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Datagram Transport Layer Security (DTLS) over Stream Control Transmission Protocol (SCTP)
draft-ietf-tsvwg-dtls-over-sctp-bis-04

Document Type Active Internet-Draft (tsvwg WG)
Authors Magnus Westerlund , John Preuß Mattsson , Claudio Porfiri
Last updated 2022-06-23
Replaces draft-westerlund-tsvwg-dtls-over-sctp-bis
Stream Internet Engineering Task Force (IETF)
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Revised I-D Needed - Issue raised by WG
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Mar 2023
Submit "DTLS over SCTP" as a Proposed Standard RFC
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Send notices to gorry@erg.abdn.ac.uk
draft-ietf-tsvwg-dtls-over-sctp-bis-04
TSVWG                                                      M. Westerlund
Internet-Draft                                         J. Preuß Mattsson
Intended status: Standards Track                              C. Porfiri
Expires: 25 December 2022                                       Ericsson
                                                            23 June 2022

      Datagram Transport Layer Security (DTLS) over Stream Control
                      Transmission Protocol (SCTP)
                 draft-ietf-tsvwg-dtls-over-sctp-bis-04

Abstract

   This document describes the usage of the Datagram Transport Layer
   Security (DTLS) protocol to protect user messages sent over the
   Stream Control Transmission Protocol (SCTP).  It is an improved
   alternative to the existing rfc6083.

   DTLS over SCTP provides mutual authentication, confidentiality,
   integrity protection, and replay protection for applications that use
   SCTP as their transport protocol and allows client/server
   applications to communicate in a way that is designed to give
   communications privacy and to prevent eavesdropping and detect
   tampering or message forgery.

   Applications using DTLS over SCTP can use almost all transport
   features provided by SCTP and its extensions.  This document is an
   improved alternative to RFC 6083 and removes the 16 kB limitation on
   protected user message size by defining a secure user message
   fragmentation so that multiple DTLS records can be used to protect a
   single user message.  It further updates the DTLS versions to use, as
   well as the HMAC algorithms for SCTP-AUTH, and simplifies secure
   implementation by some stricter requirements on the establishment
   procedures.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/gloinul/draft-westerlund-tsvwg-dtls-over-sctp-bis.

Status of This Memo

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

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   This Internet-Draft will expire on 25 December 2022.

Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
       1.1.1.  Comparison with TLS over SCTP . . . . . . . . . . . .   5
       1.1.2.  Changes from RFC 6083 . . . . . . . . . . . . . . . .   5
     1.2.  DTLS Version  . . . . . . . . . . . . . . . . . . . . . .   6
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.4.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   7
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  DTLS Considerations . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Version of DTLS . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Cipher Suites and Cryptographic Parameters  . . . . . . .   8
     3.3.  Message Sizes . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Replay Protection . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Path MTU Discovery  . . . . . . . . . . . . . . . . . . .  10
     3.6.  Retransmission of Messages  . . . . . . . . . . . . . . .  10
   4.  SCTP Considerations . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Mapping of DTLS Records . . . . . . . . . . . . . . . . .  10
     4.2.  DTLS Connection Handling  . . . . . . . . . . . . . . . .  13
     4.3.  Payload Protocol Identifier Usage . . . . . . . . . . . .  13
     4.4.  Stream Usage  . . . . . . . . . . . . . . . . . . . . . .  14

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     4.5.  Chunk Handling  . . . . . . . . . . . . . . . . . . . . .  15
     4.6.  SCTP-AUTH Hash Function . . . . . . . . . . . . . . . . .  15
     4.7.  Parallel DTLS connections . . . . . . . . . . . . . . . .  15
     4.8.  Renegotiation and KeyUpdate . . . . . . . . . . . . . . .  18
       4.8.1.  DTLS 1.2 Considerations . . . . . . . . . . . . . . .  18
       4.8.2.  DTLS 1.3 Considerations . . . . . . . . . . . . . . .  19
     4.9.  Handling of Endpoint-Pair Shared Secrets  . . . . . . . .  19
       4.9.1.  DTLS 1.2 Considerations . . . . . . . . . . . . . . .  19
       4.9.2.  DTLS 1.3 Considerations . . . . . . . . . . . . . . .  20
     4.10. Shutdown  . . . . . . . . . . . . . . . . . . . . . . . .  20
   5.  DTLS/SCTP Control Message . . . . . . . . . . . . . . . . . .  21
     5.1.  SHUTDOWN-Request  . . . . . . . . . . . . . . . . . . . .  22
     5.2.  Ready To Close Indication . . . . . . . . . . . . . . . .  22
   6.  DTLS over SCTP Service  . . . . . . . . . . . . . . . . . . .  22
     6.1.  Adaptation Layer Indication in INIT/INIT-ACK  . . . . . .  22
     6.2.  DTLS over SCTP Initialization . . . . . . . . . . . . . .  22
     6.3.  Client Use Case . . . . . . . . . . . . . . . . . . . . .  23
     6.4.  Server Use Case . . . . . . . . . . . . . . . . . . . . .  23
     6.5.  RFC 6083 Fallback . . . . . . . . . . . . . . . . . . . .  24
       6.5.1.  Client Fallback . . . . . . . . . . . . . . . . . . .  24
       6.5.2.  Server Fallback . . . . . . . . . . . . . . . . . . .  25
   7.  SCTP API Consideration  . . . . . . . . . . . . . . . . . . .  25
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     8.1.  TLS Exporter Label  . . . . . . . . . . . . . . . . . . .  26
     8.2.  SCTP Adaptation Layer Indication Code Point . . . . . . .  26
     8.3.  SCTP Payload Protocol Identifiers . . . . . . . . . . . .  26
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
     9.1.  Cryptographic Considerations  . . . . . . . . . . . . . .  27
     9.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .  29
     9.3.  Targeting DTLS Messages . . . . . . . . . . . . . . . . .  29
     9.4.  Authentication and Policy Decisions . . . . . . . . . . .  29
     9.5.  Resumption and Tickets  . . . . . . . . . . . . . . . . .  30
     9.6.  Privacy Considerations  . . . . . . . . . . . . . . . . .  30
     9.7.  Pervasive Monitoring  . . . . . . . . . . . . . . . . . .  30
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  31
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     12.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Appendix A.  Motivation for Changes . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

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1.1.  Overview

   This document describes the usage of the Datagram Transport Layer
   Security (DTLS) protocol, as defined in DTLS 1.2 [RFC6347], and DTLS
   1.3 [RFC9147], over the Stream Control Transmission Protocol (SCTP),
   as defined in [RFC9260] with Authenticated Chunks for SCTP (SCTP-
   AUTH) [RFC4895].

   This specification provides mutual authentication of endpoints,
   confidentiality, integrity protection, and replay protection of user
   messages for applications that use SCTP as their transport protocol.
   Thus, it allows client/server applications to communicate in a way
   that is designed to give communications privacy and to prevent
   eavesdropping and detect tampering or message forgery.  DTLS/SCTP
   uses DTLS for mutual authentication, key exchange with forward
   secrecy for SCTP-AUTH, and confidentiality of user messages.  DTLS/
   SCTP use SCTP and SCTP-AUTH for integrity protection and replay
   protection of all SCTP Chunks that can be authenticated, including
   user messages.

   Applications using DTLS over SCTP can use almost all transport
   features provided by SCTP and its extensions.  DTLS/SCTP supports:

   *  preservation of message boundaries.

   *  a large number of unidirectional and bidirectional streams.

   *  ordered and unordered delivery of SCTP user messages.

   *  the partial reliability extension as defined in [RFC3758].

   *  the dynamic address reconfiguration extension as defined in
      [RFC5061].

   *  User messages of any size.

   The method described in this document requires that the SCTP
   implementation supports the optional feature of fragmentation of SCTP
   user messages as defined in [RFC9260].  The implementation is
   required to have an SCTP API (for example the one described in
   [RFC6458]) that supports partial user message delivery and also
   recommended that I-DATA chunks as defined in [RFC8260] is used to
   efficiently implement and support larger user messages.

   To simplify implementation and reduce the risk for security holes,
   limitations have been defined such that STARTTLS as specified in
   [RFC3788] is no longer supported.

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1.1.1.  Comparison with TLS over SCTP

   TLS, from which DTLS was derived, is designed to run on top of a
   byte-stream-oriented transport protocol providing a reliable, in-
   sequence delivery.  TLS over SCTP as described in [RFC3436] has some
   serious limitations:

   *  It does not support the unordered delivery of SCTP user messages.

   *  It does not support partial reliability as defined in [RFC3758].

   *  It only supports the usage of the same number of streams in both
      directions.

   *  It uses a TLS connection for every bidirectional stream, which
      requires a substantial amount of resources and message exchanges
      if a large number of streams is used.

1.1.2.  Changes from RFC 6083

   The DTLS over SCTP solution defined in RFC 6083 had the following
   limitations:

   *  The maximum user message size is 2^14 (16384) bytes, which is a
      single DTLS record limit.

   *  DTLS 1.0 has been deprecated for RFC 6083 requiring at least DTLS
      1.2 [RFC8996].  This creates additional limitation as discussed in
      Section 1.2.

   *  DTLS messages that don't contain protected user message data where
      limited to only be sent on Stream 0 and requiring that stream to
      be in-order delivery which could potentially impact applications.

   This specification defines the following changes compared with RFC
   6083:

   *  Removes the limitations on user messages sizes by defining a
      secure fragmentation mechanism.  It is optional to support message
      sizes over 2^64-1 bytes.

   *  Enable DTLS key-change without requiring draining all inflight
      user message from SCTP.

   *  Mandates that more modern DTLS version are used (DTLS 1.2 or 1.3)

   *  Mandates support of modern HMAC algorithm (SHA-256) in the SCTP
      authentication extension [RFC4895].

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   *  Recommends support of [RFC8260] to enable interleaving of large
      SCTP user messages to avoid scheduling issues.

   *  Applies stricter requirements on always using DTLS for all user
      messages in the SCTP association.

   *  Requires that SCTP-AUTH is applied to all SCTP Chunks that can be
      authenticated.

   *  Requires support of partial delivery of user messages.

1.2.  DTLS Version

   Using DTLS 1.2 instead of using DTLS 1.0 limits the lifetime of a
   DTLS connection and the data volume which can be transferred over a
   DTLS connection.  This is caused by:

   *  The number of renegotiations in DTLS 1.2 is limited to 65534
      compared to unlimited in DTLS 1.0.

   *  While the AEAD limits in DTLS 1.3 does not formally apply to DTLS
      1.2 the mathematical limits apply equally well to DTLS 1.2.

   DTLS 1.3 comes with a large number of significant changes.

   *  Renegotiations are not supported and instead partly replaced by
      KeyUpdates.  The number of KeyUpdates is limited to 2^48.

   *  Strict AEAD significantly limits on how much many packets can be
      sent before rekeying.

   Many applications using DTLS/SCTP are of semi-permanent nature and
   use SCTP associations with expected lifetimes of months or even
   years, and where there is a significant cost of bringing down the
   SCTP association in order to restart it.  Such DTLS/SCTP usages that
   need:

   *  Periodic re-authentication and transfer of revocation information
      of both endpoints (not only the DTLS client).

   *  Periodic rerunning of Diffie-Hellman key-exchange to provide
      forward secrecy and mitigate static key exfiltration attacks.

   *  Perform SCTP-AUTH rekeying.

   At the time of publication DTLS 1.3 does not support any of these,
   where DTLS 1.2 renegotiation functionality can provide this
   functionality in the context of DTLS/SCTP.  To address these

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   requirements from semi-permanent applications, this document use
   several overlapping DTLS connections with either DTLS 1.2 or 1.3.
   Having uniform procedures reduces the impact when upgrading from 1.2
   to 1.3 and avoids using the renegotiation mechanism which is disabled
   by default in many DTLS implementations.

   To address known vulnerabilities in DTLS 1.2 this document describes
   and mandates implementation constraints on ciphers and protocol
   options.  The DTLS 1.2 renegotiation mechanism is forbidden to be
   used as it creates need for additional mechanism to handle race
   conditions and interactions between using DTLS connections in
   parallel.

   Secure negotiation of the DTLS version is handled by the DTLS
   handshake.  If the endpoints do not support a common DTLS version the
   DTLS handshake will be aborted.

   In the rest of the document, unless the version of DTLS is
   specifically called out the text applies to both versions of DTLS.

1.3.  Terminology

   This document uses the following terms:

   Association: An SCTP association.

   Connection: An DTLS connection.  It is uniquely identified by a
   connection identifier.

   Stream: A unidirectional stream of an SCTP association.  It is
   uniquely identified by a stream identifier.

1.4.  Abbreviations

   AEAD: Authenticated Encryption with Associated Data

   DTLS: Datagram Transport Layer Security

   HMAC: Keyed-Hash Message Authentication Code

   MTU: Maximum Transmission Unit

   PPID: Payload Protocol Identifier

   SCTP: Stream Control Transmission Protocol

   SCTP-AUTH: Authenticated Chunks for SCTP

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   TCP: Transmission Control Protocol

   TLS: Transport Layer Security

   ULP: Upper Layer Protocol

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  DTLS Considerations

3.1.  Version of DTLS

   This document defines the usage of either DTLS 1.3 [RFC9147], or DTLS
   1.2 [RFC6347].  Earlier versions of DTLS MUST NOT be used (see
   [RFC8996]).  DTLS 1.3 is RECOMMENDED for security and performance
   reasons.  It is expected that DTLS/SCTP as described in this document
   will work with future versions of DTLS.

3.2.  Cipher Suites and Cryptographic Parameters

   For DTLS 1.2, the cipher suites forbidden by [RFC9113] MUST NOT be
   used.  For all versions of DTLS, cryptographic parameters giving
   confidentiality and forward secrecy MUST be used.

3.3.  Message Sizes

   DTLS/SCTP, automatically fragments and reassembles user messages.
   This specification defines how to fragment the user messages into
   DTLS records, where each DTLS record allows a maximum of 2^14
   protected bytes.  Each DTLS record adds some overhead, thus using
   records of maximum possible size are recommended to minimize the
   transmitted overhead.  DTLS 1.3 has much less overhead than DTLS 1.2
   per record.

   The sequence of DTLS records is then fragmented into DATA or I-DATA
   Chunks to fit the path MTU by SCTP.  These changes ensure that DTLS/
   SCTP has the same capability as SCTP to support user messages of any
   size.  However, to simplify implementations it is OPTIONAL to support
   user messages larger than 2^64-1 bytes.  This is to allow
   implementation to assume that 64-bit length fields and offset
   pointers will be sufficient.

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   The SCTP-API defined in [RFC6458] results in an implementation
   limitation when it comes to support any user message sizes.  That API
   does not allow the changing of the SCTP-AUTH key used to protect the
   sending of a particular user message.  Thus, user messages that will
   be transmitted over periods of time on the order or longer than the
   interval between rekeying can't be supported.  Beyond delaying the
   completion of a rekeying until the message has been transmitted, the
   session can deadlock if the DTLS connection used to protect this long
   user message reaches the limit of number of bytes transmitted with a
   particular key.  However, this is not an interoperability issue as it
   is the sender side's API that limits what can be sent and thus the
   sender implementation will have to address this issue.

   The security operations and reassembly process requires that the
   protected user message, i.e., with DTLS record overhead, is stored in
   the receiver's buffer.  This buffer space will thus put a limit on
   the largest size of plain text user message that can be transferred
   securely.  However, by mandating the use of the partial delivery of
   user messages from SCTP and assuming that no two messages received on
   the same stream are interleaved (as it is the case when using the API
   defined in [RFC6458]) the minimally required buffering prior to DTLS
   processing is a single DTLS record per used incoming stream.  This
   enables the DTLS/SCTP implementation to provide the Upper Layer
   Protocol (ULP) with each DTLS record's content when it has been
   decrypted and its integrity been verified enabling partial user
   message delivery to the ULP.  Implementations can trade-off buffer
   memory requirements in the DTLS layer with transport overhead by
   using smaller DTLS records.  However, for efficient operation and
   avoiding flow control stalls if user message fragments are not
   frequently and expiendtly moved to upper layer memory buffers, the
   receiver buffer needs to be larger.

   The DTLS/SCTP implementation is expected to behave very similar to
   just SCTP when it comes to handling of user messages and dealing with
   large user messages and their reassembly and processing.  Making it
   the ULP responsible for handling any resource contention related to
   large user messages.

3.4.  Replay Protection

   SCTP-AUTH [RFC4895] does not have explicit replay protection.
   However, the combination of SCTP-AUTH's protection of DATA or I-DATA
   chunks and SCTP user message handling will prevent third party
   attempts to inject or replay SCTP packets resulting in impact on the
   received protected user message.  In fact, this document's solution
   is dependent on SCTP-AUTH and SCTP to prevent reordering,
   duplication, and removal of the DTLS records within each protected
   user message.  This includes detection of changes to what DTLS

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   records start and end the SCTP user message, and removal of DTLS
   records before an increment to the epoch.  Without SCTP-AUTH, these
   would all have required explicit handling.

   DTLS optionally supports record replay detection.  Such replay
   detection could result in the DTLS layer dropping valid messages
   received outside of the DTLS replay window.  As DTLS/SCTP provides
   replay protection even without DTLS replay protection, the replay
   detection of DTLS MUST NOT be used.

3.5.  Path MTU Discovery

   DTLS Path MTU Discovery MUST NOT be used.  Since SCTP provides Path
   MTU discovery and fragmentation/reassembly for user messages, and
   specified in Section 3.3, DTLS can send maximum sized DTLS Records.

3.6.  Retransmission of Messages

   SCTP provides a reliable and in-sequence transport service for DTLS
   messages that require it.  See Section 4.4.  Therefore, DTLS
   procedures for retransmissions MUST NOT be used.

4.  SCTP Considerations

4.1.  Mapping of DTLS Records

   The SCTP implementation MUST support fragmentation of user messages
   using DATA [RFC9260], and optionally I-DATA [RFC8260] chunks.

   DTLS/SCTP works as a shim layer between the user message API and
   SCTP.  On the sender side a user message is split into fragments m0,
   m1, m2, each no larger than 2^14 = 16384 bytes.

      m0 | m1 | m2 | ... = user_message

   The resulting fragments are protected with DTLS and the records are
   concatenated

      user_message' = DTLS( m0 ) | DTLS( m1 ) | DTLS( m2 ) ...

   The new user_message', i.e., the protected user message, is the input
   to SCTP.

   On the receiving side, the length field in each DTLS record can be
   used to determine the boundaries between DTLS records.  DTLS can
   decrypt individual records or a concatenated sequence of records.
   The last DTLS record can be found by subtracting the length of
   individual records from the length of user_message'.  Whether to

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   decrypt individual records, sequences of records, or the whole
   user_message' is left to the implementation.  The output from the
   DTLS decryption(s) is the fragments m0, m1, m2 ...  The user_message
   is reassembled from decrypted DTLS records as user_message = m0 |
   m1 | m2 ... There are three failure cases an implementation needs to
   detect and then act on:

   1.  Failure in decryption and integrity verification process of any
       DTLS record.  Due to SCTP-AUTH preventing delivery of injected or
       corrupt fragments of the protected user message this should only
       occur in case of implementation errors or internal hardware
       failures or the necessary security context has been prematurely
       discarded.

   2.  In case the SCTP layer indicates an end to a user message, e.g.,
       when receiving a MSG_EOR in a recvmsg() call when using the API
       described in [RFC6458], and the last buffered DTLS record length
       field does not match, i.e., the DTLS record is incomplete.

   3.  Unable to perform the decryption processes due to lack of
       resources, such as memory, and have to abandon the user message
       fragment.  This specification is defined such that the needed
       resources for the DTLS/SCTP operations are bounded for a given
       number of concurrent transmitted SCTP streams or unordered user
       messages.

   The above failure cases all result in the receiver failing to
   recreate the full user message.  This is a failure of the transport
   service that is not possible to recover from in the DTLS/SCTP layer
   and the sender could believe the complete message have been
   delivered.  This error MUST NOT be ignored, as SCTP lacks any
   facility to declare a failure on a specific stream or user message,
   the DTLS connection and the SCTP association SHOULD be terminated.  A
   valid exception to the termination of the SCTP association is if the
   receiver is capable of notifying the ULP about the failure in
   delivery and the ULP is capable of recovering from this failure.

   Note that if the SCTP extension for Partial Reliability (PR-SCTP)
   [RFC3758] is used for a user message, user message may be partially
   delivered or abandoned.  These failures are not a reason for
   terminating the DTLS connection and SCTP association.

   The DTLS Connection ID MUST be negotiated ([RFC9146] or Section 9 of
   [RFC9147]).  If DTLS 1.3 is used, the length field in the record
   layer MUST be included in all records.  A 16-bit sequence number
   SHOULD be used rather than 8-bit to minimize issues with DTLS record
   sequence number wrapping.

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   The ULP may use multiple messages simultaneous, and the progress and
   delivery of these messages are progressing independently, thus the
   receiving DTLS/SCTP implementation may not receive records in order
   in case of packet loss.  Assuming that the sender will send the DTLS
   records in order the DTLS records where created (which may not be
   certain in some implementations), then there is a risk that DTLS
   sequence number have wrapped if the amount of data in flight is more
   than the sequence number covers.  Thus, for 8-bit sequence number
   space with 16384 bytes records the receiver window only needs to be
   256*16384 = 4,194,304 bytes for this risk to definitely exist.  While
   a 16-bit sequence number should not have any sequence number wraps
   for receiver windows up to 1 GB.  The DTLS/SCTP may not be tightly
   integrated and the DTLS records may not be requested to be sent in
   strict sequence order, in these case additional guard ranges are
   needed.

   Also, if smaller DTLS records are used, this limit will be
   correspondingly reduced.  The DTLS/SCTP Sender needs to choose
   sequence number length and DTLS Record size so that the product is
   larger than the used receiver window, preferably twice as large.
   Receiver implementations that are offering receiver windows larger
   than the product 65536*16384 bytes MUST be capable of handling
   sequence number wraps through trial decoding with a lower values in
   the higher bits of the extended sequence number.

   Section 4 of [RFC9146] states "If, however, an implementation chooses
   to receive different lengths of CID, the assigned CID values must be
   self-delineating since there is no other mechanism available to
   determine what connection (and thus, what CID length) is in use.".
   As this solution requires multiple connection IDs, using a zero-
   length CID will be highly problematic as it could result in that any
   DTLS records with a zero length CID ends up in another DTLS
   connection context, and there fail the decryption and integrity
   verification.  And in that case to avoid losing the DTLS record, it
   would have to be forwarded to the zero-length CID using DTLS
   Connection and decryption and validation must be tried.  Resulting in
   higher resource utilization.  Thus, it is REQUIRED to use non-zero
   length CID values, and instead RECOMMENDED to use a single common
   length for the CID values.  A single byte should be sufficient, as
   reuse of old CIDs is possible as long as the implementation ensure
   they are not used in near time to the previous usage.

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4.2.  DTLS Connection Handling

   DTLS/SCTP is negotiated on SCTP level as an adaptation layer
   (Section 6).  After a succesful negotiation of the DTLS/SCTP during
   SCTP association establishment, a DTLS connection MUST be established
   prior to transmission of any ULP user messages.  All DTLS connections
   are terminated when the SCTP association is terminated.  A DTLS
   connection MUST NOT span multiple SCTP associations.

   As it is required to establish the DTLS connection at the beginning
   of the SCTP association, either of the peers should never send any
   SCTP user messages that are not protected by DTLS.  So, the case that
   an endpoint receives data that is not either DTLS messages or
   protected user messages in the form of a sequence of DTLS Records on
   any stream is a protocol violation.  The receiver MAY terminate the
   SCTP association due to this protocol violation.  Implementations
   that do not have a DTLS endpoint immediately ready on SCTP handshake
   completion will have to ensure correct caching of the messages until
   the DTLS endpoint is ready.

   Whenever a mutual authentication, updated security parameters, rerun
   of Diffie-Hellman key-exchange, or SCTP-AUTH rekeying is needed, a
   new DTLS connection is instead setup in parallel with the old
   connection (i.e., there may be up to two simultaneous DTLS
   connections within one association).

4.3.  Payload Protocol Identifier Usage

   SCTP Payload Protocol Identifiers are assigned by IANA.  Application
   protocols using DTLS over SCTP SHOULD register and use a separate
   Payload Protocol Identifier (PPID) and SHOULD NOT reuse the PPID that
   they registered for running directly over SCTP.

   Using the same PPID does no harm as DTLS/SCTP requires all user
   messages being DTLS protected and knows that DTLS is used.  However,
   for protocol analyzers, for example, it is much easier if a separate
   PPID is used and avoids different behavior from [RFC6083].

   Messages that are exchanged between DTLS/SCTP peers not containing
   ULP user messages shall use PPID = 0 according to section 3.3.1 of
   [RFC9260] as no application identifier can be specified by the upper
   layer for this payload data.  With the exception for the DTLS/SCTP
   Control Messagess (Section 5) that uses its own PPID.

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4.4.  Stream Usage

   DTLS 1.3 protects the actual content type of the DTLS record and have
   therefore omitted the non-protected content type field.  Thus, it is
   not possible to determine which content type the DTLS record has on
   SCTP level.  For DTLS 1.2 ULP user messages will be carried in DTLS
   records with content type "application_data".

   DTLS Records carrying protected user message fragments MUST be sent
   in the by ULP indicated SCTP stream and user message.  The ULP has no
   limitations in using SCTP facilities for stream and user messages.
   DTLS records of other types MAY be sent on any stream.  It MAY also
   be sent in its own SCTP user message as well as interleaved with
   other DTLS records carrying protected user messages.  Thus, it is
   allowed to insert between protected user message fragments DTLS
   records of other types as the DTLS receiver will process these and
   not result in any user message data being inserted into the ULP's
   user message.  However, DTLS messages of other types than protected
   user message MUST be sent reliable, so the DTLS record can only be
   interleaved in case the ULP user message is sent as reliable.

   DTLS is capable of handling reordering of the DTLS records.  However,
   depending on stream properties and which user message DTLS records of
   other types are sent in may impact in which order and how quickly
   they are possible to process.  Using the same stream with in-order
   delivery for the different messages will ensure that the DTLS Records
   are delivered in the order they are sent in user messages.  Thus,
   ensuring that if there are DTLS records that need to be delivered in
   particular order it can be ensured.  Alternatively, if it is desired
   that a DTLS record is delivered as early as possible avoiding in-
   order streams with queued messages and considering stream priorities
   can result in faster delivery.

   A simple solution avoiding any protocol issue are to send all DTLS
   messages that are not protected user message fragments is to pick a
   stream not used by the ULP, send the DTLS messages in their own user
   messages with in order delivery.  That mimics the RFC 6083 behavior
   without impacting the ULP.  However, it assumes that there are
   available streams to be used based on the SCTP association handshake
   allowed streams (Section 5.1.1 of [RFC9260]).

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4.5.  Chunk Handling

   DATA chunks of SCTP MUST be sent in an authenticated way as described
   in SCTP-AUTH [RFC4895].  All other chunks that can be authenticated,
   i.e., all chunk types that can be listed in the Chunk List Parameter
   [RFC4895], MUST also be sent in an authenticated way.  This makes
   sure that an attacker cannot modify the stream in which a message is
   sent or affect the ordered/unordered delivery of the message.

   If PR-SCTP as defined in [RFC3758] is used, FORWARD-TSN chunks MUST
   also be sent in an authenticated way as described in [RFC4895].  This
   makes sure that it is not possible for an attacker to drop messages
   and use forged FORWARD-TSN, SACK, and/or SHUTDOWN chunks to hide this
   dropping.

   I-DATA chunk type as defined in [RFC8260] is RECOMMENDED to be
   supported to avoid some of the down sides that large user messages
   have on blocking transmission of later arriving high priority user
   messages.  However, the support is not mandated and negotiated
   independently from DTLS/SCTP.  If I-DATA chunks are used, then they
   MUST be sent in an authenticated way as described in [RFC4895].

4.6.  SCTP-AUTH Hash Function

   When using DTLS/SCTP, the SHA-256 Message Digest Algorithm MUST be
   supported in the SCTP-AUTH [RFC4895] implementation.  SHA-1 MUST NOT
   be used when using DTLS/SCTP.  [RFC4895] requires support and
   inclusion of SHA-1 in the HMAC-ALGO parameter, thus, to meet both
   requirements the HMAC-ALGO parameter will include both SHA-256 and
   SHA-1 with SHA-256 listed prior to SHA-1 to indicate the preference.

4.7.  Parallel DTLS connections

   To enable SCTP-AUTH rekeying, periodic authentication of both
   endpoints, and force attackers to dynamic key extraction [RFC7624],
   DTLS/SCTP per this specification defines the usage of parallel DTLS
   connections over the same SCTP association.  This solution ensures
   that there are no limitations to the lifetime of the SCTP association
   due to DTLS, it also avoids dependency on version specific DTLS
   mechanisms such as renegotiation in DTLS 1.2, which is disabled by
   default in many DTLS implementations, or post-handshake messages in
   DTLS 1.3, which does not allow periodic mutual endpoint re-
   authentication or re-keying of SCTP-AUTH.  Parallel DTLS connections
   enable opening a new DTLS connection performing a handshake, while
   the existing DTLS connection is kept in place.  In DTLS 1.3 the
   handshake MAY be a full handshake or a resumption handshake and
   resumption can be done while the original connection is still open.
   In DTLS 1.2 the handshake MUST be a full handshake.  The new parallel

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   connection MUST use the same DTLS version as the existing connection.
   On handshake completion switch to the security context of the new
   DTLS connection for protection of user message and then ensure
   delivery of all the SCTP chunks using the old DTLS connections
   security context.  When that has been achieved close the old DTLS
   connection and discard the related security context.

   As specified in Section 4.1 the usage of DTLS connection ID is
   required to ensure that the receiver can correctly identify the DTLS
   connection and its security context when performing its de-protection
   operations.  There is also only a single SCTP-AUTH key exported per
   DTLS connection ensuring that there is clear mapping between the DTLS
   connection ID and the SCTP-AUTH security context for each key-id.

   Application writers should be aware that establishing a new DTLS
   connections may result in changes of security parameters.  See
   Section 9 for security considerations regarding rekeying.

   A DTLS/SCTP Endpoint MUST NOT have more than two DTLS connections
   open at the same time.  Either of the endpoints MAY initiate a new
   DTLS connection by performing a full DTLS handshake.  As either
   endpoint can initiate a DTLS handshake on either side at the same
   time, either endpoint may receive a DTLS ClientHello when it has sent
   its own ClientHello.  In this case the ClientHello from the endpoint
   that had the DTLS Client role in the establishment of the existing
   DTLS connection shall be continued to be processed and the other
   dropped.

   When performing the DTLS handshake the endpoint MUST verify that the
   peer identifies using the same identity as in the previous DTLS
   connection.

   When the DTLS handshake has been completed, the new DTLS connection
   MUST be used for the DTLS protection of any new ULP user messages,
   and SHOULD be switched to for protection of not yet protected user
   message fragments of partially transmitted user messages.  Also after
   the completion of the DTLS handshake a new SCTP-AUTH key will be
   exported per Section 4.9.  To enable the sender and receiver to
   correctly identify when the old DTLS connection is no longer in use,
   the SCTP-AUTH key used to protect a SCTP packet MUST NOT be from a
   newer DTLS conncetion than produced any included DTLS record
   fragment.

   The SCTP API defined in [RFC6458] has limitation in changing the
   SCTP-AUTH key until the whole SCTP user message has been delivered.
   However, the DTLS/SCTP implementation can switch the DTLS connection
   used to protect the user message fragments to a newever, even if the
   older DTLS connections exported key is used for the SCTP-AUTH.  And

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   for SCTP implementations where the SCTP-AUTH key can be switched in
   the middle of a user message the SCTP-AUTH key should be changed as
   soon as all DTLS record fragments included in an SCTP packet have
   been protected by the newer DTLS connection.  Any SCTP-AUTH receiver
   implementation is expected to be able to select key on SCTP packet
   basis.

   The DTLS/SCTP endpoint timely indicates to its peer when the previous
   DTLS connection and its context are no longer needed for receiving
   any more data from this endpoint.  This is done by sending a DTLS/
   SCTP Control Message Section 5 of type "Ready_To_Close" Section 5.2
   to its peer.  The endpoint MUST NOT send the Ready_To_Close until the
   following two conditions are fulfilled:

   1.  All SCTP packets containing part of any DTLS record or message
       protected using the security context of this DTLS connection have
       been acknowledged in a non-renegable way.

   2.  All SCTP packets using the SCTP-AUTH key associated with the
       security context of this DTLS connection have been acknowledged
       in a non-renegable way.

   A DTLS/SCTP endpoint that fulfills the above conditions for the SCTP
   packets it sends and have received a Ready_To_Close message SHALL
   immediately initiate closing of this DTLS connection by sending a
   DTLS close_notify.  Then when it have received the peer's
   close_notify terminate the DTLS connection and expunge the associated
   security context and SCTP-AUTH key.  Note that it is not required for
   a DTLS/SCTP implementation that has received a Ready_To_Close
   messsage to send that message itself when it fulfills the conditions.
   However, in some situation both endpoints will fulfill the conditions
   close enough in time that both endpoints will send its Ready_To_Close
   prior to receiving the indication from its peer, that works as both
   endpoints will then initiate DTLS close_notify and terminate the DTLS
   connections upon the reception of the peers close_notify.

   SCTP implementations exposing APIs like [RFC6458] fulfilling these
   conditions require draining the SCTP association of all outstanding
   data after having completed all the user messages using the previous
   SCTP-AUTH key identifier.  Relying on the SCTP_SENDER_DRY_EVENT to
   know when delivery has been accomplished.  A richer API could also be
   used that allows user message level tracking of delivery, see
   Section 7 for API considerations.

   For SCTP implementations exposing APIs like [RFC6458] where it is not
   possible to change the SCTP-AUTH key for a partial SCTP message
   initiated before the change of security context will be forced to
   track the SCTP messages and determine when all using the old security

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   context has been transmitted.  This maybe be impossible to do
   completely reliable without tighter integration between the DTLS/SCTP
   layer and the SCTP implementation.  This type of implementations also
   has an implicit limitation in how large SCTP messages it can support.
   Each SCTP message needs have completed delivery and enabling closing
   of the previous DTLS connection prior to the need to create yet
   another DTLS connection.  Thus, SCTP messages can't be larger than
   that the transmission completes in less than the duration between the
   rekeying or re-authentications needed for this SCTP association.

   The consequences of sending a DTLS close_notify alert in the old DTLS
   connection prior to the receiver having received the data can result
   in failure case 1 described in Section 4.1, which likely result in
   SCTP association termination.

4.8.  Renegotiation and KeyUpdate

   DTLS 1.2 renegotiation enables rekeying (with ephemeral Diffie-
   Hellman) of DTLS as well as mutual reauthentication and transfer of
   revocation information inside an DTLS 1.2 connection.  Renegotiation
   has been removed from DTLS 1.3 and partly replaced with post-
   handshake messages such as KeyUpdate.  The parallel DTLS connection
   solution was specified due to lack of necessary features with DTLS
   1.3 considered needed for long lived SCTP associations, such as
   rekeying (with ephemeral Diffie-Hellman) as well as mutual
   reauthentication.

   This specification does not allow usage of DTLS 1.2 renegotiation to
   avoid race conditions and corner cases in the interaction between the
   parallel DTLS connection mechanism and the keying of SCTP-AUTH.  In
   addition renegotiation is also disabled in some implementations, as
   well as dealing with the epoch change reliable have similar or worse
   application impact.

   This specification also recommends against using DTLS 1.3 KeyUpdate
   and instead rely on parallel DTLS connections.  For DTLS 1.3 there
   isn't feature parity.  It also has the issue that a DTLS
   implementation following the RFC may assume a too limited window for
   SCTP where the previous epoch's security context is maintained and
   thus changes to epoch handling would be necessary.  Thus, unless the
   below specified more application impacting draining is used there
   exist risk of losing data that the sender will have assumed has been
   reliably delivered.

4.8.1.  DTLS 1.2 Considerations

   The endpoint MUST NOT use DTLS 1.2 renegotiation.

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4.8.2.  DTLS 1.3 Considerations

   Before sending a KeyUpdate message, the DTLS endpoint MUST ensure
   that all DTLS messages have been acknowledged by the SCTP peer in a
   non-revokable way.  After sending the KeyUpdate message, it stops
   sending DTLS messages until the corresponding Ack message has been
   processed.

   Prior to processing a received KeyUpdate message, all other received
   SCTP user messages that are buffered in the SCTP layer and can be
   delivered to the DTLS layer MUST be read and processed by DTLS.

4.9.  Handling of Endpoint-Pair Shared Secrets

   SCTP-AUTH [RFC4895] is keyed using Endpoint-Pair Shared Secrets.  In
   SCTP associations where DTLS is used, DTLS is used to establish these
   secrets.  The endpoints MUST NOT use another mechanism for
   establishing shared secrets for SCTP-AUTH.  The endpoint-pair shared
   secret for Shared Key Identifier 0 is empty and MUST be used when
   establishing the first DTLS connection.

   The initial DTLS connection will be used to establish a new shared
   secret as specified per DTLS version below, and which MUST use shared
   key identifier 1.  After sending the DTLS Finished message for the
   initial DTLS connection, the active SCTP-AUTH key MUST be switched
   from key identifier 0 to key identifier 1.  Once the initial Finished
   message from the peer has been processed by DTLS, the SCTP-AUTH key
   with Shared Key Identifier 0 MUST be removed.

   When a subsequent DTLS connection is setup, a new a 64-byte shared
   secret is derived using the TLS-Exporter.  The shared secret
   identifiers form a sequence.  If the previous shared secret used
   Shared Key Identifier n, the new one MUST use Shared Key Identifier
   n+1, unless n = 65535, in which case the new Shared Key Identifier is
   1.

   After sending the DTLS Finished message, the new SCTP-AUTH key can be
   used according to Section 4.7.  When the endpoint has both sent and
   received a close_notify on the old DTLS connection then the endpoint
   SHALL remove the shared secret and the SCTP-AUTH key related to old
   DTLS connection.

4.9.1.  DTLS 1.2 Considerations

   Whenever a new DTLS connection is established, a 64-byte endpoint-
   pair shared secret is derived using the TLS-Exporter described in
   [RFC5705].

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   The 64-byte shared secret MUST be provided to the SCTP stack as soon
   as the computation is possible.  The exporter MUST use the label
   given in Section 8 and no context.

4.9.2.  DTLS 1.3 Considerations

   When the exporter_secret can be computed, a 64-byte shared secret is
   derived from it and provided as a new endpoint-pair shared secret by
   using the TLS-Exporter described in [RFC8446].

   The 64-byte shared secret MUST be provided to the SCTP stack as soon
   as the computation is possible.  The exporter MUST use the label
   given in Section Section 8 and no context.

4.10.  Shutdown

   To prevent DTLS from discarding DTLS user messages while it is
   shutting down, the below procedure has been defined.  Its goal is to
   avoid the need for APIs requiring per user message data level
   acknowledgments and utilizes existing SCTP protocol behavior to
   ensure delivery of the protected user messages data.

   To support DTLS 1.2 close_notify behavior and avoid any uncertainty
   related to rekeying, a DTLS/SCTP protocol message (Section 5) sent as
   protected SCTP user message is defined with it own PPID to enable the
   DTLS/SCTP layer to know that it is targeting the remote DTLS/SCTP
   function and act on the request to close in a controlled fashion.

   The interaction between peers (local and remote) and protocol stacks
   is as follows:

   1.  Local instance of ULP asks for terminating the DTLS/SCTP
       Association.

   2.  Local DTLS/SCTP acknowledge the request, from this time on no new
       data from local instance of ULP will be accepted.  In case a DTLS
       connection handshake is ongoing this needs to be aborted
       conclusively at this step to ensure that the necessary DTLS
       message exchange happens prior to draining any outstanding data
       in the SCTP association from this endpoint.

   3.  Local DTLS/SCTP finishes any protection operation on buffered
       user messages and ensures that all protected user message data
       has been successfully transferred to the remote peer.

   4.  Local DTLS/SCTP sends a DTLS/SCTP Control Message Section 5 of
       type "SHUTDOWN_Request" Section 5.1 to its peer.

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   5.  The remote DTLS/SCTP when receiving the SHUTDOWN-Request informs
       its ULP that shutdown has been initiated.  No more ULP user
       message data to be sent to peer can be accepted by DTLS/SCTP.  In
       case this endpoint has initiated a DTLS connection handshake this
       MUST be aborted as the peer is unable to respond to avoid
       additional case of draining.

   6.  Remote DTLS/SCTP finishes any protection operation on buffered
       user messages and ensures that all protected user message data
       has been successfully transferred to the remote ULP.

   7.  Remote DTLS/SCTP sends DTLS close_notify to Local DTLS/SCTP for
       each and all DTLS connections.  Then it initiates the SCTP
       shutdown procedure (section 9.2 of [RFC9260]).

   8.  When the local DTLS/SCTP receives a close_notify on a DTLS
       connection, in case it is DTLS 1.3 it SHALL send its
       corresponding DTLS close_notify on each open DTLS connection.
       When the last open DTLS connection has received close_notify and
       any if needed corresponding close_notify have been sent the local
       DTLS/SCTP initiates the SCTP shutdown procedure (section 9.2 of
       [RFC9260]).

   9.  Upon receiving the information that SCTP has closed the
       Association, independently the local and remote DTLS/SCTP
       entities destroy the DTLS connection completing the shutdown.

   The verification in step 3 and 6 that all user data message has been
   successfully delivered to the remote ULP can be provided by the SCTP
   stack that implements [RFC6458] by means of SCTP_SENDER_DRY event
   (section 6.1.9 of [RFC6458]).

   A successful SCTP shutdown will indicate successful delivery of all
   data.  However, in cases of communication failures and extensive
   packet loss the SCTP shutdown procedure can time out and result in
   SCTP association termination where its unknown if all data has been
   delivered.  The DTLS/SCTP should indicate to ULP successful
   completion or failure to shutdown gracefully.

5.  DTLS/SCTP Control Message

   The DTLS/SCTP Control Message is defined as its own upper layer
   protocol for DTLS/SCTP identified by its own PPID.  The control
   message is single 32-bit unsigned integer value in network byte
   order.  Each message is sent as its own SCTP user message after
   having been protected by an open DTLS connection on any SCTP stream
   and MUST be marked with SCTP Payload Protocol Identifier (PPID) value
   TBD1 Section 8.3.

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   The DTLS/SCTP implementation MUST consume all SCTP messages received
   with the PPID value of TBD1.  If the message is not 32-bit long the
   message MUST be discarded and the error SHOULD be logged.  In case
   the message has an unknown value the message is discarded and the
   event SHOULD be logged.

   Two control messages are defined in this specfication.

5.1.  SHUTDOWN-Request

   The value "1" is defined as a request to the peer to initiate
   controlled shutdown.  This is used per step 4 and 5 in Section 4.10.

5.2.  Ready To Close Indication

   The value "2" is defined as an indication to the peer that from its
   perspective all SCTP packets with user message or using the SCTP-AUTH
   key associated with the oldest DTLS connection has been sent and
   acknowledged as received in a non-renegable way.  This is used per
   Section 4.7 to initiate the closing of the DTLS connections during
   rekeying.

6.  DTLS over SCTP Service

   The adoption of DTLS over SCTP according to the current specification
   is meant to add to SCTP the option for transferring encrypted data.
   When DTLS over SCTP is used, all data being transferred MUST be
   protected by chunk authentication and DTLS encrypted.  Chunks that
   need to be received in an authenticated way will be specified in the
   CHUNK list parameter according to [RFC4895].  Error handling for
   authenticated chunks is according to [RFC4895].

6.1.  Adaptation Layer Indication in INIT/INIT-ACK

   At the initialization of the association, a sender of the INIT or
   INIT ACK chunk that intends to use DTLS/SCTP as specified in this
   specification MUST include an Adaptation Layer Indication Parameter
   [RFC5061] with the IANA assigned value TBD (Section 8.2) to inform
   its peer that it is able to support DTLS over SCTP per this
   specification.

6.2.  DTLS over SCTP Initialization

   Initialization of DTLS/SCTP requires all the following options to be
   part of the INIT/INIT-ACK handshake:

   RANDOM: defined in [RFC4895]

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   CHUNKS: defined in [RFC4895]

   HMAC-ALGO: defined in [RFC4895]

   ADAPTATION-LAYER-INDICATION: defined in [RFC5061]

   When all the above options are present and having acceptable
   parameters, the Association will start with support of DTLS/SCTP.
   The set of options indicated are the DTLS/SCTP Mandatory Options.  No
   data transfer is permitted before DTLS handshake is complete.  Chunk
   bundling is permitted according to [RFC9260].  The DTLS handshake
   will enable authentication of both the peers.

   The extension described in this document is given by the following
   message exchange.

      --- INIT[RANDOM; CHUNKS; HMAC-ALGO; ADAPTATION-LAYER-IND] --->
      <- INIT-ACK[RANDOM; CHUNKS; HMAC-ALGO; ADAPTATION-LAYER-IND] -
      ------------------------ COOKIE-ECHO ------------------------>
      <------------------------ COOKIE-ACK -------------------------
      ---------------- AUTH; DATA[DTLS Handshake] ----------------->
                                  ...
                                  ...
      <--------------- AUTH; DATA[DTLS Handshake] ------------------

6.3.  Client Use Case

   When a client initiates an SCTP Association with DTLS protection,
   i.e., the SCTP INIT containing DTSL/SCTP Mandatory Options, it can
   receive an INIT-ACK also containing DTLS/SCTP Mandatory Options, in
   that case the Association will proceed as specified in the previous
   Section 6.2 section.  If the peer replies with an INIT-ACK not
   containing all DTLS/SCTP Mandatory Options, the client SHOULD reply
   with an SCTP ABORT.

6.4.  Server Use Case

   If a SCTP Server supports DTLS/SCTP, i.e., per this specification,
   when receiving an INIT chunk with all DTLS/SCTP Mandatory Options it
   will reply with an INIT-ACK also containing all the DTLS/SCTP
   Mandatory Options, following the sequence for DTLS initialization
   Section 6.2 and the related traffic case.  If a SCTP Server that
   supports DTLS and configured to use it, receives an INIT chunk
   without all DTLS/SCTP Mandatory Options, it SHOULD reply with an SCTP
   ABORT.

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6.5.  RFC 6083 Fallback

   This section discusses how an endpoint supporting this specification
   can fallback to follow the DTLS/SCTP behavior in RFC6083.  It is
   recommended to define a setting that represents the policy to allow
   fallback or not.  However, the possibility to use fallback is based
   on the ULP can operate using user messages that are no longer than
   16384 bytes and where the security issues can be mitigated or
   considered acceptable.  Fallback is NOT RECOMMEND to be enabled as it
   enables downgrade attacks to weaker algorithms and versions of DTLS.

   An SCTP endpoint that receives an INIT chunk or an INIT-ACK chunk
   that does not contain the SCTP-Adaptation-Indication parameter with
   the DTLS/SCTP adaptation layer codepoint, see Section 8.2, may in
   certain cases potentially perform a fallback to RFC 6083 behavior.
   However, the fallback attempt should only be performed if policy says
   that is acceptable.

   If fallback is allowed, it is possible that the client will send
   plain text user messages prior to DTLS handshake as it is allowed per
   RFC 6083.  So that needs to be part of the consideration for a policy
   allowing fallback.

6.5.1.  Client Fallback

   A DTLS/SCTP client supporting this specification encountering an
   server not compatible with this specification MAY attempt RFC 6083
   fallback per this procedure.

   1.  Fallback procedure, if enabled, is initiated when receiving an
       SCTP INIT-ACK that does not contain the DTLS/SCTP Adaptation
       Layer indication.  If fallback is not enabled the SCTP handshake
       is aborted.

   2.  The client checks that the SCTP INIT-ACK contained the necessary
       chunks and parameters to establish SCTP-AUTH per RFC 6083 with
       this endpoint.  If not all necessary parameters or support
       algorithms don't match the client MUST abort the handshake.
       Otherwise it completes the SCTP handshake.

   3.  Client performs DTLS connection handshake per RFC 6083 over
       established SCTP association.  If successful authenticating the
       targeted server the client has successful fallen back to use RFC
       6083.  If not terminate the SCTP association.

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6.5.2.  Server Fallback

   A DTLS/SCTP Server that supports both this specification and RFC 6083
   and where fallback has been enabled for the ULP can follow this
   procedure.receiving

   1.  When receiving an SCTP INIT message without the DTLS/SCTP
       adaptation layer indication fallback procedure is initiated.

   2.  Verify that the SCTP INIT contains SCTP-AUTH parameters required
       by RFC 6083 and compatible with this server.  If that is not the
       case abort the SCTP handshake.

   3.  Send an SCTP INIT ACK with the required SCTP-AUTH chunks and
       parameters to the client.

   4.  Complete the SCTP Handshake.  Await DTLS handshake per RFC 6083.
       Plain text SCTP messages MAY be received.

   5.  Upon successful completion of DTLS handshake successful fallback
       to RFC 6083 have been accomplished.

7.  SCTP API Consideration

   DTLS/SCTP needs certain functionality on the API that the SCTP
   implementation provide to the ULP to function optimally.  A DTLS/SCTP
   implementation will need to provide its own API to the ULP, while
   itself using the SCTP API.  This discussion is focused on the needed
   functionality on the SCTP API.

   The following functionality is needed:

   *  Controlling SCPT-AUTH negotiation so that SHA-256 algorithm is
      inlcluded, and determine that SHA-1 is not selected when the
      association is established.

   *  Determine when all SCTP packets that uses an SCTP-auth key or
      contains DTLS records associated to a particular DTLS connection
      has been acknowledged non-renegable.

   *  Determine when all SCTP packets have been acknowledged non-
      renegable.

   *  Negotiate the adaptation layer indication that indicates DTLS/SCTP
      and determine if it was agreed or not.

   *  Partial user messages transmission and reception.

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8.  IANA Considerations

8.1.  TLS Exporter Label

   RFC 6083 defined a TLS Exporter Label registry as described in
   [RFC5705].  IANA is requested to update the reference for the label
   "EXPORTER_DTLS_OVER_SCTP" to also include this specification.

8.2.  SCTP Adaptation Layer Indication Code Point

   [RFC5061] defined an IANA registry for Adaptation Code Points to be
   used in the Adaptation Layer Indication parameter.  The registry was
   at time of writing located: https://www.iana.org/assignments/sctp-
   parameters/sctp-parameters.xhtml#sctp-parameters-27 IANA is requested
   to assign one Adaptation Code Point for DTLS/SCTP per the below
   proposed entry in Table 1.

         +============================+=============+===========+
         | Code Point (32-bit number) | Description | Reference |
         +============================+=============+===========+
         | 0x00000002                 | DTLS/SCTP   | [RFC-TBD] |
         +----------------------------+-------------+-----------+

                      Table 1: Adaptation Code Point

   RFC-Editor Note: Please replace [RFC-TBD] with the RFC number given
   to this specification.

8.3.  SCTP Payload Protocol Identifiers

   This document registers one Payload Protocol Identifier (PPID) to be
   used to idenfity the DTLS/SCTP control messages (Section 5).

             +=======+===========================+===========+
             | Value | SCTP PPID                 | Reference |
             +=======+===========================+===========+
             | TBD1  | DTLS/SCTP Control Message | [RFC-TBD] |
             +-------+---------------------------+-----------+

                 Table 2: SCTP Payload Protocol Identifier

   RFC-Editor Note: Please replace [RFC-TBD] with the RFC number given
   to this specification.

9.  Security Considerations

   The security considerations given in [RFC9147], [RFC4895], and
   [RFC9260] also apply to this document.

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9.1.  Cryptographic Considerations

   Over the years, there have been several serious attacks on earlier
   versions of Transport Layer Security (TLS), including attacks on its
   most commonly used ciphers and modes of operation.  [RFC7457]
   summarizes the attacks that were known at the time of publishing and
   BCP 195 [RFC7525] [RFC8996] provide recommendations for improving the
   security of deployed services that use TLS.

   When DTLS/SCTP is used with DTLS 1.2 [RFC6347], DTLS 1.2 MUST be
   configured to disable options known to provide insufficient security.
   HTTP/2 [RFC9113] gives good minimum requirements based on the attacks
   that where publicly known in 2022.  DTLS 1.3 [RFC9147] only define
   strong algorithms without major weaknesses at the time of
   publication.  Many of the TLS registries have a "Recommended" column.
   Parameters not marked as "Y" are NOT RECOMMENDED to support.  DTLS
   1.3 is preferred over DTLS 1.2 being a newer protocol that addresses
   known vulnerabilities and only defines strong algorithms without
   known major weaknesses at the time of publication.

   DTLS 1.3 requires rekeying before algorithm specific AEAD limits have
   been reached.  The AEAD limits equations are equally valid for DTLS
   1.2 and SHOULD be followed for DTLS/SCTP, but are not mandated by the
   DTLS 1.2 specification.

   HMAC-SHA-256 as used in SCTP-AUTH has a very large tag length and
   very good integrity properties.  The SCTP-AUTH key can be used longer
   than the current algorithms in the TLS record layer.  The SCTP-AUTH
   key is rekeyed when a new DTLS connection is set up at which point a
   new SCTP-AUTH key is derived using the TLS-Exporter.

   (D)TLS 1.3 [RFC8446] discusses forward secrecy from EC(DHE),
   KeyUpdate, and tickets/resumption.  Forward secrecy limits the effect
   of key leakage in one direction (compromise of a key at time T2 does
   not compromise some key at time T1 where T1 < T2).  Protection in the
   other direction (compromise at time T1 does not compromise keys at
   time T2) can be achieved by rerunning EC(DHE).  If a long-term
   authentication key has been compromised, a full handshake with
   EC(DHE) gives protection against passive attackers.  If the
   resumption_master_secret has been compromised, a resumption handshake
   with EC(DHE) gives protection against passive attackers and a full
   handshake with EC(DHE) gives protection against active attackers.  If
   a traffic secret has been compromised, any handshake with EC(DHE)
   gives protection against active attackers.

   The document "Confidentiality in the Face of Pervasive Surveillance:
   A Threat Model and Problem Statement" [RFC7624] defines key
   exfiltration as the transmission of cryptographic keying material for

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   an encrypted communication from a collaborator, deliberately or
   unwittingly, to an attacker.  Using the terms in RFC 7624, forward
   secrecy without rerunning EC(DHE) still allows an attacker to do
   static key exfiltration.  Rerunning EC(DHE) forces and attacker to
   dynamic key exfiltration (or content exfiltration).

   When using DTLS 1.3 [RFC9147], AEAD limits and forward secrecy can be
   achieved by sending post-handshake KeyUpdate messages, which triggers
   rekeying of DTLS.  Such symmetric rekeying gives significantly less
   protection against key leakage than re-running Diffie-Hellman as
   explained above.  After leakage of application_traffic_secret_N, an
   attacker can passively eavesdrop on all future data sent on the
   connection including data encrypted with
   application_traffic_secret_N+1, application_traffic_secret_N+2, etc.
   Note that KeyUpdate does not update the exporter_secret.

   DTLS/SCTP is in many deployments replacing IPsec.  For IPsec, NIST
   (US), BSI (Germany), and ANSSI (France) recommends very frequent re-
   run of Diffie-Hellman to provide forward secrecy and force attackers
   to dynamic key extraction [RFC7624].  ANSSI writes "It is recommended
   to force the periodic renewal of the keys, e.g., every hour and every
   100 GB of data, in order to limit the impact of a key compromise."
   [ANSSI-DAT-NT-003].

   For many DTLS/SCTP deployments the SCTP association is expected to
   have a very long lifetime of months or even years.  For associations
   with such long lifetimes there is a need to frequently re-
   authenticate both client and server.  TLS Certificate lifetimes
   significantly shorter than a year are common which is shorter than
   many expected DTLS/SCTP associations.

   SCTP-AUTH re-rekeying, periodic authentication of both endpoints, and
   frequent re-run of Diffie-Hellman to force attackers to dynamic key
   extraction is in DTLS/SCTP per this specification achieved by setting
   up new DTLS connections over the same SCTP association.
   Implementations SHOULD set up new connections frequently to force
   attackers to dynamic key extraction.  Implementations MUST set up new
   connections before any of the certificates expire.  It is RECOMMENDED
   that all negotiated and exchanged parameters are the same except for
   the timestamps in the certificates.  Clients and servers MUST NOT
   accept a change of identity during the setup of a new connections,
   but MAY accept negotiation of stronger algorithms and security
   parameters, which might be motivated by new attacks.

   Allowing new connections can enable denial-of-service attacks.  The
   endpoints SHOULD limit the frequency of new connections.

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   When DTLS/SCTP is used with DTLS 1.2 [RFC6347], the TLS Session Hash
   and Extended Master Secret Extension [RFC7627] MUST be used to
   prevent unknown key-share attacks where an attacker establishes the
   same key on several connections.  DTLS 1.3 always prevents these
   kinds of attacks.  The use of SCTP-AUTH then cryptographically binds
   new connections to the old connection.  This together with mandatory
   mutual authentication (on the DTLS layer) and a requirement to not
   accept new identities mitigates MITM attacks that have plagued
   renegotiation [TRISHAKE].

9.2.  Downgrade Attacks

   A peer supporting DTLS/SCTP according to this specification, DTLS/
   SCTP according to [RFC6083] and/or SCTP without DTLS may be
   vulnerable to downgrade attacks where on on-path attacker interferes
   with the protocol setup to lower or disable security.  If possible,
   it is RECOMMENDED that the peers have a policy only allowing DTLS/
   SCTP according to this specification.

9.3.  Targeting DTLS Messages

   The DTLS handshake messages and other control messages, i.e., not
   application data can easily be identified when using DTLS 1.2 as
   their content type is not encrypted.  With DTLS 1.3 there is no
   unprotected content type.  However, they will be sent with an PPID of
   0 if sent in their own SCTP user messages.  Section 4.4 proposes a
   basic behavior that will still make it easily for anyone to detect
   the DTLS messages that are not protected user messages.

9.4.  Authentication and Policy Decisions

   DTLS/SCTP MUST be mutually authenticated.  Authentication is the
   process of establishing the identity of a user or system and
   verifying that the identity is valid.  DTLS only provides proof of
   possession of a key.  DTLS/SCTP MUST perform identity authentication.
   It is RECOMMENDED that DTLS/SCTP is used with certificate-based
   authentication.  When certificates are used the application using
   DTLS/SCTP is responsible for certificate policies, certificate chain
   validation, and identity authentication (HTTPS does for example match
   the hostname with a subjectAltName of type dNSName).  The application
   using DTLS/SCTP MUST define what the identity is and how it is
   encoded and the client and server MUST use the same identity format.
   Guidance on server certificate validation can be found in [RFC6125].
   DTLS/SCTP enables periodic transfer of mutual revocation information
   (OSCP stapling) every time a new parallel connection is set up.  All
   security decisions MUST be based on the peer's authenticated
   identity, not on its transport layer identity.

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   It is possible to authenticate DTLS endpoints based on IP addresses
   in certificates.  SCTP associations can use multiple IP addresses per
   SCTP endpoint.  Therefore, it is possible that DTLS records will be
   sent from a different source IP address or to a different destination
   IP address than that originally authenticated.  This is not a problem
   provided that no security decisions are made based on the source or
   destination IP addresses.

9.5.  Resumption and Tickets

   In DTLS 1.3 any number of tickets can be issued in a connection and
   the tickets can be used for resumption as long as they are valid,
   which is up to seven days.  The nodes in a resumed connection have
   the same roles (client or server) as in the connection where the
   ticket was issued.  In DTLS/SCTP, there are no significant
   performance benefits with resumption and an implementation can chose
   to never issue any tickets.  If tickets and resumption are used it is
   enough to issue a single ticket per connection.

9.6.  Privacy Considerations

   [RFC6973] suggests that the privacy considerations of IETF protocols
   be documented.

   For each SCTP user message, the user also provides a stream
   identifier, a flag to indicate whether the message is sent ordered or
   unordered, and a payload protocol identifier.  Although DTLS/SCTP
   provides privacy for the actual user message, the other three
   information fields are not confidentiality protected.  They are sent
   as cleartext because they are part of the SCTP DATA chunk header.

   It is RECOMMENDED that DTLS/SCTP is used with certificate based
   authentication in DTLS 1.3 [RFC9147] to provide identity protection.
   DTLS/SCTP MUST be used with a key exchange method providing forward
   secrecy.

9.7.  Pervasive Monitoring

   As required by [RFC7258], work on IETF protocols needs to consider
   the effects of pervasive monitoring and mitigate them when possible.

   Pervasive Monitoring is widespread surveillance of users.  By
   encrypting more information including user identities, DTLS 1.3
   offers much better protection against pervasive monitoring.

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   Massive pervasive monitoring attacks relying on key exchange without
   forward secrecy has been reported.  By mandating forward secrecy,
   DTLS/SCTP effectively mitigate many forms of passive pervasive
   monitoring and limits the amount of compromised data due to key
   compromise.

   An important mitigation of pervasive monitoring is to force attackers
   to do dynamic key exfiltration instead of static key exfiltration.
   Dynamic key exfiltration increases the risk of discovery for the
   attacker [RFC7624].  DTLS/SCTP per this specification encourages
   implementations to frequently set up new DTLS connections with
   (EC)DHE over the same SCTP association to force attackers to do
   dynamic key exfiltration.

   In addition to the privacy attacks discussed above, surveillance on a
   large scale may enable tracking of a user over a wider geographical
   area and across different access networks.  Using information from
   DTLS/SCTP together with information gathered from other protocols
   increase the risk of identifying individual users.

10.  Contributors

   Michael Tuexen contributed as co-author to the intitial versions this
   draft.  Michael's contributions include:

   *  The use of the Adaptation Layer Indication.

   *  Many editorial improvements.

11.  Acknowledgments

   The authors of RFC 6083 which this document is based on are Michael
   Tuexen, Eric Rescorla, and Robin Seggelmann.

   The RFC 6083 authors thanked Anna Brunstrom, Lars Eggert, Gorry
   Fairhurst, Ian Goldberg, Alfred Hoenes, Carsten Hohendorf, Stefan
   Lindskog, Daniel Mentz, and Sean Turner for their invaluable
   comments.

   The authors of this document want to thank Daria Ivanova, Li Yan, and
   GitHub user vanrein for their contribution.

12.  References

12.1.  Normative References

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

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758,
              DOI 10.17487/RFC3758, May 2004,
              <https://www.rfc-editor.org/info/rfc3758>.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
              2007, <https://www.rfc-editor.org/info/rfc4895>.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              DOI 10.17487/RFC5061, September 2007,
              <https://www.rfc-editor.org/info/rfc5061>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

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

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <https://www.rfc-editor.org/info/rfc7627>.

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

   [RFC8260]  Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
              "Stream Schedulers and User Message Interleaving for the
              Stream Control Transmission Protocol", RFC 8260,
              DOI 10.17487/RFC8260, November 2017,
              <https://www.rfc-editor.org/info/rfc8260>.

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

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

   [RFC9113]  Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/info/rfc9113>.

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

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9260]  Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
              Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
              June 2022, <https://www.rfc-editor.org/info/rfc9260>.

12.2.  Informative References

   [RFC3436]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
              Layer Security over Stream Control Transmission Protocol",
              RFC 3436, DOI 10.17487/RFC3436, December 2002,
              <https://www.rfc-editor.org/info/rfc3436>.

   [RFC3788]  Loughney, J., Tuexen, M., Ed., and J. Pastor-Balbas,
              "Security Considerations for Signaling Transport (SIGTRAN)
              Protocols", RFC 3788, DOI 10.17487/RFC3788, June 2004,
              <https://www.rfc-editor.org/info/rfc3788>.

   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083,
              DOI 10.17487/RFC6083, January 2011,
              <https://www.rfc-editor.org/info/rfc6083>.

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   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458,
              DOI 10.17487/RFC6458, December 2011,
              <https://www.rfc-editor.org/info/rfc6458>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
              February 2015, <https://www.rfc-editor.org/info/rfc7457>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [ANSSI-DAT-NT-003]
              Agence nationale de la sécurité des systèmes
              d'information, "Recommendations for securing networks with
              IPsec", ANSSI Technical Report DAT-NT-003 , August 2015,
              <<https://www.ssi.gouv.fr/uploads/2015/09/
              NT_IPsec_EN.pdf>>.

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   [TRISHAKE] Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
              A., and P. Strub, "Triple Handshakes and Cookie Cutters:
              Breaking and Fixing Authentication over TLS", IEEE
              Symposium on Security & Privacy , April 2016,
              <https://hal.inria.fr/hal-01102259/file/triple-handshakes-
              and-cookie-cutters-oakland14.pdf>.

Appendix A.  Motivation for Changes

   This document proposes a number of changes to RFC 6083 that have
   various different motivations:

   Supporting Large User Messages: RFC 6083 allowed only user messages
   that could fit within a single DTLS record. 3GPP has run into this
   limitation where they have at least four SCTP using protocols (F1,
   E1, Xn, NG-C) that can potentially generate messages over the size of
   16384 bytes.

   New Versions: Almost 10 years has passed since RFC 6083 was written,
   and significant evolution has happened in the area of DTLS and
   security algorithms.  Thus, DTLS 1.3 is the newest version of DTLS
   and also the SHA-1 HMAC algorithm of RFC 4895 is getting towards the
   end of usefulness.  Use of DTLS 1.3 with long lived associations
   require parallel DTLS connections.  Thus, this document mandates
   usage of relevant versions and algorithms.

   Allowing DTLS Messages on any stream: RFC6083 requires DTLS messages
   that are not user message data to be sent on stream 0 and that this
   stream is used with in-order delivery.  That can actually limit the
   applications that can use DTLS/SCTP.  In addition with DTLS 1.3
   encrypting the actual message type it is anyway not available.
   Therefore a more flexible rule set is used that relies on DTLS
   handling reordering.

   Clarifications: Some implementation experiences have been gained that
   motivates additional clarifications on the specification.

   *  Avoid unsecured messages prior to DTLS handshake have completed.

   *  Make clear that all messages are encrypted after DTLS handshake.

Authors' Addresses

   Magnus Westerlund
   Ericsson
   Email: magnus.westerlund@ericsson.com

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   John Preuß Mattsson
   Ericsson
   Email: john.mattsson@ericsson.com

   Claudio Porfiri
   Ericsson
   Email: claudio.porfiri@ericsson.com

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