Network Working Group                                          M. Tuexen
Internet-Draft                                             R. Seggelmann
Intended status: Standards Track      Muenster Univ. of Applied Sciences
Expires: April 25, 2009 January 9, 2010                                     E. Rescorla
                                                              RTFM, Inc.
                                                        October 22, 2008
                                                            July 8, 2009

   Datagram Transport Layer Security for Stream Control Transmission

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   This document describes the usage of the Datagram Transport Layer
   Security (DTLS) protocol over the Stream Control Transmission
   Protocol (SCTP).

   The user of DTLS over SCTP can take advantage of all features
   provided by SCTP and its extensions, especially support of

   o  multi-homing to provide network level fault tolerance.

   o  multiple streams to avoid head of line blocking.

   o  multi-homing to provide network level fault tolerance.

   o  unordered delivery.

   o  dynamic reconfiguration of streams.

   o  partially reliable data transfer.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5
   3.  DTLS considerations Considerations . . . . . . . . . . . . . . . . . . . . . . 4 5
   4.  SCTP considerations Considerations . . . . . . . . . . . . . . . . . . . . . . 5 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6 7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

1.  Introduction

1.1.  Overview

   This document describes the usage of the Datagram Transport Layer
   Security (DTLS) protocol, as defined in [RFC4347], over the Stream
   Control Transmission Protocol (SCTP), as defined in [RFC4960].

   TLS is designed to run on top of a byte-stream oriented transport
   protocol providing a reliable, in-sequence delivery.  Thus, TLS is
   currently mainly being used on top of the Transmission Control
   Protocol (TCP), as defined in [RFC0793].

   TLS over SCTP as described in [RFC3436] has some serious limitations:

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

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

   o  It only supports the usage of the same number of streams in both

   o  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.

   DTLS over SCTP as described in this document overcomes these
   limitations of TLS over SCTP.  The user of DTLS over SCTP can use all
   services provided by SCTP and its partial reliability extension.  The
   dynamic modification of the IP-addresses used by the SCTP end-points
   is also supported.

   The method described in this document requires that the SCTP
   implementation supports the optional feature of fragmentation of SCTP
   user messages and the SCTP authentication extension defined in

1.2.  Terminology

   This document uses the following terms:

   Association:  An SCTP association.

   Connection:  A TLS connection.

   Session:  A TLS session.

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

1.3.  Abbreviations

   DTLS:  Datagram Transport Layer Security Security.

   MTU:  Maximum Transmission Unit Unit.

   PPID:  Payload Protocol Identifier.

   SCTP:  Stream Control Transmission Protocol Protocol.

   TCP:  Transmission Control Protocol Protocol.

   TLS:  Transport Layer Security Security.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  DTLS considerations Considerations

3.1.  Message sizes Sizes

   DTLS limits the DTLS user message size to the current Path MTU. MTU minus
   the header sizes.  This limit SHOULD be increased to 2^14 Bytes for DTLS/SCTP.
   DTLS over SCTP.

3.2.  Message fragmentation Fragmentation

   The DTLS layer MUST NOT perform message fragmentation.  The SCTP
   layer will perform this task.  Thus the supported maximum length of
   SCTP user messages MUST be at least 2^14 + 2048 + 5 = 18437 bytes.
   Every DTLS message MUST be handled as one SCTP user message for SCTP. message.

3.3.  Replay detection Detection

   Replay detection of DTLS MUST NOT be used.

3.4.  Path MTU Discovery

   Path MTU discovery of DTLS MUST NOT be used.

3.5.  Retransmission of Messages

   DTLS procedures for retransmissions MUST NOT be used.

4.  SCTP considerations Considerations

4.1.  Payload Protocol Identifier Usage

   Application protocols running over DTLS over SCTP SHOULD register and
   use a separate payload protocol identifier (PPID) and SHOULD NOT
   reuse the PPID which they registered for running directly over SCTP.

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

4.2.  Stream usage Usage

   All DTLS control messages of the ChangeCipherSpec, Alert, or Handshake
   protocol MUST be transported on stream 0 with unlimited reliability
   and with the ordered delivery feature.

   User data

   All DTLS messages of the ApplicationData protocol MAY be transported
   over stream 0 but users SHOULD use other streams for better


4.3.  Chunk handling Handling

   The DATA, SACK and FORWARD-TSN chunks of SCTP MUST be sent in an
   authenticated way as described in [RFC4895].  Other chunks MAY be
   sent in an authenticated way.

   This makes sure that an attacker can not modify the stream a message
   is sent in or affect the ordered/unordered delivery of the message.
   It is also not possible for an attacker to drop messages and use
   forged FORWARD-TSN and SACK chunks to hide this dropping.


4.4.  Handshake

   To prevent DTLS from discarding DTLS user messages while
   renegotiating, before sending a ClientHello ChangeCipherSpec message all
   outstanding SCTP user messages MUST have been acknowledged by the
   SCTP peer and can not MUST NOT be revoked anymore by the SCTP peer.

   Prior to sending processing a HelloVerifyRequest, received ChangeCipherSpec all other received
   SCTP user messages which are buffered in the SCTP layer MUST be read
   from the transport layer or user
   and processed by DTLS.

   User messages arriving between ChangeCipherSpec and Finished using
   the new epoch have probably passed the Finished and MUST be allowed during

4.4. buffered
   by DTLS until the Finished is read.

4.5.  Handling of endpoint-pair shared secrets Endpoint-pair Shared Secrets

   The endpoint-pair shared secret for Shared Key Identifier 0 is empty.
   Whenever the master key changes, a 64 byte shared secret is derived
   from every master secret and provided as a new end-point pair shared
   secret by using the algorithm described in [I-D.ietf-tls-extractor].

   The Shared Key Identifier MUST be incremented by 1.  If it is 65535,
   the next value MUST be 1.

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

   Once the corresponding Finished message MUST NOT from the peer has been
   received the old key SHOULD be sent removed.

4.6.  Shutdown

   To prevent DTLS from discarding DTLS user messages while shutting
   down, before sending a CloseNotify message all outstanding SCTP user
   messages except the ones from this handshake MUST have been acknowledged by the SCTP peer and can not MUST NOT be
   revoked anymore by the SCTP peer.

   Once the corresponding Finished message from the peer has been

   Prior to processing a received CloseNotify all other received SCTP
   user messages which are buffered in the old key SHOULD SCTP layer MUST be removed. read and
   processed by DTLS.

5.  IANA Considerations

   IANA needs to add a value to the TLS Extractor Exporter Label registry as
   described in [I-D.ietf-tls-extractor].  The label suggested is
   EXTRACTOR_DTLS_OVER_SCTP.  The reference should refer to this

6.  Security Considerations

   This section is document does not complete yet. add any additional security considerations in
   addition to the ones given in [RFC4347] and [RFC4895].

7.  Acknowledgments

   The authors wish to thank Carsten Hohendorf, and Alfred Hoenes for
   their invaluable comments.

8.  References

8.1.  Normative References

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

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758, May 2004.

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

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.

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

              Rescorla, E., "Keying Material Extractors Exporters for Transport
              Layer Security (TLS)", draft-ietf-tls-extractor-02 draft-ietf-tls-extractor-05 (work
              in progress), September 2008. March 2009.

8.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC3436]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
              Layer Security over Stream Control Transmission Protocol",
              RFC 3436, December 2002.

Authors' Addresses

   Michael Tuexen
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt


   Robin Seggelmann
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt


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


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