Datagram Transport Layer Security (DTLS) over Stream Control Transmission Protocol (SCTP)

Document Type Active Internet-Draft (candidate for tsvwg WG)
Authors Magnus Westerlund  , John Preuß Mattsson  , Claudio Porfiri  , Michael Tüxen 
Last updated 2021-03-25 (latest revision 2021-02-22)
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TSVWG                                                      M. Westerlund
Internet-Draft                                         J. Preuß Mattsson
Obsoletes: 6083 (if approved)                                 C. Porfiri
Intended status: Standards Track                                Ericsson
Expires: 26 August 2021                                         M. Tüxen
                                         Münster Univ. of Appl. Sciences
                                                        22 February 2021

      Datagram Transport Layer Security (DTLS) over Stream Control
                      Transmission Protocol (SCTP)


   This document describes a proposed update for the usage of the
   Datagram Transport Layer Security (DTLS) protocol to protect user
   messages sent over the Stream Control Transmission Protocol (SCTP).

   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 intends
   to obsolete RFC 6083 and removes the 16 kB limitation on 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 the implementation by some
   stricter requirements on the establishment procedures.

Discussion Venues

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

   Discussion of this document takes place on the TSVWG Working Group
   mailing list (, which is archived at

   Source for this draft and an issue tracker can be found at

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Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

   Copyright (c) 2021 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
       1.1.1.  Comparison with TLS for SCTP  . . . . . . . . . . . .   4
       1.1.2.  Changes from RFC 6083 . . . . . . . . . . . . . . . .   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  DTLS Considerations . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Version of DTLS . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Message Sizes . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Replay Protection . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Path MTU Discovery  . . . . . . . . . . . . . . . . . . .   8
     3.6.  Retransmission of Messages  . . . . . . . . . . . . . . .   8
   4.  SCTP Considerations . . . . . . . . . . . . . . . . . . . . .   8

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     4.1.  Mapping of DTLS Records . . . . . . . . . . . . . . . . .   8
     4.2.  DTLS Connection Handling  . . . . . . . . . . . . . . . .   9
     4.3.  Payload Protocol Identifier Usage . . . . . . . . . . . .   9
     4.4.  Stream Usage  . . . . . . . . . . . . . . . . . . . . . .  10
     4.5.  Chunk Handling  . . . . . . . . . . . . . . . . . . . . .  10
     4.6.  SCTP-AUTH Hash Function . . . . . . . . . . . . . . . . .  11
     4.7.  Renegotiation . . . . . . . . . . . . . . . . . . . . . .  11
     4.8.  DTLS Epochs . . . . . . . . . . . . . . . . . . . . . . .  11
     4.9.  Handling of Endpoint-Pair Shared Secrets  . . . . . . . .  11
     4.10. Shutdown  . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  DTLS over SCTP Service  . . . . . . . . . . . . . . . . . . .  12
     5.1.  Adaptation Layer Indication in INIT/INIT-ACK  . . . . . .  12
     5.2.  DTLS/SCTP "dtls_over_sctp_maximum_message_size"
           Extension . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  DTLS over SCTP Initialization . . . . . . . . . . . . . .  13
     5.4.  Client Use Case . . . . . . . . . . . . . . . . . . . . .  14
     5.5.  Server Use Case . . . . . . . . . . . . . . . . . . . . .  14
     5.6.  RFC 6083 Fallback . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  TLS Exporter Label  . . . . . . . . . . . . . . . . . . .  15
     6.2.  DTLS "dtls_over_sctp_buffer_size_limit" Extension . . . .  15
     6.3.  SCTP Parameter  . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     7.1.  Cryptographic Considerations  . . . . . . . . . . . . . .  16
     7.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .  17
     7.3.  DTLS/SCTP Message Sizes . . . . . . . . . . . . . . . . .  17
     7.4.  Authentication and Policy Decisions . . . . . . . . . . .  17
     7.5.  Privacy Considerations  . . . . . . . . . . . . . . . . .  18
     7.6.  Pervasive Monitoring  . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Motivation for Changes . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

1.1.  Overview

   This document describes the usage of the Datagram Transport Layer
   Security (DTLS) protocol, as defined in [I-D.ietf-tls-dtls13], over
   the Stream Control Transmission Protocol (SCTP), as defined in
   [RFC4960] 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.

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   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 perfect
   forward secrecy for SCTP-AUTH, and confidentiality of user messages.
   DTLS/SCTP use SCTP and SCTP-AUTH for integrity protection and replay
   protection of 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

   *  large user messages.

   The method described in this document requires that the SCTP
   implementation supports the optional feature of fragmentation of SCTP
   user messages as defined in [RFC4960].  To efficiently implement and
   support larger user messages it is also recommended that I-DATA
   chunks as defined in [RFC8260] as well as an SCTP API that supports
   partial user message delivery as discussed in [RFC6458].

1.1.1.  Comparison with TLS for 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

   *  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.

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1.1.2.  Changes from RFC 6083

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

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

   This update that replaces RFC6083 defines the following changes:

   *  Removes the limitations on user messages sizes by defining a
      secure fragmentation mechanism.

   *  Defines a DTLS extension for the endpoints to declare the user
      message size supported to be received.

   *  Mandates that more modern DTLS version are required (DTLS 1.2 or

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

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

   *  Recommends support of partial message delivery API, see [RFC6458]
      if larger usage messages are intended to be used.

   *  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

1.2.  Terminology

   This document uses the following terms:

   Association: An SCTP association.

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

1.3.  Abbreviations

   DTLS: Datagram Transport Layer Security

   MTU: Maximum Transmission Unit

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   PPID: Payload Protocol Identifier

   SCTP: Stream Control Transmission Protocol

   TCP: Transmission Control Protocol

   TLS: Transport Layer Security

2.  Conventions

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

3.  DTLS Considerations

3.1.  Version of DTLS

   This document is based on DTLS 1.3 [I-D.ietf-tls-dtls13], but works
   also for DTLS 1.2 [RFC6347].  Earlier versions of DTLS MUST NOT be
   used.  It is expected that DTLS/SCTP as described in this document
   will work with future versions of DTLS.

3.2.  Cipher Suites

   For DTLS 1.2, the cipher suites forbidden by [RFC7540] MUST NOT be
   used.  Cipher suites without encryption MUST NOT 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 1.3 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

   The sequence of DTLS records is then fragmented into DATA or I-DATA
   Chunks to fit the path MTU by SCTP.  The largest possible user
   messages using the mechanism defined in this specification is 2^64-1

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   The security operations and reassembly process requires that the
   protected user message, i.e. with DTLS record overhead, is buffered
   in the receiver.  This buffer space will thus put a limit on the
   largest size of plain text user message that can be transferred

   A receiver that doesn't support partial delivery of user messages
   from SCTP [RFC6458] will advertise its largest supported protected
   message using SCTP's mechanism for Advertised Receiver Window Credit
   (a_rwnd) as specified in Section 3.3.2 of [RFC4960].  Note that the
   a_rwnd value is across all user messages being delivered.

   For a receiver supporting partial delivery of user messages a_rwnd
   will not limit the maximum size of the DTLS protected user message
   because the receiver can move parts of the DTLS protected user
   message from the SCTP receiver buffer into a buffer for DTLS
   processing.  When each complete DTLS record have been received from
   SCTP, it can be processed and the plain text fragment can, in its
   turn, be partially delivered to the user application.

   Thus, the limit of the largest user message is dependent on buffering
   allocated for DTLS processing as well as the DTLS/SCTP API to the
   application.  To ensure that the sender have some understanding of
   the maximum receiver size a TLS extension
   "dtls_over_sctp_maximum_message_size" Section 5.2 is used to signal
   the endpoints receiver capability when it comes to user message size.

   All implementors of this specification MUST support user messages of
   at least 16383 bytes.  Where 16383 bytes is the supported message
   size in RFC 6083.  By requiring this message size in this document,
   we ensure compatibility with existing usage of RFC 6083, not
   requiring the upper layer protocol to implement additional features
   or requirements.

   Due to SCTP's capability to transmit concurrent user messages the
   total memory consumption in the receiver is not bounded.  In cases
   where one or more user messages are affected by packet loss, the DATA
   chunks may require more data in the receiver's buffer.

   The necessary buffering space for a single user message of
   dtls_over_sctp_maximum_message_size (MMS) is dependent on the

   When no partial data delivery is supported, the message size is
   limited by the a_rwnd as this is the largest protected user message
   that can be received and then processed by DTLS and where the plain
   text user message is expected to be no more than the signalled MMS.

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   With partial processing it is possible to have a receiver
   implementation that is bound to use no more buffer space than MMS
   (for the plaintext) plus one maximum size DTLS record.  The later
   assumes that one can realign the start of the buffer after each DTLS
   record has been consumed.  A more realistic implementation is two
   maximum DTLS record sizes.

   If an implementation supports partial delivery in both the SCTP API
   and the ULP API, and also partial processing in the DTLS/SCTP
   implementation, then the buffering space in the DTLS/SCTP layer ought
   to be no more than two DTLS records.  In which case the MMS to set is
   dependent on the ULP and the endpoints capabilities.

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 of the DTLS
   records within each protected user message.

   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 own
   Path MTU discovery and fragmentation/reassembly for user messages,
   and according to Section 3.3, DTLS can send maximum sized DTLS

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 [RFC4960], and optionally I-DATA [RFC8260] chunks.

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   DTLS/SCTP works as a shim layer between the user message API and
   SCTP.  The fragmentation works similar as the DTLS fragmentation of
   handshake messages.  On the sender side a user message fragmented
   into fragments m0, m1, m2, each no larger than 2^14 - 1 = 16383

      m0 | m1 | m2 | ... = user_message

   The resulting fragments are protected with DTLS and the records are

      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 DTLS is used to decrypt the records.  If a DTLS
   decryption fails, the DTLS connection and the SCTP association are
   terminated.  Due to SCTP-AUTH preventing delivery of corrupt
   fragments of the protected user message this should only occur in
   case of implementation errors or internal hardware failures.

   The DTLS Connection ID SHOULD NOT be negotiated (Section 9 of
   [I-D.ietf-tls-dtls13]).  If DTLS 1.3 is used, the length field MUST
   NOT be omitted and a 16 bit sequence number SHOULD be used.

4.2.  DTLS Connection Handling

   The DTLS connection MUST be established at the beginning of the SCTP
   association and be terminated when the SCTP association is
   terminated, (i.e. there's only one DTLS connection within one
   association).  A DTLS connection MUST NOT span multiple SCTP

   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 on Strea 0
   or protecetd 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.

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.

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   Using the same PPID does not harm as long as the application can
   determine whether or not DTLS is used.  However, for protocol
   analyzers, for example, it is much easier if a separate PPID is used.

   This means, in particular, that there is no specific PPID for DTLS.

4.4.  Stream Usage

   All DTLS Handshake, Alert, or ChangeCipherSpec (DTLS 1.2 only)
   messages MUST be transported on stream 0 with unlimited reliability
   and with the ordered delivery feature.

   DTLS messages of the record protocol, which carries the protected
   user messages, SHOULD use multiple streams other than stream 0; they
   MAY use stream 0 as long as the ordered message semantics is
   acceptable.  On stream 0 protected user messages as well as any DTLS
   messages that isn't record protocol will be mixed, thus the
   additional head of line blocking can occur.

4.5.  Chunk Handling

   DATA chunks of SCTP MUST be sent in an authenticated way as described
   in [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

   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].

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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 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.  Renegotiation

   Renegotiation enables rekeying and reauthentication inside an DTLS
   1.2 connection.  It is up to the upper layer to use/allow it or not.
   Application writers should be aware that allowing renegotiations may
   result in changes of security parameters.  Renegotiation has been
   removed from DTLS 1.3 and partly replaced with Post-Handshake
   messages such as KeyUpdate.  See Section 7 for security
   considerations regarding rekeying.

4.8.  DTLS Epochs

   In general, DTLS implementations SHOULD discard records from earlier
   epochs, as described in Section 4.2.1 of [I-D.ietf-tls-dtls13].  To
   avoid discarding messages, the processing guidelines in Section 4.2.1
   of DTLS 1.3 [I-D.ietf-tls-dtls13] or Section 4.1 or DTLS 1.2
   [RFC6347] should be followed.

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 a DTLS connection.  In DTLS 1.2,
   whenever the main secret changes, a 64-byte shared secret is derived
   from every main secret and provided as a new endpoint-pair shared
   secret by using the TLS-Exporter.  In DTLS 1.3, the exporter_secret
   never change.  For DTLS 1.3, the exporter is described in [RFC8446].
   For DTLS 1.2, the exporter is described in [RFC5705].  The exporter
   MUST use the label given in Section Section 6 and no context.  The
   new Shared Key Identifier MUST be the old Shared Key Identifier
   incremented by 1.  If the old one is 65535, the new one MUST be 1.

   Before sending the DTLS Finished message, the active SCTP-AUTH key
   MUST be switched to the new one.

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   Once the corresponding Finished message from the peer has been
   received, the old SCTP-AUTH key SHOULD be removed.

4.10.  Shutdown

   To prevent DTLS from discarding DTLS user messages while it is
   shutting down, a CloseNotify message MUST only be sent after all
   outstanding SCTP user messages have been acknowledged by the SCTP
   peer and MUST NOT be revoked by the SCTP peer.

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

5.  DTLS over SCTP Service

   The adoption of DTLS over SCTP according to the current description
   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].

5.1.  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
   with the IANA assigned value TBD to inform its peer that it is able
   to support DTLS over SCTP per this specification.

5.2.  DTLS/SCTP "dtls_over_sctp_maximum_message_size" Extension

   The endpoint's DTLS/SCTP maximum message size is declared in the
   "dtls_over_sctp_maximum_message_size" TLS extension.  The
   ExtensionData of the extension is MessageSizeLimit:

      uint64 MessageSizeLimit;

   The value of MessageSizeLimit is the maximum plaintext user message
   size in octets that the endpoint is willing to receive.  When the
   "dtls_over_sctp_maximum_message_size" extension is negotiated, an
   endpoint MUST NOT send a user message larger than the
   MessageSizeLimit value it receives from its peer.

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   This value is the length of the user message before DTLS
   fragmentation and protection.  The value does not account for the
   expansion due to record protection, record padding, or the DTLS

   The "dtls_over_sctp_maximum_message_size" MUST be used to negotiate
   maximum message size for DTLS/SCTP.  A DTLS/SCTP endpoint MUST treat
   the omission of "dtls_over_sctp_maximum_message_size" as a fatal
   error unless supporting RFC 6083 fallback Section 5.6, and it SHOULD
   generate an "illegal_parameter" alert.  Endpoints MUST NOT send a
   "dtls_over_sctp_maximum_message_size" extension with a value smaller
   than 16383.  An endpoint MUST treat receipt of a smaller value as a
   fatal error and generate an "illegal_parameter" alert.

   The "dtls_over_sctp_maximum_message_size" MUST NOT be send in TLS or
   in DTLS versions earlier than 1.2.  In DTLS 1.3, the server sends the
   "dtls_over_sctp_maximum_message_size" extension in the
   EncryptedExtensions message.

   During resumption, the maximum message size is renegotiated.

5.3.  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]

   CHUNKS: list of permitted chunks, defined in [RFC4895]

   HMAC-ALGO: defined in [RFC4895]


   When all the above options are present, 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
   [RFC4960].  The DTLS handshake will enable authentication of both the
   peers and also have the declare their support message size.

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

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      ------------------------ COOKIE-ECHO ------------------------>
      <------------------------ COOKIE-ACK -------------------------
      ---------------- AUTH; DATA[DTLS Handshake] ----------------->
      <--------------- AUTH; DATA[DTLS Handshake] ------------------

5.4.  Client Use Case

   When a SCTP Client initiates an Association with DTLS/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 5.3 section.  If the peer replies
   with an INIT-ACK not containing all DTLS/SCTP Mandatory Options, the
   Client can decide to keep on working with RFC 6083 fallback, plain
   data only, or to ABORT the association.

5.5.  Server Use Case

   If a SCTP Server supports DTLS/SCTP, when receiving an INIT chunk
   with all DTLS/SCTP Mandatory Options it must reply with INIT-ACK also
   containing the all DTLS/SCTP Mandatory Options, then it must follow
   the sequence for DTLS initialization Section 5.3 and the related
   traffic case.  If a SCTP Server supports DTLS, when receiving an INIT
   chunk with not all DTLS/SCTP Mandatory Options, it can decide to
   continue by creating an Association with RFC 6083 fallback, plain
   data only or to ABORT it.

5.6.  RFC 6083 Fallback

   This section discusses how an endpoint supporting this specification
   can fallback to follow the DTLS/SCTP behavior in RFC 6083.  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
   16383 bytes and where the security issues can be mitigated or
   considerd acceptable.  Fallback is NOT RECOMMEND to be enabled as it
   enables downgrade to weaker algorithms and versions of DTLS.

   A SCTP client that receives an INIT-ACK that is not compliant
   according this specification may in certain cases potentially perform
   an fallback to RFC 6083 behavior.  The first case is when the SCTP
   client receives an INIT-ACK doesn't contain the SCTP-Adaptation-
   Indication parameter with the DTLS/SCTP adaptation layer codepoint
   but do include the SCTP-AUTH parameters on a server that are expected
   to provide services using DTLS.  The second case is when the INIT-ACK

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   do contain the SCTP-Adaptation-Indication parameter with the correct
   code point, however the HMAC-ALGO or the Chunks parameters values are
   such that do not fullfil the requirement of this specification but do
   meet the requirements of RFC 6083.  In either of these cases the
   client could attempt DTLS per RFC 6083 as fallback.  However, the
   fallback attempt should only be performed if policy says that is

   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.  When performing the the DTLS handshake, the
   server is required accepting that lack of the TLS extension
   "dtls_over_sctp_maximum_message_size" and can't treat it as fatal
   error.  In case the "dtls_over_sctp_maximum_message_size" TLS
   extension is present in the handshake the server SHALL continue the
   handshake including the extension with its value also, and from that
   point follow this specification.  In case the TLS option is missing
   RFC 6083 applies.

6.  IANA Considerations

6.1.  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 this specification.

6.2.  DTLS "dtls_over_sctp_buffer_size_limit" Extension

   This document registers the "dtls_over_sctp_maximum_message_size"
   extension in the TLS "ExtensionType Values" registry established in
   [RFC5246].  The "dtls_over_sctp_maximum_message_size" extension has
   been assigned a code point of TBD.  This entry [[will be|is]] marked
   as recommended ([RFC8447] and marked as "Encrypted" in (D)TLS 1.3
   [I-D.ietf-tls-dtls13].  The IANA registry [RFC8447] [[will
   list|lists]] this extension as "Recommended" (i.e., "Y") and
   indicates that it may appear in the ClientHello (CH) or
   EncryptedExtensions (EE) messages in (D)TLS 1.3

6.3.  SCTP Parameter

   IANA is requested to assign a Adaptation Code Point for DTLS/SCTP.

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7.  Security Considerations

   The security considerations given in [I-D.ietf-tls-dtls13],
   [RFC4895], and [RFC4960] also apply to this document.

7.1.  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] provides 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 [RFC7540] gives good minimum requirements based on the attacks
   that where publicly known in 2015.  DTLS 1.3 [I-D.ietf-tls-dtls13]
   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 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 until the DTLS handshake is re-run at which point a
   new SCTP-AUTH key is derived using the TLS-Exporter.

   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 Perfect Forward Secrecy.  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 DTLS connections are expected to
   have very long lifetimes of months or even years.  For connections
   with such long lifetimes there is a need to frequently re-
   authenticate both client and server.

   When using DTLS 1.2 [RFC6347], AEAD limits, frequant re-
   authentication and frequent re-run of Diffie-Hellman can be achieved
   with frequent renegotiation, see TLS 1.2 [RFC5246].  When
   renegotiation is used both clients and servers MUST use the
   renegotiation_info extension [RFC5746] and MUST follow the
   renegotiation guidelines in BCP 195 [RFC7525].

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   In DTLS 1.3 renegotiation has been removed from DTLS 1.3 and partly
   replaced with Post-Handshake KeyUpdate.  When using DTLS 1.3
   [I-D.ietf-tls-dtls13], AEAD limits and frequent rekeying can be
   achieved by sending frequent Post-Handshake KeyUpdate messages.
   Symmetric rekeying gives less protection against key leakage than re-
   running Diffie-Hellman.  After leakage of
   application_traffic_secret_N, a passive attacker can passively
   eavesdrop on all future application data sent on the connection
   including application data encrypted with
   application_traffic_secret_N+1, application_traffic_secret_N+2, etc.
   The is no way to do Post-Handshake server authentication or Ephemeral
   Diffie-Hellman inside a DTLS 1.3 connection.  Note that KeyUpdate
   does not update the exporter_secret.

7.2.  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.

7.3.  DTLS/SCTP Message Sizes

   The DTLS/SCTP maximum message size extension enables secure negation
   of a message sizes that fit in the DTLS/SCTP buffer, which improves
   security and availability.  Very small plain text user fragment sizes
   might generate additional work for senders and receivers, limiting
   throughput and increasing exposure to denial of service.

   The maximum message size extension does not protect against peer
   nodes intending to negatively affect the peer node through flooding
   attacks.  The attacking node can both send larger messages than the
   expressed capability as well as initiating a large number of
   concurrent user message transmissions that never are concluded.  For
   the target of the attack it is more straight forward to determine
   that a peer is ignoring the node's stated limitation.

7.4.  Authentication and Policy Decisions

   DTLS/SCTP MUST be mutually authenticated.  It is RECOMMENDED that
   DTLS/SCTP is used with certificate based authentication.  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.

7.5.  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 clear text, 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 [I-D.ietf-tls-dtls13] to provide identity
   protection.  DTLS/SCTP MUST be used with a key exchange method
   providing Perfect Forward Secrecy.  Perfect Forward Secrecy
   significantly limits the amount of data that can be compromised due
   to key compromise.

7.6.  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.

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

   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
   increases the risk of identifying individual users.

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8.  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

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [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, <>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,

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   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

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

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-40, 20 January 2021, <

9.2.  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,

   [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,

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   [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,

   [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,

   [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,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [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, <>.

   [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, <>.

              Agence nationale de la sécurité des systèmes
              d'information, ., "Recommendations for securing networks
              with IPsec", ANSSI Technical Report DAT-NT-003 , August

Appendix A.  Motivation for Changes

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

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   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.  Thus, this document mandates usage of relevant
   versions and algorithms.

   Clarifications: Some implementation experiences has 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


   John Preuß Mattsson


   Claudio Porfiri


   Michael Tüxen
   Münster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt


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