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Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets


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Internet Engineering Task Force (IETF)                         M. Tuexen
Request for Comments: 8261              Muenster Univ. of Appl. Sciences
Category: Standards Track                                     R. Stewart
ISSN: 2070-1721                                            Netflix, Inc.
                                                                R. Jesup
                                                WorldGate Communications
                                                               S. Loreto
                                                           November 2017

 Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets


   The Stream Control Transmission Protocol (SCTP) is a transport
   protocol originally defined to run on top of the network protocols
   IPv4 or IPv6.  This document specifies how SCTP can be used on top of
   the Datagram Transport Layer Security (DTLS) protocol.  Using the
   encapsulation method described in this document, SCTP is unaware of
   the protocols being used below DTLS; hence, explicit IP addresses
   cannot be used in the SCTP control chunks.  As a consequence, the
   SCTP associations carried over DTLS can only be single-homed.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2017 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Encapsulation and Decapsulation Procedure . . . . . . . . . .   3
   4.  General Considerations  . . . . . . . . . . . . . . . . . . .   4
   5.  DTLS Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  SCTP Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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1.  Overview

   The Stream Control Transmission Protocol (SCTP) as defined in
   [RFC4960] is a transport protocol running on top of the network
   protocols IPv4 [RFC0791] or IPv6 [RFC8200].  This document specifies
   how SCTP is used on top of the Datagram Transport Layer Security
   (DTLS) protocol.  DTLS 1.0 is defined in [RFC4347], and the latest
   version when this RFC was published, DTLS 1.2, is defined in
   [RFC6347].  This encapsulation is used, for example, within the
   WebRTC protocol suite (see [RTC-OVERVIEW] for an overview) for
   transporting non-SRTP data between browsers.  The architecture of
   this stack is described in [DATA-CHAN].

                               |   SCTP   |
                               |   DTLS   |
                               | ICE/UDP  |

                       Figure 1: Basic Stack Diagram

   This encapsulation of SCTP over DTLS over UDP or ICE/UDP (see
   [RFC5245]) can provide a NAT traversal solution in addition to
   confidentiality, source authentication, and integrity-protected
   transfers.  Please note that using ICE does not necessarily imply
   that a different packet format is used on the wire.

   Please note that the procedures defined in [RFC6951] for dealing with
   the UDP port numbers do not apply here.  When using the encapsulation
   defined in this document, SCTP is unaware about the protocols used
   below DTLS.

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.  Encapsulation and Decapsulation Procedure

   When an SCTP packet is provided to the DTLS layer, the complete SCTP
   packet, consisting of the SCTP common header and a number of SCTP
   chunks, is handled as the payload of the application-layer protocol
   of DTLS.  When the DTLS layer has processed a DTLS record containing

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   a message of the application-layer protocol, the payload is passed to
   the SCTP layer.  The SCTP layer expects an SCTP common header
   followed by a number of SCTP chunks.

4.  General Considerations

   An implementation of SCTP over DTLS MUST implement and use a path
   maximum transmission unit (MTU) discovery method that functions
   without ICMP to provide SCTP/DTLS with an MTU estimate.  An
   implementation of "Packetization Layer Path MTU Discovery" [RFC4821]
   either in SCTP or DTLS is RECOMMENDED.

   The path MTU discovery is performed by SCTP when SCTP over DTLS is
   used for data channels (see Section 5 of [DATA-CHAN]).

5.  DTLS Considerations

   The DTLS implementation MUST support DTLS 1.0 [RFC4347] and SHOULD
   support the most recently published version of DTLS, which was DTLS
   1.2 [RFC6347] when this RFC was published.  In the absence of a
   revision to this document, the latter requirement applies to all
   future versions of DTLS when they are published as RFCs.  This
   document will only be revised if a revision to DTLS or SCTP makes a
   revision to the encapsulation necessary.

   SCTP performs segmentation and reassembly based on the path MTU.
   Therefore, the DTLS layer MUST NOT use any compression algorithm.

   The DTLS MUST support sending messages larger than the current path
   MTU.  This might result in sending IP-level fragmented messages.

   If path MTU discovery is performed by the DTLS layer, the method
   described in [RFC4821] MUST be used.  For probe packets, the
   extension defined in [RFC6520] MUST be used.

   If path MTU discovery is performed by the SCTP layer and IPv4 is used
   as the network-layer protocol, the DTLS implementation SHOULD allow
   the DTLS user to enforce that the corresponding IPv4 packet is sent
   with the Don't Fragment (DF) bit set.  If controlling the DF bit is
   not possible (for example, due to implementation restrictions), a
   safe value for the path MTU has to be used by the SCTP stack.  It is
   RECOMMENDED that the safe value not exceed 1200 bytes.  Please note
   that [RFC1122] only requires that end hosts be able to reassemble
   fragmented IP packets up to 576 bytes in length.

   The DTLS implementation SHOULD allow the DTLS user to set the
   Differentiated Services Code Point (DSCP) used for IP packets being
   sent (see [RFC2474]).  This requires the DTLS implementation to pass

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   the value through and the lower layer to allow setting this value.
   If the lower layer does not support setting the DSCP, then the DTLS
   user will end up with the default value used by the protocol stack.
   Please note that only a single DSCP value can be used for all packets
   belonging to the same SCTP association.

   Using Explicit Congestion Notification (ECN) in SCTP requires the
   DTLS layer to pass the ECN bits through and its lower layer to expose
   access to them for sent and received packets (see [RFC3168]).  The
   implementations of DTLS and its lower layer have to provide this
   support.  If this is not possible (for example, due to implementation
   restrictions), ECN can't be used by SCTP.

6.  SCTP Considerations

   This section describes the usage of the base protocol and the
   applicability of various SCTP extensions.

6.1.  Base Protocol

   This document uses SCTP [RFC4960] with the following restrictions,
   which are required to reflect that the lower layer is DTLS instead of
   IPv4 and IPv6 and that SCTP does not deal with the IP addresses or
   the transport protocol used below DTLS:

   o  A DTLS connection MUST be established before an SCTP association
      can be set up.

   o  Multiple SCTP associations MAY be multiplexed over a single DTLS
      connection.  The SCTP port numbers are used for multiplexing and
      demultiplexing the SCTP associations carried over a single DTLS

   o  All SCTP associations are single-homed, because DTLS does not
      expose any address management to its upper layer.  Therefore, it
      is RECOMMENDED to set the SCTP parameter path.max.retrans to

   o  The INIT and INIT-ACK chunk MUST NOT contain any IPv4 Address or
      IPv6 Address parameters.  The INIT chunk MUST NOT contain the
      Supported Address Types parameter.

   o  The implementation MUST NOT rely on processing ICMP or ICMPv6
      packets, since the SCTP layer most likely is unable to access the
      SCTP common header in the plain text of the packet, which
      triggered the sending of the ICMP or ICMPv6 packet.  This applies
      in particular to path MTU discovery when performed by SCTP.

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   o  If the SCTP layer is notified about a path change by its lower
      layers, SCTP SHOULD retest the path MTU and reset the congestion
      state to the initial state.  The window-based congestion control
      method specified in [RFC4960] resets the congestion window and
      slow-start threshold to their initial values.

6.2.  Padding Extension

   When the SCTP layer performs path MTU discovery as specified in
   [RFC4821], the padding extension defined in [RFC4820] MUST be
   supported and used for probe packets (HEARTBEAT chunks bundled with
   PADDING chunks [RFC4820]).

6.3.  Dynamic Address Reconfiguration Extension

   If the dynamic address reconfiguration extension defined in [RFC5061]
   is used, ASCONF chunks MUST use wildcard addresses only.

6.4.  SCTP Authentication Extension

   The SCTP authentication extension defined in [RFC4895] can be used
   with DTLS encapsulation, but does not provide any additional benefit.

6.5.  Partial Reliability Extension

   Partial reliability as defined in [RFC3758] can be used in
   combination with DTLS encapsulation.  It is also possible to use
   additional Partially Reliable Stream Control Transmission Protocol
   (PR-SCTP) policies, for example, the ones defined in [RFC7496].

6.6.  Stream Reset Extension

   The SCTP stream reset extension defined in [RFC6525] can be used with
   DTLS encapsulation.  It is used to reset SCTP streams and add SCTP
   streams during the lifetime of the SCTP association.

6.7.  Interleaving of Large User Messages

   SCTP as defined in [RFC4960] does not support the interleaving of
   large user messages that need to be fragmented and reassembled by the
   SCTP layer.  The protocol extension defined in [RFC8260] overcomes
   this limitation and can be used with DTLS encapsulation.

7.  IANA Considerations

   This document does not require any IANA actions.

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8.  Security Considerations

   Security considerations for DTLS are specified in [RFC4347] and for
   SCTP in [RFC4960], [RFC3758], and [RFC6525].  The combination of SCTP
   and DTLS introduces no new security considerations.

   SCTP should not process the IP addresses used for the underlying
   communication since DTLS provides no guarantees about them.

   It should be noted that the inability to process ICMP or ICMPv6
   messages does not add any security issue.  When SCTP is carried over
   a connection-less lower layer like IPv4, IPv6, or UDP, processing of
   these messages is required to protect other nodes not supporting
   SCTP.  Since DTLS provides a connection-oriented lower layer, this
   kind of protection is not necessary.

9.  References

9.1.  Normative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

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

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,

   [RFC4820]  Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
              Parameter for the Stream Control Transmission Protocol
              (SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

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

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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, <>.

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,

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

9.2.  Informative References

              Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data
              Channels", Work in Progress, draft-ietf-rtcweb-data-
              channel-13, January 2015.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [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, <>.

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   [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,

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,

   [RFC6525]  Stewart, R., Tuexen, M., and P. Lei, "Stream Control
              Transmission Protocol (SCTP) Stream Reconfiguration",
              RFC 6525, DOI 10.17487/RFC6525, February 2012,

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951,
              DOI 10.17487/RFC6951, May 2013,

   [RFC7496]  Tuexen, M., Seggelmann, R., Stewart, R., and S. Loreto,
              "Additional Policies for the Partially Reliable Stream
              Control Transmission Protocol Extension", RFC 7496,
              DOI 10.17487/RFC7496, April 2015,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 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, November

              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", Work in Progress, draft-ietf-
              rtcweb-overview-18, March 2017.

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   The authors wish to thank David Black, Benoit Claise, Spencer
   Dawkins, Francis Dupont, Gorry Fairhurst, Stephen Farrell, Christer
   Holmberg, Barry Leiba, Eric Rescorla, Tom Taylor, Joe Touch, and
   Magnus Westerlund for their invaluable comments.

Authors' Addresses

   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt


   Randall R. Stewart
   Netflix, Inc.
   Chapin, SC  29036
   United States of America


   Randell Jesup
   WorldGate Communications
   3800 Horizon Blvd, Suite #103
   Trevose, PA  19053-4947
   United States of America

   Phone: +1-215-354-5166

   Salvatore Loreto
   Hirsalantie 11
   Jorvas  02420