Internet Engineering Task Force (IETF) J. Snijders
Request for Comments: 9697 Fastly
Updates: 8182 T. de Kock
Category: Standards Track RIPE NCC
ISSN: 2070-1721 December 2024
Detecting RPKI Repository Delta Protocol (RRDP) Session
Desynchronization
Abstract
This document describes an approach for Resource Public Key
Infrastructure (RPKI) Relying Parties to detect a particular form of
RPKI Repository Delta Protocol (RRDP) session desynchronization and
how to recover. This document updates RFC 8182.
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
https://www.rfc-editor.org/info/rfc9697.
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Immutability of RRDP Files
3. Detection of Desynchronization
3.1. Example
4. Recovery After Desynchronization
5. Changes to RFC 8182
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
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 (RPs) 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 RPs to detect a form of RRDP
session desynchronization where the hash of a delta for a given
serial number and session_id have mutated from the previous Update
Notification File and how to recover.
1.1. Requirements Language
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.
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
precalculate files and make use of caching infrastructure.
Even though the global RPKI is understood to present a loosely
consistent view that depends on the cache's timing of updates (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.
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 only the server-provided session_id
and latest serial number across distinct RP instances would not bring
such divergence to light.
The RRDP specification [RFC8182] alludes to immutability being a
property of RRDP files, but it doesn't make it clear that
immutability is an absolute requirement for the RRDP to work well.
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
fetching 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 differences are
detected, this means that the Delta files were unexpectedly mutated,
and the RP should proceed to Section 4.
3.1. Example
This section contains two versions of an Update Notification File to
demonstrate an unexpected mutation. The initial Update Notification
File is as follows:
<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" uri="https://rrdp.example.net/1774/delta.xml"
hash=
"effac94afd30bbf1cd6e180e7f445a4d4653cb4c91068fa9e7b669d49b5aaa00"
/>
<delta serial="1773" uri="https://rrdp.example.net/1773/delta.xml"
hash=
"731169254dd5de0ede94ba6999bda63b0fae9880873a3710e87a71bafb64761a"
/>
<delta serial="1772" uri="https://rrdp.example.net/1772/delta.xml"
hash=
"d4087585323fd6b7fd899ebf662ef213c469d39f53839fa6241847f4f6ceb939"
/>
</notification>
Figure 1
Based on the above Update Notification File, an RP implementation
could record the following state:
fe528335-db5f-48b2-be7e-bf0992d0b5ec
1774 effac94afd30bbf1cd6e180e7f445a4d4653cb4c91068fa9e7b669d49b5aaa00
1773 731169254dd5de0ede94ba6999bda63b0fae9880873a3710e87a71bafb64761a
1772 d4087585323fd6b7fd899ebf662ef213c469d39f53839fa6241847f4f6ceb939
Figure 2
A new version of the Update Notification File is published as
follows:
<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" uri="https://rrdp.example.net/1775/delta.xml"
hash=
"d199376e98a9095dbcf14ccd49208b4223a28a1327669f89566475d94b2b08cc"
/>
<delta serial="1774" uri="https://rrdp.example.net/1774/delta.xml"
hash=
"10ca28480a584105a059f95df5ca8369142fd7c8069380f84ebe613b8b89f0d3"
/>
<delta serial="1773" uri="https://rrdp.example.net/1773/delta.xml"
hash=
"731169254dd5de0ede94ba6999bda63b0fae9880873a3710e87a71bafb64761a"
/>
</notification>
Figure 3
Using its previously recorded state (see Figure 2), the RP can
compare the hash values for serials 1773 and 1774. For serial 1774,
compared to the earlier version of the Update Notification File, a
different hash value is now listed, meaning an unexpected delta
mutation occurred.
4. Recovery After Desynchronization
Following the detection of RRDP session desynchronization, in order
to return to a synchronized state, RP implementations SHOULD issue a
warning and SHOULD download the latest Snapshot File and process it
as described in Section 3.4.3 of [RFC8182].
See Section 6 for an overview of risks associated with
desynchronization.
5. Changes to RFC 8182
The following paragraph is added to Section 3.4.1 of [RFC8182],
"Processing the Update Notification File", after the paragraph that
ends "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."
NEW
| If the session_id matches the last known session_id, the Relying
| Party SHOULD compare whether hash values associated with
| previously seen files for serials match the hash values of the
| corresponding serials in the newly fetched Update Notification
| File. If any differences are detected, this means that files were
| unexpectedly mutated (see [RFC9697]). The Relying Party SHOULD
| then download and process the Snapshot File specified in the
| downloaded Update Notification File as described in Section 3.4.3.
6. Security Considerations
Due to the lifetime of RRDP sessions (often measured in months),
desynchronization can persist for an extended period if undetected.
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 withdraw. As a result, due to the
desynchronized state, route decision making processes might consider
route announcements intended to be marked valid as "unknown" or
"invalid" for an indeterminate period.
Missing Validated Payloads negatively impact the ability to validate
BGP announcements using mechanisms such as those described in
[RFC6811] and [ASPA].
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.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8182] Bruijnzeels, T., Muravskiy, O., Weber, B., and R. Austein,
"The RPKI Repository Delta Protocol (RRDP)", RFC 8182,
DOI 10.17487/RFC8182, July 2017,
<https://www.rfc-editor.org/info/rfc8182>.
8.2. Informative References
[ASPA] Azimov, A., Bogomazov, E., Bush, R., Patel, K., Snijders,
J., and K. Sriram, "BGP AS_PATH Verification Based on
Autonomous System Provider Authorization (ASPA) Objects",
Work in Progress, Internet-Draft, draft-ietf-sidrops-aspa-
verification-19, 27 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
aspa-verification-19>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7115] Bush, R., "Origin Validation Operation Based on the
Resource Public Key Infrastructure (RPKI)", BCP 185,
RFC 7115, DOI 10.17487/RFC7115, January 2014,
<https://www.rfc-editor.org/info/rfc7115>.
[RFC9286] Austein, R., Huston, G., Kent, S., and M. Lepinski,
"Manifests for the Resource Public Key Infrastructure
(RPKI)", RFC 9286, DOI 10.17487/RFC9286, June 2022,
<https://www.rfc-editor.org/info/rfc9286>.
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, Mikhail Puzanov, Alberto
Leiva, Tom Harrison, Warren Kumari, Behcet Sarikaya, Murray
Kucherawy, Éric Vyncke, Roman Danyliw, Tim Bruijnzeels, and Michael
Hollyman for their careful review and feedback on this document.
Authors' Addresses
Job Snijders
Fastly
Amsterdam
Netherlands
Email: job@fastly.com