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Network Working Group                                          M. Tuexen
Internet-Draft                        Muenster Univ. of Applied Sciences
Intended status: Standards Track                            C. Hohendorf
Expires: April 25, 2007                     University of Duisburg-Essen
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                        October 22, 2006


   Datagram Transport Layer Security for Stream Control Transmission
                                Protocol
                   draft-tuexen-dtls-for-sctp-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

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




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

   o  multiple streams to avoid head of line blocking.

   o  multi-homing to provide network level fault tolerance.

   o  unordered delivery.

   o  partial reliable data transfer.


Table of Contents

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




























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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 RFC DTLS [8], over the Stream
   Control Transmission Protocol (SCTP), as defined in RFC2960 [4] and
   RFC3309 [5].

   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 [1].

   TLS over SCTP as described in RFC3436 [6] has some serious
   limitations:

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

   o  It does not support partial reliablility as defined in RFC3758
      [7].

   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.  The user of DTLS over SCTP can use all
   services provided by SCTP and its paritial reliability extension.
   The dynamic modification of the IP-addresses used by the SCTP enp-
   points is alos 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 SCTP-
   AUTH [9].

1.2.  Terminology

   This document uses the following terms:

   Association:  An SCTP association.






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   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

   MTU:  Maximum Transmission Unit

   SCTP:  Stream Control Transmission Protocol

   TCP:  Transmission Control Protocol

   TLS:  Transport Layer Security


2.  Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD.
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   RFC2119 [2].


3.  DTLS considerations

3.1.  Message 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 user message for SCTP.

3.2.  Message sizes

   DTLS imposes an limit in the user message size.  This limit applies
   also to DTLS/SCTP.

3.3.  Replay detection

   Replay detection of DTLS MUST not be used.






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3.4.  Changing of Cipher Specs

   Whenever Cipher Specs are changed a new shared secret MUST be derived
   from the master secret and used for SCTP-AUTH.  The shared key
   identifier used by SCTP-AUTH MUST be incremented.


4.  SCTP considerations

4.1.  Stream usage

   All DTLS control messages MUST be transported on stream 0 with
   unlimited reliability and with the ordered delivery feature.

   User data messages MAY be transported over stream 0 but users SHOULD
   use other streams for better performance.

4.2.  Chunk handling

   The DATA, SACK and FORWARD-TSN chunks of SCTP MUST be sent in an
   authenticated way as described in SCTP-AUTH [9].  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.

4.3.  Handling of endpoint-pair shared secrets

   The endpoint-pair shared secret for Shared Key Identifier 0 is empty.
   After DTLS cipher specs are changed, a 64 byte shared secret is
   derived from the master secret and used a the new end-point pair
   shared secret.  The shared Key identifier MUST be incremented by 1.
   If it is 65535, the next value MUST be 1.  The next version of the ID
   will specify how the shared secret is derived from the master secret.


5.  IANA Considerations

   This document does not require any actions from IANA.


6.  Security Considerations

   This section is not complete yet.





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7.  Normative References

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

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

   [3]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
         RFC 2246, January 1999.

   [4]   Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
         Paxson, "Stream Control Transmission Protocol", RFC 2960,
         October 2000.

   [5]   Stone, J., Stewart, R., and D. Otis, "Stream Control
         Transmission Protocol (SCTP) Checksum Change", RFC 3309,
         September 2002.

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

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

   [8]   Rescorla, E. and N. Modadugu, "Datagram Transport Layer
         Security", draft-rescorla-dtls-05 (work in progress),
         June 2005.

   [9]   Tuexen, M., "Authenticated Chunks for Stream Control
         Transmission Protocol (SCTP)", draft-ietf-tsvwg-sctp-auth-04
         (work in progress), September 2006.

   [10]  Stewart, R., "Stream Control Transmission Protocol (SCTP)
         Dynamic Address  Reconfiguration",
         draft-ietf-tsvwg-addip-sctp-15 (work in progress), June 2006.












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Authors' Addresses

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

   Email: tuexen@fh-muenster.de


   Carsten Hohendorf
   University of Duisburg-Essen
   Ellernstrasse 29
   Essen, SC  45326
   Germany

   Email: hohend@iem.uni-due.de


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

   Phone: +1 650-320-8549
   Email: ekr@rtfm.com























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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
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