TLS over SCTP: Transport Layer Security over Stream Control Transmission Protocol

Network Working Group                                       A. Jungmaier
Request for Comments: 3436                           University of Essen
Category: Standards Track                                    E. Rescorla
                                                               RTFM Inc.
                                                               M. Tuexen
                                                              Siemens AG
                                                           December 2002


                     Transport Layer Security over
                  Stream Control Transmission Protocol

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document describes the usage of the Transport Layer Security
   (TLS) protocol, as defined in RFC 2246, over the Stream Control
   Transmission Protocol (SCTP), as defined in RFC 2960 and RFC 3309.

   The user of TLS can take advantage of the features provided by SCTP,
   namely the support of multiple streams to avoid head of line blocking
   and the support of multi-homing to provide network level fault
   tolerance.

   Additionally, discussions of extensions of SCTP are also supported,
   meaning especially the support of dynamic reconfiguration of IP-
   addresses.

1.  Introduction

1.1.  Overview

   This document describes the usage of the Transport Layer Security
   (TLS) protocol, as defined in [RFC2246], over the Stream Control
   Transmission Protocol (SCTP), as defined in [RFC2960] and [RFC3309].

   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 [RFC793].

   Comparing TCP and SCTP, the latter provides additional features and
   this document shows how TLS should be used with SCTP to provide some
   of these additional features to the TLS user.

   This document defines:

   -   how to use the multiple streams feature of SCTP.

   -   how to handle the message oriented nature of SCTP.

   It should be noted that the TLS user can take advantage of the multi-
   homing support of SCTP.  The dynamic reconfiguration of IP-addresses,
   as currently being discussed, can also be used with the described
   solution.

   The method described in this document does not require any changes of
   TLS or SCTP.  It is only required that SCTP implementations support
   the optional feature of fragmentation of SCTP user messages.

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

   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", in this document are to be interpreted as described in
   BCP 14, RFC 2119 [RFC2119].

3.  SCTP Requirements

3.1.  Number of Inbound and Outbound Streams

   An association between the endpoints A and Z provides n streams from
   A to Z and m streams from Z to A.

   A pair consisting of two streams with the same stream identifier is
   considered and used as one bi-directional stream.

   Thus an SCTP association can be considered as a set of min(n,m) bi-
   directional streams and (max(n,m) - min(n,m)) uni-directional
   streams.

3.2.  Fragmentation of User Messages

   To avoid the knowledge and handling of the MTU inside TLS, SCTP MUST
   provide fragmentation of user messages, which is an optional feature
   of [RFC2960].  Since SCTP is a message oriented protocol, it must be
   able to transmit all TLS records as SCTP user messages.  Thus the
   supported maximum length of SCTP user messages MUST be at least 2^14
   + 2048 + 5 = 18437 bytes, which is the maximum length of a
   TLSCiphertext, as defined in [RFC2246].

   Please note that an SCTP implementation might need to support the
   partial delivery API to be able to support the transport of user
   messages of this size.

   Therefore, SCTP takes care of fragmenting and reassembling the TLS
   records in order to avoid IP-fragmentation.

4. TLS Requirements

4.1 Supported Ciphersuites

   A TLS implementation for TLS over SCTP MUST support at least the
   ciphersuite TLS_RSA_WITH_AES_128_CBC_SHA as defined in [RFC3268].

5.  Connections and Bi-directional Streams

   TLS makes use of a bi-directional stream by establishing a connection
   over it.  This means that the number of connections for an
   association is limited by the number of bi-directional streams.

   The TLS handshake protocol is used on each bi-directional stream
   separately.  Each handshake can be:

   -  a full handshake or

   -  an abbreviated handshake that resumes a TLS session with a session
      id from another connection (on the same or another association).

   After completing the handshake for a connection, the bi-directional
   stream can be used for TLS-based user data transmission.  It should
   also be noted that the handshakes for the different connections are
   independent and can be delayed until the bi-directional stream is
   used for user data transmission.

6.  Usage of bi-directional streams

   It is not required that all bi-directional streams are used for TLS-
   based user data transmission.  If TLS is not used, it is called SCTP-
   based user data transmission.

6.1.  SCTP-based user data transmission

   If a bi-directional stream is not used for TLS-based communication
   there are no restrictions on the features provided by SCTP for SCTP-
   based user data transmission.

6.2.  TLS-based user data transmission

   In general, the bi-directional stream will be used for TLS-based user
   data transmission and it SHOULD NOT be used for SCTP-based user data
   transmission.  The exception to this rule is for protocols which
   contain upgrade-to-TLS mechanisms, such as those of HTTP upgrade
   [RFC2817] and SMTP over TLS [RFC3207].

   TLS requires that the underlying transport delivers TLS records in
   strict sequence.  Thus, the 'unordered delivery' feature of SCTP MUST
   NOT be used on streams which are used for TLS based user data
   transmission.  For the same reason, TLS records delivered to SCTP for
   transmission MUST NOT have limited lifetimes.

7.  Usage of uni-directional streams

   The uni-directional streams can not be used for TLS-based user data
   transmission.  Nevertheless, they can be used without any
   restrictions for SCTP-based communication.

8.  Examples

   In these examples we consider the case of an association with two
   bi-directional streams.

8.1.  Two Bi-directional Streams with Full Handshake

   Just after the association has been established, the client sends two
   ClientHello messages on the bi-directional streams 0 and 1.  After a
   full handshake has been completed on each bi-directional stream,
   TLS-based user data transmission can take place on that stream.  It
   is possible that on the bi-directional stream 0, the handshake has
   been completed, and user data transmission is ongoing, while on the
   bi-directional stream 1, the handshake has not been completed, or
   vice versa.

8.2.  Two Bi-directional Streams with an Abbreviated Handshake

   After establishing the association, the client starts a full
   handshake on the bi-directional stream 0.  The server provides a
   session identifier which allows session resumption.  After the full
   handshake has been completed, the client initiates an abbreviated
   handshake on the bi-directional stream 1, using the session
   identifier from the handshake on the bi-directional stream 0.  User
   data can be transmitted on the bi-directional stream 0, but not on
   the bi-directional stream stream 1 in that state.  After completion
   of the abbreviated handshake on the bi-directional stream 1, user
   data can be transmitted on both streams.

   Whether or not to use abbreviated handshakes during the setup phase
   of a TLS connection over an SCTP association depends on several
   factors:

   -  the complexity and duration of the initial handshake processing
      (also determined by the number of connections),
   -  the network performance (round-trip times, bandwidth).

   Abbreviated handshakes can reduce computational complexity of the
   handshake considerably, in case this is a limiting resource.  If a
   large number of connections need to be established, it may be
   advantageous to use the TLS session resumption feature.  On the other
   hand, before an abbreviated handshake can take place, a full
   handshake needs to have been completed.  In networks with large
   round-trip time delays, it may be favorable to perform a number of
   full handshakes in parallel. Therefore, both possibilities are
   allowed.

8.3.  Two Bi-directional Streams with a Delayed Abbreviated Handshake

   This example resembles the last one, but after the completion of the
   full handshake on the bi-directional stream 0, the abbreviated
   handshake on the bi-directional stream 1 is not started immediately.
   The bi-directional stream 0 can be used for user data transmission.
   It is only when the user also wants to transmit data on the bi-
   directional stream 1 that the abbreviated handshake for the bi-
   directional stream 1 is initiated.

   This allows the user of TLS to request a large number of bi-
   directional streams without having to provide all the resources at
   association start-up if not all bi-directional streams are used right
   from the beginning.

8.4.  Two Bi-directional Streams without Full Handshakes

   This example is like the second and third one, but an abbreviated
   handshake is used for both bi-directional streams.  This requires the
   existence of a valid session identifier from connections handled by
   another association.

9.  Security Considerations

   Using TLS on top of SCTP does not provide any new security issues
   beside the ones discussed in [RFC2246] and [RFC2960].

   It is possible to authenticate TLS endpoints based on IP-addresses in
   certificates.  Unlike TCP, SCTP associations can use multiple
   addresses per SCTP endpoint.  Therefore it is possible that TLS
   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.

10.  Acknowledgements

   The authors would like to thank P. Calhoun, J. Wood, and many others
   for their invaluable comments and suggestions.

11.  References

11.1.  Normative References

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

   [RFC2246]   Diercks, T. and  C. Allen, "The TLS Protocol Version
               1.0", RFC 2246, January 1999.

   [RFC2960]   Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
               Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
               Zhang, L. and V. Paxon, "Stream Control Transmission
               Protocol", RFC 2960, October 2000.

   [RFC3268]   Chown, P., "Advanced Encryption Standard (AES)
               Ciphersuites for Transport Layer Security (TLS)", RFC
               3268, June 2002.

   [RFC3309]   Stone, J., Stewart, R., Otis, D., "Stream Control
               Transmission Protocol (SCTP) Checksum Change", RFC 3309,
               September 2002.

11.2.  Informative References

   [RFC793]    Postel, J. (ed.), "Transmission Control Protocol", STD 7,
               RFC 793, September 1981.

   [RFC2026]   Bradner, S., "The Internet Standards Process -- Revision
               3", BCP 9, RFC 2026, October 1996.

   [RFC2817]   Khare, R. and S. Lawrence, "Upgrading to TLS Within
               HTTP/1.1", RFC 2817, May 2000.

   [RFC3207]   Hoffman, P., "SMTP Service Extension for Secure SMTP over
               TLS", RFC 3207, February 2002.

12.  Authors' Addresses

   Andreas Jungmaier
   University of Essen
   Networking Technology Group at the IEM
   Ellernstrasse 29
   D-45326 Essen
   Germany

   Phone: +49 201 1837667
   EMail: ajung@exp-math.uni-essen.de


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

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


   Michael Tuexen
   Siemens AG
   D-81359 Munich
   Germany

   Phone: +49 89 722 47210
   EMail: Michael.Tuexen@siemens.com

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