Network Working Group M. Tuexen
Internet-Draft Muenster Univ. of Applied Sciences
Expires: August 28, 2006 C. Hohendorf
University of Duisburg-Essen
E. Rescorla
RTFM, Inc.
February 24, 2006
Datagram Transport Layer Security for Stream Control Transmission
Protocol
draft-tuexen-dtls-for-sctp-00.txt
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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 . . . . . . . . . . . . . . . . . . . . . . . . 7
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-01
(work in progress), October 2005.
[10] Stewart, R., "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration",
draft-ietf-tsvwg-addip-sctp-13 (work in progress),
November 2005.
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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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