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
                       +area51.research@acp.example.com
  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.¶