<pre>Internet Engineering Task Force (IETF)                       E. Rescorla
Request for Comments: 6347                                    RTFM, Inc.
Obsoletes: <a href="./rfc4347">4347</a>                                              N. Modadugu
Category: Standards Track                                   Google, Inc.
ISSN: 2070-1721                                             January 2012


             <span class="h1">Datagram Transport Layer Security Version 1.2</span>

Abstract

   This document specifies version 1.2 of the Datagram Transport Layer
   Security (DTLS) protocol.  The DTLS protocol provides communications
   privacy for datagram protocols.  The protocol allows client/server
   applications to communicate in a way that is designed to prevent
   eavesdropping, tampering, or message forgery.  The DTLS protocol is
   based on the Transport Layer Security (TLS) protocol and provides
   equivalent security guarantees.  Datagram semantics of the underlying
   transport are preserved by the DTLS protocol.  This document updates
   DTLS 1.0 to work with TLS version 1.2.

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 <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6347">http://www.rfc-editor.org/info/rfc6347</a>.

















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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

























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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-4">4</a>
      <a href="#section-1.1">1.1</a>. Requirements Terminology ...................................<a href="#page-5">5</a>
   <a href="#section-2">2</a>. Usage Model .....................................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. Overview of DTLS ................................................<a href="#page-5">5</a>
      <a href="#section-3.1">3.1</a>. Loss-Insensitive Messaging .................................<a href="#page-6">6</a>
      <a href="#section-3.2">3.2</a>. Providing Reliability for Handshake ........................<a href="#page-6">6</a>
           <a href="#section-3.2.1">3.2.1</a>. Packet Loss .........................................<a href="#page-6">6</a>
           <a href="#section-3.2.2">3.2.2</a>. Reordering ..........................................<a href="#page-7">7</a>
           <a href="#section-3.2.3">3.2.3</a>. Message Size ........................................<a href="#page-7">7</a>
      <a href="#section-3.3">3.3</a>. Replay Detection ...........................................<a href="#page-7">7</a>
   <a href="#section-4">4</a>. Differences from TLS ............................................<a href="#page-7">7</a>
      <a href="#section-4.1">4.1</a>. Record Layer ...............................................<a href="#page-8">8</a>
           <a href="#section-4.1.1">4.1.1</a>. Transport Layer Mapping ............................<a href="#page-10">10</a>
                  <a href="#section-4.1.1.1">4.1.1.1</a>. PMTU Issues ...............................<a href="#page-10">10</a>
           <a href="#section-4.1.2">4.1.2</a>. Record Payload Protection ..........................<a href="#page-12">12</a>
                  <a href="#section-4.1.2.1">4.1.2.1</a>. MAC .......................................<a href="#page-12">12</a>
                  <a href="#section-4.1.2.2">4.1.2.2</a>. Null or Standard Stream Cipher ............<a href="#page-13">13</a>
                  <a href="#section-4.1.2.3">4.1.2.3</a>. Block Cipher ..............................<a href="#page-13">13</a>
                  <a href="#section-4.1.2.4">4.1.2.4</a>. AEAD Ciphers ..............................<a href="#page-13">13</a>
                  <a href="#section-4.1.2.5">4.1.2.5</a>. New Cipher Suites .........................<a href="#page-13">13</a>
                  <a href="#section-4.1.2.6">4.1.2.6</a>. Anti-Replay ...............................<a href="#page-13">13</a>
                  <a href="#section-4.1.2.7">4.1.2.7</a>. Handling Invalid Records ..................<a href="#page-14">14</a>
      <a href="#section-4.2">4.2</a>. The DTLS Handshake Protocol ...............................<a href="#page-14">14</a>
           <a href="#section-4.2.1">4.2.1</a>. Denial-of-Service Countermeasures ..................<a href="#page-15">15</a>
           <a href="#section-4.2.2">4.2.2</a>. Handshake Message Format ...........................<a href="#page-18">18</a>
           <a href="#section-4.2.3">4.2.3</a>. Handshake Message Fragmentation and Reassembly .....<a href="#page-19">19</a>
           <a href="#section-4.2.4">4.2.4</a>. Timeout and Retransmission .........................<a href="#page-20">20</a>
                  <a href="#section-4.2.4.1">4.2.4.1</a>. Timer Values ..............................<a href="#page-24">24</a>
           <a href="#section-4.2.5">4.2.5</a>. ChangeCipherSpec ...................................<a href="#page-25">25</a>
           <a href="#section-4.2.6">4.2.6</a>. CertificateVerify and Finished Messages ............<a href="#page-25">25</a>
           <a href="#section-4.2.7">4.2.7</a>. Alert Messages .....................................<a href="#page-25">25</a>
           4.2.8. Establishing New Associations with Existing
                  Parameters .........................................<a href="#page-25">25</a>
      <a href="#section-4.3">4.3</a>. Summary of New Syntax .....................................<a href="#page-26">26</a>
           <a href="#section-4.3.1">4.3.1</a>. Record Layer .......................................<a href="#page-26">26</a>
           <a href="#section-4.3.2">4.3.2</a>. Handshake Protocol .................................<a href="#page-27">27</a>
   <a href="#section-5">5</a>. Security Considerations ........................................<a href="#page-27">27</a>
   <a href="#section-6">6</a>. Acknowledgments ................................................<a href="#page-28">28</a>
   <a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-28">28</a>
   <a href="#section-8">8</a>. Changes since DTLS 1.0 .........................................<a href="#page-29">29</a>
   <a href="#section-9">9</a>. References .....................................................<a href="#page-30">30</a>
      <a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-30">30</a>
      <a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-31">31</a>






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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   TLS [<a href="#ref-TLS" title="&quot;The TLS Protocol Version 1.0&quot;">TLS</a>] is the most widely deployed protocol for securing network
   traffic.  It is widely used for protecting Web traffic and for e-mail
   protocols such as IMAP [<a href="#ref-IMAP" title="&quot;INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1&quot;">IMAP</a>] and POP [<a href="#ref-POP" title="&quot;Post Office Protocol - Version 3&quot;">POP</a>].  The primary advantage
   of TLS is that it provides a transparent connection-oriented channel.
   Thus, it is easy to secure an application protocol by inserting TLS
   between the application layer and the transport layer.  However, TLS
   must run over a reliable transport channel -- typically TCP [<a href="#ref-TCP" title="&quot;Transmission Control Protocol&quot;">TCP</a>].
   Therefore, it cannot be used to secure unreliable datagram traffic.

   An increasing number of application layer protocols have been
   designed that use UDP transport.  In particular, protocols such as
   the Session Initiation Protocol (SIP) [<a href="#ref-SIP" title="&quot;SIP: Session Initiation Protocol&quot;">SIP</a>] and electronic gaming
   protocols are increasingly popular.  (Note that SIP can run over both
   TCP and UDP, but that there are situations in which UDP is
   preferable.)  Currently, designers of these applications are faced
   with a number of unsatisfactory choices.  First, they can use IPsec
   [<a href="./rfc4301" title="&quot;Security Architecture for the Internet Protocol&quot;">RFC4301</a>].  However, for a number of reasons detailed in [<a href="#ref-WHYIPSEC" title="&quot;Guidelines for Specifying the Use of IPsec Version 2&quot;">WHYIPSEC</a>],
   this is only suitable for some applications.  Second, they can design
   a custom application layer security protocol.  Unfortunately,
   although application layer security protocols generally provide
   superior security properties (e.g., end-to-end security in the case
   of S/MIME), they typically require a large amount of effort to design
   -- in contrast to the relatively small amount of effort required to
   run the protocol over TLS.

   In many cases, the most desirable way to secure client/server
   applications would be to use TLS; however, the requirement for
   datagram semantics automatically prohibits use of TLS.  This memo
   describes a protocol for this purpose: Datagram Transport Layer
   Security (DTLS).  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 [<a href="#ref-DTLS1" title="&quot;Datagram Transport Layer Security&quot;">DTLS1</a>] was originally defined as a delta from [<a href="#ref-TLS11" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.1&quot;">TLS11</a>].
   This document introduces a new version of DTLS, DTLS 1.2, which is
   defined as a series of deltas to TLS 1.2 [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>].  There is no DTLS
   1.1; that version number was skipped in order to harmonize version
   numbers with TLS.  This version also clarifies some confusing points
   in the DTLS 1.0 specification.

   Implementations that speak both DTLS 1.2 and DTLS 1.0 can
   interoperate with those that speak only DTLS 1.0 (using DTLS 1.0 of
   course), just as TLS 1.2 implementations can interoperate with
   previous versions of TLS (see <a href="#appendix-E.1">Appendix E.1</a> of [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>] for details),
   with the exception that there is no DTLS version of SSLv2 or SSLv3,
   so backward compatibility issues for those protocols do not apply.



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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Requirements Terminology</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="#ref-REQ" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">REQ</a>].

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Usage Model</span>

   The DTLS protocol is designed to secure data between communicating
   applications.  It is designed to run in application space, without
   requiring any kernel modifications.

   Datagram transport does not require or provide reliable or in-order
   delivery of data.  The DTLS protocol preserves this property for
   payload data.  Applications such as media streaming, Internet
   telephony, and online gaming use datagram transport for communication
   due to the delay-sensitive nature of transported data.  The behavior
   of such applications is unchanged when the DTLS protocol is used to
   secure communication, since the DTLS protocol does not compensate for
   lost or re-ordered data traffic.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Overview of DTLS</span>

   The basic design philosophy of DTLS is to construct "TLS over
   datagram transport".  The reason that TLS cannot be used directly in
   datagram environments is simply that packets may be lost or
   reordered.  TLS has no internal facilities to handle this kind of
   unreliability; therefore, TLS implementations break when rehosted on
   datagram transport.  The purpose of DTLS is to make only the minimal
   changes to TLS required to fix this problem.  To the greatest extent
   possible, DTLS is identical to TLS.  Whenever we need to invent new
   mechanisms, we attempt to do so in such a way that preserves the
   style of TLS.

   Unreliability creates problems for TLS at two levels:

      1. TLS does not allow independent decryption of individual
         records.  Because the integrity check depends on the sequence
         number, if record N is not received, then the integrity check
         on record N+1 will be based on the wrong sequence number and
         thus will fail.  (Note that prior to TLS 1.1, there was no
         explicit IV and so decryption would also fail.)

      2. The TLS handshake layer assumes that handshake messages are
         delivered reliably and breaks if those messages are lost.

   The rest of this section describes the approach that DTLS uses to
   solve these problems.



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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Loss-Insensitive Messaging</span>

   In TLS's traffic encryption layer (called the TLS Record Layer),
   records are not independent.  There are two kinds of inter-record
   dependency:

      1. Cryptographic context (stream cipher key stream) is retained
         between records.

      2. Anti-replay and message reordering protection are provided by a
         MAC that includes a sequence number, but the sequence numbers
         are implicit in the records.

   DTLS solves the first problem by banning stream ciphers.  DTLS solves
   the second problem by adding explicit sequence numbers.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Providing Reliability for Handshake</span>

   The TLS handshake is a lockstep cryptographic handshake.  Messages
   must be transmitted and received in a defined order; any other order
   is an error.  Clearly, this is incompatible with reordering and
   message loss.  In addition, TLS handshake messages are potentially
   larger than any given datagram, thus creating the problem of IP
   fragmentation.  DTLS must provide fixes for both of these problems.

<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>.  Packet Loss</span>

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

         Client                                   Server
         ------                                   ------
         ClientHello           ------&gt;

                                 X&lt;-- HelloVerifyRequest
                                                  (lost)

         [Timer Expires]

         ClientHello           ------&gt;
         (retransmit)

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



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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


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

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

<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>.  Reordering</span>

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

<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>.  Message Size</span>

   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 &lt;1500 bytes if IP fragmentation is not
   desired.  In order to compensate for this limitation, each DTLS
   handshake message may be fragmented over several DTLS records, each
   of which is intended to fit in a single IP datagram.  Each DTLS
   handshake message contains both a fragment offset and a fragment
   length.  Thus, a recipient in possession of all bytes of a handshake
   message can reassemble the original unfragmented message.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Replay Detection</span>

   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.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Differences from TLS</span>

   As mentioned in <a href="#section-3">Section 3</a>, DTLS is intentionally very similar to TLS.
   Therefore, instead of presenting DTLS as a new protocol, we present
   it as a series of deltas from TLS 1.2 [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>].  Where we do not
   explicitly call out differences, DTLS is the same as in [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>].




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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Record Layer</span>

   The DTLS record layer is extremely similar to that of TLS 1.2.  The
   only change is the inclusion of an explicit sequence number in the
   record.  This sequence number allows the recipient to correctly
   verify the TLS MAC.  The DTLS record format is shown below:

      struct {
           ContentType type;
           ProtocolVersion version;
           uint16 epoch;                                    // New field
           uint48 sequence_number;                          // New field
           uint16 length;
           opaque fragment[DTLSPlaintext.length];
         } DTLSPlaintext;

   type
      Equivalent to the type field in a TLS 1.2 record.

   version
      The version of the protocol being employed.  This document
      describes DTLS version 1.2, which uses the version { 254, 253 }.
      The version value of 254.253 is the 1's complement of DTLS version
      1.2.  This maximal spacing between TLS and DTLS version numbers
      ensures that records from the two protocols can be easily
      distinguished.  It should be noted that future on-the-wire version
      numbers of DTLS are decreasing in value (while the true version
      number is increasing in value.)

   epoch
      A counter value that is incremented on every cipher state change.

   sequence_number
      The sequence number for this record.

   length
      Identical to the length field in a TLS 1.2 record.  As in TLS 1.2,
      the length should not exceed 2^14.

   fragment
      Identical to the fragment field of a TLS 1.2 record.

   DTLS uses an explicit sequence number, rather than an implicit one,
   carried in the sequence_number field of the record.  Sequence numbers
   are maintained separately for each epoch, with each sequence_number
   initially being 0 for each epoch.  For instance, if a handshake
   message from epoch 0 is retransmitted, it might have a sequence
   number after a message from epoch 1, even if the message from epoch 1



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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   was transmitted first.  Note that some care needs to be taken during
   the handshake to ensure that retransmitted messages use the right
   epoch and keying material.

   If several handshakes are performed in close succession, there might
   be multiple records on the wire with the same sequence number but
   from different cipher states.  The epoch field allows recipients to
   distinguish such packets.  The epoch number is initially zero and is
   incremented each time a ChangeCipherSpec message is sent.  In order
   to ensure that any given sequence/epoch pair is unique,
   implementations MUST NOT allow the same epoch value to be reused
   within two times the TCP maximum segment lifetime.  In practice, TLS
   implementations rarely rehandshake; therefore, we do not expect this
   to be a problem.

   Note that because DTLS records may be reordered, a record from epoch
   1 may be received after epoch 2 has begun.  In general,
   implementations SHOULD discard packets from earlier epochs, but if
   packet loss causes noticeable problems they MAY choose to retain
   keying material from previous epochs for up to the default MSL
   specified for TCP [<a href="#ref-TCP" title="&quot;Transmission Control Protocol&quot;">TCP</a>] to allow for packet reordering.  (Note that
   the intention here is that implementors use the current guidance from
   the IETF for MSL, not that they attempt to interrogate the MSL that
   the system TCP stack is using.)  Until the handshake has completed,
   implementations MUST accept packets from the old epoch.

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

   Note that in the special case of a rehandshake on an existing
   association, it is safe to process a data packet immediately, even if
   the ChangeCipherSpec or Finished messages have not yet been received
   provided that either the rehandshake resumes the existing session or
   that it uses exactly the same security parameters as the existing
   association.  In any other case, the implementation MUST wait for the
   receipt of the Finished message to prevent downgrade attack.

   As in TLS, implementations MUST either abandon an association or
   rehandshake prior to allowing the sequence number to wrap.



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<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   Similarly, implementations MUST NOT allow the epoch to wrap, but
   instead MUST establish a new association, terminating the old
   association as described in <a href="#section-4.2.8">Section 4.2.8</a>.  In practice,
   implementations rarely rehandshake repeatedly on the same channel, so
   this is not likely to be an issue.

<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>.  Transport Layer Mapping</span>

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

   Note that unlike IPsec, DTLS records do not contain any association
   identifiers.  Applications must arrange to multiplex between
   associations.  With UDP, this is presumably done with the host/port
   number.

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

   Some transports, such as DCCP [<a href="#ref-DCCP" title="&quot;Datagram Congestion Control Protocol (DCCP)&quot;">DCCP</a>] 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.  In the
   future, extensions to DTLS may be specified that allow the use of
   only one set of sequence numbers for deployment in constrained
   environments.

   Some transports, such as DCCP, 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. [<a href="#ref-DCCPDTLS" title="&quot;Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)&quot;">DCCPDTLS</a>]
   defines a mapping of DTLS to DCCP that takes these issues into
   account.

<span class="h5"><a class="selflink" id="section-4.1.1.1" href="#section-4.1.1.1">4.1.1.1</a>.  PMTU Issues</span>

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



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   -  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
      [<a href="./rfc1191" title="&quot;Path MTU discovery&quot;">RFC1191</a>] "Datagram Too Big" indications or ICMPv6 [<a href="./rfc4443" title="&quot;Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification&quot;">RFC4443</a>]
      "Packet Too Big" indications.

   -  The DTLS handshake messages can exceed the PMTU.

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

   If PMTU estimates are available from the underlying transport
   protocol, they should be made available to upper layer protocols.  In
   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 allow the upper layer protocol to
   discover the amount of record expansion expected by the DTLS
   processing.  Note that this number is only an estimate because of
   block padding and the potential use of DTLS compression.

   If there is a transport protocol indication (either via ICMP or via a
   refusal to send the datagram as in Section 14 of [<a href="#ref-DCCP" title="&quot;Datagram Congestion Control Protocol (DCCP)&quot;">DCCP</a>]), 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 [<a href="./rfc1191" title="&quot;Path MTU discovery&quot;">RFC1191</a>] or [<a href="./rfc4821" title="&quot;Packetization Layer Path MTU Discovery&quot;">RFC4821</a>]
   mechanisms.  In particular:

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

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



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


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

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

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

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>.  Record Payload Protection</span>

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

<span class="h5"><a class="selflink" id="section-4.1.2.1" href="#section-4.1.2.1">4.1.2.1</a>.  MAC</span>

   The DTLS MAC is the same as that of TLS 1.2. However, rather than
   using TLS's implicit sequence number, the sequence number used to
   compute the MAC is the 64-bit value formed by concatenating the epoch
   and the sequence number in the order they appear on the wire.  Note
   that the DTLS epoch + sequence number is the same length as the TLS
   sequence number.

   TLS MAC calculation is parameterized on the protocol version number,
   which, in the case of DTLS, is the on-the-wire version, i.e., {254,
   253} for DTLS 1.2.

   Note that one important difference between DTLS and TLS MAC handling
   is that in TLS, MAC errors must result in connection termination.  In
   DTLS, the receiving implementation MAY simply discard the offending
   record and continue with the connection.  This change is possible
   because DTLS records are not dependent on each other in the way that
   TLS records are.

   In general, DTLS implementations SHOULD silently discard records with
   bad MACs or that are otherwise invalid.  They MAY log an error.  If a
   DTLS implementation chooses to generate an alert when it receives a
   message with an invalid MAC, it MUST generate a bad_record_mac alert



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   with level fatal and terminate its connection state.  Note that
   because errors do not cause connection termination, DTLS stacks are
   more efficient error type oracles than TLS stacks.  Thus, it is
   especially important that the advice in Section 6.2.3.2 of [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>] be
   followed.

<span class="h5"><a class="selflink" id="section-4.1.2.2" href="#section-4.1.2.2">4.1.2.2</a>.  Null or Standard Stream Cipher</span>

   The DTLS NULL cipher is performed exactly as the TLS 1.2 NULL cipher.

   The only stream cipher described in TLS 1.2 is RC4, which cannot be
   randomly accessed.  RC4 MUST NOT be used with DTLS.

<span class="h5"><a class="selflink" id="section-4.1.2.3" href="#section-4.1.2.3">4.1.2.3</a>.  Block Cipher</span>

   DTLS block cipher encryption and decryption are performed exactly as
   with TLS 1.2.

<span class="h5"><a class="selflink" id="section-4.1.2.4" href="#section-4.1.2.4">4.1.2.4</a>.  AEAD Ciphers</span>

   TLS 1.2 introduced authenticated encryption with additional data
   (AEAD) cipher suites.  The existing AEAD cipher suites, defined in
   [<a href="#ref-ECCGCM" title="&quot;TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)&quot;">ECCGCM</a>] and [<a href="#ref-RSAGCM" title="&quot;AES Galois Counter Mode (GCM) Cipher Suites for TLS&quot;">RSAGCM</a>], can be used with DTLS exactly as with TLS 1.2.

<span class="h5"><a class="selflink" id="section-4.1.2.5" href="#section-4.1.2.5">4.1.2.5</a>.  New Cipher Suites</span>

   Upon registration, new TLS cipher suites MUST indicate whether they
   are suitable for DTLS usage and what, if any, adaptations must be
   made (see <a href="#section-7">Section 7</a> for IANA considerations).

<span class="h5"><a class="selflink" id="section-4.1.2.6" href="#section-4.1.2.6">4.1.2.6</a>.  Anti-Replay</span>

   DTLS records contain 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 [<a href="#ref-ESP" title="&quot;IP Encapsulating Security Payload (ESP)&quot;">ESP</a>].

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

   Duplicates are rejected through the use of a sliding receive window.
   (How the window is implemented is a local matter, but the following
   text describes the functionality that the implementation must
   exhibit.)  A minimum window size of 32 MUST be supported, but a



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   window size of 64 is preferred and SHOULD be employed as the default.
   Another window size (larger than the minimum) MAY be chosen by the
   receiver.  (The receiver does not notify the sender of the window
   size.)

   The "right" edge of the window represents the highest validated
   sequence number value received on this session.  Records that contain
   sequence numbers lower than the "left" edge of the window are
   rejected.  Packets falling within the window are checked against a
   list of received packets within the window.  An efficient means for
   performing this check, based on the use of a bit mask, is described
   in Section 3.4.3 of [<a href="#ref-ESP" title="&quot;IP Encapsulating Security Payload (ESP)&quot;">ESP</a>].

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

<span class="h5"><a class="selflink" id="section-4.1.2.7" href="#section-4.1.2.7">4.1.2.7</a>.  Handling Invalid Records</span>

   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 level alerts to avoid attacks where the attacker
   repeatedly probes the implementation to see how it responds to
   various types of error.  Note that if DTLS is run over UDP, then any
   implementation which does this will be extremely susceptible to
   denial-of-service (DoS) attacks because UDP forgery is so easy.
   Thus, this practice is NOT RECOMMENDED for such transports.

   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.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  The DTLS Handshake Protocol</span>

   DTLS uses all of the same handshake messages and flows as TLS, with
   three principal changes:

      1. A stateless cookie exchange has been added to prevent denial-
         of-service attacks.






<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


      2. Modifications to the handshake header to handle message loss,
         reordering, and DTLS message fragmentation (in order to avoid
         IP fragmentation).

      3. Retransmission timers to handle message loss.

   With these exceptions, the DTLS message formats, flows, and logic are
   the same as those of TLS 1.2.

<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  Denial-of-Service Countermeasures</span>

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

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

      2. An attacker can use the server as an amplifier by sending
         connection initiation messages with a forged source of the
         victim.  The server then sends its next message (in DTLS, a
         Certificate message, which can be quite large) to the victim
         machine, thus flooding it.

   In order to counter both of these attacks, DTLS borrows the stateless
   cookie technique used by Photuris [<a href="#ref-PHOTURIS" title="&quot;Photuris: Session-Key Management Protocol&quot;">PHOTURIS</a>] and IKE [<a href="#ref-IKEv2" title="&quot;Internet Key Exchange Protocol Version 2 (IKEv2)&quot;">IKEv2</a>].  When
   the client sends its ClientHello message to the server, the server
   MAY respond with a HelloVerifyRequest message.  This message contains
   a stateless cookie generated using the technique of [<a href="#ref-PHOTURIS" title="&quot;Photuris: Session-Key Management Protocol&quot;">PHOTURIS</a>].  The
   client MUST retransmit the ClientHello with the cookie added.  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.















<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   The exchange is shown below:

      Client                                   Server
      ------                                   ------
      ClientHello           ------&gt;

                            &lt;----- HelloVerifyRequest
                                   (contains cookie)

      ClientHello           ------&gt;
      (with cookie)

      [Rest of handshake]

   DTLS therefore modifies the ClientHello message to add the cookie
   value.

   struct {
     ProtocolVersion client_version;
     Random random;
     SessionID session_id;
     opaque cookie&lt;0..2^8-1&gt;;                             // New field
     CipherSuite cipher_suites&lt;2..2^16-1&gt;;
           CompressionMethod compression_methods&lt;1..2^8-1&gt;;
   } ClientHello;

   When sending the first ClientHello, the client does not have a cookie
   yet; in this case, the Cookie field is left empty (zero length).

   The definition of HelloVerifyRequest is as follows:

   struct {
     ProtocolVersion server_version;
     opaque cookie&lt;0..2^8-1&gt;;
   } HelloVerifyRequest;

   The HelloVerifyRequest message type is hello_verify_request(3).

   The server_version field has the same syntax as in TLS.  However, in
   order to avoid the requirement to do version negotiation in the
   initial handshake, DTLS 1.2 server implementations SHOULD use DTLS
   version 1.0 regardless of the version of TLS that is expected to be
   negotiated.  DTLS 1.2 and 1.0 clients MUST use the version solely to
   indicate packet formatting (which is the same in both DTLS 1.2 and
   1.0) and not as part of version negotiation.  In particular, DTLS 1.2
   clients MUST NOT assume that because the server uses version 1.0 in
   the HelloVerifyRequest that the server is not DTLS 1.2 or that it
   will eventually negotiate DTLS 1.0 rather than DTLS 1.2.



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   When responding to a HelloVerifyRequest, the client MUST use the same
   parameter values (version, random, session_id, cipher_suites,
   compression_method) as it did in the original ClientHello.  The
   server SHOULD use those values to generate its cookie and verify that
   they are correct upon cookie receipt.  The server MUST use the same
   version number in the HelloVerifyRequest that it would use when
   sending a ServerHello.  Upon receipt of the ServerHello, the client
   MUST verify that the server version values match.  In order to avoid
   sequence number duplication in case of multiple HelloVerifyRequests,
   the server MUST use the record sequence number in the ClientHello as
   the record sequence number in the HelloVerifyRequest.

   Note: This specification increases the cookie size limit to 255 bytes
   for greater future flexibility.  The limit remains 32 for previous
   versions of DTLS.

   The DTLS server SHOULD generate cookies in such a way that they can
   be verified without retaining any per-client state on the server.
   One technique is to have a randomly generated secret and generate
   cookies as:

      Cookie = HMAC(Secret, Client-IP, Client-Parameters)

   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.  In order to avoid sequence number duplication in
   case of multiple cookie exchanges, the server MUST use the record
   sequence number in the ClientHello as the record sequence number in
   its initial ServerHello.  Subsequent ServerHellos will only be sent
   after the server has created state and MUST increment normally.

   One potential attack on this scheme is for the attacker to collect a
   number of cookies from different addresses 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 that legitimate clients be able to
   handshake through the transition (e.g., they received a cookie with
   Secret 1 and then sent the second ClientHello after the server has
   changed to Secret 2), the server can have a limited window during
   which it accepts both secrets.  [<a href="#ref-IKEv2" title="&quot;Internet Key Exchange Protocol Version 2 (IKEv2)&quot;">IKEv2</a>] suggests adding a version
   number to cookies to detect this case.  An alternative approach is
   simply to try verifying with both secrets.

   DTLS servers SHOULD perform a cookie exchange whenever a new
   handshake is being performed.  If the server is being operated in an
   environment where amplification is not a problem, the server MAY be
   configured not to perform a cookie exchange.  The default SHOULD be
   that the exchange is performed, however.  In addition, the server MAY



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   choose not to do a cookie exchange when a session is resumed.
   Clients MUST be prepared to do a cookie exchange with every
   handshake.

   If HelloVerifyRequest is used, the initial ClientHello and
   HelloVerifyRequest are not included in the calculation of the
   handshake_messages (for the CertificateVerify message) and
   verify_data (for the Finished message).

   If a server receives a ClientHello with an invalid cookie, it SHOULD
   treat it the same as a ClientHello with no cookie.  This avoids
   race/deadlock conditions if the client somehow gets a bad cookie
   (e.g., because the server changes its cookie signing key).

   Note to implementors: This may result in clients receiving multiple
   HelloVerifyRequest messages with different cookies.  Clients SHOULD
   handle this by sending a new ClientHello with a cookie in response to
   the new HelloVerifyRequest.

<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  Handshake Message Format</span>

   In order to support message loss, reordering, and message
   fragmentation, DTLS modifies the TLS 1.2 handshake header:

   struct {
     HandshakeType msg_type;
     uint24 length;
     uint16 message_seq;                               // New field
     uint24 fragment_offset;                           // New field
     uint24 fragment_length;                           // New field
     select (HandshakeType) {
       case hello_request: HelloRequest;
       case client_hello:  ClientHello;
       case hello_verify_request: HelloVerifyRequest;  // New type
       case server_hello:  ServerHello;
       case certificate:Certificate;
       case server_key_exchange: ServerKeyExchange;
       case certificate_request: CertificateRequest;
       case server_hello_done:ServerHelloDone;
       case certificate_verify:  CertificateVerify;
       case client_key_exchange: ClientKeyExchange;
       case finished: Finished;
     } body;
   } Handshake;

   The first message each side transmits in each handshake always has
   message_seq = 0.  Whenever each new message is generated, the
   message_seq value is incremented by one.  Note that in the case of a



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   rehandshake, this implies that the HelloRequest will have message_seq
   = 0 and the ServerHello will have message_seq = 1.  When a message is
   retransmitted, the same message_seq value is used.  For example:

         Client                             Server
         ------                             ------
         ClientHello (seq=0)  ------&gt;

                                 X&lt;-- HelloVerifyRequest (seq=0)
                                                 (lost)

         [Timer Expires]

         ClientHello (seq=0)  ------&gt;
         (retransmit)

                              &lt;------ HelloVerifyRequest (seq=0)

         ClientHello (seq=1)  ------&gt;
         (with cookie)

                              &lt;------        ServerHello (seq=1)
                              &lt;------        Certificate (seq=2)
                              &lt;------    ServerHelloDone (seq=3)

         [Rest of handshake]

   Note, however, that from the perspective of the DTLS record layer,
   the retransmission is a new record.  This record will have a new
   DTLSPlaintext.sequence_number value.

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

<span class="h4"><a class="selflink" id="section-4.2.3" href="#section-4.2.3">4.2.3</a>.  Handshake Message Fragmentation and Reassembly</span>

   As noted in <a href="#section-4.1.1">Section 4.1.1</a>, each DTLS message MUST fit within a single
   transport layer datagram.  However, handshake messages are
   potentially bigger than the maximum record size.  Therefore, DTLS
   provides a mechanism for fragmenting a handshake message over a
   number of records, each of which can be transmitted separately, thus
   avoiding IP fragmentation.



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


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

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

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

<span class="h4"><a class="selflink" id="section-4.2.4" href="#section-4.2.4">4.2.4</a>.  Timeout and Retransmission</span>

   DTLS messages are grouped into a series of message flights, according
   to the diagrams below.  Although each flight of messages may consist
   of a number of messages, they should be viewed as monolithic for the
   purpose of timeout and retransmission.





















<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


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

   ClientHello             --------&gt;                           Flight 1

                           &lt;-------    HelloVerifyRequest      Flight 2

   ClientHello             --------&gt;                           Flight 3

                                              ServerHello    \
                                             Certificate*     \
                                       ServerKeyExchange*      Flight 4
                                      CertificateRequest*     /
                           &lt;--------      ServerHelloDone    /

   Certificate*                                              \
   ClientKeyExchange                                          \
   CertificateVerify*                                          Flight 5
   [<a href="#ref-ChangeCipherSpec">ChangeCipherSpec</a>]                                         /
   Finished                --------&gt;                         /

                                       [<a id="ref-ChangeCipherSpec">ChangeCipherSpec</a>]    \ Flight 6
                           &lt;--------             Finished    /

               Figure 1. Message Flights for Full Handshake


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

   ClientHello             --------&gt;                          Flight 1

                                              ServerHello    \
                                       [<a href="#ref-ChangeCipherSpec">ChangeCipherSpec</a>]     Flight 2
                            &lt;--------             Finished    /

   [<a id="ref-ChangeCipherSpec">ChangeCipherSpec</a>]                                         \Flight 3
   Finished                 --------&gt;                         /

         Figure 2. Message Flights for Session-Resuming Handshake
                           (No Cookie Exchange)

   DTLS uses a simple timeout and retransmission scheme with the
   following state machine.  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.





<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


                      +-----------+
                      | PREPARING |
                +---&gt; |           | &lt;--------------------+
                |     |           |                      |
                |     +-----------+                      |
                |           |                            |
                |           | Buffer next flight         |
                |           |                            |
                |          \|/                           |
                |     +-----------+                      |
                |     |           |                      |
                |     |  SENDING  |&lt;------------------+  |
                |     |           |                   |  | Send
                |     +-----------+                   |  | HelloRequest
        Receive |           |                         |  |
           next |           | Send flight             |  | or
         flight |  +--------+                         |  |
                |  |        | Set retransmit timer    |  | Receive
                |  |       \|/                        |  | HelloRequest
                |  |  +-----------+                   |  | Send
                |  |  |           |                   |  | ClientHello
                +--)--|  WAITING  |-------------------+  |
                |  |  |           |   Timer expires   |  |
                |  |  +-----------+                   |  |
                |  |         |                        |  |
                |  |         |                        |  |
                |  |         +------------------------+  |
                |  |                Read retransmit      |
        Receive |  |                                     |
           last |  |                                     |
         flight |  |                                     |
                |  |                                     |
               \|/\|/                                    |
                                                         |
            +-----------+                                |
            |           |                                |
            | FINISHED  | -------------------------------+
            |           |
            +-----------+
                 |  /|\
                 |   |
                 |   |
                 +---+

              Read retransmit
           Retransmit last flight

          Figure 3. DTLS Timeout and Retransmission State Machine



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   The state machine has three basic states.

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

   In the SENDING state, the implementation transmits the buffered
   flight of messages.  Once the messages have been sent, the
   implementation then enters the FINISHED state if this is the last
   flight in the handshake.  Or, if the implementation expects to
   receive more messages, it sets a retransmit timer and then enters the
   WAITING state.

   There are three ways to exit the WAITING state:

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

   2. The implementation reads a retransmitted flight from the peer: the
      implementation transitions to the SENDING state, where it
      retransmits the flight, resets the retransmit timer, and returns
      to the WAITING state.  The rationale here is that the receipt of a
      duplicate message is the likely result of timer expiry on the peer
      and therefore suggests that part of one's previous flight was
      lost.

   3. The implementation receives 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) do
      not cause state transitions or timer resets.

   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.

   When the server desires a rehandshake, it transitions from the
   FINISHED state to the PREPARING state to transmit the HelloRequest.
   When the client receives a HelloRequest, it transitions from FINISHED
   to PREPARING to transmit the ClientHello.

   In addition, for at least twice the default MSL defined for [<a href="#ref-TCP" title="&quot;Transmission Control Protocol&quot;">TCP</a>],
   when in the FINISHED state, the node that transmits the last flight
   (the server in an ordinary handshake or the client in a resumed
   handshake) MUST respond to a retransmit of the peer's last flight



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   with a retransmit of the last flight.  This avoids deadlock
   conditions if the last flight gets lost.  This requirement applies to
   DTLS 1.0 as well, and though not explicit in [<a href="#ref-DTLS1" title="&quot;Datagram Transport Layer Security&quot;">DTLS1</a>], it was always
   required for the state machine to function correctly.  To see why
   this is necessary, consider what happens in an ordinary handshake if
   the server's Finished message is lost: the server believes the
   handshake is complete but it actually is not.  As the client is
   waiting for the Finished message, the client's retransmit timer will
   fire and it will retransmit the client's Finished message.  This will
   cause the server to respond with its own Finished message, completing
   the handshake.  The same logic applies on the server side for the
   resumed handshake.

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

<span class="h5"><a class="selflink" id="section-4.2.4.1" href="#section-4.2.4.1">4.2.4.1</a>. Timer Values</span>

   Though timer values are the choice of the implementation, mishandling
   of the timer can lead to serious congestion problems; for example, if
   many instances of a DTLS time out early and retransmit too quickly on
   a congested link.  Implementations SHOULD use an initial timer value
   of 1 second (the minimum defined in <a href="./rfc6298">RFC 6298</a> [<a href="./rfc6298" title="&quot;Computing TCP's Retransmission Timer&quot;">RFC6298</a>]) and double
   the value at each retransmission, up to no less than the <a href="./rfc6298">RFC 6298</a>
   maximum of 60 seconds.  Note that we recommend a 1-second timer
   rather than the 3-second <a href="./rfc6298">RFC 6298</a> default in order to improve latency
   for time-sensitive applications.  Because DTLS only uses
   retransmission for handshake and not dataflow, the effect on
   congestion should be minimal.

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








<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


<span class="h4"><a class="selflink" id="section-4.2.5" href="#section-4.2.5">4.2.5</a>.  ChangeCipherSpec</span>

   As with TLS, the ChangeCipherSpec message is not technically a
   handshake message but MUST be treated as part of the same flight as
   the associated Finished message for the purposes of timeout and
   retransmission.  This creates a potential ambiguity because the order
   of the ChangeCipherSpec cannot be established unambiguously with
   respect to the handshake messages in case of message loss.

   This is not a problem with any current TLS mode because the expected
   set of handshake messages logically preceeding the ChangeCipherSpec
   is predictable from the rest of the handshake state.  However, future
   modes MUST take care to avoid creating ambiguity.

<span class="h4"><a class="selflink" id="section-4.2.6" href="#section-4.2.6">4.2.6</a>.  CertificateVerify and Finished Messages</span>

   CertificateVerify and Finished messages have the same format as in
   TLS.  Hash calculations include entire handshake messages, including
   DTLS-specific fields: message_seq, fragment_offset, and
   fragment_length.  However, in order to remove sensitivity to
   handshake message fragmentation, the Finished MAC MUST be computed as
   if each handshake message had been sent as a single fragment.  Note
   that in cases where the cookie exchange is used, the initial
   ClientHello and HelloVerifyRequest MUST NOT be included in the
   CertificateVerify or Finished MAC computations.

<span class="h4"><a class="selflink" id="section-4.2.7" href="#section-4.2.7">4.2.7</a>.  Alert Messages</span>

   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.

<span class="h4"><a class="selflink" id="section-4.2.8" href="#section-4.2.8">4.2.8</a>.  Establishing New Associations with Existing Parameters</span>

   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



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   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.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Summary of New Syntax</span>

   This section includes specifications for the data structures that
   have changed between TLS 1.2 and DTLS 1.2. See [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>] for the
   definition of this syntax.

<span class="h4"><a class="selflink" id="section-4.3.1" href="#section-4.3.1">4.3.1</a>.  Record Layer</span>

   struct {
        ContentType type;
        ProtocolVersion version;
        uint16 epoch;                                     // New field
        uint48 sequence_number;                           // New field
        uint16 length;
        opaque fragment[DTLSPlaintext.length];
      } DTLSPlaintext;

      struct {
        ContentType type;
        ProtocolVersion version;
        uint16 epoch;                                     // New field
        uint48 sequence_number;                           // New field
        uint16 length;
        opaque fragment[DTLSCompressed.length];
      } DTLSCompressed;

      struct {
        ContentType type;
        ProtocolVersion version;
        uint16 epoch;                                     // New field
        uint48 sequence_number;                           // New field
        uint16 length;
        select (CipherSpec.cipher_type) {
          case block:  GenericBlockCipher;
          case aead:   GenericAEADCipher;                 // New field
        } fragment;
      } DTLSCiphertext;







<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


<span class="h4"><a class="selflink" id="section-4.3.2" href="#section-4.3.2">4.3.2</a>.  Handshake Protocol</span>

   enum {
     hello_request(0), client_hello(1), server_hello(2),
     hello_verify_request(3),                          // New field
     certificate(11), server_key_exchange (12),
     certificate_request(13), server_hello_done(14),
     certificate_verify(15), client_key_exchange(16),
     finished(20), (255) } HandshakeType;

   struct {
     HandshakeType msg_type;
     uint24 length;
     uint16 message_seq;                               // New field
     uint24 fragment_offset;                           // New field
     uint24 fragment_length;                           // New field
     select (HandshakeType) {
       case hello_request: HelloRequest;
       case client_hello:  ClientHello;
       case server_hello:  ServerHello;
       case hello_verify_request: HelloVerifyRequest;  // New field
       case certificate:Certificate;
       case server_key_exchange: ServerKeyExchange;
       case certificate_request: CertificateRequest;
       case server_hello_done:ServerHelloDone;
       case certificate_verify:  CertificateVerify;
       case client_key_exchange: ClientKeyExchange;
       case finished: Finished;
     } body; } Handshake;

   struct {
     ProtocolVersion client_version;
     Random random;
     SessionID session_id;
     opaque cookie&lt;0..2^8-1&gt;;                             // New field
     CipherSuite cipher_suites&lt;2..2^16-1&gt;;
     CompressionMethod compression_methods&lt;1..2^8-1&gt;; } ClientHello;

   struct {
     ProtocolVersion server_version;
     opaque cookie&lt;0..2^8-1&gt;; } HelloVerifyRequest;

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Security Considerations</span>

   This document describes a variant of TLS 1.2; therefore, most of the
   security considerations are the same as those of TLS 1.2 [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>],
   described in Appendices D, E, and F.




<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


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

   Unlike TLS implementations, DTLS implementations SHOULD NOT respond
   to invalid records by terminating the connection.  See <a href="#section-4.1.2.7">Section</a>
   <a href="#section-4.1.2.7">4.1.2.7</a> for details on this.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Acknowledgments</span>

   The authors would like to thank Dan Boneh, Eu-Jin Goh, Russ Housley,
   Constantine Sapuntzakis, and Hovav Shacham for discussions and
   comments on the design of DTLS.  Thanks to the anonymous NDSS
   reviewers of our original NDSS paper on DTLS [<a href="#ref-DTLS" title="&quot;The Design and Implementation of Datagram TLS&quot;">DTLS</a>] for their
   comments.  Also, thanks to Steve Kent for feedback that helped
   clarify many points.  The section on PMTU was cribbed from the DCCP
   specification [<a href="#ref-DCCP" title="&quot;Datagram Congestion Control Protocol (DCCP)&quot;">DCCP</a>].  Pasi Eronen provided a detailed review of this
   specification.  Peter Saint-Andre provided the list of changes in
   <a href="#section-8">Section 8</a>.  Helpful comments on the document were also received from
   Mark Allman, Jari Arkko, Mohamed Badra, Michael D'Errico, Adrian
   Farrell, Joel Halpern, Ted Hardie, Charlia Kaufman, Pekka Savola,
   Allison Mankin, Nikos Mavrogiannopoulos, Alexey Melnikov, Robin
   Seggelmann, Michael Tuexen, Juho Vaha-Herttua, and Florian Weimer.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IANA Considerations</span>

   This document uses the same identifier space as TLS [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>], so no
   new IANA registries are required.  When new identifiers are assigned
   for TLS, authors MUST specify whether they are suitable for DTLS.
   IANA has modified all TLS parameter registries to add a DTLS-OK flag,
   indicating whether the specification may be used with DTLS.  At the
   time of publication, all of the [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>] registrations except the
   following are suitable for DTLS.  The full table of registrations is
   available at [<a href="#ref-IANA" title="&quot;Transport Layer Security (TLS) Parameters&quot;">IANA</a>].

   From the TLS Cipher Suite Registry:

      0x00,0x03 TLS_RSA_EXPORT_WITH_RC4_40_MD5        [<a href="./rfc4346">RFC4346</a>]
      0x00,0x04 TLS_RSA_WITH_RC4_128_MD5              [<a href="./rfc5246">RFC5246</a>]
      0x00,0x05 TLS_RSA_WITH_RC4_128_SHA              [<a href="./rfc5246">RFC5246</a>]
      0x00,0x17 TLS_DH_anon_EXPORT_WITH_RC4_40_MD5    [<a href="./rfc4346">RFC4346</a>]



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


      0x00,0x18 TLS_DH_anon_WITH_RC4_128_MD5          [<a href="./rfc5246">RFC5246</a>]
      0x00,0x20 TLS_KRB5_WITH_RC4_128_SHA             [<a href="./rfc2712">RFC2712</a>]
      0x00,0x24 TLS_KRB5_WITH_RC4_128_MD5             [<a href="./rfc2712">RFC2712</a>]
      0x00,0x28 TLS_KRB5_EXPORT_WITH_RC4_40_SHA       [<a href="./rfc2712">RFC2712</a>]
      0x00,0x2B TLS_KRB5_EXPORT_WITH_RC4_40_MD5       [<a href="./rfc2712">RFC2712</a>]
      0x00,0x8A TLS_PSK_WITH_RC4_128_SHA              [<a href="./rfc4279">RFC4279</a>]
      0x00,0x8E TLS_DHE_PSK_WITH_RC4_128_SHA          [<a href="./rfc4279">RFC4279</a>]
      0x00,0x92 TLS_RSA_PSK_WITH_RC4_128_SHA          [<a href="./rfc4279">RFC4279</a>]
      0xC0,0x02 TLS_ECDH_ECDSA_WITH_RC4_128_SHA       [<a href="./rfc4492">RFC4492</a>]
      0xC0,0x07 TLS_ECDHE_ECDSA_WITH_RC4_128_SHA      [<a href="./rfc4492">RFC4492</a>]
      0xC0,0x0C TLS_ECDH_RSA_WITH_RC4_128_SHA         [<a href="./rfc4492">RFC4492</a>]
      0xC0,0x11 TLS_ECDHE_RSA_WITH_RC4_128_SHA        [<a href="./rfc4492">RFC4492</a>]
      0xC0,0x16 TLS_ECDH_anon_WITH_RC4_128_SHA        [<a href="./rfc4492">RFC4492</a>]
      0xC0,0x33 TLS_ECDHE_PSK_WITH_RC4_128_SHA        [<a href="./rfc5489">RFC5489</a>]

   From the TLS Exporter Label Registry:

      client EAP encryption       [<a href="./rfc5216">RFC5216</a>]
      ttls   keying material      [<a href="./rfc5281">RFC5281</a>]
      ttls   challenge            [<a href="./rfc5281">RFC5281</a>]

   This document defines a new handshake message, hello_verify_request,
   whose value has been allocated from the TLS HandshakeType registry
   defined in [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>].  The value "3" has been assigned by the IANA.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Changes since DTLS 1.0</span>

   This document reflects the following changes since DTLS 1.0 [<a href="#ref-DTLS1" title="&quot;Datagram Transport Layer Security&quot;">DTLS1</a>].

   -  Updated to match TLS 1.2 [<a href="#ref-TLS12" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.2&quot;">TLS12</a>].

   -  Addition of AEAD Ciphers in <a href="#section-4.1.2.3">Section 4.1.2.3</a> (tracking changes in
      TLS 1.2.

   -  Clarifications regarding sequence numbers and epochs in <a href="#section-4.1">Section</a>
      <a href="#section-4.1">4.1</a> and a clear procedure for dealing with state loss in <a href="#section-4.2.8">Section</a>
      <a href="#section-4.2.8">4.2.8</a>.

   -  Clarifications and more detailed rules regarding Path MTU issues
      in <a href="#section-4.1.1.1">Section 4.1.1.1</a>. Clarification of the fragmentation text
      throughout.

   -  Clarifications regarding handling of invalid records in <a href="#section-4.1.2.7">Section</a>
      <a href="#section-4.1.2.7">4.1.2.7</a>.

   -  A new paragraph describing handling of invalid cookies at the end
      of <a href="#section-4.2.1">Section 4.2.1</a>.




<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   -  Some new text describing how to avoid handshake deadlock
      conditions at the end of <a href="#section-4.2.4">Section 4.2.4</a>.

   -  Some new text about CertificateVerify messages in <a href="#section-4.2.6">Section 4.2.6</a>.

   -  A prohibition on epoch wrapping in <a href="#section-4.1">Section 4.1</a>.

   -  Clarification of the IANA requirements and the explicit
      requirement for a new IANA registration flag for each parameter.

   -  Added a record sequence number mirroring technique for handling
      repeated ClientHello messages.

   -  Recommend a fixed version number for HelloVerifyRequest.

   -  Numerous editorial changes.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  References</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Normative References</span>

   [<a id="ref-REQ">REQ</a>]       Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC1191">RFC1191</a>]   Mogul, J. and S. Deering, "Path MTU discovery", <a href="./rfc1191">RFC 1191</a>,
               November 1990.

   [<a id="ref-RFC4301">RFC4301</a>]   Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", <a href="./rfc4301">RFC 4301</a>, December 2005.

   [<a id="ref-RFC4443">RFC4443</a>]   Conta, A., Deering, S., and M. Gupta, Ed., "Internet
               Control Message Protocol (ICMPv6) for the Internet
               Protocol Version 6 (IPv6) Specification", <a href="./rfc4443">RFC 4443</a>, March
               2006.

   [<a id="ref-RFC4821">RFC4821</a>]   Mathis, M. and J. Heffner, "Packetization Layer Path MTU
               Discovery", <a href="./rfc4821">RFC 4821</a>, March 2007.

   [<a id="ref-RFC6298">RFC6298</a>]   Paxson, V., Allman, M., Chu, J., and M. Sargent,
               "Computing TCP's Retransmission Timer", <a href="./rfc6298">RFC 6298</a>, June
               2011.

   [<a id="ref-RSAGCM">RSAGCM</a>]    Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
               Counter Mode (GCM) Cipher Suites for TLS", <a href="./rfc5288">RFC 5288</a>,
               August 2008.

   [<a id="ref-TCP">TCP</a>]       Postel, J., "Transmission Control Protocol", STD 7, <a href="./rfc793">RFC</a>
               <a href="./rfc793">793</a>, September 1981.



<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   [<a id="ref-TLS12">TLS12</a>]     Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>, August 2008.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Informative References</span>

   [<a id="ref-DCCP">DCCP</a>]      Kohler, E., Handley, M., and S. Floyd, "Datagram
               Congestion Control Protocol (DCCP)", <a href="./rfc4340">RFC 4340</a>, March
               2006.

   [<a id="ref-DCCPDTLS">DCCPDTLS</a>]  Phelan, T., "Datagram Transport Layer Security (DTLS)
               over the Datagram Congestion Control Protocol (DCCP)",
               <a href="./rfc5238">RFC 5238</a>, May 2008.

   [<a id="ref-DTLS">DTLS</a>]      Modadugu, N. and E. Rescorla, "The Design and
               Implementation of Datagram TLS", Proceedings of ISOC NDSS
               2004, February 2004.

   [<a id="ref-DTLS1">DTLS1</a>]     Rescorla, E. and N. Modadugu, "Datagram Transport Layer
               Security", <a href="./rfc4347">RFC 4347</a>, April 2006.

   [<a id="ref-ECCGCM">ECCGCM</a>]    Rescorla, E., "TLS Elliptic Curve Cipher Suites with
               SHA-256/384 and AES Galois Counter Mode (GCM)", <a href="./rfc5289">RFC 5289</a>,
               August 2008.

   [<a id="ref-ESP">ESP</a>]       Kent, S., "IP Encapsulating Security Payload (ESP)", <a href="./rfc4303">RFC</a>
               <a href="./rfc4303">4303</a>, December 2005.

   [<a id="ref-IANA">IANA</a>]      IANA, "Transport Layer Security (TLS) Parameters",
               <a href="http://www.iana.org/assignments/tls-parameters">http://www.iana.org/assignments/tls-parameters</a>.

   [<a id="ref-IKEv2">IKEv2</a>]     Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
               "Internet Key Exchange Protocol Version 2 (IKEv2)", <a href="./rfc5996">RFC</a>
               <a href="./rfc5996">5996</a>, September 2010.

   [<a id="ref-IMAP">IMAP</a>]      Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
               4rev1", <a href="./rfc3501">RFC 3501</a>, March 2003.

   [<a id="ref-PHOTURIS">PHOTURIS</a>]  Karn, P. and W. Simpson, "Photuris: Session-Key
               Management Protocol", <a href="./rfc2522">RFC 2522</a>, March 1999.

   [<a id="ref-POP">POP</a>]       Myers, J. and M. Rose, "Post Office Protocol - Version
               3", STD 53, <a href="./rfc1939">RFC 1939</a>, May 1996.

   [<a id="ref-SIP">SIP</a>]       Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
               A., Peterson, J., Sparks, R., Handley, M., and E.
               Schooler, "SIP: Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>,
               June 2002.




<span class="grey">Rescorla &amp; Modadugu          Standards Track                   [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc6347">RFC 6347</a>                          DTLS                      January 2012</span>


   [<a id="ref-TLS">TLS</a>]       Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
               <a href="./rfc2246">RFC 2246</a>, January 1999.

   [<a id="ref-TLS11">TLS11</a>]     Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.1", <a href="./rfc4346">RFC 4346</a>, April 2006.

   [<a id="ref-WHYIPSEC">WHYIPSEC</a>]  Bellovin, S., "Guidelines for Specifying the Use of IPsec
               Version 2", <a href="https://www.rfc-editor.org/bcp/bcp146">BCP 146</a>, <a href="./rfc5406">RFC 5406</a>, February 2009.

Authors' Addresses

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA 94303

   EMail: ekr@rtfm.com


   Nagendra Modadugu
   Google, Inc.

   EMail: nagendra@cs.stanford.edu




























Rescorla &amp; Modadugu          Standards Track                   [Page 32]
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