Guidance to Avoid Carrying RPKI Validation States in Transitive BGP Path Attributes
draft-ietf-sidrops-avoid-rpki-state-in-bgp-00

Network Working Group                                        J. Snijders
Internet-Draft                                                    Fastly
Intended status: Best Current Practice                         T. Fiebig
Expires: 11 October 2024                                         MPI-INF
                                                           M. A. Stucchi
                                                             AS58280.net
                                                            9 April 2024

Guidance to Avoid Carrying RPKI Validation States in Transitive BGP Path
                               Attributes
             draft-ietf-sidrops-avoid-rpki-state-in-bgp-00

Abstract

   This document provides guidance to avoid carrying Resource Public Key
   Infrastructure (RPKI) derived Validation States in Transitive Border
   Gateway Protocol (BGP) Path Attributes.  Annotating routes with
   attributes signaling validation state may flood needless BGP UPDATE
   messages through the global Internet routing system, when, for
   example, Route Origin Authorizations are issued, revoked, or RPKI-To-
   Router sessions are terminated.

   Operators SHOULD ensure Validation States are not signalled in
   transitive BGP Path Attributes.  Specifically, Operators SHOULD NOT
   group BGP routes by their Prefix Origin Validation state into
   distinct BGP Communities.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 11 October 2024.

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Copyright Notice

   Copyright (c) 2024 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 (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Risks of Signaling Validation State With Transitive
           Attributes  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Triggers for Large-Scale Validation Changes . . . . . . .   4
       3.1.1.  ROA Issuance  . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  ROA Revocation  . . . . . . . . . . . . . . . . . . .   5
       3.1.3.  Loss of Authoritative Validation Information  . . . .   5
       3.1.4.  Outage Scenario Summary . . . . . . . . . . . . . . .   7
     3.2.  Scaling issues  . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Flooding and Cascading of BGP UPDATES . . . . . . . . . .   7
       3.3.1.  Flooding of BGP UPDATES . . . . . . . . . . . . . . .   7
       3.3.2.  Cascading of BGP UPDATES  . . . . . . . . . . . . . .   8
     3.4.  Observed data . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Lacking Value of Signaling Validation State . . . . . . .   9
   4.  Advantages of Dissociating Validation States and BGP Path
           Attributes  . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   The Resource Public Key Infrastructure (RPKI) [RFC6480] allows for
   validating received routes, e.g., for their Route Origin Validation
   (ROV) state.  Some operators and vendors suggest to use distinct BGP
   Communities [RFC1997] [RFC8092] to annotate received routes with
   their validations state.  The claim is that this practice is useful,
   as validation state can be signalled, e.g., to iBGP speakers, without
   requiring each iBGP speaker to perform their own route origin
   validation.

   However, annotating a route with a transitive attribute means that a
   BGP update message has to be send to each neighbor if such an
   attribute changes.  This means that when, for example, Route Origin
   Authorizations [RFC6482] are issued, revoked, or RPKI-To-Router
   [RFC8210] sessions are terminated, a BGP UPDATE message will be sent
   for a route that was previously annotated with a BGP Community.
   Furthermore, given that BGP Communities are a transitive attribute,
   this BGP UPDATE will have to propagate through the whole default free
   zone (DFZ).

   Hence, this document provides guidance to avoid carrying Resource
   Public Key Infrastructure (RPKI) [RFC6480] derived Validation States
   in Transitive Border Gateway Protocol (BGP) Path Attributes Section 5
   of [RFC4271].  Specifically, Operators SHOULD NOT group BGP routes by
   their Prefix Origin Validation state [RFC6811] into distinct BGP
   Communities [RFC1997] [RFC8092].  Not using BGP Communities to signal
   RPKI validation state prevent needless BGP UPDATE messages from being
   flooded through the global Internet routing system.

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.  Scope

   This document discusses signaling of RPKI validation state to BGP
   neighbors using transitive BGP attributes.  At the time of writing,
   this pertains to the use of BGP Communities [RFC1997] [RFC8092] to
   signal RPKI ROV using ROAs.  Note that this includes all operator
   specific BGP Communities to signal validation state, as well as any
   current or future documented well-known BGP Communities marking
   validation state, as, e.g., described for extended BGP Communities in
   [RFC8097].

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   However, beyond that, this document also applies to all current and
   future transitive BGP attributes that may be implicitly or explicitly
   used to signal validation state to neighbors.  Similarly, it applies
   to all future validation mechanics of RPKI, e.g., ASPA
   [I-D.ietf-sidrops-aspa-profile] and any other future validation
   mechanic build upon the RPKI.

3.  Risks of Signaling Validation State With Transitive Attributes

   This section outlines the risks of signaling RPKI Validation State
   using BGP Communities.  While the current description is specific to
   BGP communities, the observations hold similar for all transitive
   attributes that may be added to a route.  Furthermore, we will
   present data on the measured current impact of BGP Communities being
   used to signal RPKI Validation state.

3.1.  Triggers for Large-Scale Validation Changes

   Here, we describe examples for how a large amount of RPKI ROV changes
   may occur in a short time, leading to a large amount of BGP Updates
   being send.

3.1.1.  ROA Issuance

   Large-Scale ROA issuance should be a comparatively rare event for
   individual networks.  However, several cases exist where issuance by
   individual operators or (malicious) coordinated issuance of ROAs by
   multiple operators may lead to a high churn triggering a continuous
   flow of BGP Update messages caused by operators using transitive BGP
   attributes to signal RPKI validation state.

   Specifically:

   *  When one large operator newly starts issuing ROAs for their
      netblocks, possibly by issuing one ROA with a long maxLength
      covering a large number of prefixes.  This may also occur when
      incorrectly migrating to minimally covering ROAs [RFC9319], i.e.,
      when the previous ROA is first revoked (see Section 3.1.2) and the
      new ROAs are only issued after this revocation has been
      propagated, e.g., due to an operational error or bug in the
      issuance pipeline used by the operator.

   *  When multiple smaller operators coordinate to issue new ROAs at
      the same time.

   *  When a CA has been unavailable or unable to publish for some time,
      but then publishes all updates at once, or--as unlikely as it is--
      if a key-rollover encounters issues.

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3.1.2.  ROA Revocation

   Large-Scale ROA revocation should be a comparatively rare event for
   individual networks.  However, several cases exist where revocations
   by individual operators or (malicious) coordinated revocation of ROAs
   by multiple operators may lead to a high churn triggering a
   continuous flow of BGP Update messages caused by operators using
   transitive BGP attributes to signal RPKI validation state.

   Specifically:

   *  When one large operator revokes all ROAs for their netblocks at
      once, for example, when migrating to minimally covering ROAs
      [RFC9319], or when revoking one ROA with a long maxLength covering
      a large number of prefixes.

   *  When multiple smaller operators coordinate to revoke ROAs at the
      same time.

   *  When a CA becomes unavailable or unable to publish for some time,
      e.g., due to the CA expiring ([CA-Outage1], [CA-Outage2],
      [CA-Outage3], [CA-Outage4]).

3.1.3.  Loss of Authoritative Validation Information

   Similar to the issuance/revocation of routes, the validation pipeline
   of an operator may encounter issues.  Issues may occur on the router
   side or on the validator side, with network connectivity issues
   having specific impact on either of the two.

   While, in general, implementations should not have bugs, operators
   should not make mistakes, and the network should be reliable, this is
   usually not the case in practice.  Instead, the worst-case of sudden
   and unexpected, yet unintentional, loss of validation state is an
   event that, however unlikely in a specific system, may and will
   happen.  Hence, systems should be resilliant in case of unexpected
   issues, and not further amplify issues by creating a BGP UPDATE
   storm.

   Below, we provide examples of events for both categories that may
   lead to the validation state of routes in one or multiple routers of
   an operator changing from Valid to NotFound.  This list serves
   illustrative purposes and does not claim completeness.

3.1.3.1.  Validator Issues

   The following events may impact a validator's ability to provide
   validation information to routers:

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   *  The RTR service may have to be taken offline temporarily for
      maintenance.  While operators should, in general, take care to
      provision sufficient redundancy, critical vulnerabilities may
      necessitate the immediate simultaneous shutdown of all RTR
      instances.

   *  A validator may crash due to bugs when ingesting unexpected data
      from the RPKI, or run into performance issues due to insufficient
      available memory or limited I/O performance on the host.  In the
      worst case, especially memory issues, can lead to a flapping
      validator, e.g., when the system runs out of memory after a few
      minutes of communicating validation state to routers.

   *  Validation state may seemingly lapse due to issues with time
      synchronization if, e.g., the clock of the validator diverts
      significantly, starting to consider CA's certificates invalid.

   *  The validator may lose its network connectivity in general, or to
      specific CAs.  While, in general, the validator should be able to
      serve from cache, an operator may have to shutdown the validator
      in such a case, to prevent dropping prefixes as invalid due to
      stale data.

3.1.3.2.  Router Ingestion Issues

   *  The RTR client, especially when implemented as a dedicated daemon,
      may fail to start, or terminate when receiving unexpected data.
      Especially when this leads to a flapping client, e.g., due to a
      bug in the handling of incremental updates leading to a crash,
      while the initial retrieval is successful, this will lead to
      flapping between routes being Valid and NotFound.

   *  A misconfiguration may impact a router's ability to communicate
      with the RTR service.  For example, the RTR client may lose its
      credentials or may not receive updated credentials in time when
      these are changed, or the address of the RTR service changes and
      is not updated on the router in time.

   *  An RTR client may lose network connectivity to the RTR service.
      While, in general, caches should prevent this from having
      immediate impact, an RTR clients behavior in case of a flapping
      network connection with frequent interruptions may lead to
      unexpected behavior and cache invalidation.  Similarly, after
      cache expirery, routes will change from Valid to NotFound.

   *  As an extension of the previous point, multiple operators might be
      using one central RTR service hosted by an external party, or
      depend on a similar validator, which becomes unavailable, e.g.,

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      due to maintenance or an outage.  If local instances are not able
      to handle loss of this external service without changing
      validation state, i.e., do not serve from cache or the outage
      extends beyond cache expirery, routes will change their validation
      state from Valid to NotFound Naturally, the negative impact in
      such a case is significantly larger in comparison to each operator
      running their own validator.

3.1.4.  Outage Scenario Summary

   The above non-exhaustive listing suggests that issues in general
   operations, CA operations, and RPKI cache implementations simply are
   unavoidable.  Hence, Operators MUST plan and design accordingly.

3.2.  Scaling issues

   For each change in validation state of a route, an Autonomous System
   in which the local routing policy sets a BGP Community based on the
   ROV-Valid validation state, would need to send BGP UPDATE messages
   for roughly half the global Internet routing table if the validation
   state changes to ROV-NotFound.  The same, reversed case, would be
   true for every new ROA created by the address space holders, whereas
   a new BGP update would be generated, as the validation state would
   change to ROV-Valid.

   Furthermore, adding additional attributes to routes increases their
   size and memory consumption in the RIB of BGP routers.  Given the
   continuous growth of the global routing table, operators should be--
   in general--conservative regarding the additional information they
   add to routes.

3.3.  Flooding and Cascading of BGP UPDATES

   The aforementioned scaling issues are not confined to singular UPDATE
   events.  Instead, changes in validation state may lead to floods and/
   or cascades of BGP UPDATES throughout the Internet.

3.3.1.  Flooding of BGP UPDATES

   Flooding events are caused by an individual operator losing
   validation state.  If that operator annotates validation state using
   BGP communities, the operator will send updates for all routes that
   changed from Valid to NotFound to its downstreams, as well as updates
   for routes received from downstreams to its upstreams.

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   Following an RPKI service affecting outage (Section 3.1), given that
   half the global Internet routing table with close to 1,000,000
   prefixes [CIDR_Report] nowadays is covered by RPKI ROAs [NIST], such
   convergence events represent a significant burden.  See
   [How-to-break] for an elaboration on this phenomenon.

3.3.2.  Cascading of BGP UPDATES

   For events that are not specific to one operator, e.g., a malicious
   widthdrawel of a ROA, loss of a major CA, or an unexpected downtime
   of a major centralized RTR service, events can also cascade for ASes
   annotating validation state using BGP communities.  Given that
   routers' view of the RPKI with RTR is only eventually consistent,
   update messages may cascade, i.e., one event affecting validation
   state may actually trigger multiple subsequent BGP UPDATE floods.

   Assume, for example, that AS65536 is a downstream of AS65537 (both
   annotating validation state with BGP Communities and using a 300
   second RTR cycle), and a centralized RTR service fails.  In the
   example, AS65536 has their routers updated from that cache a second
   before the service went down, while AS65537 was due for a refresh a
   second thereafter.

   This means that a second after the RTR service went down, AS65537
   will trigger a BGP UPDATE flood down its cone.  AS65536 will ingest
   and propagate these BGP UPDATES down its own cone as well.

   When, rughly 300 seconds later, AS65536 fails to retrieve validation
   state as well, he community of AS65536 will again change for ROA
   covered routes, and it will again trigger a BGP UPDATE flood and
   propagate this down its cone.

   Even if either or both of AS65536 and AS65537 use a cache after RTR
   expirery, the underlying issue would not change, assuming the RTR
   service downtime spans beyond the cache TTL.  Assuming a 30 minute
   cache TTL, both ASes using a cache would only move the cascading
   event 30 minutes later.  If only one of the two uses a cache, the two
   flood events get moved further apart.  However, the overall issue of
   two independent floods due to one event remains.

3.4.  Observed data

   In February 2024, a data-gathering initiative [Side-Effect] reported
   that between 8% and 10% of BGP updates seen on the Routing
   Information Service - RIS, contained well-known communities from
   large ISPs signaling either ROV-NotFound or ROV-Valid BGP Validation
   states.  The study also demonstrated that the creation or removal of
   a ROA object triggered a chain of updates in a period of circa 1 hour

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   following the change.

   Such a high percentage of unneeded BGP updates constitutes a
   considerable level of noise, impacting the capacity of the global
   routing system while generating load on router CPUs and occupying
   more RAM than necessary.  Keeping this information inside the realms
   of the single autonomous system would help reduce the burden on the
   rest of the global routing platform, reducing workload and noise.

3.5.  Lacking Value of Signaling Validation State

   RTR has been developed to communicate validation information to
   routers.  BGP Attributes are not signed, and provide no assurance
   against third parties adding them, apart from BGP communities--
   ideally--being filtered at a networks edge.  So, even in iBGP
   scenarios, their benefit in comparison to using RTR on all BGP
   speakers is limited.

   For eBGP, given they are not signed, they provide even less
   information to other parties except introspection into an ASes
   internal validation mechanics.  Crucially, they provide no actionable
   information for BGP neighbors.  If an AS validates and enforces based
   on RPKI, Invalid routes should never be imported and, hence, never be
   send to neighbors.  Hence, the argument that adding validation state
   to communities enables, e.g., downstreams to filter RPKI Invalid
   routes is mute, as the only routes a downstream should see are
   NotFound and Valid.  Furthermore, in any case, the operators SHOULD
   run their own validation infrastructure and not rely on centralized
   services or attributes communicated by their neighbors.  Everything
   else circumvents the purpose of RPKI.

4.  Advantages of Dissociating Validation States and BGP Path Attributes

   As outlined in Section 3, signaling validation state with transitive
   attributes carries significant risks for the stability of the global
   routing ecosystem.  Not signaling validation state, hence, has
   tangible benefits, specifically:

   *  Reduction of memory consumption on customer/peer facing PE routers
      (less BGP communities == less memory pressure).

   *  No effect on the age of a BGP route when a ROA or ASPA
      [I-D.ietf-sidrops-aspa-profile] is issued or revoked.

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   *  Avoids having to resend, e.g., more than 500,000 BGP routes
      towards BGP neighbors (for the own cone to peers and upstreams,
      for the full table towards customers) if the RPKI cache crashes
      and RTR sessions are terminated, or if flaps in validation are
      caused by other events.

   Hence, operators SHOULD NOT signal RPKI validation state using
   transitive BGP attributes.

5.  Security Considerations

   The use of transitive attributes to signal RPKI validation state may
   enable attackers to cause notable route churn by issuing and
   withdrawing, e.g., ROAs for their prefixes.  DFZ routers may not be
   equipped to handle churn in all directions at global scale,
   especially if said churn cascades or repeats periodically.

   To prevent this, operators SHOULD NOT signal validation state to
   neighbors.  Furthermore, validation state signaling SHOULD NOT be
   accepted from a neighbor AS.  Instead, the validation state of a
   received announcement has only local scope due to issues such as
   scope of trust and RPKI synchrony.

6.  IANA Considerations

   None.

7.  Acknowledgements

   The authors would like to thank Aaron Groom and Wouter Prins for
   their helpful review of this document.

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>.

8.2.  Informative References

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   [CA-Outage1]
              ARIN, "RPKI Service Notice Update", August 2020,
              <https://www.arin.net/announcements/20200813/>.

   [CA-Outage2]
              RIPE NCC, "Issue affecting rsync RPKI repository
              fetching", April 2021,
              <https://www.ripe.net/ripe/mail/archives/routing-
              wg/2021-April/004314.html>.

   [CA-Outage3]
              Snijders, J., "problemas con el TA de RPKI de LACNIC",
              April 2023, <https://mail.lacnic.net/pipermail/
              lacnog/2023-April/009471.html>.

   [CA-Outage4]
              Snijders, J., "AFRINIC RPKI VRP graph for November 2023 -
              heavy fluctuations affecting 2 members", November 2023,
              <https://lists.afrinic.net/pipermail/
              dbwg/2023-November/000493.html>.

   [CIDR_Report]
              Huston, G., "CIDR REPORT", January 2024,
              <https://www.cidr-report.org/as2.0/>.

   [How-to-break]
              Snijders, J., "How to break the Internet: a talk about
              outages that never happened", CERN Academic Training
              Lecture Regular Programme; 2021-2022, March 2022,
              <https://cds.cern.ch/record/2805326>.

   [I-D.ietf-sidrops-aspa-profile]
              Azimov, A., Uskov, E., Bush, R., Snijders, J., Housley,
              R., and B. Maddison, "A Profile for Autonomous System
              Provider Authorization", Work in Progress, Internet-Draft,
              draft-ietf-sidrops-aspa-profile-17, 7 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
              aspa-profile-17>.

   [NIST]     NIST, "NIST RPKI Monitor", January 2024,
              <https://rpki-monitor.antd.nist.gov/>.

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <https://www.rfc-editor.org/info/rfc1997>.

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   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/info/rfc6480>.

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482,
              DOI 10.17487/RFC6482, February 2012,
              <https://www.rfc-editor.org/info/rfc6482>.

   [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>.

   [RFC8092]  Heitz, J., Ed., Snijders, J., Ed., Patel, K., Bagdonas,
              I., and N. Hilliard, "BGP Large Communities Attribute",
              RFC 8092, DOI 10.17487/RFC8092, February 2017,
              <https://www.rfc-editor.org/info/rfc8092>.

   [RFC8097]  Mohapatra, P., Patel, K., Scudder, J., Ward, D., and R.
              Bush, "BGP Prefix Origin Validation State Extended
              Community", RFC 8097, DOI 10.17487/RFC8097, March 2017,
              <https://www.rfc-editor.org/info/rfc8097>.

   [RFC8210]  Bush, R. and R. Austein, "The Resource Public Key
              Infrastructure (RPKI) to Router Protocol, Version 1",
              RFC 8210, DOI 10.17487/RFC8210, September 2017,
              <https://www.rfc-editor.org/info/rfc8210>.

   [RFC9319]  Gilad, Y., Goldberg, S., Sriram, K., Snijders, J., and B.
              Maddison, "The Use of maxLength in the Resource Public Key
              Infrastructure (RPKI)", BCP 185, RFC 9319,
              DOI 10.17487/RFC9319, October 2022,
              <https://www.rfc-editor.org/info/rfc9319>.

   [Side-Effect]
              Stucchi, M., "A BGP Side Effect of RPKI", February 2024,
              <https://labs.ripe.net/author/stucchimax/a-bgp-side-
              effect-of-rpki/>.

Authors' Addresses

Snijders, et al.         Expires 11 October 2024               [Page 12]
Internet-Draft           Avoid RPKI State in BGP              April 2024

   Job Snijders
   Fastly
   Amsterdam
   Netherlands
   Email: job@fastly.com

   Tobias Fiebig
   Max-Planck-Institut fuer Informatik
   Campus E14
   66123 Saarbruecken
   Germany
   Phone: +49 681 9325 3527
   Email: tfiebig@mpi-inf.mpg.de

   Massimiliano Stucchi
   AS58280.net
   CH- Bruettisellen
   Switzerland
   Email: max@stucchi.ch

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