| Network Working Group | T. Bruijnzeels |
| Internet-Draft | O. Muravskiy |
| Intended status: Standards Track | RIPE NCC |
| Expires: April 2, 2017 | B. Weber |
| Cobenian | |
| R. Austein | |
| Dragon Research Labs | |
| September 29, 2016 |
RPKI Repository Delta Protocol
draft-ietf-sidr-delta-protocol-04
In the Resource Public Key Infrastructure (RPKI), certificate authorities publish certificates, including end entity certificates, Certificate Revocation Lists (CRL), and RPKI signed objects to repositories. Relying Parties (RP) retrieve the published information from those repositories. This document specifies a delta protocol which provides relying parties with a mechanism to query a repository for incremental updates, thus enabling the RP to keep its state in sync with the repository.
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This Internet-Draft will expire on April 2, 2017.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 [RFC2119].
In the Resource Public Key Infrastructure (RPKI), Certificate Authorities (CAs) publish certificates [RFC6487], RPKI signed objects [RFC6488], manifests [RFC6486], and CRLs to repositories. CAs may have an embedded mechanism to publish to these repositories, or they may use a separate repository server and publication protocol. RPKI repositories are currently accessible using the rsync protocol, allowing Relying Parties (RPs) to synchronise a local copy of the RPKI repository used for validation with the remote repositories [RFC6481].
This document specifies an alternative repository access protocol based on notification, snapshot and delta files that a RP can retrieve over the HTTPS protocol. This allows RPs to perform either a full (re-)synchronisation of their local copy of the repository using snapshot files, or use delta files to keep their local repository updated after initial synchronisation.
This protocol is designed to be consistent (in terms of data structures) with the publication protocol [I-D.ietf-sidr-publication] and treats publication events of one or more repository objects as discrete events that can be communicated to relying parties. This approach helps to minimize the amount of data that traverses the network and thus helps minimize the amount of time until repository convergence occurs. This protocol also provides a standards based way to obtain consistent, point in time views of a single repository, eliminating a number of consistency related issues. Finally, this approach allows these discrete events to be communicated as immutable files, so that caching infrastructure can be used to reduce the load on a repository server when a large number of relying parties are querying it.
Certification Authorities (CA) in the RPKI use a repository server to publish their RPKI products, such as manifests, CRLs, signed certificates and RPKI signed objects. This repository server may be remote, or embedded in the CA engine itself. Certificates in the RPKI that use a repository server that supports this delta protocol include a special Subject Information Access (SIA) pointer referring to a notification file.
The notification file includes a globally unique session_id in the form of a version 4 UUID, and serial number that can be used by the Relying Party (RP) to determine if it and the repository are synchronised. Furthermore it includes a link to the most recent complete snapshot of current objects that are published by the repository server, and a list of links to delta files, for each revision starting at a point determined by the repository server, up to the current revision of the repository.
A RP that learns about a notification file location for the first time can download it, and then proceed to download the latest snapshot file, and thus create a local copy of the repository that is in sync with the repository server. The RP should remember the location of this notification file, the session_id and current serial number.
RPs are encouraged to re-fetch this notification file at regular intervals, but not more often than once per minute. After re-fetching the notification file, the RP may find that there are one or more delta files available that allow it to synchronise its local repository with the current state of the repository server. If no contiguous chain of deltas from RP's serial to the latest repository serial is available, or if the session_id has changed, the RP should perform a full resynchronisation instead.
As soon as the RP fetches new content in this way it should start a validation process. An example of a reason why a RP may not do this immediately is because it has learned of more than one notification location and it prefers to complete all its updates before validating.
The repository server may use caching infrastructure to reduce its load. It should be noted that snapshots and deltas for any given session_id and serial number contain an immutable record of the state of the repository server at a certain point in time. For this reason these files can be cached indefinitely. Notification files are polled by RPs to discover if updates exist, and for this reason notification files may not be cached for longer than one minute.
Certificate Authorities that use this delta protocol MUST include an instance of an SIA AccessDescription extension in resource certificates they produce, in addition to the ones defined in [RFC6487],
AccessDescription ::= SEQUENCE {
accessMethod OBJECT IDENTIFIER,
accessLocation GeneralName }
This extension MUST use an accessMethod of id-ad-rpkiNotify, see: [IANA-AD-NUMBERS],
id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }
id-ad-rpkiNotify OBJECT IDENTIFIER ::= { id-ad 13 }
The accessLocation MUST be an HTTPS URI as defined in [RFC2818], that will point to the update notification file for the repository server that publishes the products of this CA certificate.
Relying Parties that do not support this delta protocol MUST NOT reject a CA certificate merely because it has an SIA extension containing this new kind of AccessDescription.
When the repository server initialises it must perform the following actions:
Whenever the repository server receives updates from a CA it SHOULD generate new snapshot and delta files. However, if a publication server services a large number of CAs it MAY choose to combine updates from multiple CAs. If a publication server combines updates in this way, it MUST NOT postpone publishing for longer than one minute.
Updates must be processed as follows:
If the repository server is not capable of performing the above for some reason, then it MUST perform a full re-initialisation, as explained above in Section 3.3.1.
When a Relying Party (RP) performs RPKI validation and learns about a valid certificate with an SIA entry for the RRDP protocol, it SHOULD prefer to use this protocol as follows.
The RP SHOULD download the update notification file, unless an update notification file was already downloaded and processed from the same location in this validation run.
The RP MAY use a "User-Agent" header explained in section 5.5.3. of [RFC7231] to identify the name and version of the RP software used. This is not required, but would be useful to help track capabilities of Relying Parties in the event of changes to the RPKI standards.
When the RP downloads an update notification file it MUST verify the file format and validation steps described in section Section 3.5.1.3. If this verification fails, the file MUST be rejected.
The RP MUST verify whether the session_id in this update notification file matches the last known session_id for this update notification file location. If the session_id matches the last known session_id, then an RP MAY download and process missing delta files as described in section Section 3.4.3, provided that all delta files for serial numbers between the last processed serial number and the current serial number in the notification file can be processed this way.
If the session_id was not previously known, or if delta files could not be used, then the RP MUST update its last known session_id to this session_id and download and process snapshot file on the update notification file as described in section Section 3.4.2.
If neither update notification file and one snapshot file or delta files could be processed this way, the RP MUST issue an operator error, and SHOULD use an alternate repository retrieval mechanism if it is available.
When the RP downloads a snapshot file it MUST verify the file format and validation steps described in Section 3.5.2.3. If this verification fails, the file MUST be rejected.
Furthermore the RP MUST verify that the hash of the contents of this file matches the hash on the update notification file that referenced it. In case of a mismatch of this hash, the file MUST be rejected.
If an RP retrieved a snapshot file that is valid according to the above criteria, it should perform the following actions:
If an update notification file contains a contiguous chain of links to delta files from the last processed serial number to the current serial number, then RPs MUST attempt to download and process all delta files in order of serial number as follows.
When the RP downloads a delta file it MUST verify the file format and perform validation steps described in Section 3.5.3.3. If this verification fails, the file MUST be rejected.
Furthermore the RP MUST verify that the hash of the contents of this file matches the hash on the update notification file that referenced it. In case of a mismatch of this hash, the file MUST be rejected.
If an RP retrieved a delta file that is valid according to the above criteria, it should perform the following actions:
Once a Relying Party has learned about the location, session_id and last processed serial number of repository that uses the RRDP protocol, the RP MAY start polling the repository server for updates. However the RP MUST NOT poll for updates more often than once every 1 minute, and in order to reduce data usage RPs MUST use the "If-Modified-Since" header explained in section 3.3 of [RFC7232]in requests.
If an RP finds that updates are available it SHOULD download and process the file as described in Section 3.4.1, and initiate a new validation process. A detailed description of the validation process itself is out of scope of this document.
The update notification file is used by RPs to discover whether any changes exist between the state of the repository and the RP's cache. It describes the location of the files containing the snapshot and incremental deltas which can be used by the RP to synchronise with the repository.
A repository server MAY use caching infrastructure to cache the notification file and reduce the load of HTTPS requests. However, since this file is used by RPs to determine whether any updates are available the repository server MUST ensure that this file is not cached for longer than 1 minute. An exception to this rule is that it is better to serve a stale notification file, then no notification file.
How this is achieved exactly depends on the caching infrastructure used. In general a repository server may find certain HTTP headers to be useful, such as: Cache-Control: max-age=60. Another approach can be to have the repository server push out new versions of the notification file to the caching infrastructure when appropriate.
Relying Parties SHOULD NOT cache the notification file for longer than 1 minute, regardless of the headers set by the repository server or CDN.
Example notification file:
<notification xmlns="http://www.ripe.net/rpki/rrdp"
version="1"
session_id="9df4b597-af9e-4dca-bdda-719cce2c4e28"
serial="3">
<snapshot uri="https://host/9d-8/3/snapshot.xml" hash="AB"/>
<delta serial="3" uri="https://host/9d-8/3/delta.xml" hash="CD"/>
<delta serial="2" uri="https://host/9d-8/2/delta.xml" hash="EF"/>
</notification>
Note: URIs and hash values in this example are shortened because of formatting.
The following validation rules must be observed when creating or parsing notification files:
A snapshot is intended to reflect the complete and current contents of the repository for a specific session and version. Therefore it MUST contain all objects from the repository current as of the time of the publication.
A snapshot reflects the content of the repository at a specific point in time, and for that reason can be considered immutable data. Snapshot files MUST be published at a URL that is unique to the specific session and serial.
Because these files never change, they MAY be cached indefinitely. However, in order to prevent that these files use a lot of space in caching infrastructure it is RECOMMENDED that a limited interval is used in the order of hours or days.
To avoid race conditions where an RP downloads a notification file moments before it's updated, Repository Servers SHOULD retain old snapshot files for at least 5 minutes after a new notification file is published.
Example snapshot file:
<snapshot xmlns="http://www.ripe.net/rpki/rrdp"
version="1"
session_id="9df4b597-af9e-4dca-bdda-719cce2c4e28"
serial="2">
<publish uri="rsync://rpki.ripe.net/Alice/Bob.cer">
ZXhhbXBsZTE=
</publish>
<publish uri="rsync://rpki.ripe.net/Alice/Alice.mft">
ZXhhbXBsZTI=
</publish>
<publish uri="rsync://rpki.ripe.net/Alice/Alice.crl">
ZXhhbXBsZTM=
</publish>
</snapshot>
The following rules must be observed when creating or parsing snapshot files:
An incremental delta file contains all changes for exactly one serial increment of the repository server. In other words a single delta will typically include all the new objects, updated objects and withdrawn objects that a Certification Authority sent to the repository server. In its simplest form the update could concern only a single object, but it is recommended that CAs send all changes for one of their key pairs: i.e. updated objects as well as a new manifest and CRL as one atomic update message.
Deltas reflect the difference between two consecutive versions of a repository for a given session. For that reason deltas can be considered immutable data. Delta files MUST be published at a URL that is unique to the specific session and serial.
Because these files never change, they MAY be cached indefinitely. However, in order to prevent these files from using a lot of space in caching infrastructure it is RECOMMENDED that a limited interval is used in the order of hours or days.
To avoid race conditions where an RP downloads a notification file moments before it's updated, Repository Servers SHOULD retain old delta files for at least 5 minutes after they are no no longer included in the latest notification file.
Example delta file:
<delta xmlns="http://www.ripe.net/rpki/rrdp"
version="1"
session_id="9df4b597-af9e-4dca-bdda-719cce2c4e28"
serial="3">
<publish uri="rsync://rpki.ripe.net/repo/Alice/Alice.mft"
hash="50d8...545c">
ZXhhbXBsZTQ=
</publish>
<publish uri="rsync://rpki.ripe.net/repo/Alice/Alice.crl"
hash="5fb1...6a56">
ZXhhbXBsZTU=
</publish>
<withdraw uri="rsync://rpki.ripe.net/repo/Alice/Bob.cer"
hash="caeb...15c1"/>
</delta>
Note that a formal RELAX NG specification of this file format is included later in this document. A RP MUST NOT process any delta file that is incomplete or not well-formed.
The following validation rules must be observed when creating or parsing delta files:
The following is a RELAX NG compact form schema describing version 1 of this protocol.
#
# RelaxNG schema for RPKI Repository Delta Protocol (RRDP).
#
default namespace = "http://www.ripe.net/rpki/rrdp"
version = xsd:positiveInteger { maxInclusive="1" }
serial = xsd:nonNegativeInteger
uri = xsd:anyURI
uuid = xsd:string { pattern = "[\-0-9a-fA-F]+" }
hash = xsd:string { pattern = "[0-9a-fA-F]+" }
base64 = xsd:base64Binary
# Notification file: lists current snapshots and deltas
start |= element notification {
attribute version { version },
attribute session_id { uuid },
attribute serial { serial },
element snapshot {
attribute uri { uri },
attribute hash { hash }
},
element delta {
attribute serial { serial },
attribute uri { uri },
attribute hash { hash }
}*
}
# Snapshot segment: think DNS AXFR.
start |= element snapshot {
attribute version { version },
attribute session_id { uuid },
attribute serial { serial },
element publish {
attribute uri { uri },
base64
}*
}
# Delta segment: think DNS IXFR.
start |= element delta {
attribute version { version },
attribute session_id { uuid },
attribute serial { serial },
delta_element+
}
delta_element |= element publish {
attribute uri { uri },
attribute hash { hash }?,
base64
}
delta_element |= element withdraw {
attribute uri { uri },
attribute hash { hash }
}
# Local Variables:
# indent-tabs-mode: nil
# comment-start: "# "
# comment-start-skip: "#[ \t]*"
# End:
It is RECOMMENDED that Relying Parties and Publication Servers follow the Best Current Practices outlined in [RFC7525] on the use of HTTP over TLS (https).
Note that a Man-in-the-Middle (MITM) cannot produce validly signed RPKI data, but they can perform withhold or replay attacks targeting an RP, and keep the RP from learning about changes in the RPKI. Because of this RPs SHOULD do TLS certificate and host name validation when they fetch from an RRDP Publication Server
However, such validation issues are often due to configuration errors, or a lack of a common TLS trust anchor. In these cases it would be better that the RP retrieves the signed RPKI data regardless, and performs validation on it.
Therefore RPs SHOULD log any TLS certificate or host name validation issues they find, so that an operator can investigate the cause. But the RP MUST continue to retrieve the data. The RP MAY choose to log this issue only when fetching the notification update file, but not when it subsequently fetches snapshot or delta files from the same host. Furthermore the RP MAY provide a way for operators to accept untrusted connections for a given host, after the cause has been identified.
RRDP deals exclusively with transfer of RPKI objects from a repository server to a relying party. The trust relation between a CA and its repository server is out of scope for this document. However, it should be noted the from a relying party point of view all RPKI objects (certificates, CRLs, and CMS-wrapped objects) are already covered by object security mechanisms including signed manifests. This allows validation of these objects even though the repository server itself is not trusted. This document makes no change to RPKI validation procedures per se.
The original RPKI transport mechanism is rsync, which offers no channel security mechanism. RRDP replaces the use of rsync by HTTPS; while the channel security mechanism underlying RRDP (HTTPS) is not a cure-all, it does make some forms of denial of service attack more difficult for the attacker. HTTPS issues are discussed in more detail in Section 4.
Supporting both RRDP and rsync necessarily increases the number of opportunities for a malicious RPKI CA to perform denial of service attacks on relying parties, by expanding the number of URIs which the RP may need to contact in order to complete a validation run. However, other than the relative cost of HTTPS versus rsync, adding RRDP to the mix does not change this picture significantly: with either RRDP or rsync a malicious CA can supply an effectively infinite series of URIs for the RP to follow. The only real solution to this is for the RP to apply some kind of bound to the amount of work it is willing to do. Note also that the attacker in this scenario must be an RPKI CA, since otherwise the normal RPKI object security checks would reject the malicious URIs.
Processing costs for objects retrieved using RRDP may be somewhat different from the same objects retrieved using rsync: because RRDP treats an entire set of changes as a unit (one "delta"), it may not be practical to start processing any of the objects in the delta until the entire delta has been received. With rsync, by contrast, incremental processing may be easy, but the overall cost of transfer may be higher, as may be the number of corner cases in which the RP retrieves some but not all of the updated objects. Overall, RRDP's behavior is closer to a proper transactional system, which (probably) leads to an overall reliability increase.
RRDP is designed to scale much better than rsync. In particular, RRDP is designed to allow use of HTTPS caching infrastructure to reduce load on primary publication servers and increase resilience against denial of service attacks on the RPKI publication service.
IANA is requested to update the reference for id-ad-rpkiNotify to this document in the PKIX Access Descriptor registry [IANA-AD-NUMBERS].
The authors would like to thank David Mandelberg for reviewing this document.