RFC 3379           DPV and DPD Protocol Requirements      September 2002   policy, time-stamp tokens from TSAs responders trusted under the   validation policy, or a DPV response from a DPV server that is   trusted under the validation policy.  When the certificate is valid   according to the validation policy, the server MUST, upon request,   include that information in the response.  However, the server MAY   omit that information when the certificate is invalid or when it   cannot determine the validity.   The DPV server MUST be able, upon request, copy a text field provided   by the client into the DPV response.  As an example, this field may   relate to the nature or reason for the DPV query.   The DPV response MUST be bound to the DPV request so that the client   can be sure that all the parameters from the request have been taken   into consideration by the DPV server to build the response.  This can   be accomplished by including a one-way hash of the request in the   response.   In some environments it may be necessary to present only a DPV   response to another relying party without the corresponding request.   In this case the response MUST be self contained.  This can be   accomplished by repeating only the important components from the   request in the response.   For the client to be confident that the certificate validation was   handled by the expected DPV server, the DPV response MUST be   authenticated, unless an error is reported (such as a badly formatted   request or unknown validation policy).   For the client to be able prove to a third party that trusts the same   DPV server that the certificate validation was handled correctly, the   DPV response MUST be digitally signed, unless an error is reported.   The DPV server's certificate MUST authenticate the DPV server.   The DPV server MAY require client authentication, therefore, the DPV   request MUST be able to be authenticated.   When the DPV request is authenticated, the client SHOULD be able to   include a client identifier in the request for the DPV server to copy   into the response.  Mechanisms for matching this identifier with the   authenticated identity depends on local DPV server conditions and/or   the validation policy.  The DPV server MAY choose to blindly copy the   identifier, omit the identifier, or return an error response.   There are no specific confidentiality requirements within this   application layer protocol.  However, when confidentiality is needed,   it can be achieved with a lower-layer security protocol.Pinkas & Housley             Informational                      [Page 6]RFC 3379           DPV and DPD Protocol Requirements      September 20024.2. Relaying, Re-direction and Multicasting   In some network environments, especially ones that include firewalls,   a DPV server might not be able to obtain all of the information that   it needs to process a request.  However, the DPV server might be   configured to use the services of one or more other DPV servers to   fulfill all requests.  In such cases, the client is unaware that the   queried DPV server is using the services of other DPV servers, and   the client-queried DPV server acts as a DPV client to another DPV   server.  Unlike the original client, the DPV server is expected to   have moderate computing and memory resources, enabling the use of   relay, re-direct or multicasting mechanisms.  The requirements in   this section support DPV server-to-DPV server exchanges without   imposing them on DPV client-to-DPV server exchanges.   Protocols designed to satisfy these requirements MAY include optional   fields and/or extensions to support relaying, re-direction or   multicasting.  However, DPV clients are not expected to support   relay, re-direct or multicast.  If the protocol supports such   features, the protocol MUST include provisions for DPV clients and   DPV servers that do not support such features, allowing them to   conform to the basic set of requirements.   - When a server supports a relay mechanism, a mechanism to detect     loops or repetition MUST be provided.   - When a protocol provides the capability for a DPV server to re-     direct a request to another DPV server (that is, the protocol     chooses to provide a referral mechanism), a mechanism to provide     information to be used for the re-direction SHOULD be supported.     If such re-direction information is sent back to clients, then the     protocol MUST allow conforming clients to ignore it.   - Optional parameters in the protocol request and/or response MAY be     provide support for relaying, re-direction or multicasting.  DPV     clients that ignore any such optional parameters MUST be able to     use the DPV service.  DPV servers that ignore any such optional     parameters MUST still be able to offer the DPV service, although     they might not be able to overcome the limitations imposed by the     network topology.  In this way, protocol implementers do not need     to understand the syntax or semantics of any such optional     parameters.5. Delegated Path Discovery Protocol Requirements   The Delegated Path Discovery (DPD) protocol allows the client to use   a single request to collect at one time from a single server the data   elements available at the current time that might be collected usingPinkas & Housley             Informational                      [Page 7]RFC 3379           DPV and DPD Protocol Requirements      September 2002   different protocols (such as LDAP, HTTP, FTP, or OCSP) or by querying   multiple servers, to locally validate a public key certificate   according to a single path discovery policy.  The returned   information can be used to locally validate one or more certificates   for the current time.   Clients MUST be able to specify whether they want, in addition to the   certification path, the revocation information associated with the   path, for the end-entity certificate, for the CA certificates, or for   both.   If the DPD server does not support the client requested path   discovery policy, the DPD server MUST return an error.  Some forms of   path discovery policy can be simple.  In that case it is acceptable   to pass the parameters from the path discovery policy with each   individual request.  For example, the client might provide a set of   trust anchors and separate revocation status conditions for the end-   entity certificate and for the other certificates.  The DPD request   MUST allow more elaborated path discovery policies to be referenced.   However, it is expected that most of the time clients will only be   aware of the referenced path discovery policy for a given   application.   The DPD server response includes zero, one, or several certification   paths.  Each path consists of a sequence of certificates, starting   with the certificate to be validated and ending with a trust anchor.   If the trust anchor is a self-signed certificate, that self-signed   certificate MUST NOT be included.  In addition, if requested, the   revocation information associated with each certificate in the path   MUST also be returned.   By default, the DPD server MUST return a single certification path   for each end-entity certificate in the DPD request.  However, the   returned path may need to match some additional local criteria known   only to the client.  For example, the client might require the   presence of a particular certificate extension or a particular name   form.  Therefore, the DPD client MUST have a means of obtaining more   than one certification path for each end-entity certificate in the   DPD request.  At the same time, the mechanism for obtaining   additional certification paths MUST NOT impose protocol state on the   DPD server.  Avoiding the maintenance of state information associated   with previous requests minimizes potential denial of service attacks   and other problems associated with server crashes.   Path discovery MUST be performed according to the path discovery   policy.  The DPD response MUST indicate one of the following status   alternatives:Pinkas & Housley             Informational                      [Page 8]RFC 3379           DPV and DPD Protocol Requirements      September 2002   1) one or more certification paths was found according to the path      discovery policy, with all of the requested revocation information      present.   2) one or more certification paths was found according to the path      discovery policy, with a subset of the requested revocation      information present.   3) one or more certification paths was found according to the path      discovery policy, with none of the requested revocation      information present.   4) no certification path was found according to the path discovery      policy.   5) path construction could not be performed due to an error.   When no errors are detected, the information that is returned   consists of one or more certification paths and, if requested, its   associated revocation status information for each certificate in the   path.   For the client to be confident that all of the elements from the   response originate from the expected DPD server, an authenticated   response MAY be required.  For example, the server might sign the   response or data authentication might also be achieved using a   lower-layer security protocol.   The DPD server MAY require client authentication, allowing the DPD   request MUST to be authenticated.   There are no specific confidentiality requirement within the   application layer protocol.  However, when confidentiality is needed,   it can be achieved with a lower-layer security protocol.6. DPV and DPD Policy Query   Using a separate request/response pair, the DPV or DPD client MUST be   able to obtain references for the default policy or for all of the   policies supported by the server.  The response can include   references to previously defined policies or to a priori known   policies.7. Validation Policy   A validation policy is a set of rules against which the validation of   the certificate is performed.Pinkas & Housley             Informational                      [Page 9]RFC 3379           DPV and DPD Protocol Requirements      September 2002   A validation policy MAY include several trust anchors.  A trust   anchor is defined as one public key, a CA name, and a validity time   interval; a trust anchor optionally includes additional constraints.   The use of a self-signed certificate is one way to specify the public   key to be used, the issuer name, and the validity period of the   public key.   Additional constraints for each trust anchor MAY be defined.  These   constraints might include a set of certification policy constraints   or a set of naming constraints.  These constraints MAY also be   included in self-signed certificates.   Additional conditions that apply to the certificates in the path MAY   also be specified in the validation policy.  For example, specific   values could be provided for the inputs to the certification path   validation algorithm in [PKIX-1], such as user-initial-policy-set,   initial-policy-mapping-inhibit, initial-explicit-policy, or initial-   any-policy-inhibit.   Additional conditions that apply to the end-entity certificate MAY   also be specified in the validation policy.  For example, a specific   name form might be required.   In order to succeed, one valid certification path (none of the   certificates in the path are expired or revoked) MUST be found   between an end-entity certificate and a trust anchor and all   constraints that apply to the certification path MUST be verified.7.1. Components for a Validation Policy   A validation policy is built from three components:   1. Certification path requirements,   2. Revocation requirements, and   3. End-entity certificate specific requirements.   Note:  [ES-P] defines ASN.1 data elements that may be useful while   defining the components of a validation policy.7.2. Certificate Path Requirements   The path requirements identify a sequence of trust anchors used to   start certification path processing and initial conditions for   certification path validation as defined in [PKIX-1].Pinkas & Housley             Informational                     [Page 10]RFC 3379           DPV and DPD Protocol Requirements      September 2002