Datagram Transport Layer Security (DTLS) over Stream Control Transmission Protocol (SCTP)
draft-ietf-tsvwg-dtls-over-sctp-bis-07
| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Magnus Westerlund , John Preuß Mattsson , Claudio Porfiri | ||
| Last updated | 2023-10-23 | ||
| Replaces | draft-westerlund-tsvwg-dtls-over-sctp-bis | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Associated WG milestone |
|
||
| Document shepherd | Gorry Fairhurst | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | gorry@erg.abdn.ac.uk |
draft-ietf-tsvwg-dtls-over-sctp-bis-07
TSVWG M. Westerlund
Internet-Draft J. Preuß Mattsson
Intended status: Standards Track C. Porfiri
Expires: 25 April 2024 Ericsson
23 October 2023
Datagram Transport Layer Security (DTLS) over Stream Control
Transmission Protocol (SCTP)
draft-ietf-tsvwg-dtls-over-sctp-bis-07
Abstract
This document describes the usage of the Datagram Transport Layer
Security (DTLS) protocol to protect user messages sent over the
Stream Control Transmission Protocol (SCTP). It is an improved
alternative to the existing RFC 6083.
DTLS over SCTP provides mutual authentication, confidentiality,
integrity protection, and partial replay protection for applications
that use SCTP as their transport protocol and allows client/server
applications to communicate in a way that is designed to give
communications privacy and 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. This document is an
improved alternative to RFC 6083 and removes the 16 kbytes limitation
on protected user message size by defining a secure user message
fragmentation so that multiple DTLS records can be used to protect a
single user message. It further contains a large number of security
fixes and improvements. It updates the DTLS versions and SCTP-AUTH
HMAC algorithms to use. It mitigates reflection attacks of data and
control chunks and replay attacks of data chunks. It simplifies
secure implementation by some stricter requirements on the
establishment procedures as well as rekeying to align with zero trust
principles.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-tsvwg-dtls-over-sctp-
bis/.
Westerlund, et al. Expires 25 April 2024 [Page 1]
Internet-Draft DTLS over SCTP October 2023
Discussion of this document takes place on the Transport Area Working
Group (tsvwg) Working Group mailing list (mailto:tsvwg@ietf.org),
which is archived at https://mailarchive.ietf.org/arch/browse/tsvwg/.
Subscribe at https://www.ietf.org/mailman/listinfo/tsvwg/.
Source for this draft and an issue tracker can be found at
https://github.com/gloinul/draft-westerlund-tsvwg-dtls-over-sctp-bis.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 April 2024.
Copyright Notice
Copyright (c) 2023 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 (https://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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. SCTP-AUTH Assumptions . . . . . . . . . . . . . . . . . . 5
1.3. Protocol Overview . . . . . . . . . . . . . . . . . . . . 6
1.4. DTLS/SCTP Buffering and Flow Control . . . . . . . . . . 7
1.5. Comparison with TLS over SCTP . . . . . . . . . . . . . . 8
1.6. Changes from RFC 6083 . . . . . . . . . . . . . . . . . . 9
Westerlund, et al. Expires 25 April 2024 [Page 2]
Internet-Draft DTLS over SCTP October 2023
1.7. DTLS Version . . . . . . . . . . . . . . . . . . . . . . 10
1.8. Terminology . . . . . . . . . . . . . . . . . . . . . . . 11
1.9. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 11
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. DTLS Considerations . . . . . . . . . . . . . . . . . . . . . 12
3.1. Version of DTLS . . . . . . . . . . . . . . . . . . . . . 12
3.2. Cipher Suites and Cryptographic Parameters . . . . . . . 12
3.3. Authentication . . . . . . . . . . . . . . . . . . . . . 13
3.4. Renegotiation and Key Update . . . . . . . . . . . . . . 13
3.5. DTLS Connection Identifier . . . . . . . . . . . . . . . 13
3.6. DTLS Sequence number size . . . . . . . . . . . . . . . . 14
3.7. Message Sizes . . . . . . . . . . . . . . . . . . . . . . 14
3.8. Replay Protection . . . . . . . . . . . . . . . . . . . . 15
3.9. Path MTU Discovery . . . . . . . . . . . . . . . . . . . 16
3.10. Retransmission of Messages . . . . . . . . . . . . . . . 16
4. SCTP Considerations . . . . . . . . . . . . . . . . . . . . . 16
4.1. Mapping of DTLS Records . . . . . . . . . . . . . . . . . 16
4.2. DTLS Connection Handling . . . . . . . . . . . . . . . . 18
4.3. Payload Protocol Identifier Usage . . . . . . . . . . . . 18
4.4. Stream Usage . . . . . . . . . . . . . . . . . . . . . . 19
4.5. Chunk Handling . . . . . . . . . . . . . . . . . . . . . 20
4.6. SCTP-AUTH Hash Function . . . . . . . . . . . . . . . . . 20
4.7. Parallel DTLS connections . . . . . . . . . . . . . . . . 20
4.8. Handling of Endpoint Pair Shared Secrets . . . . . . . . 23
4.8.1. DTLS 1.2 Considerations . . . . . . . . . . . . . . . 24
4.8.2. DTLS 1.3 Considerations . . . . . . . . . . . . . . . 25
4.9. Shutdown . . . . . . . . . . . . . . . . . . . . . . . . 25
4.10. Transmission Limitations . . . . . . . . . . . . . . . . 26
4.10.1. Preventing DTLS sequence number wraps . . . . . . . 27
4.10.2. SCTP API Limitations . . . . . . . . . . . . . . . . 28
5. DTLS/SCTP Control Message . . . . . . . . . . . . . . . . . . 28
5.1. SHUTDOWN-Request . . . . . . . . . . . . . . . . . . . . 29
5.2. Ready To Close Indication . . . . . . . . . . . . . . . . 29
6. DTLS over SCTP Service . . . . . . . . . . . . . . . . . . . 29
6.1. Adaptation Layer Indication in INIT/INIT ACK . . . . . . 29
6.2. DTLS over SCTP Initialization . . . . . . . . . . . . . . 29
6.3. Client Use Case . . . . . . . . . . . . . . . . . . . . . 30
6.4. Server Use Case . . . . . . . . . . . . . . . . . . . . . 30
6.5. RFC 6083 Fallback . . . . . . . . . . . . . . . . . . . . 31
6.5.1. Client Fallback . . . . . . . . . . . . . . . . . . . 31
6.5.2. Server Fallback . . . . . . . . . . . . . . . . . . . 32
6.5.3. Authenticated Fallback . . . . . . . . . . . . . . . 32
7. SCTP API Consideration . . . . . . . . . . . . . . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
8.1. Transport Layer Security (TLS) Extensions . . . . . . . . 33
8.2. TLS Exporter Labels . . . . . . . . . . . . . . . . . . . 34
8.3. SCTP Adaptation Layer Indication Code Point . . . . . . . 34
8.4. SCTP Payload Protocol Identifiers . . . . . . . . . . . . 35
Westerlund, et al. Expires 25 April 2024 [Page 3]
Internet-Draft DTLS over SCTP October 2023
9. Security Considerations . . . . . . . . . . . . . . . . . . . 35
9.1. Cryptographic Considerations . . . . . . . . . . . . . . 35
9.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 37
9.3. Targeting DTLS Messages . . . . . . . . . . . . . . . . . 38
9.4. Authentication and Policy Decisions . . . . . . . . . . . 38
9.5. Resumption and Tickets . . . . . . . . . . . . . . . . . 38
9.6. Privacy Considerations . . . . . . . . . . . . . . . . . 39
9.7. Pervasive Monitoring . . . . . . . . . . . . . . . . . . 39
9.8. Replay attacks . . . . . . . . . . . . . . . . . . . . . 40
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 40
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 40
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
12.1. Normative References . . . . . . . . . . . . . . . . . . 40
12.2. Informative References . . . . . . . . . . . . . . . . . 42
Appendix A. Motivation for Changes . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
1.1. Overview
This document describes the usage of the Datagram Transport Layer
Security (DTLS) protocol, as defined in DTLS 1.2 [RFC6347], and DTLS
1.3 [RFC9147], over the Stream Control Transmission Protocol (SCTP),
as defined in [RFC9260] combined with Authenticated Chunks for SCTP
(SCTP-AUTH) [RFC4895].
Once the SCTP-AUTH assumptions are fulfilled (see Section 1.2), this
specification provides mutual authentication of endpoints, data
confidentiality, data origin authentication, data integrity
protection, and a certain level of data replay protection of user
messages for applications that use SCTP as their transport protocol
(see in this regard what stated in Section 9.8). Thus, it allows
client/server applications to communicate in a way that is designed
to give communications privacy and to prevent eavesdropping and
detect tampering or message forgery. DTLS/SCTP uses DTLS for mutual
authentication, key exchange with forward secrecy for SCTP-AUTH, and
confidentiality of user messages. DTLS/SCTP use SCTP and SCTP-AUTH
for integrity protection and partial replay protection of all SCTP
Chunks that can be authenticated, including user messages.
Applications using DTLS over SCTP can use almost all transport
features provided by SCTP and its extensions. DTLS/SCTP supports:
* preservation of message boundaries.
* a large number of unidirectional and bidirectional streams.
Westerlund, et al. Expires 25 April 2024 [Page 4]
Internet-Draft DTLS over SCTP October 2023
* 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].
* User messages of any size.
The method described in this document requires that the SCTP
implementation supports the optional feature of fragmentation of SCTP
user messages as defined in [RFC9260]. The implementation is
required to have an SCTP API (for example the one described in
[RFC6458]) that supports partial user message delivery and also
recommended that I-DATA chunks as defined in [RFC8260] is used to
efficiently implement and support larger user messages.
To simplify implementation and reduce the risk for security holes,
limitations have been defined such that STARTTLS as specified in
[RFC3788] is no longer supported.
1.2. SCTP-AUTH Assumptions
In this document it is assumed that SCTP-AUTH is provided with
periodic rekeying by periodic usage the mechanism for DTLS rekeying
and re-authentication defined in this document. It is also assumed
that SCTP-AUTH specification [RFC4895] has been updated to address
most of the known issues.
SCTP-AUTH as defined by RFC4895 has been identified as weak in the
following parts:
1. Reflection of authenticated data chunks
2. Replay of authenticated data chunks
3. Single key used with different HMAC algorithms
4. Reflection of authenticated control chunks
5. Replay of authenticated control chunks
Westerlund, et al. Expires 25 April 2024 [Page 5]
Internet-Draft DTLS over SCTP October 2023
We are expecting the SCTP-AUTH update to fully address issue 1, 3 and
4. Issue 2 will be partially addressed in this specification through
periodic rekeying to prevent replay to inject data and affect
availability, but that is based on SCTP implementation correctly
handling replayed or duplicated packets. SCTP-AUTH issues mitigated
and having a periodic rekeying is a condicio sine qua non
(indispensable condition) for this document to provide a working
solution.
1.3. Protocol Overview
The DTLS/SCTP protection is defined as an SCTP adaptation layer
[RFC5061] that is implemented on top of an SCTP API for an SCTP
implementation with SCTP-AUTH [RFC4895] support. DTLS/SCTP is
expected to provide an SCTP like API towards the upper layer protocol
with some additions for controlling the DTLS/SCTP security parameters
and policies. This minimizes the impact on the SCTP implementation
and wire image.
+---------------------+
| |
| ULP |
| |
+---------------------+ <- SCTP API + Security Parameters
| |
| DTLS/SCTP | +------+
| Adaptation +----------| DTLS |
| Layer | +------+
| |
+---------------------+ <- SCTP API + SCTP-AUTH API
| |
| SCTP + SCTP-AUTH |
| |
+---------------------+
Figure 1: DTLS/SCTP layering in regard to SCTP and upper layer
protocol
DTLS/SCTP performs protection operations on ULP data as it is
provided to DTLS/SCTP, as whole or a part of a SCTP user messages to
be transported to the peer. DTLS/SCTP uses the regular SCTP
multiplexing mechanisms for data using streams and individual user
messages. The protection operation for a ULP user message larger
than the maximum DTLS record size is performed by first splitting the
user message into suitable fragments that fit into individual DTLS
records. Each fragment is encrypted and provided with authentication
tag by DTLS.
Westerlund, et al. Expires 25 April 2024 [Page 6]
Internet-Draft DTLS over SCTP October 2023
m0 | m1 | m2 | ... = user_message
user_message' = DTLS( m0 ) | DTLS( m1 ) | DTLS( m2 ) ...
The sequence of protected user message fragments (user_message') are
then transmitted as a SCTP user message. SCTP-AUTH provides
authentication of the SCTP packets and prevents injection of data or
reordering of DTLS fragments thus ensuring that each protected user
message can be de-protected in the receiver in order and reassembled.
Partial transmission and delivery of user messages are supported on a
per fragment basis.
SCTP's capability for multi-stream concurrent transmission of
different SCTP user messages, where each SCTP user message can
potentially be very large, results in some challenges for any change
of the keys used to protect the ULP data. SCTP-AUTH API, defined in
[RFC6458], provides additional limitations that needs to be
considered when supported. These issues and the related limitations
will be discussed more in details below.
RFC6083 dealt with the above limitations by requiring that the peers
drained all outstanding data before updating the key to prevent
issues. This can have significant impact on a ULP that requires
timely and frequent exchange of user messages. This specification
uses another solution to these problems assuming a sufficient capable
SCTP and SCTP-AUTH implementations and with rich enough APIs.
The solution that ensures the current keying material will not be
prematurely discarded on renegotiation or key update, is based on not
using these mechanisms and instead establishing a second DTLS
connection over the SCTP association. This creates a parallel DTLS
connections, where the DTLS connection ID feature is used to identify
the originating DTLS connection for each DTLS record or message.
When a new DTLS connection has been established and its keying
material is made available, the sender starts using it to protect the
ULP data. When all protected user message fragments protected by the
old key have been delivered in a non-renegable way then the old DTLS
connection can be terminated and the associated keying material
discarded.
1.4. DTLS/SCTP Buffering and Flow Control
With DTLS/SCTP as a layer above SCTP stacks on both sender and
receiver side some consideration is needed for buffering and resource
contention, and how back pressure is applied in cases the receiving
application is not keeping up with the sender. The ULP may use
multiple user messages simultaneous, and the progress and delivery of
these messages are progressing independently, thus the receiving
Westerlund, et al. Expires 25 April 2024 [Page 7]
Internet-Draft DTLS over SCTP October 2023
DTLS/SCTP implementation may not receive DTLS records in order in
case of packet loss.
On the sender side the DTLS/SCTP layer will need to accept data from
the ULP of at least one maximum DTLS record size. The maximum DTLS
record size is 2^14 bytes per default or a lower negotiated value
using the DTLS extension defined in [RFC8449]. The user message
fragment is then protected by DTLS and assumed to immediately after
be dispatched for transmission by SCTP.
As SCTP schedules the DTLS record for transmission as SCTP packets it
will become part of the data tracked by the send/receive buffer in
the SCTP stacks. The maximum receiver buffer size is negotiated and
provides an upper limit of how much outstanding data can exist on the
SCTP layer. For example, if an DTLS record part of user message N
experience repeated packet losses, it may not be delivered, despite
several later user messages fragments has been delivered.
Next, we assume that the receiver side DTLS/SCTP will read partial
user messages from the SCTP receiver stack as they become available
unless it can't keep up or has run out of intermediate buffer space
for reassembly of the DTLS records in each user message. Thus, in
case the receiver falls behind it will eventually block the receiver
buffer by not consuming data from it and thus creating back pressure
towards the sender. But, at any time it is assumed that the receiver
side DTLS/SCTP layer will not buffer multiple DTLS records, and
instead process each as soon as possible. Buffering multiple DTLS
records prior to DTLS decryption would increase the total number of
DTLS records in flight, counted between DTLS encryption and
decryption, and thus risk overlapping DTLS sequence numbers.
To avoid overlapping sequence number the DTLS sender should first of
all use 16-bit sequence number to enable a larger space. Secondly,
it should track which DTLS records has been non-renegable ACKed by
the receiver and always maintain a certain safety buffer in number of
DTLS records. Thirdly, the implementation needs to attempt to
minimize usage of buffers that exist after the DTLS encryption until
the DTLS Decryption in its sender and receiver implementation.
1.5. Comparison with TLS over SCTP
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. TLS over SCTP as described in [RFC3436] has some
serious limitations:
* It does not support the unordered delivery of SCTP user messages.
Westerlund, et al. Expires 25 April 2024 [Page 8]
Internet-Draft DTLS over SCTP October 2023
* 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.
1.6. Changes from RFC 6083
The DTLS over SCTP solution defined in RFC 6083 had the following
limitations:
* The maximum user message size is 2^14 (16384) bytes, which is a
single DTLS record limit.
* DTLS 1.0 has been deprecated for RFC 6083 requiring at least DTLS
1.2 [RFC8996]. This creates additional limitations as discussed
in Section 1.7.
* DTLS messages that don't contain protected user message data where
limited to only be sent on Stream 0, which could potentially
impact applications.
* An on-path attacker can reflect the authenticated part of a SCTP
packet back to the sender as well as replaying data and control
chunks.
This specification defines the following changes compared with RFC
6083:
* Removes the limitations on user messages sizes by defining a
secure fragmentation mechanism. It is optional to support message
sizes over 2^64-1 bytes.
* Update DTLS key material without requiring draining all in-flight
user message from SCTP.
* Mandates that more modern DTLS version are used (DTLS 1.2 or 1.3)
* Mandates support of stronger HMAC algorithm (SHA-256) in the SCTP
authentication extension [RFC4895].
* Recommends support of [RFC8260] to enable interleaving of large
SCTP user messages to avoid scheduling issues.
Westerlund, et al. Expires 25 April 2024 [Page 9]
Internet-Draft DTLS over SCTP October 2023
* Applies stricter requirements on always using DTLS for all user
messages in the SCTP association.
* Requires that SCTP-AUTH is applied to all SCTP Chunks that can be
authenticated.
* Requires support of partial delivery of user messages.
* Requires an updates SCTP-AUTH specification to mitigate packet
reflection attacks that can impact the SCTP association
availability.
* Mandates SCTP-AUTH rekeying before the TSN cycles back to the
Initial TSN to mitigate replay of data chunks.
1.7. DTLS Version
Using DTLS 1.2 instead of using DTLS 1.0 limits the lifetime of a
DTLS connection and the data volume which can be transferred over a
DTLS connection. This is caused by:
* The number of renegotiations in DTLS 1.2 is limited to 65534
compared to unlimited in DTLS 1.0.
* While the AEAD limits in DTLS 1.3 does not formally apply to DTLS
1.2 the mathematical limits apply equally well to DTLS 1.2.
DTLS 1.3 comes with a large number of significant changes.
* Renegotiations are not supported and instead partly replaced by
key updates. The number of key updates is limited to 2^48.
* Strict AEAD significantly limits on how many DTLS records can be
sent before rekeying.
Many applications using DTLS/SCTP are of semi-permanent nature.
Semi-permanent term comes from telecom and referres to connections
that start at a certain time and are rarely closed. Semi-permanent
connections use SCTP associations with expected lifetimes of months
or even years where there is a significant cost for bringing them
down in order to restart it. Such DTLS/SCTP usages that need:
* Periodic re-authentication and transfer of revocation information
of both endpoints (not only the DTLS client).
* Periodic rerunning of Diffie-Hellman Key Exchange to provide
forward secrecy and mitigate static key exfiltration attacks.
Westerlund, et al. Expires 25 April 2024 [Page 10]
Internet-Draft DTLS over SCTP October 2023
* Perform SCTP-AUTH rekeying.
At the time of publication, DTLS 1.3 does not support any of these,
where DTLS 1.2 renegotiation functionality can provide these
functionality in the context of DTLS/SCTP. To address these
requirements from semi-permanent applications, this document uses
several overlapping DTLS connections with either DTLS 1.2 or 1.3.
Having uniform procedures reduces the impact when upgrading from DTLS
1.2 to DTLS 1.3 and avoids using the renegotiation mechanism which is
disabled by default in many DTLS implementations.
To address known vulnerabilities in DTLS 1.2 this document describes
and mandates implementation constraints on ciphers and protocol
options. The DTLS 1.2 renegotiation mechanism is forbidden to be
used as it creates the need for additional mechanism to handle race
conditions and interactions between using DTLS connections in
parallel.
Secure negotiation of the DTLS version is handled by the DTLS
handshake. If the endpoints do not support a common DTLS version the
DTLS handshake will be aborted.
In the rest of the document, unless the version of DTLS is
specifically called out, the text applies to both versions of DTLS.
DTLS/SCTP requires the maximum DTLS record size to be known, and not
being changed during the lifetime of the Association.
1.8. Terminology
This document uses the following terms:
Association: An SCTP association.
Connection: A DTLS connection. It is uniquely identified by a
connection identifier.
Stream: A unidirectional stream of an SCTP association. It is
uniquely identified by a stream identifier.
1.9. Abbreviations
AEAD: Authenticated Encryption with Associated Data
DTLS: Datagram Transport Layer Security
HMAC: Keyed-Hash Message Authentication Code
Westerlund, et al. Expires 25 April 2024 [Page 11]
Internet-Draft DTLS over SCTP October 2023
MTU: Maximum Transmission Unit
PPID: Payload Protocol Identifier
SCTP: Stream Control Transmission Protocol
SCTP-AUTH: Authenticated Chunks for SCTP [RFC4895]
TCP: Transmission Control Protocol
TLS: Transport Layer Security
ULP: Upper Layer Protocol
2. Conventions
The key words "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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. DTLS Considerations
3.1. Version of DTLS
This document defines the usage of either DTLS 1.3 [RFC9147], or DTLS
1.2 [RFC6347]. Earlier versions of DTLS MUST NOT be used (see
[RFC8996]). DTLS 1.3 is RECOMMENDED for security and performance
reasons. It is expected that DTLS/SCTP as described in this document
will work with future versions of DTLS.
Only one version of DTLS MUST be used during the lifetime of an SCTP
Association, meaning that the procedure for replacing the DTLS
version in use requires the existing SCTP Association to be
terminated and a new SCTP Association with the desired DTLS version
to be instantiated.
3.2. Cipher Suites and Cryptographic Parameters
For DTLS 1.2, the cipher suites forbidden by [RFC9113] MUST NOT be
used. For all versions of DTLS, cryptographic parameters giving
confidentiality and forward secrecy MUST be used.
There are potential for aligning used hash algorithms between SCTP-
AUTH and the DTLS cipher suit. If the otherwise considered to be
used SCTP-AUTH hash algorithms and DTLS Cipher suits have matching
hashing algorithms it is RECOMMENDED to indicate a preference for
Westerlund, et al. Expires 25 April 2024 [Page 12]
Internet-Draft DTLS over SCTP October 2023
such algorithms. Note, however as the SCTP-AUTH hashing algorithm is
chosen during SCTP association handshake it can't be changed once it
is known what is supported in DTLS by the peer endpoint.
3.3. Authentication
The DTLS handshakes MUST use mutual authentication.
3.4. Renegotiation and Key Update
DTLS 1.2 renegotiation enables rekeying (with ephemeral Diffie-
Hellman) of DTLS as well as mutual reauthentication and transfer of
revocation information inside an DTLS 1.2 connection. Renegotiation
has been removed from DTLS 1.3 and partly replaced with post-
handshake mechanism for key update. The parallel DTLS connection
solution was specified due to lack of necessary features with DTLS
1.3 considered needed for long lived SCTP associations, such as
rekeying (with ephemeral Diffie-Hellman) as well as mutual
reauthentication.
This specification does not allow usage of DTLS 1.2 renegotiation to
avoid race conditions and corner cases in the interaction between the
parallel DTLS connection mechanism and the keying of SCTP-AUTH. In
addition, renegotiation is also disabled in some implementations, as
well as dealing with the epoch change reliable have similar or worse
application impact.
This specification also forbids against using DTLS 1.3 key update and
instead rely on parallel DTLS connections. For DTLS 1.3 there isn't
feature parity. It also has the issue that a DTLS implementation
following the RFC may assume a too limited window for SCTP where the
previous epoch's security context is maintained and thus, changes to
epoch handling would be necessary.
A DTLS 1.2 endpoint MUST NOT use renegotiation and a DTLS 1.3
endpoint MUST NOT send any KeyUpdate message. The endpoint MUST
instead initiate a new DTLS connection before the old one reaches the
used cipher suit's key lifetime. The AEAD limits given in section
4.5.3 of [RFC9147] SHOULD be followed.
3.5. DTLS Connection Identifier
The DTLS Connection ID MUST be negotiated, according to [RFC9146] for
DTLS 1.2, and Section 9 of [RFC9147] for DTLS 1.3.
Section 4 of [RFC9146] states "If, however, an implementation chooses
to receive different lengths of CID, the assigned CID values must be
self-delineating since there is no other mechanism available to
Westerlund, et al. Expires 25 April 2024 [Page 13]
Internet-Draft DTLS over SCTP October 2023
determine what connection (and thus, what CID length) is in use.".
As this solution requires multiple connection IDs, using a zero-
length CID will be highly problematic as it could result in that any
DTLS records with a zero length CID ends up in another DTLS
connection context, and there fail the decryption and integrity
verification. And in that case to avoid losing the DTLS record, it
would have to be forwarded to another zero-length CID using DTLS
Connection, where decryption and validation must be tried, resulting
in higher resource utilization. Thus, it is REQUIRED to use non-zero
length CID values, and RECOMMENDED to use a single common length for
the CID values. A single byte should be sufficient, as reuse of old
CIDs is possible as long as the implementation ensures that they are
not used in near time to the previous usage.
3.6. DTLS Sequence number size
16-bit sequence number SHOULD be used rather than 8-bit to avoid
limitations in number of inflight DTLS records. Overlapping sequence
number due to wrapping of the sequence number MUST be prevented as it
otherwise can lead to decryption failure that result in failure of
the transport service. See Section 4.10.1 for how to prevent
sequence number wraps.
3.7. Message Sizes
If DTLS 1.3 is used, the length field in the record layer MUST be
included in all records.
DTLS/SCTP, automatically fragments and reassembles user messages.
This specification defines how to fragment the user messages into
DTLS records, where each DTLS record allows a maximum of 2^14
protected bytes. It is mandated that DTLS supports the maximum
record size of 2^14 bytes. DTLS/SCTP MAY exploit maximum DTLS record
size less than 2^14 bytes due to implementation choice, in such case
maximum record size MUST be negotiated according to [RFC8449]. The
negotiated value MUST be known to DTLS/SCTP and SHALL NOT be changed
during the SCTP Association lifetime.
The sequence of DTLS records is then fragmented into DATA or I-DATA
Chunks to fit the path MTU by SCTP. These changes ensure that DTLS/
SCTP has the same capability as SCTP to support user messages of any
size. However, to simplify implementations it is OPTIONAL to support
user messages larger than 2^64-1 bytes. This is to allow
implementation to assume that 64-bit length fields and offset
pointers will be sufficient.
Westerlund, et al. Expires 25 April 2024 [Page 14]
Internet-Draft DTLS over SCTP October 2023
The security operations and reassembly process requires that the
protected user message, i.e., with DTLS record overhead, is stored in
the receiver's buffer. This buffer space will thus put a limit on
the largest size of plain text user message that can be transferred
securely. However, by mandating the use of the partial delivery of
user messages from SCTP and assuming that no two messages received on
the same stream are interleaved (as it is the case when using the API
defined in [RFC6458]) the minimally required buffering prior to DTLS
processing is a single DTLS record per used incoming stream. This
enables the DTLS/SCTP implementation to provide the Upper Layer
Protocol (ULP) with each DTLS record's content, when it has been
decrypted and its integrity been verified, enabling partial user
message delivery to the ULP. However, for efficient operation and
avoiding flow control stalls if user message fragments are not
frequently and expediently moved to upper layer memory buffers, the
receiver buffer needs to be larger.
Implementations can trade-off buffer memory requirements in the DTLS
layer with transport overhead by using smaller DTLS records, in this
case the record size limit extension for DTLS according to [RFC8449]
MUST be used and the negotiated record size SHALL be communicated to
DTLS/SCTP. The maximum record size SHALL be the same during the
lifetime of the Association, i.e., renegotiated to the same value in
all subsequent DTLS connections.
The DTLS/SCTP implementation is expected to behave very similar to
just SCTP when it comes to handling of user messages and dealing with
large user messages and their reassembly and processing. Making it
the ULP responsible for handling any resource contention related to
large user messages.
3.8. Replay Protection
SCTP-AUTH [RFC4895] does not have explicit replay protection.
However, the combination of SCTP-AUTH's protection of DATA or I-DATA
chunks and SCTP user message handling will prevent third party
attempts to inject or replay SCTP data chunks as long as the
Transmission Sequence Numbers (TSNs) are unique. In fact, this
document's solution is dependent on SCTP-AUTH and SCTP to prevent
reordering, duplication, and removal of the DTLS records within each
protected user message. This includes detection of changes to what
DTLS records start and end the SCTP user message, and removal of DTLS
records before an increment to the epoch. Without SCTP-AUTH, these
would all have required explicit handling.
Westerlund, et al. Expires 25 April 2024 [Page 15]
Internet-Draft DTLS over SCTP October 2023
To prevent replay of DATA or I-DATA chunks resulting in impact on the
received protected user message, the SCTP-AUTH key MUST be retired
before it has been used with more than 2^32 TSNs. Implementations
MUST therefore setup a new parallel DTLS connection to rekey well
before 2^32 TSNs have been used with a SCTP-AUTH key.
DTLS/SCTP does not provide replay protection for authenticated
control chunks such as ERROR, RE-CONFIG [RFC6525], or SACK. An on-
path attacker can replay control chunks as long as the receiving
endpoint still has the endpoint pair shared secret. Such replay
could disrupt the SCTP association and could therefore be a denial-
of-service attack.
DTLS optionally supports record replay detection. Such replay
detection could result in the DTLS layer dropping valid messages
received outside of the DTLS replay window. As DTLS/SCTP provides
the necessary replay protection even without DTLS replay protection,
the replay detection of DTLS MUST NOT be used.
3.9. Path MTU Discovery
DTLS Path MTU Discovery MUST NOT be used. Since SCTP provides Path
MTU discovery and fragmentation/reassembly for user messages as
specified in Section 3.7, DTLS can send maximum sized DTLS Records.
3.10. 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 SCTP implementation MUST support fragmentation of user messages
using DATA [RFC9260], and optionally I-DATA [RFC8260] chunks.
DTLS/SCTP as an SCTP adaptation layer exist between the ULP user
message API and SCTP. On the sender side a user message is split
into fragments m0, m1, m2, each no larger than 2^14 = 16384 bytes or
the negotiated maximum DTLS record size (Section 3.7).
m0 | m1 | m2 | ... = user_message
The resulting fragments are protected with DTLS and the records are
concatenated
Westerlund, et al. Expires 25 April 2024 [Page 16]
Internet-Draft DTLS over SCTP October 2023
user_message' = DTLS( m0 ) | DTLS( m1 ) | DTLS( m2 ) ...
The new user_message', i.e., the protected user message, is the input
to SCTP.
On the receiving side, the length field in each DTLS record can be
used to determine the boundaries between DTLS records. DTLS/SCTP
SHOULD request decryption of each individual records as soon as
possible. The last DTLS record can be found by subtracting the
length of individual records from the length of user_message'. The
output from the DTLS decryption(s) is the fragments m0, m1, m2 ...
The user_message is reassembled from decrypted DTLS records as
user_message = m0 | m1 | m2 ...
There are four failure cases an DTLS/SCTP implementation needs to
detect and then act on:
1. Failure in decryption and integrity verification process of any
DTLS record. Due to SCTP-AUTH preventing delivery of injected or
corrupt fragments of the protected user message this should only
occur in case of implementation errors or internal hardware
failures or the necessary security context has been prematurely
discarded.
2. In case the SCTP layer indicates an end to a user message, e.g.,
when receiving a MSG_EOR in a recvmsg() call when using the API
described in [RFC6458], and the last buffered DTLS record length
field does not match, i.e., the DTLS record is incomplete.
3. Unable to perform the decryption processes due to lack of
resources, such as memory, and have to abandon the user message
fragment. This specification is defined such that the needed
resources for the DTLS/SCTP operations are bounded for a given
number of concurrent transmitted SCTP streams or unordered user
messages.
4. DTLS Replay protection. This specification mandates that replay
protection shall not be used, otherwise the sequence number in a
delayed DTLS record might be beyond what the replay window
accepts and thus be dropped. If such a discard would happen the
user message would be compromised as the data has been lost.
The above failure cases all result in the receiver failing to
recreate the full user message. This is a failure of the transport
service that is not possible to recover from the DTLS/SCTP layer and
the sender could believe the complete message have been delivered.
This error MUST NOT be ignored, as SCTP lacks any facility to declare
a failure on a specific stream or user message, the DTLS connection
Westerlund, et al. Expires 25 April 2024 [Page 17]
Internet-Draft DTLS over SCTP October 2023
and the SCTP association SHOULD be terminated. A valid exception to
the termination of the SCTP association is if the receiver is capable
of notifying the ULP about the failure in delivery and the ULP is
capable of recovering from this failure.
Note that if the SCTP extension for Partial Reliability (PR-SCTP)
[RFC3758] is used for a user message, user message may be partially
delivered or abandoned. These failures are not a reason for
terminating the DTLS connection and SCTP association.
4.2. DTLS Connection Handling
DTLS/SCTP is negotiated on SCTP level as an adaptation layer
(Section 6). After a successful negotiation of the DTLS/SCTP
adaptation layer during SCTP association establishment, a DTLS
connection MUST be established prior the transmission of any ULP user
messages. All DTLS connections are terminated when the SCTP
association is terminated. A DTLS connection MUST NOT span multiple
SCTP associations.
As it is required to establish the DTLS connection at the beginning
of the SCTP association, either of the peers should never send any
SCTP user message that is not protected by DTLS. So, the case that
an endpoint receives data that is neither DTLS messages nor protected
user messages in the form of a sequence of DTLS Records on any stream
is a protocol violation. The receiver MAY terminate the SCTP
association due to this protocol violation. Implementations that do
not have a DTLS endpoint in a state where application_data record can
be accepted on SCTP handshake completion, will have to ensure correct
caching of the messages until the DTLS endpoint is ready.
Whenever a mutual authentication, updated security parameters, rerun
of Diffie-Hellman Key Exchange, or SCTP-AUTH rekeying is needed, a
new DTLS connection is instead setup in parallel with the old
connection (i.e., there may be up to two simultaneous DTLS
connections within one association).
4.3. Payload Protocol Identifier Usage
SCTP Payload Protocol Identifiers are assigned by IANA. Application
protocols using DTLS over SCTP SHOULD register and use a separate
Payload Protocol Identifier (PPID) and SHOULD NOT reuse the PPID that
they registered for running directly over SCTP.
Using the same PPID does no harm as DTLS/SCTP requires all user
messages being DTLS protected and knows that DTLS is used. However,
for protocol analyzers, for example, it is much easier if a separate
PPID is used and avoids different behavior from [RFC6083].
Westerlund, et al. Expires 25 April 2024 [Page 18]
Internet-Draft DTLS over SCTP October 2023
Messages that are exchanged between DTLS/SCTP peers not containing
ULP user messages shall use PPID = 0 according to section 3.3.1 of
[RFC9260] as no application identifier can be specified by the upper
layer for this payload data. With the exception for the DTLS/SCTP
Control Messages (Section 5) that uses its own PPID.
4.4. Stream Usage
DTLS 1.3 protects the actual content type of the DTLS record and have
therefore omitted the non-protected content type field. Thus, it is
not possible to determine which content type the DTLS record has on
SCTP level. For DTLS 1.2 ULP user messages will be carried in DTLS
records with content type "application_data".
DTLS Records carrying protected user message fragments MUST be sent
in by the ULP indicated SCTP stream and user message and additional
properties, such as PPID. The ULP has no limitations in using SCTP
facilities for stream and user messages. DTLS records of other types
MAY be sent on any SCTP stream. It MAY also be sent in its own SCTP
user message as well as interleaved with other DTLS records carrying
protected user message fragments. Thus, it is allowed to insert
between protected user message fragments DTLS records of other types
as the DTLS receiver will process these and not result in any user
message data being inserted into the ULP's user message. However,
DTLS messages of other types than protected user message MUST be sent
reliable, so the DTLS record can only be interleaved in case the ULP
user message is sent as reliable.
DTLS is capable of handling reordering of the DTLS records. However,
depending on stream properties and which user message DTLS records of
other types are sent in may impact in which order and how quickly
they are possible to process. Using the same stream with in-order
delivery for the different messages will ensure that the DTLS Records
are delivered in the order they are sent in user messages. Thus,
ensuring that if there are DTLS records that need to be delivered in
particular order it can be ensured. Alternatively, if it is desired
that a DTLS record is delivered as early as possible, avoiding in-
order streams with queued messages and considering stream priorities
can result in faster delivery.
A simple solution avoiding any protocol issue with sending DTLS
messages, that are not protected user message fragments, is to pick a
stream not used by the ULP, and send the DTLS messages in their own
SCTP user messages with in order delivery. That mimics the RFC 6083
behavior without impacting the ULP. However, it assumes that there
are available streams to be used based on the SCTP association
handshake parameter for "maximum inbound streams" (Section 5.1.1 of
[RFC9260]).
Westerlund, et al. Expires 25 April 2024 [Page 19]
Internet-Draft DTLS over SCTP October 2023
4.5. Chunk Handling
All chunks types that can be listed in the Chunk List Parameter
[RFC4895], i.e., all chunks types except INIT, INIT ACK, and
SHUTDOWN-COMPLETE, MUST be sent in an authenticated way as described
in [RFC4895]. 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. Note that COOKIE ECHO and COOKIE ACK are
protected with an empty key. This is not a problem as everything in
these chunks are determined by earlier chunks or ignored on receipt.
If PR-SCTP as defined in [RFC3758] is used, the FORWARD-TSN chunks
are sent in an authenticated way which 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.
I-DATA chunk type as defined in [RFC8260] is RECOMMENDED to be
supported to avoid some of the down sides that large user messages
have on blocking transmission of later arriving high priority user
messages. However, the support is not mandated and negotiated
independently from DTLS/SCTP.
4.6. SCTP-AUTH Hash Function
When using DTLS/SCTP, the SHA-256 Message Digest Algorithm MUST be
supported in the SCTP-AUTH [RFC4895] implementation. SHA-1 MUST NOT
be used when using DTLS/SCTP. [RFC4895] requires support and
inclusion of SHA-1 in the HMAC-ALGO parameter, thus, to meet both
requirements the HMAC-ALGO parameter will include both SHA-256 and
SHA-1 with SHA-256 listed prior to SHA-1 to indicate the preference.
When using DTLS/SCTP, each endpoint MUST use a single SCTP-AUTH
Message Digest Algorithm during the whole SCTP association. This
guarantees that an association shared key is only used with a single
algorithm.
4.7. Parallel DTLS connections
To enable SCTP-AUTH rekeying, periodic authentication of both
endpoints, and force attackers to dynamic key extraction [RFC7624],
DTLS/SCTP per this specification defines the usage of parallel DTLS
connections over the same SCTP association. This solution ensures
that there are no limitations to the lifetime of the SCTP association
due to DTLS, it also avoids dependency on version specific DTLS
mechanisms such as renegotiation in DTLS 1.2, which is disabled by
default in many DTLS implementations, or post-handshake messages in
DTLS 1.3, which does not allow periodic mutual endpoint re-
authentication or re-keying of SCTP-AUTH.
Westerlund, et al. Expires 25 April 2024 [Page 20]
Internet-Draft DTLS over SCTP October 2023
Parallel DTLS connections enable opening a new DTLS connection
performing an handshake, while the existing DTLS connection is kept
in place. In DTLS 1.3 the handshake MAY be a full handshake or a
resumption handshake, and resumption can be done while the original
connection is still open. In DTLS 1.2 the handshake MUST be a full
handshake. The new parallel connection MUST use the same DTLS
version as the existing connection.
On DTLS handshake completion, DTLS/SCTP starts using the security
context of the new DTLS connection for protection of ULP user
messages and then ensure delivery of all the SCTP chunks using the
old DTLS connections security context. When that has been achieved
DTLS/SCTP shall close the old DTLS connection and discard the related
security context.
As specified in Section 4.1 the usage of DTLS connection ID is
required to ensure that the receiver can correctly identify the DTLS
connection and its security context when performing its de-protection
operations. There is also only a single SCTP-AUTH key exported per
DTLS connection and transmission direction ensuring that there is
clear mapping between the DTLS connection ID and the SCTP-AUTH
security context for each Key Identifier.
Application writers should be aware that establishing a new DTLS
connection may result in changes of security parameters. See
Section 9 for security considerations regarding rekeying.
A DTLS/SCTP Endpoint MUST NOT have more than two DTLS connections
open at the same time. Either of the endpoints MAY initiate a new
DTLS connection by performing a DTLS handshake. To support this
implementations and certificates need to support both DTLS client and
server roles. Note that resumption is not possible between DTLS
connections unless the endpoints have the same roles. As either
endpoint can initiate a DTLS handshake on either side at the same
time, either endpoint may receive a DTLS ClientHello message when it
has sent its own ClientHello. In this case the ClientHello from the
endpoint that had the DTLS Client role in the establishment of the
existing DTLS connection shall be continued to be processed and the
other dropped.
When performing the DTLS handshake the endpoint MUST verify that the
peer identifies using the same identity as in the previous DTLS
connection.
When the DTLS handshake has been completed, the new DTLS connection
MUST be used for the DTLS protection of any new ULP user message, and
SHOULD be switched to for protection of not yet protected user
message fragments of partially transmitted user messages. Also,
Westerlund, et al. Expires 25 April 2024 [Page 21]
Internet-Draft DTLS over SCTP October 2023
after the completion of the DTLS handshake, a new SCTP-AUTH key will
be exported per Section 4.8. To enable the sender and receiver to
correctly identify when the old DTLS connection is no longer in use,
the SCTP-AUTH key used to protect a SCTP packet MUST NOT be from a
newer DTLS connection than produced any included DTLS record
fragment.
The SCTP API defined in [RFC6458] has limitation in changing the
SCTP-AUTH key until the whole SCTP user message has been delivered.
However, the DTLS/SCTP implementation can switch the DTLS connection
used to protect the user message fragments to a newer, even if the
older DTLS connections exported key is used for the SCTP-AUTH. And
for SCTP implementations where the SCTP-AUTH key can be switched in
the middle of a user message the SCTP-AUTH key should be changed as
soon as all DTLS record fragments included in an SCTP packet have
been protected by the newer DTLS connection. Any SCTP-AUTH receiver
implementation is expected to be able to select key on per SCTP
packet basis.
The DTLS/SCTP endpoint timely indicates to its peer when the previous
DTLS connection and its context are no longer needed for receiving
any more data from this endpoint. This is done by sending a DTLS/
SCTP Control Message (Section 5) of type "Ready_To_Close"
(Section 5.2) to its peer. The endpoint MUST NOT send the
Ready_To_Close until the following two conditions are fulfilled:
1. All SCTP packets containing part of any DTLS record or message
protected using the security context of this DTLS connection have
been acknowledged in a non-renegable way.
2. All SCTP packets using the SCTP-AUTH key associated with the
security context of this DTLS connection have been acknowledged
in a non-renegable way.
A DTLS/SCTP endpoint that fulfills the above conditions for the SCTP
packets it sends, and have received a Ready_To_Close message, SHALL
immediately initiate closing of this DTLS connection by sending a
DTLS close_notify. Then when it has received the peer's close_notify
terminate the DTLS connection and expunges the associated security
context and SCTP-AUTH key. Note that it is not required for a DTLS/
SCTP implementation that has received a Ready_To_Close message to
send that message itself when it fulfills the conditions. However,
in some situations both endpoints will fulfill the conditions close
enough in time that both endpoints will send their Ready_To_Close
prior to receiving the indication from the peer, that works as both
endpoints will then initiate DTLS close_notify and terminate the DTLS
connections upon the reception of the peers close_notify.
Westerlund, et al. Expires 25 April 2024 [Page 22]
Internet-Draft DTLS over SCTP October 2023
SCTP implementations exposing APIs like [RFC6458] fulfilling these
conditions require draining the SCTP association of all outstanding
data after having completed all the user messages using the previous
SCTP-AUTH key identifier, relying on the SCTP_SENDER_DRY_EVENT to
know when delivery has been accomplished. A richer API could also be
used that allows user message level tracking of delivery, see
Section 7 for API considerations.
For SCTP implementations exposing APIs like [RFC6458] where it is not
possible to change the SCTP-AUTH key for a partial SCTP message
initiated before the change of security context, it will be forced to
track the SCTP messages and determine when all using the old security
context has been transmitted. This maybe be impossible to do
completely reliable without tighter integration between the DTLS/SCTP
layer and the SCTP implementation. This type of implementations also
has an implicit limitation in how large SCTP messages it can support.
Each SCTP message needs to have completed delivery and enabling
closing of the previous DTLS connection prior to the need to create
yet another DTLS connection. Thus, SCTP messages can't be larger
than that the transmission completes in less than the duration
between the rekeying or re-authentications needed for this SCTP
association.
The consequences of sending a DTLS close_notify alert in the old DTLS
connection prior to the receiver having received the data can result
in failure case 1 described in Section 4.1, which likely result in
SCTP association termination.
4.8. Handling of Endpoint Pair Shared Secrets
Editor's Note: Assuming that RFC 4895 is updates to address the
security issues this section is expected to be able to be updated to
not require generating two different keys.
SCTP-AUTH [RFC4895] is keyed using endpoint pair shared secrets. In
DTLS/SCTP, DTLS is used to establish these secrets. The endpoint
pair shared secrets MUST be provided to the SCTP stack as soon as the
computation is possible. The endpoints MUST NOT use another
mechanism for establishing endpoint pair shared secrets for SCTP-
AUTH. The endpoint pair shared secret for Shared Key Identifier zero
(0) is empty, it is used by both endpoints when establishing the
first DTLS connection and MUST NOT be used to protect ULP data.
Westerlund, et al. Expires 25 April 2024 [Page 23]
Internet-Draft DTLS over SCTP October 2023
The initial DTLS connection will be used to establish two new
endpoint pair shared secrets which MUST use shared key identifier 2
and 3. The endpoint pair shared secrets are derived using the TLS
exporter interface using the ASCII strings "EXPORTER-DTLS-OVER-SCTP-
CLIENT-WRITE" and "EXPORTER-DTLS-OVER-SCTP-SERVER-WRITE" with no
terminating NUL, no context, and length 64.
TLS-Exporter("EXPORTER-DTLS-OVER-SCTP-CLIENT-WRITE", , 64)
TLS-Exporter("EXPORTER-DTLS-OVER-SCTP-SERVER-WRITE", , 64)
Keys derived with the label "EXPORTER-DTLS-OVER-SCTP-CLIENT-WRITE"
always have an even Shared Key Identifier. They are used by the TLS
client for sending AUTH chunks and MUST NOT be used by the TLS client
for receiving AUTH chunks. Keys derived with the label "EXPORTER-
DTLS-OVER-SCTP-SERVER-WRITE" always have an odd Shared Key
Identifier. They are used by the TLS server for sending AUTH chunks
and MUST NOT be used by the TLS server for receiving AUTH chunks.
These directional keys change the behavior of SCTP-AUTH [RFC4895] and
requires extensions to the SCTP API defined in [RFC6458].
When a subsequent DTLS connection is setup, two new 64-byte endpoint
pair shared secrets are derived using the TLS-Exporter as defined
above. The Shared Key Identifiers form a sequence. If the previous
endpoint pair shared secrets used Shared Key Identifiers 2n and 2n+1,
the new ones MUST use Shared Key Identifier 2n+2 and 2n+3, unless 2n
= 65534, in which case the new Shared Key Identifiers are 2 and 3.
A DTLS connection MUST NOT be used be used for protection of ULP data
before the two SCTP-AUTH endpoint pair shared secrets has been
exported and the other endpoint has been authenticated.
4.8.1. DTLS 1.2 Considerations
Whenever a new DTLS connection is established, two 64-byte endpoint
pair shared secrets are derived using the TLS-Exporter described in
[RFC5705].
After sending or receiving the DTLS client Finished message for the
initial DTLS connection, the active SCTP-AUTH key MUST be switched
from key identifier zero (0) to key identifiers 2 and 3 and the SCTP-
AUTH Shared Key Identifier zero MUST NOT be used.
When the endpoint has sent or received a close_notify on the old DTLS
connection then the endpoint SHALL remove the two SCTP-AUTH endpoint
pair shared secrets derived from the old DTLS connection.
Westerlund, et al. Expires 25 April 2024 [Page 24]
Internet-Draft DTLS over SCTP October 2023
4.8.2. DTLS 1.3 Considerations
Whenever a new exporter_secret can be computed, two 64-byte endpoint
pair shared secrets are derived using the TLS-Exporter described in
Section 7.5 of [RFC8446].
After sending or receiving the DTLS server Finished message for the
initial DTLS connection, the active SCTP-AUTH key MUST be switched
from key identifier zero (0) to key identifiers 2 and 3 and the SCTP-
AUTH Shared Key Identifier zero MUST NOT be used.
When the endpoint has sent or received a close_notify in one
direction on the old DTLS connection then the endpoint SHALL remove
the SCTP-AUTH endpoint pair shared secret associated with that
direction in the old DTLS connection.
4.9. Shutdown
To prevent DTLS from discarding DTLS user messages while it is
shutting down, the below procedure has been defined. Its goal is to
avoid the need for APIs requiring per user message data level
acknowledgments and utilizes existing SCTP protocol behavior to
ensure delivery of the protected user messages data.
To support DTLS 1.2 close_notify behavior and avoid any uncertainty
related to rekeying, a DTLS/SCTP protocol message (Section 5) sent as
protected SCTP user message is defined, with its own PPID, to inform
the DTLS/SCTP layer that it is targeting the remote DTLS/SCTP
function and act on the request to close in a controlled fashion.
The shutdown procedure is initiated by any of the two peers,
targeting the closure of the SCTP Association and the DTLS
connections. In order to ensure that shutdown is completed without
data lost, DTLS/SCTP must control that both SCTP Tx buffers are empty
first, then it must ensure that all data in SCTP Rx buffer has been
fetched and delivered to ULP and finally it shall shutdown the DTLS
connections and the SCTP Association.
The interaction between peers (local and remote) and protocol stacks
is as follows:
1. Local instance of ULP asks for terminating the DTLS/SCTP
Association.
2. Local DTLS/SCTP acknowledges the request, from this time on no
new data from local instance of ULP will be accepted.
Westerlund, et al. Expires 25 April 2024 [Page 25]
Internet-Draft DTLS over SCTP October 2023
3. Local DTLS/SCTP finishes any protection operation on buffered
user messages and ensures that all protected user message data
has been successfully transferred to the remote peer.
4. Local DTLS/SCTP sends a DTLS/SCTP Control Message (Section 5) of
type "SHUTDOWN_Request" (Section 5.1) to its peer.
5. The remote DTLS/SCTP, when receiving the SHUTDOWN-Request,
informs its ULP that shutdown has been initiated. No more ULP
user message data to be sent to the peer can be accepted by DTLS/
SCTP.
6. Remote DTLS/SCTP finishes any protection operation on buffered
user messages and ensures that all protected user message data
has been successfully transferred to the remote ULP.
7. Remote DTLS/SCTP sends DTLS close_notify to Local DTLS/SCTP for
each and all DTLS connections. Then it initiates the SCTP
shutdown procedure (section 9.2 of [RFC9260]).
8. When the local DTLS/SCTP receives a close_notify on a DTLS
connection, in case it is DTLS 1.3 it SHALL send its
corresponding DTLS close_notify on each open DTLS connection.
When the last open DTLS connection has received close_notify and
any if needed corresponding close_notify have been sent, the
local DTLS/SCTP initiates the SCTP shutdown procedure (section
9.2 of [RFC9260]).
9. Upon receiving the information that SCTP has closed the
Association, independently the local and remote DTLS/SCTP
entities destroy the DTLS connection completing the shutdown.
The verification in step 3 and 6 that all user data message has been
successfully delivered to the remote ULP can be provided by the SCTP
stack that implements [RFC6458] by means of SCTP_SENDER_DRY event
(section 6.1.9 of [RFC6458]).
A successful SCTP shutdown will indicate successful delivery of all
data. However, in cases of communication failures and extensive
packet loss the SCTP shutdown procedure can time out and result in
SCTP association termination where its unknown if all data has been
delivered. The DTLS/SCTP should indicate to ULP successful
completion or failure to shutdown gracefully.
4.10. Transmission Limitations
Westerlund, et al. Expires 25 April 2024 [Page 26]
Internet-Draft DTLS over SCTP October 2023
4.10.1. Preventing DTLS sequence number wraps
To avoid failures in DTLS record decryption it is necessary to ensure
that the sequence number space never wraps for the DTLS records that
are outstanding between the DTLS encryption and decryption. As
discussed in Section 1.4 the amount of packets this include is a
combination of any buffering in the endpoint and the amount of data
in the SCTP sender/receiver buffer for the transmission.
To avoid overlapping sequence number the DTLS sender should first of
all use 16-bit sequence number to enable a larger space. Secondly,
it should track which DTLS records has been non-renegable ACKed by
the receiver and always maintain a certain safety buffer in number of
DTLS records. Thirdly, the implementation needs to attempt to
minimize usage of buffers that exist after the DTLS encryption until
the DTLS Decryption in its sender and receiver implementation.
If the receiver implementation keeps with the assumption to timely
decrypt DTLS records after it has been completely received, the
tracking of when a records has been fully received can maintain a
good view of the total number of outstanding records in regard to the
DTLS sequence number space and prevent wrapping of the sequence
number space by not protecting additional user message fragments
until further DTLS records has been acknowledged.
Assuming a that a quarter of the sequence number space is used as
safety margin it will limit the number of simultaneous in-flight DTLS
records to 49152, and thus also the number of simultaneous user
messages. Technically, if the DTLS implementation supports trial
decoding, overlap of the sequence number but that results in both
implementation requirements, need to signal the window it supports,
and additional decryption overhead due to trial decoding and will be
left for future extension.
So, what size of SCTP receiver window this limitation corresponds to
is highly dependent on the SCTP user message size. If all SCTP user
messages are large, e.g., 1 MB, then most DTLS Records will be close
to maximum DTLS record size. Thus, the SCTP receiver window size
required before this becomes an issue becomes fairly close to 49152
times 16384, i.e., approximately 800 MB. While SCTP user messages of
almost exclusively 100 bytes would only need a receiver window of
approximately 5 MB.
Westerlund, et al. Expires 25 April 2024 [Page 27]
Internet-Draft DTLS over SCTP October 2023
4.10.2. SCTP API Limitations
The SCTP-API defined in [RFC6458] results in an implementation
limitation when it comes to support transmission of user messages of
arbitrary sizes. That API does not allow changing the SCTP-AUTH key
used for protecting the sending of a particular user message. Thus,
user messages that will be transmitted over periods of time on the
order or longer than the interval between rekeying can't be
supported. Beyond delaying the completion of a rekeying until the
message has been transmitted, the session can deadlock if the DTLS
connection used to protect this long user message reaches the limit
of number of bytes transmitted with a particular key. However, this
is not an interoperability issue as it is the sender side's API that
limits what can be sent and thus the sender implementation will have
to address this issue.
5. DTLS/SCTP Control Message
The DTLS/SCTP Control Message is defined as its own upper layer
protocol for DTLS/SCTP identified by its own PPID. The control
message is sent in network byte order.
The first 32 bits are split in two 16-bit integers where the first
contains the Control Message Number and the next 16-bit integer
contains the length of the optional Variable Parameter. Granularity
of Variable Parameter is 32-bit with trailing zeroes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Message No | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Variable Parameter /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Each message is sent as its own SCTP user message after having been
protected by an open DTLS connection on any SCTP stream and MUST be
marked with SCTP Payload Protocol Identifier (PPID) value TBD1
(Section 8.4).
The DTLS/SCTP implementation MUST consume all SCTP messages received
with the PPID value of TBD1. If the message is not 32-bit long the
message MUST be discarded and the error SHOULD be logged. In case
the message has an unknown value the message is discarded and the
event SHOULD be logged.
Westerlund, et al. Expires 25 April 2024 [Page 28]
Internet-Draft DTLS over SCTP October 2023
Two control messages are defined in this specification.
5.1. SHUTDOWN-Request
The value "1" is defined as a request to the peer to initiate
controlled shutdown. This is used per step 4 and 5 in Section 4.9.
Control Message 1 "Shutdown request" has Parameter Length = 0.
5.2. Ready To Close Indication
The value "2" is defined as an indication to the peer that from its
perspective all SCTP packets with user message or using the SCTP-AUTH
key associated with the indicated DTLS connection have been sent and
acknowledged as received in a non-renegable way. This is used per
Section 4.7 to initiate the closing of the DTLS connections during
rekeying. Control Message 2 "Ready To Close" has Parameter Length
equal to the size of the DTLS Connection ID parameter in bytes. The
Variable Parameter contains the DTLS Connection ID that is to be
closed.
6. DTLS over SCTP Service
The adoption of DTLS over SCTP according to the current specification
is meant to add to SCTP the option for transferring encrypted data.
When DTLS over SCTP is used, all data being transferred MUST be
protected by chunk authentication and DTLS encrypted. Chunks that
need to be received in an authenticated way will be specified in the
CHUNK list parameter according to [RFC4895]. Error handling for
authenticated chunks is according to [RFC4895].
6.1. Adaptation Layer Indication in INIT/INIT ACK
At the initialization of the association, a sender of the INIT or
INIT ACK chunk that intends to use DTLS/SCTP as specified in this
specification MUST include an Adaptation Layer Indication Parameter
[RFC5061] with the IANA assigned value TBD (Section 8.3) to inform
its peer that it is able to support DTLS over SCTP per this
specification.
6.2. DTLS over SCTP Initialization
Initialization of DTLS/SCTP requires all the following options to be
part of the INIT/INIT ACK handshake:
RANDOM: defined in [RFC4895]
CHUNKS: defined in [RFC4895]
Westerlund, et al. Expires 25 April 2024 [Page 29]
Internet-Draft DTLS over SCTP October 2023
HMAC-ALGO: defined in [RFC4895]
ADAPTATION-LAYER-INDICATION: defined in [RFC5061]
When all the above options are present and having acceptable
parameters, the Association will start with support of DTLS/SCTP.
The set of options indicated are the DTLS/SCTP Mandatory Options. No
data transfer is permitted before DTLS handshake is completed. Chunk
bundling is permitted according to [RFC9260]. The DTLS handshake
will enable authentication of both the peers.
The extension described in this document is given by the following
message exchange.
--- INIT[RANDOM; CHUNKS; HMAC-ALGO; ADAPTATION-LAYER-IND] --->
<- INIT ACK[RANDOM; CHUNKS; HMAC-ALGO; ADAPTATION-LAYER-IND] -
--------------------- AUTH; COOKIE ECHO --------------------->
<-------------------- AUTH; COOKIE ACK -----------------------
---------------- AUTH; DATA[DTLS Handshake] ----------------->
...
...
<--------------- AUTH; DATA[DTLS Handshake] ------------------
6.3. Client Use Case
When a client initiates an SCTP Association with DTLS protection,
i.e., the SCTP INIT containing DTLS/SCTP Mandatory Options, it can
receive an INIT ACK also containing DTLS/SCTP Mandatory Options, in
that case the Association will proceed as specified in the previous
Section 6.2 section. If the peer replies with an INIT ACK not
containing all DTLS/SCTP Mandatory Options, the client SHOULD reply
with an SCTP ABORT.
6.4. Server Use Case
If a SCTP Server supports DTLS/SCTP, i.e., per this specification,
when receiving an INIT chunk with all DTLS/SCTP Mandatory Options it
will reply with an INIT ACK also containing all the DTLS/SCTP
Mandatory Options, following the sequence for DTLS initialization
Section 6.2 and the related traffic case. If a SCTP Server that
supports DTLS and configured to use it, receives an INIT chunk
without all DTLS/SCTP Mandatory Options, it SHOULD reply with an SCTP
ABORT.
Westerlund, et al. Expires 25 April 2024 [Page 30]
Internet-Draft DTLS over SCTP October 2023
6.5. RFC 6083 Fallback
This section discusses how an endpoint supporting this specification
can fallback to follow the DTLS/SCTP behavior in RFC6083. It is
recommended to define a setting that represents the policy to allow
fallback or not. However, the possibility to use fallback is based
on the ULP can operate using user messages that are no longer than
16384 bytes and where the security issues can be mitigated or
considered acceptable. If fallback is enabled, implementations MUST
use the dtls_sctp_ext extension (Section 6.5.3) to authenticate the
fallback. This mitigates on-path attacker to trigger fallback to RFC
6083. Fallback is NOT RECOMMENDED to be enabled as it permits weaker
algorithms and versions of DTLS.
An SCTP endpoint that receives an INIT chunk or an INIT ACK chunk
that does not contain the SCTP-Adaptation-Indication parameter with
the DTLS/SCTP adaptation layer codepoint, see Section 8.3, may in
certain cases potentially perform a fallback to RFC 6083 behavior.
However, the fallback attempt should only be performed if policy says
that is acceptable.
If fallback is allowed, it is possible that the client will send
plain text user messages prior to DTLS handshake as it is allowed per
RFC 6083. So that needs to be part of the consideration for a policy
allowing fallback.
6.5.1. Client Fallback
A DTLS/SCTP client supporting this specification encountering a
server not compatible with this specification MAY attempt RFC 6083
fallback per this procedure.
1. Fallback procedure, if enabled, is initiated when receiving an
SCTP INIT ACK that does not contain the DTLS/SCTP Adaptation
Layer indication. If fallback is not enabled the SCTP handshake
is aborted.
2. The client checks that the SCTP INIT ACK contained the necessary
chunks and parameters to establish SCTP-AUTH per RFC 6083 with
this endpoint. If not all necessary parameters or support
algorithms don't match the client MUST abort the handshake.
Otherwise it completes the SCTP handshake.
3. Client performs DTLS connection handshake per RFC 6083 over
established SCTP association. If successful authenticating the
targeted server the client has successful fallen back to use RFC
6083. If not terminate the SCTP association.
Westerlund, et al. Expires 25 April 2024 [Page 31]
Internet-Draft DTLS over SCTP October 2023
6.5.2. Server Fallback
A DTLS/SCTP Server that supports both this specification and RFC 6083
and where fallback has been enabled for the ULP can follow this
procedure.
1. When receiving an SCTP INIT message without the DTLS/SCTP
adaptation layer indication fallback procedure is initiated.
2. Verify that the SCTP INIT contains SCTP-AUTH parameters required
by RFC 6083 and compatible with this server. If that is not the
case abort the SCTP handshake.
3. Send an SCTP INIT ACK with the required SCTP-AUTH chunks and
parameters to the client.
4. Complete the SCTP Handshake. Await DTLS handshake per RFC 6083.
Plain text SCTP messages MAY be received.
5. Upon successful completion of DTLS handshake successful fallback
to RFC 6083 have been accomplished.
6.5.3. Authenticated Fallback
A DTLS/SCTP implementation supporting this document MUST include the
dtls_sctp_ext extension in all DTLS Client Hello used in DTLS/SCTP
according to RFC 6083. A DTLS/SCTP implementation supporting this
document MUST abort the SCTP association if the dtls_sctp_ext
extension is received when DTLS/SCTP according to RFC 6083 is used.
This mechanism provides authenticated fallback to RFC 6083.
The dtls_sctp_ext extention is defined as follows:
enum {
dtls_sctp_ext(TBD2), (65535)
} ExtensionType;
Clients MAY send this extention in ClientHello. It contains the
following structure:
struct {
Empty;
} DTLSOverSCTPExt;
Westerlund, et al. Expires 25 April 2024 [Page 32]
Internet-Draft DTLS over SCTP October 2023
7. SCTP API Consideration
DTLS/SCTP needs certain functionality on the API that the SCTP
implementation provides to the ULP to function optimally. A DTLS/
SCTP implementation will need to provide its own API to the ULP,
while itself using the SCTP API. This discussion is focused on the
needed functionality on the SCTP API.
The following functionality is needed:
* Controlling SCTP-AUTH negotiation so that SHA-256 algorithm is
included, and determine that SHA-1 is not selected when the
association is established.
* Determining when all SCTP packets that uses an SCTP-AUTH key or
contains DTLS records associated to a particular DTLS connection
has been acknowledged non-renegable.
* Install SCTP-AUTH keys with directionality
* Determining when all SCTP packets have been acknowledged non-
renegable.
* Negotiating the adaptation layer indication that indicates DTLS/
SCTP and determine if it was agreed or not.
* Partial user messages transmission and reception.
8. IANA Considerations
This document registers a number of protocol values per the below.
RFC-Editor Note: Please replace [RFC-TBD] with the RFC number given
to this specification.
8.1. Transport Layer Security (TLS) Extensions
IANA is requested to add a value to the Transport Layer Security
(TLS) Extensions' "TLS ExtensionType Values" registry defined by
[RFC8447]. At time of writing located at: TLS ExtensionType Values
Registry (https://www.iana.org/assignments/tls-extensiontype-values/
tls-extensiontype-values.xhtml#tls-extensiontype-values-1)
Westerlund, et al. Expires 25 April 2024 [Page 33]
Internet-Draft DTLS over SCTP October 2023
+=======+================+=====+=========+=============+===========+
| Value | Extension Name | TLS | DTLS-OK | Recommended | Reference |
| | | 1.3 | | | |
+=======+================+=====+=========+=============+===========+
| TBD2 | dtls_sctp_ext | CH | Y | Y | [RFC-TBD] |
+-------+----------------+-----+---------+-------------+-----------+
Table 1: TLS Extension
8.2. TLS Exporter Labels
IANA is requested to add two values to the TLS Exporter Label
registry as defined by [RFC5705], and [RFC8447]. At time of writing
located at: TLS Exporter Label registry
(https://www.iana.org/assignments/tls-parameters/tls-
parameters.xhtml#exporter-labels)
+====================================+=======+===========+=========+
|Value |DTLS-OK|Recommended|Reference|
+====================================+=======+===========+=========+
|EXPORTER-DTLS-OVER-SCTP-CLIENT-WRITE|Y |Y |[RFC-TBD]|
+------------------------------------+-------+-----------+---------+
|EXPORTER-DTLS-OVER-SCTP-SERVER-WRITE|Y |Y |[RFC-TBD]|
+------------------------------------+-------+-----------+---------+
Table 2: TLS Exporter Label
8.3. SCTP Adaptation Layer Indication Code Point
IANA is requested to assign an Adaptation Code Point to DTLS/SCTP for
usage in the Adaptation Layer Indication Parameter. The Adaptation
Code Point is registered in the SCTP Adaptation Code Points registry
defined by [RFC5061]. The registry was at time of writing located:
Adaptation Code Point registry (https://www.iana.org/assignments/
sctp-parameters/sctp-parameters.xhtml#sctp-parameters-27)
+============================+=============+===========+
| Code Point (32-bit number) | Description | Reference |
+============================+=============+===========+
| TBD | DTLS/SCTP | [RFC-TBD] |
+----------------------------+-------------+-----------+
Table 3: Adaptation Code Point
Westerlund, et al. Expires 25 April 2024 [Page 34]
Internet-Draft DTLS over SCTP October 2023
8.4. SCTP Payload Protocol Identifiers
IANA is requested to assign one SCTP Payload Protocol Identifier
(PPID) to be used to identify the DTLS/SCTP control messages
(Section 5). This PPID is registered in the SCTP Payload Protocol
Identifiers registry defined by [RFC9260]. The registry was at time
of writing located: SCTP Payload Protocol Identifiers
(https://www.iana.org/assignments/sctp-parameters/sctp-
parameters.xhtml#sctp-parameters-25)
+=======+===========================+===========+
| Value | SCTP PPID | Reference |
+=======+===========================+===========+
| TBD1 | DTLS/SCTP Control Message | [RFC-TBD] |
+-------+---------------------------+-----------+
Table 4: SCTP Payload Protocol Identifier
9. Security Considerations
The security considerations given in [RFC9147], [RFC4895], and
[RFC9260] also apply to this document.
9.1. Cryptographic Considerations
Over the years, there have been several serious attacks on earlier
versions of Transport Layer Security (TLS), including attacks on its
most commonly used ciphers and modes of operation. [RFC7457]
summarizes the attacks that were known at the time of publishing and
BCP 195 [RFC7525] [RFC8996] provide recommendations for improving the
security of deployed services that use TLS.
When DTLS/SCTP is used with DTLS 1.2 [RFC6347], DTLS 1.2 MUST be
configured to disable options known to provide insufficient security.
HTTP/2 [RFC9113] gives good minimum requirements based on the attacks
that where publicly known in 2022. DTLS 1.3 [RFC9147] only defines
strong algorithms without major weaknesses at the time of
publication. Many of the TLS registries have a "Recommended" column.
Parameters not marked as "Y" are NOT RECOMMENDED to support. DTLS
1.3 is preferred over DTLS 1.2 being a newer protocol that addresses
known vulnerabilities and only defines strong algorithms without
known major weaknesses at the time of publication.
DTLS 1.3 requires rekeying before algorithm specific AEAD limits have
been reached. The AEAD limits equations are equally valid for DTLS
1.2 and SHOULD be followed for DTLS/SCTP, but are not mandated by the
DTLS 1.2 specification.
Westerlund, et al. Expires 25 April 2024 [Page 35]
Internet-Draft DTLS over SCTP October 2023
HMAC-SHA-256 as used in SCTP-AUTH has a very large tag length and
very good integrity properties. The SCTP-AUTH key can be used longer
than the current algorithms in the TLS record layer. The SCTP-AUTH
key is rekeyed when a new DTLS connection is set up at which point a
new SCTP-AUTH key is derived using the TLS-Exporter.
(D)TLS 1.3 [RFC8446] discusses forward secrecy from (EC)DHE, Key
Update, and tickets/resumption. Forward secrecy limits the effect of
key leakage in one direction (compromise of a key at time T2 does not
compromise some key at time T1 where T1 < T2). Protection in the
other direction (compromise at time T1 does not compromise keys at
time T2) can be achieved by rerunning (EC)DHE. If a long-term
authentication key has been compromised, a full handshake with
(EC)DHE gives protection against passive attackers. If the
resumption_master_secret has been compromised, a resumption handshake
with (EC)DHE gives protection against passive attackers and a full
handshake with (EC)DHE gives protection against active attackers. If
a traffic secret has been compromised, any handshake with (EC)DHE
gives protection against active attackers.
The document “Confidentiality in the Face of Pervasive Surveillance:
A Threat Model and Problem Statement” [RFC7624] defines key
exfiltration as the transmission of cryptographic keying material for
an encrypted communication from a collaborator, deliberately or
unwittingly, to an attacker. Using the terms in RFC 7624, forward
secrecy without rerunning (EC)DHE still allows an attacker to do
static key exfiltration. Rerunning (EC)DHE forces and attacker to
dynamic key exfiltration (or content exfiltration).
When using DTLS 1.3 [RFC9147], AEAD limits and forward secrecy can be
achieved by sending post-handshake KeyUpdate messages, which triggers
rekeying of DTLS. Such symmetric rekeying gives significantly less
protection against key leakage than re-running Diffie-Hellman as
explained above. After leakage of application_traffic_secret_N, an
attacker can passively eavesdrop on all future data sent on the
connection including data encrypted with
application_traffic_secret_N+1, application_traffic_secret_N+2, etc.
Note that Key Update does not update the exporter_secret.
DTLS/SCTP is in many deployments replacing IPsec. For IPsec, NIST
(US), BSI (Germany), and ANSSI (France) recommends very frequent re-
run of Diffie-Hellman to provide forward secrecy and force attackers
to dynamic key extraction [RFC7624]. ANSSI writes "It is recommended
to force the periodic renewal of the keys, e.g., every hour and every
100 GB of data, in order to limit the impact of a key compromise."
[ANSSI-DAT-NT-003].
Westerlund, et al. Expires 25 April 2024 [Page 36]
Internet-Draft DTLS over SCTP October 2023
For many DTLS/SCTP deployments the SCTP association is expected to
have a very long lifetime of months or even years. For associations
with such long lifetimes there is a need to frequently re-
authenticate both client and server. TLS Certificate lifetimes
significantly shorter than a year are common which is shorter than
expected to be used DTLS/SCTP associations lifetimes.
SCTP-AUTH re-rekeying, periodic authentication of both endpoints, and
frequent re-run of Diffie-Hellman to force attackers to dynamic key
extraction is in DTLS/SCTP per this specification achieved by setting
up new DTLS connections over the same SCTP association.
Implementations SHOULD set up new connections frequently to force
attackers to dynamic key extraction. Implementations MUST set up new
connections before any of the certificates expire. It is RECOMMENDED
that all negotiated and exchanged parameters are the same except for
the timestamps in the certificates. Clients and servers MUST NOT
accept a change of identity during the setup of a new connections,
but MAY accept negotiation of stronger algorithms and security
parameters, which might be motivated by new attacks.
Allowing new connections can enable denial-of-service attacks. The
endpoints MUST limit the number of simultaneous connections to two.
The implementor shall take into account that an existing DTLS
connection can only be closed after "Ready_To_Close" Section 5.2
indication.
When DTLS/SCTP is used with DTLS 1.2 [RFC6347], the TLS Session Hash
and Extended Master Secret Extension [RFC7627] MUST be used to
prevent unknown key-share attacks where an attacker establishes the
same key on several connections. DTLS 1.3 always prevents these
kinds of attacks. The use of SCTP-AUTH then cryptographically binds
new connections to the old connections. This together with mandatory
mutual authentication (on the DTLS layer) and a requirement to not
accept new identities mitigates MITM attacks that have plagued
renegotiation [TRISHAKE].
9.2. Downgrade Attacks
A peer supporting DTLS/SCTP according to this specification, DTLS/
SCTP according to [RFC6083] and/or SCTP without DTLS may be
vulnerable to downgrade attacks where on on-path attacker interferes
with the protocol setup to lower or disable security. If possible,
it is RECOMMENDED that the peers have a policy only allowing DTLS/
SCTP according to this specification.
Westerlund, et al. Expires 25 April 2024 [Page 37]
Internet-Draft DTLS over SCTP October 2023
9.3. Targeting DTLS Messages
The DTLS handshake messages and other control messages, i.e., not
application data can easily be identified when using DTLS 1.2 as
their content type is not encrypted. With DTLS 1.3 there is no
unprotected content type. However, they will be sent with an PPID of
0 if sent in their own SCTP user messages. Section 4.4 proposes a
basic behavior that will still make it easily for anyone to detect
the DTLS messages that are not protected user messages.
9.4. Authentication and Policy Decisions
DTLS/SCTP MUST be mutually authenticated. Authentication is the
process of establishing the identity of a user or system and
verifying that the identity is valid. DTLS only provides proof of
possession of a key. DTLS/SCTP MUST perform identity authentication.
It is RECOMMENDED that DTLS/SCTP is used with certificate-based
authentication. When certificates are used the application using
DTLS/SCTP is responsible for certificate policies, certificate chain
validation, and identity authentication (HTTPS does for example match
the hostname with a subjectAltName of type dNSName). The application
using DTLS/SCTP MUST define what the identity is and how it is
encoded and the client and server MUST use the same identity format.
Guidance on server certificate validation can be found in [RFC6125].
DTLS/SCTP enables periodic transfer of mutual revocation information
(OSCP stapling) every time a new parallel connection is set up. All
security decisions MUST be based on the peer's authenticated
identity, not on its transport layer identity.
It is possible to authenticate DTLS endpoints based on IP addresses
in certificates. SCTP associations can use multiple IP addresses per
SCTP endpoint. Therefore, it is possible that DTLS records will be
sent from a different source IP address or to a different destination
IP address than that originally authenticated. This is not a problem
provided that no security decisions are made based on the source or
destination IP addresses.
9.5. Resumption and Tickets
In DTLS 1.3 any number of tickets can be issued in a connection and
the tickets can be used for resumption as long as they are valid,
which is up to seven days. The nodes in a resumed connection have
the same roles (client or server) as in the connection where the
ticket was issued. Resumption can have significant latency benefits
for quickly restarting a broken DTLS/SCTP association. If tickets
and resumption are used it is enough to issue a single ticket per
connection.
Westerlund, et al. Expires 25 April 2024 [Page 38]
Internet-Draft DTLS over SCTP October 2023
9.6. Privacy Considerations
[RFC6973] suggests that the privacy considerations of IETF protocols
be documented.
For each SCTP user message, the 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/SCTP
provides privacy for the actual user message, the other three
information fields are not confidentiality protected. They are sent
as clear text because they are part of the SCTP DATA chunk header.
It is RECOMMENDED that DTLS/SCTP is used with certificate-based
authentication in DTLS 1.3 [RFC9147] to provide identity protection.
DTLS/SCTP MUST be used with a key exchange method providing forward
secrecy.
9.7. Pervasive Monitoring
As required by [RFC7258], work on IETF protocols needs to consider
the effects of pervasive monitoring and mitigate them when possible.
Pervasive Monitoring is widespread surveillance of users. By
encrypting more information including user identities, DTLS 1.3
offers much better protection against pervasive monitoring.
Massive pervasive monitoring attacks relying on key exchange without
forward secrecy has been reported. By mandating forward secrecy,
DTLS/SCTP effectively mitigate many forms of passive pervasive
monitoring and limits the amount of compromised data due to key
compromise.
An important mitigation of pervasive monitoring is to force attackers
to do dynamic key exfiltration instead of static key exfiltration.
Dynamic key exfiltration increases the risk of discovery for the
attacker [RFC7624]. DTLS/SCTP per this specification encourages
implementations to frequently set up new DTLS connections with
(EC)DHE over the same SCTP association to force attackers to do
dynamic key exfiltration.
In addition to the privacy attacks discussed above, surveillance on a
large scale may enable tracking of a user over a wider geographical
area and across different access networks. Using information from
DTLS/SCTP together with information gathered from other protocols
increase the risk of identifying individual users.
Westerlund, et al. Expires 25 April 2024 [Page 39]
Internet-Draft DTLS over SCTP October 2023
9.8. Replay attacks
Replay attack breaks data origin authentication, data integrity
protection, and data confidentiality. The peculiar architecture of
rfc6083 makes hard to predict how a replay attack can get success.
What is clear is that replay attack hasn't been considered when
rfc6083 has been specified, making it weak from the beginning. In
rfc6083 the replay window is open during the lifetime of the SCTP-
AUTH key validity and being TSN visible it's relatively easy to
inject an old Data Chunk that passes validation. Since DTLS replay
protection is not used and because a single chunk is also a single
DTLS record, the attack surface of rfc6083 is large and even if SCTP-
AUTH will be fixed in regards to replay attack, the combination of
SCTP-AUTH and DTLS as described in rfc6083 is not by architecture.
Details are described in Section 3.8
10. Contributors
Michael Tüxen contributed as co-author to the initial versions this
draft. Michael's contributions include:
* The use of the Adaptation Layer Indication.
* Many editorial improvements.
11. Acknowledgments
The authors of RFC 6083 which this document is based on are Michael
Tüxen, Eric Rescorla, and Robin Seggelmann.
The RFC 6083 authors thanked Anna Brunstrom, Lars Eggert, Gorry
Fairhurst, Ian Goldberg, Alfred Hoenes, Carsten Hohendorf, Stefan
Lindskog, Daniel Mentz, and Sean Turner for their invaluable
comments.
The authors of this document want to thank Daria Ivanova, Li Yan, and
GitHub user vanrein for their contribution.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Westerlund, et al. Expires 25 April 2024 [Page 40]
Internet-Draft DTLS over SCTP October 2023
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758,
DOI 10.17487/RFC3758, May 2004,
<https://www.rfc-editor.org/info/rfc3758>.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for the Stream Control Transmission
Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
2007, <https://www.rfc-editor.org/info/rfc4895>.
[RFC5061] Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
Kozuka, "Stream Control Transmission Protocol (SCTP)
Dynamic Address Reconfiguration", RFC 5061,
DOI 10.17487/RFC5061, September 2007,
<https://www.rfc-editor.org/info/rfc5061>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8260] Stewart, R., Tuexen, M., Loreto, S., and R. Seggelmann,
"Stream Schedulers and User Message Interleaving for the
Stream Control Transmission Protocol", RFC 8260,
DOI 10.17487/RFC8260, November 2017,
<https://www.rfc-editor.org/info/rfc8260>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
Westerlund, et al. Expires 25 April 2024 [Page 41]
Internet-Draft DTLS over SCTP October 2023
[RFC8449] Thomson, M., "Record Size Limit Extension for TLS",
RFC 8449, DOI 10.17487/RFC8449, August 2018,
<https://www.rfc-editor.org/info/rfc8449>.
[RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[RFC9146] Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and
A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146,
DOI 10.17487/RFC9146, March 2022,
<https://www.rfc-editor.org/info/rfc9146>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control
Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260,
June 2022, <https://www.rfc-editor.org/info/rfc9260>.
12.2. Informative References
[RFC3436] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission Protocol",
RFC 3436, DOI 10.17487/RFC3436, December 2002,
<https://www.rfc-editor.org/info/rfc3436>.
[RFC3788] Loughney, J., Tuexen, M., Ed., and J. Pastor-Balbas,
"Security Considerations for Signaling Transport (SIGTRAN)
Protocols", RFC 3788, DOI 10.17487/RFC3788, June 2004,
<https://www.rfc-editor.org/info/rfc3788>.
[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083,
DOI 10.17487/RFC6083, January 2011,
<https://www.rfc-editor.org/info/rfc6083>.
Westerlund, et al. Expires 25 April 2024 [Page 42]
Internet-Draft DTLS over SCTP October 2023
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458,
DOI 10.17487/RFC6458, December 2011,
<https://www.rfc-editor.org/info/rfc6458>.
[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/info/rfc6525>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7525, DOI 10.17487/RFC7525, May 2015,
<https://www.rfc-editor.org/info/rfc7525>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
Westerlund, et al. Expires 25 April 2024 [Page 43]
Internet-Draft DTLS over SCTP October 2023
[ANSSI-DAT-NT-003]
Agence nationale de la sécurité des systèmes
d'information, "Recommendations for securing networks with
IPsec", ANSSI Technical Report DAT-NT-003 , August 2015,
<<https://www.ssi.gouv.fr/uploads/2015/09/
NT_IPsec_EN.pdf>>.
[TRISHAKE] Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P. Strub, "Triple Handshakes and Cookie Cutters:
Breaking and Fixing Authentication over TLS", IEEE
Symposium on Security & Privacy , April 2016,
<https://hal.inria.fr/hal-01102259/file/triple-handshakes-
and-cookie-cutters-oakland14.pdf>.
Appendix A. Motivation for Changes
This document proposes a number of changes to RFC 6083 that have
various different motivations:
Supporting Large User Messages: RFC 6083 allowed only user messages
that could fit within a single DTLS record. 3GPP has run into this
limitation where they have at least four SCTP using protocols (F1,
E1, Xn, NG-C) that can potentially generate messages over the size of
16384 bytes.
New Versions: 10 years has passed since RFC 6083 was written, and
significant evolution has happened in the area of DTLS and security
algorithms. Thus, DTLS 1.3 is the newest version of DTLS and also
the SHA-1 HMAC algorithm of RFC 4895 is getting towards the end of
usefulness. Use of DTLS 1.3 with long lived associations require a
solution to enable mutual re-authentication and (EC)DHE based
rekeying to ensure forward secrecy. Thus, this document mandates
usage of relevant versions and algorithms as well as defining the
parallel DTLS connection solution.
Allowing DTLS Messages on any stream: RFC6083 requires DTLS messages
that are not user message data to be sent on stream 0 and that this
stream is used with in-order delivery. That can actually limit the
applications that can use DTLS/SCTP. In addition, with DTLS 1.3
encrypting the actual message type it is anyway not available.
Therefore, a more flexible rule set is used that relies on DTLS
handling reordering.
Clarifications: Some implementation experiences have been gained that
motivates additional clarifications on the specification.
* Avoid unsecured messages prior to DTLS handshake have completed.
Westerlund, et al. Expires 25 April 2024 [Page 44]
Internet-Draft DTLS over SCTP October 2023
* Make clear that all messages are encrypted after DTLS handshake.
Authors' Addresses
Magnus Westerlund
Ericsson
Email: magnus.westerlund@ericsson.com
John Preuß Mattsson
Ericsson
Email: john.mattsson@ericsson.com
Claudio Porfiri
Ericsson
Email: claudio.porfiri@ericsson.com
Westerlund, et al. Expires 25 April 2024 [Page 45]