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Versions: (draft-tuexen-tsvwg-dtls-for-sctp) 00 01 02 03 04 05 06 RFC 6083

Network Working Group                                          M. Tuexen
Internet-Draft                                             R. Seggelmann
Intended status: Standards Track      Muenster Univ. of Applied Sciences
Expires: September 23, 2010                                  E. Rescorla
                                                              RTFM, Inc.
                                                          March 22, 2010


Datagram Transport Layer Security (DTLS) for Stream Control Transmission
                            Protocol (SCTP)
                 draft-ietf-tsvwg-dtls-for-sctp-05.txt

Abstract

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

   Security features provided by DTLS over SCTP include authentication,
   message integrity and privacy of user messages.  Applications using
   DTLS over SCTP can use almost all transport features provided by SCTP
   and its extensions.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 23, 2010.

Copyright Notice




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   Copyright (c) 2010 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
   (http://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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  DTLS Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   4.  SCTP Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9
























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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].

   Security features provided by DTLS over SCTP include authentication,
   message integrity and privacy of user messages.  Applications using
   DTLS over SCTP can use almost all transport features provided by SCTP
   and its extensions.

   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.  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
      directions.

   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.  Especially DTLS/SCTP supports

   o  preservation of message boundaries.

   o  a large number of uni-directional and bi-directional streams.

   o  ordered and unordered delivery of SCTP user messages.

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

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

   However, the following limitations still apply:





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   o  The maximum user message size is 2^14 bytes, which is the DTLS
      limit.

   o  The DTLS user can not perform the SCTP-AUTH key management,
      because this is done by the DTLS layer.

   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] and the SCTP authentication
   extension defined in [RFC4895].

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.

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  DTLS Considerations

3.1.  Future Versions of DTLS

   This document is based on [RFC4347].  If a new RFC updates or
   obsoletes [RFC4347], this documents also applies to the newer



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   document defining DTLS unless this document also gets updated or
   revised.

3.2.  Message Sizes

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

3.3.  Replay 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

4.1.  Mapping of DTLS Records

   The supported maximum length of SCTP user messages MUST be at least
   2^14 + 2048 + 13 = 18445 bytes (2^14 + 2048 is the maximum length of
   the DTLSCiphertext.fragment and 13 is the size of the DTLS record
   header).  In particular, the SCTP implementation MUST support
   fragmentation of user messages.

   Every SCTP user message MUST consist of exactly one DTLS record.

4.2.  DTLS connection handling

   Each DTLS connection MUST be established and terminated within the
   same SCTP association.  A DTLS connection MUST NOT span multiple SCTP
   associations.

4.3.  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 that they registered for running directly over SCTP.

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




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4.4.  Stream Usage

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

   All DTLS messages of the ApplicationData protocol MAY be transported
   over stream 0, but users SHOULD use other streams to avoid possible
   performance problems due to head of line blocking.

4.5.  Chunk Handling

   DATA 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 in which a
   message is sent in 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.

4.6.  Handshake

   A DTLS implementation discards DTLS messages from older epochs after
   some time, as described in section 4.1 of [RFC4347].  This is not
   acceptable when the DTLS user performs a reliable data transfer.  To
   avoid discarding messages, the following procedures are required.

   Before sending a ChangeCipherSpec message all outstanding SCTP user
   messages MUST have been acknowledged by the SCTP peer and MUST NOT be
   revoked anymore by the SCTP peer.

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

   User messages arriving between ChangeCipherSpec and Finished using
   the new epoch have probably passed the Finished and MUST be buffered
   by DTLS until the Finished is read.

4.7.  Handling of Endpoint-pair Shared Secrets

   The endpoint-pair shared secret for Shared Key Identifier 0 is empty
   and MUST be used when establishing a DTLS connection.  Whenever the
   master key changes, a 64 byte shared secret is derived from every



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   master secret and provided as a new end-point pair shared secret by
   using the exporter described in [RFC5705].  The exporter MUST use the
   label given in Section 5 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 Finished message the active SCTP-AUTH key MUST be
   switched to the new one.

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

4.8.  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 still 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.  IANA Considerations

   IANA needs to add a value to the TLS Exporter Label registry as
   described in [RFC5705].  The label suggested is
   "EXPORTER_DTLS_OVER_SCTP".  The reference should refer to this
   document.


6.  Security Considerations

   The security considerations given in [RFC4347], [RFC4895], and
   [RFC4960] also apply to this document.

   It is possible to authenticate DTLS endpoints based on IP-addresses
   in certificates.  SCTP associations can use multiple addresses per
   SCTP endpoint.  Therefore it is possible that DTLS records will be
   sent from a different IP-address than that originally authenticated.
   This is not a problem provided that no security decisions are made
   based on that IP-address.  This is a special case of a general rule:
   all decisions should be based on the peer's authenticated identity,
   not on its transport layer identity.

   For each message, the SCTP user also provides a stream identifier, a
   flag to indicate whether the message is sent ordered or unordered and



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   a payload protocol identifier.  Although DTLS can be used to provide
   privacy for the actual user message, none of these three are
   protected by DTLS.  They are sent as clear text, because they are
   part of the SCTP DATA chunk header.


7.  Acknowledgments

   The authors wish to thank Carsten Hohendorf, Alfred Hoenes, Daniel
   Mentz, Ian Goldberg, Anna Brunstrom, Stefan Lindskog, and Gorry
   Fairhurst 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.

   [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.

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

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

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.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)



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              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.


Authors' Addresses

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

   Email: tuexen@fh-muenster.de


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

   Email: seggelmann@fh-muenster.de


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

   Email: ekr@networkresonance.com




















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