Updated by:
Keywords: SCTP, Stream Control Transmission Protocol, TLS, Transport Layer Security
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.
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RFC 3436 TLS over SCTP December 2002
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.
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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.
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RFC 3436 TLS over SCTP December 2002
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].
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RFC 3436 TLS over SCTP December 2002
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),
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RFC 3436 TLS over SCTP December 2002
- 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.
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RFC 3436 TLS over SCTP December 2002
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.
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RFC 3436 TLS over SCTP December 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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RFC 3436 TLS over SCTP December 2002
13. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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