Stream Control Transmission Protocol (SCTP) DTLS Chunk

Introduction and Protocol Overview This document extends the Stream Control Transmission Protocol (SCTP), as specified in , by defining the DTLS chunk format and the procedures for negotiating and managing its usage. DTLS chunk support is integrated into the SCTP implementation, enabling the secure transfer of SCTP packets (including both control and DATA chunks). The DTLS chunk protects a sequence of SCTP chunks by encrypting the plain text into encrypted ciphertext using the DTLS record format and its processing. This processing is based on DTLS 1.3, as specified in . Resulting in a protected SCTP association instead of an SCTP association where all the SCTP protocol details are in plain text. Key management is performed outside of the SCTP implementation and is out of scope of this document. This process is referred to as the DTLS Key Management Method. While these methods can also be based on DTLS 1.3, it is not a requirement. The DTLS chunk in combination with the DTLS Key Management Method can provide mutual authentication, confidentiality, DTLS based data origin authentication, integrity, and replay protection for SCTP packets. The DTLS Key Management Method utilizes an API to provision the SCTP association's DTLS chunk protection with key material to enable and rekey the protection operations. is an example illustrating the DTLS chunk processing in regard to SCTP and the Upper Layer Protocol (ULP) using DTLS 1.3 as the crytptographic core of the DTLS Key Management Method. Here the DTLS Key Management Method provides validation, i.e. using certificates, handshaking, updating policies etc.

DTLS Chunk Layering in Regard to SCTP and ULP + Key Exporter +-+-+ | | | | | | +---------------------+ | | | | | | | | | | | | | | | | +---------------------+ | | | | | | | +--+ Key Management + | | | | | | | | +----------+----------+ | | | | | | | | | | | | | | | | | ContentType | | | | | | | | | +----------+----------+ | | | | | | | +->| Record | | | | | | | | | Protection Operator | | | | | | | | +----------+----------+ | | | | | | +----------------------------+ | | +-------+-------+ +--------------------------------+-+ ^ ^ | | | | +--+----------------------+ | keys PPID | | V V +--------------------------------------------------+-+ | +---------------------+ | | | SCTP | Chunk |<----+ | | | Protection Operator | | | +---------------------+ | +----------------------------------------------------+ ]]> After receiving an SCTP packet and identifying the association using the SCTP common header, the DTLS chunk is processed by the Chunk Protection Operator. The Chunk Protection Operator performs replay protection, decryption, and authentication. If this processing is successful, the encapsulated SCTP chunks are further processed. For outgoing traffic, after the SCTP stack has created the unprotected SCTP packet containing control and/or DATA chunks, these SCTP chunks will be processed by the Chunk Protection Operator for protection. This results in a DTLS 1.3 record encapsulated in a DTLS chunk. The method of secure key-management, e.g. based on DTLS 1.3, providing initial mutual authentication, key establishment, and periodic re-authentication and rekeying of the DTLS chunk protection is defined in separate documents (see ). To prevent downgrade attacks affecting the DTLS Key Management Method negotiation, the DTLS Key Management Method should implement specific procedures when deriving keys. The Chunk Protection Operator performs protection operations on all chunks of an SCTP packet. No information is sent in plain text except for the following:

The initial SCTP handshake.
The DTLS Key Management Method's initial traffic.
The SCTP common header and the SCTP DTLS chunk header of protected packets.
The INIT and INIT ACK chunks during an SCTP restart procedure.

Support of the DTLS chunk and the selection of a DTLS Key Management Method are negotiated during the SCTP handshake using a new parameter. DTLS Key Management and application traffic are then multiplexed using the Payload Protocol Identifier (PPID). Applications using the DTLS chunk can leverage most transport features provided by SCTP and its extensions. However, the following limitations apply:

Performing an SCTP restart without knowing the restart key material is not supported.
The use of the lookup address in the Dynamic Address Reconfiguration extension as specified in is not supported.

Relationship to RFC 6083 This document obsoletes , which defined the use of DTLS over SCTP by encapsulating user data in DTLS records and sending the DTLS records as SCTP user data using the SCTP-AUTH extension () for integrity protection of the SCTP packets. That approach suffered from several limitations: it could not support SCTP user messages larger than 16384 bytes, it could not encrypt SCTP control chunks, and it exposed significant SCTP metadata in clear text. The mechanism defined in this document replaces 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.

Relationship to RFC 5061 This document updates by restricting the use of the Dynamic Address Reconfiguration extension when the DTLS chunk is in use. Specifically, the lookup address feature of 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 relies on for authenticating ASCONF chunks, is incompatible with this extension.

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 when, and only when, they appear in all capitals, as shown here.

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.
Protected SCTP Association:: an SCTP Association implementing DTLS Chunk as described in this document providing encryption, integrity protection and replay protection to all SCTP chunks in SCTP packets.
SCTP Association:: an association as defined in .

Protocol Considerations

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 is identified by means of the Association identifiers and the Epoch. 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 due to the 32-bit alignment of 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 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.

Mandatory-to-Implement Cipher Suites In the absence of an application profile standard specifying otherwise: A SCTP DTLS chunk implementation MUST implement the TLS_AES_128_GCM_SHA256 cipher suite and SHOULD implement the TLS_AES_256_GCM_SHA384 and TLS_CHACHA20_POLY1305_SHA256 cipher suites, using the identifiers defined by . In general any TLS 1.3 cipher suite that is marked as DTLS-OK in the TLS cipher suite table is expected to be usable.

SCTP Considerations The SCTP authentication extension (SCTP-AUTH) defined in 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 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 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. Note 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.

New Chunk, Parameter and Error Causes

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 .

DTLS Key Management Parameter

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 ) 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 () 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 will determine the selected roles and chosen method.

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.

DTLS Chunk

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 ) 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 .
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 , the DTLSCiphertext has variable length and is depicted in .

DTLS DTLSCiphertext 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 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 is shown in .

DTLSCiphertext used structure

New Error Causes This specification defines four new error causes.

Missing DTLS Chunk Support The DTLS Chunk Support Required error cause can be sent by the receiver of the packet containing the INIT chunk to indicate that the receiver requires the support of the DTLS chunk, but no DTLS Key Management Parameter was included in the INIT chunk.

Error Missing DTLS Chunk Support

Cause Code: 16 bits (unsigned integer): This value MUST be set to 100.
Cause Length: 16 bits (unsigned integer): This value MUST be set to 4.

This error cause MAY be included in an ABORT chunk. It MUST NOT be included in any other chunk.

No Common DTLS Key Management Method The No Common DTLS Key Management Method error cause can be used by the receiver of the packet containing the INIT chunk to indicate that the receiver does not support any of the DTLS Key Management Methods offered by the sender of the packet containing the INIT chunk. The format of this error cause is depicted in .

Error Cause No Common DTLS Key Management Method

Cause Code: 16 bits (unsigned integer): This value MUST be set to 101.
Cause Length: 16 bits (unsigned integer): This value MUST be set to 4.

This error cause MAY be included in an ABORT chunk. It MUST NOT be included in any other chunk.

DTLS Key Management Tie Breaker Collision The DTLS Key Management Tie Breaker Collision error cause can be used to indicate that both sides chose the same tie breaker. The format of this error cause is depicted in .

Error Cause DTLS Key Management Tie Breaker Collision

Cause Code: 16 bits (unsigned integer): This value MUST be set to 102.
Cause Length: 16 bits (unsigned integer): This value MUST be set to 4.

This error cause MAY be included in an ABORT chunk. It MUST NOT be included in any other chunk.

Incompatible DTLS Key Management Roles The Incompatible DTLS Key Management Roles error cause can be used to indicate that the roles chosen are incompatible. The format of this error cause is depicted in .

Error Cause Incompatible DTLS Key Management Roles

Cause Code: 16 bits (unsigned integer): This value MUST be set to 103.
Cause Length: 16 bits (unsigned integer): This value MUST be set to 4.

This error cause MAY be included in an ABORT chunk. It MUST NOT be included in any other chunk.

Procedures

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:

At least one of the DTLS Key Management Identifiers listed in the parameter matches a locally supported one.
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 ). This parameter lists the supported DTLS Key Management Identifiers (see ) 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 ), "No Common DTLS Key Management Method" (see ), or "Incompatible DTLS Key Management Roles" (see ). 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 of the parameters provided by each endpoints. This falls into the following cases:

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.
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 ). 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.

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.

Unprotected SCTP Packet The diagram shown in describes the structure of an unprotected SCTP packet as described in .

Protected SCTP Packets The diagram shown in 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 . Then the DTLSCiphertext is computed per the DTLS 1.3 specification . The cipher suite from the Primary DTLS Key Context is used, if configured. Otherwise, the cipher suite from the Restart DTLS Key Context is used. The resulting DTLSCiphertext is the chunk value of the DTLS chunk. 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 or key managment method has restricted the SCTP packet handling to protected SCTP packets only, an SCTP packet not containing a DTLS chunk MUST be silently discarded unless they contain an INIT or INIT-ACK chunk. The later may be received during a SCTP restart procedure, see . 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 . After the encrypted_record has been verified and decrypted, the replay protection is performed. If a replay of the DTLS record is detected, further processing of the DTLSInnerPlaintext.content MUST NOT be performed. Otherwise, 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.

Termination of a Protected Association Note that the state of any DTLS Key Management Method doesn't impact 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.

SCTP Restart Considerations This section deals with the handling of an unexpected INIT chunk during an association lifetime as described in 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 , 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 (). 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 , 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 ). 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 . A DTLS chunk using the restart DTLS key context is identified by having the R bit (Restart Indicator) set in the DTLS chunk (see ). 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.

Handshake of SCTP Restart for DTLS in SCTP | | Plain |<-----------------(INIT ACK)-----------------+ +------- | | -' | | -. | | +------- +---------[DTLS CHUNK(COOKIE ECHO)]---------->| | Protected |<--------[DTLS CHUNK(COOKIE ACK)]------------+ +------- | | -' | | ]]> The 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 and MUST remove all primary DTLS key contexts. Therefore, the restart DTLS key context is used 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 are expected to 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 what is expected to be an COOKIE ECHO chunk protected by DTLS chunk as described above, in other words if it receives a SCTP packet with a DTLS chunk having the R bit (Restart Indicator) set in the DTLS chunk (see ), MUST silently discard it.

DTLS Key Management Method Considerations This document specifies the mechanisms for protecting SCTP associations using DTLS chunks, including an API for managing the corresponding key material. While this document defines the interface, the upper layer is responsible for the actual key management. DTLS Key Management Methods define the procedures for initial key generation, key updates and peer authentication. These procedures are out of scope for this document. Currently two DTLS Key Management Methods with different properties (such as mutual authentication and rekeying) are defined:

An SCTP endpoint MAY support multiple DTLS Key Management Methods subject to implementation requirements and local security policies. Every DTLS Key Management Method:

MUST be registered in the IANA Registry to receive a unique identifier, enabling negotiation during the SCTP handshake.
SHOULD use its own PPIDs to ensure that the DTLS Key Management Method related user messages are processed by the relevant entity.
SHOULD ensure that the local receive keys are installed before the peer installs the corresponding send keys.
MUST include in its key derivation the DTLS Key Management Parameter (including the parameter header and excluding the optional padding) as the sequence of bytes sent and received over the network during the SCTP handshake, to mitigate downgrade attacks.

Abstract API This section describes an abstract API between the upper layer, including a DTLS Key Management Method, and the SCTP implementation with the DTLS chunk. Please note that this section is an informational example API only and there are alternative implementations. This API enables the cryptographic protection operations performed to allow transmission and reception of the DTLS Records in the DTLS chunk. This API includes the information necessary to handle any AEAD cipher suite defined to work with DTLS. The API enable setting record payload key, sequence number keys, and initialization vector (IV) for primary and restart DTLS contexts in both send and receive direction. This is the traffic keying material required for the record protection per Section 5.2 and 5.3 of TLS 1.3 and the record sequence number protection per Section 4.2.3 of DTLS 1.3 . As this API references the key material as for send or receive, it will be the responsibility of the Key Management Method used to define how it maps the endpoint's role to which keys are for send and receive direction respectively.

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 that are supported, from the most preferred to the least preferred.

Reply: Success or Error Parameters: None

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 separable records.

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 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

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 have to 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 needs to be 3. Any subsequent epoch uses 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

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

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

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

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 can 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

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

Read Indication of Protection The API functions for receiving data should explicitly indicate if the data provided to the upper layer protocol was protected by the SCTP association using DTLS chunk.

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

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

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

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

Socket API Considerations This section describes how the socket API defined in needs to be extended to provide a way for the application to control the usage of the DTLS chunk. A 'Socket API Considerations' section is contained in all SCTP-related specifications published after describing an extension for which implementations using the socket API as specified in would require some extension of the socket API. Please note that this section is informational only. Please also note, that the API described in this section can change in a non-backwards compatible way during the evolution of this document due to changed functionality or gained experience during the implementation. A socket API implementation based on is extended by supporting several new IPPROTO_SCTP-level socket options and a new flag for recvmsg().

SCTP_ASSOC_CHANGE Notification When an SCTP_ASSOC_CHANGE notification (specified in ) 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.

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.

Functions

sctp_dtls_nr_cipher_suites() sctp_dtls_nr_cipher_suites() returns the number of cipher suites supported by the SCTP implementation. The function prototype is: This function can be used in combination with sctp_dtls_cipher_suites().

sctp_dtls_cipher_suites() sctp_dtls_cipher_suites() returns the cipher suites supported by the SCTP implementation. The function prototype is:

and the arguments are
cipher_suites:: An array where the supported cipher suites are stored. A cipher suite is represented by two uint8_t using the IANA assigned values in the TLS cipher suite registry .
n:: The number of cipher suites which can be stored in cipher_suites.

sctp_dtls_cipher_suites returns -1, if n is smaller than the number of cipher suites supported by the stack. If n is equal to or larger than the number of cipher suites supported by the SCTP implementation, the cipher suites are stored in cipher_suites and the number of supported cipher suites is returned.

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

Option Name	Data Type	Set	Get
`SCTP_DTLS_LOCAL_CONFIG`	`struct sctp_dtls_config`	X	X
`SCTP_DTLS_GET_CONFIG`	`struct sctp_dtls_config`		X
`SCTP_DTLS_GET_LOCAL_KM_PARAM`	`struct sctp_dtls_kmp`		X
`SCTP_DTLS_GET_PEER_KM_PARAM`	`struct sctp_dtls_kmp`		X
`SCTP_DTLS_SET_SEND_KEYS`	`struct sctp_dtls_keys`	X
`SCTP_DTLS_ADD_RECV_KEYS`	`struct sctp_dtls_keys`	X
`SCTP_DTLS_DEL_RECV_KEYS`	`struct sctp_dtls_keys_id`	X
`SCTP_DTLS_ENFORCE_PROTECTION`	`struct sctp_assoc_value`	X	X
`SCTP_DTLS_REPLAY_WINDOW`	`struct sctp_assoc_value`	X	X
`SCTP_DTLS_GET_STATS`	`struct sctp_dtls_stats`		X

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.

Get or Set the Local DTLS Key Management Configuration (SCTP_DTLS_LOCAL_CONFIG) This socket option allows to get and set the DTLS Key Management identifiers being sent to the peer during the handshake, the supported DTLS roles, and determines whether the restart operation is supported and the use of the DTLS chunk is required. The following structure is used as the option_value:

sdc_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. For setsockopt(), only SCTP_FUTURE_ASSOC can be used.

sdc_flags:

This field may contain any of the following flags and is composed of a
bitwise OR of these values.
SCTP_DTLS_CLIENT:: All Key Management Methods in sdc_kmids can operate as a DTLS client.
SCTP_DTLS_SERVER:: All Key Management Methods in sdc_kmids can operate as a DTLS server.
SCTP_DTLS_RESTART:: All Key Management Methods in sdc_kmids support the restart operation.
SCTP_DTLS_REQUIRED:: If this flag is set, the SCTP endpoint operates in strict DTLS mode. Otherwise, it operates in loose DTLS mode.

sdc_nr_kmids:

The number of entries in sdc_kmids.

sdc_kmids:

The DTLS Key Management identifiers which will be sent to the peer in the DTLS Key Management Parameter in the sequence provided.

This socket option can be used with setsockopt() only for SCTP endpoints in the SCTP_CLOSED state for configuration. It can be used with getsockopt() in any state.

Get the DTLS Key Management Configuration (SCTP_DTLS_GET_CONFIG) This socket option reports the DTLS Key Management Parameters negotiated during the handshake. The following structure is used as the option_value:

sdc_assoc_id:

This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.

sdc_flags:

This field contains any of the following flags and is composed of a bitwise
OR of these values.
SCTP_DTLS_CLIENT:: Indicates that the Key Management Method provided in sdc_kmids acts as a client.
SCTP_DTLS_SERVER:: Indicates that the Key Management Method provided in sdc_kmids acts as a server.
SCTP_DTLS_RESTART:: Indicates that the Key Management Method provided in sdc_kmids supports the restart operation.

sdc_nr_kmids:

The number of entries in sdc_kmids. A value of 1 indicates that DTLS chunk support was successfully negotiated, while a value of 0 indicates negotiation failed.

sdc_kmids:

The DTLS Key Management identifier which was negotiated.

This socket option will fail on any SCTP endpoint in state SCTP_CLOSED, SCTP_COOKIE_WAIT and SCTP_COOKIE_ECHOED.

Get the Local DTLS Key Management Parameter (SCTP_DTLS_GET_LOCAL_KM_PARAM) This socket option provides the DTLS Key Management Parameter sent by the endpoint during the handshake. The following structure is used as the option_value:

sdk_assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.
sdk_nr_bytes:: The number of entries in sdk_bytes.
sdk_bytes:: The DTLS Key Management Parameter sent by the endpoint as the sequence of bytes sent over the network. This includes the parameter header and excludes the optional parameter padding.

This socket option will fail on any SCTP endpoint in state SCTP_CLOSED, SCTP_COOKIE_WAIT and SCTP_COOKIE_ECHOED.

Get the Peer DTLS Key Management Parameter (SCTP_DTLS_GET_PEER_KM_PARAM) This socket option provides the DTLS Key Management Parameter received by the endpoint during the handshake. The following structure is used as the option_value:

sdk_assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.
sdk_nr_bytes:: The number of entries in sdk_bytes.
sdk_bytes:: The DTLS Key Management Parameter received by the endpoint as the sequence of bytes received over the network. This includes the parameter header and excludes the optional parameter padding.

This socket option will fail on any SCTP endpoint in state SCTP_CLOSED, SCTP_COOKIE_WAIT and SCTP_COOKIE_ECHOED.

Set Send Keys (SCTP_DTLS_SET_SEND_KEYS) Using this socket option allows to add a particular set of keys used for sending DTLS chunks. The following structure is used as the option_value:

sdk_assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.
sdk_cipher_suite:: The cipher suite for which the keys are used.
sdk_restart:: If the value is 0, the primary keys are added, if a value different from 0 is used, the restart keys are added.
sdk_unused:: This field is ignored.
sdk_epoch:: The epoch for which the keys are added.
sdk_key_len:: The length of the key specified in sdk_keys.
sdk_iv_len:: The length of the initialization vector specified in sdk_keys.
sdk_sn_key_len:: The length of the sequence number key specified in sdk_keys.
sdk_keys:: The key of length sdk_key_len directly followed by the initialization vector of length sdk_iv_len directly followed by the sequence number key of length sdk_sn_key_len.

This socket option can only be used to set a primary key on SCTP endpoints in states other than SCTP_LISTEN, SCTP_COOKIE_WAIT, and SCTP_COOKIE_ECHOED. A restart key can always be set on SCTP endpoints in the SCTP_CLOSED state. Only if a primary key is set on an SCTP endpoint, a restart key can be set in states other than SCTP_CLOSED, SCTP_LISTEN, SCTP_COOKIE_WAIT, and SCTP_COOKIE_ECHOED. If the socket option is successful, all affected DTLS chunks sent will use the specified keys until the keys are changed again by another call of this socket option.

Add Receive Keys (SCTP_DTLS_ADD_RECV_KEYS) Using this socket option allows to add a particular set of keys used for receiving DTLS chunks. The following structure is used as the option_value:

sdk_assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.
sdk_cipher_suite:: The cipher suite for which the keys are used.
sdk_restart:: If the value is 0, the primary keys are added, if a value different from 0 is used, the restart keys are added.
sdk_unused:: This field is ignored.
sdk_epoch:: The epoch for which the keys are added.
sdk_key_len:: The length of the key specified in sdk_keys.
sdk_iv_len:: The length of the initialization vector specified in sdk_keys.
sdk_sn_key_len:: The length of the sequence number key specified in sdk_keys.
sdk_keys:: The key of length sdk_key_len directly followed by the initialization vector of length sdk_iv_len directly followed by the sequence number key of length sdk_sn_key_len.

This socket option can only be used on SCTP endpoints in states other than SCTP_LISTEN, SCTP_COOKIE_WAIT and SCTP_COOKIE_ECHOED.

Delete Receive Keys (SCTP_DTLS_DEL_RECV_KEYS) Using this socket option allows to remove a particular set of keys used for receiving DTLS chunks. The following structure is used as the option_value:

sdki_assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.
sdki_restart:: If the value is 0, the primary keys are removed, if a value different from 0 is used, the restart keys are removed.
sdki_epoch:: The epoch for which the keys are removed.

This socket option can only be used on SCTP endpoints in states other than SCTP_CLOSED, SCTP_LISTEN, SCTP_COOKIE_WAIT and SCTP_COOKIE_ECHOED.

Get or Set Protection Enforcement (SCTP_DTLS_ENFORCE_PROTECTION) Enabling this socket option on an SCTP endpoint enforces that received SCTP packets are only processed, if they are protected. All received packets with the first chunk not being an INIT chunk, INIT ACK chunk, or DTLS chunk will be silently discarded. The following structure is used as the option_value:

assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.

assoc_value: The value 0 represents that the option is off, any other value represents that the option is on. This socket option is off by default on any SCTP endpoint. Once protection has been enforced by enabling this socket option on an SCTP endpoint, it cannot be disabled again. Whether protection has been enforced on an SCTP endpoint can be queried in any state other than SCTP_CLOSED. Protection can be enforced in any state other than SCTP_CLOSED, SCTP_COOKIE_WAIT and SCTP_COOKIE_ECHOED.

Get Statistic Counters (SCTP_DTLS_GET_STATS) This socket options allows to get various statistic counters for a specific SCTP endpoint. The following structure is used as the option_value:

sds_assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{FUTURE|CURRENT|ALL}_ASSOC.
sds_dropped_unprotected:: The number of unprotected packets received for this SCTP endpoint after protection was enforced.
sds_aead_failures:: The number of AEAD failures when processing received DTLS chunks.
sds_recv_protected:: The number of DTLS chunks successfully processed.
sds_sent_protected:: The number of DTLS chunks sent.

Get or Set the Replay Protection Window Size (SCTP_DTLS_REPLAY_WINDOW) This socket option can be used to configure the replay protection for SCTP endpoints. The following structure is used as the option_value:

assoc_id:: This parameter is ignored for one-to-one style sockets. For one-to-many style sockets, this parameter indicates upon which SCTP association the caller is performing the action. It is an error to use SCTP_{CURRENT|ALL}_ASSOC.

assoc_value: The maximum number of DTLS chunks in the replay protection window.

IANA Considerations This document adds registry entries or registries in the Stream Control Transmission Protocol (SCTP) Parameters group handled by IANA:

One new registry for the Key Management IDs
One new SCTP Chunk Types and its corresponding Chunk Type Flags registry
One new SCTP Chunk Parameter Type
Four new SCTP Error Cause Codes

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. DTLS Key Management Method Identifiers

Identifier	Key Management Method Name	Reference	Contact
0	DTLS Chunk with Pre-shared cryptographic parameters	RFC-To-Be	Draft Authors
1-191	Available for Assignment using Specification Required policy
192-254	Available for Assignment using First Come, First Served policy
255	Reserved for extension of the identifier space

New entries in the range 1-191 are registered following the Specification Required policy as defined by . 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.

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 with a reference to this document. DTLS Chunk Type

ID Value	Chunk Type	Reference
0x41	DTLS Chunk (DTLS)	RFC-To-Be

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

Chunk Flag Value	Chunk Flag Name	Reference
0x01	R bit	RFC-To-Be
0x02	Unassigned
0x04	Unassigned
0x08	Unassigned
0x10	Unassigned
0x20	Unassigned
0x40	Unassigned
0x80	Unassigned

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 with a reference to this document. DTLS Key Management Chunk Parameter

ID Value	Chunk Parameter Type	Reference
0x8006	DTLS Key Management	RFC-To-Be

SCTP Error Cause Codes In the Stream Control Transmission Protocol (SCTP) Parameters group's "Error Cause Codes" registry, IANA is requested to update the reference for the four error cause codes depicted in with a reference to this document. Error Cause Codes

ID Value	Error Cause Codes	Reference
100	Missing DTLS Chunk Support	RFC-To-Be
101	No Common DTLS Key Management Method	RFC-To-Be
102	DTLS Key Management Tie Breaker Collision	RFC-To-Be
103	Incompatible DTLS Key Management Roles	RFC-To-Be

Security Considerations All the security and privacy considerations of the security protocol used as the Chunk Protection Operator apply. The record layer security considerations from apply including rekeying.

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.

AEAD Limit Considerations 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.

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.

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.

Acknowledgments The authors thank Hannes Tschofenig and Tirumaleswar Reddy for their participation in the design team and their contributions to this document. We also like to thank Xin Long for his contributions to this document and Amanda Baber with IANA for feedback on our IANA registry. We also like to thank Russ Housley for his review.