Network Working Group M. Tuexen
Internet-Draft R. Seggelmann
Intended status: Standards Track Muenster Univ. of Applied Sciences
Expires: April 26, 2010 E. Rescorla
RTFM, Inc.
October 23, 2009
Datagram Transport Layer Security for Stream Control Transmission
Protocol
draft-ietf-tsvwg-dtls-for-sctp-02.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).
The user of DTLS over SCTP can take advantage of most of the features
provided by SCTP and its extensions, especially support of
o multi-homing to provide network level fault tolerance.
o dynamic reconfiguration of IPv4 and IPv6 addresses.
o multiple streams to avoid head of line blocking.
o unordered delivery.
o dynamic reconfiguration of streams.
o partially reliable data transfer.
However, the DTLS maximum user message size limit of 2^14 bytes
applies also to DTLS over SCTP. Since DTLS over SCTP uses the SCTP-
AUTH extension, the DTLS user can not manage the keying material,
since this is done by the DTLS layer.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. DTLS Considerations . . . . . . . . . . . . . . . . . . . . . . 5
4. SCTP Considerations . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
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 [I-D.ietf-tls-rfc4347-bis],
over the Stream Control Transmission Protocol (SCTP), as defined in
[RFC4960].
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. The user of DTLS over SCTP can use
almost all services provided by SCTP and its partial reliability
extension. However, DTLS limits the user message size to 2^14 bytes.
The dynamic modification of the IP-addresses used by the SCTP end-
points is also supported. The same applies to the dynamic
reconfiguration of streams. The DTLS user can request SCTP chunk
types to be authenticated by using SCTP-AUTH as defined in [RFC4895].
However, 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 and the SCTP authentication extension defined in
[RFC4895].
1.2. Terminology
This document uses the following terms:
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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. 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.
3.2. Replay Detection
Replay detection of DTLS MUST NOT be used.
3.3. Path MTU Discovery
Path MTU discovery of DTLS MUST NOT be used.
3.4. Retransmission of Messages
DTLS procedures for retransmissions MUST NOT be used.
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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. 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 which they registered for running directly over SCTP.
This means in particular that there is no specific PPID for DTLS.
4.3. 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.
4.4. Chunk Handling
The DATA, SACK, SHUTDOWN, and FORWARD-TSN 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 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, SACK, and/or SHUTDOWN chunks to hide this
dropping.
4.5. Handshake
A DTLS implementation discards DTLS messages from older epochs after
some time as described in section 4.1 of [I-D.ietf-tls-rfc4347-bis].
This is not acceptable when the DTLS user performs a reliable data
transfer. To avoid the discarding of messages, the following
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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 which 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.
4.6. Handling of Endpoint-pair Shared Secrets
The endpoint-pair shared secret for Shared Key Identifier 0 is empty.
Whenever the master key changes, a 64 byte shared secret is derived
from every master secret and provided as a new end-point pair shared
secret by using the algorithm described in [I-D.ietf-tls-extractor].
The Shared Key Identifier MUST be incremented by 1. If it is 65535,
the next value 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.
4.7. Shutdown
To prevent DTLS from discarding DTLS user messages while shutting
down, before sending a CloseNotify 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 CloseNotify all other received SCTP
user messages which 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 [I-D.ietf-tls-extractor]. The label suggested is
EXTRACTOR_DTLS_OVER_SCTP. The reference should refer to this
document.
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6. Security Considerations
The security considerations given in [I-D.ietf-tls-rfc4347-bis],
[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.
The SCTP user provides for each user message also a stream
identifier, a flag whether the message is sent ordered or unordered
and 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.
If future SCTP extensions define chunk types which processing affect
the handling of TSNs, these chunk types MUST be sent in an
authenticated way as described in [RFC4895]. One example would be an
extension providing an alternate way of acknowledging TSNs.
7. Acknowledgments
The authors wish to thank Carsten Hohendorf, Alfred Hoenes, Daniel
Mentz, Ian Goldberg, Anna Brunstrom, and Stefan Lindskog 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.
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[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[]
Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", draft-ietf-tls-extractor-07 (work
in progress), September 2009.
[I-D.ietf-tls-rfc4347-bis]
Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security version 1.2", draft-ietf-tls-rfc4347-bis-03 (work
in progress), October 2009.
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.
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
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Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Email: ekr@networkresonance.com
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