Stream Control Transmission Protocol (SCTP) DTLS Chunk

1.1. Relationship to RFC 6083 and RFC 5061

This document obsoletes [RFC6083], which defined the use of DTLS over SCTP by encapsulating DTLS records as SCTP user data using the SCTP-AUTH extension ([RFC4895]) for integrity protection of the SCTP headers. That approach suffered from several limitations: it could not support SCTP user messages above 16384 bytes, it could not protect SCTP control chunks, and it exposed significant SCTP metadata in clear text. The mechanism defined in this document replaces [RFC6083] by integrating DTLS 1.3 record protection directly at the chunk level, providing confidentiality and integrity for both user data and SCTP control chunks without dependency on SCTP-AUTH.¶

This document updates [RFC5061] by restricting the use of the Dynamic Address Reconfiguration extension when the DTLS chunk is in use. Specifically, the lookup address feature of [RFC5061] MUST NOT be used, and the dynamic address reconfiguration extension MUST NOT be used unless DTLS chunk handling is enabled in both directions. These restrictions are necessary because SCTP-AUTH, which [RFC5061] relies on for authenticating ASCONF chunks, is incompatible with this extension.¶

2.1. Terminology

Chunk Protection Operator:

The component within the SCTP implementation that performs DTLS record protection (encryption and authentication) on outgoing SCTP chunks and the corresponding verification and decryption on incoming DTLS chunks.¶

DTLS Key Context:

DTLS key context includes key material (record payload key, sequence number protection key, and initialization vector (IV)) for sending and receiving, replay window for receiving, and last used sequence number for sending.¶

Primary DTLS Key Context:

The DTLS key context used for normal SCTP association traffic, as distinguished from the restart DTLS key context.¶

Restart DTLS Key Context:

A dedicated DTLS key context maintained for the sole purpose of protecting the SCTP restart procedure.¶

DTLS Key Management Method:

A protocol or procedure, external to this specification, that performs mutual authentication, key establishment, and periodic rekeying for the DTLS chunk. Each method is identified by a DTLS Key Management Identifier.¶

DTLS Key Management Identifier:

An 8-bit unsigned integer assigned by IANA that uniquely identifies a DTLS Key Management Method, enabling negotiation during the SCTP handshake via the DTLS Key Management Parameter.¶

Key Material:

Key material is all cryptographic information needed for protection operation in one direction.¶

3.1. DTLS Considerations

Once the DTLS Key Management Method has established its context, it derives primary and restart key material for sending and receiving and configures the Chunk Protection Operator via an API. This establishes the necessary DTLS key contexts for SCTP chunk encryption and decryption. A DTLS key context for primary operations MUST be created, while a DTLS key context for SCTP association restart SHOULD be created.¶

DTLS key context includes key material such as record payload key, sequence number protection key, and IV each for sending and receiving, replay window for receiving, and last used sequence number for sending. Each DTLS key context is associated with a three-value tuple identifying the context, consisting of SCTP Association, the restart indicator, and the DTLS epoch.¶

The DTLS Chunk uses a single configuration of the DTLS record format. The DTLS Connection ID in the DTLS Record layer MUST NOT be used in the DTLS Chunk as the full DTLS connection state is not used in the DTLS Chunk and the DTLS key context anyway can be identified. The length field MUST NOT be used as the DTLS chunk provides record length information. Finally 16-bit Sequence Numbers are used as they give maximum support for reordering and there are no byte savings possible to ensure the 32-bit alignment for the encrypted record.¶

The first DTLS key context established for any SCTP association MUST use epoch 3. Each subsequent DTLS key context will use the next consecutive epoch value. Following the DTLS 1.3 specification [RFC9147] the first DTLS record for each epoch will use sequence number 0.¶

The replay window for the DTLS Sequence Number needs to account for the concurrent transmission of packets on multiple paths in multihomed associations. In particular, this applies to packets containing HEARTBEAT chunks. The window size must be sufficiently large to accommodate these latency differences.¶

Endpoints implementing DTLS chunk MUST support DTLS records containing up to 2¹⁴ (16384) bytes of plain text.¶

3.2. SCTP Considerations

The SCTP authentication extension (SCTP-AUTH) defined in [RFC4895] is incompatible with the extension defined in this document. Therefore, its support MUST NOT be negotiated in combination with the support of the DTLS chunk.¶

In particular, the dynamic address reconfiguration defined in [RFC5061] cannot use SCTP-AUTH. Instead, the DTLS chunk is used for authentication. This introduces the following limitations:¶

• The lookup address MUST NOT be used.¶

• The dynamic address reconfiguration extension MUST NOT be used unless DTLS chunk handling is enabled in both directions.¶

To mitigate potential information leakage from packet size variations, implementations MAY pad SCTP packets to uniform sizes. However, the padding MUST be applied within the encryption envelope to ensure the padding itself is protected.¶

Both SCTP and DTLS provide mechanisms for padding packets. If padding of SCTP packets is desired to hide actual message sizes, it is RECOMMENDED to use the SCTP Padding Chunk [RFC4820] to generate a consistent SCTP payload size. Support of this chunk is only required on the sender side; any SCTP receiver will safely ignore the PAD Chunk. However, if the PAD chunk is not supported DTLS padding MAY be used.¶

It should be noted that regardless of whether SCTP padding or DTLS padding is used, the additional bytes are not accounted for by the SCTP congestion control. Extensive use of padding has potential to worsen congestion situations, as the SCTP association will consume more bandwidth than its fair share as determined by congestion control.¶

The use of the SCTP PAD chunk is preferred, as it remains visible at the SCTP layer. This enables future extensions or SCTP implementations to correctly account for padding within congestion control mechanisms. In contrast, DTLS padding conceals this packet expansion from the SCTP layer.¶

4.1. DTLS Key Management Parameter

The DTLS Key Management Parameter is used to negotiate the support of the DTLS chunk and the Key Management Method used for the DTLS chunks. The format of this chunk parameter is depicted in Figure 2.¶

Parameter Type: 16 bits (unsigned integer)

This value MUST be set to 0x8006. Note that this parameter type requires the receiver to ignore the parameter and continue processing if the parameter type is not supported. This is accomplished (as described Section 3.2.1 of [RFC9260]) by the use of the upper bits of the parameter type.¶

Parameter Length: 16 bits (unsigned integer)

This value holds the length of the parameter in bytes, which is the number of DTLS Key Management identifiers (N) plus 9.¶

Tie Breaker: 32 bits (unsigned integer)

This is a 32-bit random number to be used to determine the client and server role for the Key Management Method.¶

Reserved: 5 bits (unsigned integer)

The reserved bits MUST be set to 0 by the sender and MUST be ignored by the receiver.¶

R bit: 1 bit

The (R)estart supported bit.¶

S bit: 1 bit

The (S)erver role supported bit.¶

C bit: 1 bit

The (C)lient role supported bit.¶

DTLS Key Management Identifier: 8 bits (unsigned integer)

Each DTLS Key Management Identifier (Section 9.1) is an 8-bit unsigned integer value indicating a DTLS Key Management Method. The DTLS Key Management Methods are listed in descending order of preference, i.e. the first listed in the parameter is the most preferred one and the last listed one is the least preferred by the sender of the parameter. The parameter MUST include at least one DTLS Key Management Identifier.¶

Padding: 0, 8, 16, or 24 bits (unsigned integer)

The padding MUST be set to 0 by the sender and MUST be ignored by the receiver.¶

The DTLS Key Management Parameter MAY be included in the INIT and INIT ACK chunk and MUST NOT be included in any other chunk. Both peers include their respective preference list and the procedure in Section 5.1 will determine the selected roles and chosen method.¶

4.2. DTLS Chunk (DTLS)

The DTLS chunk is used to hold the DTLS 1.3 record with the protected payload of a plain text SCTP packet without the SCTP common header.¶

Type: 8 bits (unsigned integer)

This value MUST be set to 0x41. It should be noted that the chunk type requires the receiver stop processing this SCTP packet, discard the unrecognized chunk and all further chunks, and report the unrecognized chunk in an ERROR chunk using the 'Unrecognized Chunk Type' error cause, if the receiver does not support this chunk type. This is accomplished (as described in Section 3.2 of [RFC9260]) by the use of the upper bits of the chunk type.¶

reserved: 7 bits

Reserved bits for future use. These bits MUST be set to 0 by the sender and MUST be ignored by the receiver.¶

R: 1 bit (boolean)

Restart indicator. If this bit is set this DTLS chunk is protected with a restart DTLS key context.¶

Chunk Length: 16 bits (unsigned integer)

This value holds the length of the Payload in bytes plus 4 plus the Payload Pre-Padding length.¶

Pre-Padding: 8 bits

The sender MUST pad with one zero byte and the receiver MUST ignore the padding bytes.¶

Payload: variable length

This MUST contain exactly one DTLSCiphertext as specified in DTLS 1.3 [RFC9147].¶

Post-Padding: 0, 8, 16, or 24 bits

If the length of the Payload is not a multiple of 4 bytes, the sender MUST pad the chunk with all zero bytes to make the chunk 32-bit aligned. The Padding MUST NOT be longer than 3 bytes and it MUST be ignored by the receiver.¶

From Section 4 of [RFC9147], the DTLS record header has variable length and is depicted in Figure 4.¶

The DTLSCiphertext contains the unified_hdr followed by the encrypted_record, where unified_hdr has variable format but a single selected configuration is used in the DTLS Chunk. The use of one byte of Pre-Padding ensures 32-bit alignment of the encrypted_record in relation to the start of the DTLS chunk, which allows a receiver to perform an in-place decryption and then process the sequence of chunks. SCTP as specified in [RFC9260] guarantees that chunks start on a 32-bit boundary.¶

The used DTLSCiphertext configuration contains the unified_hdr with flags and the two least significant bits of the DTLS Epoch, a 16-bit sequence number (S=1), no length field (L=0), and no Connection ID (C=0). This results in a 3-byte unified_hdr (1 byte fixed header plus 2 bytes sequence number) and consequently 1 byte of Pre-Padding to achieve 32-bit alignment of the encrypted_record. The used DTLSCiphertext are shown in Figure 5.¶

4.3. New Error Causes

This specification defines four new error causes.¶

5.1. Establishment of a Protected Association

An SCTP endpoint wanting to use the DTLS chunk can operate in one of two modes:¶

Strict DTLS mode:

The SCTP endpoint wants to use the DTLS chunk and is not willing to operate without DTLS chunk protection.¶

Loose DTLS mode:

The SCTP endpoint want to use the DTLS chunk but is willing to operate without DTLS chunk protection.¶

A received DTLS Key Management Parameter is compatible, if the following two conditions are satisfied:¶

1. At least one of the DTLS Key Management Identifiers listed in the parameter matches a locally supported one.¶

2. At least one of the roles (client or server) indicated in the parameter complements a local role.¶

To initiate an SCTP association, an SCTP endpoint wanting to use the DTLS chunk MUST send an SCTP packet with an INIT chunk containing the DTLS Key Management Parameter (see Section 4.1). This parameter lists the supported DTLS Key Management Identifiers (see Figure 2) in descending order of preference.¶

If an SCTP endpoint operating in strict DTLS mode receives an SCTP packet containing an INIT chunk with a compatible DTLS Key Management Parameter, it MUST reply with a packet containing an INIT ACK containing its own DTLS Key Management Parameters.¶

If an SCTP endpoint operates in strict DTLS mode and receives an SCTP packet with an INIT or INIT ACK chunk does not contain any DTLS Key Management Parameter or only an incompatible one, the endpoint MUST send an SCTP packet with an ABORT chunk. It MAY include the appropriate error cause "Missing DTLS Chunk Support" (see Section 4.3.1), "No Common DTLS Key Management Method" (see Section 4.3.2), or "No Common DTLS Key Management Method" (see Section 4.3.4).¶

If an SCTP endpoint operates in loose DTLS mode, it MAY continue with the handshake in any case.¶

To ensure that each endpoint's Key Management Method knows which role it has and both endpoints agree on which method that was chosen the below procedure MUST be executed by both endpoints before entering the ESTABLISHED state.¶

First the Key Management role of each endpoint is determined. This is done by evaluating the S and C bits in the two endpoints' parameters. This falls into the following cases:¶

1. At least one endpoint indicates a single role (client or server) and the peer supports the other role. In this case the endpoint indicating a single role takes that role, and the other endpoint takes the reverse role.¶

2. Both endpoints indicate both roles, this is to be expected for endpoints supporting simultaneous open. In this case the role needs to be determined using the parameter's Tie breaker. The endpoint with the larger value SHALL be the server, and the other endpoint takes the client role. In case both endpoints have the same tie breaker value, the selection has failed and the association MUST be aborted. The ABORT chunk MAY include the SCTP error cause "DTLS Key Management Tie Breaker Collision" (see Section 4.3.3). Endpoints are RECOMMENDED to attempt establishing a new SCTP association.¶

Once the key management roles have been established, the endpoints determine the method to be used. The prioritized list of DTLS Key Management Identifiers provided by the endpoint acting as the server is evaluated, and the first identifier also supported by the peer endpoint is selected.¶

When the SCTP association has been established, the process defined by the selected DTLS Key Management Method MUST be followed for establishing DTLS key contexts and installing them.¶

5.2. DTLS Chunk Handling

The DTLS chunk MUST NOT be bundled with any other chunk. Specifically, it MUST appear as the first and only chunk in the packet.¶

The diagram shown in Figure 10 describes the structure of an unprotected SCTP packet as described in [RFC9260].¶

The diagram shown in Figure 11 describes the structure of a protected SCTP packet being sent. Such packets contain only the SCTP common header and one DTLS chunk.¶

Once the part of the DTLS key context responsible for sending DTLS chunks has been configured by the application, all SCTP packets SHALL be sent using a DTLS chunk.¶

When an SCTP packet needs to be sent, the sequence of chunks is used as DTLSInnerPlaintext.content and DTLSInnerPlaintext.type is set to application_data [RFC9147]. Then the DTLSCiphertext is computed per the DTLS 1.3 specification [RFC9147] and the configured cipher suite, and the result is used as the payload. Finally the SCTP common header is prepended.¶

When the DTLS chunk is used, the endpoint MUST consider the DTLS chunk header and the overhead of DTLS to ensure that the final SCTP packet does not exceed the PMTU.¶

Upon receipt of an SCTP packet in which a DTLS chunk is bundled with any other chunk, the entire packet MUST be silently discarded.¶

After the application has restricted the SCTP packet handling to protected SCTP packets only, an SCTP packet not containing a DTLS chunk MUST be silently discarded.¶

When processing the payload of the DTLS chunk (i.e. the DTLSCiphertext), the Restart flag in addition to the unified_hdr is used to find the keys for processing the encrypted_record following DTLS 1.3 [RFC9147].¶

After the encrypted_record has been verified and decrypted, the corresponding chunks (the DTLSInnerPlaintext.content) are processed as defined in the corresponding specifications.¶

If the Chunk Protection Operator experiences a non-critical error, it MUST NOT abort the association.¶

5.3. Termination of a Protected Association

Note that the closure of any DTLS Key Management Method doesn't compromise the capability of terminating the SCTP association gracefully as that capability only relies on the Key Context and not on the DTLS Key Management Method from where it has been derived.¶

5.4. SCTP Restart Considerations

This section deals with the handling of an unexpected INIT chunk during an association lifetime as described in Section 5.2 of [RFC9260] with the purpose of defining a protected restart procedure.¶

When the upper layer protocols require support of SCTP restart for associations using the DTLS chunk, as in case of 3GPP NG-C protocol [ETSI-TS-38.413], the endpoint needs to support also the protected SCTP restart procedure described below. Implementation of the protected restart procedure is RECOMMENDED; however, it is not required, as it relies on the availability of persistent secure storage for the restart DTLS key context. An endpoint will know that its peer supports this protected SCTP restart procedure from the DTLS Key Management Parameter's R bit Figure 2.¶

The protected SCTP restart procedure keeps the security characteristics of an SCTP association using DTLS chunks.¶

In protected SCTP restart, INIT and INIT ACK chunks are sent according to [RFC9260], but COOKIE ECHO and COOKIE ACK chunks are encrypted using DTLS chunks and the restart DTLS key contexts. The endpoints MUST include the DTLS Key Management Parameter in the INIT and INIT ACK, using the same method list, but with a new random Tie Breaker.¶

In order to support protected SCTP restart, the SCTP endpoints need to allocate and maintain dedicated restart DTLS key contexts. SCTP packets protected by these contexts will be identified in the DTLS chunk with the R (Restart) bit set (see Section 4.2). Both SCTP endpoints need to ensure that restart DTLS key contexts are preserved for supporting the protected SCTP restart use case.¶

For a protected SCTP endpoint to be available for protected SCTP restart, the DTLS chunk requires access to a DTLS key context associated with the SCTP association. This key context MUST be maintained in a well-defined state such that both endpoints have a consistent view of the DTLS sequence numbers and replay window (i.e., initialized but never used). The SCTP restart DTLS key MUST NOT be used for any purpose other than SCTP association restart.¶

An SCTP endpoint wanting to be able to initiate a protected SCTP restart needs to store securely and persistently the restart keys, and related DTLS epoch, indexed so that when performing a restart with a peer endpoint, the restarting endpoint can identify the right restart key and DTLS epoch and initialize the restart DTLS key context for when restarting the SCTP association.¶

The keys and epoch need to be stored securely and persistently so that they survive the events that are causing the protected SCTP restart procedure to be used, for instance a crash of the SCTP stack. The security considerations for persistent secure storage of key material are further discussed in Section 10.4.¶

A DTLS chunk using the restart DTLS key context is identified by having the R bit (Restart Indicator) set in the DTLS chunk (see Figure 3). There's exactly one active restart DTLS key context at a time, the newest. However, a crash at the time having completed the DTLS Key Management exchange but failing to commit the DTLS key context to persistent secure storage could result in loss of the latest DTLS key context. Therefore, the endpoints SHOULD retain the old restart DTLS key context until the DTLS Key Management confirms the new ones are committed to secure storage. For example, this can ensure that signals to terminate the old DTLS key contexts (including the restart context) are never sent until the new restart DTLS key context has been committed to storage.¶

Implementations will also need to consider the risk of two-time pads, i.e. the usage of the same nonce twice. An SCTP endpoint restarted and sent a number of packets using the restart key context thus using sequence number 1 to 8. If this endpoint restarts again, it is crucial that the restarted endpoint does not reuse sequence number 1 to 8 a second time. Ensuring that a sequence number is never reused in the context of a crash may be hard, thus the simpler solution is to remove a restart key context that is being used from the persistent storage to prevent reuse.¶

The Figure 12 shows how the control chunks being used for SCTP association restart are transported within DTLS in SCTP.¶

A restarted SCTP association MUST continue to use the restart DTLS key context, for user traffic until a new primary DTLS key context will be available. The implementors SHOULD initiate a rekeying as soon as possible, and derive the primary and restart keys so that the time when no restart DTLS key context is available is kept to a minimum. Note that another restart attempt prior to having created new restart DTLS key context for the new SCTP association will result in the endpoints being unable to restart the SCTP association.¶

After restart, the next primary DTLS key context MUST use epoch 3, i.e. the epoch value is reset. After having derived a new primary DTLS key context, the endpoint installs it and starts using it. The new restart DTLS key context is only installed after all old in-flight restart packets have been received.¶

An SCTP endpoint supporting only normal SCTP restart and involved in an SCTP association using DTLS chunks SHOULD NOT attempt to restart the association. The effect will be that the restart initiator will receive INIT ACK but then all sent packets with COOKIE ECHO will be dropped until the peer endpoint times out the SCTP association from lack of any response from the restarting node.¶

An SCTP endpoint supporting only legacy SCTP restart and involved in an SCTP association using DTLS chunks, when receiving a COOKIE ECHO chunk protected by DTLS chunk as described above, thus having the R bit (Restart Indicator) set in the DTLS chunk (see Figure 3), MUST silently discard it.¶

7.1. Set Supported DTLS Key Management Parameters

Prior to attempting to establish an SCTP association an SCTP endpoint needs to configure which DTLS Key Management Methods it supports, as well as the roles and if restart is supported. This information is included in the DTLS Key Management Parameter, which, when these parameters are set, will be included in either INIT or INIT ACK chunks.¶

An endpoint can either support: client only, server only, or client and server. The latter is for situations where both endpoints may attempt to establish an SCTP association towards each other, potentially causing a simultaneous sending of INIT chunks. Depending on port configuration SCTP supports this happening during the association establishment, and the result will be a single SCTP association. However, to select the same Key Management Method on both sides, the SCTP stack will resolve the key management role for this association.¶

Request: Set Supported DTLS Key Management Methods¶

Parameters:

¶

* SCTP Association Handle:

The handle to what may become an SCTP Association or a server port accepting association establishment.¶

• Key Management Roles Supported:

  The endpoint indicates if it can act as client only, server only, or client and server.¶

• Support SCTP Restart (boolean):

  Indicate if the endpoint is capable of supporting SCTP restart when DTLS chunk has been negotiated. If both endpoints indicate flag then a restart has the potential to succeed.¶

• Requires DTLS Chunk security (boolean):

  SCTP association is only established if both endpoints support DTLS Chunk and have included the DTLS Key Management Parameter.¶

• List of Identifiers:

  A prioritized list of DTLS Key Management identifiers Section 9.1 that are supported, from the most preferred to the least preferred.¶

Reply: Success or Error¶

Parameters: None¶

7.2. Get Agreed DTLS Key Management Method and Role

After an SCTP association has been established the key management function needs to know which method was agreed on and which role this endpoint will have. The function provides both endpoints' actual values from the DTLS Key Management Parameter, used in key derivation to prevent downgrade attacks.¶

Request: Get Agreed DTLS Key Management Method and Role¶

Parameters:

¶

* SCTP Association Handle:

The handle to an established SCTP Association.¶

Reply: Success or Error¶

Parameters:¶

• Key Management Role:

  The role that was selected for this endpoint, one of client or server.¶

• Key Management Method:

  The selected Key Management Method as a DTLS Key Management Identifier.¶

• Down Grade Prevention Data:

  In network byte order the whole of the DTLS Key Management Parameter including header and excluding padding that the endpoint with the client role offered, followed by the corresponding parameter content of the endpoint with the server role in seperable records.¶

7.3. Cipher Suite Capabilities

The DTLS Key Management Method needs to know which cipher suites defined for use with DTLS 1.3 that are supported by the DTLS chunk and its protection operations block. All TLS cipher suites that are defined are listed in the TLS cipher suite registry [TLS-CIPHER-SUITES] at IANA and are identified by a 2-byte value. Thus this needs to return a list of all supported cipher suites to the higher layer.¶

Request : Get Cipher Suites¶

Parameters : none¶

Reply : Cipher Suites¶

Parameters : list of cipher suites¶

7.4. Establish Send Key Material

The DTLS chunk can use one out of multiple sets of cipher suite and corresponding key materials. Some limitations do exists when establishing Key Material to avoid issues:¶

• Primary Keys MUST be installed prior to Restart Keys for each epoch to avoid them being used unless this endpoint is attempting a restart.¶

• With the exception of restart keys when this endpoint attempts restart of the SCTP association, the first DTLS epoch set in an association MUST be 3. Any subsequent epoch must be the next consecutive number compared to the previously used.¶

The following information needs to be provided when setting send key material:¶

Request : Establish Send Key Material¶

Parameters :¶

• SCTP Association:

  Reference to the SCTP association to set the key material for.¶

• Restart indication:

  A bit indicating whether the key material is for restart purposes¶

• DTLS Epoch:

  The DTLS epoch this key material is valid for. Note that Epoch lower than 3 are not expected as they are used during DTLS handshake.¶

• Cipher Suite:

  2 bytes cipher suite identification for the DTLS 1.3 cipher suite used to identify the DTLS AEAD algorithm to perform the DTLS record protection. The cipher suite is fixed for a (SCTP Association, Key) pair.¶

• Key Material:

  The cipher suite specific binary object containing all necessary information for protection operations such as Record Payload Key, Sequence Number Key and IV. The secret will be used by the DTLS 1.3 client to encrypt the record. Binary arbitrary long object depending on the cipher suite used.¶

Reply: Established or Failed¶

7.5. Establish Receive Key Material

The DTLS chunk can use one out of multiple sets of cipher suite and corresponding key materials.¶

The following information needs to be provided when setting receive key material:¶

Request : Establish Receive Key Material¶

Parameters :¶

• SCTP Association:

  Reference to the SCTP association to set the key material for.¶

• Restart indication:

  A bit indicating whether the key material is for restart purposes¶

• DTLS Epoch:

  The DTLS epoch these keys are valid for. Note that Epoch lower than 3 are not expected as they are used during DTLS handshake. The DTLS epoch must be the next in turn.¶

• Cipher Suite:

  2 bytes cipher suite identification for the DTLS 1.3 cipher suite used to identify the DTLS AEAD algorithm to perform the DTLS record protection. The cipher suite is fixed for a (SCTP Association, Key) pair.¶

• Key Material:

  The cipher suite specific binary object containing all necessary information for protection operations such as Record Payload Key, Sequence Number Key and IV. The secret will be used by the DTLS 1.3 client to encrypt the record. Binary arbitrary long object depending on the cipher suite used.¶

Reply : Established or Failed¶

7.6. Destroy Send Key Material

A function to destroy the send key material for a given epoch for the Primary or Restart DTLS Key Context for a given SCTP Association.¶

Request : Destroy Send Key Material¶

Parameters :¶

• SCTP Association¶

• Restart indication¶

• DTLS Epoch¶

Reply: Destroyed¶

Parameters : Success or Error¶

7.7. Destroy Receive Key Material

A function to destroy the receive key material for a given epoch for the Primary or Restart DTLS Key Context for a given SCTP Association.¶

Request: Destroy Receive Key Material¶

Parameters:¶

• SCTP Association¶

• Restart indication¶

• DTLS Epoch¶

Reply: Destroyed¶

Parameters: Success or Error¶

7.8. Set Key to Use

Set which key to use to protect the future SCTP packets sent by the SCTP Association. This function can be replace by Establish Send Key Material which immediately enables the primary key for the next epoch when installed.¶

Setting the restart keys to be used instead of primary keys SHALL only be done when attempting to perform a restart of the SCTP association.¶

Request: Set Key used¶

Parameters:¶

• SCTP Association¶

• Restart indication¶

• DTLS Epoch¶

Reply: Key set¶

Parameters: true or false¶

7.9. Require Protected SCTP Packets

A function to configure an SCTP association to require that all SCTP packets, excepts those with INIT or INIT ACK chunks, being received on this endpoint are required to be protected in a DTLS chunk going forward. Any unprotected SCTP packets to this SCTP association will be discarded.¶

Parameters:¶

• SCTP Association¶

Reply: Acknowledgement¶

7.10. Get AEAD Encryption Invocations

Get the number of AEAD encryption invocations (protected SCTP packets) for a given epoch.¶

Request : Get AEAD Encryption Invocations¶

Parameters :¶

• SCTP Association¶

• Restart indication¶

• DTLS Epoch¶

Reply: AEAD Encryption Invocations¶

Parameters : non-negative integer¶

7.11. Get AEAD Decryption Invocations

Get the number of AEAD decryption invocations for a given epoch.¶

Request : Get AEAD Decryption Invocations¶

Parameters :¶

• SCTP Association¶

• Restart indication¶

• DTLS Epoch¶

Reply: AEAD Decryption Invocations¶

Parameters : non-negative integer¶

7.12. Get Failed AEAD Decryption Invocations

Get the number of failed AEAD decryption invocations for a given epoch.¶

Request : Get Failed AEAD Decryption Invocations¶

Parameters :¶

• SCTP Association¶

• Restart indication¶

• DTLS Epoch¶

Reply: Failed AEAD Decryption Invocations¶

Parameters : non-negative integer¶

7.13. Configure Replay Protection

The DTLS replay protection in this usage is expected to be fairly robust. Its depth of handling is related to maximum network path reordering that the receiver expects to see during the SCTP association. However as the actual reordering in number of packets is a combination of how delayed one packet may be compared to another, times the actual packet rate. This value may become larger for some applications and may need to be tuned. Thus, having the potential for setting this a more suitable value depending on the use case should be supported.¶

Note this sets the configuration across any DTLS key context for a given SCTP Association and both primary and restart usages.¶

Request : Configure Replay Protection¶

Parameters :¶

• SCTP Association¶

• Restart indication¶

• Configuration parameters¶

Reply: Replay Protection Configured¶

Parameters : true or false¶

8.1. SCTP_ASSOC_CHANGE Notification

When an SCTP_ASSOC_CHANGE notification (specified in Section 6.1.1 of [RFC6458]) is delivered indicating a sac_state of SCTP_COMM_UP or SCTP_RESTART for an SCTP association where both peers support the DTLS chunk, SCTP_ASSOC_SUPPORTS_DTLS should be listed in the sac_info field.¶

8.2. A New Flag for recvmsg() (MSG_PROTECTED)

This flag is returned by recvmsg() in msg_flags for all user messages for which all DATA chunks were received in protected SCTP packets. This also means that if sctp_recvv() is used, MSG_PROTECTED is returned in the *flags argument.¶

8.3. Functions

unsigned int
sctp_dtls_nr_cipher_suites(void);

int
sctp_dtls_cipher_suites(uint8_t cipher_suites[][2], unsigned int n);

8.4. Socket Options

The following table provides an overview of the IPPROTO_SCTP-level socket options defined by this section.¶

sctp_opt_info() needs to be extended to support:¶

• SCTP_DTLS_LOCAL_CONFIG,¶

• SCTP_DTLS_GET_CONFIG,¶

• SCTP_DTLS_GET_LOCAL_KM_PARAM,¶

• SCTP_DTLS_GET_PEER_KM_PARAM,¶

• SCTP_DTLS_ENFORCE_PROTECTION,¶

• SCTP_DTLS_REPLAY_WINDOW, and¶

• SCTP_DTLS_GET_STATS.¶

struct sctp_dtls_config {
        sctp_assoc_t sdc_assoc_id;
        uint16_t sdc_flags;
        uint16_t sdc_nr_kmids;
        uint8_t sdc_kmids[];
};

struct sctp_dtls_config {
        sctp_assoc_t sdc_assoc_id;
        uint16_t sdc_flags;
        uint16_t sdc_nr_kmids;
        uint8_t sdc_kmids[];
};

struct sctp_dtls_kmp {
        sctp_assoc_t sdk_assoc_id;
        uint32_t sdk_nr_bytes;
        uint8_t sdk_bytes[];
};

struct sctp_dtls_kmp {
        sctp_assoc_t sdk_assoc_id;
        uint32_t sdk_nr_bytes;
        uint8_t sdk_bytes[];
};

struct sctp_dtls_keys {
        sctp_assoc_t sdk_assoc_id;
        uint8_t sdk_cipher_suite[2];
        uint16_t sdk_restart;
        uint64_t sdk_epoch;
        uint16_t sdk_key_len;
        uint16_t sdk_iv_len;
        uint16_t sdk_sn_key_len;
        uint16_t sdk_unused;
        uint8_t sdk_keys[];
};

struct sctp_dtls_keys {
        sctp_assoc_t sdk_assoc_id;
        uint8_t sdk_cipher_suite[2];
        uint16_t sdk_restart;
        uint64_t sdk_epoch;
        uint16_t sdk_key_len;
        uint16_t sdk_iv_len;
        uint16_t sdk_sn_key_len;
        uint16_t sdk_unused;
        uint8_t sdk_keys[];
};

struct sctp_dtls_keys_id {
   sctp_assoc_t sdki_assoc_id;
   uint32_t sdki_restart;
   uint64_t sdki_epoch;
}

struct sctp_assoc_value {
   sctp_assoc_t assoc_id;
   uint32_t assoc_value;
};

struct sctp_dtls_stats {
   sctp_assoc_t sds_assoc_id;
   uint64_t sds_dropped_unprotected;
   uint64_t sds_aead_failures;
   uint64_t sds_recv_protected;
   uint64_t sds_sent_protected;
   /* There will be added more fields before the WGLC. */
   /* There might be additional platform specific counters. */
};

struct sctp_assoc_value {
   sctp_assoc_t assoc_id;
   uint32_t assoc_value;
};

9.1. DTLS Key Management Method Identifiers

Note: The RFC Editor is requested to replace RFC-To-Be with a reference to this document.¶

IANA is requested to create a new registry called "DTLS Key Management Method". This registry is part of the Stream Control Transmission Protocol (SCTP) Parameters grouping.¶

The purpose of this registry is to assign DTLS Key Management Method Identifier for any DTLS Key Management Method used for the extension described in this document. Each entry will be assigned an 8-bit unsigned integer value from the suitable range.¶

New entries in the range 1-191 are registered following the Specification Required policy as defined by [RFC8126]. New entries in the range 192-254 are first come, first served with expert review. The expert reviewers' primary purpose is to ensure that the registration is relevant and not performed to consume the number space.¶

9.2. SCTP Chunk Type

In the Stream Control Transmission Protocol (SCTP) Parameters group's "Chunk Types" registry, IANA is requested to update the reference for the DTLS chunk as depicted in Table 3 with a reference to this document.¶

IANA is requested to add the corresponding registration table for the chunk flags of the DTLS chunk with the initial contents shown in Table 4:¶

9.3. SCTP Chunk Parameter Types

In the Stream Control Transmission Protocol (SCTP) Parameters group's "Chunk Parameter Types" registry, IANA is requested to update the reference for the DTLS Key Management as depicted in Table 5 with a reference to this document.¶

9.4. SCTP Error Cause Codes

In the Stream Control Transmission Protocol (SCTP) Parameters group's "Error Cause Codes" registry, IANA is requested to add the new entries depicted below in Table 6 with a reference to this document.¶

The suggested cause code will need to be confirmed by IANA.¶

9.5. SCTP Payload Protocol Identifier

In the Stream Control Transmission Protocol (SCTP) Parameters group's "Payload Protocol Identifiers" registry, IANA is requested to update the reference for the PPID 4242 as depicted in Table 7 with a reference to this document.¶

10.1. Privacy Considerations

Use of the SCTP DTLS chunk provides privacy to SCTP by protecting user data and much of the SCTP control signaling. The SCTP association is identifiable based on the 5-tuple where the destination IP and port and VTag are fixed for each direction. Advanced privacy features such as sequence number encryption might therefore have limited effect.¶

10.2. AEAD Limit Considerations

Section 4.5.3 of [RFC9147] defines limits on the number of records q that can be protected using the same key as well as limits on the number of received packets v that fail authentication with each key. To adhere to these limits the DTLS Key Management Method can periodically poll the DTLS protection operation function to see when a limit has been reached or is close to being reached. Instead of periodic polling, a callback can be used.¶

10.3. Downgrade Attacks

Downgrade attacks may attempt to force the DTLS Key Management Method by altering the content of INIT chunk, for instance by removing all offered DTLS Key Management Methods but the one desired. This is possible if the attacker is an on-path attacker that can modify packets because INIT and INIT ACK chunks are plain text.¶

Preventing the downgrade attacks is implemented by using the content of the DTLS Key Management Parameter sent in the INIT chunk plus the content of the DTLS Key Management Parameter received in the INIT ACK chunk from the responder for deriving the keys from the handshake secrets obtained during the DTLS initial handshake. Using the whole content of the parameters ensures that the role selection and thus the method selection isn't possible to manipulate.¶

If the attacker succeeds in changing the DTLS Key Management Parameter in either INIT, INIT ACK or both chunks, the peers will not be able to derive the same keys and the association will fail to complete. Any modification will result in association failure, thus preventing down-grade.¶

In case any DTLS Key Management Method does not include the parameter content in its key-derivation down-grade would be possible if that DTLS Key Management Method is selected. It is up to endpoint policies to determine what level of protection it deems necessary against down-grade attacks.¶

10.4. Persistent Secure Storage of Restart Key Context

The restart DTLS key context needs to be stored securely and persistently. Securely as access to this security context may enable an attacker to perform a restart, resulting in a denial of service on the existing SCTP association. It can also give the attacker access to the ULP. Thus the storage needs to provide at least as strong resistance against exfiltration as the main DTLS key context store.¶

How to realize persistent storage is highly dependent on the ULP and how it utilizes restarted SCTP associations. One way can be to have an actual secure persistent storage solution accessible to the endpoint. In other use cases the persistence part might be accomplished by keeping the current restart DTLS key context with the ULP State if that is sufficiently secure.¶

12.1. Normative References

[RFC4820]

Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007, <https://www.rfc-editor.org/info/rfc4820>.

[RFC4895]

Tuexen, M., Stewart, R., Lei, P., and E. Rescorla, "Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August 2007, <https://www.rfc-editor.org/info/rfc4895>.

[RFC5061]

Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M. Kozuka, "Stream Control Transmission Protocol (SCTP) Dynamic Address Reconfiguration", RFC 5061, DOI 10.17487/RFC5061, September 2007, <https://www.rfc-editor.org/info/rfc5061>.

[RFC8126]

Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.

[RFC9147]

Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.

[RFC9260]

Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260, June 2022, <https://www.rfc-editor.org/info/rfc9260>.

[TLS-CIPHER-SUITES]

"TLS Cipher Suites", November 2023, <https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2. Informative References

[RFC6083]

Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)", RFC 6083, DOI 10.17487/RFC6083, January 2011, <https://www.rfc-editor.org/info/rfc6083>.

[RFC6458]

Stewart, R., Tuexen, M., Poon, K., Lei, P., and V. Yasevich, "Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)", RFC 6458, DOI 10.17487/RFC6458, December 2011, <https://www.rfc-editor.org/info/rfc6458>.

[RFC8446]

Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.

[I-D.ietf-tsvwg-rfc4895-bis]

Tüxen, M., Stewart, R. R., Lei, P., and H. Tschofenig, "Authenticated Chunks for the Stream Control Transmission Protocol (SCTP)", Work in Progress, Internet-Draft, draft-ietf-tsvwg-rfc4895-bis-05, 21 April 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-rfc4895-bis-05>.

[I-D.ietf-tsvwg-dtls-chunk-key-management]

Tüxen, M., Tschofenig, H., and T. Reddy.K, "Using Datagram Transport Layer Security (DTLS) for Key Management for the Stream Control Transmission Protocol (SCTP) DTLS Chunk", Work in Progress, Internet-Draft, draft-ietf-tsvwg-dtls-chunk-key-management-01, 2 March 2026, <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-dtls-chunk-key-management-01>.

[I-D.westerlund-tsvwg-sctp-DTLS-handshake]

Westerlund, M., Preuß Mattsson, J., and C. Porfiri, "Datagram Transport Layer Security (DTLS) in the Stream Control Transmission Protocol (SCTP) DTLS Chunk", July 2025, <https://datatracker.ietf.org/doc/draft-westerlund-tsvwg-sctp-dtls-handshake/>.

[ETSI-TS-38.413]

"NG Application Protocol (NGAP) version 18.5.0 Release 18", March 2025, <https://www.etsi.org/deliver/etsi_ts/138400_138499/138413/18.05.00_60/ts_138413v180500p.pdf>.