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The Datagram Transport Layer Security (DTLS) Protocol Version 1.3
RFC 9147

Document Type RFC - Proposed Standard (April 2022)
Obsoletes RFC 6347
Authors Eric Rescorla , Hannes Tschofenig , Nagendra Modadugu
Last updated 2022-04-21
Replaces draft-rescorla-tls-dtls13
Stream Internet Engineering Task Force (IETF)
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OPSDIR Last Call Review Incomplete, due 2021-02-22
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RFC 9147


Internet Engineering Task Force (IETF)                       E. Rescorla
Request for Comments: 9147                                       Mozilla
Obsoletes: 6347                                            H. Tschofenig
Category: Standards Track                                    Arm Limited
ISSN: 2070-1721                                              N. Modadugu
                                                            Google, Inc.
                                                              April 2022

   The Datagram Transport Layer Security (DTLS) Protocol Version 1.3

Abstract

   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 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.

   This document obsoletes RFC 6347.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9147.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

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

Table of Contents

   1.  Introduction
   2.  Conventions and Terminology
   3.  DTLS Design Rationale and Overview
     3.1.  Packet Loss
     3.2.  Reordering
     3.3.  Fragmentation
     3.4.  Replay Detection
   4.  The DTLS Record Layer
     4.1.  Demultiplexing DTLS Records
     4.2.  Sequence Number and Epoch
       4.2.1.  Processing Guidelines
       4.2.2.  Reconstructing the Sequence Number and Epoch
       4.2.3.  Record Number Encryption
     4.3.  Transport Layer Mapping
     4.4.  PMTU Issues
     4.5.  Record Payload Protection
       4.5.1.  Anti-Replay
       4.5.2.  Handling Invalid Records
       4.5.3.  AEAD Limits
   5.  The DTLS Handshake Protocol
     5.1.  Denial-of-Service Countermeasures
     5.2.  DTLS Handshake Message Format
     5.3.  ClientHello Message
     5.4.  ServerHello Message
     5.5.  Handshake Message Fragmentation and Reassembly
     5.6.  EndOfEarlyData Message
     5.7.  DTLS Handshake Flights
     5.8.  Timeout and Retransmission
       5.8.1.  State Machine
       5.8.2.  Timer Values
       5.8.3.  Large Flight Sizes
       5.8.4.  State Machine Duplication for Post-Handshake Messages
     5.9.  Cryptographic Label Prefix
     5.10. Alert Messages
     5.11. Establishing New Associations with Existing Parameters
   6.  Example of Handshake with Timeout and Retransmission
     6.1.  Epoch Values and Rekeying
   7.  ACK Message
     7.1.  Sending ACKs
     7.2.  Receiving ACKs
     7.3.  Design Rationale
   8.  Key Updates
   9.  Connection ID Updates
     9.1.  Connection ID Example
   10. Application Data Protocol
   11. Security Considerations
   12. Changes since DTLS 1.2
   13. Updates Affecting DTLS 1.2
   14. IANA Considerations
   15. References
     15.1.  Normative References
     15.2.  Informative References
   Appendix A.  Protocol Data Structures and Constant Values
     A.1.  Record Layer
     A.2.  Handshake Protocol
     A.3.  ACKs
     A.4.  Connection ID Management
   Appendix B.  Analysis of Limits on CCM Usage
     B.1.  Confidentiality Limits
     B.2.  Integrity Limits
     B.3.  Limits for AEAD_AES_128_CCM_8
   Appendix C.  Implementation Pitfalls
   Contributors
   Authors' Addresses

1.  Introduction

   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 the
   Datagram Transport Layer Security (DTLS) protocol has been developed
   to offer communication security protection for those applications.
   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
   as a delta from TLS 1.3 [TLS13].  It obsoletes DTLS 1.2.

   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 [RFC7525].  [DEPRECATE]
   forbids the use of DTLS 1.0.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described 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.

   association:  Shared state between two endpoints established with a
      DTLS handshake.

   connection:  Synonym for association.

   endpoint:  Either the client or server of the connection.

   epoch:  One set of cryptographic keys used for encryption and
      decryption.

   handshake:  An initial negotiation between client and server that
      establishes the parameters of the connection.

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

   receiver:  An endpoint that is receiving records.

   sender:  An endpoint that is transmitting records.

   server:  The endpoint that did not initiate the DTLS connection.

   CID:  Connection ID.

   MSL:  Maximum Segment Lifetime.

   The reader is assumed to be familiar with [TLS13].  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.

   DTLS 1.3 uses network byte order (big-endian) format for encoding
   messages based on the encoding format defined in [TLS13] and earlier
   (D)TLS specifications.

   The reader is also assumed to be familiar with [RFC9146], 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
         message.

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

   '{}'  indicates messages protected using keys derived from a
         [sender]_handshake_traffic_secret.

   '[]'  indicates messages protected using keys derived from
         traffic_secret_N.

3.  DTLS Design Rationale and Overview

   The basic design philosophy of DTLS is to construct "TLS over
   datagram transport".  Datagram transport neither requires nor
   provides reliable or in-order delivery of data.  The DTLS protocol
   preserves this property for application data.  Applications such as
   media streaming, Internet telephony, and online gaming use datagram
   transport for communication due to the delay-sensitive nature of
   transported data.  The behavior of such applications is unchanged
   when the DTLS protocol is used to secure communication, since the
   DTLS protocol does not compensate for lost or reordered data traffic.
   Note that while low-latency streaming and gaming use DTLS to protect
   data (e.g., for protection of a WebRTC data channel), telephony
   utilizes DTLS for key establishment and the Secure Real-time
   Transport Protocol (SRTP) for protection of data [RFC5763].

   TLS cannot be used directly over datagram transports for the
   following four 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
       numbers to records.

   2.  The TLS handshake is a lock-step cryptographic protocol.
       Messages must be transmitted and received in a defined order; any
       other order is an error.  The DTLS handshake includes message
       sequence numbers to enable fragmented message reassembly and in-
       order delivery in case datagrams are lost or reordered.

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

   4.  Datagram transport protocols are susceptible to abusive behavior
       effecting denial-of-service (DoS) attacks against
       nonparticipants.  DTLS adds a return-routability check and DTLS
       1.3 uses the TLS 1.3 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:

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

                                    X<-- HelloRetryRequest
                                                     (lost)

            [Timer Expires]

            ClientHello           ------>
            (retransmit)

                   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 timer expires, the client knows that either the
   ClientHello or the response from the server has been lost, which
   causes the client to retransmit the ClientHello.  When the server
   receives the retransmission, it knows to retransmit its
   HelloRetryRequest or ServerHello.

   The server also maintains a retransmission timer for messages it
   sends (other than HelloRetryRequest) and retransmits when that timer
   expires.  Not applying retransmissions to the HelloRetryRequest
   avoids the need to create 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.

   For more detail on timeouts and retransmission, see Section 5.8.

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.  Fragmentation

   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 (see Section 4.4 for guidance).  Each DTLS handshake
   message contains both a fragment offset and a fragment length.  Thus,
   a recipient in possession of all bytes of a handshake message can
   reassemble the original unfragmented message.

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 decrypt
       and verify DTLS records.  However, the number of bits used for
       the epoch and sequence number fields in the DTLSCiphertext
       structure has been reduced from those in previous versions.

   3.  The DTLS epoch serialized in DTLSPlaintext is 2 octets long for
       compatibility with DTLS 1.2.  However, this value is set as the
       least significant 2 octets of the connection epoch, which is an 8
       octet counter incremented on every KeyUpdate.  See Section 4.2
       for details.  The sequence number is set to be the low order 48
       bits of the 64 bit sequence number.  Plaintext records MUST NOT
       be sent with sequence numbers that would exceed 2^48-1, so the
       upper 16 bits will always be 0.

   4.  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
   versions.

       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 Formats

   legacy_record_version:  This value MUST be set to {254, 253} for all
      records other than the initial ClientHello (i.e., one not
      generated after a HelloRetryRequest), where it may also be {254,
      255} for compatibility purposes.  It MUST be ignored for all
      purposes.  See [TLS13], Appendix D.1 for the rationale for this.

   epoch:  The least significant 2 bytes of the connection epoch value.

   unified_hdr:  The unified header (unified_hdr) is a structure of
      variable length, shown in Figure 3.

   encrypted_record:  The encrypted form of the serialized
      DTLSInnerPlaintext structure.

       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 Unified Header

   Fixed Bits:  The three high bits of the first byte of the unified
      header are set to 001.  This ensures that the value will fit
      within the DTLS region when multiplexing is performed as described
      in [RFC7983].  It also ensures that distinguishing encrypted DTLS
      1.3 records from encrypted DTLS 1.2 records is possible when they
      are carried on the same host/port quartet; such multiplexing is
      only possible when CIDs [RFC9146] are in use, in which case DTLS
      1.2 records will have the content type tls12_cid (25).

   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.  Implementations
      MAY mix sequence numbers of different lengths on the same
      connection.

   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
      epoch.

   Connection ID:  Variable-length CID.  The CID functionality is
      described in [RFC9146].  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 headers.

    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)
   |               |
   +-+-+-+-+-+-+-+-+

    DTLSPlaintext
      Structure

                     Figure 4: DTLS 1.3 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.  Implementations MAY mix records with and
   without length fields on the same connection.

   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:

       struct {
           uint64 epoch;
           uint64 sequence_number;
       } RecordNumber;

   This 128-bit value is used in the ACK message as well as in the
   "record_sequence_number" input to the Authenticated Encryption with
   Associated Data (AEAD) function.  The entire header value shown in
   Figure 4 (but prior to record number encryption; see Section 4.2.3)
   is used as the additional data value for the AEAD function.  For
   instance, if the minimal variant is used, the Associated Data (AD) 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 IDs.  In DTLS 1.3 the 64-bit sequence_number is used as
   the sequence number for the AEAD computation; unlike DTLS 1.2, the
   epoch is not included.

4.1.  Demultiplexing DTLS Records

   DTLS 1.3's header format is more complicated to demux than DTLS 1.2,
   which always carried the content type as the first byte.  As
   described in Figure 5, the first byte determines how an incoming DTLS
   record is demultiplexed.  The first 3 bits of the first byte
   distinguish a DTLS 1.3 encrypted record from record types used in
   previous DTLS versions and plaintext DTLS 1.3 record types.  Hence,
   the range 32 (0b0010 0000) to 63 (0b0011 1111) needs to be excluded
   from future allocations by IANA to avoid problems while
   demultiplexing; see Section 14.  Implementations can demultiplex DTLS
   1.3 records by examining the first byte as follows:

   *  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.

   Figure 5 shows this demultiplexing procedure graphically, taking DTLS
   1.3 and earlier versions of DTLS into account.

                +----------------+
                | Outer Content  |
                |   Type (OCT)   |
                |                |
                |   OCT == 20   -+--> ChangeCipherSpec (DTLS <1.3)
                |   OCT == 21   -+--> Alert (Plaintext)
                |   OCT == 22   -+--> DTLSHandshake (Plaintext)
                |   OCT == 23   -+--> Application Data (DTLS <1.3)
                |   OCT == 24   -+--> Heartbeat (DTLS <1.3)
   packet  -->  |   OCT == 25   -+--> DTLSCiphertext with CID (DTLS 1.2)
                |   OCT == 26   -+--> ACK (DTLS 1.3, Plaintext)
                |                |
                |                |   /+----------------+\
                | 31 < OCT < 64 -+--> |DTLSCiphertext  |
                |                |    |(header bits    |
                |      else      |    | start with 001)|
                |       |        |   /+-------+--------+\
                +-------+--------+            |
                        |                     |
                        v          Decryption |
                  +---------+          +------+
                  |  Reject |          |
                  +---------+          v
                               +----------------+
                               | Decrypted      |
                               | Content Type   |
                               | (DCT)          |
                               |                |
                               |     DCT == 21 -+--> Alert
                               |     DCT == 22 -+--> DTLSHandshake
                               |     DCT == 23 -+--> Application Data
                               |     DCT == 24 -+--> Heartbeat
                               |     DCT == 26 -+--> ACK
                               |     else ------+--> Error
                               +----------------+

           Figure 5: Demultiplexing DTLS 1.2 and DTLS 1.3 Records

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.  Implementations
   SHOULD discard records from earlier epochs but MAY choose to retain
   keying material from previous epochs for up to the default MSL
   specified for TCP [RFC0793] to allow for packet reordering.  (Note
   that the intention here is that implementers use the current guidance
   from the IETF for MSL, as specified in [RFC0793] or successors, not
   that they attempt to interrogate the MSL that the system TCP stack is
   using.)

   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 rekey 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, the recipient does not have a
   full epoch or sequence number value in the record and so there is
   some opportunity for ambiguity.  Because the full sequence number is
   used to compute the per-record nonce and the epoch determines the
   keys, 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 follow.

   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 in the
   current epoch.

   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 for that epoch as described
   above.

4.2.3.  Record 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 per-epoch Secret value to be
   used is described in Section 7.3 of [TLS13].  Note that a new key is
   used for each epoch: because the epoch is sent in the clear, this
   does not result in ambiguity.

   The encrypted sequence number is computed by XORing the leading bytes
   of the mask with the on-the-wire representation of the sequence
   number.  Decryption is accomplished by the same process.

   This procedure requires the ciphertext length to 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 that 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
   inputs.

   Future cipher suites, which are not based on AES or ChaCha20, MUST
   define their own record sequence number encryption in order to be
   used with DTLS.

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

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 Path MTU (PMTU) estimates
   obtained from the record layer.  For more information about PMTU
   issues, see Section 4.4.

   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 without CIDs.

   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
   account.

4.4.  PMTU Issues

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

   *  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
      "Datagram Too Big" indications [RFC1191] or ICMPv6 "Packet Too
      Big" indications [RFC4443].

   *  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
   particular:

   *  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.

   The DTLS record layer SHOULD also allow the upper layer protocol to
   discover the amount of record expansion expected by the DTLS
   processing; alternately, it MAY report PMTU estimates minus the
   estimated expansion from the transport layer and DTLS record framing.

   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.

   If there is a transport protocol indication that the PMTU was
   exceeded (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] and [RFC4821] for
   IPv4 or via [RFC8201] for IPv6.  In particular:

   *  Where allowed by the underlying transport protocol, the upper
      layer protocol SHOULD be allowed to set the state of the Don't
      Fragment (DF) bit (in IPv4) or prohibit local fragmentation (in
      IPv6).

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

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

   *  If the DTLS record layer informs the DTLS handshake layer that a
      message is too big, the handshake layer SHOULD immediately attempt
      to fragment the message, 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 specification does not specify an exact number
      of retransmits to attempt before backing off, but 2-3 seems
      appropriate.

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].  Because each epoch resets the sequence number space, a
   separate sliding window is needed for each epoch.

   The received record counter for an epoch MUST be initialized to zero
   when that epoch is first used.  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
   in that epoch during the lifetime of the association.  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 computing the full record number from the partial is still
   a potential timing channel for the record number, though a less
   powerful one than whether the record was deprotected.

   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.)

   The "right" edge of the window represents the highest validated
   sequence number value received in the epoch.  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 due to a received record until that
   record has been deprotected successfully.

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 fatal 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 DoS attacks because
   UDP forgery is so easy.  Thus, generating fatal alerts 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.

   Note that because invalid records are rejected at a layer lower than
   the handshake state machine, they do not affect pending
   retransmission timers.

4.5.3.  AEAD Limits

   Section 5.5 of [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.

   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
   attempts.

   Implementations MUST count the number of received packets that fail
   authentication with each key.  If the number of packets that fail
   authentication exceeds 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
   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_8_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_8_SHA256
   MUST NOT be 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 of packets that can fail authentication before a key
   update is required.  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.

5.  The DTLS Handshake Protocol

   DTLS 1.3 reuses 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.

   In addition, DTLS reuses TLS 1.3's "cookie" extension to provide a
   return-routability check as part of connection establishment.  This
   is an important DoS prevention mechanism for UDP-based protocols,
   unlike TCP-based protocols, for which TCP establishes return-
   routability as part of the connection establishment.

   DTLS implementations do not use the TLS 1.3 "compatibility mode"
   described in Appendix D.4 of [TLS13].  DTLS servers MUST NOT echo the
   "legacy_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 address that
       belongs to a 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 is the
       case for a Certificate message.

   In order to counter both of these attacks, DTLS borrows the stateless
   cookie technique used by Photuris [RFC2522] and IKE [RFC7296].  When
   the client sends its ClientHello message to the server, the server
   MAY respond with a HelloRetryRequest message.  The HelloRetryRequest
   message, as well as the "cookie" extension, is defined in TLS 1.3.
   The HelloRetryRequest message contains a stateless cookie (see
   [TLS13], Section 4.2.2).  The client MUST send a new 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 reuses 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 6.  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 6: 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 zero-valued single byte length field).

   When responding to a HelloRetryRequest, the client MUST create a new
   ClientHello message following the description in Section 4.1.2 of
   [TLS13].

   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 content or
   hash of the initial ClientHello (and HelloRetryRequest) to be stored
   in the cookie.  The initial ClientHello is included in the handshake
   transcript as a synthetic "message_hash" message, so only the hash
   value is needed for the handshake to complete, though the complete
   HelloRetryRequest contents are needed.

   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
   lifetimes.

   Alternatively, the server can store timestamps in the cookie and
   reject cookies that were generated outside a certain interval of
   time.

   DTLS servers SHOULD perform a cookie exchange whenever a new
   handshake is being performed.  If the server is being operated in an
   environment where amplification is not a problem, e.g., where ICE
   [RFC8445] has been used to establish bidirectional connectivity, 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 and the IP address matches one associated
   with the PSK.  Servers which process 0-RTT requests and send 0.5-RTT
   responses without a cookie exchange risk being used in an
   amplification attack if the size of outgoing messages greatly exceeds
   the size of those that are received.  A server SHOULD limit the
   amount of data it sends toward a client address to three times the
   amount of data sent by the client before it verifies that the client
   is able to receive data at that address.  A client address is valid
   after a cookie exchange or handshake completion.  Clients MUST be
   prepared to do a cookie exchange with every handshake.  Note that
   cookies are only valid for the existing handshake and cannot be
   stored for future handshakes.

   If a server receives a ClientHello with an invalid cookie, it MUST
   terminate the handshake with an "illegal_parameter" alert.  This
   allows the client to restart the connection from scratch without a
   cookie.

   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).

   DTLS clients which do not want to receive a Connection ID SHOULD
   still offer the "connection_id" extension [RFC9146] unless there is
   an application profile to the contrary.  This permits a server which
   wants to receive a CID to negotiate one.

5.2.  DTLS Handshake Message Format

   DTLS uses the same Handshake messages as TLS 1.3.  However, prior to
   transmission they are converted to DTLSHandshake messages, which
   contain extra data needed to support message loss, reordering, and
   message fragmentation.

       enum {
           client_hello(1),
           server_hello(2),
           new_session_ticket(4),
           end_of_early_data(5),
           encrypted_extensions(8),
           request_connection_id(9),           /* New */
           new_connection_id(10),              /* New */
           certificate(11),
           certificate_request(13),
           certificate_verify(15),
           finished(20),
           key_update(24),
           message_hash(254),
           (255)
       } 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 (msg_type) {
               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;
               case request_connection_id: RequestConnectionId;
               case new_connection_id:     NewConnectionId;
           } body;
       } DTLSHandshake;

   In DTLS 1.3, the message transcript is computed over the original TLS
   1.3-style Handshake messages without the message_seq,
   fragment_offset, and fragment_length values.  Note that this is a
   change from DTLS 1.2 where those values were included in the
   transcript.

   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 reused, 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.

      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 trade-off).

   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 implementations MUST NOT use the HelloVerifyRequest to
   execute a return-routability check.  A dual-stack DTLS 1.2 / DTLS 1.3
   client MUST, however, be prepared to interact with a DTLS 1.2 server.

5.3.  ClientHello Message

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

       uint16 ProtocolVersion;
       opaque Random[32];

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

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

   legacy_version:  In previous versions of DTLS, this field was used
      for version negotiation and represented the highest version number
      supported by the client.  Experience has shown that many servers
      do not properly implement version negotiation, leading to "version
      intolerance" in which the server rejects an otherwise acceptable
      ClientHello with a version number higher than it supports.  In
      DTLS 1.3, the client indicates its version preferences in the
      "supported_versions" extension (see Section 4.2.1 of [TLS13]) and
      the legacy_version field MUST be set to {254, 253}, which was the
      version number for DTLS 1.2.  The supported_versions entries for
      DTLS 1.0 and DTLS 1.2 are 0xfeff and 0xfefd (to match the wire
      versions).  The value 0xfefc is used to indicate DTLS 1.3.

   random:  Same as for TLS 1.3, except that the downgrade sentinels
      described in Section 4.1.3 of [TLS13] when TLS 1.2 and TLS 1.1 and
      below are negotiated apply to DTLS 1.2 and DTLS 1.0, respectively.

   legacy_session_id:  Versions of TLS and DTLS before version 1.3
      supported a "session resumption" feature, which has been merged
      with pre-shared keys (PSK) in version 1.3.  A client which has a
      cached session ID set by a pre-DTLS 1.3 server SHOULD set this
      field to that value.  Otherwise, it MUST be set as a zero-length
      vector (i.e., a zero-valued single byte length field).

   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; only suites with DTLS-OK=Y may
      be used.

   legacy_compression_methods:  Same as for TLS 1.3.

   extensions:  Same as for TLS 1.3.

5.4.  ServerHello Message

   The DTLS 1.3 ServerHello message is the same as the TLS 1.3
   ServerHello message, except that the legacy_version field is set to
   0xfefd, indicating DTLS 1.2.

5.5.  Handshake Message Fragmentation and Reassembly

   As described in Section 4.3, one or more handshake messages may be
   carried in a single datagram.  However, handshake messages are
   potentially bigger than the size allowed by the underlying datagram
   transport.  DTLS provides a mechanism for fragmenting a handshake
   message over a number of records, each of which can be transmitted in
   separate datagrams, 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 DTLSHandshake messages, all
   with the same message_seq value as the original DTLSHandshake
   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 handshake message fragment that is
   placed into a record MUST be delivered in a single UDP datagram.

   When a DTLS implementation receives a handshake message fragment
   corresponding to the next expected handshake message sequence number,
   it MUST process it, either by buffering it until it has the entire
   handshake message or by processing any in-order portions of the
   message.  The transcript consists of complete TLS Handshake messages
   (reassembled as necessary).  Note that this requires removing the
   message_seq, fragment_offset, and fragment_length fields to create
   the Handshake structure.

   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.  Senders MUST
   NOT change handshake message bytes upon retransmission.  Receivers
   MAY check that retransmitted bytes are identical and SHOULD abort the
   handshake with an "illegal_parameter" alert if the value of a byte
   changes.

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

5.6.  EndOfEarlyData Message

   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 NOT accept records from epoch 1 indefinitely once they are
   able to process records from epoch 3.  Though reordering of IP
   packets can result in records from epoch 1 arriving after records
   from epoch 3, this is not likely to persist for very long relative to
   the round trip time.  Servers could discard epoch 1 keys after the
   first epoch 3 data arrives, or retain keys for processing epoch 1
   data for a short period.  (See Section 6.1 for the definitions of
   each epoch.)

5.7.  DTLS Handshake Flights

   DTLS handshake messages are grouped into a series of message flights.
   A flight starts with the handshake message transmission of one peer
   and ends with the expected response from the other peer.  Table 1
   contains a complete list of message combinations that constitute
   flights.

      +======+========+========+===================================+
      | Note | Client | Server | Handshake Messages                |
      +======+========+========+===================================+
      |      | x      |        | ClientHello                       |
      +------+--------+--------+-----------------------------------+
      |      |        | x      | HelloRetryRequest                 |
      +------+--------+--------+-----------------------------------+
      |      |        | x      | ServerHello, EncryptedExtensions, |
      |      |        |        | CertificateRequest, Certificate,  |
      |      |        |        | CertificateVerify, Finished       |
      +------+--------+--------+-----------------------------------+
      | 1    | x      |        | Certificate, CertificateVerify,   |
      |      |        |        | Finished                          |
      +------+--------+--------+-----------------------------------+
      | 1    |        | x      | NewSessionTicket                  |
      +------+--------+--------+-----------------------------------+

              Table 1: Flight Handshake Message Combinations

   Remarks:

   *  Table 1 does not highlight any of the optional messages.

   *  Regarding note (1): When a handshake flight is sent without any
      expected response, as is the case with the client's final flight
      or with the NewSessionTicket message, the flight must be
      acknowledged with an ACK message.

   Below are several example message exchanges illustrating the flight
   concept.  The notational conventions from [TLS13] are used.

   Client                                            Server

                                                              +--------+
    ClientHello                                               | Flight |
                          -------->                           +--------+

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

                                                              +--------+
   ClientHello                                                | Flight |
    + cookie              -------->                           +--------+

                                                 ServerHello
                                       {EncryptedExtensions}  +--------+
                                       {CertificateRequest*}  | Flight |
                                              {Certificate*}  +--------+
                                        {CertificateVerify*}
                                                  {Finished}
                          <--------      [Application Data*]

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

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

      Figure 7: Message Flights for a Full DTLS Handshake (with Cookie
                                 Exchange)

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

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

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

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

         Figure 8: Message Flights for Resumption and PSK Handshake
                         (without Cookie Exchange)

   Client                                            Server

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

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

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

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

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

            Figure 9: Message Flights for the Zero-RTT Handshake

   Client                                            Server

                                                             +--------+
                          <--------       [NewSessionTicket] | Flight |
                                                             +--------+

                                                             +--------+
   [ACK]                  -------->                          | Flight |
                                                             +--------+

        Figure 10: Message Flights for the NewSessionTicket Message

   KeyUpdate, NewConnectionId, and RequestConnectionId follow a similar
   pattern to NewSessionTicket: a single message sent by one side
   followed by an ACK by the other.

5.8.  Timeout and Retransmission

5.8.1.  State Machine

   DTLS uses a simple timeout and retransmission scheme with the state
   machine shown in Figure 11.

                                +-----------+
                                | 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 |  (Maybe Send ACK) |
              last |                   |
            flight |                   | Receive ACK
                   |                   | for last flight
                  \|/                  |
                                       |
               +-----------+           |
               |           | <---------+
               | FINISHED  |
               |           |
               +-----------+
                   |  /|\
                   |   |
                   |   |
                   +---+

             Server read retransmit
                 Retransmit ACK

          Figure 11: 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 transmission 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 acknowledged.  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, adjusts and
       re-arms the retransmit timer (see Section 5.8.2), 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 unacknowledged portion of the flight, adjusts and
       re-arms 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
       when none of the messages that it sent in response to that flight
       have been acknowledged: the implementation transitions to the
       SENDING state, where it retransmits the flight, adjusts and re-
       arms 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 peer and therefore suggests
       that part of one's previous flight was lost.

   4.  The implementation receives some or all of the 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 final flight with a retransmit of its
   ACK.

   Note that because of packet loss, it is possible for one side to be
   sending application data even though the other side has not received
   the first side's Finished message.  Implementations MUST either
   discard or buffer all application data records for epoch 3 and above
   until they have received the Finished message from the peer.
   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.

5.8.2.  Timer Values

   The configuration of timer settings varies with implementations, and
   certain deployment environments require timer value adjustments.
   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.

   Unless implementations have deployment-specific and/or external
   information about the round trip time, implementations SHOULD use an
   initial timer value of 1000 ms and double the value at each
   retransmission, up to no less than 60 seconds (the maximum as
   specified in RFC 6298 [RFC6298]).  Application-specific profiles MAY
   recommend shorter or longer timer values.  For instance:

   *  Profiles for specific deployment environments, such as in low-
      power, multi-hop mesh scenarios as used in some Internet of Things
      (IoT) networks, MAY specify longer timeouts.  See [IOT-PROFILE]
      for more information about one such DTLS 1.3 IoT profile.

   *  Real-time protocols MAY specify shorter timeouts.  It is
      RECOMMENDED that for DTLS-SRTP [RFC5764], a default timeout of 400
      ms be used; because customer experience degrades with one-way
      latencies of greater than 200 ms, real-time deployments are less
      likely to have long latencies.

   In settings where there is external information (for instance, from
   an ICE [RFC8445] handshake, or from previous connections to the same
   server) about the RTT, implementations SHOULD use 1.5 times that RTT
   estimate as the retransmit timer.

   Implementations SHOULD retain the current timer value until a message
   is transmitted and acknowledged without having to be retransmitted,
   at which time the value SHOULD be adjusted to 1.5 times the measured
   round trip time for that message.  After a long period of idleness,
   no less than 10 times the current timer value, implementations MAY
   reset the timer to the initial value.

   Note that because retransmission is for the handshake and not
   dataflow, the effect on congestion of shorter timeouts is smaller
   than in generic protocols such as TCP or QUIC.  Experience with DTLS
   1.2, which uses a simpler "retransmit everything on timeout"
   approach, has not shown serious congestion problems in practice.

5.8.3.  Large Flight Sizes

   DTLS does not have any built-in congestion control or rate control;
   in general, this is not an issue because messages tend to be small.
   However, in principle, some messages -- especially Certificate -- can
   be quite large.  If all the messages in a large flight are sent at
   once, this can result in network congestion.  A better strategy is to
   send out only part of the flight, sending more when messages are
   acknowledged.  Several extensions have been standardized to reduce
   the size of the Certificate message -- for example, the "cached_info"
   extension [RFC7924]; certificate compression [RFC8879]; and
   [RFC6066], which defines the "client_certificate_url" extension
   allowing DTLS clients to send a sequence of Uniform Resource Locators
   (URLs) instead of the client certificate.

   DTLS stacks SHOULD NOT send more than 10 records in a single
   transmission.

5.8.4.  State Machine Duplication for Post-Handshake Messages

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

   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 5.8.1.  For example, if a server sends a
   NewSessionTicket and a CertificateRequest message, two independent
   state machines will be created.

   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 messages first.  Likewise, a server MAY send
   multiple CertificateRequest messages at once without having completed
   earlier client authentication requests before.  In contrast,
   implementations MUST NOT send KeyUpdate, NewConnectionId, or
   RequestConnectionId messages if an earlier message of the same type
   has not yet been acknowledged.

      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
   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.

5.9.  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.  Note that
   alerts are not reliably transmitted; implementations SHOULD NOT
   depend on receiving alerts in order to signal errors or connection
   closure.

   Any data received with an epoch/sequence number pair after that of a
   valid received closure alert MUST be ignored.  Note: this is a change
   from TLS 1.3 which depends on the order of receipt rather than the
   epoch and sequence number.

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
   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
   ClientHellos.

      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 to avoid
      the 5-tuple-based ambiguity.

6.  Example of Handshake with Timeout and Retransmission

   The following is an example of a handshake with lost packets and
   retransmissions.  Note that the client sends an empty ACK message
   because it can only acknowledge Record 2 sent by the server once it
   has processed messages in Record 0 needed to establish epoch 2 keys,
   which are needed to encrypt or decrypt messages found in Record 2.
   Section 7 provides the necessary background details for this
   interaction.  Note: For simplicity, we are not resetting record
   numbers in this diagram, so "Record 1" is really "Epoch 2, Record 0",
   etc.

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

    Record 0                  -------->
    ClientHello
    (message_seq=0)

                                X<-----                 Record 0
                                (lost)               ServerHello
                                                 (message_seq=0)
                                                        Record 1
                                             EncryptedExtensions
                                                 (message_seq=1)
                                                     Certificate
                                                 (message_seq=2)

                              <--------                 Record 2
                                               CertificateVerify
                                                 (message_seq=3)
                                                        Finished
                                                 (message_seq=4)

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

                              <--------                 Record 3
                                                     ServerHello
                                                 (message_seq=0)
                                             EncryptedExtensions
                                                 (message_seq=1)
                                                     Certificate
                                                 (message_seq=2)

                              <--------                 Record 4
                                               CertificateVerify
                                                 (message_seq=3)
                                                        Finished
                                                 (message_seq=4)

    Record 2                  -------->
    Certificate
    (message_seq=1)
    CertificateVerify
    (message_seq=2)
    Finished
    (message_seq=3)

                              <--------               Record 5
                                                       ACK [2]

         Figure 12: Example DTLS Exchange Illustrating Message Loss

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 reordering, 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
   state:

   *  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 that 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, that post-handshake
      messages 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 values (4 to 2^64-1) are 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 record with an epoch value for which
   no corresponding cipher state can be determined SHOULD handle it as a
   record which fails deprotection.

   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 13 illustrates the epoch values in an example DTLS handshake.

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

    Record 0
    ClientHello
    (epoch=0)
                               -------->
                                                        Record 0
                               <--------       HelloRetryRequest
                                                       (epoch=0)
    Record 1
    ClientHello                -------->
    (epoch=0)
                                                        Record 1
                               <--------             ServerHello
                                                       (epoch=0)
                                           {EncryptedExtensions}
                                                       (epoch=2)
                                                   {Certificate}
                                                       (epoch=2)
                                             {CertificateVerify}
                                                       (epoch=2)
                                                      {Finished}
                                                       (epoch=2)
    Record 2
    {Certificate}              -------->
    (epoch=2)
    {CertificateVerify}
    (epoch=2)
    {Finished}
    (epoch=2)
                                                        Record 2
                               <--------                   [ACK]
                                                       (epoch=3)
    Record 3
    [Application Data]         -------->
    (epoch=3)
                                                        Record 3
                               <--------      [Application Data]
                                                       (epoch=3)

                            Some time later ...
                    (Post-Handshake Message Exchange)
                                                        Record 4
                               <--------      [NewSessionTicket]
                                                       (epoch=3)
    Record 4
    [ACK]                      -------->
    (epoch=3)

                            Some time later ...
                              (Rekeying)
                                                        Record 5
                               <--------      [Application Data]
                                                       (epoch=4)
    Record 5
    [Application Data]         -------->
    (epoch=4)

          Figure 13: 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 26.  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 lock-step protocol).
   In particular, receiving a message from a handshake flight implicitly
   acknowledges all messages from the previous flight(s).  Accordingly,
   an ACK from the server would not cover both the ClientHello and the
   client's Certificate message, because the ClientHello and client
   Certificate are in different flights.  Implementations can accomplish
   this by clearing their ACK list upon receiving the start of the next
   flight.

   For post-handshake messages, 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 records
   containing messages which are discarded because a previous copy has
   been received.

   During the handshake, ACK records MUST be sent with an epoch which 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 ServerHello
   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).  "Immediately" is hard to define.  One approach 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.  Note: The 1/4 value here is somewhat
      arbitrary.  Given that the round trip estimates in the DTLS
      handshake are generally very rough (or the default), any value
      will be an approximation, and there is an inherent compromise due
      to competition between retransmission due to over-aggressive
      ACKing and over-aggressive timeout-based retransmission.  As a
      comparison point, QUIC's loss-based recovery algorithms
      ([RFC9002], Section 6.1.2) work out to a delay of about 1/3 of the
      retransmit timer.

   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 of the
       main handshake.

   2.  The server's post-handshake CertificateRequest.

   ACKs SHOULD NOT be sent for these flights unless the responding
   flight cannot be generated immediately.  All other flights MUST be
   ACKed.  In this case, implementations MAY send explicit ACKs for the
   complete received flight even though it will eventually also be
   implicitly acknowledged through the responding flight.  A notable
   example for this is the case of client authentication in constrained
   environments, where generating the CertificateVerify message can take
   considerable time on the client.  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 request_update set
      to "update_requested" does not implicitly acknowledge the earlier
      KeyUpdate message because the two KeyUpdate messages might have
      crossed in flight.

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

   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 to which
   it is responding.

7.3.  Design Rationale

   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 or early retransmission, as opposed to the
   timeout-based whole flight retransmission in previous versions of
   DTLS.  When DTLS 1.3 is used in deployments with lossy networks, such
   as low-power, long-range radio networks as well as low-power mesh
   networks, the use of ACKs is recommended.

   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 13 acknowledges receipt and processing of Record
   4 (containing the NewSessionTicket message), and if it is not sent,
   the server will continue retransmission of the NewSessionTicket
   indefinitely until its maximum retransmission count is reached.

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 records 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 reordering, 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
   keys.

   Figure 14 shows an example exchange illustrating that successful ACK
   processing updates the keys of the KeyUpdate message sender, which is
   reflected in the change of epoch values.

   Client                                             Server

         /-------------------------------------------\
        |                                             |
        |             Initial Handshake               |
         \-------------------------------------------/

    [Application Data]         -------->
    (epoch=3)

                               <--------      [Application Data]
                                                       (epoch=3)

         /-------------------------------------------\
        |                                             |
        |              Some time later ...            |
         \-------------------------------------------/

    [Application Data]         -------->
    (epoch=3)

    [KeyUpdate]
    (+ update_requested        -------->
    (epoch 3)

                               <--------      [Application Data]
                                                       (epoch=3)

                                                           [ACK]
                               <--------               (epoch=3)

    [Application Data]
    (epoch=4)                  -------->

                               <--------             [KeyUpdate]
                                                       (epoch=3)

    [ACK]                      -------->
    (epoch=4)

                               <--------      [Application Data]
                                                       (epoch=4)

                     Figure 14: Example DTLS Key Update

   With a 128-bit key as in AES-128, rekeying 2^64 times has a high
   probability of key reuse within a given connection.  Note that even
   if the key repeats, the IV is also independently generated.  In order
   to provide an extra margin of security, sending implementations MUST
   NOT allow the epoch to exceed 2^48-1.  In order to allow this value
   to be changed later, receiving implementations MUST NOT enforce this
   rule.  If a sending implementation receives a KeyUpdate with
   request_update set to "update_requested", it MUST NOT send its own
   KeyUpdate if that would cause it to exceed these limits and SHOULD
   instead ignore the "update_requested" flag.  Note: this overrides the
   requirement in TLS 1.3 to always send a KeyUpdate in response to
   "update_requested".

9.  Connection ID Updates

   If the client and server have negotiated the "connection_id"
   extension [RFC9146], either side can send a new CID that it wishes
   the other side to use in a NewConnectionId message.

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

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

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

   cids:  Indicates the set of CIDs that the sender wishes the peer to
      use.

   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 CIDs MUST be used immediately for all future records.  If it
      is set to "cid_spare", then either an existing or new CID MAY be
      used.

   Endpoints SHOULD use receiver-provided CIDs in the order they were
   provided.  Implementations which receive more spare CIDs than they
   wish to maintain MAY simply discard any extra CIDs.  Endpoints MUST
   NOT have more than one NewConnectionId message outstanding.

   Implementations which either did not negotiate the "connection_id"
   extension or which have negotiated receiving an empty CID MUST NOT
   send NewConnectionId.  Implementations MUST NOT send
   RequestConnectionId when sending an empty Connection ID.
   Implementations which detect a violation of these rules MUST
   terminate the connection with an "unexpected_message" alert.

   Implementations SHOULD use a new CID whenever sending on a new path
   and SHOULD request new CIDs for this purpose if path changes are
   anticipated.

       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_cids CIDs as
   soon as possible.  Endpoints MUST NOT send a RequestConnectionId
   message when an existing request is still unfulfilled; this implies
   that endpoints need to request new CIDs well in advance.  An endpoint
   MAY handle requests which it considers excessive by responding with a
   NewConnectionId message containing fewer than num_cids CIDs,
   including no CIDs at all.  Endpoints MAY handle an excessive number
   of RequestConnectionId messages by terminating the connection using a
   "too_many_cids_requested" (alert number 52) alert.

   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
   direction.

      Note: The "connection_id" extension, which is used in ClientHello
      and ServerHello messages, is defined in [RFC9146].

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

   ClientHello
   (connection_id=5)
                               -------->

                               <--------       HelloRetryRequest
                                                        (cookie)

   ClientHello                 -------->
   (connection_id=5)
     + cookie

                               <--------             ServerHello
                                             (connection_id=100)
                                             EncryptedExtensions
                                                         (cid=5)
                                                     Certificate
                                                         (cid=5)
                                               CertificateVerify
                                                         (cid=5)
                                                        Finished
                                                         (cid=5)

   Certificate                -------->
   (cid=100)
   CertificateVerify
   (cid=100)
   Finished
   (cid=100)
                              <--------                      ACK
                                                         (cid=5)

   Application Data           ========>
   (cid=100)
                              <========         Application Data
                                                         (cid=5)

               Figure 15: 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.

10.  Application Data Protocol

   Application data messages are carried by the record layer and are
   split into records 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 by excessive resource consumption.  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.

   Some key properties required of the cookie for the cookie-exchange
   mechanism to be functional are described in Section 3.3 of [RFC2522]:

   *  The cookie MUST depend on the client's address.

   *  It MUST NOT be possible for anyone other than the issuing entity
      to generate cookies that are accepted as valid by that entity.
      This typically entails an integrity check based on a secret key.

   *  Cookie generation and verification are triggered by
      unauthenticated parties, and as such their resource consumption
      needs to be restrained in order to avoid having the cookie-
      exchange mechanism itself serve as a DoS vector.

   Although the cookie must allow the server to produce the right
   handshake transcript, it SHOULD be constructed so that knowledge of
   the cookie is insufficient to reproduce the ClientHello contents.
   Otherwise, this may create problems with future extensions such as
   Encrypted Client Hello [TLS-ECH].

   When cookies are generated using a keyed authentication mechanism, it
   should be possible to rotate the associated secret key, so that
   temporary compromise of the key does not permanently compromise the
   integrity of the cookie-exchange mechanism.  Though this secret is
   not as high-value as, e.g., a session-ticket-encryption key, rotating
   the cookie-generation key on a similar timescale would ensure that
   the key rotation functionality is exercised regularly and thus in
   working order.

   The cookie exchange provides address validation during the initial
   handshake.  DTLS with Connection IDs allows for endpoint addresses to
   change during the association; any such updated addresses are not
   covered by the cookie exchange during the handshake.  DTLS
   implementations MUST NOT update the address they send to in response
   to packets from a different address unless they first perform some
   reachability test; no such test is defined in this specification and
   a future specification would need to specify a complete procedure for
   how and when to update addresses.  Even with such a test, an active
   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.

   TLS 1.3 requires replay protection for 0-RTT data (or rather, for
   connections that use 0-RTT data; see Section 8 of [TLS13]).  DTLS
   provides an optional per-record replay-protection mechanism, since
   datagram protocols are inherently subject to message reordering and
   replay.  These two replay-protection mechanisms are orthogonal, and
   neither mechanism meets the requirements for the other.

   DTLS 1.3's handshake transcript does not include the new DTLS fields,
   which makes it have the same format as TLS 1.3.  However, the DTLS
   1.3 and TLS 1.3 transcripts are disjoint because they use different
   version numbers.  Additionally, the DTLS 1.3 key schedule uses a
   different label and so will produce different keys for the same
   transcript.

   The security and privacy properties of the CID for DTLS 1.3 build on
   top of what is described for DTLS 1.2 in [RFC9146].  There are,
   however, several differences:

   *  In both versions of DTLS, extension negotiation is used to agree
      on the use of the CID feature and the CID values.  In both
      versions, the CID is carried in the DTLS record header (if
      negotiated).  However, the way the CID is included in the record
      header differs between the two versions.

   *  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.

   *  The ability to use multiple CIDs allows for improved privacy
      properties in multihomed scenarios.  When only a single CID is in
      use on multiple paths from such a host, an adversary can correlate
      the communication interaction across 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.

   *  The mechanism for encrypting sequence numbers (Section 4.2.3)
      prevents trivial tracking by on-path adversaries that attempt to
      correlate the pattern of sequence numbers received on different
      paths; such tracking could occur even when different CIDs are used
      on each path, in the absence of sequence number encryption.
      Switching CIDs based on certain events, or even regularly, helps
      against tracking by on-path adversaries.  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 because it acts as a key
      identifier, which 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
      adversary.

12.  Changes since DTLS 1.2

   Since TLS 1.3 introduces a large number of changes with respect 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 cipher suite negotiation.

   *  New session resumption mechanism.

   *  PSK authentication redefined.

   *  New key derivation hierarchy utilizing a new key derivation
      construct.

   *  Improved version negotiation.

   *  Optimized record layer encoding and thereby its size.

   *  Added CID functionality.

   *  Sequence numbers are encrypted.

13.  Updates Affecting DTLS 1.2

   This document defines several changes that optionally affect
   implementations of DTLS 1.2, including those which do not also
   support DTLS 1.3.

   *  A version downgrade protection mechanism as described in [TLS13],
      Section 4.1.3 and applying to DTLS as described in Section 5.3.

   *  The updates described in [TLS13], Section 1.3.

   *  The new compliance requirements described in [TLS13], Section 9.3.

14.  IANA Considerations

   IANA has allocated the content type value 26 in the "TLS ContentType"
   registry for the ACK message, defined in Section 7.  The value for
   the "DTLS-OK" column is "Y".  IANA has reserved the content type
   range 32-63 so that content types in this range are not allocated.

   IANA has allocated value 52 for the "too_many_cids_requested" alert
   in the "TLS Alerts" registry.  The value for the "DTLS-OK" column is
   "Y".

   IANA has allocated two values in the "TLS HandshakeType" registry,
   defined in [TLS13], for request_connection_id (9) and
   new_connection_id (10), as defined in this document.  The value for
   the "DTLS-OK" column is "Y".

   IANA has added this RFC as a reference to the "TLS Cipher Suites"
   registry along with the following Note:

   |  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, as
   |  specified in Section 4.5.3 of RFC 9147.

15.  References

15.1.  Normative References

   [CHACHA]   Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
              <https://www.rfc-editor.org/info/rfc8439>.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,
              <https://www.rfc-editor.org/info/rfc6298>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9146]  Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
              A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146,
              DOI 10.17487/RFC9146, March 2022,
              <https://www.rfc-editor.org/info/rfc9146>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

15.2.  Informative References

   [AEAD-LIMITS]
              Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
              AEAD Algorithms", Work in Progress, Internet-Draft, draft-
              irtf-cfrg-aead-limits-04, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              aead-limits-04>.

   [AEBounds] Luykx, A. and K. Paterson, "Limits on Authenticated
              Encryption Use in TLS", 28 August 2017,
              <https://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.

   [CCM-ANALYSIS]
              Jonsson, J., "On the Security of CTR + CBC-MAC", Selected
              Areas in Cryptography pp. 76-93,
              DOI 10.1007/3-540-36492-7_7, February 2003,
              <https://doi.org/10.1007/3-540-36492-7_7>.

   [DEPRECATE]
              Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
              <https://www.rfc-editor.org/info/rfc8996>.

   [IOT-PROFILE]
              Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
              the Internet of Things", Work in Progress, Internet-Draft,
              draft-ietf-uta-tls13-iot-profile-04, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              tls13-iot-profile-04>.

   [RFC2522]  Karn, P. and W. Simpson, "Photuris: Session-Key Management
              Protocol", RFC 2522, DOI 10.17487/RFC2522, March 1999,
              <https://www.rfc-editor.org/info/rfc2522>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/info/rfc4340>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <https://www.rfc-editor.org/info/rfc4346>.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
              <https://www.rfc-editor.org/info/rfc4347>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
              the Datagram Congestion Control Protocol (DCCP)",
              RFC 5238, DOI 10.17487/RFC5238, May 2008,
              <https://www.rfc-editor.org/info/rfc5238>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <https://www.rfc-editor.org/info/rfc5763>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [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, <https://www.rfc-editor.org/info/rfc7296>.

   [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, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <https://www.rfc-editor.org/info/rfc7924>.

   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,
              <https://www.rfc-editor.org/info/rfc7983>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8879]  Ghedini, A. and V. Vasiliev, "TLS Certificate
              Compression", RFC 8879, DOI 10.17487/RFC8879, December
              2020, <https://www.rfc-editor.org/info/rfc8879>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/info/rfc9002>.

   [ROBUST]   Fischlin, M., Günther, F., and C. Janson, "Robust
              Channels: Handling Unreliable Networks in the Record
              Layers of QUIC and DTLS 1.3", received 15 June 2020, last
              revised 22 February 2021,
              <https://eprint.iacr.org/2020/718>.

   [TLS-ECH]  Rescorla, E., Oku, K., Sullivan, N., and C.A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-14, 13 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-14>.

Appendix A.  Protocol Data Structures and Constant Values

   This section provides the normative protocol types and constants
   definitions.

A.1.  Record Layer

       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;

       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)  |
       +-+-+-+-+-+-+-+-+

       struct {
           uint64 epoch;
           uint64 sequence_number;
       } RecordNumber;

A.2.  Handshake Protocol

       enum {
           hello_request_RESERVED(0),
           client_hello(1),
           server_hello(2),
           hello_verify_request_RESERVED(3),
           new_session_ticket(4),
           end_of_early_data(5),
           hello_retry_request_RESERVED(6),
           encrypted_extensions(8),
           request_connection_id(9),           /* New */
           new_connection_id(10),              /* New */
           certificate(11),
           server_key_exchange_RESERVED(12),
           certificate_request(13),
           server_hello_done_RESERVED(14),
           certificate_verify(15),
           client_key_exchange_RESERVED(16),
           finished(20),
           certificate_url_RESERVED(21),
           certificate_status_RESERVED(22),
           supplemental_data_RESERVED(23),
           key_update(24),
           message_hash(254),
           (255)
       } 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 (msg_type) {
               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;
               case request_connection_id: RequestConnectionId;
               case new_connection_id:     NewConnectionId;
           } body;
       } Handshake;

       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;

A.3.  ACKs

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

A.4.  Connection ID Management

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

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

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

       struct {
         uint8 num_cids;
       } RequestConnectionId;

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
   AEAD_AES_128_CCM.

   [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 128.

   n:  The size of the block function in bits.  For this cipher, n is
       128.

   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 bytes.

   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
   of the ciphertext in blocks, and 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 block.

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 in a single-key setting of 2^-60, which
   matches that used by TLS 1.3, as summarized in [AEAD-LIMITS], 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 a 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
   TLS 1.3, as summarized in [AEAD-LIMITS].  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 a 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).

   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
   formula:

   v <= 2^7

   As this represents attempts that 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.  Implementation Pitfalls

   In addition to the aspects of TLS that have been a source of
   interoperability and security problems (Appendix C.3 of [TLS13]),
   DTLS presents a few new potential sources of issues, noted here.

   *  Do you correctly handle messages received from multiple epochs
      during a key transition?  This includes locating the correct key
      as well as performing replay detection, if enabled.

   *  Do you retransmit handshake messages that are not (implicitly or
      explicitly) acknowledged (Section 5.8)?

   *  Do you correctly handle handshake message fragments received,
      including when they are out of order?

   *  Do you correctly handle handshake messages received out of order?
      This may include either buffering or discarding them.

   *  Do you limit how much data you send to a peer before its address
      is validated?

   *  Do you verify that the explicit record length is contained within
      the datagram in which it is contained?

Contributors

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

   Hanno Becker
   Arm Limited
   Email: Hanno.Becker@arm.com

   David Benjamin
   Google
   Email: davidben@google.com

   Thomas Fossati
   Arm Limited
   Email: thomas.fossati@arm.com

   Tobias Gondrom
   Huawei
   Email: tobias.gondrom@gondrom.org

   Felix Günther
   ETH Zurich
   Email: mail@felixguenther.info

   Benjamin Kaduk
   Akamai Technologies
   Email: kaduk@mit.edu

   Ilari Liusvaara
   Independent
   Email: ilariliusvaara@welho.com

   Martin Thomson
   Mozilla
   Email: martin.thomson@gmail.com

   Christopher A. Wood
   Cloudflare
   Email: caw@heapingbits.net

   Yin Xinxing
   Huawei
   Email: yinxinxing@huawei.com

   The sequence number encryption concept is taken from QUIC [RFC9000].
   We would like to thank the authors of RFC 9000 for their work.  Felix
   Günther and Martin Thomson contributed the analysis in Appendix B.
   We would like to thank Jonathan Hammell, Bernard Aboba, and Andy
   Cunningham for their review comments.

   Additionally, we would like to thank the IESG members for their
   review comments: Martin Duke, Erik Kline, Francesca Palombini, Lars
   Eggert, Zaheduzzaman Sarker, John Scudder, Éric Vyncke, Robert
   Wilton, Roman Danyliw, Benjamin Kaduk, Murray Kucherawy, Martin
   Vigoureux, and Alvaro Retana.

Authors' Addresses

   Eric Rescorla
   Mozilla
   Email: ekr@rtfm.com

   Hannes Tschofenig
   Arm Limited
   Email: hannes.tschofenig@arm.com

   Nagendra Modadugu
   Google, Inc.
   Email: nagendra@cs.stanford.edu