Detecting RRDP Session Desynchronization

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Table of Contents

1. Introduction

The Resource Public Key Infrastructure (RPKI) Repository Delta Protocol (RRDP) [RFC8182] is a one-way synchronization protocol for distributing RPKI data in the form of differences (deltas) between sequential repository states. Relying Parties apply a contiguous chain of deltas to synchronize their local copy of the repository with the current state of the remote Repository Server. Delta files for any given session_id and serial number are expected to contain an immutable record of the state of the Repository Server at that given point in time, but this is not always the case.¶

This document describes an approach for Relying Parties (RPs) to detect a particular form of RRDP session desynchronization and how to recover, as implemented in OpenBSD's rpki-client validator.¶

2. Immutability of RRDP files

Section 3.1 of [RFC8182] describes how discrete publication events such as the addition, modification, or deletion of one or more repository objects can be communicated as immutable files, highlighting advantages for publishers such as the ability to pre-calculate files and make use of caching infrastructure.¶

However, if an RRDP server over time serves differing data for a given session_id and serial number, distinct RP instances (depending on the moment they connected to the RRDP server) would end up with divergent local repositories. Comparing the server-provided session_id and serial numbers across distinct RP instances would not bring such divergence to light.¶

While the global RPKI is understood to present a loosely consistent view, depending on timing, updating, fetching (see Section 6 of [RFC7115]), different caches having different data for the same RRDP session at the same serial violates the principle of least astonishment.¶

The [RFC8182] specification does allude to immutability being a property of RRDP files, but doesn't make it clear immutability is an absolute requirement for the RRDP protocol to work well. A future update to [RFC8182] should set a hard rule to establish that the immutability of RRDP files must not be violated after publication, and RPs should check for unexpected mutations.¶

3. Detection of Desynchronization

Relying Parties can implement a mechanism to keep a record of the serial and hash attribute values in delta elements of the previous successful fetch of an Update Notification File. Then, after the fetch of a new Update Notification File, the Relying Party should compare if the serial and hash values of previously seen serials match those in the newly fetched file. If any difference is detected, Delta files were unexpectedly mutated, and the RP should proceed to Section 4.¶

RP implementations decide how many Delta Files to maximally process before switching to downloading the latest Snapshot File. The same upper bound can be used as a limit to the number of delta element serial and hash values to track.¶

<notification xmlns="http://www.ripe.net/rpki/rrdp" version="1"
              session_id="fe528335-db5f-48b2-be7e-bf0992d0b5ec"
              serial="1774">
  <snapshot uri="https://rrdp.example.net/1774/snapshot.xml"
       hash="4b5f27b099737b8bf288a33796bfe825fb2014a69fd6aa99080380299952f2e2"/>
  <delta serial="1774"
         hash="effac94afd30bbf1cd6e180e7f445a4d4653cb4c91068fa9e7b669d49b5aaa00"
         uri="https://rrdp.example.net/1774/delta.xml" />
  <delta serial="1773"
         hash="731169254dd5de0ede94ba6999bda63b0fae9880873a3710e87a71bafb64761a"
         uri="https://rrdp.example.net/1773/delta.xml" />
  <delta serial="1772"
         hash="d4087585323fd6b7fd899ebf662ef213c469d39f53839fa6241847f4f6ceb939"
         uri="https://rrdp.example.net/1772/delta.xml />
</notification>

fe528335-db5f-48b2-be7e-bf0992d0b5ec
1774 effac94afd30bbf1cd6e180e7f445a4d4653cb4c91068fa9e7b669d49b5aaa00
1773 731169254dd5de0ede94ba6999bda63b0fae9880873a3710e87a71bafb64761a
1772 d4087585323fd6b7fd899ebf662ef213c469d39f53839fa6241847f4f6ceb939

<notification xmlns="http://www.ripe.net/rpki/rrdp" version="1"
              session_id="fe528335-db5f-48b2-be7e-bf0992d0b5ec"
              serial="1775">
  <snapshot uri="https://rrdp.example.net/1775/snapshot.xml"
       hash="cd430c386deacb04bda55301c2aa49f192b529989b739f412aea01c9a77e5389"/>
  <delta serial="1775"
         hash="d199376e98a9095dbcf14ccd49208b4223a28a1327669f89566475d94b2b08cc"
         uri="https://rrdp.example.net/1775/delta.xml />
  <delta serial="1774"
         hash="10ca28480a584105a059f95df5ca8369142fd7c8069380f84ebe613b8b89f0d3"
         uri="https://rrdp.example.net/1774/delta.xml" />
  <delta serial="1773"
         hash="731169254dd5de0ede94ba6999bda63b0fae9880873a3710e87a71bafb64761a"
         uri="https://rrdp.example.net/1773/delta.xml" />
</notification>

4. Recovery after Desynchronization

Following the detection of RRDP session desynchronization, the RP implementation SHOULD issue a warning and SHOULD download the latest Snapshot File and process it as described in Section 3.4.3 of [RFC8182].¶

5. Security Considerations

Due to the lifetime of RRDP sessions (often measured in months), desynchronization can persist for an extended period.¶

Caches in a desynchronized state pose a risk by emitting a different set of Validated Payloads than they would otherwise emit with a consistent repository copy. Through the interaction of the desynchronization and the failed fetch mechanism described in Section 6.6 of [RFC9286], Relying Parties could spuriously omit Validated Payloads or emit Validated Payloads that the Certification Authority intended to withdrawn. In a desynchronized state, all bets are off.¶

Missing Validated Payloads negatively impact the ability to validate BGP announcements using mechanisms such as described in [RFC6811] and [I-D.ietf-sidrops-aspa-verification].¶

Section 6.6 of [RFC9286] advises RP implementations to continue to use cached versions of objects, but only until such time as they become stale. By detecting whether the remote Repository Server is in an inconsistent state and then immediately switching to using the latest Snapshot File, RPs increase the probability to successfully replace objects before they become stale.¶

6. IANA Considerations

No IANA actions required.¶

Appendix A. Acknowledgements

During the hallway track at RIPE 86, Ties de Kock shared the idea for detecting this particular form of RRDP desynchronization, after which Claudio Jeker, Job Snijders, and Theo Buehler produced an implementation based on rpki-client. Equipped with tooling to detect this particular error condition, in subsequent months it became apparent that unexpected delta mutations in the global RPKI repositories do happen from time to time.¶

The authors wish to thank Theo Buehler for their careful review and feedback.¶

Authors' Addresses