TSVWG M. Westerlund Internet-Draft J. Preuß Mattsson Intended status: Standards Track C. Porfiri Expires: 28 December 2025 Ericsson 26 June 2025 Datagram Transport Layer Security (DTLS) based key-management of the Stream Control Transmission Protocol (SCTP) DTLS Chunk draft-westerlund-tsvwg-sctp-dtls-handshake-latest Abstract This document defines a key-management solution based on Datagram Transport Layer Security (DTLS) 1.3 to protect the content of Stream Control Transmission Protocol (SCTP) packets using the packet protection framework provided by the SCTP DTLS chunk. The combination provides encryption, source authentication, integrity and replay protection for the SCTP association with in-band DTLS based key-management and mutual authentication of the peers. The specification is enabling very long-lived sessions of weeks and months and supports mutual re-authentication and rekeying with ephemeral key exchange. The key-management solution does not require any additional defined features or implementation support beyond core DTLS 1.3. This is intended as an alternative to using DTLS/SCTP (RFC6083) and SCTP-AUTH (RFC4895). About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-westerlund-tsvwg-sctp-dtls- handshake/. Discussion of this document takes place on the Transport Area Working Group (tsvwg) Working Group mailing list (mailto:tsvwg@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tsvwg/. Subscribe at https://www.ietf.org/mailman/listinfo/tsvwg/. Source for this draft and an issue tracker can be found at https://github.com/gloinul/draft-westerlund-tsvwg-sctp-dtls- handshake. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 28 December 2025. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction 1.1. Overview 1.2. Protocol Overview 1.3. Properties of DTLS in SCTP 1.3.1. Benefits Compared to DTLS/SCTP 1.4. Terminology 1.5. Abbreviations 1.6. Conventions 2. DTLS usage of DTLS Chunk 3. DTLS messages over SCTP User Messages 4. DTLS Chunk Integration 4.1. State Machine 4.1.1. PROTECTION INITILIZATION state 4.1.2. PROTECTED state 4.1.3. SHUTDOWN states 4.2. DTLS Connection Handling 4.2.1. Add a New DTLS Connection 4.2.2. Remove an existing DTLS Connection 4.2.3. Considerations about removal of DTLS Connections 4.3. DTLS Key Update 4.4. Error Cases 5. DTLS Considerations 5.1. Version of DTLS 5.2. Configuration of DTLS 5.2.1. General 5.2.2. Authentication and Policy Decisions 5.2.3. New Connections 5.2.4. Padding of DTLS Records 5.2.5. DTLS 1.3 6. Establishing DTLS in SCTP 6.1. DTLS Key Context derivation 6.2. DTLS Handshake 6.2.1. Handshake of initial DTLS connection 6.2.2. Handshake of further DTLS connections 6.3. SCTP Association Restart 6.3.1. Installation of initial Restart DTLS Key Context 6.3.2. SCTP Association Restart Procedure 7. Parallel DTLS Rekeying 7.1. Criteria for Rekeying 7.2. Procedure for Rekeying 7.3. Race Condition in Rekeying 8. PMTU Discovery Considerations 9. Implementation Considerations 10. Security Considerations 10.1. General 10.2. Privacy Considerations 11. IANA Consideration 12. References 12.1. Normative References 12.2. Informative References Authors' Addresses 1. Introduction 1.1. Overview This document describes the usage of the Datagram Transport Layer Security version 1.3 (DTLS) protocol [RFC9147] for key-managment of the Stream Control Transmission Protocol (SCTP) [RFC9260] DTLS Chunk packet protection [I-D.westerlund-tsvwg-sctp-dtls-chunk]. This combination of specification is intended as an alternative to DTLS/ SCTP [RFC6083] and usage of SCTP-AUTH [RFC4895]. The combination of SCTP DTLS Chunk and the key-management defined in this document we call DTLS in SCTP. DTLS in SCTP provides mutual authentication of endpoints, data confidentiality, data origin authentication, data integrity protection, and data replay protection of SCTP packets. Ensuring these security services to the application and its upper layer protocol over SCTP. Thus, it allows client/server applications to communicate in a way that is designed with communications privacy and preventing eavesdropping and detect tampering or message forgery. Applications using DTLS in SCTP can use all currently existing transport features provided by SCTP and its extensions, in some cases with some limitations, as specified in [I-D.westerlund-tsvwg-sctp-dtls-chunk]. DTLS in SCTP supports: * preservation of message boundaries. * no limitation on number of unidirectional and bidirectional streams. * ordered and unordered delivery of SCTP user messages. * the partial reliability extension as defined in [RFC3758]. * multi-homing of the SCTP association per [RFC9260]. * the dynamic address reconfiguration extension as defined in [RFC5061] (Limitations apply). * User messages of any size. * SCTP Packets with a protected set of chunks up to a size of 2^14 (16384) bytes. The main benefit of this key-management solution over the solution proposed by the WG is that this does not require any extensions to DTLS 1.3 to be implemented. It soley relies on the core DTLS handshake to do mutual authentication, a create a main secret, and then relies on the TLS exporter to export necessary secrets for the DTLS Chunk. 1.2. Protocol Overview DTLS in SCTP is a key management specification for the SCTP DTLS 1.3 chunk [I-D.westerlund-tsvwg-sctp-dtls-chunk] that together utilizes DTLS 1.3 for the security functions like key exchange, authentication, encryption, integrity protection, and replay protection. All key management message exchange happens inband over the SCTP assocation. In this document we use the terms DTLS Key context for indicating a Key, derived from a DTLS connection, and all relevant data that needs to be provided to the SCTP DTLS Chunk Protection Operator for DTLS encryption and decryption. DTLS Key context includes Keys for sending and receiving, replay window, last used sequence number. Each DTLS key context is associated with a four value tuple identifying the context, consisting of SCTP Association, the restart indicator, the DTLS Connection ID (if used), an the DTLS epoch. The basic functionalities and how things are related is described below. The process starts with a SCTP association where DTLS 1.3 Chunk usage has been negotiated and this key-managmenet method has been agreed in the SCTP INIT and INIT-ACK. To initilize and authenticate the peer the DTLS handshake is exchanged as SCTP user messages with the DTLS Chunk Key-Management Messages PPID (see section 10.6 of [I-D.westerlund-tsvwg-sctp-dtls-chunk]) until an initial DTLS connection has been established. If the DTLS handshake fails, the SCTP association is aborted. With succesful handshake and authentication of the peer the key material exported from the DTLS connection and configured for the DTLS 1.3 chunk. From that point until SCTP association termination the DTLS chunk will protect the SCTP packets. When the DTLS connection has been established and the DTLS Chunk configured with DTLS Key context the PVALID message is exchanged to verify that no downgrade attack between any offered protection solutions has occurred. To prevent manipulation, the PVALID message are sent encapsulated in DTLS chunks. Assuming that the PVALID validation is successful the SCTP association is established and the Upper Layer Protocol (ULP) can start sending messages over the SCTP association. All chunks are protected by encapsulating them in DTLS chunks as defined in [I-D.westerlund-tsvwg-sctp-dtls-chunk]. Using the current DTLS Key context the DTLS Chunk Protection operator protects the plain text producing a DTLS Record that is encapsualted in the DTLS chunk and then transmitted as a SCTP packet with a common header. The DTLS Chunk specifies that in the receiving SCTP endpoint each incoming SCTP packet on any of its interfaces and ports are matched to the SCTP association based on ports and VTAG in the common header. Using the current DTLS Key context the content of the DTLS chunk is attempted to be processed, including replay protection, decryption, and integrity checking. And if decryption and integrity verification was successful the produced plain text of one or more SCTP chunks are provided for normal SCTP processing in the identified SCTP association along with associated per-packet meta data such as path received on, original packet size, and ECN bits. When mutual re-authentication or rekeying is needed or desired by either endpoint a new DTLS connection handshake is performed between the SCTP endpoints and new DTLS Key contextes are created. When the handshake has completed the DTLS in SCTP implementation can simply switch to use the new DTLS Key contextes in the DTLS chunk. All rekeying will be using ephemeral key exchange and shall not use the DTLS Key-Update mechanism to avoid confusion about the properties of the DTLS Key Contextes for the DTLS chunk. After a short while (no longer than 2 min) to enable any outstanding packets to drain from the network path between the endpoints, the old DTLS Key contextes can be deleted from the DTLS chunk's key store. The DTLS connection may send alerts, handshake messages, or other non-application data to its peer at any point in time. All DTLS message will be sent by means of SCTP user messages with the DTLS Chunk Key-Management Messages PPID as specified in [I-D.westerlund-tsvwg-sctp-dtls-chunk]. However, only the DTLS close_notify is expected to be used after the handshake has been completed in this solution. +---------------+ +--------------------+ | | | DTLS 1.3 | Keys | ULP | | +-------------. | | | Key Management | | +---------------+-+---+----------------+ --+-- API | | \ User | | | +-- Level | | SCTP Chunks Handler | Messages | | | | | | +-- SCTP Unprotected Payload | | |/ | +---------------------+ +---------------------+ | | DTLS | | DTLS 1.3 | | | Chunk |<-->| |<--' | Handler | | Protection Operator | +---------------------+ +---------------------+ | |\ | SCTP Header Handler | +-- SCTP Protected Payload | | +---------------------+ Figure 1: DTLS in SCTP layer in regard to SCTP and upper layer protocol 1.3. Properties of DTLS in SCTP DTLS in SCTP (as the combination of the DTLS chunk and the in-band authentication and key-management using DTLS handshakes defined in this document) has a number of properties that are attractive. * Provides confidentiality, integrity protection, and source authentication for each SCTP packet. * Provides replay protection on SCTP packet level preventing malicious replay attacks on SCTP, both protecting the data as well as the SCTP functions themselves. * Provides mutual authentication of the endpoints based on any authentication mechanism supported by DTLS. * Uses parallel DTLS connections to enable mutual re-authentication and rekeying with ephemeral key-exchange. Thus, enabling SCTP association lifetimes without known limitations and without needing to drain the SCTP association. * Uses core of DTLS as it is and updates and fixes to DTLS security properties; can be implemented without further changes to this specification. * No reliance on DTLS implementation used for key-management having to support any features beyond core DTLS specification and the TLS exporter. * Secures all SCTP packets exchanged after SCTP association has reached the established state and the initial key-exchange has completed. Making targeted attacks against the SCTP protocol and implementation much harder. * DTLS in SCTP results in no limitations on user message transmission or message sizes, those properties are the same as for an unprotected SCTP association. * Limited overhead on a per packet basis, with 4 bytes for the DTLS chunk plus the DTLS record overhead. The DTLS overhead is dependent on the DTLS version and cipher suit. * Support of SCTP packet plain text payload sizes up to 2^14 bytes. 1.3.1. Benefits Compared to DTLS/SCTP DTLS/SCTP as defined by [I-D.ietf-tsvwg-dtls-over-sctp-bis] has several important differences most to the benefit of DTLS in SCTP. This section reviews these differences. * Replay Protection in DTLS/SCTP has some limitations due to SCTP- AUTH [RFC4895] and its interaction with the SCTP implementation and dependencies on the actual SCTP-AUTH rekeying frequency. DTLS in SCTP relies on DTLS mechanism for replay protection that can prevent duplicates from being delivered as well as preventing packets from outside the current window to be delivered. Thus, a stronger protection especially for non-DATA chunk is provided and protects the SCTP stack from replayed or duplicated packets. * Encryption in DTLS/SCTP is only applied to ULP data. For DTLS in SCTP all chunk types, after the association has reached established state and the initial DTLS handshake has compeleted, will be encrypted. This, makes protocol attacks harder as a third-party attacker will have less insight into SCTP protocol state. Also, protocol header information likes PPIDs will also be encrypted, which makes targeted attacks harder but may also make management and debugging harder. * DTLS/SCTP Rekeying is complicated and requires advanced API or user message tracking to determine when a key is no longer needed so that it can be discarded. A DTLS/SCTP key that is prematurely discarded can result in loss of parts of a user message and failure of the assumptions on the transport where the sender believes it's delivered and the receiver never gets it. This usually will result in the need to terminate the SCTP association to restart the ULP session to avoid any issues due to inconsistencies. DTLS in SCTP is robustly handling of any early discard of the DTLS Key context after having switched to a new established DTLS Key context. Any outstanding packet that has not been decoded yet will simply be treated as lost between the SCTP endpoints, and SCTP's retransmission will retransmit any user message data that requires it. Also, the algorithm for when to discard a DTLS connection can be much simpler. * DTLS/SCTP rekeying can put restrictions on user message sizes unless the right APIs exist to the SCTP implementation to determine the state of user messages. No such restriction exists in DTLS in SCTP. * By using the DTLS chunk that is acting on SCTP packet level instead of user messages the consideration for extensions are quite different. Only extensions that would affect the common header or how packets are formed would interact with this 1.4. Terminology This document uses the following terms: Association: An SCTP association. Connection: A DTLS connection. DTLS Key context: Keys, derived from a DTLS connection, and all relevant data that needs to be provided to the SCTP DTLS Chunk. Each DTLS key context is associated with a four value tuple identifying the context, consisting of SCTP Association, the restart indicator, the DTLS Connection ID (if used), an the DTLS epoch Restart DTLS Key context: A DTLS Key context to be used for an SCTP Association Restart Stream: A unidirectional stream of an SCTP association. It is uniquely identified by a stream identifier. Traffic DTLS Key context: A DTLS Key context used to protect the regular SCTP traffic, i.e. not a restart DTLS Key context. 1.5. Abbreviations AEAD: Authenticated Encryption with Associated Data DKC: DTLS Key Context DTLS: Datagram Transport Layer Security MTU: Maximum Transmission Unit PPID: Payload Protocol Identifier SCTP: Stream Control Transmission Protocol ULP: Upper Layer Protocol 1.6. Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. DTLS usage of DTLS Chunk DTLS in SCTP uses the DTLS chunk as specified in [I-D.westerlund-tsvwg-sctp-dtls-chunk]. The chunk if just repeated here for the reader's convience. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 0x4x |reserved |R| Chunk Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Payload | | | | +-------------------------------+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: DTLS Chunk Structure reserved: 7 bits Reserved bits for future use. R: 1 bit (boolean) Restart indicator. If this bit is set this DTLS chunk is protected with an restart DTLS Key context. If not set, then a traffic DTLS Key context is indicated. Payload: variable length One DTLS record. 3. DTLS messages over SCTP User Messages DTLS messages, that are not DTLS records containing protected SCTP chunk payloads, will be sent as SCTP user messages using the format defined below. A DTLS handshake message may be fragmented by DTLS to a set of DTLS records of a maximum configured fragment size. Each DTLS message fragment is sent as a SCTP user message on the same stream where each message is configured for reliable and in-order delivery with the PPID set to DTLS Chunk Key-Management Messages [I-D.westerlund-tsvwg-sctp-dtls-chunk]. These user messages MAY contain one or more DTLS records. The SCTP stream ID used MAY be any stream ID that the ULP alreay uses, and if not know Stream 0. Note that all fragments of a handshake message MUST be sent with the same stream ID to ensure the in-order delivery. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |DCI| | +-+-+-+-+-+-+-+-+ | | | | DTLS Message | | | | +-------------------------------+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: DTLS User Message Structure DCI: DTLS Connection Index 2 bits (unsigned integer) DTLS Connection Index is the lower two bits of an DTLS Connection Index counter which corresponds to the epoch used in the SCTP DTLS Chunk. This is a counter implemented in DTLS in SCTP that is used to identify which DTLS connection instance that is capable of processing the DTLS message over an user message. This counter is recommended to be the lower part of a 64-bit unsigned integer variable as this is how DTLS epoch counter is defined. DCI is unrelated to the DTLS Connection ID [RFC9147]. DTLS Message: variable length One or more DTLS records. In cases more than one DTLS record is included all DTLS records except the last MUST include a length field. Note that this matches what is specified in DTLS 1.3 [RFC9147] will always include the length field in each record. 4. DTLS Chunk Integration The [I-D.westerlund-tsvwg-sctp-dtls-chunk] contains a high-level description of the basic DTLS in SCTP architecture, this section deals with details related to the DTLS 1.3 inband key-establishment integration with SCTP. 4.1. State Machine DTLS in SCTP uses inband key-establishment, thus the DTLS handshake establishes shared keys with the remote peer. As soon as the SCTP State Machine enters PROTECTION INITILIZATION state, DTLS in SCTP is responsible for progressing to the PROTECTED state when DTLS handshake has completed. 4.1.1. PROTECTION INITILIZATION state When entering PROTECTION INITILIZATION state, DTLS will start the handshake according to Section 6.2. When a successful handshake has been completed, the Traffic DTLS Key Context and the Restart DTLS Key Context will be created by deriving the keys from the DTLS connection. At that point the DTLS chunk Handler will move to the VALIDATION state and perform validation and then move SCTP State Machine into PROTECTED state. 4.1.2. PROTECTED state In the PROTECTED state the currently active DTLS connection is used for protection operation of the payload of SCTP chunks in each packet per below specification. When necessary to meet requirements on periodic re-authentication of the peer and establishment of new forward secrecy keys, the existing DTLS 1.3 connection is being replaced with a new one by first opening a new parallel DTLS connection as further specified in Section 7 and then close the old DTLS connection. 4.1.3. SHUTDOWN states When the SCTP association leaves the ESTABLISHED state per [RFC9260] to be shutdown the DTLS connection is kept and continues to protect the SCTP packet payloads through the shutdown process. When the association reaches the CLOSED state as part of the SCTP association closing process all DTLS connections existing (traffic and restart) for this association are terminated without further transmissions, i.e. DTLS close_notify is not transmitted. 4.2. DTLS Connection Handling It's up to DTLS key-establishment function to manage the DTLS connections and their related DTLS Key Context in the DTLS chunk. 4.2.1. Add a New DTLS Connection Either peer can add a new DTLS connection to the SCTP association at any time, but no more than 2 DTLS connections can exist at the same time. Details of the handshake are described in Section 6.2. As either endpoint can initiate a DTLS handshake at the same time, either endpoint may receive a DTLS ClientHello message when it has sent its own ClientHello. In this case the ClientHello from the endpoint that had the DTLS Client role in the establishment of the previous DTLS connection shall be continued to be processed and the other dropped. When the handshake has been completed successfully, the new DTLS connection will be possible to use, if the handshake is not completed successfully, a next DTLS handshake attempt will be tried. 4.2.2. Remove an existing DTLS Connection A DTLS connection is removed when a newer DTLS connection is in use. It is RECOMMENDED to not initiate removal until at least one SCTP packet protected by the new DTLS Key Context has been received, and any transmitted packets protected using the new DTLS Key Context has been acknowledge, alternatively one Maximum Segment Lifetime (120 seconds) has passed since the last SCTP packet protected by the old DTLS connection was transmitted. Either peers can initialize the removal of a DTLS connection from the current SCTP association when needed when a new have been established. The closing of the DTLS connection when the SCTP association is in PROTECTED and ESTABLISHED state is done by having the DTLS connection send a DTLS close_notify. When DTLS closure for a DTLS connection is completed, the related DTLS Key Context in the DTLS chunk are released. 4.2.3. Considerations about removal of DTLS Connections Removal of a DTLS connection may happen under circumstances as described above in Section 4.2.2 in different states of the Association. This section describes how the implementation should take care of the DTLS connection removal in details. The initial DTLS connection exists as soon as Association reaches the PROTECTED state. As long as one DTLS connection only exists, that DTLS connection SHALL NOT be removed as it won't be possible for the Association to proceed further. In general a DTLS connection can be removed when there's another active DTLS connection with valid DTLS Key Context that can be used for negotiating further DTLS DTLS 1.3 connections. In case the DTLS connection is removed and no useable DTLS Key Context exist for DTLS 1.3 negotiation, the Association SHALL be ABORTED. It is up to the implementation to guarantee that a DTLS Key Context exists all the time, for avoiding that undesired DTLS connection closure causes the Association abortion. 4.3. DTLS Key Update DTLS Key Update MUST NOT be used. DTLS Key Context replacemente MUST be used instead, by means creating a new DTLS connection as specified in Section 7, deriving the new Traffic DTLS Key Context, the new Restart DTLS Key Context and then closing the old DTLS connection. 4.4. Error Cases As DTLS has its own error reporting mechanism by exchanging DTLS alert messages no new DTLS related cause codes are defined to use the error handling defined in [I-D.westerlund-tsvwg-sctp-dtls-chunk]. When DTLS encounters an error it may report that issue using DTLS alert message to its peer by putting the created DTLS record in a SCTP user message (Section 3). This is independent of what to do in relation to the SCTP association. Depending on the severance of the error different paths can be the result: Non-critical: the DTLS connection can continue to protect the SCTP association. In this case the issue may be worth reporting to the peer using a DTLS alert message, but otherwise continue without further action. Critical, but not immediately fatal: If the DTLS connection has a critical issue, but can still protect packets then a the endpoint SHOULD attempt to establish a new DTLS connection. If that succeeds then the SCTP association switches over to the new DTLS connection replace the DTLS Key Contextes, and can terminate the old DTLS connection including reporting the error. In case the establishment fails, then this critical issue MUST be reported to the SCTP association so that it can send an ABORT chunk with the Error in Protection cause code. This will terminate the SCTP association immediately, provide ULP with notification of the failure and speeding up any higher layer management of the failure. Critical, and immediately fatal: If the DTLS connection fails so that no further data can be protected (i.e. either sent or received) with maintained security then it is not possible to establish a new DTLS connection and DTLS will have to indicate this to the SCTP implementation so it can perform a one sides SCTP association termination. This will lead to an eventual SCTP association timeout in the peer. 5. DTLS Considerations 5.1. Version of DTLS This document defines the usage of DTLS 1.3 [RFC9147]. Earlier versions of DTLS MUST NOT be used (see [RFC8996]). It is expected that DTLS in SCTP as described in this document will work with future versions of DTLS. Only one version of DTLS MUST be used during the lifetime of an SCTP Association, meaning that the procedure for replacing the DTLS version in use requires the existing SCTP Association to be terminated and a new SCTP Association with the desired DTLS version to be instantiated. 5.2. Configuration of DTLS 5.2.1. General The DTLS Connection ID SHOULD NOT be included in the DTLS records as it is not needed, avoiding overhead and addition implementation requirements on DTLS implementation. The DTLS record length field is normally not needed as the DTLS Chunk provides a length field unless multiple records are put in same DTLS chunk payload or user message. If multiple DTLS records are included in one DTLS chunk payload or user message the DTLS record length field MUST be present in all but the last. DTLS record replay detection MUST be used. Sequence number size can be adapted based on how quickly it wraps. Many of the TLS registries have a "Recommended" column. Parameters not marked as "Y" are NOT RECOMMENDED to support. Non-AEAD cipher suites or cipher suites without confidentiality MUST NOT be supported. Cipher suites and parameters that do not provide ephemeral key-exchange MUST NOT be supported. 5.2.2. Authentication and Policy Decisions DTLS in SCTP MUST be mutually authenticated. Authentication is the process of establishing the identity of a user or system and verifying that the identity is valid. DTLS only provides proof of possession of a key. DTLS in SCTP MUST perform identity authentication. It is RECOMMENDED that DTLS in SCTP is used with certificate-based authentication. When certificates are used the application using DTLS in SCTP is responsible for certificate policies, certificate chain validation, and identity authentication (HTTPS does for example match the hostname with a subjectAltName of type dNSName). The application using DTLS in SCTP defines what the identity is and how it is encoded and the client and server MUST use the same identity format. Guidance on server certificate validation can be found in [I-D.ietf-uta-rfc6125bis]. DTLS in SCTP enables periodic transfer of mutual revocation information (OSCP stapling) every time a new parallel connection is set up. All security decisions MUST be based on the peer's authenticated identity, not on its transport layer identity. It is possible to authenticate DTLS endpoints based on IP addresses in certificates. SCTP associations can use multiple IP addresses per SCTP endpoint. Therefore, it is possible that DTLS records will be sent from a different source IP address or to a different destination IP address than that originally authenticated. This is not a problem provided that no security decisions are made based on the source or destination IP addresses. 5.2.3. New Connections Implementations MUST set up a new DTLS connection using a full handshake before any of the certificates expire. It is RECOMMENDED that all negotiated and exchanged parameters are the same except for the timestamps in the certificates. Clients and servers MUST NOT accept a change of identity during the setup of a new connections, but MAY accept negotiation of stronger algorithms and security parameters, which might be motivated by new attacks. Allowing new connections can enable denial-of-service attacks. The endpoints MUST limit the number of simultaneous connections to two. To force attackers to do dynamic key exfiltration and limit the amount of compromised data due to key compromise, implementations MUST have policies for how often to set up new connections with ephemeral key exchange such as ECDHE. Implementations SHOULD set up new connections frequently to force attackers to dynamic key extraction. E.g., at least every hour and every 100 GB of data which is a common policy for IPsec [ANSSI-DAT-NT-003]. See [I-D.ietf-tls-rfc8446bis] for a more detailed discussion on key compromise and key exfiltration in (D)TLS. As recommended in [KTH-NCSA], resumption can be used to chain the connections, increasing security by forcing an adversary to break them in sequence. For many DTLS in SCTP deployments the SCTP association is expected to have a very long lifetime of months or even years. For associations with such long lifetimes there is a need to frequently re- authenticate both client and server by setting up a new connection using a full handshake. TLS Certificate lifetimes significantly shorter than a year are common which is shorter than many expected SCTP associations protected by DTLS in SCTP. 5.2.4. Padding of DTLS Records Both SCTP and DTLS contains mechanisms to padd SCTP payloads, and DTLS records respectively. If padding of SCTP packets are desired to hide actual message sizes it RECOMMEDED to use the SCTP Padding Chunck [RFC4820] to generate a consistent SCTP payload size. Support of this chunk is only required on the sender side. However, if the PAD chunk is not supported DTLS padding MAY be used. It needs to be noted that independent if SCTP padding or DTLS padding is used the padding is not taken into account by the SCTP congestion control. Extensive use of padding has potential for worsen congestion situations as the SCTP association will consume more bandwidth than its derived share by the congestion control. The use of SCTP PAD chunk is recommened as it at least can enable future extension or SCTP implementation that account also for the padding. Use of DTLS padding hides this packet expansion from SCTP. 5.2.5. DTLS 1.3 DTLS 1.3 is used instead of DTLS 1.2 being a newer protocol that addresses known vulnerabilities and only defines strong algorithms without known major weaknesses at the time of publication. DTLS 1.3 requires rekeying before algorithm specific AEAD limits have been reached. Implementations MAY setup a new DTLS connection instead of using key-update, this document mandates the setup of a new DTLS connection. In DTLS 1.3 any number of tickets can be issued in a connection and the tickets can be used for resumption as long as they are valid, which is up to seven days. The nodes in a resumed connection have the same roles (client or server) as in the connection where the ticket was issued. Resumption can have significant latency benefits for quickly restarting a broken DTLS/SCTP association. If tickets and resumption are used it is enough to issue a single ticket per connection. The PSK key exchange mode : psk_ke MUST NOT be used as it does not provide ephemeral key exchange. 6. Establishing DTLS in SCTP This section specifies how DTLS in SCTP is established [I-D.westerlund-tsvwg-sctp-dtls-chunk]. A DTLS in SCTP Association is built up with a DTLS connection, from that connection Traffic DTLS Key Context and Restart DTLS Key Context are derived. The DTLS connection is established as part of extra procedures for the DTLS chunk initial handshake (see Section 6.2.1). 6.1. DTLS Key Context derivation This section describes how DTLS Key Contect are derived from the DTLS handshake. 6.2. DTLS Handshake 6.2.1. Handshake of initial DTLS connection The handshake of the initial DTLS connection is part of the DTLS in SCTP Association initialization. The initialization is split in three distinct phases: * SCTP Handshake * DTLS Handshake * Validation Moving towards next phase is possible only when the previous phase handshake is completed. SCTP Handshake is strictly compliant to [RFC9260]. As soon the SCTP Association has entered the SCTP state PROTECTION INITILIZATION as defined by [I-D.westerlund-tsvwg-sctp-dtls-chunk] the DTLS handshake procedure is initiated by the endpoint that has initiated the SCTP association. The DTLS endpoint will send the DTLS message in one or more SCTP user message depending if the DTLS endpoint fragments the message or not Section 3. The DTLS instance SHOULD NOT use DTLS retransmission to repair any packet losses of handshake message fragment. Note: If the DTLS implementation supports configuring a MTU larger than the actual IP MTU it MAY be used as SCTP provides reliability and fragmentation. If the DTLS handshake is successful in establishing a security context to protect further communication and the peer identity is accepted then Traffic DTLS Key Context and Restart DTLS Key Context as specified in Section 6.1. The DTLS Key Contextes are installed for the DTLS chunk. This then triggers validated of the association establishment (see Section 1.2) by handshaking PVALID messages. Once the Association has been validated, then the SCTP association is informed that it can move to the PROTECTED state. If the DTLS handshake failed the SCTP association SHALL be aborted and an ERROR chunk with the Error in Protection error cause, with the appropriate extra error causes is generated, the right selection of "Error During Protection Handshake" or "Timeout During Protection Handshake or Validation". Initiator Responder | | -. +--------------------[INIT]------------------>| | |<-----------------[INIT-ACK]-----------------+ | SCTP +----------------[COOKIE ECHO]--------------->| +----- |<----------------[COOKIE ACK]----------------+ | | | -' | | -. +----------[DATA(DTLS Client Hello)]--------->| | |<--[DATA(DTLS Server Hello ... Finished)]----+ | DTLS +---[DATA(DTLS Certificate ... Finished)]---->| +----- |<-------------[DATA(DTLS ACK)]---------------+ | | | -' | | -. |<-----------[DTLS CHUNK(PVALID)]-------------+ | VALIDATION +------------[DTLS CHUNK(PVALID)]------------>| +----------- | | -' | | -. +-------[DTLS CHUNK(DATA(APP DATA))]--------->| | APP DATA +<-------[DTLS CHUNK(DATA(APP DATA))]---------+ +--------- | ... | | | ... | | Figure 4: Handshake of initial DTLS connection The Figure 4 shows a successfull handshake and highlits the different parts of the setup. DTLS handshake messages are transported by means of DATA Chunks with the DTLS Chunk Key-Management Messages PPID. 6.2.2. Handshake of further DTLS connections When the SCTP Association has entered the PROTECTED state, each of the endpoint can initiate a DTLS handshake for rekeying when necessary. The DTLS endpoint will if necessary fragment the handshake into multiple records. Each DTLS handshake message fragment is sent as a SCTP user message Section 3. The DTLS instance SHOULD NOT use DTLS retransmission to repair any packet losses of handshake message fragment. Note: If the DTLS implementation support configuring a MTU larger than the actual IP MTU it could be used as SCTP provides reliability and fragmentation. If the DTLS handshake failed the SCTP association SHALL generate an ERROR chunk with the Error in Protection error cause, with extra error causes "Error During Protection Handshake". Initiator Responder | | +----------[DATA(DTLS Client Hello)]--------->| |<--[DATA(DTLS Server Hello ... Finished)]----+ +---[DATA(DTLS Certificate ... Finished)]---->| |<-------------[DATA(DTLS ACK)]---------------+ | | Figure 5: Handshake of further DTLS connection The Figure 5 shows a successfull handshake of a further DTLS connection. Such connections can be initiated by any of the peers. Same as during the initial handshake, DTLS handshake messages are transported by means of DATA chunks with the DTLS Chunk Key- Management Messages PPID. 6.3. SCTP Association Restart In order to achieve an Association Restart as described in [I-D.westerlund-tsvwg-sctp-dtls-chunk], a Restart DTLS Key Context dedicated to Restart SHALL exist and be available. Furthermore, both peers SHALL have safely stored both the current Restart DTLS Key Context. Here we assume that Restart DTLS Key Context is maintained across the events leading to SCTP Restart request. 6.3.1. Installation of initial Restart DTLS Key Context As soon as the Association has reached the PROTECTED INITILIZATION state, a Restart DTLS Key Context SHALL be installed. It MAY exist a time gap where the Association is in PROTECTED state but no Restart DTLS Key Context has been installed yet. If a SCTP Restart procedure will be initiated during that time, it will fail and the Association will also fail. Once installed, no traffic will be sent over the Restart DTLS Key Context so that both endpoints will have a known DTLS record state. 6.3.2. SCTP Association Restart Procedure The DTLS in SCTP Association Restart is meant to preserve the security characteristics. In order the Association Restart to proceed both Initiator and Responder SHALL use the same Restart DTLS Key Context for COOKIE- ECHO/COOKIE-ACK handshake, that implies that the Initiator must preserve the Restart DTLS Key Context and that the Responder SHALL NOT change the Restart DTLS Key Context during the Restart procedure. Initiator Responder | | -. +------------[DTLS CHUNK(INIT)]-------------->| | |<---------[DTLS CHUNK(INIT-ACK)]-------------+ +------- | | | Using | | | SCTP +---------[DTLS CHUNK(COOKIE ECHO)]---------->| | Chunks |<--------[DTLS CHUNK(COOKIE ACK)]------------+ +------- | | -' | | -. +----------[DATA(DTLS Client Hello)]--------->| | |<--[DATA(DTLS Server Hello ... Finished)]----+ | New DTLS +---[DATA(DTLS Certificate ... Finished)]---->| | Connection |<-------------[DATA(DTLS ACK)]---------------+ +---------------- | | -' | ... | -. | ... | | Derive new | ... | | Traffic and | ... | | Restart | ... | | DTLS Key | ... | | Contextes | ... | +---------------- | ... | -' | | -. +-------[DTLS CHUNK(DATA(APP DATA))]--------->| | APP DATA +<-------[DTLS CHUNK(DATA(APP DATA))]---------+ +--------- | ... | | | ... | | Figure 6: SCTP Restart sequence for DTLS in SCTP The Figure 6 shows a successfull SCTP Association Restart. From procedure viewpoint the sequence is the following: * Initiator sends INIT (VTag=0), Responder replies INIT-ACK both encrypted using Restart DTLS Key Context * Initiator sends COOKIE-ECHO using DTLS CHUNK encrypted with the Key tied to the Restart DTLS Key Context * Responder replies with COOKIE-ACK using DTLS CHUNK encrypted with the to the Restart DTLS Key Context * Initiator sends handshakes for new Traffic DTLS connnection as well as new Restart DTLS connection. * When the handshake for the a new traffic DTLS connection has been completed, new Traffic and Restart DTLS Key Contextes are derived. New Traffic DTLS Key Context is being used for traffic. User Data for any ULP traffic MAY be initiated immediately after COOKIE-ECHO/COOKIE-ACK handshake using the current Restart DTLS Key Context, that is even before a new Traffic DTLS Key Context or a Restart DTLS Key Context have been derived. If a problem occurs before the new Restart DTLS Key Context has been installed, the Association cannot be Restarted, thus it's RECOMMENDED the new Restart DTLS Key Context to be installed as early as possible. Note that, different than the initial Association establishment, the ULP traffic is permitted immediately after the COOKIE-ECHO/COOKIE-ACK handshake, the reason is that the validation has already been performed prior to the restart DTLS Key Context was created. 7. Parallel DTLS Rekeying Rekeying in this specification is implemented by replacing the DTLS connection getting old with a new one by first creating the new DTLS connection, derive the new DTLS Key contextes, start using it, then closing the old one. 7.1. Criteria for Rekeying The criteria for rekeying may vary depending on the ULP requirement on security properties, chosen cipher suits etc. Therefore it is assumed that the implementation will be configurable by the ULP to meet its demand. Likely criteria to impact the need for rekeying through the usage of new DTLS connection are: * Maximum time since last authentication of the peer * Amount of data transferred since last forward secrecy preserving rekeying * The cipher suit's maximum key usage being reached. Although for DTLS 1.3 usage of the Key Update mechanism can generate new keys not having the same security properties as opening a new DTLS connection. 7.2. Procedure for Rekeying This specification allows up to 2 DTLS connection to be active at the same time for the current SCTP Association. The following state machine applies. +---------+ +--------->| YOUNG | There's only one | +----+----+ DTLS connection until | | aging criteria are met | | | AGING | REMOTE AGING | V | +---------+ | | AGED | When in AGED state a | +----+----+ new DTLS connection | | is added with a newly derived Traffic DTLS Key Context | NEW DTLS | As well as a new Restart DTLS Key Context | | | | | V | +---------+ | | OLD | In OLD state there | +----+----+ are 2 active DTLS connections | | Traffic is switched to the new Traffic DTLS Key Context | SWITCH | | V | +---------+ | | DRAIN | The aged DTLS connection | +----+----+ is drained before being ready | | to be closed | | | DRAINED | DTLS close_notify | V | +---------+ | | DEAD | In DEAD state the aged | +----+----+ connection is closed | | | REMOVED | +---------------+ Figure 7: State Diagram for Rekeying Trigger for rekeying can either be a local AGING event, triggered by the DTLS connection meeting the criteria for rekeying, or a REMOTE AGING event, triggered by receiving a DTLS connection creation request on the Traffic DTLS Key Context that would be used for new DTLS connection. In such case a new DTLS connection shall be added according to Section 4.2.1 with a new Traffic DTLS Key Context. As soon as the new DTLS connection completes handshaking, and the Traffic and Restart DTLS Key Contextes have been derived, the traffic is moved from the old Traffic DTLS Key Context, then the procedure for closing the old DTLS connection is initiated, see Section 4.2.2. 7.3. Race Condition in Rekeying A race condition may happen when both peer experience local AGING event at the same time and start creation of a new DTLS connection. The race condition is solved as specified in Section 4.2.1. 8. PMTU Discovery Considerations Due to the DTLS record limitation for application data SCTP MUST use 2^14 as input to determine absolute maximum MTU when running PMTUD and using DTLS in SCTP. The implementor needs to handle the DTLS 1.3 record overhead. SCTP PMTUD needs to include both the DTLS record as well as the DTLS chunk overhead in this consideration and ensure that produced packets, especially those that are not PMTUD probes do not become oversized. The DTLS record size may change during the SCTP associations lifetime due to future handshakes affecting cipher suit in use, or changes to record layer configurations. Note that this implies that DTLS 1.3 is expected to accept application data payloads of potentially larger sizes than what it configured to use for messages the DTLS implementation generates itself for signaling. 9. Implementation Considerations For each DTLS connection, there are certain crypto state information that needs to be handled thread safe to avoid nonce re-use and correct replay protection. This arises as the key materials for DTLS epoch=3 and higher are shared between the DTLS chunk and the DTLS handshake parts. This issue is primarily for kernel implementation of SCTP, thus the DTLS chunk implementation resides in kernel space, and the DTLS handshake resides in user space. For user space implementations where both DTLS handshake messages and SCTP message protection can directly call the same DTLS implementation instance the issue is avoided. Different implementation strategies do exists for the kernel implementations but likely have some impact on the DTLS implementation itself as the DTLS record protection processing either need to synchronize over state variables, alternatively use the DTLS Chunk protection operation using an extended DTLS Chunk API [I-D.westerlund-tsvwg-sctp-dtls-chunk]. 10. Security Considerations 10.1. General The security considerations given in [RFC9147], [RFC6347], and [RFC9260] also apply to this document. BCP 195 [RFC9325] [RFC8996] provides recommendations and requirements for improving the security of deployed services that use DTLS. BCP 195 MUST be followed which implies that DTLS 1.0 SHALL NOT be supported and are therefore not defined. 10.2. Privacy Considerations Although DTLS in SCTP provides privacy for the actual user message as well as almost all chunks, some fields are not confidentiality protected. In addition to the DTLS record header, the SCTP common header and the DTLS chunk header are not confidentiality protected. An attacker can correlate DTLS connections over the same SCTP association using the SCTP common header. To provide identity protection it is RECOMMENDED that DTLS in SCTP is used with certificate-based authentication in DTLS 1.3 [RFC9147] and to not reuse tickets. DTLS 1.3 with external PSK authentication does not provide identity protection. By mandating ephemeral key exchange and cipher suites with confidentiality DTLS in SCTP effectively mitigate many forms of passive pervasive monitoring. By recommending implementations to frequently set up new DTLS connections with (EC)DHE force attackers to do dynamic key exfiltration and limits the amount of compromised data due to key compromise. 11. IANA Consideration This document has no IANA considerations currently. 12. References 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, . [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, . [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021, . [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, . [RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November 2022, . [RFC9260] Stewart, R., Tüxen, M., and K. Nielsen, "Stream Control Transmission Protocol", RFC 9260, DOI 10.17487/RFC9260, June 2022, . [I-D.westerlund-tsvwg-sctp-dtls-chunk] Westerlund, M., Preuß Mattsson, J., and C. Porfiri, "Stream Control Transmission Protocol (SCTP) DTLS chunk", March 2025, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 12.2. Informative References [RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P. Conrad, "Stream Control Transmission Protocol (SCTP) Partial Reliability Extension", RFC 3758, DOI 10.17487/RFC3758, May 2004, . [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, . [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, . [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, . [I-D.ietf-tls-rfc8446bis] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Work in Progress, Internet-Draft, draft- ietf-tls-rfc8446bis-12, 17 February 2025, . [I-D.ietf-tsvwg-dtls-over-sctp-bis] Westerlund, M., Mattsson, J. P., and C. Porfiri, "Datagram Transport Layer Security (DTLS) over Stream Control Transmission Protocol (SCTP)", Work in Progress, Internet- Draft, draft-ietf-tsvwg-dtls-over-sctp-bis-08, 3 May 2024, . [I-D.ietf-uta-rfc6125bis] Saint-Andre, P. and R. Salz, "Service Identity in TLS", Work in Progress, Internet-Draft, draft-ietf-uta- rfc6125bis-15, 10 August 2023, . [I-D.mattsson-tls-super-jumbo-record-limit] Mattsson, J. P., Tschofenig, H., and M. Tüxen, "Large Record Sizes for TLS and DTLS with Reduced Overhead", Work in Progress, Internet-Draft, draft-mattsson-tls-super- jumbo-record-limit-05, 5 September 2024, . [ANSSI-DAT-NT-003] Agence nationale de la sécurité des systèmes d'information, "Recommendations for securing networks with IPsec", ANSSI Technical Report DAT-NT-003 , August 2015, <>. [KTH-NCSA] Ekerå, M., "On factoring integers, and computing discrete logarithms and orders, quantumly", KTH, School of Electrical Engineering and Computer Science (EECS), Computer Science, Theoretical Computer Science, TCS. Swedish NCSA, Swedish Armed Forces. , October 2024, . Authors' Addresses Magnus Westerlund Ericsson Email: magnus.westerlund@ericsson.com John Preuß Mattsson Ericsson Email: john.mattsson@ericsson.com Claudio Porfiri Ericsson Email: claudio.porfiri@ericsson.com