INTERNET-DRAFT                                               E. Rescorla
Obsoletes (if approved): RFC 4347                             RTFM, Inc.
Intended Status: Proposed Standard                           N. Modadugu
<draft-ietf-tls-rfc4347-bis-02.txt>                  Stanford University
                                  March 7, 2009 (Expires September 2009)

             Datagram Transport Layer Security version 1.2

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  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
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   Internet-Drafts are working documents of the Internet Engineering
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      Copyright (c) 2009 IETF Trust and the persons identified as the
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      This document is subject to BCP 78 and the IETF Trust's Legal

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      Provisions Relating to IETF Documents in effect on the date of
      publication of this document (
      Please review these documents carefully, as they describe your rights
      and restrictions with respect to this document.


   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.


      This documents and the information contained therein are provided on

Table of Contents

   1.        Introduction                                             3
   1.1.      Requirements Terminology                                 4
   2.        Usage Model                                              4
   3.        Overview of DTLS                                         5
   3.1.      Loss-Insensitive Messaging                               5
   3.2.      Providing Reliability for Handshake                      5
   3.2.1.    Packet Loss                                              6
   3.2.2.    Reordering                                               6
   3.2.3.    Message Size                                             6
   3.3.      Replay Detection                                         7
   4.        Differences from TLS                                     7
   4.1.      Record Layer                                             7
   4.1.1.    Transport Layer Mapping                                  9  PMTU Issues                                              10
   4.1.2.    Record Payload Protection                                11  MAC                                                      11  Null or Standard Stream Cipher                           12  Block Cipher                                             12

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draft-ietf-tls-rfc4347-bis-00     DTLS                         June 2008  AEAD Ciphers                                             12  New Cipher Suites                                        12  Anti-replay                                              13
   4.2.      The DTLS Handshake Protocol                              13
   4.2.1.    Denial of Service Countermeasures                        14
   4.2.2.    Handshake Message Format                                 16
   4.2.3.    Message Fragmentation and Reassembly                     18
   4.2.4.    Timeout and Retransmission                               18  Timer Values                                             22
   4.2.5.    ChangeCipherSpec                                         22
   4.2.6.    CertificateVerify and Finished Messages                  22
   4.2.7.    Alert Messages                                           22
   4.3.      Summary of new syntax                                    23
   4.3.1.    Record Layer                                             24
   4.3.2.    Handshake Protocol                                       24
   5.        Security Considerations                                  25
   6.        Acknowledgements                                         26
   7.        IANA Considerations                                      26
   8.        References                                               26
   8.1.      Normative References                                     26
   8.2.      Informative References                                   27

1. Introduction

   TLS [TLS] 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 [IMAP] and POP [POP].  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 [TCP].
   It therefore cannot be used to secure unreliable datagram traffic.

   However, an increasing number of application layer protocols have
   been designed that use UDP transport.  In particular protocols such
   as the Session Initiation Protocol (SIP) [SIP] 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
   [RFC4301].  However, for a number of reasons detailed in [WHYIPSEC],
   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 requires a large amount of effort to
   design -- in contrast to the relatively small amount of effort

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   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 [DTLS1] was originally defined as a delta from [TLS11]. This
   document introduces a new version of DTLS, DTLS 1.2, which is defined
   as a series of deltas to TLS 1.2 [TLS12] 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.

1.1. Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [REQ].

2. Usage Model

   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.

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3. Overview of DTLS

   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, and 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's traffic encryption layer does not allow independent
      decryption of individual records.  If record N is not received,
      then record N+1 cannot be decrypted.

      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.

3.1. Loss-Insensitive Messaging

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

      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.

3.2. Providing Reliability for Handshake

   The TLS handshake is a lockstep cryptographic handshake.  Messages
   must be transmitted and received in a defined order, and 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

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   fragmentation.  DTLS must provide fixes for both of these problems.

3.2.1. Packet Loss

   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           ------>

                              X<-- HelloVerifyRequest

      [Timer Expires]

      ClientHello           ------>

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

   Note: timeout and retransmission do not apply to the
   HelloVerifyRequest, because this requires creating state on the

3.2.2. Reordering

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

3.2.3. Message Size

   TLS and DTLS handshake messages can be quite large (in theory up to
   2^24-1 bytes, in practice many kilobytes).  By contrast, UDP
   datagrams are often limited to <1500 bytes if fragmentation is not
   desired.  In order to compensate for this limitation, each DTLS
   handshake message may be fragmented over several DTLS records.  Each

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   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.

3.3. 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. Differences from TLS

   As mentioned in Section 3, 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 [TLS12].  Where we do not
   explicitly call out differences, DTLS is the same as in [TLS12].

4.1. Record Layer

   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;

       Equivalent to the type field in a TLS 1.2 record.

       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

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       version numbers of DTLS are decreasing in value (while the true
       version number is increasing in value.)

       A counter value that is incremented on every cipher state

       The sequence number for this record.

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

       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
   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 the ChangeCipherSpec messages 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 and we therefore 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 MAY choose to retain keying
   material from previous epochs for up to 120 seconds (the default TCP
   MSL) to allow for packet reordering.  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

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   handshake. For instance, the server may send its Finished 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

   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 CSS or Finished has 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 an
   other case, the implementation MUST wait for the receipt of the
   Finished to prevent downgrade attack.

4.1.1. Transport Layer Mapping

   Each DTLS record MUST fit within a single datagram.  In order to
   avoid fragmentation, that 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 host/port

   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 [DCCP] 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, and 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

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   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. [DCCPDTLS]
   defines a mapping of DTLS to DCCP that takes these issues into
   account. PMTU Issues

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

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

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

    - The DTLS handshake messages can exceed the PMTU.

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

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

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

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

   - 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.

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

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

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

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

   The final issue is the DTLS handshake protocol. From the perspective
   of the DTLS record layer, this is merely another upper layer
   protocol. However, DTLS handshakes occur infrequently and involve
   only a few round trips, and 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

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

4.1.2. 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. MAC

   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

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   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 data with
   bad MACs.  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 with level fatal and terminate its connection
   state. Null or Standard Stream Cipher

   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. Block Cipher

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

   TLS 1.2 introduced authenticated encryption with additional data
   (AEAD) cipher suites. The existing AEAD cipher suites, defined in
   [ECCGCM] and [RSAGCM] can be used with DTLS exactly as with TLS 1.2. New Cipher Suites

   Upon registration, new TLS cipher suites MUST indicate whether they
   are suitable for DTLS usage and what, if any, adaptations must be

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   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 [ESP].

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

   The "right" edge of the window represents the highest validated
   Sequence Number value received on 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 [ESP].

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

4.2. The DTLS Handshake Protocol

   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.

      2. Modifications to the handshake header to handle message loss,
      reordering, and fragmentation.

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      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.

4.2.1. Denial of Service Countermeasures

   Datagram security protocols are extremely susceptible to a variety of
   denial of service (DoS) attacks.  Two attacks are of particular

      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 [PHOTURIS] and IKE [IKEv2].  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 [PHOTURIS].  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.

   The exchange is shown below:

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

                               <----- HelloVerifyRequest
                                      (contains cookie)

         ClientHello           ------>
         (with cookie)

         [Rest of handshake]

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   DTLS therefore modifies the ClientHello message to add the cookie

      struct {
        ProtocolVersion client_version;
        Random random;
        SessionID session_id;
        opaque cookie<0..2^8-1>;                             // New field
        CipherSuite cipher_suites<2..2^16-1>;
        CompressionMethod compression_methods<1..2^8-1>;
      } 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<0..2^8-1>;
      } HelloVerifyRequest;

   The HelloVerifyRequest message type is hello_verify_request(3).

   The server_version field is defined as in TLS.

   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.

   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.

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   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. [IKEv2] 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
   choose not to do a cookie exchange when a session is resumed.
   Clients MUST be prepared to do a cookie exchange with every

   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 results in clients receiving multiple
   HelloVerifyRequest messages with different cookies.  Clients SHOULD
   handle this by sending a new HelloVerify in response to the new

4.2.2. Handshake Message Format

   In order to support message loss, reordering, and 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) {

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          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.  When a message is
   retransmitted, the same message_seq value is used.  For example:

      Client                             Server
      ------                             ------
      ClientHello (seq=0)  ------>

                              X<-- HelloVerifyRequest (seq=0)

      [Timer Expires]

      ClientHello (seq=0)  ------>

                           <------ HelloVerifyRequest (seq=0)

      ClientHello (seq=1)  ------>
      (with cookie)

                           <------        ServerHello (seq=1)
                           <------        Certificate (seq=2)
                           <------    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.

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   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).

4.2.3. Message Fragmentation and Reassembly

   As noted in Section 4.1.1, 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.

   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 labelled 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.

4.2.4. Timeout and Retransmission

   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.

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

    ClientHello             -------->                           Flight 1

                            <-------    HelloVerifyRequest      Flight 2

   ClientHello              -------->                           Flight 3

                                               ServerHello    \
                                              Certificate*     \
                                        ServerKeyExchange*      Flight 4
                                       CertificateRequest*     /
                            <--------      ServerHelloDone    /

    Certificate*                                              \
    ClientKeyExchange                                          \
    CertificateVerify*                                          Flight 5
    [ChangeCipherSpec]                                         /
    Finished                -------->                         /

                                        [ChangeCipherSpec]    \ Flight 6
                            <--------             Finished    /

          Figure 1. Message flights for full handshake

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

    ClientHello             -------->                          Flight 1

                                               ServerHello    \
                                        [ChangeCipherSpec]     Flight 2
                             <--------             Finished    /

    [ChangeCipherSpec]                                         \Flight 3
    Finished                 -------->                         /

   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.

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                   | PREPARING |
             +---> |           | <--------------------+
             |     |           |                      |
             |     +-----------+                      |
             |           |                            |
             |           |                            |
             |           | Buffer next flight         |
             |           |                            |
             |          \|/                           |
             |     +-----------+                      |
             |     |           |                      |
             |     |  SENDING  |<------------------+  |
             |     |           |                   |  | 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

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draft-ietf-tls-rfc4347-bis-00     DTLS                         June 2008

        Figure 3. DTLS timeout and retransmission state machine

   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

      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
   2MSL, when in the FINISHED state, the node which 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

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   flight 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 [DTLS1] but was always
   required for the state machine to function correctly. Timer Values

   Though timer values are the choice of the implementation, mishandling
   of the timer can lead to serious congestion problems; for example, if
   many instances of a DTLS time out early and retransmit too quickly on
   a congested link.  Implementations SHOULD use an initial timer value
   of 1 second (the minimum defined in RFC 2988 [RFC2988]) and double
   the value at each retransmission, up to no less than the RFC 2988
   maximum of 60 seconds.  Note that we recommend a 1-second timer
   rather than the 3-second RFC 2988 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.

4.2.5. ChangeCipherSpec

   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

4.2.6. CertificateVerify and Finished Messages

   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
   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.

4.2.7. Alert Messages

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   Note that Alert messages are not retransmitted at all, even when they
   occur in the context of a handshake.  However, a DTLS implementation
   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.

4.3. Summary of new syntax

   This section includes specifications for the data structures that
   have changed between TLS 1.2 and DTLS.

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4.3.1. Record Layer

      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;
        } fragment;
      } DTLSCiphertext;

4.3.2. Handshake Protocol

      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

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        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<0..2^8-1>;                             // New field
        CipherSuite cipher_suites<2..2^16-1>;
        CompressionMethod compression_methods<1..2^8-1>;
      } ClientHello;

      struct {
        ProtocolVersion server_version;
        opaque cookie<0..2^8-1>;
      } HelloVerifyRequest;

5. Security Considerations

   This document describes a variant of TLS 1.2 and therefore most of
   the security considerations are the same as those of TLS 1.2 [TLS12],
   described in Appendices D, E, and F.

   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 which do
   not use this cookie exchange are still vulnerable to DoS.  In
   particular, DTLS servers which 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.

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6. Acknowledgements

   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 [DTLS] 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 [DCCP].  Pasi Eronen provided a detailed review of this
   specification.  Helpful comments on the document were also received
   from Mark Allman, Jari Arkko, Joel Halpern, Ted Hardie, and Allison

7. IANA Considerations

   This document uses the same identifier space as TLS [TLS12], so no
   new IANA registries are required.  When new identifiers are assigned
   for TLS, authors MUST specify whether they are suitable for DTLS.

   This document defines a new handshake message, hello_verify_request,
   whose value has been allocated from the TLS HandshakeType registry
   defined in [TLS12].  The value "3" has been assigned by the IANA.

8. References

8.1. Normative References

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

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

   [RSAGCM]   Salowey, J., Choudhury, A., and D. McGrew, "AES-GCM Cipher
              Suites for TLS", RFC 5288, August 2008.

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

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draft-ietf-tls-rfc4347-bis-00     DTLS                         June 2008

   [TLS12]    Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, May 2008.

8.2. Informative References

   [DCCP]     Kohler, E., Handley, M., Floyd, S., Padhye, J., "Datagram
              Congestion Control Protocol", Work in Progress, 10 March

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

   [DTLS]     Modadugu, N., Rescorla, E., "The Design and Implementation
              of Datagram TLS", Proceedings of ISOC NDSS 2004, February

   [DTLS1]    Rescorla, E., and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [ECCGCM]   E. Rescorla, "TLS Elliptic Curve Cipher Suites with
              SHA-256/384 and AES Galois Counter Mode", RFC 5289, August

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

   [IKEv2]    C. Kaufman (ed), "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306,
              December 2005.

              4rev1", RFC 3501, March 2003.

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

   [POP]      Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

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

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   [SIP]      Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [TLS]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [TLS11]    Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [WHYIPSEC] Bellovin, S., "Guidelines for Mandating the Use of IPsec",
              Work in Progress, October 2003.

Authors' Addresses

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


   Nagendra Modadugu
   Computer Science Department
   Stanford University
   353 Serra Mall
   Stanford, CA 94305


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