6.2.1. ACP Certificates

ACP certificates MUST be [RFC5280] compliant X.509 v3 [X.509] certificates.¶

ACP nodes MUST support handling ACP certificates, TA certificates, and certificate chain certificates (henceforth just called certificates in this section) with RSA public keys and certificates with Elliptic Curve Cryptography (ECC) public keys.¶

ACP nodes MUST NOT support certificates with RSA public keys of less than a 2048-bit modulus or curves with group order of less than 256 bits. They MUST support certificates with RSA public keys with 2048-bit modulus and MAY support longer RSA keys. They MUST support certificates with ECC public keys using NIST P-256 curves and SHOULD support P-384 and P-521 curves.¶

ACP nodes MUST NOT support certificates with RSA public keys whose modulus is less than 2048 bits, or certificates whose ECC public keys are in groups whose order is less than 256 bits. RSA signing certificates with 2048-bit public keys MUST be supported, and such certificates with longer public keys MAY be supported. ECDSA certificates using the NIST P-256 curve MUST be supported, and such certificates using the P-384 and P-521 curves SHOULD be supported.¶

ACP nodes MUST support RSA certificates that are signed by RSA signatures over the SHA-256 digest of the contents and SHOULD additionally support SHA-384 and SHA-512 digests in such signatures. The same requirements for digest usage in certificate signatures apply to Elliptic Curve Digital Signature Algorithm (ECDSA) certificates, and additionally, ACP nodes MUST support ECDSA signatures on ECDSA certificates.¶

The ACP certificate SHOULD use an RSA key and an RSA signature when the ACP certificate is intended to be used not only for ACP authentication but also for other purposes. The ACP certificate MAY use an ECC key and an ECDSA signature if the ACP certificate is only used for ACP and ANI authentication and authorization.¶

Any secure channel protocols used for the ACP as specified in this document or extensions of this document MUST therefore support authentication (e.g., signing), starting with these types of certificates. See [RFC8422] for more information.¶

The reason for these choices are as follows: as of 2020, RSA is still more widely used than ECC, therefore the MUST-level requirements for RSA. ECC offers equivalent security at (logarithmically) shorter key lengths (see [RFC8422]). This can be beneficial especially in the presence of constrained bandwidth or constrained nodes in an ACP/ANI network. Some ACP functions such as GRASP peer-to-peer across the ACP require end-to-end/any-to-any authentication and authorization, therefore ECC can only reliably be used in the ACP when it MUST be supported on all ACP nodes. RSA signatures are mandatory to be supported also for ECC certificates because the CAs themselves may not support ECC yet.¶

The ACP certificate SHOULD be used for any authentication between nodes with ACP domain certificates (ACP nodes and NOC nodes) where a required authorization condition is ACP domain membership, such as ACP node to NOC/OAM end-to-end security and ASA to ASA end-to-end security. Section 6.2.3 defines this "ACP domain membership check". The uses of this check that are standardized in this document are for the establishment of hop-by-hop ACP secure channels (Section 6.8) and for ACP GRASP (Section 6.9.2) end to end via TLS.¶

The ACP domain membership check requires a minimum number of elements in a certificate as described in Section 6.2.3. The identity of a node in the ACP is carried via the acp-node-name as defined in Section 6.2.2.¶

To support Elliptic Curve Diffie-Hellman (ECDH) directly with the key in the ACP certificate, ACP certificates with ECC keys need to indicate that they are ECDH capable: if the X.509 v3 keyUsage extension is present, the keyAgreement bit must then be set. Note that this option is not required for any of the required ciphersuites in this document and may not be supported by all CAs.¶

Any other fields of the ACP certificate are to be populated as required by [RFC5280]. As long as they are compliant with [RFC5280], any other field of an ACP certificate can be set as desired by the operator of the ACP domain through the appropriate ACP registrar and/or ACP CA procedures. For example, other fields may be required for purposes other than those that the ACP certificate is intended to be used for (such as elements of a SubjectName).¶

For further certificate details, ACP certificates may follow the recommendations from [CABFORUM].¶

For diagnostic and other operational purposes, it is beneficial to copy the device-identifying fields of the node's IDevID certificate into the ACP certificate, such as the "serialNumber" attribute ([X.520], Section 6.2.9) in the subject field distinguished name encoding. Note that this is not the certificate serial-number. See also [RFC8995], Section 2.3.1. This can be done, for example, if it would be acceptable for the device's "serialNumber" to be signaled via the Link Layer Discovery Protocol [LLDP] because, like LLDP-signaled information, the ACP certificate information can be retrieved by neighboring nodes without further authentication and can be used either for beneficial diagnostics or for malicious attacks. Retrieval of the ACP certificate is possible via a (failing) attempt to set up an ACP secure channel, and the "serialNumber" usually contains device type information that may help to more quickly determine working exploits/attacks against the device.¶

Note that there is no intention to constrain authorization within the ACP or Autonomic Networks using the ACP to just the ACP domain membership check as defined in this document. It can be extended or modified with additional requirements. Such future authorizations can use and require additional elements in certificates or policies or even additional certificates. See Section 6.2.5 for the additional check against the id-kp-cmcRA extended key usage attribute ("Certificate Management over CMS (CMC) Updates" [RFC6402]), and see Appendix A.9.5 for possible future extensions.¶

6.2.2. ACP Certificate AcpNodeName

  acp-node-name = local-part "@" acp-domain-name
  local-part = [ acp-address ] [ "+" rsub extensions ]
  acp-address = 32HEXDIG / "0" ; HEXDIG as of [RFC5234], Appendix B.1
  rsub = [ <subdomain> ] ; <subdomain> as of [RFC1034], Section 3.5
  acp-domain-name = <domain> ; as of [RFC1034], Section 3.5
  extensions = *( "+" extension )
  extension  = 1*etext  ; future standard definition.
  etext      = ALPHA / DIGIT /  ; Printable US-ASCII
               "!" / "#" / "$" / "%" / "&" / "'" /
               "*" / "-" / "/" / "=" / "?" / "^" /
               "_" / "`" / "{" / "|" / "}" / "~"

  routing-subdomain = [ rsub "." ] acp-domain-name

Example:¶

Given an ACP address of fd89:b714:f3db:0:200:0:6400:0000, an ACP domain name of acp.example.com, and an rsub extension of area51.research, then this results in the following:¶

  acp-node-name      = fd89b714f3db00000200000064000000
                       [email protected]
  acp-domain-name    = acp.example.com
  routing-subdomain  = area51.research.acp.example.com

The acp-node-name in Figure 2 is the ABNF definition ("Augmented BNF for Syntax Specifications: ABNF" [RFC5234]) of the ACP Node Name. An ACP certificate MUST carry this information. It MUST contain an otherName field in the X.509 Subject Alternative Name extension, and the otherName MUST contain an AcpNodeName as described in Section 6.2.2.¶

Nodes complying with this specification MUST be able to receive their ACP address through the domain certificate, in which case their own ACP certificate MUST have a 32HEXDIG acp-address field. The acp-address field is case insensitive because ABNF HEXDIG is. It is recommended to encode acp-address with lowercase letters. Nodes complying with this specification MUST also be able to authenticate nodes as ACP domain members or ACP secure channel peers when they have a zero-value acp-address field and as ACP domain members (but not as ACP secure channel peers) when the acp-address field is omitted from their AcpNodeName. See Section 6.2.3.¶

The acp-domain-name is used to indicate the ACP domain across which ACP nodes authenticate and authorize each other, for example, to build ACP secure channels to each other, see Section 6.2.3. The acp-domain-name SHOULD be the FQDN of an Internet domain owned by the network administration of the ACP and ideally reserved to only be used for the ACP. In this specification, it serves as a name for the ACP that ideally is globally unique. When acp-domain-name is a globally unique name, collision of ACP addresses across different ACP domains can only happen due to ULA hash collisions (see Section 6.11.2). Using different acp-domain-names, operators can distinguish multiple ACPs even when using the same TA.¶

To keep the encoding simple, there is no consideration for internationalized acp-domain-names. The acp-node-name is not intended for end-user consumption. There is no protection against an operator picking any domain name for an ACP whether or not the operator can claim to own the domain name. Instead, the domain name only serves as a hash seed for the ULA and for diagnostics for the operator. Therefore, any operator owning only an internationalized domain name should be able to pick an equivalently unique 7-bit ASCII acp-domain-name string representing the internationalized domain name.¶

The routing-subdomain is a string that can be constructed from the acp-node-name, and it is used in the hash creation of the ULA (see Section 6.11.2). The presence of the rsub component allows a single ACP domain to employ multiple /48 ULA prefixes. See Appendix A.6 for example use cases.¶

The optional extensions field is used for future standardized extensions to this specification. It MUST be ignored if present and not understood.¶

The following points explain and justify the encoding choices described:¶

1. Formatting notes:¶

   1.1

   The rsub component needs to be in the local-part: if the format just had routing-subdomain as the domain part of the acp-node-name, rsub and acp-domain-name could not be separated from each other to determine in the ACP domain membership check which part is the acp-domain-name and which is solely for creating a different ULA prefix.¶

   1.2

   If both acp-address and rsub are omitted from AcpNodeName, the local-part will have the format "++extension(s)". The two plus characters are necessary so the node can unambiguously parse that both acp-address and rsub are omitted.¶

2. The encoding of the ACP domain name and ACP address as described in this section is used for the following reasons:¶

   2.1

   The acp-node-name is the identifier of a node's ACP. It includes the necessary components to identify a node's ACP both from within the ACP as well as from the outside of the ACP.¶

   2.2

   For manual and/or automated diagnostics and backend management of devices and ACPs, it is necessary to have an easily human-readable and software-parsable standard, single string representation of the information in the acp-node-name. For example, inventory or other backend systems can always identify an entity by one unique string field but not by a combination of multiple fields, which would be necessary if there were no single string representation.¶

   2.3

   If the encoding was not such a string, it would be necessary to define a second standard encoding to provide this format (standard string encoding) for operator consumption.¶

   2.4

   Addresses of the form <local>@<domain> have become the preferred format for identifiers of entities in many systems, including the majority of user identifiers in web or mobile applications such as multi-domain single-sign-on systems.¶

3. Compatibilities:¶

   3.1

   It should be possible to use the ACP certificate as an LDevID certificate on the system for uses besides the ACP. Therefore, the information element required for the ACP should be encoded so that it minimizes the possibility of creating incompatibilities with other such uses. The attributes of the subject field, for example, are often used in non-ACP applications and therefore should not be occupied by new ACP values.¶

   3.2

   The element should not require additional ASN.1 encoding and/or decoding because libraries for accessing certificate information, especially for embedded devices, may not support extended ASN.1 decoding beyond predefined, mandatory fields. subjectAltName / otherName is already used with a single string parameter for several otherNames (see "Extensible Messaging and Presence Protocol (XMPP): Core" [RFC6120], "Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS Based on the Network Access Identifier (NAI)" [RFC7585], "Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name" [RFC4985], "Internationalized Email Addresses in X.509 Certificates" [RFC8398]).¶

   3.3

   The element required for the ACP should minimize the risk of being misinterpreted by other uses of the LDevID certificate. It also must not be misinterpreted as an email address, hence the use of the otherName / rfc822Name option in the certificate would be inappropriate.¶

See Section 4.2.1.6 of [RFC5280] for details on the subjectAltName field.¶

ANIMA-ACP-2020
  { iso(1) identified-organization(3) dod(6)
    internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
    id-mod-anima-acpnodename-2020(97) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

IMPORTS
  OTHER-NAME
  FROM PKIX1Implicit-2009
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-pkix1-implicit-02(59) }

  id-pkix
  FROM PKIX1Explicit-2009
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-pkix1-explicit-02(51) } ;

  id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

  AcpNodeNameOtherNames OTHER-NAME ::= { on-AcpNodeName, ... }

  on-AcpNodeName OTHER-NAME ::= {
      AcpNodeName IDENTIFIED BY id-on-AcpNodeName
  }

  id-on-AcpNodeName OBJECT IDENTIFIER ::= { id-on 10 }

  AcpNodeName ::= IA5String (SIZE (1..MAX))
   -- AcpNodeName as specified in this document carries the
   -- acp-node-name as specified in the ABNF in Section 6.2.2

END

6.2.3. ACP Domain Membership Check

The following points constitute the ACP domain membership check of a candidate peer via its certificate:¶

1. The peer has proved ownership of the private key associated with the certificate's public key. This check is performed by the security association protocol used, for example, Section 2.15 of "Internet Key Exchange Protocol Version 2 (IKEv2)" [RFC7296].¶
2. The peer's certificate passes certificate path validation as defined in [RFC5280], Section 6, against one of the TAs associated with the ACP node's ACP certificate (see Section 6.2.4). This includes verification of the validity (lifetime) of the certificates in the path.¶

3. If the peer's certificate indicates a CRLDP ([RFC5280], Section 4.2.1.13) or OCSP responder ([RFC5280], Section 4.2.2.1), then the peer's certificate MUST be valid according to those mechanisms when they are available: an OCSP check for the peer's certificate across the ACP must succeed, or the peer's certificate must not be listed in the CRL retrieved from the CRLDP. These mechanisms are not available when the ACP node has no ACP or non-ACP connectivity to retrieve a current CRL or has no access an OCSP responder, and the security association protocol itself also has no way to communicate the CRL or OCSP check.¶

   Retries to learn revocation via OCSP or CRL SHOULD be made using the same backoff as described in Section 6.7. If and when the ACP node then learns that an ACP peer's certificate is invalid for which Rule 3 had to be skipped during ACP secure channel establishment, then the ACP secure channel to that peer MUST be closed even if this peer is the only connectivity to access CRL/OCSP. This applies (of course) to all ACP secure channels to this peer if there are multiple. The ACP secure channel connection MUST be retried periodically to support the case that the neighbor acquires a new, valid certificate.¶

4. The peer's certificate has a syntactically valid acp-node-name field, and the acp-domain-name in that peer's acp-node-name is the same as in this ACP node's certificate (lowercase normalized).¶

When checking a candidate peer's certificate for the purpose of establishing an ACP secure channel, one additional check is performed:¶

5. The acp-address field of the candidate peer certificate's AcpNodeName is not omitted but is either 32HEXDIG or 0, according to Figure 2.¶

Technically, ACP secure channels can only be built with nodes that have an acp-address. Rule 5 ensures that this is taken into account during ACP domain membership check.¶

Nodes with an omitted acp-address field can only use their ACP domain certificate for non-ACP secure channel authentication purposes. This includes, for example, NMS type nodes permitted to communicate into the ACP via ACP connect (Section 8.1)¶

The special value "0" in an ACP certificate's acp-address field is used for nodes that can and should determine their ACP address through mechanisms other than learning it through the acp-address field in their ACP certificate. These ACP nodes are permitted to establish ACP secure channels. Mechanisms for those nodes to determine their ACP address are outside the scope of this specification, but this option is defined here so that any ACP nodes can build ACP secure channels to them according to Rule 5.¶

The optional rsub field of the AcpNodeName is not relevant to the ACP domain membership check because it only serves to structure routing and addressing within an ACP but not to segment mutual authentication and authorization (hence the name "routing subdomain").¶

In summary:¶

• Steps 1 through 4 constitute standard certificate validity verification and private key authentication as defined by [RFC5280] and security association protocols (such as IKEv2 [RFC7296]) when leveraging certificates.¶
• Except for its public key, Steps 1 through 4 do not include the verification of any preexisting form of a certificate's identity elements, such as a web server's domain name prefix, which is often encoded in the certificate common name. Step 5 is an equivalent step for the AcpNodeName.¶
• Step 4 constitutes standard CRL and OCSP checks refined for the case of missing connectivity and limited-functionality security association protocols.¶
• Steps 1 through 4 authorize the building of any secure connection between members of the same ACP domain except for ACP secure channels.¶
• Step 5 is the additional verification of the presence of an ACP address as necessary for ACP secure channels.¶
• Steps 1 through 5 therefore authorize the building of an ACP secure channel.¶

For brevity, the remainder of this document refers to this process only as authentication instead of as authentication and authorization.¶

6.2.4. Trust Anchors (TA)

ACP nodes need TA information according to [RFC5280], Section 6.1.1 (d), typically in the form of one or more certificates of the TA to perform certificate path validation as required by Section 6.2.3, Rule 2. TA information MUST be provisioned to an ACP node (together with its ACP domain certificate) by an ACP registrar during initial enrollment of a candidate ACP node. ACP nodes MUST also support the renewal of TA information via EST as described in Section 6.2.5.¶

The required information about a TA can consist of one or more certificates as required for dealing with CA certificate renewals as explained in Section 4.4 of "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)" [RFC4210]).¶

A certificate path is a chain of certificates starting at the ACP certificate (the leaf and/or end entity), followed by zero or more intermediate CA certificates, and ending with the TA information, which is typically one or two self-signed certificates of the TA. The CA that signs the ACP certificate is called the assigning CA. If there are no intermediate CAs, then the assigning CA is the TA. Certificate path validation authenticates that the TA associated with the ACP permits the ACP certificate, either directly or indirectly via one or more intermediate CA.¶

Note that different ACP nodes may have different intermediate CAs in their certificate path and even different TA. The set of TAs for an ACP domain must be consistent across all ACP members so that any ACP node can authenticate any other ACP node. The protocols through which the ACP domain membership check Rules 1 through 3 are performed need to support the exchange not only of the ACP nodes certificates but also the exchange of the intermediate TA.¶

For the ACP domain membership check, ACP nodes MUST support certificate path validation with zero or one intermediate CA. They SHOULD support two intermediate CAs and two TAs (to permit migration from one TA to another TA).¶

Certificates for an ACP MUST only be given to nodes that are allowed to be members of that ACP. When the signing CA relies on an ACP registrar, the CA MUST only sign certificates with acp-node-name through trusted ACP registrars. In this setup, any existing CA, unaware of the formatting of acp-node-name, can be used.¶

These requirements can be achieved by using a TA private to the owner of the ACP domain or potentially through appropriate contractual agreements between the involved parties (registrar and CA). Using a public CA is out of scope of this document.¶

A single owner can operate multiple, independent ACP domains from the same set of TAs. Registrars must then know into which ACP a node needs to be enrolled.¶

6.2.5. Certificate and Trust Anchor Maintenance

ACP nodes MUST support renewal of their certificate and TA information via EST and MAY support other mechanisms. See Section 6.1 for TLS requirements. An ACP network MUST have at least one ACP node supporting EST server functionality across the ACP so that EST renewal is usable.¶

ACP nodes SHOULD remember the GRASP O_IPv6_LOCATOR parameters of the EST server with which they last renewed their ACP certificate. They SHOULD provide the ability for these EST server parameters to also be set by the ACP registrar (see Section 6.11.7) that initially enrolled the ACP device with its ACP certificate. When BRSKI is used (see [RFC8995]), the IPv6 locator of the BRSKI registrar from the BRSKI TLS connection SHOULD be remembered and used for the next renewal via EST if that registrar also announces itself as an EST server via GRASP on its ACP address (see Section 6.2.5.1).¶

The EST server MUST present a certificate that passes the ACP domain membership check in its TLS connection setup (Section 6.2.3, rules 1 through 4, not rule 5 as this is not for an ACP secure channel setup). The EST server certificate MUST also contain the id-kp-cmcRA extended key usage attribute [RFC6402], and the EST client MUST check its presence.¶

The additional check against the id-kp-cmcRA extended key usage extension field ensures that clients do not fall prey to an illicit EST server. While such illicit EST servers should not be able to support certificate signing requests (as they are not able to elicit a signing response from a valid CA), such an illicit EST server would be able to provide faked CA certificates to EST clients that need to renew their CA certificates when they expire.¶

Note that EST servers supporting multiple ACP domains will need to have a separate certificate for each of these ACP domains and respond on a different transport address (IPv6 address and/or TCP port). This is easily automated on the EST server if the CA allows registrars to request certificates with the id-kp-cmcRA extended usage extension for themselves.¶

   [M_FLOOD, 12340815, h'fd89b714f3db0000200000064000001', 210000,
       [["SRV.est", 4, 255 ],
       [O_IPv6_LOCATOR,
            h'fd89b714f3db0000200000064000001', IPPROTO_TCP, 443]]
   ]

flood-message = [M_FLOOD, session-id, initiator, ttl,
                 +[objective, (locator-option / [])]]
                             ; See example above and explanation
                             ; below for initiator and ttl.

objective = ["SRV.est", objective-flags, loop-count,
                                       objective-value]

objective-flags = sync-only  ; As in [RFC8990].
sync-only       = 4          ; M_FLOOD only requires synchronization.
loop-count      = 255        ; Recommended as there is no mechanism
                             ; to discover network diameter.
objective-value = any        ; Reserved for future extensions.

6.8.1. General Considerations

Due to channel selection (Section 6.6), ACP can support an evolving set of security association protocols and does not require support for a single network-wide MTI. ACP nodes only need to implement those protocols required to interoperate with their candidate peers, not with potentially any node in the ACP domain. See Section 6.8.5 for an example of this.¶

The degree of security required on every hop of an ACP network needs to be consistent across the network so that there is no designated "weakest link" because it is that "weakest link" that would otherwise become the designated point of attack. When the secure channel protection on one link is compromised, it can be used to send and/or receive packets across the whole ACP network. Therefore, even though the security association protocols can be different, their minimum degree of security should be comparable.¶

Secure channel protocols do not need to always support arbitrary Layer 3 (L3) connectivity between peers, but can leverage the fact that the standard use case for ACP secure channels is an L2 adjacency. Hence, L2 dependent mechanisms could be adopted for use as secure channel association protocols.¶

L2 mechanisms such as strong encrypted radio technologies or [MACSEC] may offer equivalent encryption, and the ACP security association protocol may only be required to authenticate ACP domain membership of a peer and/or derive a key for the L2 mechanism. Mechanisms that leverage such underlying L2 security to auto-discover and associate ACP peers are possible and desirable to avoid duplication of encryption, but none are specified in this document.¶

Strong physical security of a link may stand in where cryptographic security is infeasible. As there is no secure mechanism to automatically discover strong physical security solely between two peers, it can only be used with explicit configuration, and that configuration too could become an attack vector. This document therefore specifies with ACP connect (Section 8.1) only one explicitly configured mechanism without any secure channel association protocol for the case where both the link and the nodes attached to it have strong physical security.¶

6.8.2. Common Requirements

The authentication of peers in any security association protocol MUST use the ACP certificate according to Section 6.2.3. Because auto-discovery of candidate ACP neighbors via GRASP (see Section 6.4) as specified in this document does not communicate the neighbor's ACP certificate, and ACP nodes may not (yet) have any other network connectivity to retrieve certificates, any security association protocol MUST use a mechanism to communicate the certificate directly instead of relying on a referential mechanism such as communicating only a hash and/or URL for the certificate.¶

A security association protocol MUST use Forward Secrecy (whether inherently or as part of a profile of the security association protocol).¶

Because the ACP payload of legacy protocol payloads inside the ACP and hop-by-hop ACP flooded GRASP information is unencrypted, the ACP secure channel protocol requires confidentiality. Symmetric encryption for the transmission of secure channel data MUST use encryption schemes considered to be security wise equal to or better than 256-bit key strength, such as AES-256. There MUST NOT be support for NULL encryption.¶

Security association protocols typically only signal the end entity certificate (e.g., the ACP certificate) and any possible intermediate CA certificates for successful mutual authentication. The TA has to be mutually known and trusted, and therefore its certificate does not need to be signaled for successful mutual authentication. Nevertheless, for use with ACP secure channel setup, there SHOULD be the option to include the TA certificate in the signaling to aid troubleshooting, see Section 9.1.1.¶

Signaling of TA certificates may not be appropriate when the deployment relies on a security model where the TA certificate content is considered confidential, and only its hash is appropriate for signaling. ACP nodes SHOULD have a mechanism to select whether the TA certificate is signaled or not, assuming that both options are possible with a specific secure channel protocol.¶

An ACP secure channel MUST immediately be terminated when the lifetime of any certificate in the chain used to authenticate the neighbor expires or becomes revoked. This may not be standard behavior in secure channel protocols because the certificate authentication may only influence the setup of the secure channel in these protocols, but may not be revalidated during the lifetime of the secure connection in the absence of this requirement.¶

When specifying an additional security association protocol for ACP secure channels beyond those covered in this document, any protocol options that are unnecessary for the support of devices that are expected to be able to support the ACP SHOULD be eliminated in order to minimize implementation complexity. For example, definitions for security protocols often include old and/or inferior security options required only to interoperate with existing devices that cannot update to the currently preferred security options. Such old and/or inferior security options do not need to be supported when a security association protocol is first specified for the ACP, thus strengthening the "weakest link" and simplifying ACP implementation overhead.¶

6.8.3. ACP via IPsec

An ACP node announces its ability to support IPsec, negotiated via IKEv2, as the ACP secure channel protocol using the "IKEv2" 'objective-value' in the "AN_ACP" GRASP objective.¶

The ACP usage of IPsec and IKEv2 mandates a profile with a narrow set of options of the current Standards Track usage guidance for IPsec ("Cryptographic Algorithm Implementation Requirements and Usage Guidance for Encapsulating Security Payload (ESP) and Authentication Header (AH)" [RFC8221]) and IKEv2 ("Algorithm Implementation Requirements and Usage Guidance for the Internet Key Exchange Protocol Version 2 (IKEv2)" [RFC8247]). These options result in stringent security properties and can exclude deprecated and legacy algorithms because there is no need for interoperability with legacy equipment for ACP secure channels. Any such backward compatibility would lead only to an increased attack surface and implementation complexity, for no benefit.¶

TSi = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
TSr = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

TSi = (47, 0-65535, Initiator-IPv6-LL-addr ... Initiator-IPv6-LL-addr)
TSr = (47, 0-65535, Responder-IPv6-LL-addr ... Responder-IPv6-LL-addr)

6.8.4. ACP via DTLS

This document defines the use of ACP via DTLS on the assumption that it is likely the first transport encryption supported in some classes of constrained devices: DTLS is commonly used in constrained devices when IPsec is not. Code space on those devices may be also be too limited to support more than the minimum number of required protocols.¶

An ACP node announces its ability to support DTLS version 1.2 ("Datagram Transport Layer Security Version 1.2" [RFC6347]) compliant with the requirements defined in this document as an ACP secure channel protocol in GRASP through the "DTLS" 'objective-value' in the "AN_ACP" objective (see Section 6.4).¶

To run ACP via UDP and DTLS, a locally assigned UDP port is used that is announced as a parameter in the GRASP "AN_ACP" objective to candidate neighbors. This port can also be any newer version of DTLS as long as that version can negotiate a DTLS 1.2 connection in the presence of a peer that only supports DTLS 1.2.¶

All ACP nodes supporting DTLS as a secure channel protocol MUST adhere to the DTLS implementation recommendations and security considerations of BCP 195 [RFC7525] except with respect to the DTLS version. ACP nodes supporting DTLS MUST support DTLS 1.2. They MUST NOT support older versions of DTLS.¶

Unlike for IPsec, no attempts are made to simplify the requirements of the recommendations in BCP 195 [RFC7525] because the expectation is that DTLS would use software-only implementations where the ability to reuse widely adopted implementations is more important than the ability to minimize the complexity of a hardware-accelerated implementation, which is known to be important for IPsec.¶

DTLS 1.3 [TLS-DTLS13] is "backward compatible" with DTLS 1.2 (see Section 1 of [TLS-DTLS13]). A DTLS implementation supporting both DTLS 1.2 and DTLS 1.3 does comply with the above requirements of negotiating to DTLS 1.2 in the presence of a DTLS 1.2 only peer, but using DTLS 1.3 when booth peers support it.¶

Version 1.2 is the MTI version of DTLS in this specification because of the following:¶

• There is more experience with DTLS 1.2 across the spectrum of target ACP nodes.¶
• Firmware of lower-end, embedded ACP nodes may not support a newer version for a long time.¶
• There are significant changes with DTLS 1.3, such as a different record layer requiring time to gain implementation and deployment experience especially on lower-end devices with limited code space.¶
• The existing BCP [RFC7525] for DTLS 1.2 may take an equally longer time to be updated with experience from a newer DTLS version.¶
• There are no significant benefits of DTLS 1.3 over DTLS 1.2 that are use-case relevant in the context of the ACP options for DTLS. For example, signaling performance improvements for session setup in DTLS 1.3 is not important for the ACP given the long-lived nature of ACP secure channel connections and the fact that DTLS connections are mostly link local (short RTT).¶

Nevertheless, newer versions of DTLS, such as DTLS 1.3, have stricter security requirements, and the use of the latest standard protocol version is in general recommended for IETF security standards. Therefore, ACP implementations are advised to support all the newer versions of DTLS that can still negotiate down to DTLS 1.2.¶

There is no additional session setup or other security association besides this simple DTLS setup. As soon as the DTLS session is functional, the ACP peers will exchange ACP IPv6 packets as the payload of the DTLS transport connection. Any DTLS-defined security association mechanisms such as rekeying are used as they would be for any transport application relying solely on DTLS.¶

6.8.5. ACP Secure Channel Profiles

As explained in the beginning of Section 6.6, there is no single secure channel mechanism mandated for all ACP nodes. Instead, this section defines two ACP profiles, "baseline" and "constrained", for ACP nodes that do introduce such requirements.¶

An ACP node supporting the baseline profile MUST support IPsec natively and MAY support IPsec via GRE. An ACP node supporting the constrained profile that cannot support IPsec MUST support DTLS. An ACP node connecting an area of constrained ACP nodes with an area of baseline ACP nodes needs to support both IPsec and DTLS and therefore supports both the baseline and constrained profiles.¶

Explanation: not all types of ACP nodes are able to or need to connect directly to each other, nor are they able to support or prefer all possible secure channel mechanisms. For example, IoT devices with limited code space may only support DTLS because that code already exists on them for end-to-end security, but high-end core routers may not want to support DTLS because they can perform IPsec in accelerated hardware, but they would need to support DTLS in an underpowered CPU forwarding path shared with critical control plane operations. This is not a deployment issue for a single ACP across these types of nodes as long as there are also appropriate gateway ACP nodes that sufficiently support many secure channel mechanisms to allow interconnecting areas of ACP nodes with a more constrained set of secure channel protocols. On the edge between IoT areas and high-end core networks, general-purpose routers that act as those gateways and that can support a variety of secure channel protocols are the norm already.¶

Native IPsec with tunnel mode provides the shortest encapsulation overhead. GRE may be preferred by legacy implementations because, in the past, the virtual interfaces required by ACP design in conjunction with secure channels have been implemented more often for GRE than purely for native IPsec.¶

ACP nodes need to specify the set of secure ACP mechanisms they support in documentation and should declare which profile they support according to the above requirements.¶

6.9.1. GRASP as a Core Service of the ACP

The ACP MUST run an instance of GRASP inside of it. It is a key part of the ACP services. The function in GRASP that makes it fundamental as a service of the ACP is the ability to provide ACP-wide service discovery (using objectives in GRASP).¶

ACP provides IP unicast routing via RPL (see Section 6.12).¶

The ACP does not use IP multicast routing nor does it provide generic IP multicast services (the handling of GRASP link-local multicast messages is explained in Section 6.9.2). Instead, the ACP provides service discovery via the objective discovery/announcement and negotiation mechanisms of the ACP GRASP instance (services are a form of objectives). These mechanisms use hop-by-hop reliable flooding of GRASP messages for both service discovery (GRASP M_DISCOVERY messages) and service announcement (GRASP M_FLOOD messages).¶

See Appendix A.5 for discussion about this design choice of the ACP.¶

6.9.2. ACP as the Security and Transport Substrate for GRASP

In the terminology of GRASP [RFC8990], the ACP is the security and transport substrate for the GRASP instance run inside the ACP ("ACP GRASP").¶

This means that the ACP is responsible for ensuring that this instance of GRASP is only sending messages across the ACP GRASP virtual interfaces. Whenever the ACP adds or deletes such an interface because of new ACP secure channels or loss thereof, the ACP needs to indicate this to the ACP instance of GRASP. The ACP exists also in the absence of any active ACP neighbors. It is created when the node has a domain certificate, and it continues to exist even if all of its neighbors cease operation.¶

In this case, ASAs using GRASP running on the same node still need to be able to discover each other's objectives. When the ACP does not exist, ASAs leveraging the ACP instance of GRASP via APIs MUST still be able to operate, and they MUST be able to understand that there is no ACP and that therefore the ACP instance of GRASP cannot operate.¶

How the ACP acts as the security and transport substrate for GRASP is shown in Figure 8.¶

GRASP unicast messages inside the ACP always use the ACP address. Link-local addresses from the ACP VRF MUST NOT be used inside objectives. GRASP unicast messages inside the ACP are transported via TLS. See Section 6.1 for TLS requirements. TLS mutual authentication MUST use the ACP domain membership check defined in Section 6.2.3.¶

GRASP link-local multicast messages are targeted for a specific ACP virtual interface (as defined Section 6.13.5), but they are sent by the ACP to an ACP GRASP virtual interface that is constructed from the TCP connection(s) to the IPv6 link-local neighbor address(es) on the underlying ACP virtual interface. If the ACP GRASP virtual interface has two or more neighbors, the GRASP link-local multicast messages are replicated to all neighbor TCP connections.¶

TCP and TLS connections for GRASP in the ACP use the IANA-assigned TCP port for GRASP (7017). Effectively, the transport stack is expected to be TLS for connections to and from the ACP address (e.g., global-scope address(es)) and TCP for connections to and from the link-local addresses on the ACP virtual interfaces. The latter ones are only used for the flooding of GRASP messages.¶

    ..............................ACP..............................
    .                                                             .
    .         /-GRASP-flooding-\         ACP GRASP instance       .
    .        /                  \                                 A
    .    GRASP      GRASP      GRASP                              C
    .  link-local   unicast  link-local                           P
    .   multicast  messages   multicast                           .
    .   messages      |       messages                            .
    .      |          |          |                                .
    ...............................................................
    .      v          v          v    ACP security and transport  .
    .      |          |          |    substrate for GRASP         .
    .      |          |          |                                .
    .      |       ACP GRASP     |       - ACP GRASP              A
    .      |       loopback      |         loopback interface     C
    .      |       interface     |       - ACP-cert auth          P
    .      |         TLS         |                                .
    .   ACP GRASP     |       ACP GRASP  - ACP GRASP virtual      .
    .   subnet1       |       subnet2      interfaces             .
    .     TCP         |         TCP                               .
    .      |          |          |                                .
    ...............................................................
    .      |          |          |   ^^^ Users of ACP (GRASP/ASA) .
    .      |          |          |   ACP interfaces/addressing    .
    .      |          |          |                                .
    .      |          |          |                                A
    .      | ACP loopback interf.|      <- ACP loopback interface C
    .      |      ACP-address    |       - address (global ULA)   P
    .    subnet1      |        subnet2  <- ACP virtual interfaces .
    .  link-local     |      link-local  - link-local addresses   .
    ...............................................................
    .      |          |          |   ACP VRF                      .
    .      |     RPL-routing     | virtual routing and forwarding .
    .      |   /IP-Forwarding\   |                                A
    .      |  /               \  |                                C
    .  ACP IPv6 packets   ACP IPv6 packets                        P
    .      |/                   \|                                .
    .    IPsec/DTLS        IPsec/DTLS  - ACP-cert auth            .
    ...............................................................
             |                   |   Data Plane
             |                   |
             |                   |     - ACP secure channel
         link-local        link-local  - encapsulation addresses
           subnet1            subnet2  - data plane interfaces
             |                   |
          ACP-Nbr1            ACP-Nbr2

6.11.1. Fundamental Concepts of Autonomic Addressing

• Usage: autonomic addresses are exclusively used for self-management functions inside a trusted domain. They are not used for user traffic. Communications with entities outside the trusted domain use another address space, for example, a normally managed routable address space (called "data plane" in this document).¶
• Separation: autonomic address space is used separately from user address space and other address realms. This supports the robustness requirement.¶
• Loopback only: only ACP loopback interfaces (and potentially those configured for ACP connect, see Section 8.1) carry routable address(es); all other interfaces (called ACP virtual interfaces) only use IPv6 link-local addresses. The usage of IPv6 link-local addressing is discussed in "Using Only Link-Local Addressing inside an IPv6 Network" [RFC7404].¶
• Use of ULA: for loopback interfaces of ACP nodes, we use ULA with the L bit set to 1 (as defined in Section 3.1 of [RFC4193]). Note that the random hash for ACP loopback addresses uses the definition in Section 6.11.2 and not the one in [RFC4193], Section 3.2.2.¶
• No external connectivity: the addresses do not provide access to the Internet. If a node requires further connectivity, it should use another, traditionally managed addressing scheme in parallel.¶
• Addresses in the ACP are permanent and do not support temporary addresses as defined in "Temporary Address Extensions for Stateless Address Autoconfiguration in IPv6" [RFC8981].¶
• Addresses in the ACP are not considered sensitive on privacy grounds because ACP nodes are not expected to be end-user hosts, and therefore ACP addresses do not represent end users or groups of end users. All ACP nodes are in one (potentially federated) administrative domain. For ACP traffic, the nodes are assumed to be either candidate hosts or transit nodes. There are no transit nodes with fewer privileges to know the identity of other hosts in the ACP. Therefore, ACP addresses do not need to be pseudorandom as discussed in "Security and Privacy Considerations for IPv6 Address Generation Mechanisms" [RFC7721]. Because they are not propagated to untrusted (non-ACP) nodes and stay within a domain (of trust), we also do not consider them to be subject to scanning attacks.¶

The ACP is based exclusively on IPv6 addressing for a variety of reasons:¶

• Simplicity, reliability, and scale: if other network-layer protocols were supported, each would have to have its own set of security associations, routing table, and process, etc.¶
• Autonomic functions do not require IPv4: autonomic functions and autonomic service agents are new concepts. They can be exclusively built on IPv6 from day one. There is no need for backward compatibility.¶
• OAM protocols do not require IPv4: the ACP may carry OAM protocols. All relevant protocols (SNMP, TFTP, SSH, SCP, RADIUS, Diameter, NETCONF, etc.) are available in IPv6. See also [RFC8368] for how ACP could be made to interoperate with IPv4-only OAM.¶

Further explanation about the addressing and routing-related reasons for the choice of the autonomous ACP addressing can be found in Section 6.13.5.1.¶

6.11.2. The ACP Addressing Base Scheme

The ULA addressing base scheme for ACP nodes has the following format:¶

  8      40                     2                     78
+--+-------------------------+------+------------------------------+
|fd| hash(routing-subdomain) | Type |     (sub-scheme)             |
+--+-------------------------+------+------------------------------+

The first 48 bits follow the ULA scheme as defined in [RFC4193], to which a Type field is added.¶

fd:

Identifies a locally defined ULA address.¶

hash(routing-subdomain):

The 40-bit ULA Global ID (a term from [RFC4193]) for ACP addresses carried in the acp-node-name in the ACP certificates are the first 40 bits of the SHA-256 hash of the routing-subdomain from the same acp-node-name. In the example of Section 6.2.2, the routing-subdomain is "area51.research.acp.example.com", and the 40-bit ULA Global ID is 89b714f3db.¶

When creating a new routing-subdomain for an existing Autonomic Network, it MUST be ensured that rsub is selected so the resulting hash of the routing-subdomain does not collide with the hash of any preexisting routing-subdomains of the Autonomic Network. This ensures that ACP addresses created by registrars for different routing subdomains do not collide with each other.¶

To allow for extensibility, the fact that the ULA Global ID is a hash of the routing-subdomain SHOULD NOT be assumed by any ACP node during normal operations. The hash function is only executed during the creation of the certificate. If BRSKI is used, then the BRSKI registrar will create the acp-node-name in response to the EST Certificate Signing Request (CSR) Attributes Request message sent by the pledge.¶

Establishing connectivity between different ACPs (different acp-domain-names) is outside the scope of this specification. If it is being done through future extensions, then the rsub of all routing-subdomains across those Autonomic Networks needs to be selected so that the resulting routing-subdomain hashes do not collide. For example, a large cooperation with its own private TA may want to create different Autonomic Networks that initially do not connect but where the option to do so should be kept open. When taking this possibility into account, it is always easy to select rsub so that no collisions happen.¶

Type:

This field allows different addressing sub-schemes. This addresses the "upgradability" requirement. Assignment of types for this field will be maintained by IANA.¶

(sub-scheme):

The sub-scheme may imply a range or set of addresses assigned to the node. This is called the ACP address range/set and explained in each sub-scheme.¶

Please refer to Section 6.11.7 and Appendix A.1 for further explanations for why the following addressing sub-schemes are used and why multiple are necessary.¶

The following summarizes the addressing sub-schemes:¶

The F-bit (format bit, Section 6.11.5) and Z (Section 6.11.4) are two encoding fields that are explained in the sections covering the sub-schemes that use them. V-bits is the number of bits of addresses allocated to the ACP node. Prefix is the prefix that the ACP node is announcing into RPL.¶

6.11.3. ACP Zone Addressing Sub-Scheme (ACP-Zone)

This sub-scheme is used when the Type field of the base scheme is 0 and the Z bit is 0.¶

                 64                             64
+-----------------+---+---------++-----------------------------+---+
|  (base scheme)  | Z | Zone-ID ||           Node-ID               |
|                 |   |         || Registrar-ID |   Node-Number| V |
+-----------------+---+---------++--------------+--------------+---+
         50         1     13            48           15          1

The fields are defined as follows:¶

Type:

MUST be 0.¶

Z:

MUST be 0.¶

Zone-ID:

A value for a network zone.¶

Node-ID:

A unique value for each node.¶

The 64-bit Node-ID must be unique across the ACP domain for each node. It is derived and composed as follows:¶

Registrar-ID (48 bits):

A number unique inside the domain identifying the ACP registrar that assigned the Node-ID to the node. One or more domain-wide unique identifiers of the ACP registrar can be used for this purpose. See Section 6.11.7.2.¶

Node-Number:

A number to make the Node-ID unique. This can be sequentially assigned by the ACP registrar owning the Registrar-ID.¶

V (1 bit):

Virtualization bit:¶

0:

Indicates the ACP itself ("ACP node base system)¶

1:

Indicates the optional "host" context on the ACP node (see below).¶

In the Zone Addressing Sub-Scheme, the ACP address in the certificate has V field as all zero bits.¶

The ACP address set of the node includes addresses with any Zone-ID value and any V value. Therefore, no two nodes in the same ACP and the same hash(routing-subdomain) can have the same Node-ID with the Zone Addressing Sub-Scheme, for example, by differing only in their Zone-ID.¶

The Virtualization bit in this sub-scheme allows the easy addition of the ACP as a component to existing systems without causing problems in the port number space between the services in the ACP and the existing system. V=0 is the ACP router (autonomic node base system), V=1 is the host with preexisting transport endpoints on it that could collide with the transport endpoints used by the ACP router. The ACP host could, for example, have a P2P (peer-to-peer) virtual interface with the V=0 address as its router to the ACP. Depending on the software design of ASAs, which is outside the scope of this specification, they may use the V=0 or V=1 address.¶

The location of the V bit(s) at the end of the address allows the announcement of a single prefix for each ACP node. For example, in a network with 20,000 ACP nodes, this avoids 20,000 additional routes in the routing table.¶

It is RECOMMENDED that only Zone-ID 0 is used unless it is meant to be used in conjunction with operational practices for partial or incremental adoption of the ACP as described in Section 9.4.¶

Note: Zones and Zone-ID as defined here are not related to zones or zone_id defined in "IPv6 Scoped Address Architecture" [RFC4007]. ACP zone addresses are not scoped (i.e., reachable only from within a zone as defined by [RFC4007]) but are reachable across the whole ACP. A zone_id is a zone index that has only local significance on a node [RFC4007], whereas an ACP Zone-ID is an identifier for an ACP zone that is unique across that ACP.¶

6.11.4. ACP Manual Addressing Sub-Scheme (ACP-Manual)

This sub-scheme is used when the Type field of the base scheme is 0 and the Z bit is 1.¶

                64                             64
+---------------------+---+----------++-----------------------------+
|    (base scheme)    | Z | Subnet-ID||     Interface Identifier    |
+---------------------+---+----------++-----------------------------+
         50             1    13

The fields are defined as follows:¶

Type:

MUST be 0.¶

Z:

MUST be 1.¶

Subnet-ID:

Configured subnet identifier.¶

Interface Identifier:

Interface identifier according to [RFC4291].¶

This sub-scheme is meant for the "manual" allocation to subnets where the other addressing schemes cannot be used. The primary use case is for assignment to ACP connect subnets (see Section 8.1.1).¶

"Manual" means that allocations of the Subnet-ID need to be done with preexisting, non-autonomic mechanisms. Every subnet that uses this addressing sub-scheme needs to use a unique Subnet-ID (unless some anycast setup is done).¶

The Z bit field was added to distinguish between the Zone Addressing Sub-Scheme and the Manual Addressing Sub-Scheme without requiring one more bit in the base scheme and therefore allowing for the Vlong Addressing Sub-Scheme (described in Section 6.11.5) to have one more bit available.¶

The Manual Addressing Sub-Scheme addresses SHOULD NOT be used in ACP certificates. Any node capable of building ACP secure channels and is permitted by registrar policy to participate in building ACP secure channels SHOULD receive an ACP address (prefix) from one of the other ACP addressing sub-schemes. A node that cannot or is not permitted to participate in ACP secure channels can connect to the ACP via ACP connect interfaces of ACP edge nodes (see Section 8.1) without setting up an ACP secure channel. Its ACP certificate MUST omit the acp-address field to indicate that its ACP certificate is only usable for non-ACP secure channel authentication, such as end-to-end transport connections across the ACP or data plane.¶

Address management of ACP connect subnets is done using traditional assignment methods and existing IPv6 protocols. See Section 8.1.3 for details. Therefore, the notion of /V-bits multiple addresses assigned to the ACP nodes does not apply to this sub-scheme.¶

6.11.5. ACP Vlong Addressing Sub-Scheme (ACP-Vlong-8/ACP-Vlong-16)

This addressing sub-scheme is used when the Type field of the base scheme is 1.¶

          50                              78
+---------------------++-----------------------------+----------+
|    (base scheme)    ||           Node-ID                      |
|                     || Registrar-ID |F| Node-Number|        V |
+---------------------++--------------+--------------+----------+
          50                46         1   23/15          8/16

This addressing sub-scheme foregoes the Zone-ID field (Section 6.11.3) to allow for larger, flatter routed networks (e.g., as in IoT) with 8,421,376 Node-Numbers (2²³ + 2¹⁵). It also allows for up to 2¹⁶ (i.e., 65,536) different virtualized addresses within a node, which could be used to address individual software components in an ACP node.¶

The fields are the same as in the Zone Addressing Sub-Scheme (Section 6.11.3) with the following refinements:¶

F:

Format bit. This bit determines the format of the subsequent bits.¶

V:

Virtualization bit: this is a field that is either 8 or 16 bits. For F=0, it is 8 bits, for F=1 it is 16 bits. The V-bits are assigned by the ACP node. In the ACP certificate's ACP address (Section 6.2.2), the V-bits are always set to 0.¶

Registrar-ID:

To maximize Node-Number and V, the Registrar-ID is reduced to 46 bits. One or more domain-wide unique identifiers of the ACP registrar can be used for this purpose. See Section 6.11.7.2.¶

Node-Number:

The Node-Number is unique to each ACP node. There are two formats for the Node-Number. When F=0, the Node-Number is 23 bits, for F=1, it is 15 bits. Each format of Node-Number is considered to be in a unique number space.¶

The F=0 bit format addresses are intended to be used for "general purpose" ACP nodes that would potentially have a limited number (less than 256) of clients (ASA and/or autonomic functions or legacy services) of the ACP that require separate V(irtual) addresses.¶

The F=1 bit Node-Numbers are intended for ACP nodes that are ACP edge nodes (see Section 8.1.1) or that have a large number of clients requiring separate V(irtual) addresses, for example, large SDN controllers with container modular software architecture (see Section 8.1.2).¶

In the Vlong Addressing Sub-Scheme, the ACP address in the certificate has all V field bits as zero. The ACP address set for the node includes any V value.¶

6.11.6. Other ACP Addressing Sub-Schemes

Before further addressing sub-schemes are defined, experience with the schemes defined here should be collected. The schemes defined in this document have been devised to allow hopefully a sufficiently flexible setup of ACPs for a variety of situations. These reasons also lead to the fairly liberal use of address space: the Zone Addressing Sub-Scheme is intended to enable optimized routing in large networks by reserving bits for Zone-IDs. The Vlong Addressing Sub-Scheme enables the allocation of 8/16-bit of addresses inside individual ACP nodes. Both address spaces allow distributed, uncoordinated allocation of node addresses by reserving bits for the Registrar-ID field in the address.¶

6.11.7. ACP Registrars

ACP registrars are responsible for enrolling candidate ACP nodes with ACP certificates and associated trust anchor(s). They are also responsible for including an acp-node-name field in the ACP certificate. This field carries the ACP domain name and the ACP node's ACP address prefix. This address prefix is intended to persist unchanged through the lifetime of the ACP node.¶

Because of the ACP addressing sub-schemes, an ACP domain can have multiple distributed ACP registrars that do not need to coordinate for address assignment. ACP registrars can also be sub-CAs, in which case they can also assign ACP certificates without dependencies against a (shared) TA (except during renewals of their own certificates).¶

ACP registrars are PKI registration authorities (RA) enhanced with the handling of the ACP certificate-specific fields. They request certificates for ACP nodes from a CA through any appropriate mechanism (out of scope in this document, but this mechanism is required to be BRSKI for ANI registrars). Only nodes that are trusted to be compliant with the registrar requirements described in this section can be given the necessary credentials to perform this RA function, such as the credential for the ACP registrar to connect to the CA as a registrar.¶

6.12.1. ACP RPL Profile

The following is a description of the RPL profile that ACP nodes need to support by default. The format of this section is derived from [ROLL-APPLICABILITY].¶

6.13.1. Performance

There are no performance requirements for ACP implementations defined in this document because the performance requirements depend on the intended use case. It is expected that a fully autonomic node with a wide range of ASA can require high forwarding plane performance in the ACP, for example, for telemetry. Implementations of ACP that solely support traditional or SDN-style use cases can benefit from ACP at lower performance, especially if the ACP is used only for critical operations, e.g., when the data plane is not available. The design of the ACP as specified in this document is intended to support a wide range of performance options: it is intended to allow software-only implementations at potentially low performance, but the design can also support high-performance options. See [RFC8368] for more details.¶

6.13.2. Addressing of Secure Channels

In order to be independent of the data plane routing and addressing, the ACP secure channels discovered via GRASP use IPv6 link-local addresses between adjacent neighbors. Note: Section 8.2 specifies extensions in which secure channels are configured tunnels operating over the data plane, so those secure channels cannot be independent of the data plane.¶

To avoid impacting the operations of the IPv6 (link-local) interface/address used for ACP channels when configuring the data plane, appropriate implementation considerations are required. If the IPv6 interface/link-local address is shared with the data plane, it needs to be impossible to unconfigure and/or disable it through configuration. Instead of sharing the IPv6 interface/link-local address, a separate (virtual) interface with a separate IPv6 link-local address can be used. For example, the ACP interface could be run over a separate MAC address of an underlying L2 (Ethernet) interface. For more details and options, see Appendix A.9.2.¶

Note that other (nonideal) implementation choices may introduce additional, undesired dependencies against the data plane, for example, shared code and configuration of the secure channel protocols (IPsec and/or DTLS).¶

6.13.3. MTU

The MTU for ACP secure channels MUST be derived locally from the underlying link MTU minus the secure channel encapsulation overhead.¶

ACP secure channel protocols do not need to perform MTU discovery because they are built across L2 adjacencies: the MTUs on both sides connecting to the L2 connection are assumed to be consistent. Extensions to ACP where the ACP is, for example, tunneled need to consider how to guarantee MTU consistency. This is an issue of tunnels, not an issue of running the ACP across a tunnel. Transport stacks running across ACP can perform normal PMTUD (Path MTU Discovery). Because the ACP is meant to prioritize reliability over performance, they MAY opt to only expect IPv6 minimum MTU (1280 octets) to avoid running into PMTUD implementation bugs or underlying link MTU mismatch problems.¶

6.13.4. Multiple Links between Nodes

If two nodes are connected via several links, the ACP SHOULD be established across every link, but it is possible to establish the ACP only on a subset of links. Having an ACP channel on every link has a number of advantages, for example, it allows for a faster failover in case of link failure, and it reflects the physical topology more closely. Using a subset of links (for example, a single link), reduces resource consumption on the node because state needs to be kept per ACP channel. The negotiation scheme explained in Section 6.6 allows the Decider (the node with the higher ACP address) to drop all but the desired ACP channels to the Follower, and the Follower will not retry to build these secure channels from its side unless the Decider appears with a previously unknown GRASP announcement (e.g., on a different link or with a different address announced in GRASP).¶

6.13.5. ACP Interfaces

Conceptually, the ACP VRF has two types of interfaces: the "ACP loopback interface(s)" to which the ACP ULA address(es) are assigned and the "ACP virtual interfaces" that are mapped to the ACP secure channels.¶

8.1.1. Non-ACP Controller and/or Network Management System (NMS)

The ACP can be used by management systems, such as controllers or NMS hosts, to connect to devices (or other type of nodes) through it. For this, an NMS host needs to have access to the ACP. The ACP is a self-protecting overlay network, which allows access only to trusted, autonomic systems by default. Therefore, a traditional, non-ACP NMS does not have access to the ACP by default, such as any other external node.¶

If the NMS host is not autonomic, i.e., it does not support autonomic negotiation of the ACP, then it can be brought into the ACP by explicit configuration. To support connections to adjacent non-ACP nodes, an ACP node SHOULD support "ACP connect" (sometimes also called "autonomic connect").¶

"ACP connect" is an interface-level, configured workaround for connection of trusted non-ACP nodes to the ACP. The ACP node on which ACP connect is configured is called an "ACP edge node". With ACP connect, the ACP is accessible from those non-ACP nodes (such as NOC systems) on such an interface without those non-ACP nodes having to support any ACP discovery or ACP channel setup. This is also called "native" access to the ACP because, to those NOC systems, the interface looks like a normal network interface without any ACP secure channel that is encapsulating the traffic.¶

                                 Data Plane "native" (no ACP)
                                          .
+--------+       +----------------+       .        +-------------+
| ACP    |       |ACP Edge Node   |       .        |             |
| Node   |       |                |       v        |             |
|        |-------|...[ACP VRF]....+----------------|             |+
|        |   ^   |.               |                | NOC Device  ||
|        |   .   | .[Data Plane]..+----------------| "NMS hosts" ||
|        |   .   |  [          ]  | .         ^    |             ||
+--------+   .   +----------------+  .        .    +-------------+|
             .                        .       .     +-------------+
             .                        .       .
          Data Plane "native"         .   ACP "native" (unencrypted)
        + ACP auto-negotiated         .   "ACP connect subnet"
          and encrypted               .
                                    ACP connect interface
                                    e.g., "VRF ACP native" (config)

ACP connect has security consequences: all systems and processes connected via ACP connect have access to all ACP nodes on the entire ACP, without further authentication. Thus, the ACP connect interface and the NOC systems connected to it need to be physically controlled and/or secured. For this reason, the mechanisms described here explicitly do not include options to allow for a non-ACP router to be connected across an ACP connect interface and addresses behind such a router routed inside the ACP.¶

Physically controlled and/or secured means that attackers cannot gain access to the physical device hosting the ACP edge node, the physical interfaces and links providing the ACP connect link, nor the physical devices hosting the NOC device. In a simple case, ACP edge node and NOC device are colocated in an access-controlled room, such as a NOC, to which attackers cannot gain physical access.¶

An ACP connect interface provides exclusive access to only the ACP. This is likely insufficient for many NMS hosts. Instead, they would require a second "data plane" interface outside the ACP for connections between the NMS host and administrators, or Internet-based services, or for direct access to the data plane. The document "Using Autonomic Control Plane for Stable Connectivity of Network OAM" [RFC8368] explains in more detail how the ACP can be integrated in a mixed NOC environment.¶

An ACP connect interface SHOULD use an IPv6 address/prefix from the Manual Addressing Sub-Scheme (Section 6.11.4), letting the operator configure, for example, only the Subnet-ID and having the node automatically assign the remaining part of the prefix/address. It SHOULD NOT use a prefix that is also routed outside the ACP so that the addresses clearly indicate whether it is used inside the ACP or not.¶

The prefix of ACP connect subnets MUST be distributed by the ACP edge node into the ACP routing protocol, RPL. The NMS host MUST connect to prefixes in the ACP routing table via its ACP connect interface. In the simple case where the ACP uses only one ULA prefix, and all ACP connect subnets have prefixes covered by that ULA prefix, NMS hosts can rely on [RFC6724] to determine longest match prefix routes towards its different interfaces, ACP and data plane. With [RFC6724], the NMS host will select the ACP connect interface for all addresses in the ACP because any ACP destination address is longest matched by the address on the ACP connect interface. If the NMS host's ACP connect interface uses another prefix, or if the ACP uses multiple ULA prefixes, then the NMS host requires (static) routes towards the ACP interface for these prefixes.¶

When an ACP edge node receives a packet from an ACP connect interface, the ACP edge node MUST only forward the packet to the ACP if the packet has an IPv6 source address from that interface (this is sometimes called Reverse Path Forwarding (RPF) filtering). This filtering rule MAY be changed through administrative measures. The more any such administrative action enables reachability of non-ACP nodes to the ACP, the more this may cause security issues.¶

To limit the security impact of ACP connect, nodes supporting it SHOULD implement a security mechanism to allow configuration and/or use of ACP connect interfaces only on nodes explicitly targeted to be deployed with it (those in physically secure locations such as a NOC). For example, the registrar could disable the ability to enable ACP connect on devices during enrollment, and that property could only be changed through reenrollment. See also Appendix A.9.5.¶

ACP edge nodes SHOULD have a configurable option to prohibit packets with RPI headers (see Section 6.12.1.13) across an ACP connect interface. These headers are outside the scope of the RPL profile in this specification but may be used in future extensions of this specification.¶

8.1.2. Software Components

The previous section assumed that the ACP edge node and NOC devices are separate physical devices and that the ACP connect interface is a physical network connection. This section discusses the implication when these components are instead software components running on a single physical device.¶

The ACP connect mechanism can be used not only to connect physically external systems (NMS hosts) to the ACP but also other applications, containers, or virtual machines. In fact, one possible way to eliminate the security issue of the external ACP connect interface is to colocate an ACP edge node and an NMS host by making one a virtual machine or container inside the other; therefore converting the unprotected external ACP subnet into an internal virtual subnet in a single device. This would ultimately result in a fully ACP-enabled NMS host with minimum impact to the NMS host's software architecture. This approach is not limited to NMS hosts but could equally be applied to devices consisting of one or more VNF (virtual network functions): an internal virtual subnet connecting out-of-band management interfaces of the VNFs to an ACP edge router VNF.¶

The core requirement is that the software components need to have a network stack that permits access to the ACP and optionally also to the data plane. Like in the physical setup for NMS hosts, this can be realized via two internal virtual subnets: one that connects to the ACP (which could be a container or virtual machine by itself), and one (or more) connecting to the data plane.¶

This "internal" use of the ACP connect approach should not be considered to be a "workaround" because, in this case, it is possible to build a correct security model: it is not necessary to rely on unprovable, external physical security mechanisms as in the case of external NMS hosts. Instead, the orchestration of the ACP, the virtual subnets, and the software components can be done by trusted software that could be considered to be part of the ANI (or even an extended ACP). This software component is responsible for ensuring that only trusted software components will get access to that virtual subnet and that only even more trusted software components will get access to both the ACP virtual subnet and the data plane (because those ACP users could leak traffic between ACP and data plane). This trust could be established, for example, through cryptographic means such as signed software packages.¶

8.1.3. Autoconfiguration

ACP edge nodes, NMS hosts, and software components that, as described in the previous section, are meant to be composed via virtual interfaces SHOULD support SLAAC [RFC4862] on the ACP connect subnet and route autoconfiguration according to "Default Router Preferences and More-Specific Routes" [RFC4191].¶

The ACP edge node acts as the router towards the ACP on the ACP connect subnet, providing the (auto)configured prefix for the ACP connect subnet and (auto)configured routes to the ACP to NMS hosts and/or software components.¶

The ACP edge node uses the Route Information Option (RIO) of [RFC4191] to announce aggregated prefixes for address prefixes used in the ACP (with normal RIO lifetimes). In addition, the ACP edge node also uses a RIO to announce the default route (::/0) with a lifetime of 0.¶

These RIOs allow the connecting of type C hosts to the ACP via an ACP connect subnet on one interface and another network (Data Plane and/or NMS network) on the same or another interface of the type C host, relying on routers other than the ACP edge node. The RIOs ensure that these hosts will only route the prefixes used in the ACP to the ACP edge node.¶

Type A and B hosts ignore the RIOs and will consider the ACP node to be their default router for all destinations. This is sufficient when the type A or type B host only needs to connect to the ACP but not to other networks. Attaching a type A or type B host to both the ACP and other networks requires explicit ACP prefix route configuration on either the host or the combined ACP and data plane interface on the ACP edge node, see Section 8.1.4.¶

Aggregated prefix means that the ACP edge node needs to only announce the /48 ULA prefixes used in the ACP but none of the actual /64 (Manual Addressing Sub-Scheme), /127 (Zone Addressing Sub-Scheme), /112 or /120 (Vlong Addressing Sub-Scheme) routes of actual ACP nodes. If ACP interfaces are configured with non-ULA prefixes, then those prefixes cannot be aggregated without further configured policy on the ACP edge node. This explains the above recommendation to use ACP ULA prefixes for ACP connect interfaces: they allow for a shorter list of prefixes to be signaled via [RFC4191] to NMS hosts and software components.¶

The ACP edge nodes that have a Vlong ACP address MAY allocate a subset of their /112 or /120 address prefix to ACP connect interface(s) to eliminate the need to non-autonomically configure and/or provision the address prefixes for such ACP connect interfaces.¶

8.1.4. Combined ACP and Data Plane Interface (VRF Select)

                     Combined ACP and data plane interface
                                             .
  +--------+       +--------------------+    .   +--------------+
  | ACP    |       |ACP Edge No         |    .   | NMS Host(s)  |
  | Node   |       |                    |    .   | / Software   |
  |        |       |  [ACP  ].          |    .   |              |+
  |        |       | .[VRF  ] .[VRF   ] |    v   | "ACP Address"||
  |        +-------+.         .[Select].+--------+ "Data Plane  ||
  |        |   ^   | .[Data ].          |        |  Address(es)"||
  |        |   .   |  [Plane]           |        |              ||
  |        |   .   |  [     ]           |        +--------------+|
  +--------+   .   +--------------------+         +--------------+
               .
        Data plane "native" and + ACP auto-negotiated/encrypted

Using two physical and/or virtual subnets (and therefore interfaces) to NMS hosts (as per Section 8.1.1) or software (as per Section 8.1.2) may be seen as additional complexity, for example, with legacy NMS hosts that support only one IP interface, or it may be insufficient to support type A or type B hosts [RFC4191] (see Section 8.1.3).¶

To provide a single subnet to both the ACP and Data plane, the ACP edge node needs to demultiplex packets from NMS hosts into ACP VRF and data plane. This is sometimes called "VRF select". If the ACP VRF has no overlapping IPv6 addresses with the data plane (it should have no overlapping addresses), then this function can use the IPv6 destination address. The problem is source address selection on the NMS host(s) according to [RFC6724].¶

Consider the simple case: the ACP uses only one ULA prefix, and the ACP IPv6 prefix for the combined ACP and data plane interface is covered by that ULA prefix. The ACP edge node announces both the ACP IPv6 prefix and one (or more) prefixes for the data plane. Without further policy configurations on the NMS host(s), it may select its ACP address as a source address for data plane ULA destinations because of Rule 8 (Section 5 of [RFC6724]). The ACP edge node can pass on the packet to the data plane, but the ACP source address should not be used for data plane traffic, and return traffic may fail.¶

If the ACP carries multiple ULA prefixes or non-ULA ACP connect prefixes, then the correct source address selection becomes even more problematic.¶

With separate ACP connect and data plane subnets and prefix announcements [RFC4191] that are to be routed across the ACP connect interface, the source address selection of Rule 5 (use address of outgoing interface) (Section 5 of [RFC6724]) will be used, so that above problems do not occur, even in more complex cases of multiple ULA and non-ULA prefixes in the ACP routing table.¶

To achieve the same behavior with a combined ACP and data plane interface, the ACP edge node needs to behave as two separate routers on the interface: one link-local IPv6 address/router for its ACP reachability, and one link-local IPv6 address/router for its data plane reachability. The Router Advertisements for both are as described in Section 8.1.3: for the ACP, the ACP prefix is announced together with the prefix option [RFC4191] routed across the ACP, and the lifetime is set to zero to disqualify this next hop as a default router. For the data plane, the data plane prefix(es) are announced together with whatever default router parameters are used for the data plane.¶

As a result, source address selection Rule 5.5 (Section 5 of [RFC6724]) may result in the same correct source address selection behavior of NMS hosts without further configuration as the separate ACP connect and data plane interfaces on the host. As described in the text for Rule 5.5 (Section 5 of [RFC6724]), this is only a MAY because IPv6 hosts are not required to track next-hop information. If an NMS host does not do this, then separate ACP connect and data plane interfaces are the preferable method of attachment. Hosts implementing "First-Hop Router Selection by Hosts in a Multi-Prefix Network" [RFC8028] should (instead of may) implement Rule 5.5 (Section 5 of [RFC6724]), so it is preferred for hosts to support [RFC8028].¶

ACP edge nodes MAY support the combined ACP and data plane interface.¶

8.1.5. Use of GRASP

GRASP can and should be possible to use across ACP connect interfaces, especially in the architecturally correct solution when it is used as a mechanism to connect software (e.g., ASA or legacy NMS applications) to the ACP.¶

Given how the ACP is the security and transport substrate for GRASP, the requirements are that those devices connected via ACP connect are equivalently (if not better) secured against attacks than ACP nodes that do not use ACP connect, and they run only software that is equally (if not better) protected, known (or trusted) not to be malicious, and accordingly designed to isolate access to the ACP against external equipment.¶

The difference in security is that cryptographic security of the ACP secure channel is replaced by required physical security and/or control of the network connection between an ACP edge node and the NMS or other host reachable via the ACP connect interface. See Section 8.1.1.¶

When using the combined ACP and data plane interface, care has to be taken that only GRASP messages received from software or NMS hosts and intended for the ACP GRASP domain are forwarded by ACP edge nodes. Currently there is no definition for a GRASP security and transport substrate beside the ACP, so there is no definition how such software/NMS host could participate in two separate GRASP domains across the same subnet (ACP and data plane domains). Currently it is assumed that all GRASP packets on a combined ACP and data plane interface belong to the GRASP ACP domain. They SHOULD all use the ACP IPv6 addresses of the software/NMS hosts. The link-local IPv6 addresses of software/NMS hosts (used for GRASP M_DISCOVERY and M_FLOOD messages) are also assumed to belong to the ACP address space.¶

8.2.1. Configured Remote ACP Neighbor

On the ACP node, remote ACP neighbors are configured explicitly. The parameters of such a "connection" are described in the following ABNF. The syntax shown is non-normative (as there are no standards for configuration) but only meant to illustrate the parameters and which ones can be optional.¶

  connection = method "," local-addr "," remote-addr [ "," pmtu ]
  method =    "any"
             / ( "IKEv2" [ ":" port ] )
             / (  "DTLS"  [ ":"  port ] )
  port = 1*DIGIT
  local-addr  = [ address  [ ":" vrf  ] ]
  remote-addr =   address
  address =  "any"
             / IPv4address / IPv6address  ; From [RFC5954]
  vrf = system-dependent ; Name of VRF for local-address

Explicit configuration of a remote peer according to this ABNF provides all the information to build a secure channel without requiring a tunnel to that peer and running DULL GRASP inside of it.¶

The configuration includes the parameters otherwise signaled via DULL GRASP: local address, remote (peer) locator, and method. The differences over DULL GRASP local neighbor discovery and secure channel creation are as follows:¶

• The local and remote address can be IPv4 or IPv6 and are typically global-scope addresses.¶
• The VRF across which the connection is built (and in which local-addr exists) can be specified. If vrf is not specified, it is the default VRF on the node. In DULL GRASP, the VRF is implied by the interface across which DULL GRASP operates.¶
• If local address is "any", the local address used when initiating a secure channel connection is decided by source address selection ([RFC6724] for IPv6). As a responder, the connection listens on all addresses of the node in the selected VRF.¶
• Configuration of port is only required for methods where no defaults exist (e.g., "DTLS").¶
• If the remote address is "any", the connection is only a responder. It is a "hub" that can be used by multiple remote peers to connect simultaneously -- without having to know or configure their addresses, for example, a hub site for remote "spoke" sites reachable over the Internet.¶
• The pmtu parameter should be configurable to overcome issues or limitations of Path MTU Discovery (PMTUD).¶
• IKEv2/IPsec to remote peers should support the optional NAT Traversal (NAT-T) procedures.¶

8.2.2. Tunneled Remote ACP Neighbor

An IP-in-IP, GRE, or other form of preexisting tunnel is configured between two remote ACP peers, and the virtual interfaces representing the tunnel are configured for "ACP enable". This will enable IPv6 link-local addresses and DULL on this tunnel. As a result, the tunnel is used for normal "L2 adjacent" candidate ACP neighbor discovery with DULL and secure channel setup procedures described in this document.¶

Tunneled Remote ACP Neighbor requires two encapsulations: the configured tunnel and the secure channel inside of that tunnel. This makes it in general less desirable than Configured Remote ACP Neighbor. Benefits of tunnels are that it may be easier to implement because there is no change to the ACP functionality - just running it over a virtual (tunnel) interface instead of only native interfaces. The tunnel itself may also provide PMTUD while the secure channel method may not. Or the tunnel mechanism is permitted/possible through some firewall while the secure channel method may not.¶

Tunneling using an insecure tunnel encapsulation increases, on average, the risk of a MITM downgrade attack somewhere along the underlay path. In such an attack, the MITM filters packets for all but the most easily attacked ACP secure channel option to force use of that option. ACP nodes supporting Tunneled Remote ACP Neighbors SHOULD support configuration on such tunnel interfaces to restrict or explicitly select the available ACP secure channel protocols (if the ACP node supports more than one ACP secure channel protocol in the first place).¶

8.2.3. Summary

Configured and Tunneled Remote ACP Neighbors are less "indestructible" than L2 adjacent ACP neighbors based on link-local addressing, since they depend on more correct data plane operations, such as routing and global addressing.¶

Nevertheless, these options may be crucial to incrementally deploying the ACP, especially if it is meant to connect islands across the Internet. Implementations SHOULD support at least Tunneled Remote ACP Neighbors via GRE tunnels, which is likely the most common router-to-router tunneling protocol in use today.¶

9.1.1. Secure Channel Peer Diagnostics

When using mutual certificate authentication, the TA certificate is not required to be signaled explicitly because its hash is sufficient for certificate chain validation. In the case of ACP secure channel setup, this leads to limited diagnostics when authentication fails because of TA mismatch. For this reason, Section 6.8.2 recommends also including the TA certificate in the secure channel signaling. This should be possible to do without modifying the security association protocols used by the ACP. For example, while [RFC7296] does not mention this, it also does not prohibit it.¶

One common use case where diagnostics through the signaled TA of a candidate peer are very helpful is the multi-tenant environment, such as an office building, where different tenants run their own networks and ACPs. Each tenant is given supposedly disjoint L2 connectivity through the building infrastructure. In these environments, there are various common errors through which a device may receive L2 connectivity into the wrong tenant's network.¶

While the ACP itself is not impacted by this, the data plane to be built later may be impacted. Therefore, it is important to be able to diagnose such undesirable connectivity from the ACP so that any autonomic or non-autonomic mechanisms to configure the data plane can treat such interfaces accordingly. The information in the TA of the peer can then ease troubleshooting of such issues.¶

Another use case is the intended or accidental reactivation of equipment, such as redundant gear taken from storage, whose TA certificate has long expired.¶

A third use case is when, in a merger and acquisition case, ACP nodes have not been correctly provisioned with the mutual TA of a previously disjoint ACP. This assumes that the ACP domain names were already aligned so that the ACP domain membership check is only failing on the TA.¶

A fourth use case is when multiple registrars are set up for the same ACP but are not correctly set up with the same TA. For example, when registrars support also being CAs themselves but are misconfigured to become TAs instead of intermediate CAs.¶

9.2.1. Registrar Interactions

This section summarizes and discusses the interactions with other entities required by an ACP registrar.¶

In a simple instance of an ACP network, no central NOC component beside a TA is required. Typically, this is a root CA. One or more uncoordinated acting ACP registrars can be set up, performing the following interactions.¶

To orchestrate enrolling a candidate ACP node autonomically, the ACP registrar can rely on the ACP and use proxies to reach the candidate ACP node, therefore allowing minimal, preexisting (auto)configured network services on the candidate ACP node. BRSKI defines the BRSKI proxy, a design that can be adopted for various protocols that pledges and/or candidate ACP nodes could want to use, for example, BRSKI over CoAP (Constrained Application Protocol) or the proxying of NETCONF.¶

To reach a TA that has no ACP connectivity, the ACP registrar uses the data plane. The ACP and data plane in an ACP registrar could (and by default should) be completely isolated from each other at the network level. Only applications such as the ACP registrar would need the ability for their transport stacks to access both.¶

In non-autonomic enrollment options, the data plane between an ACP registrar and the candidate ACP node needs to be configured first. This includes the ACP registrar and the candidate ACP node. Then any appropriate set of protocols can be used between the ACP registrar and the candidate ACP node to discover the other side, and then connect and enroll (configure) the candidate ACP node with an ACP certificate. For example, NETCONF Zero Touch ("Secure Zero Touch Provisioning (SZTP)" [RFC8572]) is a protocol that could be used for this. BRSKI using optional discovery mechanisms is equally a possibility for candidate ACP nodes attempting to be enrolled across non-ACP networks, such as the Internet.¶

When a candidate ACP node, such as a BRSKI pledge, has secure bootstrap, it will not trust being configured and/or enrolled across the network unless it is presented with a voucher (see "A Voucher Artifact for Bootstrapping Protocols" [RFC8366]) authorizing the network to take possession of the node. An ACP registrar will then need a method to retrieve such a voucher, either offline or online from a MASA (Manufacturer Authorized Signing Authority). BRSKI and NETCONF Zero Touch are two protocols that include capabilities to present the voucher to the candidate ACP node.¶

An ACP registrar could operate EST for ACP certificate renewal and/or act as a CRL Distribution Point. A node performing these services does not need to support performing (initial) enrollment, but it does require the same above described connectivity as an ACP registrar: via the ACP to the ACP nodes and via the data plane to the TA and other sources of CRL information.¶

9.2.2. Registrar Parameters

The interactions of an ACP registrar outlined in Section 6.11.7 and Section 9.2.1 depend on the following parameters:¶

• A URL to the TA and credentials so that the ACP registrar can let the TA sign candidate ACP node certificates.¶
• The ACP domain name.¶
• The Registrar-ID to use. This could default to a MAC address of the ACP registrar.¶
• For recovery, the next usable Node-IDs for the Zone Addressing Sub-Scheme (Zone-ID 0) and for the Vlong Addressing Sub-Scheme (/112 and /120). These IDs would only need to be provisioned after recovering from a crash. Some other mechanism would be required to remember these IDs in a backup location or to recover them from the set of currently known ACP nodes.¶
• Policies on whether the candidate ACP nodes should receive a domain certificate or not, for example, based on the device's IDevID certificate as in BRSKI. The ACP registrar may whitelist or blacklist based on a device's "serialNumber" attribute [X.520] in the subject field distinguished name encoding of its IDevID certificate.¶
• Policies on what type of address prefix to assign to a candidate ACP device, likely based on the same information.¶
• For BRSKI or other mechanisms using vouchers: parameters to determine how to retrieve vouchers for specific types of secure bootstrap candidate ACP nodes (such as MASA URLs), unless this information is automatically learned, such as from the IDevID certificate of candidate ACP nodes (as defined in BRSKI).¶

9.2.3. Certificate Renewal and Limitations

When an ACP node renews and/or rekeys its certificate, it may end up doing so via a different registrar (e.g., EST server) than the one it originally received its ACP certificate from, for example, because that original ACP registrar is gone. The ACP registrar through which the renewal/rekeying is performed would by default trust the acp-node-name from the ACP node's current ACP certificate and maintain this information so that the ACP node maintains its ACP address prefix. In EST renewal/rekeying, the ACP node's current ACP certificate is signaled during the TLS handshake.¶

This simple scenario has two limitations:¶

1. The ACP registrar cannot directly assign certificates to nodes and therefore needs an "online" connection to the TA.¶
2. Recovery from a compromised ACP registrar is difficult. When an ACP registrar is compromised, it can insert, for example, a conflicting acp-node-name and thereby create an attack against other ACP nodes through the ACP routing protocol.¶

Even when such a malicious ACP registrar is detected, resolving the problem may be difficult because it would require identifying all the wrong ACP certificates assigned via the ACP registrar after it was compromised. Without additional centralized tracking of assigned certificates, there is no way to do this.¶

9.2.4. ACP Registrars with Sub-CA

In situations where either of the above two limitations are an issue, ACP registrars could also be sub-CAs. This removes the need for connectivity to a TA whenever an ACP node is enrolled, and it reduces the need for connectivity of such an ACP registrar to a TA to only those times when it needs to renew its own certificate. The ACP registrar would also now use its own (sub-CA) certificate to enroll and sign the ACP node's certificates, and therefore it is only necessary to revoke a compromised ACP registrar's sub-CA certificate. Alternatively, one can let it expire and not renew it when the certificate of the sub-CA is appropriately short-lived.¶

As the ACP domain membership check verifies a peer ACP node's ACP certificate trust chain, it will also verify the signing certificate, which is the compromised and/or revoked sub-CA certificate. Therefore, ACP domain membership for an ACP node enrolled by a compromised and discovered ACP registrar will fail.¶

ACP nodes enrolled by a compromised ACP registrar would automatically fail to establish ACP channels and ACP domain certificate renewal via EST and therefore revert to their role as candidate ACP members and attempt to get a new ACP certificate from an ACP registrar, for example, via BRSKI. As a result, ACP registrars that have an associated sub-CA make isolating and resolving issues with compromised registrars easier.¶

Note that ACP registrars with sub-CA functionality also can control the lifetime of ACP certificates more easily and therefore can be used as a tool to introduce short-lived certificates and to no longer rely on CRL, whereas the certificates for the sub-CAs themselves could be longer lived and subject to CRL.¶

9.2.5. Centralized Policy Control

When using multiple, uncoordinated ACP registrars, several advanced operations are potentially more complex than with a single, resilient policy control backend, for example, including but not limited to the following:¶

• Deciding which candidate ACP node is permitted or not permitted into an ACP domain. This may not be a decision to be made upfront, so that a policy per "serialNumber" attribute in the subject field distinguished name encoding can be loaded into every ACP registrar. Instead, it may better be decided in real time, potentially including a human decision in a NOC.¶
• Tracking all enrolled ACP nodes and their certificate information. For example, in support of revoking an individual ACP node's certificates.¶
• Needing more flexible policies as to which type of address prefix or even which specific address prefix to assign to a candidate ACP node.¶

These and other operations could be introduced more easily by introducing a centralized Policy Management System (PMS) and modifying ACP registrar behavior so that it queries the PMS for any policy decision occurring during the candidate ACP node enrollment process and/or the ACP node certificate renewal process, for example, which ACP address prefix to assign. Likewise, the ACP registrar would report any relevant state change information to the PMS as well, for example, when a certificate was successfully enrolled onto a candidate ACP node.¶

9.3.1. Filtering for Non-ACP/ANI Packets

Whenever this document refers to enabling an interface for ACP (or BRSKI), it only requires permitting the interface to send and receive packets necessary to operate ACP (or BRSKI) -- but not any other data plane packets. Unless the data plane is explicitly configured and enabled, all packets that are not required for ACP/BRSKI should be filtered on input and output.¶

Both BRSKI and ACP require link-local-only IPv6 operations on interfaces and DULL GRASP. IPv6 link-local operations mean the minimum signaling to auto-assign an IPv6 link-local address and talk to neighbors via their link-local addresses: SLAAC [RFC4862] and ND [RFC4861]. When the device is a BRSKI pledge, it may also require TCP/TLS connections to BRSKI proxies on the interface. When the device has keying material, and the ACP is running, it requires DULL GRASP packets and packets necessary for the secure channel mechanism it supports, e.g., IKEv2 and IPsec ESP packets or DTLS packets to the IPv6 link-local address of an ACP neighbor on the interface. It also requires TCP/TLS packets for its BRSKI proxy functionality if it supports BRSKI.¶

9.3.2. "admin down" State

Interfaces on most network equipment have at least two states: "up" and "down". These may have product-specific names. For example, "down" could be called "shutdown", and "up" could be called "no shutdown". The "down" state disables all interface operations down to the physical level. The "up" state enables the interface enough for all possible L2/L3 services to operate on top of it, and it may also auto-enable some subset of them. More commonly, the operations of various L2/L3 services are controlled via additional node-wide or interface-level options, but they all become active only when the interface is not "down". Therefore, an easy way to ensure that all L2/L3 operations on an interface are inactive is to put the interface into "down" state. The fact that this also physically shuts down the interface is just a side effect in many cases, but it may be important in other cases (see Section 9.3.2.2).¶

A common problem of remote management is the operator or SDN controller cutting its own connectivity to the remote node via configuration, impacting its own management connection to the node. The ACP itself should have no dedicated configuration other than the aforementioned enabling of the ACP on brownfield ACP nodes. This leaves configuration that cannot distinguish between the ACP and data plane as sources of configuration mistakes as these commands will impact the ACP even though they should only impact the data plane.¶

The one ubiquitous type of command that does this on many types of routers is the interface "down" command/configuration. When such a command is applied to the interface through which the ACP provides access for remote management, it cuts the remote management connection through the ACP because, as outlined above, the "down" command typically impacts the physical layer, too, and not only the data plane services.¶

To provide ACP/ANI resilience against such operator misconfiguration, this document recommends separating the "down" state of interfaces into an "admin down" state, where the physical layer is kept running and the ACP/ANI can use the interface, and a "physical down" state. Any existing "down" configurations would map to "admin down". In "admin down", any existing L2/L3 services of the data plane should see no difference to "physical down" state. To ensure that no data plane packets could be sent or received, packet filtering could be established automatically as described in Section 9.3.1.¶

An example of ANI, but not ACP, traffic that should be permitted to pass even in "admin down" state is BRSKI enrollment traffic between a BRSKI pledge and a BRSKI proxy.¶

New configuration options could be introduced as necessary (see discussion below) to issue "physical down". The options should be provided with additional checks to minimize the risk of issuing them in a way that breaks the ACP without automatic restoration. Examples of checks include not allowing the option to be issued from a control connection (NETCONF/SSH) that goes across the interface itself ("do not disconnect yourself") or only applying the option after additional reconfirmation.¶

The following subsections discuss important aspects of the introduction of "admin down" state.¶

9.3.3. Enabling Interface-Level ACP and ANI

The interface-level configuration option "ACP enable" enables ACP operations on an interface, starting with ACP neighbor discovery via DULL GRASP. The interface-level configuration option "ANI enable" on nodes supporting BRSKI and ACP starts with BRSKI pledge operations when there is no domain certificate on the node. On ACP/BRSKI nodes, only "ANI enable" may need to be supported and not "ACP enable". Unless overridden by global configuration options (see Section 9.3.4), either "ACP enable" or "ANI enable" (both abbreviated as "ACP/ANI enable") will result in the "down" state on an interface behaving as "admin down".¶

9.3.4. Which Interfaces to Auto-Enable?

Section 6.4 requires that "ACP enable" is automatically set on native interfaces, but not on non-native interfaces (reminder: a native interface is one that exists without operator configuration action, such as physical interfaces in physical devices).¶

Ideally, "ACP enable" is set automatically on all interfaces that provide access to additional connectivity, which allows more nodes of the ACP domain to be reached. The best set of interfaces necessary to achieve this is not possible to determine automatically. Native interfaces are the best automatic approximation.¶

Consider an ACP domain of ACP nodes transitively connected via native interfaces. A data plane tunnel between two of these nodes that are nonadjacent is created, and "ACP enable" is set for that tunnel. ACP RPL sees this tunnel as just as a single hop. Routes in the ACP would use this hop as an attractive path element to connect regions adjacent to the tunnel nodes. As a result, the actual hop-by-hop paths used by traffic in the ACP can become worse. In addition, correct forwarding in the ACP now depends on correct data plane forwarding configuration including QoS, filtering, and other security on the data plane path across which this tunnel runs. This is the main reason why "ACP/ANI enable" should not be set automatically on non-native interfaces.¶

If the tunnel would connect two previously disjoint ACP regions, then it likely would be useful for the ACP. A data plane tunnel could also run across nodes without ACP and provide additional connectivity for an already connected ACP network. The benefit of this additional ACP redundancy has to be weighed against the problems of relying on the data plane. If a tunnel connects two separate ACP regions, how many tunnels should be created to connect these ACP regions reliably enough? Between which nodes? These are all standard tunneled network design questions not specific to the ACP, and there are no generic, fully automated answers.¶

Instead of automatically setting "ACP enable" on these types of interfaces, the decision needs to be based on the use purpose of the non-native interface, and "ACP enable" needs to be set in conjunction with the mechanism through which the non-native interface is created and/or configured.¶

In addition to the explicit setting of "ACP/ANI enable", non-native interfaces also need to support configuration of the ACP RPL cost of the link to avoid the problems of attracting too much traffic to the link as described above.¶

Even native interfaces may not be able to automatically perform BRSKI or ACP because they may require additional operator input to become operational. Examples include DSL interfaces requiring Point-to-Point Protocol over Ethernet (PPPoE) credentials or mobile interfaces requiring credentials from a SIM card. Whatever mechanism is used to provide the necessary configuration to the device to enable the interface can also be expanded to decide whether or not to set "ACP/ANI enable".¶

The goal of automatically setting "ACP/ANI enable" on interfaces (native or not) is to eliminate unnecessary "touches" to the node to make its operation as much as possible "zero-touch" with respect to ACP/ANI. If there are "unavoidable touches" such a creating and/or configuring a non-native interface or provisioning credentials for a native interface, then "ACP/ANI enable" should be added as an option to that "touch". If an erroneous "touch" is easily fixed (does not create another high-cost touch), then the default should be not to enable ANI/ACP, and if it is potentially expensive or slow to fix (e.g., parameters on SIM card shipped to remote location), then the default should be to enable ACP/ANI.¶

9.3.5. Enabling Node-Level ACP and ANI

A node-level command "ACP/ANI enable [up-if-only]" enables ACP or ANI on the node (ANI = ACP + BRSKI). Without this command set, any interface-level "ACP/ANI enable" is ignored. Once set, ACP/ANI will operate an interface where "ACP/ANI enable" is set. Setting of interface-level "ACP/ANI enable" is either automatic (default) or explicit through operator action as described in Section 9.3.4.¶

If the option "up-if-only" is selected, the behavior of "down" interfaces is unchanged, and ACP/ANI will only operate on interfaces where "ACP/ANI enable" is set and that are "up". When it is not set, then "down" state of interfaces with "ACP/ANI enable" is modified to behave as "admin down".¶

9.3.6. Undoing "ANI/ACP enable"

Disabling ANI/ACP by undoing "ACP/ANI enable" is a risk for the reliable operations of the ACP if it can be executed by mistake or without authorization. This behavior could be influenced through some additional (future) property in the certificate (e.g., in the acp-node-name extension field): in an ANI deployment intended for convenience, disabling it could be allowed without further constraints. In an ANI deployment considered to be critical, more checks would be required. One very controlled option would be to not permit these commands unless the domain certificate has been revoked or is denied renewal. Configuring this option would be a parameter on the BRSKI registrar(s). As long as the node did not receive a domain certificate, undoing "ANI/ACP enable" should not have any additional constraints.¶

9.3.7. Summary

Node-wide "ACP/ANI enable [up-if-only]" commands enable the operation of ACP/ANI. This is only auto-enabled on ANI greenfield devices, otherwise it must be configured explicitly.¶

If the option "up-if-only" is not selected, interfaces enabled for ACP/ANI interpret the "down" state as "admin down" and not "physical down". In the "admin-down" state, all non-ACP/ANI packets are filtered, but the physical layer is kept running to permit ACP/ANI to operate.¶

(New) commands that result in physical interruption ("physical down", "loopback") of ACP/ANI-enabled interfaces should be built to protect continuance or reestablishment of ACP as much as possible.¶

Interface-level "ACP/ANI enable" commands control per-interface operations. It is enabled by default on native interfaces and has to be configured explicitly on other interfaces.¶

Disabling "ACP/ANI enable" globally and per interface should have additional checks to minimize undesired breakage of ACP. The degree of control could be a domain-wide parameter in the domain certificates.¶

10.2.1. From the Outside

As explained in Section 6, the ACP is based on secure channels built between nodes that have mutually authenticated each other with their domain certificates. The channels themselves are protected using standard encryption technologies such as DTLS or IPsec, which provide additional authentication during channel establishment, data integrity, and data confidentiality protection inside the ACP, and also provide replay protection.¶

An attacker will not be able to join the ACP unless it has a valid ACP certificate. An on-path attacker without a valid ACP certificate cannot inject packets into the ACP due to ACP secure channels. An attacker also cannot decrypt ACP traffic unless it can crack the encryption. It can attempt behavioral traffic analysis on the encrypted ACP traffic.¶

The degree to which compromised ACP nodes can impact the ACP depends on the implementation of the ACP nodes and their impairment. When an attacker has only gained administrative privileges to configure ACP nodes remotely, the attacker can disrupt the ACP only through one of the few configuration options to disable it (see Section 9.3) or by the configuring of non-autonomic ACP options if those are supported on the impaired ACP nodes (see Section 8). Injecting traffic into or extracting traffic from an impaired ACP node is only possible when an impaired ACP node supports ACP connect (see Section 8.1), and the attacker can control traffic into/from one of the ACP node's interfaces, such as by having physical access to the ACP node.¶

The ACP also serves as protection (through authentication and encryption) for protocols relevant to OAM that may not have secured protocol stack options or where implementation or deployment of those options fail due to some vendor, product, or customer limitations. This includes protocols such as SNMP ("An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks" [RFC3411]), NTP [RFC5905], PTP (Precision Time Protocol [IEEE-1588-2008]), DNS ("DNS Extensions to Support IP Version 6" [RFC3596]), DHCPv6 ("Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" [RFC3315]), syslog ("The BSD Syslog Protocol" [RFC3164]), RADIUS ("Remote Authentication Dial In User Service (RADIUS)" [RFC2865]), Diameter ("Diameter Base Protocol" [RFC6733]), TACACS ("An Access Control Protocol, Sometimes Called TACACS" [RFC1492]), IPFIX ("Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of Flow Information" [RFC7011]), NetFlow ("Cisco Systems NetFlow Services Export Version 9" [RFC3954]) -- just to name a few. Not all of these protocol references are necessarily the latest version of protocols, but they are versions that are still widely deployed.¶

Protection via the ACP secure hop-by-hop channels for these protocols is meant to be only a stopgap, though: the ultimate goal is for these and other protocols to use end-to-end encryption utilizing the domain certificate and to rely on the ACP secure channels primarily for zero-touch reliable connectivity, but not primarily for security.¶

The remaining attack vector would be to attack the underlying ACP protocols themselves, either via directed attacks or by denial-of-service attacks. However, as the ACP is built using link-local IPv6 addresses, remote attacks from the data plane are impossible as long as the data plane has no facilities to remotely send IPv6 link-local packets. The only exceptions are ACP-connected interfaces, which require greater physical protection. The ULA addresses are only reachable inside the ACP context and therefore unreachable from the data plane. Also, the ACP protocols should be implemented to be attack resistant and to not consume unnecessary resources even while under attack.¶

10.2.2. From the Inside

The security model of the ACP is based on trusting all members of the group of nodes that receive an ACP certificate for the same domain. Attacks from the inside by a compromised group member are therefore the biggest challenge.¶

Group members must be protected against attackers so that there is no easy way to compromise them or use them as a proxy for attacking other devices across the ACP. For example, management plane functions (transport ports) should be reachable only from the ACP and not from the data plane. This applies especially to those management plane functions that lack secure end-to-end transport and to which the ACP provides both automatic, reliable connectivity and protection against attacks. Protection across all potential attack vectors is typically easier to do in devices whose software is designed from the beginning with the ACP in mind than in legacy, software-based systems where the ACP is added on as another feature.¶

As explained above, traffic across the ACP should still be end-to-end encrypted whenever possible. This includes traffic such as GRASP, EST, and BRSKI inside the ACP. This minimizes man-in-the-middle attacks by compromised ACP group members. Such attackers cannot eavesdrop or modify communications, but they can just filter them (which is unavoidable by any means).¶

See Appendix A.9.8 for further considerations on avoiding and dealing with compromised nodes.¶

A.3.1. LLDP

LLDP and Cisco's earlier Cisco Discovery Protocol (CDP) are examples of L2 discovery protocols that terminate their messages on L2 ports. If those protocols had been chosen for ACP neighbor discovery, ACP neighbor discovery would also have terminated on L2 ports. This would have prevented ACP construction over non-ACP-capable, but LLDP- or CDP-enabled L2 switches. LLDP has extensions using different MAC addresses, and this could have been an option for ACP discovery as well, but the additional required IEEE standardization and definition of a profile for such a modified instance of LLDP seemed to be more work than the benefit of "reusing the existing protocol" LLDP for this very simple purpose.¶

A.3.2. mDNS and L2 Support

Multicast DNS (mDNS) "Multicast DNS" [RFC6762] with DNS Service Discovery (DNS-SD) Resource Records (RRs) as defined in "DNS-Based Service Discovery" [RFC6763] was a key contender as an ACP discovery protocol. Because it relies on link-local IP multicast, it operates at the subnet level and is also found in L2 switches. The authors of this document are not aware of an mDNS implementation that terminates its mDNS messages on L2 ports instead of on the subnet level. If mDNS was used as the ACP discovery mechanism on an ACP-capable (L3)/L2 switch as outlined in Section 7, then this would be necessary to implement. It is likely that termination of mDNS messages could only be applied to all mDNS messages from such a port, which would then make it necessary to software forward any non-ACP-related mDNS messages to maintain prior non-ACP mDNS functionality. Adding support for ACP to such L2 switches with mDNS could therefore create regression problems for prior mDNS functionality on those nodes. With low performance of software forwarding in many L2 switches, this could also make the ACP risky to support on such L2 switches.¶

A.3.3. Why DULL GRASP?

LLDP was not considered because of the above mentioned issues. mDNS was not selected because of the above L2 mDNS considerations and because of the following additional points.¶

If mDNS was not already existing in a node, it would be more work to implement than DULL GRASP, and if an existing implementation of mDNS was used, it would likely be more code space than a separate implementation of DULL GRASP or a shared implementation of DULL GRASP and GRASP in the ACP.¶

A.9.1. Auto-Aggregation of Routes

Routing in the ACP according to this specification only leverages the standard RPL mechanism of route optimization, e.g., keeping only the routes that are not towards the RPL root. This is known to scale to networks with 20,000 or more nodes. There is no auto-aggregation of routes for /48 ULA prefixes (when using rsub in the acp-node-name) and/or Zone-ID based prefixes.¶

Automatic assignment of Zone-ID and auto-aggregation of routes could be achieved, for example, by configuring zone boundaries, announcing via GRASP into the zones the zone parameters (Zone-ID and /48 ULA prefix), and auto-aggregating routes on the zone boundaries. Nodes would assign their Zone-ID and potentially even the /48 prefix based on the GRASP announcements.¶

A.9.2. More Options for Avoiding IPv6 Data Plane Dependencies

As described in Section 6.13.2, the ACP depends on the data plane to establish IPv6 link-local addressing on interfaces. Using a separate MAC address for the ACP allows the full isolation of the ACP from the data plane in a way that is compatible with this specification. It is also an ideal option when using single-root input/output virtualization (SR-IOV, see [SR]) in an implementation to isolate the ACP because different SR-IOV interfaces use different MAC addresses.¶

When additional MAC address(es) are not available, separation of the ACP could be done at different demux points. The same subnet interface could have a separate IPv6 interface for the ACP and data plane and therefore separate link-local addresses for both, where the ACP interface is not configurable on the data plane. This too would be compatible with this specification and not impact interoperability.¶

An option that would require additional specification is to use a different Ethertype from 0x86DD (IPv6) to encapsulate IPv6 packets for the ACP. This would be a similar approach as used for IP authentication packets in [IEEE-802.1X], which uses the Extensible Authentication Protocol over Local Area Network (EAPoL) Ethertype (0x88A2).¶

Note that in the case of ANI nodes, all of the above considerations equally apply to the encapsulation of BRSKI packets including GRASP used for BRSKI.¶

A.9.3. ACP APIs and Operational Models (YANG)

Future work should define a YANG data model [RFC7950] and/or node-internal APIs to monitor and manage the ACP.¶

Support for the ACP adjacency table (Section 6.3) and ACP GRASP needs to be included in such model and/or API.¶

A.9.4. RPL Enhancements

   ..... USA ......              ..... Europe ......

        NOC1                           NOC2
         |                              |
         |            metric 100        |
       ACP1 --------------------------- ACP2  .
         |                              |     . WAN
         | metric 10          metric 20 |     . Core
         |                              |     .
       ACP3 --------------------------- ACP4  .
         |            metric 100        |
         |                              |     .
         |                              |     . Sites
       ACP10                           ACP11  .

The profile for RPL specified in this document builds only one spanning-tree path set to a root, typically a registrar in one NOC. In the presence of multiple NOCs, routing toward the non-root NOCs may be suboptimal. Figure 17 shows an extreme example. Assuming that node ACP1 becomes the RPL root, traffic between ACP11 and NOC2 will pass through ACP4-ACP3-ACP1-ACP2 instead of ACP4-ACP2 because the RPL-calculated DODAG and routes are shortest paths towards the RPL root.¶

To overcome these limitations, extensions and/or modifications to the RPL profile can optimize for multiple NOCs. This requires utilizing data plane artifacts, including IP-in-IP encapsulation/decapsulation on ACP routers and processing of IPv6 RPI headers. Alternatively, (Src,Dst) routing table entries could be used.¶

Flooding of ACP GRASP messages can be further constrained and therefore optimized by flooding only via links that are part of the RPL DODAG.¶

A.9.5. Role Assignments

ACP connect is an explicit mechanism to "leak" ACP traffic explicitly (for example, in a NOC). It is therefore also a possible security gap when it is easy to enable ACP connect on arbitrary compromised ACP nodes.¶

One simple solution is to define an extension in the ACP certificate's ACP information field indicating the permission for ACP connect to be configured on that ACP node. This could similarly be done to decide whether a node is permitted to be a registrar or not.¶

Tying the permitted "roles" of an ACP node to the ACP certificate provides fairly strong protection against misconfiguration, but it is still subject to code modifications.¶

Another interesting role to assign to certificates is that of a NOC node. This would allow the limiting of certain types of connections, such as OAM TLS connections to only the NOC initiators or responders.¶

A.9.6. Autonomic L3 Transit

In this specification, the ACP can only establish autonomic connectivity across L2 hops but requires non-autonomic configuration to tunnel across L3 paths. Future work should specify mechanisms to automatically tunnel ACP across L3 networks. A hub-and-spoke option would allow tunneling across the Internet to a cloud or central instance of the ACP; a peer-to-peer tunneling mechanism could tunnel ACP islands across an L3VPN infrastructure.¶

A.9.7. Diagnostics

Section 9.1 describes diagnostics options that can be applied without changing the external, interoperability-affecting characteristics of ACP implementations.¶

Even better diagnostics of ACP operations are possible with additional signaling extensions, such as the following:¶

1. Consider if LLDP should be a recommended functionality for ANI devices to improve diagnostics, and if so, which information elements it should signal (noting that such information is conveyed in an insecure manner). This includes potentially new information elements.¶
2. As an alternative to LLDP, a DULL GRASP diagnostics objective could be defined to carry these information elements.¶
3. The IDevID certificate of BRSKI pledges should be included in the selected insecure diagnostics option. This may be undesirable when exposure of device information is seen as too much of a security issue (the ability to deduce possible attack vectors from device model, for example).¶
4. A richer set of diagnostics information should be made available via the secured ACP channels, using either single-hop GRASP or network-wide "topology discovery" mechanisms.¶

A.9.8. Avoiding and Dealing with Compromised ACP Nodes

Compromised ACP nodes pose the biggest risk to the operations of the network. The most common types of compromise are the leakage of credentials to manage and/or configure the device and the application of malicious configuration, including the change of access credentials, but not the change of software. Most of today's networking equipment should have secure boot/software infrastructure anyhow, so attacks that introduce malicious software should be a lot harder.¶

The most important aspect of security design against these types of attacks is to eliminate password-based configuration access methods and instead rely on certificate-based credentials handed out only to nodes where it is clear that the private keys cannot leak. This limits unexpected propagation of credentials.¶

If password-based credentials to configure devices still need to be supported, they must not be locally configurable, but only be remotely provisioned or verified (through protocols like RADIUS or Diameter), and there must be no local configuration permitting the change of these authentication mechanisms, but ideally they should be autoconfiguring across the ACP. See [NOC-AUTOCONFIG].¶

Without physical access to the compromised device, attackers with access to configuration should not be able to break the ACP connectivity, even when they can break or otherwise manipulate (spoof) the data plane connectivity through configuration. To achieve this, it is necessary to avoid providing configuration options for the ACP, such as enabling/disabling it on interfaces. For example, there could be an ACP configuration that locks down the current ACP configuration unless factory reset is done.¶

With such means, the valid administration has the best chances to maintain access to ACP nodes, to discover malicious configuration though ongoing configuration tracking from central locations, for example, and to react accordingly.¶

The primary reaction is to withdraw or change credentials, terminate malicious existing management sessions, and fix the configuration. Ensuring that management sessions using invalidated credentials are terminated automatically without recourse will likely require new work.¶

Only when these steps are infeasible, would it be necessary to revoke or expire the ACP certificate credentials and consider the node kicked off the network until the situation can be further rectified, likely requiring direct physical access to the node.¶

Without extensions, compromised ACP nodes can only be removed from the ACP at the speed of CRL/OCSP information refresh or expiry (and non-removal) of short-lived certificates. Future extensions to the ACP could, for example, use the GRASP flooding distribution of triggered updates of CRL/OCSP or the explicit removal indication of the compromised node's domain certificate.¶

A.9.9. Detecting ACP Secure Channel Downgrade Attacks

The following text proposes a mechanism to protect against downgrade attacks without introducing a new specialized GRASP secure channel mechanism. Instead, it relies on running GRASP after establishing a secure channel protocol to verify if the established secure channel option could have been the result of a MITM downgrade attack.¶

MITM attackers can force downgrade attacks for ACP secure channel selection by filtering and/or modifying DULL GRASP messages and/or actual secure channel data packets. For example, if at some point in time, DTLS traffic could be more easily decrypted than traffic of IKEv2, the MITM could filter all IKEv2 packets to force ACP nodes to use DTLS (assuming that the ACP nodes in question supported both DTLS and IKEv2).¶

For cases where such MITM attacks are not capable of injecting malicious traffic (but only of decrypting the traffic), a downgrade attack could be discovered after a secure channel connection is established, for example, by using the following type of mechanism.¶

After the secure channel connection is established, the two ACP peers negotiate, via an appropriate (to be defined) GRASP negotiation, which ACP secure channel protocol should have been selected between them (in the absence of a MITM attacker). This negotiation would signal the ACP secure channel options announced by DULL GRASP by each peer followed by an announcement of the preferred secure channel protocol by the ACP peer that is the Decider in the secure channel setup, i.e., the ACP peer that decides which secure channel protocol to use. If that chosen secure channel protocol is different from the one that actually was chosen, then this mismatch is an indication that there is a MITM attacker or other similar issue (e.g., a firewall prohibiting the use of specific protocols) that caused a non-preferred secure channel protocol to be chosen. This discovery could then result in mitigation options such as logging and ensuing investigations.¶