RFC 6083

Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)

📅 published January 2011, from ietf

RFC 6083: Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP) 🎀

Okay so this one is the spec for running DTLS on top of SCTP, and it's giving main character energy 💅 It's a Standards Track document (ISSN 2070-1721) from January 2011, written by M. Tuexen and R. Seggelmann at Muenster Univ. of Applied Sciences, with E. Rescorla at RTFM, Inc. The header doesn't list anything it obsoletes or updates, so nothing gets replaced or amended here 🌸

Abstract ✨

This doc spells out how to use DTLS over SCTP 🦋

DTLS over SCTP gives apps that use SCTP as their transport protocol communications privacy 🔒 It lets client/server apps talk in a way that's designed to stop eavesdropping and catch tampering or message forgery 💖

Apps using DTLS over SCTP get to use almost every transport feature SCTP and its extensions offer 🧚

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.

Info about the current status of this document, any errata, and how to give feedback on it can be found at http://www.rfc-editor.org/info/rfc6083 💜

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 has to include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Simplified BSD License 🫶

Table of Contents 🌈

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

1. Introduction 💖

1.1. Overview

This document describes the usage of the Datagram Transport Layer Security (DTLS) protocol, as defined in [RFC4347], over the Stream Control Transmission Protocol (SCTP), as defined in [RFC4960].

DTLS over SCTP gives apps that use SCTP as their transport protocol communications privacy, and it lets client/server apps talk in a way designed to prevent eavesdropping and detect tampering or message forgery 🔐

Apps using DTLS over SCTP can use almost every transport feature SCTP offers, plus its extensions ✨

Okay, so TLS is where DTLS comes from, and TLS is built to run on top of a byte-stream-oriented transport protocol that gives reliable, in-sequence delivery. That's why TLS is mostly used on top of the Transmission Control Protocol (TCP), as defined in [RFC0793] 🌸

TLS over SCTP as described in [RFC3436] has some serious limitations 😭

  • It does not support the unordered delivery of SCTP user messages.
  • It does not support partial reliability as defined in [RFC3758].
  • It only supports the usage of the same number of streams in both directions.
  • 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:

  • preservation of message boundaries.
  • a large number of unidirectional and bidirectional streams.
  • ordered and unordered delivery of SCTP user messages.
  • the partial reliability extension as defined in [RFC3758].
  • the dynamic address reconfiguration extension as defined in [RFC5061].

However, the following limitations still apply 🙈

  • The maximum user message size is 2^14 bytes, which is the DTLS limit.
  • 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] 🔥

1.2. Terminology

This document uses the following terms:

  • 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 📝

Okay so 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. Version of DTLS

This document is based on [RFC4347], and it's expected that DTLS/SCTP as described here will work with future versions of DTLS 🦋

3.2. Message Sizes

DTLS limits the DTLS user message size to the current Path MTU minus the header sizes. For running over SCTP, the DTLS path MTU MUST be considered to be 2^14. 🎀

3.3. Replay Detection

The replay detection of DTLS may result in the DTLS layer dropping messages. Since DTLS/SCTP provides a reliable service if the application requests it, replay detection cannot be used. Therefore, replay detection of DTLS MUST NOT be used 🙅‍♀️

3.4. Path MTU Discovery

SCTP provides Path MTU discovery and fragmentation/reassembly for user messages. According to Section 3.2, DTLS can send maximum sized messages. Therefore, Path MTU discovery of DTLS MUST NOT be used 💅

3.5. Retransmission of Messages

SCTP provides a reliable and in-sequence transport service for DTLS messages that require it. See Section 4.4. Therefore, DTLS procedures for retransmissions MUST NOT be used 🫶

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. DTLS Connection Handling

Each DTLS connection MUST be established and terminated within the same SCTP association. A DTLS connection MUST NOT span multiple SCTP associations 🌈

4.3. Payload Protocol Identifier Usage

Application protocols using DTLS over SCTP SHOULD register and use a separate payload protocol identifier (PPID), and they SHOULD NOT reuse the PPID they registered for running directly over SCTP 🏷️

Using the same PPID doesn't harm as long as the application can determine whether or not DTLS is used. However, for protocol analyzers, for example, it's way easier if a separate PPID is used 🙈

This means, in particular, that there is no specific PPID for DTLS 💜

4.4. 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 🔒

DTLS messages of the ApplicationData protocol SHOULD use multiple streams other than stream 0. They MAY use stream 0 for everything if they don't care about minimizing head of line blocking ✨

4.5. Chunk Handling

DATA 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't modify the stream a message is sent in, 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 it's not possible for an attacker to drop messages and then use forged FORWARD-TSN, SACK, and/or SHUTDOWN chunks to hide that dropping 💀

4.6. Renegotiation

DTLS supports renegotiation, so this feature is also available by DTLS/SCTP. It's up to the upper layer to use or allow it, or not. Application writers should be aware that allowing renegotiations may result in changes of security parameters 👀

4.7. Handshake

A DTLS implementation discards DTLS messages from older epochs after some time, as described in 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 they 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 they MUST be buffered by DTLS until the Finished message is read 🫧

4.8. Handling of Endpoint-Pair Shared Secrets

The endpoint-pair shared secret for Shared Key Identifier 0 is empty, and it MUST be used when establishing a DTLS connection. Whenever the master key changes, a 64-byte shared secret is derived from every master secret and provided as a new endpoint-pair shared secret by using the exporter described in [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 🗑️

4.9. Shutdown

To prevent DTLS from discarding DTLS user messages while it's shutting down, a CloseNotify message MUST only be sent after all outstanding SCTP user messages have been acknowledged by the SCTP peer, and it MUST NOT still be revoked by the SCTP peer 🚪

Prior to processing a received CloseNotify, all other received SCTP user messages that are buffered in the SCTP layer MUST be read and processed by DTLS 🥹

5. IANA Considerations 🌷

IANA added a value to the TLS Exporter Label registry as described in [RFC5705]. The label is "EXPORTER_DTLS_OVER_SCTP" ✨

6. Security Considerations 🔐

Okay bestie, this section is SERIOUS but also kind of iconic 💅 The security considerations from [RFC4347], [RFC4895], and [RFC4960] also apply to this document. So those three are part of the security story too 📚✨

It is possible to authenticate DTLS endpoints based on IP addresses in certificates 🌸 SCTP associations can use multiple addresses per SCTP endpoint 🦋 So it's totally possible that DTLS records will be sent from a different IP address than the one that was originally authenticated 👀 This is not a problem provided that no security decisions are made based on that IP address. Think of it as a special case of a general rule: all decisions should be based on the peer's authenticated identity, not on its transport layer identity 💖 Main character energy is trusting WHO someone is, not WHERE their packet came from 🫶

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 ✨ Here's the tea 🍵: even though 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. So anyone watching can see them 🙈

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. Not me choosing NULL and thinking I'm protected 😤 No privacy means NO privacy, periodt 🚫🔥

7. Acknowledgments 🎀

The authors wish to thank Anna Brunstrom, Lars Eggert, Gorry Fairhurst, Ian Goldberg, Alfred Hoenes, Carsten Hohendorf, Stefan Lindskog, Daniel Mentz, and Sean Turner for their invaluable comments 🫶💜 These legends gave their time and brainpower, so big thank-yous to all of them ✨

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.
  • [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.

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.
  • [RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, September 2007.

Authors' Addresses 🗺️

Michael Tuexen Muenster University of Applied Sciences Stegerwaldstr. 39 48565 Steinfurt Germany

EMail: tuexen@fh-muenster.de

Robin Seggelmann Muenster University of Applied Sciences Stegerwaldstr. 39 48565 Steinfurt Germany

EMail: seggelmann@fh-muenster.de

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

EMail: ekr@networkresonance.com

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