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Versions: (draft-reynolds-rpki-ltamgmt) 00 01 02 03 04 05 06 07 08

Secure Inter-Domain Routing                                  M. Reynolds
Internet-Draft                                                   S. Kent
Intended status: Standards Track                                     BBN
Expires: May 13, 2011                                   November 9, 2010


Local Trust Anchor Management for the Resource Public Key Infrastructure
                    <draft-ietf-sidr-ltamgmt-00.txt>


Status of this Memo

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   This Internet-Draft will expire on May 13, 2011.

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Abstract

   This document describes a facility to enable a relying party (RP) to
   manage trust anchors (TAs) in the context of the Resource Public Key
   Infrastructure (RPKI). It is common to allow an RP to import TA
   material in the form of self-signed certificates. The facility
   described in this document allows an RP to impose constraints on such
   TAs. Because this mechanism is designed to operate in the RPKI
   context, the relevant constraints are the RFC 3779 extensions that
   bind address spaces and/or autonomous system (AS) numbers to
   entities. The primary motivation for this facility is to enable an RP
   to ensure that resource allocation information that it has acquired
   via some trusted channel is not overridden by the information
   acquired from the RPKI repository system or by the putative TAs that
   the RP imports. Specifically, the mechanism allows an RP to specify a
   set of bindings between public key identifiers and RFC 3779 extension
   data and will override any conflicting bindings expressed via the
   putative TAs and the certificates downloaded from the RPKI repository
   system. Although this mechanism is designed for local use by an RP,
   an entity that is accorded administrative control over a set of RPs
   may use this mechanism to convey its view of the RPKI to a set of RPs
   within its jurisdiction. The means by which this latter use case is
   effected is outside the scope of this document.



Table of Contents

   1  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 4
      1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
   2  Overview of Certificate Processing . . . . . . . . . . . . . . . 5
      2.1  Target Certificate Processing . . . . . . . . . . . . . . . 5
      2.2  Perforation . . . . . . . . . . . . . . . . . . . . . . . . 5
      2.3  TA Re-parenting . . . . . . . . . . . . . . . . . . . . . . 6
      2.4  Paracertificates  . . . . . . . . . . . . . . . . . . . . . 6
   3  Format of the constraints file . . . . . . . . . . . . . . . . . 8
      3.1  Relying party subsection  . . . . . . . . . . . . . . . . . 8
      3.2  Flags subsection  . . . . . . . . . . . . . . . . . . . . . 8
      3.3  Tags subsection . . . . . . . . . . . . . . . . . . . . . . 9
         3.3.1  Xvalidity_dates tag  . . . . . . . . . . . . . . . .  10
         3.3.2  Xcrldp tag . . . . . . . . . . . . . . . . . . . . .  10
         3.3.3  Xcp tag  . . . . . . . . . . . . . . . . . . . . . .  11
         3.3.4  Xaia tag . . . . . . . . . . . . . . . . . . . . . .  11
      3.4  Blocks subsection . . . . . . . . . . . . . . . . . . . .  12
   4  Certificate Processing Algorithm . . . . . . . . . . . . . . .  13
      4.1  Proofreading algorithm  . . . . . . . . . . . . . . . . .  14
      4.2  TA processing algorithm . . . . . . . . . . . . . . . . .  15
         4.2.1  Preparatory processing (stage 0) . . . . . . . . . .  16



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         4.2.2  Target processing (stage 1)  . . . . . . . . . . . .  17
         4.2.3  Ancestor processing (stage 2)  . . . . . . . . . . .  18
         4.2.4  Tree processing (stage 3)  . . . . . . . . . . . . .  19
         4.2.5  TA re-parenting (stage 4)  . . . . . . . . . . . . .  20
      4.3  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  21
   5  Implications for Path Discovery  . . . . . . . . . . . . . . .  21
      5.1  Two answers . . . . . . . . . . . . . . . . . . . . . . .  21
      5.2  One answer  . . . . . . . . . . . . . . . . . . . . . . .  22
      5.3  No answer . . . . . . . . . . . . . . . . . . . . . . . .  22
   6  Implications for Revocation  . . . . . . . . . . . . . . . . .  22
      6.1  No state bits set . . . . . . . . . . . . . . . . . . . .  22
      6.2  ORIGINAL state bit set  . . . . . . . . . . . . . . . . .  23
      6.3  PARA state bit set  . . . . . . . . . . . . . . . . . . .  23
      6.4  Both ORIGINAL and PARA state bits set . . . . . . . . . .  23
   7  Security Considerations  . . . . . . . . . . . . . . . . . . .  24
   8  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  24
   9  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  24
   10  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
      10.1  Normative References . . . . . . . . . . . . . . . . . .  24
      10.2  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25
   Appendix A: Sample Constraints File . . . . . . . . . . . . . . .  26
   Appendix B: Optional Sorting Algorithm for Ancestor Processing  .  27




























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1  Introduction

   The Resource Public Key Infrastructure (RPKI) [I-D.sidr-arch] is a
   PKI in which certificates are issued to facilitate management of IP
   addresses and autonomous system number resources. Such resources are
   expressed in the form of X.509v3 "resource" certificates with
   extensions as defined by RFC 3779 [I-D.sidr-res-cert-prof].
   Validation of a resource certificate is preceded by path discovery.
   Path discovery is effected by constructing a certificate path
   (upward) from a target certificate to a trust anchor. Path validation
   proceeds from the TA in question to the target certificate, using the
   public key from each certificate along the path to verify the
   signature of its subordinate certificate. In the RPKI it is
   anticipated that one or more putative TAs, aligned with the resource
   allocation hierarchy, will be available in the form of self-signed
   certificates configured by an RP. There are circumstances under which
   an RP may wish to override the resource specifications obtained
   through the RPKI distributed repository system [I-D.sidr-repos-
   struct]. This document describes a mechanism by which an RP may
   override any conflicting information expressed via the putative TAs
   and the certificates downloaded from the RPKI repository system.

   To effect this local control, this document calls for a relying party
   to specify a set of bindings between public key identifiers and
   resources (IP resources and/or AS number resources) through a text
   file known as a constraints file. The constraints expressed in this
   file then take precedence over any competing claims expressed by
   resource certificates acquired from the distributed repository
   system. (The means by which a relying party acquires the key
   identifier and the RFC 3779 extension data used to populate the
   constraints file is outside the scope of this document.) The relying
   party also may use a local publication point (the root of a local
   directory tree that is made available as if it were a remote
   repository) as a source of certificates and CRLs (and other RPKI
   signed objects, e.g. ROAs and manifests) that do not appear in the
   RPKI repository system.

   In order to allow reuse of existing, standard path validation
   mechanisms, the RP-imposed constraints are realized by having the RP
   itself represented as the only TA known in the local certificate
   validation context. To ensure that all RPKI certificates can be
   validated relative to this TA, this RP TA certificate must contain
   all-encompassing resource allocations, i.e. 0/0 for IPv4, 0::/0 for
   IPv6 and 0-4294967295 for AS numbers. Thus, a conforming
   implementation of this mechanism must be able to cause a self-signed
   certification authority (CA) certificate to be created with a locally
   generated key pair. It also must be able to issue CA certificates
   subordinate to this TA. Finally, a conforming implementation of this



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   mechanism must process the constraints file and modify certificates
   as needed in order to enforce the constraints asserted in the file.

   The remainder of this document describes in detail the types of
   certificate modification that may occur, the semantics of the
   constraints file, and the implications of certificate modification on
   path discovery and revocation.

1.1  Terminology

   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" [RFC5280] and "X.509
   Extensions for IP Addresses and AS Identifiers" [RFC3779].

   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.

2  Overview of Certificate Processing

   The fundamental aspect of the facility described in this document is
   one of certificate modification. The constraints file, described in
   more detail in the next section, contains assertions about resources
   that are to be specially processed. As a result of this processing,
   certificates in the local copy of the RPKI repository are transformed
   into new certificates satisfying the resource constraints so
   specified. This enables the RP to override conflicting assertions
   about resource holdings as acquired from the RPKI repository system.
   Three forms of certificate modification can occur.

2.1  Target Certificate Processing

   If a certificate is acquired from the RPKI repository system and it's
   SKI is listed in the constraints file, it will be reissued directly
   under the RP TA certificate, with (possibly) modified RFC 3779
   extensions. The modified extensions will include any RFC 3779 data
   expressed in the constraints file. In Section 4.2, target certificate
   processing corresponds to stage one of the algorithm.

2.2  Perforation

   Any certificate acquired from the RPKI repository that contains an
   RFC 3779 extension that intersects the resource data in the
   constraints file will be reissued directly under the RP TA, with
   modified RFC 3779 extensions. We refer to the process of modifying
   the RFC 3779 extension in an affected certificate as "perforation"
   (because the process will create "holes" in these extensions). The



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   modified extensions will exclude any RFC 3779 data expressed in the
   constraints file. In the certificate processing algorithm described
   in Section 4.2, perforation corresponds to stage two of the algorithm
   ("ancestor processing") and also to stage three of the algorithm
   ("tree processing").

2.3  TA Re-parenting

   For consistency, all valid, self-signed certificates that would have
   been regarded as TAs in the public RPKI certificate hierarchy, e.g.
   self-signed certificates issued by IANA or the RIRs, will be re-
   issued under the RP TA certificate. This processing is done even
   though all but one of these certificates might not intersect any
   resources specified in the constraints file. We refer to this
   reissuance as "re-parenting" since the Issuer (parent) of the
   certificate has been changed. In the certificate processing algorithm
   described in Section 4.2, TA re-parenting corresponds to stage four
   of the algorithm.

2.4  Paracertificates

   If a certificate is subject to any of the three forms of processing
   just described, that certificate will be referred to as an "original"
   certificate and the processed (output) certificate will be referred
   to as a paracertificate. When an original certificate is transformed
   into a paracertificate all the fields and extensions from the
   original certificate will be retained, except as indicated in Table
   1, below.























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      Original Certificate Field         Action

          Version                     unchanged
          Serial number               created per note A
          Signature                   replaced if needed
                                        with RP's signing alg
          Issuer                      replaced with RP's name
          Validity dates              replaced per note B
          Subject                     unchanged
          Subject public key info     unchanged
          Extensions
            Subject key identifier    unchanged
            Key usage                 unchanged
            Basic constraints         unchanged
            CRL distribution points   replaced per note B
            Certificate policy        replaced per note B
            Authority info access     replaced per note B
            Authority key ident       replaced with RP's
            IP address block          modified as described
            AS number block           modified as described
            Subject info access       unchanged
            All other extensions      unchanged
          Signature Algorithm         same as above
          Signature value             new

                Table 1  Certificate Field Modifications



   Note A. The serial number will be created by concatenating the
   current time (the number of seconds since Jan 1, 1970) with a count
   of the certificates created in the current run.

   Note B. These fields are derived (as described in section 3.3 below)
   from parameters in the constraints file (if present); otherwise, they
   take on values from the certificates from which the paracertificates
   are derived.














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3  Format of the constraints file

   This section describes a general model for the syntax of the
   constraints file. The model described below is nominal;
   implementations need not match details of this model as presented,
   but the external behavior of implementations MUST correspond to the
   externally observable characteristics of this model in order to be
   compliant.

   The constraints file consists of four logical subsections: the
   replying party subsection, the flags subsection, the tags subsection
   and the blocks subsection. The relying party subsection and the
   blocks subsection are REQUIRED and MUST be present; the flags and
   tags subsections are OPTIONAL. Each subsection is described in more
   detail below. Note that the semicolon (;) character acts as the
   comment character, to enable annotating constraints files. All
   characters from a semicolon to the end of that line are ignored. In
   addition, lines consisting only of whitespace are ignored. The
   subsections MUST occur in the order indicated. An example constraints
   file is given in Appendix A.

3.1  Relying party subsection

   The relying party subsection is a REQUIRED subsection of the
   constraints file. It MUST be the first subsection of the constraints
   file, and it MUST consist of two lines of the form:

      PRIVATEKEYMETHOD      value [ ... value ]
      TOPLEVELCERTIFICATE   value

   The first line provides guidance to the certificate processing
   algorithm on the method that will be used to gain access to the RP's
   private key. This line consists of the string literal
   PRIVATEKEYMETHOD, followed by one or more whitespace delimited string
   values. These values are passed to the certificate processing
   algorithm as described below. Note that this entry, as for all
   entries in the constraints file, is case sensitive.

   The second line of this subsection consists of the string literal
   TOPLEVELCERTIFICATE, followed by exactly one string value. This value
   is the name of a file containing the relying party's TA certificate.
   The file name is passed to the certificate processing algorithm as
   described below.

3.2  Flags subsection

   The flags subsection of the constraints file is an OPTIONAL
   subsection. If present it MUST immediately follow the relying party



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   subsection. The flags subsection consists of one or more lines of the
   form

      CONTROL  flagname  booleanvalue

   Each such line is referred to as a control line. Each control line
   MUST contain exactly three whitespace delimited strings. The first
   string MUST be the literal CONTROL. The second string MUST be one of
   the following three literals:

            resource_nounion
            intersection_always
            treegrowth

   The third string denotes a boolean value, and MUST be one of the
   literals TRUE or FALSE. Control flags influence the global operation
   of the certificate processing algorithm; the semantics of the flags
   is described in detail in Section 4.2. Note that each flag has a
   default value, so that if the corresponding CONTROL line does not
   appear in the constraints file, the algorithm flag is considered to
   take the corresponding default value. The default value for each flag
   is FALSE. Thus, if any flag is not named in a control line it takes
   the value FALSE. Further, if the flags subsection is absent, all
   three flags take the value FALSE.

3.3  Tags subsection

   The tags subsection is an OPTIONAL subsection in the constraints
   file. If present it MUST immediately follow the relying party
   subsection (if the flags subsection is absent) or the flags
   subsection (if it is present). The tags subsection consists of one or
   more lines of the form

      TAG  tagname  tagvalue [ ... tagvalue ]

   Each such line is referred to as a tag line. Each tag line MUST
   consist of at least three whitespace delimited string values, the
   first of which must be the literal TAG. The second string value gives
   the name of the tag, and subsequent string(s) give the value(s) of
   the tag. The tag name MUST be one of the following four string
   literals:

            Xvalidity_dates
            Xcrldp
            Xcp
            Xaia

   The purpose of the tag lines is to provide an indication of the means



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   by which paracertificate fields, specifically those indicated above
   under "Note B", are constructed. Each tag has a default, so that if
   the corresponding tag line is not present in the constraints file,
   the default behavior is used when constructing the paracertificates.
   The syntax and semantics of each tag line is described next.

   Note that the tag lines are considered to be global; the action of
   each tag line (or the default action, if that tag line is not
   present) applies to all paracertificates that are created as part of
   the certificate processing algorithm.

3.3.1  Xvalidity_dates tag

   This tag line is used to control the value of the notBefore and
   notAfter fields in paracertificates. If this tag line is specified
   and there is a single tagvalue which is the literal string C, the
   paracertificate validity interval is copied from the original
   certificate validity interval from which it is derived. If this tag
   is specified and there is a single tagvalue which is the literal
   string R, the paracertificate validity interval is copied from the
   validity interval of the relying party's top level (TA) certificate.
   If this tag is specified and the tagvalue is neither of these
   literals, then exactly two tagvalues MUST be specified. Each must be
   a Generalized Time string of the form YYYYMMDDHHMMSSZ. The first
   tagvalue is assigned to the notBefore field and the second tagvalue
   is assigned to the notAfter field. It MUST be the case that the
   tagvalues may be parsed as valid Generalized Time strings such that
   notBefore is less than notAfter, and also such that notAfter
   represents a time in the future (i.e., the paracertificate has not
   already expired).

   If this tag line is not present in the constraints file the default
   behavior is to copy the validity interval from the original
   certificate to the corresponding paracertificate.

3.3.2  Xcrldp tag

   This tag line is used to control the value of the CRL distribution
   point extension in paracertificates. If this tag line is specified
   and there is a single tagvalue that is the string literal C, the
   CRLDP of the paracertificate is copied from the CRLDP of the original
   certificate from which it is derived. If this tag line is specified
   and there is a single tagvalue that is the string literal R, the
   CRLDP of the paracertificate is copied from the CRLDP of the RP's top
   level certificate. If this tag line is specified and there is a
   single tagvalue that is not one of these two reserved literals, or if
   there is more than one tagvalue, then each tagvalue is interpreted as
   a URL that will be placed in the CRLDP sequence in the



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   paracertificate.

   If this tag line is not present in the constraints file the default
   behavior is to copy the CRLDP from the original certificate to the
   corresponding paracertificate.

3.3.3  Xcp tag

   This tag line is used to control the value of the policyQualifierId
   field in paracertificates. If this tag line is specified there MUST
   be exactly one tagvalue. If the tagvalue is the string literal C, the
   paracertificate value is copied from the value in the corresponding
   original certificate. If the tagvalue is the string literal R, the
   paracertificate value is copied from the value in the RP's top level
   TA certificate. If the tagvalue is the string literal D, the
   paracertificate value is set to the default OID. If the tagvalue is
   not one of these reserved string literals, then the tagvalue MUST be
   an OID specified using the standard dotted notation. The value in the
   paracertificate's policyQualifierId field is set to this OID. Note
   the RFC 5280 specifies that only a single policy may be specified in
   a certificate, so only a single tagvalue is permitted in this tag
   line, even though the CertificatePolicy field is an ASN.1 sequence.

   If this tag line is not specified the default behavior is to use the
   default OID in creating the paracertificate.

   This option permits the RP to convert a value of the
   policyQualifierId field in a certificate (that would not be in
   conformance with the RPKI CP) to a conforming value in the
   paracertificate. This conversion enables use of RPKI validation
   software that checks the policy field against that specified in the
   RPKI CP [ID.sidr-res-cert-prof].

3.3.4  Xaia tag

   This tag line is used to control the value of the Authority
   Information Access (AIA) extension in the paracertificate. If this
   tag line is present then it MUST have exactly one tagvalue. If this
   tagvalue is the string literal C, then the AIA field in the
   paracertificate is copied from the AIA field in the original
   certificate from which it is derived. If this tag line is present and
   the tagvalue is not the reserved string literal, then the tagvalue
   MUST be a URL. This URL is set as the AIA extension of the
   paracertificates that are created.

   If this tag line is not specified the default behavior is to use copy
   the AIA field from the original certificate to the AIA field of the
   paracertificate.



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3.4  Blocks subsection

   The blocks subsection is a REQUIRED subsection of the constraints
   file. If the tags subsection is present, the blocks subsection MUST
   appear immediately after it. If the tags subsection is absent, but
   the flags subsection is present, the block subsection MUST appear
   immediately after it. Otherwise, the blocks subsection MUST appear
   immediately after the relying party subsection. The blocks subsection
   consists of one or more blocks, known as target blocks. A target
   block is used to specify an association between a certificate (given
   by a hash of its public key information) and a set of resource
   assertions. Each target block contains four regions, an SKI region,
   an IPv4 region, an IPv6 region and an AS number region. All regions
   are REQUIRED to be present in a target block.

   The SKI region contains a single line beginning with the string
   literal SKI and followed by forty hexadecimal characters giving the
   subject key identifier of a certificate, known as the target
   certificate. The hex character string MAY contain embedded whitespace
   or colon characters (included to improve readability), which are
   ignored. The IPv4 region consists of a line containing only the
   string literal IPv4. This line is followed by zero or more lines
   containing IPv4 prefixes in the format described in RFC 3779. The
   IPv6 region consists of a line containing only the string literal
   IPv6, followed by zero or more lines containing IPv6 prefixes using
   the format described in RFC 3513. (The presence of the IPv4 and IPv6
   literals is to simplify parsing of the constraints file.) Finally,
   the AS number region consists of a line containing only the string
   literal AS#, followed by zero or more lines containing AS numbers
   (one per line). The AS numbers are specified in decimal notation as
   recommended in RFC 5396. A target block is terminated by either the
   end of the constraints file, or by the beginning of the next target
   block, as signaled by its opening SKI region line. An example target
   block is shown below. See also the complete constraints file example
   given in Appendix A. Note that whitespace, as always, is ignored.

        SKI 00:12:33:44:00:BA:BA:DE:EB:EE:00:99:88:77:66:55:44:33:22:11
        IPv4
          10.2.3/24
          10.8/16
        IPv6
          1:2:3:4:5:6/112
        AS#
          123
          567

   The blocks subsection MUST contain at least one target block. Note
   that it is OPTIONAL that the SKI refer to a certificate that is known



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   or resolvable within the context of the local RPKI repository. Also,
   there is no REQUIRED or implied ordering of target blocks within the
   block subsection. As a result of the fact that blocks may occur in
   any order, it MAY result that the outcome of processing a constraints
   file depends on the order in which target blocks occur within the
   constraints file. The next section of this document contains a
   detailed description of the certificate processing algorithm.

4  Certificate Processing Algorithm

   The section describes the certificate processing algorithm through
   which paracertificates are created from original certificates in the
   local RPKI repository. For the purposes of describing this algorithm,
   it will be assumed that certificates may be persistently associated
   with state (or metadata) information. This state information will be
   further construed as having the form of any array of named bits that
   are associated with each certificate. No specific implementation of
   this functionality is mandated by this document. Any implementation
   that provides the indicated functionality is acceptable, and need not
   actually consist of a bit field associated with each certificate.

   The state bits used in certificate processing are

         NOCHAIN
         ORIGINAL
         PARA
         TARGET

   If the NOCHAIN bit is set, this indicates that a full path between
   the given certificate and a TA has not yet been discovered. If the
   ORIGINAL bit is set, this indicates that the certificate is question
   has been processed by some part of the processing algorithm described
   in Section 4.2. If it was processed as part of stage one processing,
   as described in section 4.2.2, the TARGET bit will also be set.
   Finally, any paracertificate will have the PARA bit set.

   At the beginning of algorithm processing each certificate in the
   local RPKI repository has the ORIGINAL, PARA and TARGET bits clear.
   If a certificate has a complete, validated path to a TA, or is itself
   a TA, then that certificate will have the NOCHAIN bit clear,
   otherwise it will have the NOCHAIN bit set. As the certificate
   processing algorithm is executed, the bit state of original
   certificates may changed. In addition, since the certificate
   processing algorithm may also be creating paracertificates, it is
   responsible for actively setting or clearing the state of these four
   bits on those paracertificates.

   The certificate processing algorithm consists of two sub-algorithms:



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   "proofreading" and "TA processing". Conceptually, the proofreading
   sub-algorithm performs syntactic checks on the constraints file,
   while the TA processing sub-algorithm performs the actual certificate
   transformation processing. If the proofreading sub-algorithm does not
   succeed in parsing the constraints file, the TA processing sub-
   algorithm is not executed. Note also that if the constraints file is
   not present, neither sub-algorithm is executed and the local RPKI
   repository is not modified. Each of the constituent algorithms will
   now be described in detail.

4.1  Proofreading algorithm

   The goal of the proofreading algorithm is to check the constraints
   file for syntactic errors, such as missing REQUIRED subsections, or
   malformed addresses such as 1.2.300/24. It also performs a set of
   heuristic checks, such as checking for prefixes that are too large
   (larger than /8). The proofreading algorithm SHOULD also examine
   resource regions (IPv4, IPv6 and AS# regions) within the blocks
   subsection, and reorder such resources within a region in ascending
   numeric order. On encountering any error the proofreading algorithm
   SHOULD provide an error message indicating the line on which the
   error occurred as well as informative text that is sufficiently
   descriptive as to allow the user to identify and correct the error.
   An implementation of the proofreading algorithm MUST NOT assume that
   is has access to the local RPKI repository (even read-only access).
   An implementation of the proofreading algorithm MUST NOT alter the
   local RPKI repository in any way; it also MUST NOT change any of the
   state/metadata information associated with certificates in that
   repository. (Recall that the processing described here is creating a
   copy of that local repository.) Finally, the proofreading algorithm
   MAY produce a transformed output file containing the same syntactic
   information as in the text version of the constraints file, so long
   as the format of the transformed file is understood by the TA
   processing algorithm.

   The proofreading algorithm performs the following syntactic checks on
   the constraints file. It checks for the presence of the REQUIRED
   relying party subsection and the REQUIRED blocks subsection. It
   checks that the order of the two, three or four subsections is as
   stated above. It checks that the relying party subsection conforms to
   the specification given in section 3.1 above. If present, it checks
   that the tags and flags subsections conform to the specifications in
   sections 3.2 and 3.3 above. It then checks the blocks subsection. It
   splits the blocks subsection into constituent target blocks, as
   delimited by the SKI region line(s), and verifies that at least one
   target block is present. It verifies that each SKI region line
   contains exactly forty hexadecimal digits and contains no additional
   characters other than whitespace or colon characters. For each target



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   block, it then verifies the presence of the IPv4, IPv6 and AS#
   regions, and also verifies that at least one such resource is
   present. For each IPv4 prefix, IPv6 prefix and autonomous system
   number given, it checks that the indicated resource is syntactically
   valid according to the appropriate RFC definition, as described in
   section 3.4. It also verifies that no IPv4 resource has a prefix
   larger than /8. The proofreading algorithm SHOULD performing
   reordering within each of the three resource regions so that stated
   resource occur in ascending numerical order. If the proofreading
   algorithm has performed any reordering of information it MAY
   overwrite the constraints file. If it does so, however, it MUST
   preserve all information contained within the file, including
   information that is not parsed (such as comments). If the
   proofreading algorithm has performed any reordering of information
   but has not overwritten the constraints file, it MAY produce a
   transformed output file, as described above. If the proofreading
   algorithm has performed any reordering of information, but has
   neither overwritten the constraints file nor produced a transformed
   output file, it MUST provide an error message to the user indicating
   what reordering was performed.

4.2  TA processing algorithm

   The TA processing algorithm acts on the constraints file (or the
   output file produced by the proofreading algorithm) and the contents
   of the local RPKI repository to produce paracertificates for the
   purpose of enforcing the resource allocations as expressed in the
   constraints file. The TA processing algorithm operates in five
   stages, a preparatory stage (stage 0), target processing (stage 1),
   ancestor processing (stage 2), tree processing (state 3) and TA re-
   parenting (stage 4). Conceptually, during the preparatory stage the
   constraints (or proofreader output) file is read and a set of
   internal RP, tag and flag variables are set based on the contents of
   that file. (If the constraint file has not specified one or more of
   the tags and/or flags, those tags and flags are set to default
   values). During target processing all certificates specified by a
   target block are processed, and the resources for those certificates
   are (potentially) expanded; for each target found a new
   paracertificate is manufactured with its various fields set, as shown
   in Table 1, using the values of the internal variables set in the
   preparatory stage and also, of course, the fields of the original
   certificate (and, potentially, fields of the RP's TA certificate). In
   stage 2 (ancestor) processing, all ancestors of the each target
   certificate are found, and the claimed resources are then removed
   (perforated). A new paracertificate with these diminished resources
   is crafted, with its fields generated based on internal variable
   settings, original certificate field values, and, potentially, the
   fields of the RP's TA certificate. In tree processing (stage 3), the



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   entire local RPKI repository is searching for any other certificates
   that have resources that intersect a target resource, and that were
   not otherwise processed during a preceding stage. Perforation is
   again performed for any such intersecting certificates, and
   paracertificates created as in stage 2. Finally, in the fourth and
   last stage, TA re-parenting, any TA certificates in the local RPKI
   repository that have not already been processed are now re-parented
   under the RP's TA certificate. This transformation will create
   paracertificates; however, these paracertificates may have RFC 3779
   resources that were not altered during algorithm processing. The
   final output of algorithm processing will be threefold. First, the
   state/metadata information on some (original) certificates in the
   repository MAY be altered. Second, paracertificates will be created,
   with the appropriate metadata, and entered into the repository.
   Finally, the TA processing algorithm SHOULD produce a human readable
   log of its actions, indicating which paracertificates were created
   and why. The remainder of this section describes the processing
   stages of the algorithm in detail.

4.2.1  Preparatory processing (stage 0)

   During preparatory processing, the constraints file, or the
   corresponding output file of the proofreader algorithm, is read.
   Internal variables are set corresponding to each tag and flag, if
   present, or to their defaults, if absent. Internal variables are also
   set corresponding to the PRIVATEKEYMETHOD value string(s) and the
   TOPLEVELCERTIFICATE string. The TA processing algorithm is queried to
   determine if it supports the indicated private key access
   methodology. This query is performed in an implementation-specific
   manner. In particular, an implementation is free to vacuously return
   success to this query. The TA processing algorithm next uses the
   value string for the TOPLEVELCERTIFICATE to locate this certificate,
   again in an implementation=specific manner. The certificate in
   question may already be present in the local RPKI repository, or it
   may be located elsewhere. The implementation is also free to create
   the top level certificate at this time, and then assign to this
   newly-created certificate the name indicated. It is necessary only
   that, at the conclusion of this processing, a valid trust anchor
   certificate for the relying party has been created or otherwise
   obtained.

   Some form of access to the RP's private key and top level certificate
   are required for subsequent correct operation of the algorithm.
   Therefore, stage 0 processing MUST terminate if one or both
   conditions are not satisfied. In the error case, the implementation
   SHOULD provide an error message of sufficient detail that the user
   can correct the error(s). If stage 0 processing does not succeed, no
   further stages of TA processing are executed.



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4.2.2  Target processing (stage 1)

   During target processing, the TA processing algorithm reads all
   target blocks in the constraints file or corresponding proofreader
   output file. It then processes each target block in the order
   specified in the file. In the description that follows, except where
   noted, the operation of the algorithm on a single target block will
   be described. Note, however, that all stage 1 processing is executed
   before any processing in subsequent stages is performed.

   The algorithm first obtains the SKI region of the target block. It
   then locates, in an implementation-dependent manner, the certificate
   the SKI extension field of which contains that value. Note that if
   paracertificates have been created by virtue of previous target
   blocks being processed, those paracertificates are not searched in
   attempting to locate a certificate with a matching SKI; only original
   certificates are searched. If more than one original certificate is
   found matching this SKI, there are two possible scenarios. If a
   resource holder has two certificates issued by the same CA, with
   overlapping validity intervals and the same key, but distinct subject
   names (typically, by virtue of the SerialNumber parts being
   different), then these two certificates are both consider to be
   (distinct) targets, and are both processed. If, however, a resource
   holder has certificates issued by two different CAs, containing
   different resources, but using the same key, there is no unambiguous
   method to decide which of the certificates is intended as the target.
   In this latter case the algorithm MUST issue a warning to that
   effect, mark the target block in question as unavailable for
   processing by subsequent stages and proceed to the next target block.
   If no certificate is found then the algorithm SHOULD issue a warning
   to that effect and proceed to process the next target block.

   If a single original certificate is found matching the indicated SKI,
   then the algorithm takes the following actions. First, it sets the
   ORIGINAL state bit for the certificate found. Second, it sets the
   TARGET state bit for the certificate found. Third, it extracts the
   RFC 3779 resources from the certificate. If the global
   resource_nounion flag is TRUE, it compares the extracted certificate
   resources with the resources specified in the constraints file. If
   the two resource sets are different, the algorithm SHOULD issue a
   warning noting the difference. An output resource set is then formed
   that is identical to the resource set extracted from the certificate.
   If, however, the resource_nounion flag is FALSE, then the output
   resource set is calculated by forming the union of the resources
   extracted from the certificate and the resources specified for this
   target block in the constraints file. A paracertificate is then
   constructed according to Table 1, using fields from the original
   certificate, the tags that had been set during stage 0, and, if



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   necessary, fields from the RP's TA certificate. The RFC 3779
   resources of the paracertificate are equated to the derived output
   resource set. The PARA state bit is set for the newly created
   paracertificate.

4.2.3  Ancestor processing (stage 2)

   The goal of ancestor processing is to discover all ancestors of
   target certificates and remove from those ancestors the resources
   specified in the target blocks corresponding to the targets being
   processed. Note that it is possible that, for a given chain from a
   target certificate to a trust anchor, another target might be
   encountered. This is handled by removing all the target resources of
   all descendents. The set of all targets that are descendants of the
   given certificate is formed. The union of all the target resources of
   the corresponding target blocks is computed, and this union in then
   removed from the shared ancestor.

   In detail, the algorithm is as follows. First, all original target
   certificates processed during stage 1 processing are collected.
   Second, any such certificates that have the NOCHAIN state bit set are
   eliminated from the collection. (Note that, as a result of
   eliminating such certificates, the resulting collection may be empty,
   in which case this stage of algorithm processing terminates, and
   processing advances to stage 3.) Next, an implementation MAY sort the
   collection. The optional sorting algorithm is described in Appendix
   B. Note that all stage 2 processing is completed before any stage 3
   processing.

   Two levels of nested iteration are performed. The outer iteration is
   effected over all certificates in the collection; the inner iteration
   is over all ancestors of the designated certificate being processed.
   The first certificate in the collection is chosen, and a resource set
   R is initialized based on the resources of the target block for that
   certificate (since the certificate is in collection, it must be a
   target certificate, and thus correspond to a target block). The
   parent of the certificate is then located using ordinary path
   discovery over original certificates only. The ancestor's certificate
   resources A are then extracted. These resources are then perforated
   with respect to R. That is, an output set of resources is created by
   forming the intersection I of A and R, and then taking the set
   difference A - I as the output resources. A paracertificate is then
   created containing resources tat are these output resources, and
   containing other fields and extensions from the original certificate
   (and possibly the RP's TA certificate) according to the procedure
   given in Table 1. The PARA state bit is set on this paracertificate
   and the ORIGINAL state bit is set on A. If A is also a target
   certificate, as indicated by its TARGET state bit being set, then



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   there will already have been a paracertificate created for it. This
   previous paracertificate is destroyed in favor of the newly created
   paracertificate. In this case also, the set R is augmented by adding
   into it the set of resources of the target block for A. The algorithm
   then proceeds to process the parent of A. This inner iteration
   continues until the self-signed certificate at the root of the path
   is encountered and processed. The outer iteration then continues by
   clearing R and proceeding to the next certificate in the target
   collection.

   Note that ancestor processing has the potential for order dependency
   as mentioned earlier in this document. If sorting is not implemented,
   or if the sorting algorithm fails to completely process the
   collection of target certificates because the allotted maximum number
   of iterations has been realized, it may be the case that an ancestor
   of a certificate logically occurs before that certificate in the
   collection. Whenever an existing paracertificate is replaced by a
   newly created paracertificate during ancestor processing, the
   algorithm SHOULD alert the user, and SHOULD log sufficient detail
   such that the user is able to determine which resources were
   perforated from the original certificate in order to create the (new)
   paracertificate.

   In addition, implementations MUST provide for conflict detection and
   notification during ancestor processing. In particular, if a
   certificate is encountered two or more times during any part of the
   ancestor processing algorithm, and the modifications dictated by the
   ancestor processing algorithm are in conflict, the implementation
   MUST refrain from processing that certificate. Further, the
   implementation MUST present the user with an error message that
   contains enough detail that the user can locate those directives in
   the constraints file that are creating the conflict. For example,
   during one stage of the processing algorithm it may be directed that
   resources R1 be added to a certificate C, while during a different
   stage of the processing algorithm it may be directed that resources
   R2 be removed from certificate C. If the resource sets R1 and R2 have
   a non-empty intersection, that is a conflict.

4.2.4  Tree processing (stage 3)

   The goal of tree processing is to locate other certificates the
   resources of which might conflict with the resources allocated to a
   target by virtue of their being mentioned in the constraints file. In
   this stage of processing, certificates that are not ancestors of any
   target are considered. In detail, the algorithm used is as follows.
   First, all target certificates are again collected. Second, all
   target certificates that have the NOCHAIN state bit set are
   eliminated from this collection. Third, if the intersection_always



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   global flag is set, those target blocks that occur in the constraints
   file, but that did not correspond to a certificate in the local
   repository, are also added to the collection. In tree processing,
   unlike ancestor processing, this collection is not sorted. An
   iteration is now performed over each certificate (or set of target
   block resources) in the collection. Note that the collection may be
   empty, in which case this stage of algorithm processing terminates,
   and processing advances to stage 4. Note also that all stage 3
   processing is performed before any stage 4 processing.

   Given a certificate or target resource block, each top level original
   TA certificate is examined. If that TA certificate has an
   intersection with the target block resources, then the certificate is
   perforated with respect to those resources. A paracertificate is
   created based on the contents of the original certificate (and
   possibly the RP's TA certificate, as indicated in Table 1) using the
   perforated resources. The ORIGINAL state bit is set on the original
   certificate processed in this manner, and the PARA state bit is set
   on the paracertificate just created. An inner iteration then begins
   on the descendants of the original certificate just processed. There
   are two ways in which this iteration may proceed. If the treegrowth
   global flag is clear, then examination of the children proceeds until
   all children are exhausted, or until one child is found with
   intersecting resources. If the treegrowth global flag is set, all
   children are examined. Since a transfer of resources may be in
   process such that more than one child possesses intersecting
   resources, it is RECOMMENDED that the treegrowth flag be set. The
   inner iteration proceeds until all descendants have been examined and
   no further intersecting resources are found. The outer iteration then
   continues with the next certificate or target resource block in the
   collection. Note that unlike ancestor processing, there is no concept
   of a potentially cumulating resource collection R; only the resources
   in the target block are used for perforation.

4.2.5  TA re-parenting (stage 4)

   In the final stage of TA algorithm processing, all TA certificates
   (other than the RP's TA certificate) that have not already been
   processed in a previous stage are now processed. It will be the case
   that all such unprocessed TA certificates have no intersection with
   any target resource blocks. As such, in creating the corresponding
   paracertificates, the output resource set is identical to the input
   resource set. Other transformations as described in Table 1 are
   performed. The original TA certificates have the ORIGINAL state bit
   set; the newly created paracertificates have the PARA state bit set.
   Note that once stage four processing is completely, only a single TA
   certificate will remain in an unprocessed state, namely the relying
   party's own TA certificate.



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4.3  Discussion

   The algorithm described in this document effectively creates two
   coexisting certificate hierarchies: the original certificate
   hierarchy and the paracertificate hierarchy. Note that original
   certificates are not removed during any of the processing described
   in the previous section. Some original certificates may move from
   having no state bits set (or only the NOCHAIN state bit set) to
   having one or both of the ORIGINAL and TARGET state bits set. In
   addition, the NOCHAIN state bit will still be set if it was set
   before any processing. The paracertificate hierarchy, however, is
   intended to supersede the original hierarchy for the purposes of ROA
   validation. The presence of two hierarchies has implications for the
   handling of path discovery, and also for the handling of revocation.
   If one thinks of a certificate as being "named" by its SKI, then
   there can now be two certificates with the same name, one an original
   certificate and the other a paracertificate. The next two sections
   discuss the implications of this duality in detail. Before
   proceeding, it is worth noting that even without the existence of the
   paracertificate hierarchy, cases may exist in which two or more
   original certificates have the same SKI. As noted earlier, in Section
   4.2.2, these cases may be subdivided into the case in which such
   certificates are distinguishable by virtue of having different
   subject names, but identical issuers and resource sets, versus all
   other cases. In the distinguishable case, the path discovery
   algorithm treats the original certificates as separate certificates,
   and processes them separately. In all other cases, the original
   certificates should be treated as indistinguishable, and path
   validation should fail.

5  Implications for Path Discovery

   Path discovery proceeds from a child certificate C by asking for a
   parent certificate P such that the AKI of C is equal to the SKI of P.
   With one hierarchy this question would produce at most one answer.
   With two hierarchies, the original certificate hierarchy and the
   paracertificate hierarchy, the question may produce two answers, one
   answer, or no answer. Each of these cases is considered in turn.

5.1  Two answers

   In this case, it SHOULD be the case that one of the matches is a
   certificate with the ORIGINAL state bit set and the PARA state bit
   clear, while the other match inversely has the ORIGINAL state bit
   clear and the PARA state bit set. If any other combination of
   ORIGINAL and PARA state bits obtains, the path discovery algorithm
   MUST alert the user. In addition, the path discovery algorithm SHOULD
   refrain from attempting to make a choice as to which of the two



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   certificates is the putative parent. In the no-error case, with the
   state bits are as indicated, the certificate with the PARA state bit
   set is chosen as the parent P. Note this means, in effect, that all
   children of the original certificate have been re-parented under the
   paracertificate.

5.2  One answer

   If the matching certificate has neither the ORIGINAL state bit set
   nor the PARA state bit set, this certificate is the parent. If the
   matching certificate has the PARA state bit set but the ORIGINAL
   state bit not set, this certificate is the parent. (This situation
   would arise, for example, if the original certificate had been
   revoked by its issuer but the paracertificate had not been revoked by
   the RP.) If the matching certificate has the ORIGINAL state bit set
   but the PARA state bit not set, this is not an error but it is a
   situation in which path discovery MUST be forced to fail. The parent
   P MUST be set to NULL, and the NOCHAIN state bit must be set on C and
   all its descendants; the user SHOULD be warned. Even if the RP has
   revoked the paracertificate, the original certificate MAY persist.
   Forcing path discovery to unsuccessfully terminate is a reflection of
   the RP's preference for path discovery to fail as opposed to using
   the original hierarchy. Finally, if the matching certificate has both
   the ORIGINAL and PARA state bits set, this is an error. The parent P
   MUST be set to NULL, and the user MUST be warned.

5.3  No answer

   This situation occurs when C has no parent in either the original
   hierarchy or the paracertificate hierarchy. In this case the parent P
   is NULL and path discovery terminates unsuccessfully. The NOCHAIN
   state bit must be set on C and all its descendants.

6  Implications for Revocation

   In a standard implementation of revocation in a PKI, a valid CRL
   names a (sibling) certificate by serial number. That certificate is
   revoked and is purged from the local RPKI repository. In the
   mechanism described in this document, the original certificate
   hierarchy and the paracertificate hierarchy are closely related. It
   can thus be asked how revocation is handled in the presence of these
   two hierarchies, in particular with regard to whether changes in one
   of the hierarchies triggers corresponding changes in the other
   hierarchy. There are four cases.

6.1  No state bits set

   If the CRL names a certificate that has neither the ORIGINAL state



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   bit set nor the PARA state bit set, revocation proceeds normally. All
   children of the revoked certificate have their state modified so that
   the NOCHAIN state bit is set.

6.2  ORIGINAL state bit set

   If the CRL names a certificate with the ORIGINAL state bit set and
   the PARA state bit clear, then this certificate is revoked as usual.
   If this original certificate also has the TARGET state bit set, then
   the corresponding paracertificate (if it exists) is not revoked; if
   this original certificate has the TARGET state bit clear, then the
   corresponding paracertificate is revoked as well. Note that since all
   the children of the original certificate have been re-parented to be
   children of the corresponding paracertificate, as described above,
   the revocation algorithm MUST NOT set the NOCHAIN state bit on these
   children unless the paracertificate is also revoked. Note also that
   if the original certificate is revoked but the paracertificate is not
   revoked, the paracertificate retains its PARA state bit. This is to
   ensure that path discovery proceeds preferentially through the
   paracertificate hierarchy, as described above.

6.3  PARA state bit set

   If the CRL names a certificate with the PARA state bit set and the
   ORIGINAL state bit clear, this CRL must have been issued, perforce,
   by the RP itself. This is because all the paracertificates are
   children of the RP's TA certificate. (Recall that a TA is not revoked
   via a CRL; it is merely removed from the repository.) The
   paracertificate is revoked and all children of the paracertificate
   have the NOCHAIN state bit set. No action is taken on the
   corresponding original certificate; in particular, its ORIGINAL state
   bit is not cleared.

   Note that the serial numbers of paracertificates are synthesized
   according to the procedure given in Table 1, rather than being
   assigned by an algorithm under the control of the (original) issuer.

6.4  Both ORIGINAL and PARA state bits set

   This is an error. The revocation algorithm MUST alert the user and
   take no further action.










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7  Security Considerations

   The goal of the algorithm described in this document is to enable an
   RP to impose its own constraints on its view of the RPKI, which
   itself is a security function. An RP using a constraints file is
   trusting the assertions made in that file. Errors in the constraints
   file used by an RP can undermine the security offered by the RPKI, to
   that RP. In particular, since the paracertificate hierarchy is
   intended to trump the original certificate hierarchy for the purposes
   of path discovery, an improperly constructed paracertificate
   hierarchy could validate origin attestations that would otherwise be
   invalid, or could declare as invalid origin attestations that would
   otherwise be valid. As a result, an RP must carefully consider the
   security implications of the constraints file being used.

8  IANA Considerations

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

9  Acknowledgements

   The authors would like to acknowledge the significant contributions
   of Charles Gardiner, who was the original author of an internal
   version of this document, and who contributed significantly to its
   evolution into the current version.

10  References

10.1  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3513]   Hinden, R., and S. Deering, "Internet Protocol Version 6
               (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [RFC3779]   Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
               Addresses and AS Identifiers", RFC 3779, June 2004.

   [RFC5280]   Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
               Housley, R., and W. Polk, "Internet X.509 Public Key
               Infrastructure Certificate and Certificate Revocation
               List (CRL) Profile", RFC 5280, May 2008.

   [RFC5396]   Huston, G., and G. Michaelson, "Textual Representation of
               Autonomous System (AS) Numbers", RFC 5396, December 2008.




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   [I-D. sidr-arch]
               Lepinski, M. and S. Kent, "An Infrastructure to Support
               Secure Internet Routing", draft-ietf-sidr-arch-11.txt
               (work in progress), September 2010.

   [I-D. sidr-repos-struct]
               Huston, G., Loomans, R., and G. Michaelson, "A Profile
               for Resource Certificate Policy Structure", draft-ietf-
               sidr-repos-struct-05.txt (work in progress), October
               2010.

   [I-D. sidr-res-cert-prof]
               Huston, G., Michaelson, G., and R. Loomans, "A Profile
               for X.509 PKIX Resource Certificates", draft-ietf-sidr-
               res-certs-19.txt (work in progress), October 2010.


10.2  Informative References

   None.

Authors' Addresses


     Stephen Kent
     BBN Technologies
     10 Moulton St.
     Cambridge, MA 02138

     Email: kent@bbn.com

     Mark Reynolds
     BBN Technologies
     10 Moulton St.
     Cambridge, MA 02138

     Email: mreynold@bbn.com














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Appendix A: Sample Constraints File


   ;
   ; Sample constraints file for TBO LTA Test Corporation.
   ;
   ; TBO manages its own local (10.x.x.x) address space
   ; via the target blocks in this file.
   ;

   ;
   ; Relying party subsection. TBO uses ssh-agent as
   ; a software cryptographic agent.
   ;

   PRIVATEKEYMETHOD         OBO(ssh-agent)
   TOPLEVELCERTIFICATE      tbomaster.cer

   ;
   ; Flags subsection
   ;
   ; Always use the resources in this file to augment
   ;   certificate resources.
   ; Always process resource conflicts in the tree, even
   ;   if the target certificate is missing.
   ; Always search the entire tree.
   ;

   CONTROL  resource_nounion      FALSE
   CONTROL  intersection_always   TRUE
   CONTROL  treegrowth            TRUE

   ;
   ; Tags subsection
   ;
   ; Copy the original cert's validity dates.
   ; Use the default policy OID.
   ; Use our own CRLDP.
   ; Use our own AIA.
   ;

   TAG   Xvalidity_dates         C
   TAG   Xcp                     D
   TAG   Xcrldp       rsync://tbo_lta_test.com/pub/CRLs
   TAG   Xaia         rsync://tbo_lta_test.com/pub/repos

   ;
   ; Block subsection



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   ;

   ;
   ; First block: TBO corporate
   ;

   SKI 00112233445566778899998877665544332211
     IPv4
       10.2.3/24
       10.8/16
     IPv6
       2000:2:3:4:5:6/112
     AS#
       60123
       5507

   ;
   ; Second block: TBO LTA Test enforcement division
   ;

   SKI 653420AF758421CF600029FF857422AA6833299F
     IPv4
       10.2.8/24
       10.47/16
     IPv6
     AS#
       60124

   ;
   ; Third block: TBO LTA Test Acceptance Corporation
   ; Quality financial services since sometime
   ; late yesterday.
   ;

   SKI 19:82:34:90:8b:a0:9c:ef:00:af:a0:98:23:09:82:4b:ef:ab:98:09
     IPv4
       10.3.3/24
     IPv6
     AS#
       60125

   ; End of TBO constraints file


Appendix B: Optional Sorting Algorithm for Ancestor Processing

   Sorting is performed in an effort to eliminate any order dependencies
   in ancestor processing, as described in section 4.2.3 of this



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   document. The sorting algorithm does this by rearranging the
   processing of certificates such that if A is an ancestor of B, B is
   processed before A. The sorting algorithm is an OPTIONAL part of
   ancestor processing. Sorting proceeds as follows. The collection
   created at the beginning of ancestor processing is traversed and any
   certificate in the collection that is visited as a result of path
   discovery is temporarily marked. After the traversal, all unmarked
   certificates are moved to the beginning of the collection. The
   remaining marked certificates are unmarked, and a traversal again
   performed through this sub-collection of previously marked
   certificates. The sorting algorithm proceeds iteratively until all
   certificates have been sorted or until a predetermined fixed number
   of iterations has been performed. (Eight is suggested as a munificent
   value for the upper bound, since the number of sorting steps need
   should be no greater than the maximum depth of the tree). Finally,
   the ancestor processing algorithm is applied in turn to each
   certificate in the remaining sorted collection. If the sorting
   algorithm fails to converge, that is if the maximum number of
   iterations has been reached and unsorted certificates remain, the
   implementation SHOULD warn the user.































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