Internet Engineering Task Force (IETF) M. Tuexen Request for Comments: 6083 R. Seggelmann Category: Standards Track Muenster Univ. of Applied Sciences ISSN: 2070-1721 E. Rescorla RTFM, Inc. January 2011 Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)
AbstractThis document describes the usage of the Datagram Transport Layer Security (DTLS) protocol over the Stream Control Transmission Protocol (SCTP). DTLS over SCTP provides communications privacy for applications that use SCTP as their transport protocol and allows client/server applications to communicate in a way that is designed to prevent eavesdropping and detect tampering or message forgery. Applications using DTLS over SCTP can use almost all transport features provided by SCTP and its extensions. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6083.
Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. DTLS Considerations . . . . . . . . . . . . . . . . . . . . . . 4 4. SCTP Considerations . . . . . . . . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 RFC4347], over the Stream Control Transmission Protocol (SCTP), as defined in [RFC4960]. DTLS over SCTP provides communications privacy for applications that use SCTP as their transport protocol and allows client/server applications to communicate in a way that is designed to prevent eavesdropping and detect tampering or message forgery. Applications using DTLS over SCTP can use almost all transport features provided by SCTP and its extensions. 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. In particular, DTLS/SCTP supports: o preservation of message boundaries. o a large number of unidirectional and bidirectional streams. o ordered and unordered delivery of SCTP user messages. o the partial reliability extension as defined in [RFC3758]. o the dynamic address reconfiguration extension as defined in [RFC5061]. However, the following limitations still apply: o The maximum user message size is 2^14 bytes, which is the DTLS limit. o The DTLS user cannot 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 as defined in [RFC4960] and the SCTP authentication extension defined in [RFC4895].
RFC2119]. RFC4347], and it is expected that DTLS/ SCTP as described in this document will work with future versions of DTLS. Section 3.2, DTLS can send maximum sized messages. Therefore, Path MTU discovery of DTLS MUST NOT be used.
Section 4.4. Therefore, DTLS procedures for retransmissions MUST NOT be used.
RFC4895]. Other chunks MAY be sent in an authenticated way. This makes sure that an attacker cannot modify the stream in which a message is sent or affect the ordered/unordered delivery of the message. If PR-SCTP as defined in [RFC3758] is used, FORWARD-TSN chunks MUST also be sent in an authenticated way as described in [RFC4895]. This makes sure that it is not possible for an attacker to drop messages and use forged FORWARD-TSN, SACK, and/or SHUTDOWN chunks to hide this dropping. Section 4.1 of [RFC4347]. This is not acceptable when the DTLS user performs a reliable data transfer. To avoid discarding messages, the following 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 by the SCTP peer. Prior to processing a received ChangeCipherSpec, all other received SCTP user messages that are buffered in the SCTP layer MUST be read and processed by DTLS. User messages that arrive between ChangeCipherSpec and Finished messages and use the new epoch have probably passed the Finished message and MUST be buffered by DTLS until the Finished message is read. RFC5705]. The exporter MUST use the
label given in Section 5 and no context. The new Shared Key Identifier MUST be the old Shared Key Identifier incremented by 1. If the old one is 65535, the new one 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. RFC5705]. The label is "EXPORTER_DTLS_OVER_SCTP". RFC4347], [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. For each message, the SCTP user also provides a stream identifier, a flag to indicate 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.
DTLS supports cipher suites that contain a NULL cipher algorithm. Negotiating a NULL cipher algorithm will not provide communications privacy for applications and will not provide privacy for user messages. [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. [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, April 2006. [RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, "Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)", RFC 4895, August 2007. [RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. [RFC5705] Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, March 2010. [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.