Stream Control Transmission Protocol (SCTP) CRYPTO Chunk

Introduction This document defines a CRYPTO chunk for the Stream Control Transmission Protocol (SCTP), as defined in . This specification defines the actual CRYPTO chunk. How to enable it usage, how it interacts with the SCTP association establishment to enable endpoint authentication, key-establishment, and other features require a separate protection engine specification. This specification is intended to be capable of enabling mutual authentication of endpoints, data confidentiality, data origin authentication, data integrity protection, and data replay protection for SCTP packets after the SCTP association has been established. The exact properties will depend on the companion specification defining the protection engine used with the CRYPTO chunk. The protection engine specification might be based on an existing security protocol. Applications using SCTP CRYPTO chunk can use all transport features provided by SCTP and its extensions. However, there can be some limitations or additional requirements for them to function such as those noted for SCTP restart and use of Dynamic Address Reconfiguration, see and . Due to its level of integration as discussed in next section it will provide its security functions on all content of the SCTP packet, and will thus not impact the potential to utilize any SCTP functionalities or extensions.

Overview

Protocol Overview The CRYPTO chunk is defined as a method for secure and confidential transfer for SCTP packets. This is implemented inside the SCTP protocol, in a sublayer between the SCTP common header handling and the SCTP chunk handling. Once an SCTP packet has been received and the SCTP common header has been used to identify the SCTP association, the CRYPTO chunk is sent to the chosen protection engine that will return the SCTP payload containing the unprotected SCTP chunks, those chunks will then be handled according to current SCTP protocol specification. illustrates the CRYPTO chunk layering in regard to SCTP and the Upper Layer Protocol (ULP).

CRYPTO Chunk Layering in Regard to SCTP and ULP | |<--' | Handler | | Protection Operator | +---------------------+ +---------------------+ | |\ | SCTP Header Handler | +-- SCTP Protected Payload | | +---------------------+ ]]> Use of the CRYPTO chunk is defined per SCTP association and a SCTP association uses a single protection engine. Different associations within the same SCTP endpoint may use or not use the CRYPTO chunk, and different associations may use different protection engines. On the outgoing direction, once the SCTP stack has created the unprotected SCTP packet payload containing control and/or DATA chunks, that payload will be sent to the protection engine to be protected. The format of the protected payload depends on the protection engine but the unprotected payload will typically be encrypted and integrity tagged before being encapsulated in a CRYPTO chunk. The SCTP protection engine performs protection operations on the whole unprotected SCTP packet payload, i.e., all chunks after the SCTP common header. Information protection is kept during the lifetime of the association and no information is sent unprotected except than the initial SCTP handshake, the SCTP common Header, the SCTP CRYPTO chunk header and the SHUTDOWN-COMPLETE chunk. SCTP CRYPTO chunk capability is agreed by the peers at the initialization of the SCTP association, during that phase the peers exchange information about the protection engines available. Once the peers have agreed on what protection to use, the SCTP endpoints start sending Protection Engine's payloads in SCTP DATA chunks containing the initialization information related to the protection engine including key agreement and endpoint authentication. This is depending on the chosen protection engine thus is not being detailed in the current specification. When the endpoint authentication has been completed, the association is meant to be initialized and the ULP is informed about that, from this time on it's possible for the ULPs to exchange data. CRYPTO chunks will never be retransmitted, retransmission is implemented by SCTP endpoint at chunk level as in the legacy. Duplicated CRYPTO chunks, whenever they will be accepted by the protection engine, will result in duplicated SCTP chunks and will be handled as duplicated chunks by SCTP endpoint in the same way a duplicated SCTP packet with those SCTP chunks would have been.

Protection Engines Considerations The protection engine, independently from the security characteristics, needs to be capable working on an unreliable transport mechanism same as UDP in regards to the payloads of the CRYPTO chunk, and have its own key management capability. SCTP is capable of providing reliable transport of key-management messages. SCTP CRYPTO chunk directly exploits the protection engine by requesting protection and unprotection of a buffer, in particular the protection buffer should never exceed the possible SCTP packet size thus protection engine needs to be aware of the PMTU (see ). The key management part of the protection engine is the set of data and procedures that take care of key distribution, verification, and update. For key management the Protection Engines uses SCTP DATA chunks identified with a dedicated Payload Protocol Identifier. The protection engine can specify if the transmission of any key-managment messages are non-reliable or reliable transmitted by SCTP. During initialization, that is before Association reaches the ESTABLISHED state (see ), inband Key Management use DATA chunks that SHALL use the Protection Engine PPID (see ). These DATA chunks SHALL be sent unprotected by the protection engine as no keys have been established yet. As soon as the SCTP Association reaches PROTECTED state, any protection engine that uses inband key management, i.e. sent using SCTP DATA chunks with the Protection Engine PPID, will have their message protected inside SCTP CRYPTO chunk protected with the currently established key. Key management MAY use other mechanism than what provided by SCTP CRYPTO chunks, in any case the mechanism for key management MUST be detailed in the specification for that protection engine. The protection engines MAY exploit the Flags byte provided by the CRYPTO chunk header (see ) for its needs. Details of the use of Flags, if different from what described in the current document, MUST be specified in the Protection Engine Specification document for that specific protection engine. The SCTP common header is assumed to be implicitly protected by the protection engine. This protection is based on the assumption that there will be a one-to-one mapping between SCTP association and individually established security contexts. If the protection engine does not meet that assumption further protection of the common header is likely required. An example of protection engine can be DTLS as specified in .

SCTP CRYPTO Chunk Buffering and Flow Control Protection engine and SCTP are asynchronous, meaning that the protection engine may deliver the decrypted SCTP Payload to the SCTP endpoint without respecting the reception order. It's up to SCTP endpoint to reorder the chunks in the reception buffer and to take care of the flow control according to what specified in . From SCTP perspective the CRYPTO chunk processing is part of the transport network. Even though the above allows the implementors to adopt a multithreading design of the protection engines, the actual implementation should consider that out-of-order handling of SCTP chunks is not desired and may cause false congestions and retransmissions.

PMTU Considerations The addition of the CRYPTO chunk to SCTP reduces the room for payload, due to the size of the CRYPTO chunk header and plain text expansion due to ciphering algorithm and any authentication tag. Thus, the SCTP layer creating the plain text payload needs to know about the overhead to adjust its target payload size appropriately. On the other hand, the protection engine needs to be informed about the PMTU by removing from the value the sum of the common SCTP header and the CRYPTO chunk header. This implies that SCTP can propagate the computed PMTU at run time specifically. The way protection engine provides the primitive for PMTU communication shall be part of the protection engine specification. From SCTP perspective, if there is a maximum size of plain text data that can be protected by the protection engine that must be communicated to SCTP. As such a limit will limit the PMTU for SCTP to the maximum plain text plus CRYPTO chunk and algorithm overhead plus the SCTP common header.

Congestion Control Considerations The SCTP mechanism for handling congestion control does depend on successful data transfer for enlarging or reducing the congestion window CWND (see section 7.2). It may happen that protection engine discards packets due to internal checks or because it has detected a malicious attempt. As those packets do not represent what the peer sent, it is acceptable to ignore them, although in-situ modification on the path of a packet resulting in discarding due to integrity failure will leave a gap, but has to be accepted as part of the path behavior. The protection engine shall not interfere with the SCTP congestion control mechanism, this basically means that from SCTP perspective the congestion control is exactly the same as how specified in .

ICMP Considerations The SCTP implementation will be responsible for handling ICMP messages and their validation as specified in Section 10. This means that the ICMP validation needs to be done in relation to the actual sent SCTP packets with the CRYPTO chunk and not the unprotected payload. However, valid ICMP errors or information may indirectly be provided to the protection engine, such as an update to PMTU values based on packet to big ICMP messages.

Path Selection Considerations When an Association is multihomed there are multiple paths between Endpoints. The selection of the specific path to be used at a certain time belongs to SCTP protocol that will decide according to . The Protection Engine shall not influence the path selection algorithm, actually the Protection Engine will not even know what path is being used.

Dynamic Address Reconfiguration Considerations When using Dynamic Address Reconfiguration in an SCTP association using CRYPTO Chunk the ASCONF chunk is protected, thus it needs to be unprotected first, furthermore it MAY come from an unknown IP Address. In order to properly address the ASCONF chunk to the relevant Association for being unprotected, Destination Address, Source and Destination ports and VTag shall be exploited. If the combination of those parameters is not unique the implementor MAY choose to send the Crypto Chunk to all Associations that fit with the parameters in order to find the right one. The association will attempt de-protection operations on the crypto chunk, and if that is successful the ASCONF chunk can be processed. The recommendation specifies (section 4.1.1 of ) that ASCONF message are required to be sent authenticated with SCTP-AUTH . For SCTP associations using Crypto Chunks, when the Protection Engine provides strong Authentication such for instance in case of DTLS, results in the use of redundant mechanism for Authentication with both SCTP-AUTH and the Crypto Chunk. We recommend to amend for including Crypto Chunks as Authentication mechanism for ASCONF chunks.

SCTP Restart Considerations SCTP Restart procedure allows an endpoint to recover an association when the remote endpoint still has the state for the association. However, an SCTP association that has an established protection engine will not accept a unprotected INIT to restart it, instead it would discard it. Thus, an endpoint attempting to restart an SCTP association which was using Crypto Chunks MUST have the security context for this association, and use it to encapsulate the INIT in a CRYPTO chunk. Note, the SCTP common header will contain a verification tag equal to 0, combined with CRYPTO chunk when carrying an INIT to restart. An endpoint implementing Crypto Chunk MUST accept receiving SCTP packets with a verification tag with value 0 and containing a Crypto Chunk. The endpoit will attempt to map the packet to an association based on source IP address, destination address and port. If the combination of those parameters is not unique the implementor MAY choose to send the Crypto Chunk to all Associations that fit with the parameters in order to find the right one. The association will attempt de-protection operations on the crypto chunk, and if that is successful the INIT chunk can be processed as a restart attempt. Note that this will update the verification tags for both this endpoint and the peer. Failure to authenticate the crypto chunk payload will result in it being discarded. Association restart when using crypto chunks has increased requirements on the endpoint maintaining state across the restart. In cases this is not possible or failed to be done successfully the endpoint will be need to fall back to initiating a new SCTP association.

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.

New Parameter Type This section defines the new parameter type that will be used to negotiate the use of the CRYPTO chunk and protection engines during association setup. illustrates the new parameter type. New INIT/INIT-ACK Parameter

Parameter Type	Parameter Name
0x80xx	Protected Association

Note that the parameter format requires the receiver to ignore the parameter and continue processing if the parameter is not understood. This is accomplished (as described in , Section 3.2.1.) by the use of the upper bits of the parameter type.

Protected Association Parameter This parameter is used to carry the list of proposed protection engines and the chosen protection engine during INIT/INIT-ACK handshake.

Protected Association Parameter

Parameter Type: 16 bits (unsigned integer): This value MUST be set to 0x80XX.
Parameter Length: 16 bits (unsigned integer): This value holds the length of the Protection Engines field in bytes plus 4.
Protection Engines: variable length: In the INIT chunk this holds the list of protection engines in descending order of preference, i.e. the most preferred comes first, and the least preferred last in this field. In the INIT-ACK chunk this holds a single chosen protection engine. Each protection engine is specified by a 16-bit unsigned integer.
Padding: 0 or 16 bits: If the length of the Protection Engines field 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 2 bytes and it MUST be ignored by the receiver.

RFC-Editor Note: Please replace 0x08XX with the actual parameter type value assigned by IANA and then remove this note.

New Chunk Types

Crypto Chunk (CRYPTO) This section defines the new chunk type that will be used to transport protected SCTP payload. illustrates the new chunk type. CRYPTO Chunk Type

Chunk Type	Chunk Name
0x4X	Crypto Chunk (CRYPTO)

RFC-Editor Note: Please replace 0x4x with the actual chunk type value assigned by IANA and then remove this note. It should be noted that the CRYPTO chunk format 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. This is accomplished (as described in Section 3.2.) by the use of the upper bits of the chunk type. The CRYPTO chunk is used to hold the protected payload of a plain SCTP packet.

CRYPTO Chunk Structure

Chunk Type: 8 bits (unsigned integer): This value MUST be set to 0x4X for all CRYPTO chunks.
Chunk Flags: 8 bits: This is used by the protection engine and ignored by SCTP.
Chunk Length: 16 bits (unsigned integer): This value holds the length of the Payload in bytes plus 4.
Payload: variable length: This holds the encrypted data.
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.

Protected Association Parameter Validation Chunk (PVALID) This section defines the new chunk types that will be used to validate the negotiation of the protection engine selected for CRYPTO chunk. This to prevent down grade attacks on the negotiation of protection engines. illustrates the new chunk type. PVALID Chunk Type

Chunk Type	Chunk Name
0x4X	Protected Association Parameter Validation (PVALID)

It should be noted that the PVALID chunk format 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. This is accomplished (as described in Section 3.2.) by the use of the upper bits of the chunk type. The PVALID chunk is used to hold the protection engines list.

PVALID Chunk Structure

Chunk Type: 8 bits (unsigned integer): This value MUST be set to 0x4X.
Chunk Flags: 8 bits: MUST be set to zero on transmit and MUST be ignored on receipt.
Chunk Length: 16 bits (unsigned integer): This value holds the length of the Protection Engines field in bytes plus 4.
Protection Engines: variable length: This holds the list of protection engines where each protection engine is specified by a 16-bit unsigned integer. The field MUST be identical to the content of the Protected Association Parameter () Protection Engines field that the endpoint sent in the INIT or INIT-ACK chunk.
Padding: 0 or 16 bits: If the length of the Protection Engines field 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 2 bytes and it MUST be ignored by the receiver.

RFC-Editor Note: Please replace 0x4X with the actual chunk type value assigned by IANA and then remove this note.

Error Handling This specification introduces a new set of error causes that are to be used when SCTP endpoint detects a faulty condition. The special case is when the error is detected by the protection engine that may provide additional information.

Mandatory Protected Association Parameter Missing When an initiator SCTP endpoint sends an INIT chunk that doesn't contain the Protected Association parameter towards an SCTP endpoint that only accepts protected associations, the responder endpoint SHALL raise a Missing Mandatory Parameter error. The ERROR chunk will contain the cause code 'Missing Mandatory Parameter' (2) (see section 3.3.10.7) and the protected association parameter identifier in the missing param Information field.

ERROR Missing Protected Association Paramater Note: Cause Length is equal to the number of missing parameters 8 + N * 2 according to , section 3.3.10.2. Also the Protection Association ID may be present in any of the N missing params, no order implied by the example in .

Error in Protection A new Error Type is defined for Crypto Chunk, it's used for any error related to the Protection mechanism described in this document and has a structure that allows detailed information to be added as extra causes. This specification describes some of the causes whilst the Protection Engine Specification MAY add further Causes related to the related Protection Engine. When detecting an error, SCTP will send an ABORT chunk containing the relevant Error Type and Causes.

Error in Protection Cause Format

Casuse Code: 16 bits (unsigned integer): The SCTP Error Chunk Cause Code indicating "Error in Protection" is TBA9.
Cause Length: 16 bits (unsigned integer): Is for N extra Causes equal to 4 + N * 2
Extra Cause: 16 bits (unsigned integer): Each Extra Cause indicate an additional piece of information as part of the error. There MAY be zero to as many as can fit in the extra cause field in the ERROR Chunk (A maximum of 32764).

Editor's Note: Please replace TBA9 above with what is assigned by IANA. Below a number of defined Error Causes are defined, additional causes can be registered with IANA following the rules in .

No Supported Protection Engine If list of protection engines contained in the INIT signal doesn't contain at least an entry that fits the list of protection engines at the responder, the responder will reply with an ABORT chunk with error in protection cause code (specified in ) and the "No Supported Protection Engine" extra cause code identifier 0x00.

Error During Protection Handshake If the protection engine specifies a handshake for example for authentication, and key management is implemented inband, it may happen that the procedure has errors. In such case an ABORT chunk will be sent with error in protection cause code (specified in ) and extra cause "Error During Protection Handshake" identifier 0x01.

Failure in Protection Engines Validation A Failure may occur during protection engine Validation (see ). In such case an ABORT chunk will be sent with error in protection cause code (specified in ) and extra cause "Failure in Protection Engines Validation" identifier 0x02 to indicate this failure.

Timeout During Protection Handshake or Validation Whenever a T-valid timeout occurs, the SCTP endpoint will send an ABORT chunk with "Error in Protection" cause (specified in ) and extra cause "Timeout During Protection Handshake or Validation" identifier 0x03 to indicate this failure. To indicate in which phase the timeout occurred an additional extra cause code is added. If the protection engine specifies that key management is implemented inband and the T-valid timeout occurs during the handshake the Cause-Specific code to add is "Error During Protection Handshake" identifier 0x01. If the T-valid timeout occurs during the protection association parameter validation, the extra cause code to use is "Failure in Protection Engines Validation" identifier 0x02.

Critical Error from Protection Engine Protection engine MAY inform local SCTP endpoint about errors, in such case it's to be defined in the protection engine specification document. When an Error in the protection engine compromises the protection mechanism, the protection engine may stop processing data altogether, thus the local SCTP endpoint will not be able to send or receive any chunk for the specified Association. This will cause the Association to be closed by legacy timer-based mechanism. Since the Association protection is compromised no further data will be sent and the remote peer will also experience timeout on the Association.

Non-critical Error in the Protection Engine A non-critical error in the protection engine means that the protection engine is capable of recovering without the need of the whole Association to be restarted. From SCTP perspective, a non-critical error will be perceived as a temporary problem in the transport and will be handled with retransmissions and SACKS according to . When the protection engine will experience a non-critical error, an ABORT chunk SHALL NOT be sent. This way non-critical errors are handled and how the protection engine will recover from these errors is being described in the Protection Engine Specifications.

Protected SCTP State Diagram The shows the changes of the SCTP association state machine as described in section 4.

SCTP State Diagram with Crypto

New States This section describes details on the amendment to the SCTP association establishment state machine.

PROTECTION PENDING The presence of a Protected Association Parameter in the INIT or INIT-ACK chunk makes the State Machine entering PROTECTION PENDING state instead of ESTABLISHED. When entering PROTECTION PENDING state, a T-valid timer is started that will cover the whole validation time including the in-band key establishment. The value of T-valid is dependent on the protection engine and may also be further adjusted based if expected RTT values are outside of the ones commonly occurring on the general Internet, see . If key establishment is in-band, the protection engine will start the handshake with its peer and in case of failure or T-valid timeout, the Crypto Chunk will generate an ABORT chunk. The ERROR handling follows what specified in . When Handshake has been successfully completed, the association state machine will enter PROTECTED state. The protection engine specification MUST specify when PROTECTED state can be entered for each endpoint. If key establishment is out-of-band, after starting T-valid timer the SCTP association will enter PROTECTED state per protection engine specification when the necessary security context is in place. Whilst in PROTECTION PENDING state, only User Layer Protocol data belonging to the Protection Engine will be handled, such data will be transferred as SCTP DATA chunks with the Protection Engine PPID (see ) for the Protection Engine handshake.

PROTECTED The association state machine can only reach PROTECTED state from PROTECTION PENDING state (see ). When entering into PROTECTED state the T-valid timer is running and the protection engine has completed the key establishment so that protected data can be sent to the peer. Whilst in PROTECTION PENDING state, only User Layer Protocol data belonging to the Protection Engine will be handled, such data will be transferred as SCTP DATA chunks with the Protection Engine PPID (see ) for the Protection Engine handshake. In PROTECTED state the association initiating SCTP Endpoint (initiator) MUST validate the INIT sent protected association parameter, thus the initiator will send a PVALID chunk that will contain exactly the same list of Protection Engines as previously sent in protected association parameter of INIT chunk and in the same order. When the responder will receive PVALID, it will compare the list of protection engines with the list received in the INIT chunk, if they are identical it will reply to the initiator with a PVALID chunk containing the Protection Engine previously sent as protected association parameter in INIT-ACK chunk, it will clear the T-valid timer and will move into ESTABLISHED state. Once in ESTABLISHED state, only CRYPTO chunks can be sent to the remote peer and any other type of plain text SCTP chunks coming from the remote peer will be silently discarded with the exception of SHUTDOWN-COMPLETE chunk. If the lists of Protection Engines don't match, it will generate an ABORT chunk. ERROR CAUSE will indicate "Failure in Protection Engines Validation" and the SCTP association will be terminated. After sending PVALID, the initiator will wait for the responder to reply with the PVALID confirmation. The initiator will compare the Protection Engine received from the responder, if the value is the same it will clear the T-valid timer and move into ESTABLISHED state. If the chosen Protection Engines don't match, it will generate an ABORT chunk. ERROR CAUSE will indicate "Failure in Protection Engines Validation" that is critical. If T-valid timer expires either at initiator or responder, it will generate an ABORT chunk. The ERROR handling follows what specified in .

Considerations on key management When the Association is in PROTECTION PENDING state, in-band key management shall exploit SCTP DATA chunk with the Protection Engine PPID (see ) that will be sent unencrypted. When the Association is in PROTECTED, ESTABLISHED or in any of the states that can be reached after ESTABLISHED state, in-band key management shall exploit SCTP DATA chunk that will be protected by the Protection Engine and encapsulated in CRYPTO chunks. In-band key management shall use a dedicated Payload Protocol Identifier assigned by IANA and defined in the specific Protection Engine Specification.

Consideration on T-valid The timer T-Valid supervises initializations that depend on how the handshake is specified for the Protection Engine and also on the characteristics of the transport network. This specification recommends a default value of 30 seconds for T-valid. This value is expected to be superseded by recommendations in the Protection Engine Specification for each Protection Engine.

Procedures

Establishment of a Protected Association An SCTP Endpoint acting as initiator willing to create a protected association shall send to the remote peer an INIT chunk containing the Protected Association parameter (see ) where all the supported Protection Engines are listed, given in descending order of preference (see ). As alternative, an SCTP Endpoint acting as responder willing to support only protected associations shall consider INIT chunk not containing the Protected Association parameter as an error, thus it will reply with an ABORT chunk according to what specified in indicating that for this endpoint mandatory protected association parameter is missing. An SCTP Endpoint acting as responder, when receiving an INIT chunk with protected association parameter, will search the list of protection engines for the most preferred commonly supported choice and will reply with INIT-ACK containing the protected association parameter with the chosen protection engine. When the responder cannot find a supported protection engine, it will reply with ABORT containing Error in Protection with the extra cause code for "No Supported Protection Engine" (). When initiator and responder have agreed on a protected association by means of handshaking INIT/INIT-ACK with a common protection engine, only control chunks, CRYPTO chunks and DATA Chunks with the Protection Engine PPID will be accepted. Any other DATA chunk being sent on a Protected association will be silently discarded. After completion of initial handshake, that is after COOKIE-ECHO and COOKIE-ACK, the Protection Engine shall initialize itself by transferring its own data as payload of the DATA chunk if necessary. At completion of Protection Engine initialization, the setup of the Protected association is complete and from that time on only CRYPTO chunks will be exchanged. Any plain text chunks will be silently discarded. After completion of protected association initialization, the initiator MUST send to the responder a PVALID chunk (see ) containing the list of Protection Engines previously sent in the protected association parameter of the INIT chunk. The responder receiving the PVALID chunk will compare the Protection Engines list with the one previously received in the INIT chunk, if they are exactly the same, with the same Protection engine in the same position, it will reply to the initiator with a PVALID chunk containing the chosen Protection Engine, otherwise it will reply with an ABORT chunk. If the association was not aborted the protected association is considered successfully established. When the initiator receive the PVALID chunk, it will compare with the previous chosen Protection Engine and in case of mismatch with the one received previously in the protected association parameter in the INIT-ACK chunk, it will reply with ABORT, otherwise the protected association is successfully established.

Termination of a Protected Association Besides the procedures for terminating an association explained in , the protection engine SHALL ask SCTP endpoint for terminating an association when having an internal error or by detecting a security violation, using the procedure described in . The internal design of Protection Engines and their capability is out of the scope of the current document.

Protected Data Chunk Handling With reference to the State Diagram as shown in and Figure 3 of , the handling of Control chunks, Data chunks and Crypto chunks follows the rules defined below:

When the association is in states CLOSED, COOKIE-WAIT, and COOKIE-ECHOED, any Control chunk is sent unprotected (i.e. plain text). No DATA chunks shall be sent in these states and DATA chunks received shall be silently discarded.
When the association is in state PROTECTION PENDING, any Control chunk is sent unprotected (i.e. plain text). No DATA chunks, except than the one used by the Protection Engine for handshake, thus being identified with the Protection Engine PPID (see ), SHALL be sent in these states and DATA chunks with different PPID being received shall be silently discarded. DATA Chunks with the Protection Engine PPID (see ) shall be sent by the Protection Engine to establish its security context.
When the association is in state PROTECTED, any SCTP chunk except for CRYPTO chunks, will be used to create an SCTP payload that will be encrypted by the Protection Engine and the result from that encryption will be the used as payload for a CRYPTO chunk that will be the only chunk in the SCTP packet to be sent. DATA chunks received with PPID different than Protection engine PPID shall be silently discarded. Note that User Data in PROTECTED state are not used for creating DATA chunks, the only DATA chunk being handled are the ones with Protection Engine PPID (see ) related to Protection Engine handshake.
When the association is in states ESTABLISHED and in the states for association shutdown, i.e. SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED, SHUTDOWN-ACK-SENT as defined by , any SCTP chunk including DATA chunks, but excluding CRYPTO chunk, will be used to create an SCTP payload that will be encrypted by the Protection Engine and the result from that encryption will be the used as payload for a CRYPTO chunk that will be the only chunk in the SCTP packet to be sent. DATA chunks are accepted and handled according to section 4 of .

Plain Text SCTP Packet The diagram shown in describes the structure of any plain text SCTP packet being sent or received when the association has not reached the PROTECTED state yet. SCTP packet as depicted in may also be sent in PROTECTION PENDING state and in any later state of the association.

Protected SCTP Packets The diagram shown in describes the structure of an SCTP packet being sent after the PROTECTED state has been reached. Such packets are built with the SCTP common header. Only one CRYPTO chunk can be sent in a SCTP packet.

Protected Data Chunk Transmission When the association state machine (see ) has reached the PROTECTION PENDING state, it MAY perform protection engine key management inband depending on how the specification for the chosen Protection Engine has been defined. In such case, the CRYPTO chunk Handler will receive plain control and DATA chunks from the SCTP chunk handler. When the association state machine (see ) has reached the PROTECTED state, the CRYPTO chunk handler will receive control chunks and DATA chunks from the SCTP chunk handler as a complete SCTP payload with maximum size limited by PMTU reduced by the size of the SCTP common header and the CRYPTO chunk overhead. That plain payload will be sent to the protection engine in use for that specific association, the protection engine will return an encrypted payload. Depending on the specification for the chosen protection engine, when forming the CRYPTO chunk header the CRYPTO chunk handler MAY set the chunk header flags (see ). An SCTP packet containing an SCTP CRYPTO chunk SHALL be delivered without delay and SCTP bundling SHALL NOT be performed.

Protected Data Chunk Reception When the association state machine (see ) has reached the PROTECTION PENDING state, it MAY handle key management inband depending on how the specification for the chosen protection engine has been defined. In such case, the CRYPTO chunk handler will receive plain control chunks and DATA chunks with Protection Engine PPID from the SCTP Header Handler. Those plain control chunks will be forwarded to SCTP chunk handler. When the association state machine (see ) has reached the PROTECTED state, the CRYPTO chunk handler will receive CRYPTO chunks from the SCTP Header Handler. Payload from CRYPTO chunks will be forwarded to the protection engine in use for that specific association for decryption, the protection engine will return a plain SCTP Payload. The plain SCTP payload will be forwarded to SCTP Chunk Handler that will split it in separated chunks and will handle them according to . Depending on the specification for the chosen protection engine, when receiving the CRYPTO chunk header the CRYPTO Chunk Handler MAY handle the Flags (see ) according to that specification. Meta data belonging to the SCTP packet received SHALL be tied to the relevant chunks and forwarded transparently to the SCTP endpoint.

SCTP Header Handler The SCTP Header Handler is responsible for correctness of the SCTP common header, it receives the SCTP packet from the lower transport layer, discriminates among associations and forwards the payload and relevant data to the SCTP protection engine for handling. In the opposite direction it creates the SCTP common header and fills it with the relevant information for the specific association and delivers it towards the lower transport layer.

IANA Considerations This document defines two new registries in the Stream Control Transmission Protocol (SCTP) Parameters group that IANA maintains. Theses registries are for the protection engine identifiers and extra cause codes for protection related errors. It also adds registry entries into several other registries in the Stream Control Transmission Protocol (SCTP) Parameters group:

Two new SCTP Chunk Types
One new SCTP Chunk Parameter Type
One new SCTP Error Cause Codes
One new SCTP Payload Protocol Identifier

Protection Engine Identifier Registry IANA is requested to create a new registry called "CRYPTO Chunk Protection Engine Identifiers". This registry is part of the Stream Control Transmission Protocol (SCTP) Parameters grouping. The purpose of this registry is to enable identification of different protection engines used by the CRYPTO chunk when performing the SCTP handshake and negotiating support. Entries in the registry requires a protection engine name, a reference to the specification for the protection engine, and a contact. Each entry will be assigned by IANA a unique 16-bit unsigned integer identifier for their protection engine. Values 0-65534 are available for assignment. Value 65535 is reserved for future extension. The proposed general form of the registry is depicted below in . Protection Engine Identifier Registry

ID Value	Name	Reference	Contact
0-65534	Available for Assignment	RFC-To-Be
65535	Reserved	RFC-To-Be	Authors

New entries are registered following the Specification Required policy as defined by .

Protection Error Cause Codes Registry IANA is requested to create a new registry called "Protection Error Cause Codes". This registry is part of the Stream Control Transmission Protocol (SCTP) Parameters grouping. The purpose of this registry is to enable identification of different protection related errors when using CRYPTO chunk and a protection engine. Entries in the registry requires a Meaning, a reference to the specification defining the error, and a contact. Each entry will be assigned by IANA a unique 16-bit unsigned integer identifier for their protection engine. Values 0-65534 are available for assignment. Value 65535 is reserved for future extension. The proposed general form of the registry is depicted below in . Protection Error Cause Code

Cause Code	Meaning	Reference	Contact
0	Error in the Protection Engine List	RFC-To-Be	Authors
1	Error During Protection Handshake	RFC-To-Be	Authors
2	Failure in Protection Engines Validation	RFC-To-Be	Authors
3	Timeout During KEY Handshake or Validation	RFC-To-Be	Authors
4-65534	Available for Assignment	RFC-To-Be	Authors
65535	Reserved	RFC-To-Be	Authors

New entries are registered following the Specification Required policy as defined by .

SCTP Chunk Types In the Stream Control Transmission Protocol (SCTP) Parameters group's "Chunk Types" registry, IANA is requested to add the two new entries depicted below in in with a reference to this document. The registry at time of writing was available at: https://www.iana.org/assignments/sctp-parameters/sctp-parameters.xhtml#sctp-parameters-1 New Chunk Types Registered

ID Value	Chunk Type	Reference
TBA6	Crypto Chunk (CRYPTO)	RFC-To-Be
TBA7	Protected Association Parameter Validation (PVALID)	RFC-To-Be

SCTP Chunk Parameter Types In the Stream Control Transmission Protocol (SCTP) Parameters group's "Chunk Parameter Types" registry, IANA is requested to add the new entry depicted below in in with a reference to this document. The registry at time of writing was available at: https://www.iana.org/assignments/sctp-parameters/sctp-parameters.xhtml#sctp-parameters-2 New Chunk Type Parameters Registered

ID Value	Chunk Parameter Type	Reference
TBA8	Protected Association	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 add the new entry depicted below in in with a reference to this document. The registry at time of writing was available at: https://www.iana.org/assignments/sctp-parameters/sctp-parameters.xhtml#sctp-parameters-24 Error Cause Codes Parameters Registered

ID Value	Error Cause Codes	Reference
TBA9	Protection Engine Error	RFC-To-Be

SCTP Payload Protocol Identifier In the Stream Control Transmission Protocol (SCTP) Parameters group's "Payload Protocol Identifiers" registry, IANA is requested to add the new entry depicted below in in with a reference to this document. The registry at time of writing was available at: https://www.iana.org/assignments/sctp-parameters/sctp-parameters.xhtml#sctp-parameters-25 Protection Engine Protocol Identifier Registered

ID Value	SCTP Payload Protocol Identifier	Reference
TBA10	Protection Engine Protocol Identifier	RFC-To-Be

Security Considerations All the security and privacy considerations of the security protocol used as the protection engine applies.

Privacy Considerations Using a security protocol in the SCTP CRYPTO chunk might lower the privacy properties of the security protocol as the SCTP Verification Tag is an unique identifier for the association.

Downgrade Attacks The CRYPTO chunk provides a mechanism for preventing downgrade attacks that detects downgrading attempts between protection engines and terminates the association. The chosen protection engine is the same as if the peers had been communicating in the absence of an attacker. The protection engine initial handshake is verified before the association is set as ESTABLISHED, thus no user data are sent before validation. The downgrade protection is only as strong as the weakest of the supported protection engines as an active attacker can trick the endpoints to negotiate the weakest protection engine and then modify the weakly protected CRYPTO chunks to deceive the endpoints that the negotiation of the protection engines is validated. This is similar to the downgrade protection in TLS 1.3 specified in Section 4.1.3. of where downgrade protection is not provided when TLS 1.2 with static RSA is used. It is RECOMMENDED to only support a limited set of strongly profiled protection engines.

Requirements towards the protection engines This section specifies what is to be specified in the description of a protection engine.

Define how to protect the plain text set of chunks and encapsulate them in the CRYPTO Chunk payload.
Can define its usage of the 8-bit chunk Flags field in the CRYPTO chunk
Is required to register the defined protection engine(s) with IANA per .
Detail the state transition between PROTECTION PENDING and PROTECTED state (see ).

Acknowledgments The authors thank Michael Tüxen for his invaluable comments helping to cope with Association Restart, ASCONF handling and restructuring the Protection Engine architecture.