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RFC 8052

Group Domain of Interpretation (GDOI) Protocol Support for IEC 62351 Security Services

Pages: 25
Proposed Standard

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Internet Engineering Task Force (IETF)                           B. Weis
Request for Comments: 8052                                    M. Seewald
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                                  H. Falk
                                                                   SISCO
                                                               June 2017


             Group Domain of Interpretation (GDOI) Protocol
                Support for IEC 62351 Security Services

Abstract

The IEC 61850 power utility automation family of standards describes methods using Ethernet and IP for distributing control and data frames within and between substations. The IEC 61850-90-5 and IEC 62351-9 standards specify the use of the Group Domain of Interpretation (GDOI) protocol (RFC 6407) to distribute security transforms for some IEC 61850 security protocols. This memo defines GDOI payloads to support those security protocols. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8052.
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Copyright Notice

   Copyright (c) 2017 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
   (http://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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . 4 2. IEC 61850 Protocol Information . . . . . . . . . . . . . . . 5 2.1. ID Payload . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. SA TEK Payload . . . . . . . . . . . . . . . . . . . . . 6 2.3. KD Payload . . . . . . . . . . . . . . . . . . . . . . . 11 3. Security Considerations . . . . . . . . . . . . . . . . . . . 12 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.1. Normative References . . . . . . . . . . . . . . . . . . 16 5.2. Informative References . . . . . . . . . . . . . . . . . 16 Appendix A. Example ID, SA TEK, and KD Payloads for IEC 61850 . 19 Appendix B. Implementation Considerations . . . . . . . . . . . 23 B.1. DER Length Fields . . . . . . . . . . . . . . . . . . . . 23 B.2. Groups with Multiple Senders . . . . . . . . . . . . . . 23 Appendix C. Data Attribute Format . . . . . . . . . . . . . . . 23 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25

1. Introduction

Power substations use Generic Object Oriented Substation Events (GOOSE) protocol [IEC-61850-8-1] to distribute control information to groups of devices using a multicast strategy. Sources within the power substations also distribute IEC 61850-9-2 sampled values data streams [IEC-61850-9-2]. The IEC 62351-9 standard [IEC-62351-9] describes key management methods for the security methods protecting these IEC 61850 messages, including methods of device authentication and authorization, and methods of policy and keying material
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   agreement for IEC 61850 message encryption and data integrity
   protection.  These key management methods include the use of GDOI
   [RFC6407] to distribute the security policy and session keying
   material used to protect IEC 61850 messages when the messages are
   sent to a group of devices.

   The protection of the messages is defined in IEC 62351-6
   [IEC-62351-6], IEC 61850-8-1 [IEC-61850-8-1], and IEC 61850-9-2
   [IEC-61850-9-2].  Protected IEC 61850 messages typically include the
   output of a Message Authentication Code (MAC) and may also be
   encrypted using a symmetric cipher such as the Advanced Encryption
   Standard (AES).

   Section 5.5.2 of RFC 6407 specifies that the following information
   needs to be provided in order to fully define a new security
   protocol:

   o  The Protocol-ID for the particular security protocol

   o  The SPI Size

   o  The method of SPI generation

   o  The transforms, attributes, and keys needed by the security
      protocol

   This document defines GDOI payloads to distribute policy and keying
   material to protect IEC 61850 messages and defines the necessary
   information to ensure interoperability between IEC 61850
   implementations.

   This memo extends RFC 6407 in order to define extensions needed by
   IEC 62351-9.  With the current IANA registry rules set up by RFC
   6407, this requires "Standards Action" [RFC5226] by the IETF; this
   document satisfies that requirement.  As the relevant IEC
   specifications are not available to the IETF community, it is not
   possible for this RFC to fully describe the security considerations
   that apply.  Therefore, implementers need to depend on the security
   analysis within the IEC specifications.  As two different Standards
   Development Organizations are involved here, and since group key
   management is inherently complex, it is possible that some security
   issues have not been identified, so additional analysis of the
   security of the combined set of specifications may be advisable.
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1.1. Requirements Language

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.

1.2. Terminology

The following key terms are used throughout this document: Generic Object Oriented Substation Events: Power substation control model defined as per IEC 61850. IEC 61850 message: A message in the IEC 61850 family of protocols carrying control or data frames between substation devices.

1.3. Acronyms

The following acronyms are used throughout this document: AES Advanced Encryption Standard GCKS Group Controller/Key Server GDOI Group Domain of Interpretation GM Group Member GOOSE Generic Object Oriented Substation Events KD Key Download KEK Key Encryption Key MAC Message Authentication Code SA Security Association SPI Security Parameter Index TEK Traffic Encryption Key
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2. IEC 61850 Protocol Information

The following subsections describe the GDOI payload extensions that are needed in order to distribute security policy and keying material for the IEC 62351 Security Services. The Identification (ID) Payload is used to describe an IEC 62351 GDOI group. The Security Association (SA) Traffic Encryption Key (TEK) payload is used to describe the policy defined by a Group Controller/Key Server (GCKS) for a particular IEC 62351 traffic selector. No changes are required to the Key Download (KD) Payload, but a mapping of IEC 62351 keys to the KD payload key types is included. All multi-octet fields are in network byte order.

2.1. ID Payload

The ID payload in a GDOI GROUPKEY-PULL exchange allows the Group Member (GM) to declare the group it would like to join. A group is defined by an ID payload as defined in GDOI [RFC6407] and reproduced in Figure 1. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! Next Payload ! RESERVED ! Payload Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! ID Type ! DOI-Specific ID Data = 0 ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ~ Identification Data ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! Figure 1: RFC 6407 Identification Payload An ID Type name of ID_OID (value 13) is defined in this memo to specify an Object Identifier (OID) [ITU-T-X.683] encoded using Distinguished Encoding Rules (DER) [ITU-T-X.690]. Associated with the OID may be an OID-Specific Payload DER encoded as further defining the group. Several OIDs are specified in [IEC-62351-9] for use with IEC 61850. Each OID represents a GOOSE or Sampled Value protocol, and in some cases IEC 61850 also specifies a particular multicast destination address to be described in the OID-Specific Payload field. The format of the ID_OID Identification Data is specified as shown in Figure 2.
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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID Length   !                       OID                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID-Specific Payload Length  !     OID-Specific Payload      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                   Figure 2: ID_OID Identification Data

   The ID_OID Identification Data fields are defined as follows:

   o  OID Length (1 octet) -- Length of the OID field.

   o  OID (variable) -- An ASN.1 ObjectIdentifier encoded using DER
      [ITU-T-X.690].

   o  OID-Specific Payload Length (2 octets) -- Length of the OID-
      Specific payload.  Set to zero if the OID does not require an OID-
      Specific payload.

   o  OID-Specific Payload (variable) -- OID-specific selector encoded
      in DER.  If OID-Specific Payload Length is set to zero, this field
      does not appear in the ID payload.

2.2. SA TEK Payload

The SA TEK payload contains security attributes for a single set of policy associated with a group TEK. The type of policy to be used with the TEK is described by a Protocol-ID field included in the SA TEK. As shown in Figure 3 reproduced from RFC 6407, each Protocol-ID describes a particular TEK Protocol-Specific Payload definition. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! Next Payload ! RESERVED ! Payload Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! Protocol-ID ! TEK Protocol-Specific Payload ~ +-+-+-+-+-+-+-+-+ ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! Figure 3: RFC 6407 SA TEK Payload The Protocol-ID name of GDOI_PROTO_IEC_61850 (value 3) is defined in this memo for the purposes of distributing IEC 61850 policy. A GDOI_PROTO_IEC_61850 SA TEK includes an OID and (optionally) an OID-
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   Specific payload that together define the selectors for the network
   traffic.  The selector fields are followed by security policy fields
   indicating how the specified traffic is to be protected.  The
   GDOI_PROTO_IEC_61850 TEK Protocol-Specific Payload is defined as
   shown in Figure 4.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID Length   !                       OID                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  OID-Specific Payload Length  !     OID-Specific Payload      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                              SPI                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !           Auth Alg            !            Enc Alg            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                    Remaining Lifetime Value                   !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                      SA Data Attributes                       ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                    Figure 4: IEC 61850 SA TEK Payload

   The GDOI_PROTO_IEC_61850 SA TEK payload fields are defined as
   follows:

   o  OID Length (1 octet) -- Length of the OID field.

   o  OID (variable) -- An ASN.1 ObjectIdentifier encoded using DER.
      OIDs defined in IEC 61850 declare the type of IEC 61850 message to
      be protected, as defined by [IEC-62351-9].

   o  OID-Specific Payload Length (2 octets) -- Length of the OID-
      Specific payload.  This field is set to zero if the policy does
      not include an OID-Specific payload.

   o  OID-Specific Payload (variable) -- The traffic selector (e.g.,
      multicast address) specific to the OID encoded using DER.  Some
      OID policy settings do not require the use of an OID-Specific
      payload, in which case this field is not included in the TEK and
      the OID-Specific Payload Length is set to zero.

   o  SPI (4 octets) -- Identifier for the Current Key. This field
      represents an SPI.

   o  Auth Alg (2 octets) -- Authentication Algorithm ID.  Valid values
      are defined in Section 2.2.2.
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   o  Enc Alg (2 octets) -- Confidentiality Algorithm ID.  Valid values
      are defined in Section 2.2.3.

   o  Remaining Lifetime value (4 octets) -- The number of seconds
      remaining before this TEK expires.  A value of zero (0) shall
      indicate that the TEK does not have an expire time.

   o  SA Data Attributes (variable length) -- Contains zero or more
      attributes associated with this SA.  Section 2.2.4 defines
      attributes.

2.2.1. Selectors

The OID and (optionally) an OID-Specific payload together define the selectors for the network traffic. While they may match the OID and OID-Specific payload that the GM had previously requested in the ID payload, there is no guarantee that this will be the case. Including selectors in the SA TEK is important for at least the following reasons: o The Key Server (KS) policy may direct the KS to return multiple TEKs, each representing different traffic selectors, and it is important that every GM receiving the set of TEKs explicitly identify the traffic selectors associated with the TEK. o The KS policy may include the use of a GDOI GROUPKEY-PUSH message, which distributes new or replacement TEKs to group members. Since the GROUPKEY-PUSH message does not contain an ID payload, the TEK definition must include the traffic selectors.

2.2.2. Authentication Algorithms

This memo defines the following authentication algorithms for use with this TEK. These algorithms are defined in [IEC-TR-61850-90-5], including requirements on one or more algorithms defined as mandatory to implement. o NONE. Specifies that an authentication algorithm is not required, or when the accompanying confidentiality algorithm includes authentication (e.g., AES-GCM-128). See Section 3 for cautionary notes regarding using this value without any confidentiality algorithm. o HMAC-SHA256-128. Specifies the use of SHA-256 [FIPS180-4] combined with HMAC [RFC2104]. The output is truncated to 128 bits, as per [RFC2104]. The key size is the size of the hash value produced by SHA-256 (256 bits).
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   o  HMAC-SHA256.  Specifies the use of SHA-256 [FIPS180-4] combined
      with HMAC [RFC2104].  The key size is the size of the hash value
      produced by SHA-256 (256 bits).

   o  AES-GMAC-128.  Specifies the use of AES [FIPS197] in the Galois
      Message Authentication Code (GMAC) mode [SP.800-38D] with a
      128-bit key size.

   o  AES-GMAC-256.  Specifies the use of AES [FIPS197] in the Galois
      Message Authentication Code (GMAC) mode [SP.800-38D] with a
      256-bit key size.

2.2.3. Confidentiality Algorithms

This memo defines the following confidentiality algorithms for use with this TEK. These algorithms are defined in [IEC-TR-61850-90-5], including requirements on one or more algorithms defined as mandatory to implement. o NONE. Specifies that confidentiality is not required. Note: See Section 3 for guidance on cautionary notes regarding using this value. o AES-CBC-128. Specifies the use of AES [FIPS197] in the Cipher Block Chaining (CBC) mode [SP.800-38A] with a 128-bit key size. This encryption algorithm does not provide authentication and MUST NOT be used with the NONE authentication algorithm. o AES-CBC-256. Specifies the use of AES [FIPS197] in the Cipher Block Chaining (CBC) mode [SP.800-38A] with a 256-bit key size. This encryption algorithm does not provide authentication and MUST NOT be used with the NONE authentication algorithm. o AES-GCM-128. Specifies the use of AES [FIPS197] in the Galois/ Counter Mode (GCM) mode [SP.800-38D] with a 128-bit key size. This encryption algorithm provides authentication and is used with a NONE authentication algorithm. o AES-GCM-256. Specifies the use of AES [FIPS197] in the Galois/ Counter Mode (GCM) mode [SP.800-38D] with a 256-bit key size. This encryption algorithm provides authentication and is used with a NONE authentication algorithm.
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2.2.4. SA Attributes

The following attributes may be present in an SA TEK. The attributes must follow the format described in Appendix C).
2.2.4.1. SA Time Activation Delay (SA_ATD)
A GCKS will sometimes distribute an SA TEK in advance of when it is expected to be used. This is communicated to group members using the SA Activation Time Delay (SA_ATD) attribute. When a GM receives an SA_TEK with this attribute, it waits for the number of seconds contained within the attribute before installing it for either transmitting or receiving. This Activation Time Delay attribute applies only this SA, and MAY be used in either a GROUPKEY-PULL or GROUPKEY-PUSH exchange. RFC 6407 also describes an ACTIVATION_TIME_DELAY attribute for the Group Associated Policy (GAP) payload, which is applied to all Security Associations and is restricted to use in a GROUPKEY-PUSH message. If both attributes are included in a GROUPKEY-PUSH payload, the value contained in SA_ATD will be used.
2.2.4.2. Key Delivery Assurance (SA_KDA)
Group policy can include notifying a multicast source ("Publisher") of an indication of whether multicast receivers ("Subscribers") have previously received the SA TEK. This notification allows a Publisher to set a policy as to whether to activate the new SA TEK or not based on the percentage of Subscribers that are able to receive packets protected by the SA TEK. The attribute value is a number between 0 and 100 (inclusive).

2.2.5. SPI Discussion

As noted in Section 1, RFC 6407 requires that characteristics of an SPI must be defined. An SPI in a GDOI_PROTO_IEC_61850 SA TEK is represented as a Key Identifier (KeyID). The SPI size is 4 octets. The SPI is unilaterally chosen by the GCKS using any method chosen by the implementation. However, an implementation needs to take care not to duplicate an SPI value that is currently in use for a particular group.
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2.3. KD Payload

The KD payload contains group keys for the policy specified in the SA Payload. It is comprised of a set of Key Packets, each of which hold the keying material associated with an SPI (i.e., an IEC 61850 Key Identifier). The RFC 6407 KD payload format is reproduced in Figure 5. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! Next Payload ! RESERVED ! Payload Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! Number of Key Packets ! RESERVED2 ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ~ Key Packets ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! Figure 5: KD Payload Each Key Packet holds the keying material associated with a particular IEC 61850 Key Identifier, although GDOI refers to it as an SPI. The keying material is described in a set of attributes indicating an encryption key, integrity key, etc., in accordance with the security policy of the group as defined by the associated SA Payload. Each Key Packet has the following format, reproduced in Figure 6. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! KD Type ! RESERVED ! Key Packet Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! SPI Size ! SPI (variable) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ~ Key Packet Attributes ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! Figure 6: Key Packet No changes are needed to GDOI in order to distribute IEC 61850 keying material, but the keys MUST be distributed as defined in Section 5.6 of RFC 6407. The KD Type MUST be TEK (1). A key associated with an IEC 61850 authentication algorithm (distributed in the Auth Alg field) MUST be distributed as a TEK_INTEGRITY_KEY attribute. The value of the attribute is interpreted according to the type of key distributed in the SA TEK:
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   o  HMAC-SHA256-128, HMAC-SHA256.  The value is 32 octets.

   o  AES-GMAC-128.  The value is 20 octets.  The first 16 octets are
      the 128-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

   o  AES-GMAC-256.  The value is 36 octets.  The first 32 octets are
      the 256-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

   A key associated with an IEC 61850 confidentiality algorithm
   (distributed in the Enc Alg SA TEK field) MUST be distributed as a
   TEK_ALGORITHM_KEY attribute.  The value of the attribute is
   interpreted according to the type of key distributed in the SA TEK:

   o  AES-CBC-128.  The value is 16 octets.

   o  AES-CBC-256.  The value is 32 octets.

   o  AES-GCM-128.  The value is 20 octets.  The first 16 octets are the
      128-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

   o  AES-GCM-256.  The value is 36 octets.  The first 32 octets are the
      256-bit AES key, and the remaining four octets are used as the
      salt value in the nonce.

3. Security Considerations

GDOI is a Security Association (SA) management protocol for groups of senders and receivers. This protocol performs authentication of communicating protocol participants (Group Member, Group Controller/ Key Server). GDOI provides confidentiality of key management messages, and it provides source authentication of those messages. GDOI includes defenses against man-in-middle, connection-hijacking, replay, reflection, and denial-of-service (DOS) attacks on unsecured networks. GDOI assumes that the network is not secure and may be under the complete control of an attacker. The Security Considerations described in RFC 6407 are relevant to the distribution of GOOSE and sampled values policy as defined in this memo. Message Authentication is an optional property for IEC 62351 Security Services; however, when encryption is used, authentication MUST also be provided by using an authenticated encryption algorithm such as AES-GCM-128 or by using a specific authentication algorithm such as HMAC-SHA-256. Setting the authentication algorithm to NONE but setting the confidentiality algorithm to an algorithm that does not
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   include authentication (i.e., is marked with an N in the
   "Authenticated Encryption" column of the "IEC 62351-9 Confidentiality
   Values" registry) is not safe and MUST NOT be done.

   When Message Authentication is used, a common practice is to truncate
   the output of a MAC and include some of the bits in the integrity
   protection field of the data security transform.  Current guidance in
   [RFC2104] is to truncate no less than half of the length of the hash
   output.  The authentication algorithm HMAC-SHA256-128 defined in this
   memo truncates the output to exactly half of the output, which
   follows this guidance.

   Confidentiality is an optional security property for IEC 62351
   Security Services.  Confidentiality Algorithm IDs SHOULD be included
   in the IEC 61850 SA TEK payload if the IEC 61850 messages are
   expected to traverse public network links and are not protected by
   another level of encryption (e.g., an encrypted Virtual Private
   Network).  Current cryptographic advice indicates that the use of
   AES-CBC-128 for confidentiality is sufficient for the foreseeable
   future [SP.800-131A], but some security policies may require the use
   of AES-CBC-256.

   IEC 62351 Security Services describe a variety of policy choices for
   protecting network traffic, including the option of specifying no
   protection at all.  This is enabled with the use of NONE as an
   authentication algorithm and/or confidentiality algorithm.  The
   following guidance is given regarding the use of NONE.

   o  Setting both the authentication algorithm and confidentiality
      algorithm to NONE is possible but NOT RECOMMENDED.  Setting such a
      policy is sometimes necessary during a migration period, when
      traffic is being protected incrementally and some traffic has not
      yet been scheduled for protection.  Alternatively, site security
      policy for some packet flows requires inspection of packet data on
      the private network followed by network-layer encryption before
      delivery to a public network.

   o  Setting the confidentiality algorithm to NONE but setting the
      authentication algorithm to a MAC can be an acceptable policy in
      the following conditions: the disclosed information in the data
      packets is comprised of raw data values and the disclosure of the
      data files is believed to be of no more value to an observer than
      traffic analysis on the frequency and size of packets protected
      for confidentiality.  Alternatively, site security policy for some
      packet flows requires inspection of packet data on the private
      network followed by network-layer encryption before delivery to a
      public network.
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   o  Setting the authentication algorithm to NONE but setting the
      confidentiality algorithm to an algorithm that does not include
      authentication is not safe and MUST NOT be done.

4. IANA Considerations

The "Group Domain of Interpretation (GDOI) Payloads" registry [GDOI-REG] has been updated as described below. The terms "Expert Review", "Reserved", and "Private Use" are used as defined in [RFC5226]. o GDOI_PROTO_IEC_61850 (value 3) has been added to the "SA TEK Payload Values - Protocol-ID" registry. o A new "IEC 62351-9 Authentication Values" registry has been created. This registry defines Auth Alg values. Initial values for the registry are given below; future assignments are to be made through "Expert Review" [RFC5226]. Name Value ---- ----- Reserved 0 NONE 1 HMAC-SHA256-128 2 HMAC-SHA256 3 AES-GMAC-128 4 AES-GMAC-256 5 Unassigned 6-61439 Reserved for Private Use 61440-65535 o A new "IEC 62351-9 Confidentiality Values" registry has been created. This registry defines Enc Alg values. Initial values for the registry are given below; future assignments are to be made through "Expert Review" [RFC5226]. Name Value Authenticated Encryption ---- ----- ------------------------ Reserved 0 NONE 1 AES-CBC-128 2 N AES-CBC-256 3 N AES-GCM-128 4 Y AES-GCM-256 5 Y Unassigned 6-61439 Reserved for Private Use 61440-65535
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   o  A new "GDOI SA TEK Attributes" registry has been created.  This
      registry defines SA TEK attributes.  Initial values for the
      registry are given below; future assignments are to be made
      through "Expert Review" [RFC5226].  In the table, attributes that
      are defined as Type/Value (TV) are marked as Basic (B); attributes
      that are defined as Type/Length/Value (TLV) are marked as Variable
      (V).

      Attribute                    Value           Type
      ---------                    -----           ----
      Reserved                       0
      SA_ATD                         1               V
      SA_KDA                         2               B
      Unassigned                  3-28671
      Reserved for Private Use   28672-32767

   o  A new "ID Types" registry has been created for the Identification
      Payload when the DOI is GDOI.  This registry is taken from the
      "IPSEC Identification Type" registry for the IPsec DOI
      [IPSEC-DOI-REG].  Values 1-12 are defined identically to the
      equivalent values in the "IPSEC Identification Type" registry.
      Value 13 (ID_OID) is defined in this memo.  Initial values for the
      registry are given below; future assignments are to be made
      through "Expert Review" [RFC5226].

      Name                          Value
      ----                          -----
      Reserved                        0
      ID_IPV4_ADDR                    1
      ID_FQDN                         2
      ID_USER_FQDN                    3
      ID_IPV4_ADDR_SUBNET             4
      ID_IPV6_ADDR                    5
      ID_IPV6_ADDR_SUBNET             6
      ID_IPV4_ADDR_RANGE              7
      ID_IPV6_ADDR_RANGE              8
      ID_DER_ASN1_DN                  9
      ID_DER_ASN1_GN                  10
      ID_KEY_ID                       11
      ID_LIST                         12
      ID_OID                          13
      Unassigned                   14-61439
      Reserved for Private Use   61440-65535
Top   ToC   RFC8052 - Page 16

5. References

5.1. Normative References

[IEC-62351-9] International Electrotechnical Commission, "Power systems management and associated information exchange - Data and communications security - Part 9: Cyber security key management for power system equipment", IEC 62351-9:2017, May 2017. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain of Interpretation", RFC 6407, DOI 10.17487/RFC6407, October 2011, <http://www.rfc-editor.org/info/rfc6407>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <http://www.rfc-editor.org/info/rfc8174>.

5.2. Informative References

[FIPS180-4] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-4, DOI 10.6028/NIST.FIPS.180-4, August 2015, <http://nvlpubs.nist.gov/nistpubs/FIPS/ NIST.FIPS.180-4.pdf>. [FIPS197] National Institute of Standards and Technology, "Advanced Encryption Standard (AES)", FIPS PUB 197, November 2001, <http://csrc.nist.gov/publications/fips/fips197/ fips-197.pdf>. [GDOI-REG] IANA, "Group Domain of Interpretation (GDOI) Payloads", <http://www.iana.org/assignments/gdoi-payloads>.
Top   ToC   RFC8052 - Page 17
   [IEC-61850-8-1]
              International Electrotechnical Commission, "Communication
              networks and systems for power utility automation - Part
              8-1: Specific communication service mapping (SCSM) -
              Mappings to MMS (ISO 9506-1 and ISO 9506-2) and to ISO/IEC
              8802-3", IEC 61850-8-1, June 2011.

   [IEC-61850-9-2]
              International Electrotechnical Commission, "Communication
              networks and systems for power utility automation - Part
              9-2: Specific communication service mapping (SCSM) -
              Sampled values over ISO/IEC 8802-3", IEC 61850-2,
              September 2011.

   [IEC-62351-6]
              International Electrotechnical Commission, "Power systems
              management and associated information exchange - Data and
              communications security - Part 6: Security for IEC 61850",
              IEC 62351-6, June 2007.

   [IEC-TR-61850-90-5]
              International Electrotechnical Commission, "Communication
              networks and systems for power utility automation - Part
              90-5: Use of IEC 61850 to transmit synchrophasor
              information according to IEEE C37.118", IEC TR 62351-90-5,
              May 2012.

   [IPSEC-DOI-REG]
              IANA, "'Magic Numbers' for ISAKMP Protocol",
              <http://www.iana.org/assignments/isakmp-registry>.

   [ITU-T-X.683]
              International Telecommunications Union, "Information
              technology - Abstract Syntax Notation One (ASN.1):
              Parameterization of ASN.1 specifications",
              ITU-T Recommendation X.683, August 2015,
              <https://www.itu.int/rec/T-REC-X.683-201508-I/en>.

   [ITU-T-X.690]
              International Telecommunications Union, "Information
              technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, August 2015,
              <https://www.itu.int/rec/T-REC-X.690-201508-I/en>.
Top   ToC   RFC8052 - Page 18
   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [SP.800-131A]
              Barker, E. and A. Roginsky, "Transitions: Recommendation
              for Transitioning the Use of Cryptographic Algorithms and
              Key Lengths", NIST Special Publication 800-131A,
              DOI 10.6028/NIST.SP.800-131Ar1, November 2015,
              <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-131Ar1.pdf>.

   [SP.800-38A]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Methods and Techniques", NIST Special
              Publication 800-38A, DOI 10.6028/NIST.SP.800-38A, December
              2001, <http://nvlpubs.nist.gov/nistpubs/Legacy/SP/
              nistspecialpublication800-38a.pdf>.

   [SP.800-38D]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Galois/Counter Mode (GCM) and GMAC", NIST
              Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D,
              November 2007,
              <http://nvlpubs.nist.gov/nistpubs/Legacy/SP/
              nistspecialpublication800-38d.pdf>.
Top   ToC   RFC8052 - Page 19

Appendix A. Example ID, SA TEK, and KD Payloads for IEC 61850

An Intelligent Electronic Device (IED) begins a GROUPKEY-PULL exchange and requests keys and security policy for 61850_UDP_ADDR_GOOSE (OID = 1.2.840.10070.61850.8.1.2 as defined in [IEC-61850-9-2]) and IP multicast address 233.252.0.1 encoded as specified in [IEC-61850-9-2]. OID and OID-Specific Payload protocol fields are variable-length fields. To improve readability, their representations in Figures 7 and 8 are "compressed", as indicated by a trailing "~" for these fields. Implementations should be aware that because these fields are variably sized, some payload fields may not be conveniently aligned on an even octet. Note: The actual DER for the OID-Specific Payload field is defined in [IEC-62351-6]. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! Next Payload ! RESERVED ! Payload Length ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! ID Type=13 ! DOI-Specific ID Data = 0 ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! OID Len=13 ! OID=<06 0B 2A 86 48 CE 56 83 E3 1A 08 01 02> ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! ! OID-Specific Payload Len ! OID SP=<DER for 233.252.0.1> ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-! Figure 7: Sample Identification Payload The Key Server responds with the following SA TEK payload including two GDOI_PROTO_IEC_61850 Protocol-Specific TEK payloads in the second GROUPKEY-PULL message. The first one is to be activated immediately and has a lifetime of 3600 seconds (0x0E10) remaining. The second has a lifetime of 12 hours (0xA8C0) and should be activated in 3300 seconds (0x0CE4), which gives a 5-minute (300-second) overlap of the two SAs.
Top   ToC   RFC8052 - Page 20
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Next Payload  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                             DOI = 2                           !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                         Situation = 0                         !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! SA Attr NP=16 (SA TEK)        !          RESERVED2            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! NP=16 (SA TEK)!   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Prot-ID=3     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID Len=13    ! OID=<06 0B 2A 86 48 CE 56 83 E3 1A 08 01 02>  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID-Specific Payload Len      !OID SP=<DER for 233.252.0.1>   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=1                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !  AuthAlg=1 (HMAC-SHA256-128)  !    EncAlg=2  (AES-CBC-128)    !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !              Remaining Lifetime=0x0E01                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! SA Attr NP=16 (SA TEK)        !          RESERVED2            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! NP=0          !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Prot-ID=3     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID Len=13    ! OID=<06 0B 2A 86 48 CE 56 83 E3 1A 08 01 02>  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! OID-Specific Payload Len      !OID SP=<DER for 233.252.0.1>   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=2                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !       AuthAlg=0 (NONE)        !    EncAlg=4 (AES-GCM-128)     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !              Remaining Lifetime=0xA8C0                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !       Type=1 (SA_ATD)         !           Length=4            !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                        Value=0x0CE4                           !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                   Figure 8: Sample IEC 61850 SA Payload
Top   ToC   RFC8052 - Page 21
   The IED acknowledges that it is capable and willing to use this
   policy in the third GROUPKEY-PULL message.  In response, the KS sends
   a KD payload to the requesting IED.  This concludes the GROUPKEY-PULL
   exchange.
Top   ToC   RFC8052 - Page 22
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Next Payload  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! Number of Key Packets=2       !            RESERVED2          !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   KD Type=1   !   RESERVED    !        Key Packet Length      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   SPI Size=4  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=1                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! TYPE=TEK_INTEGRITY_KEY (2)    ! LENGTH=32 (256-bit key)       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                                                               !
     !                                                               !
     !                                                               !
     !                        HMAC-SHA256 Key                        !
     !                                                               !
     !                                                               !
     !                                                               !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! TYPE=TEK_ALGORITHM_KEY (1)    ! LENGTH=16                     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                                                               !
     !                        AES-CBC-128 Key                        !
     !                                                               !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   KD Type=1   !   RESERVED    !        Key Packet Length      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !   SPI Size=4  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                            SPI=2                              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     ! TYPE=TEK_ALGORITHM_KEY (1)    ! LENGTH=20                     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
     !                                                               !
     !                    AES-GCM-128 Key & Salt                     !
     !                                                               !
     !                                                               !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!

                        Figure 9: Sample KD Payload
Top   ToC   RFC8052 - Page 23

Appendix B. Implementation Considerations

Several topics have been suggested as useful for implementers.

B.1. DER Length Fields

The ID and SA TEK payloads defined in this memo include explicit lengths for fields formatted as DER. This includes the OID Length and OID-Specific Payload Length fields shown in Figures 2 and 4. Strictly speaking, these lengths are redundant since the length of the DER value is also encoded within the DER fields. It would be possible to determine the lengths of the fields from those encoded values. However, many implementations will find the explicit length fields convenient when constructing and sanity checking the GDOI messages including these payloads. Implementations will thus be spared from manipulating the DER itself when performing activities that do not otherwise require parsing in order to obtain values therein.

B.2. Groups with Multiple Senders

GCKS policy may specify more than one protected type of IEC 61850 message within a GDOI group. This is represented within a GDOI SA Payload by the presence of an SA TEK payload for each multicast group that is protected as part of group policy. The OID contained in each of the SA TEK payloads may be identical, but the value of each OID- Specific Payload would be unique. Typically, the OID-Specific payload defines a destination address, and there is typically a single sender to that destination address.

Appendix C. Data Attribute Format

Data attributes attached to an SA TEK following the data attribute format are described in this section. Data attributes can be in Type/Value (TV) format (useful when a value is defined to be less than two octets in size) or in Type/Length/Value (TLV) form. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ !A! Attribute Type ! AF=0 Attribute Length ! !F! ! AF=1 Attribute Value ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . AF=0 Attribute Value . . AF=1 Not Transmitted . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Data Attributes
Top   ToC   RFC8052 - Page 24
   The Data Attributes fields are defined as follows:

   o  Attribute Type (2 octets) -- Unique identifier for each type of
      attribute.  These attributes are defined as part of the DOI-
      specific information.  The most significant bit, or Attribute
      Format (AF), indicates whether the data attributes follow the
      Type/Length/Value (TLV) format or a shortened Type/Value (TV)
      format.  If the AF bit is a zero (0), then the data attributes are
      of the Type/Length/Value (TLV) form.  If the AF bit is a one (1),
      then the data attributes are of the Type/Value form.

   o  Attribute Length (2 octets) -- Length in octets of the Attribute
      Value.  When the AF bit is a one (1), the Attribute Value is only
      2 octets, and the Attribute Length field is not present.

   o  Attribute Value (variable length) -- Value of the attribute
      associated with the DOI-specific Attribute Type.  If the AF bit is
      a zero (0), this field has a variable length defined by the
      Attribute Length field.  If the AF bit is a one (1), the Attribute
      Value has a length of 2 octets.

Acknowledgements

The authors thank Sean Turner, Steffen Fries, Yoav Nir, Vincent Roca, Dennis Bourget, and David Boose for their thoughtful reviews, each of which resulted in substantial improvements to this memo. Joe Salowey provided valuable guidance as document shepherd during the publication process. The authors are indebted to Kathleen Moriarty for her agreement to sponsor the publication of the document.
Top   ToC   RFC8052 - Page 25

Authors' Addresses

Brian Weis Cisco Systems 170 W. Tasman Drive San Jose, California 95134-1706 United States of America Phone: +1 408 526 4796 Email: bew@cisco.com Maik Seewald Cisco Systems Am Soeldnermoos 17 D-85399 Hallbergmoos Germany Phone: +49 619 6773 9655 Email: maseewal@cisco.com Herb Falk SISCO 6605 19-1/2 Mile Road Sterling Heights, MI 48314 United States of America Phone: +1 586 254 0020 x105 Email: herb@sisconet.com