnectionId message
   when an existing request is still unfulfilled; this implies that
   endpoints needs to request new CIDs well in advance.  An endpoint MAY
   ignore requests, which it considers excessive (though they MUST be
   acknowledged as usual).

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   Endpoints MUST NOT send either of these messages if they did not
   negotiate a CID.  If an implementation receives these messages when
   CIDs were not negotiated, it MUST abort the connection with an
   unexpected_message alert.

9.1.  Connection ID Example

   Below is an example exchange for DTLS 1.3 using a single CID in each

   Note: The connection_id extension is defined in
   [I-D.ietf-tls-dtls-connection-id], which is used in ClientHello and
   ServerHello messages.

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


                               <--------       HelloRetryRequest

   ClientHello                 -------->

                               <--------             ServerHello

   Certificate                -------->
                              <--------                      Ack

   Application Data           ========>
                              <========         Application Data

              Figure 13: Example DTLS 1.3 Exchange with CIDs

   If no CID is negotiated, then the receiver MUST reject any records it
   receives that contain a CID.

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10.  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.

11.  Security Considerations

   Security issues are discussed primarily in [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.

   DTLS implementations MUST NOT update their sending address in
   response to packets from a different address unless they first
   perform some reachability test; no such test is defined in this
   specification.  Even with such a test, an on-path adversary can also
   black-hole traffic or create a reflection attack against third
   parties because a DTLS peer has no means to distinguish a genuine
   address update event (for example, due to a NAT rebinding) from one
   that is malicious.  This attack is of concern when there is a large
   asymmetry of request/response message sizes.

   With the exception of order protection and non-replayability, the
   security guarantees for DTLS 1.3 are the same as TLS 1.3.  While TLS
   always provides order protection and non-replayability, DTLS does not
   provide order protection and may not provide replay protection.

   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 records 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.

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   The security and privacy properties of the CID for DTLS 1.3 builds on
   top of what is described in [I-D.ietf-tls-dtls-connection-id].  There
   are, however, several improvements:

   -  The use of the Post-Handshake message allows the client and the
      server to update their CIDs and those values are exchanged with
      confidentiality protection.

   -  With multi-homing, an adversary is able to correlate the
      communication interaction over the two paths, which adds further
      privacy concerns.  In order to prevent this, implementations
      SHOULD attempt to use fresh CIDs whenever they change local
      addresses or ports (though this is not always possible to detect).
      The RequestConnectionId message can be used by a peer to ask for
      new CIDs to ensure that a pool of suitable CIDs is available.

   -  Switching CID based on certain events, or even regularly, helps
      against tracking by on-path adversaries but the sequence numbers
      can still allow linkability.  For this reason this specification
      defines an algorithm for encrypting sequence numbers, see
      Section 4.2.3.  Note that sequence number encryption is used for
      all encrypted DTLS 1.3 records irrespective of whether a CID is
      used or not.  Unlike the sequence number, the epoch is not
      encrypted.  This may improve correlation of packets from a single
      connection across different network paths.

   -  DTLS 1.3 encrypts handshake messages much earlier than in previous
      DTLS versions.  Therefore, less information identifying the DTLS
      client, such as the client certificate, is available to an on-path

12.  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

   -  Only AEAD ciphers are supported.  Additional data calculation has
      been simplified.

   -  Removed support for weaker and older cryptographic algorithms

   -  HelloRetryRequest of TLS 1.3 used instead of HelloVerifyRequest

   -  More flexible ciphersuite negotiation

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   -  New session resumption mechanism

   -  PSK authentication redefined

   -  New key derivation hierarchy utilizing a new key derivation

   -  Improved version negotiation

   -  Optimized record layer encoding and thereby its size

   -  Added CID functionality

   -  Sequence numbers are encrypted.

13.  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
   26.  The value for the "DTLS-OK" column is "Y".  IANA is requested to
   reserve the content type range 32-63 so that content types in this
   range are not allocated.

   IANA is requested to allocate two values in the "TLS Handshake Type"
   registry, defined in [TLS13], for RequestConnectionId (TBD), and
   NewConnectionId (TBD), as defined in this document.  The value for
   the "DTLS-OK" columns are "Y".

14.  References

14.1.  Normative References

   [CHACHA]   Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,

              Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
              Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
              id-07 (work in progress), October 2019.

   [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,

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   [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,

   [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, <>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

14.2.  Informative References

              Luykx, A. and K. Paterson, "Limits on Authenticated
              Encryption Use in TLS", March 2016,

              Jonsson, J., "On the Security of CTR + CBC-MAC", Selected
              Areas in Cryptography pp. 76-93,
              DOI 10.1007/3-540-36492-7_7, 2003.

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

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   [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,

   [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, <>.

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   [ROBUST]   Fischlin, M., Guenther, F., and C. Janson, "Robust
              Channels: Handling Unreliable Networks in the Record
              Layers of QUIC and DTLS 1.3", June 2020,

14.3.  URIs




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Appendix A.  Protocol Data Structures and Constant Values

   This section provides the normative protocol types and constants

   %%## Record Layer %%## Handshake Protocol %%## ACKs %%## Connection
   ID Management

Appendix B.  Analysis of Limits on CCM Usage

   TLS [TLS13] and [AEBounds] do not specify limits on key usage for
   AEAD_AES_128_CCM.  However, any AEAD that is used with DTLS requires
   limits on use that ensure that both confidentiality and integrity are
   preserved.  This section documents that analysis for

   [CCM-ANALYSIS] is used as the basis of this analysis.  The results of
   that analysis are used to derive usage limits that are based on those
   chosen in [TLS13].

   This analysis uses symbols for multiplication (*), division (/), and
   exponentiation (^), plus parentheses for establishing precedence.
   The following symbols are also used:

   t: The size of the authentication tag in bits.  For this cipher, t is

   n: The size of the block function in bits.  For this cipher, n is

   l: The number of blocks in each packet (see below).

   q: The number of genuine packets created and protected by endpoints.
      This value is the bound on the number of packets that can be
      protected before updating keys.

   v: The number of forged packets that endpoints will accept.  This
      value is the bound on the number of forged packets that an
      endpoint can reject before updating keys.

   The analysis of AEAD_AES_128_CCM relies on a count of the number of
   block operations involved in producing each message.  For simplicity,
   and to match the analysis of other AEAD functions in [AEBounds], this
   analysis assumes a packet length of 2^10 blocks and a packet size
   limit of 2^14.

   For AEAD_AES_128_CCM, the total number of block cipher operations is
   the sum of: the length of the associated data in blocks, the length

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   of the ciphertext in blocks, the length of the plaintext in blocks,
   plus 1.  In this analysis, this is simplified to a value of twice the
   maximum length of a record in blocks (that is, "2l = 2^11").  This
   simplification is based on the associated data being limited to one

B.1.  Confidentiality Limits

   For confidentiality, Theorem 2 in [CCM-ANALYSIS] establishes that an
   attacker gains a distinguishing advantage over an ideal pseudorandom
   permutation (PRP) of no more than:

   (2l * q)^2 / 2^n

   For a target advantage of 2^-60, which matches that used by [TLS13],
   this results in the relation:

   q <= 2^23

   That is, endpoints cannot protect more than 2^23 packets with the
   same set of keys without causing an attacker to gain an larger
   advantage than the target of 2^-60.

B.2.  Integrity Limits

   For integrity, Theorem 1 in [CCM-ANALYSIS] establishes that an
   attacker gains an advantage over an ideal PRP of no more than:

   v / 2^t + (2l * (v + q))^2 / 2^n

   The goal is to limit this advantage to 2^-57, to match the target in
   [TLS13].  As "t" and "n" are both 128, the first term is negligible
   relative to the second, so that term can be removed without a
   significant effect on the result.  This produces the relation:

   v + q <= 2^24.5

   Using the previously-established value of 2^23 for "q" and rounding,
   this leads to an upper limit on "v" of 2^23.5.  That is, endpoints
   cannot attempt to authenticate more than 2^23.5 packets with the same
   set of keys without causing an attacker to gain an larger advantage
   than the target of 2^-57.

B.3.  Limits for AEAD_AES_128_CCM_8

   The TLS_AES_128_CCM_8_SHA256 cipher suite uses the AEAD_AES_128_CCM_8
   function, which uses a short authentication tag (that is, t=64).

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   The confidentiality limits of AEAD_AES_128_CCM_8 are the same as
   those for AEAD_AES_128_CCM, as this does not depend on the tag
   length; see Appendix B.1.

   The shorter tag length of 64 bits means that the simplification used
   in Appendix B.2 does not apply to AEAD_AES_128_CCM_8.  If the goal is
   to preserve the same margins as other cipher suites, then the limit
   on forgeries is largely dictated by the first term of the advantage

   v <= 2^7

   As this represents attempts to fail authentication, applying this
   limit might be feasible in some environments.  However, applying this
   limit in an implementation intended for general use exposes
   connections to an inexpensive denial of service attack.

   This analysis supports the view that TLS_AES_128_CCM_8_SHA256 is not
   suitable for general use.  Specifically, TLS_AES_128_CCM_8_SHA256
   cannot be used without additional measures to prevent forgery of
   records, or to mitigate the effect of forgeries.  This might require
   understanding the constraints that exist in a particular deployment
   or application.  For instance, it might be possible to set a
   different target for the advantage an attacker gains based on an
   understanding of the constraints imposed on a specific usage of DTLS.

Appendix C.  History


   IETF Drafts

   draft-39 - Updated Figure 4 due to misalignment with Figure 3 content

   draft-38 - Ban implicit connection IDs (*) - ACKs are processed as
   the union.

   draft-37: - Fix the other place where we have ACK.

   draft-36: - Some editorial changes.  - Changed the content type to
   not conflict with existing allocations (*)

   draft-35: - I-D.ietf-tls-dtls-connection-id became a normative
   reference - Removed duplicate reference to I-D.ietf-tls-dtls-
   connection-id.  - Fix figure 11 to have the right numbers andno
   cookie in message 1.  - Clarify when you can ACK.  - Clarify
   additional data computation.

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   draft-33: - Key separation between TLS and DTLS.  Issue #72.

   draft-32: - Editorial improvements and clarifications.

   draft-31: - Editorial improvements in text and figures.  - Added
   normative reference to ChaCha20 and Poly1305.

   draft-30: - Changed record format - Added text about end of early
   data - Changed format of the Connection ID Update message - Added
   Appendix A "Protocol Data Structures and Constant Values"

   draft-29: - Added support for sequence number encryption - Update to
   new record format - Emphasize that compatibility mode isn't used.

   draft-28: - Version bump to align with TLS 1.3 pre-RFC version.

   draft-27: - Incorporated unified header format.  - Added support for

   draft-04 - 26: - Submissions to align with TLS 1.3 draft versions

   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 records 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

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Appendix D.  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 E.  Contributors

   Many people have contributed to previous DTLS versions and they are
   acknowledged in prior versions of DTLS specifications or in the
   referenced specifications.  The sequence number encryption concept is
   taken from the QUIC specification.  We would like to thank the
   authors of the QUIC specification for their work.  Felix Guenther and
   Martin Thomson contributed the analysis in Appendix B.

   In addition, we would like to thank:

   * David Benjamin

   * Thomas Fossati
     Arm Limited

   * Tobias Gondrom

   * Felix Guenther
     ETH Zurich

   * Ilari Liusvaara

   * Martin Thomson

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   * Christopher A. Wood
     Apple Inc.

   * Yin Xinxing

   * Hanno Becker
     Arm Limited

Appendix F.  Acknowledgements

   We would like to thank Jonathan Hammell for his review comments.

Authors' Addresses

   Eric Rescorla
   RTFM, Inc.


   Hannes Tschofenig
   Arm Limited


   Nagendra Modadugu
   Google, Inc.


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