Internet-Draft DTLS over SCTP July 2021
Westerlund, et al. Expires 13 January 2022 [Page]
6083 (if approved)
Intended Status:
Standards Track
M. Westerlund
J. Preuß Mattsson
C. Porfiri
M. Tüxen
Münster Univ. of Appl. Sciences

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


This document describes a proposed update for the usage of the Datagram Transport Layer Security (DTLS) protocol to protect user messages sent over the Stream Control Transmission Protocol (SCTP).

DTLS over SCTP provides mutual authentication, confidentiality, integrity protection, and 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 intends to obsolete RFC 6083 and removes the 16 kB limitation on 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 updates the DTLS versions to use, as well as the HMAC algorithms for SCTP-AUTH, and simplifies secure implementation by some stricter requirements on the establishment procedures.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the TSVWG Working Group mailing list (, which is archived at

Source for this draft and an issue tracker can be found at

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

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 13 January 2022.

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 [I-D.ietf-tls-dtls13], over the Stream Control Transmission Protocol (SCTP), as defined in [RFC4960] with Authenticated Chunks for SCTP (SCTP-AUTH) [RFC4895].

This specification provides mutual authentication of endpoints, confidentiality, integrity protection, and replay protection of user messages for applications that use SCTP as their transport protocol. 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 perfect forward secrecy for SCTP-AUTH, and confidentiality of user messages. DTLS/SCTP use SCTP and SCTP-AUTH for integrity protection and replay protection of 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].
  • large user messages.

The method described in this document requires that the SCTP implementation supports the optional feature of fragmentation of SCTP user messages as defined in [RFC4960]. The implementation is also 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.

1.1.1. Comparison with TLS for 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.

1.1.2. Changes from RFC 6083

The DTLS over SCTP solution defined in RFC 6083 had the following limitation:

  • The maximum user message size is 2^14 bytes, which is a single DTLS record limit.

This update that replaces RFC 6083 defines the following changes:

  • Removes the limitations on user messages sizes by defining a secure fragmentation mechanism.
  • Mandates that more modern DTLS version are required (DTLS 1.2 or 1.3)
  • Mandates use of modern 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.

Mandating DTLS 1.2 or DTLS 1.3 instead to 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 cause by:

  • The number of renegotiations in DTLS 1.2 is limited to 65534 compared to unlimited in DTLS 1.0.
  • The number of KeyUpdates in DTLS 1.3 is limited to 65532 and renegotiations are not supported.

1.2. DTLS Version

DTLS/SCTP as defined by this document can use either DTLS 1.2 [RFC6347] or DTLS 1.3 [I-D.ietf-tls-dtls13]. Some crucial difference between the DTLS versions make it necessary for a user of DTLS/SCTP to make an informed choice of the DTLS version to use based on their application's requirements. In general, DTLS 1.3 is to preferred being a newer protocol that addresses known vulnerabilities and only defines strong algorithms without known major weaknesses at the time of publication.

However, some applications using DTLS/SCTP are of semi-permanent nature and use SCTP associations with lifetimes that are more than a few hours, and where there is a significant cost of bringing down the SCTP association in order to restart it. For such DTLS/SCTP usages that need either of:

  • Periodic re-authentication of both endpoints (not only the DTLS client).
  • Periodic rerunning of Diffie-Hellman key-exchange to provide Perfect Forward Secrecy (PFS) to reduce the impact any key-reveal.
  • Perform SCTP-AUTH re-keying.

At the time of publication DTLS 1.3 does not support any of these, where DTLS 1.2 renegotiation functionality can provide this functionality in the context of DTLS/SCTP. The application will have to analyze its needs and requirements on the above and based on this select the DTLS version to use.

To address known vulnerabilities in DTLS 1.2 this document describes and mandates implementation constraints on ciphers, protocol options and how to use the DTLS renegotiation mechanism.

In the rest of the document, unless the version of DTLS is specifically called out the text applies to both versions of DTLS.

1.3. Terminology

This document uses the following terms:

Association: An SCTP association.

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

1.4. Abbreviations

DTLS: Datagram Transport Layer Security

MTU: Maximum Transmission Unit

PFS: Perfect Forward Secrecy

PPID: Payload Protocol Identifier

SCTP: Stream Control Transmission Protocol

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 [I-D.ietf-tls-dtls13], or DTLS 1.2 [RFC6347]. Earlier versions of DTLS MUST NOT be used (see [RFC8996]). It is expected that DTLS/SCTP as described in this document will work with future versions of DTLS.

3.2. Cipher Suites and Cryptographic Parameters

For DTLS 1.2, the cipher suites forbidden by [RFC7540] MUST NOT be used. For all versions of DTLS, cryptographic parameters giving confidentiality and Perfect Forward Secrecy (PFS) MUST be used.

3.3. Message Sizes

DTLS/SCTP, automatically fragments and reassembles user messages. This specification defines how to fragment the user messages into DTLS records, where each DTLS 1.3 record allows a maximum of 2^14 protected bytes. Each DTLS record adds some overhead, thus using records of maximum possible size are recommended to minimize the transmitted overhead.

The sequence of DTLS records is then fragmented into DATA or I-DATA Chunks to fit the path MTU by SCTP. The largest possible user messages using the mechanism defined in this specification is 2^64-1 bytes.

The security operations and reassembly process requires that the protected user message, i.e., with DTLS record overhead, is buffered in the receiver. 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 required buffering prior to DTLS processing can be limited to 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. Implementations can trade-off buffer memory requirements in the DTLS layer with transport overhead by using smaller DTLS records.

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.

3.4. 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 packets resulting in impact on the received protected user message. 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.

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 replay protection even without DTLS replay protection, the replay detection of DTLS MUST NOT be used.

3.5. Path MTU Discovery

DTLS Path MTU Discovery MUST NOT be used. Since SCTP provides Path MTU discovery and fragmentation/reassembly for user messages, and according to Section 3.3, DTLS can send maximum sized DTLS Records.

3.6. 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 [RFC4960], and optionally I-DATA [RFC8260] chunks.

DTLS/SCTP works as a shim layer between the user message API and SCTP. The fragmentation works similar as the DTLS fragmentation of handshake messages. On the sender side a user message fragmented into fragments m0, m1, m2, each no larger than 2^14 - 1 = 16383 bytes.

   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 DTLS is used to decrypt the individual records. There are three failure cases an 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.
  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.

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 in 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.

The DTLS Connection ID SHOULD NOT be negotiated (Section 9 of [I-D.ietf-tls-dtls13]). If DTLS 1.3 is used, the length field MUST be included and a 16-bit sequence number SHOULD be used.

4.2. DTLS Connection Handling

The DTLS connection MUST be established at the beginning of the SCTP association and be terminated when the SCTP association is terminated, (i.e., there's only one DTLS connection within one association). 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 messages that are not protected by DTLS. So, the case that an endpoint receives data that is not either DTLS messages on Stream 0 or 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.

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 not harm as long as the application can determine whether or not DTLS is used. However, for protocol analyzers, for example, it is much easier if a separate PPID is used.

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

4.4. Stream Usage

All DTLS Handshake, Alert, or ChangeCipherSpec (DTLS 1.2 only) messages MUST be transported on stream 0 with unlimited reliability and with the ordered delivery feature.

DTLS messages of the record protocol, which carries the protected user messages, SHOULD use multiple streams other than stream 0; they MAY use stream 0. On stream 0 protected user messages as well as any DTLS messages that aren't record protocol will be mixed, thus the additional head of line blocking can occur.

4.5. Chunk Handling

DATA chunks of SCTP MUST be sent in an authenticated way as described in [RFC4895]. All other chunks that can be authenticated, i.e., all chunk types that can be listed in the Chunk List Parameter [RFC4895], MUST also be sent in an authenticated way. This makes sure that an attacker cannot modify the stream in which a message is sent or affect the ordered/unordered delivery of the message.

If PR-SCTP as defined in [RFC3758] is used, FORWARD-TSN chunks MUST also be sent in an authenticated way as described in [RFC4895]. This makes sure that it is not possible for an attacker to drop messages and use forged FORWARD-TSN, SACK, and/or SHUTDOWN chunks to hide this dropping.

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. If I-DATA chunks are used, then they MUST be sent in an authenticated way as described in [RFC4895].

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.

4.7. Renegotiation

DTLS 1.2 renegotiation enables rekeying (with ephemeral Diffie-Hellman) of both DTLS and SCTP-AUTH as well as mutual reauthentication inside an DTLS 1.2 connection. It is up to the upper layer to use/allow it or not. Application writers should be aware that allowing renegotiations may result in changes of security parameters. Renegotiation has been removed from DTLS 1.3 and partly replaced with post-handshake messages such as KeyUpdate. See Section 8 for security considerations regarding rekeying.

4.7.1. DTLS 1.2 Considerations

Before sending during renegotiation a ClientHello message or ServerHello message, the DTLS endpoint MUST ensure that all DTLS messages using the previous epoch have been acknowledged by the SCTP peer in a non-revokable way.

Prior to processing a received ClientHello message or ServerHello message, all other received SCTP user messages that are buffered in the SCTP layer and can be delivered to the DTLS layer MUST be read and processed by DTLS.

4.7.2. DTLS 1.3 Considerations

Before sending a KeyUpdate message, the DTLS endpoint MUST ensure that all DTLS messages have been acknowledged by the SCTP peer in a non-revokable way. After sending the KeyUpdate message, it stops sending DTLS messages until the corresponding Ack message has been processed.

Prior to processing a received KeyUpdate message, all other received SCTP user messages that are buffered in the SCTP layer and can be delivered to the DTLS layer MUST be read and processed by DTLS.

4.8. DTLS Epochs

In general, DTLS implementations SHOULD discard records from earlier epochs. However, in the context of a reliable communication this is not appropriate.

4.8.1. DTLS 1.2 Considerations

The procedures of Section 4.1 of [RFC6347] MUST NOT be followed. Instead, when currently using epoch n, for n > 1, DTLS packets from epoch n - 1 and n MUST be processed.

4.8.2. DTLS 1.3 Considerations

The procedures of Section 4.2.1 of [I-D.ietf-tls-dtls13] are irrelevant. When receiving DTLS packets using epoch n, no DTLS packets from earlier epochs are received.

4.9. Handling of Endpoint-Pair Shared Secrets

SCTP-AUTH [RFC4895] is keyed using Endpoint-Pair Shared Secrets. In SCTP associations where DTLS is used, DTLS is used to establish these secrets. The endpoints MUST NOT use another mechanism for establishing shared secrets for SCTP-AUTH. The endpoint-pair shared secret for Shared Key Identifier 0 is empty and MUST be used when establishing a DTLS connection.

4.9.1. DTLS 1.2 Considerations

Whenever the master secret changes, a 64-byte shared secret is derived from every master secret and provided as a new endpoint-pair shared secret by using the TLS-Exporter described in [RFC5705]. The 64-byte shared secret MUST be provided to the SCTP stack as soon as the computation is possible. The exporter MUST use the label given in Section Section 7 and no context. The new Shared Key Identifier MUST be the old Shared Key Identifier incremented by 1.

After sending the DTLS Finished message, the active SCTP-AUTH key MUST be switched to the new one.

Once the initial Finished message from the peer has been processed by DTLS, the SCTP-AUTH key with Shared Key Identifier 0 MUST be removed. Once the Finished message using DTLS epoch n with n > 2 has been processed by DTLS, the SCTP-AUTH key with Shared Key Identifier n - 2 MUST be removed.

4.9.2. DTLS 1.3 Considerations

When the exporter_secret can be computed, a 64-byte shared secret is derived from it and provided as a new endpoint-pair shared secret by using the TLS-Exporter described in [RFC8446]. The 64-byte shared secret MUST be provided to the SCTP stack as soon as the computation is possible. The exporter MUST use the label given in Section Section 7 and no context. This shared secret MUST use Shared Key Identifier 1.

After sending the DTLS Finished message, the active SCTP-AUTH key MUST be switched to use Shared Key Identifier 1.

Once the Finished message from the peer has been processed by DTLS, the SCTP-AUTH key with Shared Key Identifier 0 MUST be removed.

4.10. Shutdown

To prevent DTLS from discarding DTLS user messages while it is shutting down, a CloseNotify message MUST only be sent after all outstanding SCTP user messages have been acknowledged by the SCTP peer in a non-revokable way.

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

5. DTLS over SCTP Service

The adoption of DTLS over SCTP according to the current description 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].

5.1. 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 with the IANA assigned value TBD (Section 7.2) to inform its peer that it is able to support DTLS over SCTP per this specification.

5.2. 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: list of permitted chunks, defined in [RFC4895]

HMAC-ALGO: defined in [RFC4895]


When all the above options are present, 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 complete. Chunk bundling is permitted according to [RFC4960]. The DTLS handshake will enable authentication of both the peers and also have the declare their support message size.

The extension described in this document is given by the following message exchange.

   ------------------------ COOKIE-ECHO ------------------------>
   <------------------------ COOKIE-ACK -------------------------
   ---------------- AUTH; DATA[DTLS Handshake] ----------------->
   <--------------- AUTH; DATA[DTLS Handshake] ------------------

5.3. Client Use Case

When a client initiates an SCTP Association with DTLS protection, i.e., the SCTP INIT containing DTSL/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 5.2 section. If the peer replies with an INIT-ACK not containing all DTLS/SCTP Mandatory Options, the client SHOULD reply with an SCTP ABORT.

5.4. 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 5.2 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.

5.5. 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 16383 bytes and where the security issues can be mitigated or considered acceptable. Fallback is NOT RECOMMEND to be enabled as it enables downgrade to 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 7.2, 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. Socket API Considerations

This section describes how the socket API defined in [RFC6458] is extended to provide a way for the application to observe the HMAC algorithms used for sending and receiving of AUTH chunks.

Please note that this section is informational only.

A socket API implementation based on [RFC6458] is, by means of the existing SCTP_AUTHENTICATION_EVENT event, extended to provide the event notification whenever a new HMAC algorithm is used in a received AUTH chunk.

Furthermore, two new socket options for the level IPPROTO_SCTP and the name SCTP_SEND_HMAC_IDENT and SCTP_EXPOSE_HMAC_IDENT_CHANGES are defined as described below. The first socket option is used to query the HMAC algorithm used for sending AUTH chunks. The second enables the monitoring of HMAC algorithms used in received AUTH chunks via the SCTP_AUTHENTICATION_EVENT event.

Support for the SCTP_SEND_HMAC_IDENT and SCTP_EXPOSE_HMAC_IDENT_CHANGES socket options also needs to be added to the function sctp_opt_info().

6.1. Socket Option to Get the HMAC Identifier being Sent (SCTP_SEND_HMAC_IDENT)

During the SCTP association establishment a HMAC Identifier is selected which is used by an SCTP endpoint when sending AUTH chunks. An application can access the result of this selection by using this read-only socket option, which uses the level IPPROTO_SCTP and the name SCTP_SEND_HMAC_IDENT.

The following structure is used to access HMAC Identifier used for sending AUTH chunks:

struct sctp_assoc_value {
    sctp_assoc_t assoc_id;
    uint32_t assoc_value;

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, the application fills in an association identifier. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC in assoc_id.


This parameter contains the HMAC Identifier used for sending AUTH chunks.

6.2. Exposing the HMAC Identifiers being Received

Section 6.1.8 of [RFC6458] defines the SCTP_AUTHENTICATION_EVENT event, which uses the following structure:

struct sctp_authkey_event {
    uint16_t auth_type;
    uint16_t auth_flags;
    uint32_t auth_length;
    uint16_t auth_keynumber;
    uint32_t auth_indication;
    sctp_assoc_t auth_assoc_id;

This document updates this structure to

struct sctp_authkey_event {
    uint16_t auth_type;
    uint16_t auth_flags;
    uint32_t auth_length;
    uint16_t auth_identifier; /* formerly auth_keynumber */
    uint32_t auth_indication;
    sctp_assoc_t auth_assoc_id;

by renaming auth_keynumber to auth_identifier. auth_identifier just replaces auth_keynumber in the context of [RFC6458]. In addition to that, the SCTP_AUTHENTICATION_EVENT event is extended to also indicate when a new HMAC Identifier is received and such reporting is explicitly enabled as described in Section 6.3. In this case auth_indication is SCTP_AUTH_NEW_HMAC and the new HMAC identifier is reported in auth_identifier.

6.3. Socket Option to Expose HMAC Identifier Usage (SCTP_EXPOSE_HMAC_IDENT_CHANGES)

This options allows the application to enable and disable the reception of SCTP_AUTHENTICATION_EVENT events when a new HMAC Identifiers has been received in an AUTH chunk (see Section 6.2). This read/write socket option uses the level IPPROTO_SCTP and the name SCTP_EXPOSE_HMAC_IDENT_CHANGES. It is needed to provide backwards compatibility and the default is that these events are not reported.

The following structure is used to enable or disable the reporting of newly received HMAC Identifiers in AUTH chunks:

struct sctp_assoc_value {
    sctp_assoc_t assoc_id;
    uint32_t assoc_value;

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, the application may fill in an association identifier or SCTP_{FUTURE|CURRENT|ALL}_ASSOC.


Newly received HMAC Identifiers are reported if, and only if, this parameter is non-zero.

7. IANA Considerations

7.1. TLS Exporter Label

RFC 6083 defined a TLS Exporter Label registry as described in [RFC5705]. IANA is requested to update the reference for the label "EXPORTER_DTLS_OVER_SCTP" to this specification.

7.2. SCTP Adaptation Layer Indication Code Point

[RFC5061] defined a IANA registry for Adaptation Code Points to be used in the Adaptation Layer Indication parameter. The registry was at time of writing located: IANA is requested to assign one Adaptation Code Point for DTLS/SCTP per the below proposed entry in Table 1.

Table 1: Adaptation Code Point
Code Point (32-bit number) Description Reference
0x00000002 DTLS/SCTP [RFC-TBD]

RFC-Editor Note: Please replace [RFC-TBD] with the RFC number given to this specification.

8. Security Considerations

The security considerations given in [I-D.ietf-tls-dtls13], [RFC4895], and [RFC4960] also apply to this document.

8.1. 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] provides 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 [RFC7540] gives good minimum requirements based on the attacks that where publicly known in 2015. DTLS 1.3 [I-D.ietf-tls-dtls13] only define 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 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 until the DTLS handshake is re-run at which point a new SCTP-AUTH key is derived using the TLS-Exporter. As discussed below DTLS 1.3 does not currently support renegotiation and lacks the capability of updating the SCTP-AUTH key.

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 Perfect Forward Secrecy. 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 DTLS connections are expected to have very long lifetimes of months or even years. For connections with such long lifetimes there is a need to frequently re-authenticate both client and server.

When using DTLS 1.2 [RFC6347], AEAD limits, frequent re-authentication and frequent re-run of Diffie-Hellman can be achieved with frequent renegotiation, see TLS 1.2 [RFC5246]. When renegotiation is used both clients and servers MUST use the renegotiation_info extension [RFC5746] and MUST follow the renegotiation guidelines in BCP 195 [RFC7525]. In particular, both clients and servers MUST NOT accept a change of identity during renegotiation.

In DTLS 1.3 renegotiation has been removed from DTLS 1.3 and partly replaced with Post-Handshake KeyUpdate. When using DTLS 1.3 [I-D.ietf-tls-dtls13], AEAD limits and frequent rekeying can be achieved by sending frequent post-handshake KeyUpdate messages. Symmetric rekeying gives less protection against key leakage than re-running Diffie-Hellman. After leakage of application_traffic_secret_N, a passive attacker can passively eavesdrop on all future application data sent on the connection including application data encrypted with application_traffic_secret_N+1, application_traffic_secret_N+2, etc. The is no way to do post-handshake server authentication or Ephemeral Diffie-Hellman inside a DTLS 1.3 connection. Note that KeyUpdate does not update the exporter_secret.

For upper layer protocols where frequent re-run of Diffie-Hellman, rekeying of SCTP-AUTH, and server reauthentication is required and creating a new SCTP connection with DTLS 1.3 to replace the current is not a viable option it is RECOMMENDED to use DTLS 1.2.

8.2. 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.

8.3. Authentication and Policy Decisions

DTLS/SCTP MUST be mutually authenticated. It is RECOMMENDED that DTLS/SCTP is used with certificate-based authentication. 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.

8.4. 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 cleartext 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 [I-D.ietf-tls-dtls13] to provide identity protection. DTLS/SCTP MUST be used with a key exchange method providing Perfect Forward Secrecy. Perfect Forward Secrecy significantly limits the amount of data that can be compromised due to key compromise.

8.5. 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 perfect forward secrecy, DTLS/SCTP effectively mitigate many forms of passive pervasive monitoring and limits the amount of compromised data due to key compromise.

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. Acknowledgments

The authors of RFC 6083 which this document is based on are Michael Tuexen, 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 GitHub user vanrein for their contribution.

10. References

10.1. 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., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, , <>.
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, , <>.
Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, , <>.
Rescorla, E., Ray, M., Dispensa, S., and N. Oskov, "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, DOI 10.17487/RFC5746, , <>.
Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, , <>.
Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, , <>.
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, , <>.
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, , <>.
Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", Work in Progress, Internet-Draft, draft-ietf-tls-dtls13-43, , <>.

10.2. 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, , <>.
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, , <>.
Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)", RFC 6083, DOI 10.17487/RFC6083, , <>.
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, , <>.
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)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, , <>.
Agence nationale de la sécurité des systèmes d'information, ., "Recommendations for securing networks with IPsec", ANSSI Technical Report DAT-NT-003 , , <<>>.

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: Almost 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. Thus, this document mandates usage of relevant versions and algorithms.

Clarifications: Some implementation experiences have been gained that motivates additional clarifications on the specification.

Authors' Addresses

Magnus Westerlund
John Preuß Mattsson
Claudio Porfiri
Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt