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Internet Engineering Task Force (IETF)
Request for Comments: 8182
Category: Standards Track
ISSN: 2070-1721
T. Bruijnzeels
O. Muravskiy
RIPE NCC
B. Weber
Cobenian
R. Austein
Dragon Research Labs
July 2017

The RPKI Repository Delta Protocol (RRDP)

Abstract

In the Resource Public Key Infrastructure (RPKI), Certificate Authorities (CAs) publish certificates, including end-entity certificates, Certificate Revocation Lists (CRLs), and RPKI signed objects to repositories. Relying Parties retrieve the published information from those repositories. This document specifies a new RPKI Repository Delta Protocol (RRDP) for this purpose. RRDP was specifically designed for scaling. It relies on an Update Notification File which lists the current Snapshot and Delta Files that can be retrieved using HTTPS (HTTP over Transport Layer Security (TLS)), and it enables the use of Content Distribution Networks (CDNs) or other caching infrastructures for the retrieval of these files.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8182.

Copyright Notice

Copyright © 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  RPKI Repository Delta Protocol Implementation . . . . . . . .   4
     3.1.  Informal Overview . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Certificate Authority Use . . . . . . . . . . . . . . . .   5
     3.3.  Repository Server Use . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Initialization  . . . . . . . . . . . . . . . . . . .   6
       3.3.2.  Publishing Updates  . . . . . . . . . . . . . . . . .   6
     3.4.  Relying Party Use . . . . . . . . . . . . . . . . . . . .   7
       3.4.1.  Processing the Update Notification File . . . . . . .   7
       3.4.2.  Processing Delta Files  . . . . . . . . . . . . . . .   9
       3.4.3.  Processing a Snapshot File  . . . . . . . . . . . . .  10
       3.4.4.  Polling the Update Notification File  . . . . . . . .  10
       3.4.5.  Considerations Regarding Operational Failures in RRDP  11
     3.5.  File Definitions  . . . . . . . . . . . . . . . . . . . .  11
       3.5.1.  Update Notification File  . . . . . . . . . . . . . .  11
       3.5.2.  Snapshot File . . . . . . . . . . . . . . . . . . . .  13
       3.5.3.  Delta File  . . . . . . . . . . . . . . . . . . . . .  15
       3.5.4.  XML Schema  . . . . . . . . . . . . . . . . . . . . .  17
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .  18
     4.1.  Compatibility with previous standards . . . . . . . . . .  18
     4.2.  Distribution Considerations . . . . . . . . . . . . . . .  19
     4.3.  HTTPS Considerations  . . . . . . . . . . . . . . . . . .  19
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1. Introduction

In the Resource Public Key Infrastructure (RPKI), Certificate Authorities 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 [RSYNC], allowing Relying Parties to synchronize a local copy of the RPKI repository used for validation with the remote repositories [RFC6481].

rsync [RSYNC] has proven valuable in the early deployment of RPKI, because it allowed operators to gain experience without the need to invent a custom protocol. However, operational experience has brought concerns to light that we wish to address here:

  • rsync [RSYNC] is designed to limit the amount of data that needs to be transferred between client and server. However, the server needs to spend significant resources in terms of CPU and memory for every connection. This is a problem in an envisioned RPKI deployment where thousands of Relying Parties query a small number of central repositories, and it makes these repositories weak to denial-of-service attacks.
  • A secondary concern is the lack of supported rsync server and client libraries. In practice, all implementations have to make system calls to an rsync binary. This is inefficient; it introduces fragility with regards to updates of this binary, makes it difficult to catch and report problems to operators, and complicates software development and testing.

This document specifies an alternative repository access protocol based on Update Notification, Snapshot, and Delta Files that a Relying Party can retrieve over the HTTPS protocol. This allows Relying Parties to either perform a full (re-)synchronization of their local copy of the repository using Snapshot Files or use Delta Files to keep their local repository updated after initial synchronization. We call this the RPKI Repository Delta Protocol, or RRDP in short.

RRDP was designed to support scaling in RPKI's asymmetric deployment. It is consistent (in terms of data structures) with the publication protocol [RFC8181] 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. RRDP 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. This enables Repository Servers to pre-calculate these files only once for all clients, thus limiting the CPU and memory investments required, and enables the use of a caching infrastructure to reduce the load on a Repository Server when a large number of Relying Parties are querying it.

This document allows the use of RRDP as an additional repository distribution mechanism for RPKI. In time, RRDP may replace rsync [RSYNC] as the only mandatory-to-implement repository distribution mechanism. However, this transition is outside of the scope of this document.

2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. RPKI Repository Delta Protocol Implementation

3.1. Informal Overview

Certification Authorities 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 Certificate Authority engine itself. Certificates in the RPKI that use a Repository Server that supports RRDP include a special Subject Information Access (SIA) pointer referring to an Update Notification File.

The Update Notification File includes a globally unique session_id in the form of a version 4 Universally Unique IDentifier (UUID) [RFC4122] and serial number that can be used by the Relying Party to determine if it and the repository are synchronized. 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 Relying Party that learns about an Update Notification File location for the first time can download it and then proceed to download the latest Snapshot File, thus creating a local copy of the repository that is in sync with the Repository Server. The Relying Party records the location of this Update Notification File, the session_id, and the current serial number.

Relying Parties are encouraged to re-fetch this Update Notification File at regular intervals, but not more often than once per minute. After re-fetching the Update Notification File, the Relying Party may find that there are one or more Delta Files available that allow it to synchronize its local repository with the current state of the Repository Server. If no contiguous chain of deltas from the Relying Party's serial to the latest repository serial is available, or if the session_id has changed, the Relying Party performs a full resynchronization instead.

As soon as the Relying Party fetches new content in this way, it could start a validation process. An example of a reason why a Relying Party may not choose to 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 could use a caching infrastructure to reduce its load, particularly because 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. Update Notification Files are polled by Relying Parties to discover if updates exist; for this reason, Update Notification Files may not be cached for longer than one minute.

3.2. Certificate Authority Use

Certificate Authorities that use RRDP 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 Section 6: 

     id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
     
       dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
     
     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 [RFC7230] that will point to the Update Notification File for the Repository Server that publishes the products of this Certificate Authority certificate.

3.3. Repository Server Use

3.3.1. Initialization

When the Repository Server initializes, it performs the following actions:

  • The server MUST generate a new random version 4 UUID (see Section 4.1.3 of [RFC4122]) to be used as the session_id.
  • The server MUST then generate a Snapshot File for serial number ONE for this new session that includes all currently known published objects that the Repository Server is responsible for. Note that this Snapshot File may contain zero publish elements at this point if no objects have been submitted for publication yet.
  • This Snapshot File MUST be made available at a URL that is unique to this session_id and serial number, so that it can be cached indefinitely. The format and caching concerns for Snapshot Files are explained in more detail in Section 3.5.2.
  • After the Snapshot File has been published, the Repository Server MUST publish a new Update Notification File that contains the new session_id, has serial number ONE, has one reference to the Snapshot File that was just published, and contains no delta references. The format and caching concerns for Update Notification Files are explained in more detail in Section 3.5.1.

3.3.2. Publishing Updates

Whenever the Repository Server receives updates from a Certificate Authority, it MUST generate new snapshot and Delta Files within one minute. If a Repository Server services a large number of Certificate Authorities, it MAY choose to combine updates from multiple CAs. If a Repository Server combines updates in this way, it MUST ensure that publication never postponed for longer than one minute for any of the CAs involved.

Updates are processed as follows:

  • The new repository serial number MUST be one greater than the current repository serial number.
  • A new Delta File MUST be generated for this new serial. This Delta File MUST include all new, replaced, and withdrawn objects for multiple CAs, if applicable, as a single change set.
  • This Delta File MUST be made available at a URL that is unique to the current session_id and serial number, so that it can be cached indefinitely.
  • The format and caching concerns for Delta Files are explained in more detail in Section 3.5.3.
  • The Repository Server MUST also generate a new Snapshot File for this new serial. This file MUST contain all "publish" elements for all current objects.
  • The Snapshot File MUST be made available at a URL that is unique to this session and new serial, so that it can be cached indefinitely.
  • The format and caching concerns for Snapshot Files are explained in more detail in Section 3.5.2.
  • Any older Delta Files that, when combined with all more recent Delta Files, will result in the total size of deltas exceeding the size of the snapshot MUST be excluded to avoid that Relying Parties download more data than necessary.
  • A new Update Notification File MUST now be created by the Repository Server. This new Update Notification File MUST include a reference to the new Snapshot File and all Delta Files selected in the previous steps.
  • The format and caching concerns for Update Notification Files are explained in more detail in Section 3.5.1.

If the Repository Server is not capable of performing the above for some reason, then it MUST perform a full re-initialization, as explained above in Section 3.3.1.

3.4. Relying Party Use

3.4.1. Processing the Update Notification File

When a Relying Party performs RPKI validation and learns about a valid certificate with an SIA entry for the RRDP protocol, it SHOULD use this protocol as follows.

The Relying Party MUST download the Update Notification File, unless an Update Notification File was already downloaded and processed from the same location in this validation run or a polling strategy was used (see Section 3.4.4).

It is RECOMMENDED that the Relying Party uses a "User-Agent" header explained in Section 5.5.3. of [RFC7231] to identify the name and version of the Relying Party software used. It is useful to track capabilities of Relying Parties in the event of changes to the RPKI standards.

When the Relying Party downloads an Update Notification File, it MUST verify the file format and validation steps described in Section 3.5.1.3. If this verification fails, the file MUST be rejected and RRDP cannot be used. See Section 3.4.5 for considerations.

The Relying Party MUST verify whether the session_id matches the last known session_id for this Update Notification File location. Note that even though the session_id is a random UUID value, it alone MUST NOT be used by a Relying Party as a unique identifier of a session but always together with the location of the Update Notification File. The reason for this is that a malicious server can use an existing session_id from another Repository Server.

If the session_id matches the last known session_id, then a Relying Party MAY download and process missing Delta Files as described in Section 3.4.2, provided that all Delta Files for serial numbers between the last processed serial number and the current serial number in the Update Notification File can be processed this way.

If the session_id matches the last known session_id, but Delta Files were not used, then the Relying Party MUST download and process the Snapshot File on the Update Notification File as described in Section 3.4.3.

If the session_id does not match the last known session_id, the Relying Party MUST update its last known session_id to the value specified in the downloaded Update Notification File. The Relying Party MUST then download and process the Snapshot File specified in the downloaded Update Notification File as described in Section 3.4.3.

3.4.2. Processing Delta Files

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 Relying Parties MUST attempt to download and process all Delta Files in order of serial number as follows.

When the Relying Party 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 Relying Party 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 a Relying Party retrieved a Delta File that is valid according to the above criteria, it performs the following actions:

  • The Relying Party MUST verify that the session_id matches the session_id of the Update Notification File. If the session_id values do not match, the file MUST be rejected.
  • The Relying Party MUST verify that the serial number of this Delta File is exactly one greater than the last processed serial number for this session_id, and if not, this file MUST be rejected.
  • The Relying Party SHOULD add all publish elements to a local storage and update its last processed serial number to the serial number of this Delta File.
  • When a Relying Party encounters a "withdraw" element, or a "publish" element where an object is replaced, in a delta that it retrieves from a Repository Server, it MUST verify that the object to be withdrawn or replaced was retrieved from this same Repository Server before applying the appropriate action. Failing to do so will leave the Relying Party vulnerable to malicious Repository Servers instructing it to delete or change arbitrary objects.

If any Delta File is rejected, Relying Parties MUST process the current Snapshot File instead, as described in Section 3.4.3.

3.4.3. Processing a Snapshot File

Snapshot Files MUST only be used if Delta Files are unavailable or were rejected; for a description of the process, see Section 3.4.1.

When the Relying Party 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 Relying Party 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 a Relying Party retrieved a Snapshot File that is valid according to the above criteria, it performs the following actions:

  • The Relying Party MUST verify that the session_id matches the session_id of the Update Notification File. If the session_id values do not match, the file MUST be rejected.
  • The Relying Party MUST verify that the serial number of this Snapshot File is greater than the last processed serial number for this session_id. If this fails, the file MUST be rejected.
  • The Relying Party SHOULD then add all publish elements to a local storage and update its last processed serial number to the serial number of this Snapshot File.

If a Snapshot File is rejected, it means that RRDP cannot be used. See Section 3.4.5 for considerations.

3.4.4. Polling the Update Notification File

Once a Relying Party has learned about the location, session_id, and last processed serial number of the repository that uses the RRDP protocol, the Relying Party MAY start polling the Repository Server for updates. However, the Relying Party MUST NOT poll for updates more often than once every 1 minute, and in order to reduce data usage, Relying Parties MUST use the "If-Modified-Since" header explained in Section 3.3 of [RFC7232] in requests.

If a Relying Party finds that updates are available, it SHOULD download and process the file as described in Section 3.4.1 and initiate a new RPKI object validation process. However, a detailed description of the RPKI object validation process itself is out of scope of this document.

3.4.5. Considerations Regarding Operational Failures in RRDP

If a Relying Party experiences any issues with retrieving or processing any of the files used in this protocol, it will be unable to retrieve new RPKI data from the affected Repository Server.

Relying Parties could attempt to use alternative repository access mechanisms, if they are available, according to the accessMethod element value(s) specified in the SIA of the associated certificate (see Section 4.8.8 of [RFC6487]).

Furthermore, Relying Parties may wish to employ re-try strategies while fetching RRDP files. Relying Parties are also advised to keep old objects in their local cache so that validation can be done using old objects.

It is also recommendable that re-validation and retrieval is performed pro-actively before manifests or CRLs go stale, or certificates expire, to ensure that problems on the side of the Relying Party can be identified and resolved before they cause major concerns.

3.5. File Definitions

3.5.1. Update Notification File

3.5.1.1. Purpose

The Update Notification File is used by Relying Parties to discover whether any changes exist between the state of the repository and the Relying Party's cache. It describes the location of the files containing the snapshot and incremental deltas, which can be used by the Relying Party to synchronize with the repository.

3.5.1.2. Cache Concerns

A Repository Server MAY use caching infrastructure to cache the Update Notification File and reduce the load of HTTPS requests. However, since this file is used by Relying Parties to determine whether any updates are available, the Repository Server SHOULD 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 Update Notification File rather than no Update 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" (see Section 5.2 of [RFC7234]). Another approach can be to have the Repository Server push out new versions of the Update Notification File to the caching infrastructure when appropriate.

In case of a high load on a Repository Server or its distribution network, the Cache-Control HTTP header, or a similar mechanism, MAY be used to suggest an optimal (for the Repository Server) poll interval for Relying Parties. However, setting it to an interval longer than 1 hour is NOT RECOMMENDED. Relying parties SHOULD align the suggested interval with their operational practices and the expected update frequency of RPKI repository data and MAY discard the suggested value.

3.5.1.3. File Format and Validation

Example Update 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 Update Notification Files:

  • A Relying Party MUST reject any Update Notification File that is not well-formed or does not conform to the RELAX NG schema outlined in Section 3.5.4 of this document.
  • The XML namespace MUST be "http://www.ripe.net/rpki/rrdp".
  • The encoding MUST be "US-ASCII".
  • The version attribute in the notification root element MUST be "1".
  • The session_id attribute MUST be a random version 4 UUID [RFC4122], unique to this session.
  • The serial attribute MUST be an unbounded, unsigned positive integer in decimal format indicating the current version of the repository.
  • The Update Notification File MUST contain exactly one 'snapshot' element for the current repository version.
  • If delta elements are included, they MUST form a contiguous sequence of serial numbers starting at a revision determined by the Repository Server, up to the serial number mentioned in the notification element. Note that the elements may not be ordered.
  • The hash attribute in snapshot and delta elements MUST be the hexadecimal encoding of the SHA-256 [SHS] hash of the referenced file. The Relying Party MUST verify this hash when the file is retrieved and reject the file if the hash does not match.

3.5.2. Snapshot File

3.5.2.1. Purpose

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.

3.5.2.2. Cache Concerns

A snapshot reflects the content of the repository at a specific point in time; for that reason, it 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 these files from using a lot of space in the caching infrastructure, it is RECOMMENDED that a limited interval is used in the order of hours or days.

To avoid race conditions where a Relying Party downloads an Update Notification File moments before it's updated, Repository Servers SHOULD retain old Snapshot Files for at least 5 minutes after a new Update Notification File is published.

3.5.2.3. File Format and Validation

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:

  • A Relying Party MUST reject any Snapshot File that is not well- formed or does not conform to the RELAX NG schema outlined in Section 3.5.4 of this document.
  • The XML namespace MUST be "http://www.ripe.net/rpki/rrdp".
  • The encoding MUST be "US-ASCII".
  • The version attribute in the notification root element MUST be "1".
  • The session_id attribute MUST match the expected session_id in the reference in the Update Notification File.
  • The serial attribute MUST match the expected serial in the reference in the Update Notification File.
  • Note that the publish element is similar to the publish element defined in the publication protocol [RFC8181]. However, the "tag" attribute is not used here because it is not relevant to Relying Parties. The "hash" attribute is not used here because this file represents a complete current state of the repository; therefore, it is not relevant to know which existing RPKI object (if any) is updated.

3.5.3. Delta File

3.5.3.1. Purpose

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 (updated objects as well as a new manifest and CRL) as one atomic update message.

3.5.3.2. Cache Concerns

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 the caching infrastructure, it is RECOMMENDED that a limited interval is used in the order of hours or days.

To avoid race conditions where a Relying Party downloads an Update Notification File moments before it's updated, Repository Servers SHOULD retain old Delta Files for at least 5 minutes after they are no longer included in the latest Update Notification File.

3.5.3.3. File Format and Validation

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 Relying Party 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:

  • A Relying Party MUST reject any Delta File that is not well-formed or does not conform to the RELAX NG schema outlined in Section 3.5.4 of this document.
  • The XML namespace MUST be "http://www.ripe.net/rpki/rrdp".
  • The encoding MUST be "US-ASCII".
  • The version attribute in the delta root element MUST be "1".
  • The session_id attribute MUST be a random version 4 UUID unique to this session.
  • The session_id attribute MUST match the expected session_id in the reference in the Update Notification File.
  • The serial attribute MUST match the expected serial in the reference in the Update Notification File.
  • Note that the publish element is similar to the publish element defined in the publication protocol [RFC8181]. However, the "tag" attribute is not used here because it is not relevant to Relying Parties.

3.5.4. XML Schema

The following is a RELAX NG compact form schema describing version 1 of this protocol.

#
# RELAX NG schema for the RPKI Repository Delta Protocol (RRDP). #

default namespace = "http://www.ripe.net/rpki/rrdp" 

   version = xsd:positiveInteger   { maxInclusive="1" }
   serial  = xsd:positiveInteger
   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:

4. Operational Considerations

4.1. Compatibility with previous standards

This protocol has been designed to replace rsync as a distribution mechanism of an RPKI repository. However, it is also designed to coexist with existing implementations based on rsync, to enable smooth transition from one distribution mechanism to another.

For every repository object listed in the Snapshot and Delta Files, both the hash of the object's content and the rsync URI [RFC5781] of its location in the repository are listed. This makes it possible to distribute the same RPKI repository, represented by a set of files on a filesystem, using both rsync and RRDP. It also enables Relying Parties tools to query, combine, and consequently validate objects from repositories of different types.

4.2. Distribution Considerations

One of the design goals of RRDP was to minimize load on a Repository Server while serving clients. To achieve this, neither the content nor the URLs of the Snapshot and Delta Files are modified after they have been published in the Update Notification File. This allows their effective distribution by using either a single HTTP server or a CDN.

The RECOMMENDED way for Relying Parties to keep up with the repository updates is to poll the Update Notification File for changes. The content of that file is updated with every new serial version of a repository (while its URL remains stable). To effectively implement distribution of the Update Notification File, an "If-Modified-Since" HTTP request header is required to be present in all requests for the Update Notification File (see Section 3.4.4). Therefore, it is RECOMMENDED that Relying Party tools implement a mechanism to keep track of a previous successful fetch of an Update Notification File.

Implementations of RRDP should also take care of not producing new versions of the repository (and subsequently, new Update Notification, Snapshot, and Delta Files) too often. Usually the maintenance of the RPKI repository includes regular updates of manifest and CRL objects performed on a schedule. This often results in bursts of repository updates during a short period of time. Since the Relying Parties are required to poll for the Update Notification File not more often than once per minute (Section 3.4.4), it is not practical to generate new serial versions of the repository much more often than 1 per minute. It is allowed to combine multiple updates, possibly from different CAs, into a new serial repository version (Section 3.3.2). This will significantly shorten the size of the Update Notification File and total amount of data distributed to all Relying Parties.

4.3. HTTPS Considerations

Note that a Man in the Middle (MITM) cannot produce validly signed RPKI data but can perform withhold or replay attacks targeting a Relying Party and keep the Relying Party from learning about changes in the RPKI. Because of this, Relying Parties SHOULD do TLS certificate and host name validation when they fetch from an RRDP Repository Server.

Relying Party tools SHOULD log any TLS certificate or host name validation issues found, so that an operator can investigate the cause. However, such validation issues are often due to configuration errors or a lack of a common TLS trust anchor. In these cases, it is better if the Relying Party retrieves the signed RPKI data regardless and performs validation on it. Therefore, the Relying Party MUST continue to retrieve the data in case of errors. The Relying Party MAY choose to log encountered issues only when fetching the Update Notification File, but not when it subsequently fetches Snapshot or Delta Files from the same host. Furthermore, the Relying Party MAY provide a way for operators to accept untrusted connections for a given host, after the cause has been identified.

It is RECOMMENDED that Relying Parties and Repository Servers follow the Best Current Practices outlined in [RFC7525] on the use of HTTP over TLS (HTTPS) [RFC7230]. Relying Parties SHOULD do TLS certificate and host name validation using subjectAltName dNSName identities as described in [RFC6125]. The rules and guidelines defined in [RFC6125] apply here, with the following considerations:

  • Relying Parties and Repository Servers SHOULD support the DNS-ID identifier type. The DNS-ID identifier type SHOULD be present in Repository Server certificates.
  • DNS names in Repository Server certificates SHOULD NOT contain the wildcard character "*".
  • A Common Name (CN) field may be present in a Repository Server certificate's subject name but SHOULD NOT be used for authentication within the rules described in [RFC6125].
  • This protocol does not require the use of SRV-IDs.
  • This protocol does not require the use of URI-IDs.

Note, however, that this validation is done on a best-effort basis and serves to highlight potential issues, but RPKI object security does not depend on this. Therefore, Relying Parties MAY deviate from the validation steps listed above.

5. Security Considerations

RRDP deals exclusively with the transfer of RPKI objects from a Repository Server to a Relying Party. The trust relation between a Certificate Authority and its Repository Server is out of scope for this document. However, it should be noted that from a Relying Party point of view, all RPKI objects (certificates, CRLs, and objects wrapped in Cryptographic Message Syntax (CMS)) 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 protocol 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 attacks more difficult for the attacker. HTTPS issues are discussed in more detail in Section 4.3.

Supporting both RRDP and rsync necessarily increases the number of opportunities for a malicious RPKI Certificate Authority to perform denial-of-service attacks on Relying Parties, by expanding the number of URIs which the Relying Party 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 Certificate Authority can supply an effectively infinite series of URIs for the Relying Party to follow. The only real solution to this is for the Relying Party 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 Certificate Authority; 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 Relying Party 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 an HTTPS caching infrastructure to reduce load on primary Repository Servers and increase resilience against denial-of-service attacks on the RPKI publication service.

6. IANA Considerations

IANA has updated the reference for id-ad-rpkiNotify to point to this document in the "SMI Security for PKIX Access Descriptor" registry [IANA-AD-NUMBERS].

7. References

7.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <http://www.rfc-editor.org/info/rfc4122>.
   
   [RFC5781]  Weiler, S., Ward, D., and R. Housley, "The rsync URI
              Scheme", RFC 5781, DOI 10.17487/RFC5781, February 2010,
              <http://www.rfc-editor.org/info/rfc5781>.
   
   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <http://www.rfc-editor.org/info/rfc6125>.
   
   [RFC6481]  Huston, G., Loomans, R., and G. Michaelson, "A Profile for
              Resource Certificate Repository Structure", RFC 6481,
              DOI 10.17487/RFC6481, February 2012,
              <http://www.rfc-editor.org/info/rfc6481>.
   
   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              DOI 10.17487/RFC6487, February 2012,
              <http://www.rfc-editor.org/info/rfc6487>.
   
   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.
   
   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.
   
   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <http://www.rfc-editor.org/info/rfc7232>.
   
   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <http://www.rfc-editor.org/info/rfc7234>.
   
   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.
   
   [RFC8181]  Weiler, S., Sonalker, A., and R. Austein, "A Publication
              Protocol for the Resource Public Key Infrastructure
              (RPKI)", DOI 10.17487/RFC8181, July 2017,
              <http://www.rfc-editor.org/info/rfc8181>.
   
   [SHS]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

7.2. Informative References

[IANA-AD-NUMBERS] 

              IANA, "Structure of Management Information (SMI) Numbers
              (MIB Module Registrations)",
              <http://www.iana.org/assignments/smi-numbers>.
   
   [RFC6486]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 6486, DOI 10.17487/RFC6486, February 2012,
              <http://www.rfc-editor.org/info/rfc6486>.
   
   [RFC6488]  Lepinski, M., Chi, A., and S. Kent, "Signed Object
              Template for the Resource Public Key Infrastructure
              (RPKI)", RFC 6488, DOI 10.17487/RFC6488, February 2012,
              <http://www.rfc-editor.org/info/rfc6488>.
   
   [RSYNC]    "rsync", <https://rsync.samba.org>.

Acknowledgements

The authors would like to thank David Mandelberg for reviewing this document.

Authors' Addresses

Tim Bruijnzeels
RIPE NCC

Email: 

          tim@ripe.net

Oleg Muravskiy
RIPE NCC

Email: 

          oleg@ripe.net

Bryan Weber
Cobenian

Email: 

          bryan@cobenian.com

Rob Austein
Dragon Research Labs

Email: 

          sra@hactrn.net