A Profile for AS Adjacency Attestation Objects

The primary purpose of the Internet IP Address and AS Number Resource Public Key Infrastructure (RPKI) system is to improve routing security. As part of this security framework, a mechanism is defined here to allow entities to verify that an AS attests that is adjacent to one or more other AS's, with the inference that it may elect to announce routes to these adjacent AS's in the inter-domain routing environment. An AAO provides this function. An AAO is a digitally signed object that makes use of Cryptographic Message Syntax (CMS) as a standard encapsulation format. CMS was chosen to take advantage of existing open source software available for processing messages in this format. The AAO is an attestation authored by the local AS that the local AS is a routing peer to each of the AS's that are enumerated in an associated AS list. An AAO is a two part structure, containing the local AS and a list of AS's. The AAO is signed by a an EE Resource Certificate that has the local AS as the value of its AS number resource extension.

It is assumed that the reader is familiar with the terms and concepts described in "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile" , "X.509 Extensions for IP Addresses and AS Identifiers" , "Internet Protocol" , "Internet Protocol Version 6 (IPv6) Addressing Architecture" , "Internet Registry IP Allocation Guidelines" , and related regional Internet registry address management policy documents, and BGP-4 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

An AAO is an attestation on the part of a AS holder that it supports inter-domain routing adjacencies to each of the AS's listed in the AAO. The AAO does not list any prefixes that may be announced to the adjacent AS's either directly or indirectly. The AAO also does not list any local routing policies that may be applied to the routes that are advertised across this adjacency, nor any routing policies that may be applied to routes that are learned from this adjacency. The AAO is intended to provide "closure" with respect to interpretation of the AAO by relying parties, to the extent that if a valid AAO exists for a local AS, then from the perspective of that local AS all adjacencies with those AS's listed in the valid AAO can be regarded as "valid" and any other adjacency from the perspective of the local AS can be regarded as potentially "invalid". In other words an AAO is an attestation of adjacency with the AS's listed in the AAO and an implicit attestion of the denial of adjacency with all other AS's. Where an AS holder has published two or more valid AAO's, the set of "valid" adjacent AS's refers to the union of the lists of adjacent AS's and all other AS's can be regarded as "invalid" from the perspective of the local AS. It is reasonable for a relying party to infer from a valid AAO that the signing AS has the intent to advertise route objects across this adjacency, and is prepared to learn route objects that are passed to it from the adjacent AS. It is noted that an AAO is an asymmetric assertion, where one AS is asserting that an inter-domain routing adjacency with another AS is asserted to exist, but this assertion is not explicitly acknowledged by the remote AS in the context of a single AAO. Relying parties may elect to place greater levels of confidence in the existence of an inter-domain routing adjacency when both AS's have signed and published AAO objects that contain mutual references. It is also noted that there is a subtle distinction that could be drawn here between the appropriate semantic interpretation a pair of unilateral assertions of adjacency using two AAOs and a bilateral assertion of adjacency where both AS's sign a single assertion of the existence of an inter-domain routing adjacency between these AS's. This bilateral approach, using a single assertion with two digital signatures, is not defined in this document.

Using CMS syntax, an AAO is a type of signed-data object. The general format of a CMS object is:

As a AAO is a signed-data object, it uses the corresponding OID, 1.2.840.113549.1.7.2.

According to the CMS standard, the signed-data content type shall have ASN.1 type SignedData:

The version is the syntax version number. It MUST be 3, corresponding to the signerInfo structure having version number 3.

The digestAlgorithms set MUST include only SHA-256, the OID for which is 2.16.840.1.101.3.4.2.1. It MUST NOT contain any other algorithms.

encapContentInfo is the signed content, consisting of a content type identifier and the content itself.

The ContentType for a AAO is defined as id-ct-ASAdjancyAttest and has the numerical value of 1.2.840.113549.1.9.16.1.32.

The content of an AAO identifies one or more AS's that the signing AS is attesting that it has a routing adjacency with. Multiple adjacencies can be attested on one or more AAOs. The AAO contains no routing policy qualifications, nor does it reference any address prefixes that may be announced within the context of that routing adjacency. An AAO is formally defined as:

The version number of the ASAdjacencyAttestation MUST be 0.

The ASIdentifiers element is a SEQUENCE containing AS numbers for which the localASnum AS is attesting the existence of a routing adjacency. Any pair of items in the asIdentifiers SEQUENCE MUST NOT overlap. Any contiguous series of AS identifiers MUST be combined into a single range whenever possible. The AS identifiers in the asIdentifiers element MUST be sorted by increasing numeric value.

The ASIdOrRange type is a CHOICE of either a single integer (ASId) or a single sequence (ASRange).

The ASRange type is a SEQUENCE consisting of a min and a max element, and is used to specify a range of AS identifier values.

The min and max elements have type ASId. The min element is used to specify the value of the minimum AS identifier in the range, and the max element specifies the value of the maximum AS identifier in the range.

The ASId type is an INTEGER.

The localASNum field contains the AS that is making the attestation of routing adjacency to each of the AS's listed in the ASIdentifiers element.

The CertificateSet type is defined in section 10 of

The certificates element MUST be included and MUST contain only the single end entity resource certificate needed to validate this AAO.

The crls element MUST be omitted.

SignerInfo is defined under CMS as:

The version number MUST be 3, corresponding with the choice of SubjectKeyIdentifier for the sid.

The sid is defined as:

For a AAO, the sid MUST be a SubjectKeyIdentifier.

The digestAlgorithm MUST be SHA-256, the OID for which is 2.16.840.1.101.3.4.2.1.

The signedAttrs is defined as:

The signedAttr element MUST be present and MUST include the content-type and message-digest signed attributes. The signer MAY also include the signing-time signed attribute, the binary-signing-time signed attribute, or both signed attributes. Other signed attributes that are deemed appropriate by the signer MAY also be included. The intent is to allow additional signed attributes to be included if a future need is identified. This does not cause an interoperability concern because unrecognized signed attributes are ignored by the relying party. The signedAttr MUST include only a single instance of any particular attribute. Additionally, even though the syntax allows for a SET OF AttributeValue, in a AAO the attrValues must consist of only a single AttributeValue

The ContentType attribute MUST be present. The attrType OID for the ContentType attribute is 1.2.840.113549.1.9.3. The attrValues for the ContentType attribute in a AAO MUST be 1.2.840.113549.1.9.16.1.24 (matching the eContentType in the EncapsulatedContentInfo).

The MessageDigest attribute MUST be present. The attrType OID for the MessageDigest Attribute is 1.2.840.113549.1.9.4. The attrValues for the MessageDigest attribute contains the output of the digest algorithm applied to the content being signed, as specified in Section 11.1 of .

The SigningTime attribute MAY be present. If it is present it MUST be ignored by the relying party. The presence of absence of the SigningTime attribute in no way affects the validation of the AAO (as specified in Section 4). The attrType OID for the SigningTime attribute is 1.2.840.113549.1.9.5. The attrValues for the SigningTime attribute is defined as:

The Time element specifies the time, based on the local system clock, at which the digital signature was applied to the content.

The BinarySigningTime attribute MAY be present. If it is present it MUST be ignored by the relying party. The presence of absence of the BinarySigningTime attribute in no way affects the validation of the AAO (as specified in Section 3). The attrType OID for the SigningTime attribute is 1.2.840.113549.1.9.16.2.46. The attrValues for the SigningTime attribute is defined as:

The BinaryTime element specifies the time, based on the local system clock, at which the digital signature was applied to the content.

The signatureAlgorithm MUST be RSA (rsaEncryption), the OID for which is 1.2.840.113549.1.1.1.

The signature value is defined as:

The signature characteristics are defined by the digest and signature algorithms.

unsignedAttrs MUST be omitted.

Before a relying party can use an AAO, the relying party must first use the RPKI to validate the AAO by performing the following steps. Verify that the AAO syntax complies with this specification. In particular, verify the following: The contentType of the CMS object is SignedData (OID 1.2.840.113549.1.7.2). The version of the SignedData object is 3. The digestAlgorithm in the SignedData object is SHA-256 (OID 2.16.840.1.101.3.4.2.1). The certificates field in the SignedData object is present and contains an EE certificate whose Subject Key Identifier (SKI) matches the sid field of the SignerInfo object. The crls field in the SignedData object is omitted. The eContentType in the EncapsulatedContentInfo is id-ct-ADAdjacencyAttest (OID 1.2.840.113549.1.9.16.1.32) The version of the id-ct-ASAdjacencyAttest is 0. The version of the SignerInfo is 3. The digestAlgorithm in the SignerInfo object is SHA-256 (OID 2.16.840.1.101.3.4.2.1). The signatureAlgorithm in the SignerInfo object is RSA (OID 1.2.840.113549.1.1.1). The signedAttrs field in the SignerInfo object is present and contains both the ContentType attribute (OID 1.2.840.113549.1.9.3) and the MessageDigest attribute (OID 1.2.840.113549.1.9.4). The unsignedAttrs field in the SignerInfo object is omitted. Use the public key in the EE certificate to verify the signature on the AAO. Verify that the EE certificate has an Autonomous System Identifier Delegation Extension and that the Autonomous System Identifier in that extension exactly matches the Autonomous System Identifier in the localASNum element of the AAO. Verify that the EE certificate is a valid end-entity certificate in the RPKI by constructing a valid certificate path to a trust anchor. (See for more details.)

There is no assumption of confidentiality for the data in a AAO; it is anticipated that AAOs will be stored in repositories that are accessible to all ISPs, and potentially to all Internet users. There is no explicit authentication associated with a AAO, since the RPKI that is used for AAO validation provides authorization but not authentication. Although the AAO is a signed, application layer object, there is no intent to convey non-repudiation via a AAO. The purpose of a AAO is to convey a unilateral statement of intent that an AS has the intention to announce route objects via a routing adjacency with another AS and has the intention to listen for route objects that are passed to it over a routing adjacency. This should not be interpreted as an authority, nor is a relying party justified in assuming that such an adjacency exists, nor that any valid routing announcements that are passed across this routing adjacency. A relying party may be able to place greater confidence in the inferred existence of a routing adjacency in the case where both AS holders mutually generate signed AAO objects that nominate each other as an adjacent AS. The AAO object does not convey any information relating to route policies that may be applied to the adjacency by either party to a route adjacency, nor what prefixes may be advertised across that adjacency, nor any attributes that may be associated with such advertisements.

[Note to IANA, to be removed prior to publication: there are no IANA considerations stated in this version of the document.]

The authors would like to acknowledge the work of Matt Lepinski, Stephen Kent and Derrick Kong, whose work on the Route Origin Attestation Profile was used as the starting point for this document.