Internet-Draft DTLS over SCTP May 2024
Westerlund, et al. Expires 4 November 2024 [Page]
Intended Status:
Standards Track
M. Westerlund
J. Preuß Mattsson
C. Porfiri

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


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.

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Table of Contents

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.

  • 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

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.

   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 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 214 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.

  • 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 214 (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 264-1 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.

  • 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 248.

  • 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.

  • 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:


An SCTP association.


A DTLS connection. It is uniquely identified by a connection identifier.


A unidirectional stream of an SCTP association. It is uniquely identified by a stream identifier.

1.9. Abbreviations


Authenticated Encryption with Associated Data


Datagram Transport Layer Security


Keyed-Hash Message Authentication Code


Maximum Transmission Unit


Payload Protocol Identifier


Stream Control Transmission Protocol


Authenticated Chunks for SCTP [RFC4895]


Transmission Control Protocol


Transport Layer Security


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 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 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 214 protected bytes. It is mandated that DTLS supports the maximum record size of 214 bytes. DTLS/SCTP MAY exploit maximum DTLS record size less than 214 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 264-1 bytes. This is to allow implementation to assume that 64-bit length fields and offset pointers will be sufficient.

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.

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 232 TSNs. Implementations MUST therefore setup a new parallel DTLS connection to rekey well before 232 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 214 = 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

   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 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].

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]).

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.

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, 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.

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.

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.


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.

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.

  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

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.

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.

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]

HMAC-ALGO: defined in [RFC4895]


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.

   --------------------- 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.

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.

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 {

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:

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

Table 1: TLS Extension
Value Extension Name TLS 1.3 DTLS-OK Recommended Reference
TBD2 dtls_sctp_ext CH Y Y [RFC-TBD]

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

Table 2: TLS Exporter Label
Value DTLS-OK Recommended Reference

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

Table 3: Adaptation Code Point
Code Point (32-bit number) Description Reference

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

Table 4: SCTP Payload Protocol Identifier
Value SCTP PPID Reference
TBD1 DTLS/SCTP Control Message [RFC-TBD]

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.

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].

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.

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.

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.

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:

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

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, "Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, , <>.
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, , <>.
Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, , <>.
Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.
Salowey, J. and S. Turner, "IANA Registry Updates for TLS and DTLS", RFC 8447, DOI 10.17487/RFC8447, , <>.
Thomson, M., "Record Size Limit Extension for TLS", RFC 8449, DOI 10.17487/RFC8449, , <>.
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, , <>.
Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, , <>.
Rescorla, E., Ed., Tschofenig, H., Ed., Fossati, T., and A. Kraus, "Connection Identifier for DTLS 1.2", RFC 9146, DOI 10.17487/RFC9146, , <>.
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, , <>.
Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260, , <>.

12.2. Informative References

Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport Layer Security over Stream Control Transmission Protocol", RFC 3436, DOI 10.17487/RFC3436, , <>.
Loughney, J., Tuexen, M., Ed., and J. Pastor-Balbas, "Security Considerations for Signaling Transport (SIGTRAN) Protocols", RFC 3788, DOI 10.17487/RFC3788, , <>.
Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)", RFC 6083, DOI 10.17487/RFC6083, , <>.
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, , <>.
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, , <>.
Stewart, R., Tuexen, M., and P. Lei, "Stream Control Transmission Protocol (SCTP) Stream Reconfiguration", RFC 6525, DOI 10.17487/RFC6525, , <>.
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, , <>.
Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, , <>.
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, , <>.
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, , <>.
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, , <>.
Agence nationale de la sécurité des systèmes d'information, "Recommendations for securing networks with IPsec", ANSSI Technical Report DAT-NT-003 , , <<>>.
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 , , <>.

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.

Authors' Addresses

Magnus Westerlund
John Preuß Mattsson
Claudio Porfiri