RPKI Publication Server Best Current Practices

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 when, and only when, they appear in all capitals, as shown here.

Introduction describes the RPKI Publication Protocol used between RPKI Certification Authorities (CAs) and their Publication Servers. The server is responsible for handling publication requests sent by the CAs, called Publishers in this context, and ensuring that their data is made available to RPKI Relying Parties (RPs) in (public) rsync and RRDP repositories. In this document, we will describe best current practices based on the operational experience of several implementers and operators.

Glossary

Term	Description
Publication Server	Publication Server
Publishers	Publishers (Certification Authorities)
RRDP Server	Public-facing RRDP server
Rsync Server	Public-facing rsync server
rsyncd	Software daemon package providing rsync service
RIR	Regional Internet Registry
NIR	National Internet Registry

Publication Server The Publication Server handles the server side of the Publication Protocol. The Publication Server generates the content for the public-facing RRDP and Rsync Servers. It is strongly RECOMMENDED that these functions are separated from serving the repository content.

Self-Hosted Publication Server Generally, address holders that want to make use of RPKI will rely on a CA hosted by the provider of those addresses, typically an RIR or an NIR. In some instances, address holders will instead deploy and manage a CA themselves. This type of CA is commonly referred to as a self-hosted CA, or delegated CA. When operating a self-hosted CA, the address holder must decide how they will handle publication. The holder can either deploy their own Publication Server and associated infrastructure (referred to as a self-hosted repository), or rely on a third-party Publication Server. If the holder uses a self-hosted repository, then they are responsible for ensuring the availability of signed content via RRDP and rsync as described in section 5 and 6 of this document. RPs are expected to make use of cached data from a previous, successful fetch (Section 6 of ). Therefore, short outages on the server side do not need to be cause for immediate concern, provided the server operator restores access availability in a timely fashion (e.g., before objects expire). However, in practice, self-hosted repositories tend to have frequent availability issues when compared with those provided by larger organisations like RIRs and NIRs. Additionally, the greater the number of separate repositories, the greater the chance for negative impact on RPs. Therefore, CAs that act as parents of other CAs are RECOMMENDED to provide a publication service for their child CAs, and CAs with a parent who offers a publication service are RECOMMENDED to use that service, instead of running their own. If a CA's parent does not offer a publication service, but the CA operator is able to use a reliable third-party Publication Server, the CA operator SHOULD make use of that service. For the case of a 'grandchild' CA, where CA1 is a TA, CA2 is a child CA of CA1, and CA3 is a child CA of CA2, there are several options for providing publication service to CA3:

RFC 8183 defines a 'referral' mechanism as part of the out-of-band CA setup protocol. If supported by CA1 and CA2, then this simplifies the process of registering CA3 as a direct publication client of CA1.
CA1 may support the registration of multiple publishers by CA2, by using the publisher_request/repository_response XML exchange defined in RFC 8183. CA2 would then be able to register a separate publisher on behalf of CA3.
CA2 may operate a publication proxy service (per e.g. ), which acts as the Publication Server for CA3. This proxy would set aside part of CA2's namespace at CA1 for the publication of CA3's objects, adjusting and forwarding requests from CA3 to CA1 accordingly.

For options 1 and 2, CAs operating as CA1 should consider the implications of providing direct publication service to CA3 in this way: for example, CA3 may expect publication service technical support from CA1 directly.

Publication Server as a Service The Publication Server and repository content have different demands on their availability and reachability. While the repository content MUST be highly available to any RP worldwide, only publishers need to access the Publication Server. Depending on the specific setup, this may allow for additional access restrictions in this context. For example, the Publication Server can limit access to known source IP addresses or apply rate limits. If the Publication Server is unavailable for some reason, this will prevent Publishers from making updated RPKI objects available. The most immediate impact of this is that the publisher cannot distribute new issuances or revocations of ROAs, ASPAs or BGPsec Router Certificates for the duration of this outage. Thus, in effect, it cannot signal changes in its routing operations. If the outage persists for an extended period, then RPKI Manifests, CRLs, and Signed Objects might became stale, hampering for example BGP Origin Validation (). For this reason, the Publication Server MUST be highly available. Measuring the availability of the Publication Server in a round-trip fashion is recommended by monitoring the publication of objects. Maintenance windows SHOULD be planned and communicated to publishers. This makes publishers aware of the root cause for disruption in the Publication Server, as well as supporting them more generally in their administration of their RPKI CA and associated systems.

Availability Short outages of an Publication Server will not affect RPs as long as the corresponding RRDP and rsync repositories remain available. However, such outages prevent publishers from updating their ROAs and reissuing their manifests and CRLs in a timely manner. The propagation time between ROA issuance and the ultimate use of the resulting VRPs in routers is described in table 2 of , as at the time of that study. That propagation time was between 15 and 95 minutes for the CAs and associated repositories that were analysed. As seen in the study, the delay between signing and publication can be a major contributor to long propagation times. The potential unavailability of a Publication Server adds to this propagation delay. Publication Servers SHOULD therefore aim for high availability of the publication protocol service.

Data Loss Publication Servers MUST aim to minimise data loss. If a server restore is needed and a content regression has occurred, then the server MUST perform an RRDP session reset. Publishing CAs typically only check in with their Publication Server when they have changes that need to be published. As a result, they may not be aware if the server performed a restore and their content regressed to an earlier state. This could result in a number of problems:

The published ROAs no longer reflect the CA's intentions.
The CA might not reissue their Manifest or CRL in time, because they operated under the assumption that the currently-published Manifest and CRL have not yet became stale.
Changes to publishers may not have been persisted. Newly registered publishers may not be present, and recently removed publishers may still be present.

Therefore, the Publication Server SHOULD notify publishing CAs about this issue if it occurs, so that a full resynchronisation can be initiated by CAs.

Publisher Repository Synchronisation It is RECOMMENDED that publishing CAs always perform a list query as described in section 2.3 of before submitting changes to the Publication Server. This approach means that any desynchronisation issue can be resolved at least as soon as the publisher is aware of updates that it needs to publish. When publishing changes, CAs SHOULD send all of their changes using multiple PDUs in a single multi-element query message, as described in section 2.2 and section 3.7.1 of . This reduces the risk of change sets that were intended to take effect as a single unit from taking effect separately. In addition to the above, the publishing CA MAY perform regular planned synchronisation events where it issues an list query and ensures that the Publication Server has the expected state, even if the CA has no new content to publish. For Publication Servers that serve a large number of CAs (e.g., thousands) this operation could become costly from a resource consumption perspective. Unfortunately, the protocol has no proper support for rate limiting. Therefore, publishers SHOULD NOT perform this resynchronisation more frequently than once every 10 minutes unless otherwise agreed with the Publication Server.

Hostnames It is RECOMMENDED that the public RRDP Server URI have a different hostname from that of the service_uri used by publishers, as well as that of any rsync URIs (e.g. sia_base) used by the relevant Publication Server. Using a unique hostname will allow the operator to use dedicated infrastructure and/or a Content Delivery Network (CDN) for its RRDP content without interfering with the other functions. If feasible, there is merit in using different TLDs and/or subdomains for these hostnames, as DNS issues at any level could otherwise be a single point of failure affecting both RRDP and rsync. Operators need to weigh this benefit against potential increased operational risk and the burden of maintaining multiple domains. Because the usefulness of this approach is highly context-dependent, no normative recommendation is given here. Furthermore, it is RECOMMENDED that DNSSEC is used in accordance with best current practice as described in .

IP Address Space and Autonomous Systems To prevent failure scenarios which persist beyond remediation, the topological placement and reachability of Publication Servers in the global Internet routing system need to be considered very carefully. See section 6 of . An example of a problematic scenario would be when a prefix or AS path related to a repository becomes invalid because of RPKI objects published in that repository. As a result, RPs may be unable to retrieve remediating updates from that repository. With the above in mind, it is RECOMMENDED to use IP addresses for RRDP and rsync services from IP address space which is not subordinate to authorities solely dependent on those service endpoints. It is also RECOMMENDED to host RRDP and rsync services in Autonomous Systems which are not subordinate to authorities publishing through those same endpoints. It is RECOMMENDED to host RRDP and rsync services in different networks.

RRDP Server

Same Origin URIs Publication Servers need to take note of the normative updates to in section 3.1 of . In short, this means that all delta and snapshot URIs need to use the same host, and redirects to other origins are not allowed.

Endpoint Protection Repository operators SHOULD use access control to protect their RRDP endpoints. For example. if the repository operator knows HTTP GET parameters are not in use, then all requests containing GET parameters can be blocked.

Bandwidth and Data Usage The bandwidth needed for RRDP evolves over time, and depends on many parameters. These consist of three main groups:

RRDP-specific repository properties, such as the size of notification, delta, and snapshot files.
Properties of the CAs publishing through a particular server, such as the number of updates, number of objects, and size of objects.
Relying party behaviour, e.g. using HTTP compression, requiring timeouts or minimum transfer speed for downloads, and using conditional HTTP requests for notification.xml.

When an RRDP repository server is overloaded, e.g. where the bandwidth demands exceed capacity, this causes a negative feedback loop (i.e. the aggregate load increases), and the efficiency of RRDP degrades. For example, when an RP attempts to download one or more delta files, and one fails, it will typically try to download the snapshot (larger than the sum of the size of the deltas). If this also fails, the RP falls back to rsync. Furthermore, when the RP tries to use RRDP again on the next run, it typically starts by downloading the snapshot. A Publication Server SHOULD attempt to prevent these issues by closely monitoring performance (e.g. bandwidth, performance on an RP outside their network, unexpected fallback to snapshot). Besides increasing the capacity, we will discuss several other measures to reduce bandwidth demands. Publication Servers SHOULD support compression. As the RRDP XML and embedded base64 content is highly compressible, this can reduce transferred data by about 50%. Servers SHOULD at least support either deflate or gzip content encoding as described in sections 8.4.1.2 and 8.4.1.3 of , in addition to any other popular compression types that the server can support.

Content Availability Publication Servers MUST ensure that their RRDP servers are highly available. If possible, it is strongly RECOMMENDED that a CDN is used to serve the RRDP content. Care MUST be taken to ensure that the notification file is not cached for longer than 1 minute unless the backend RRDP Server is unavailable, in which case it is RECOMMENDED that stale files are served. A CDN will likely cache 404s for files not found on the backend server. Because of this, the Publication Server SHOULD use randomised, unpredictable paths for snapshot and delta Files to avoid the CDN caching such 404s for future updates. Alternatively, the Publication Server can clear the CDN cache for any new files it publishes. Note that some organisations that run a Publication Server may be able to attain a similar level of availability themselves without the use of a third-party CDN. This document makes no specific recommendations on achieving this, as this is highly dependent on local circumstances and operational preferences. Also note that small repositories that serve a single CA, and which serve a small amount of data that does not change frequently, may attain high availability using a modest setup. Short downtime would not lead to immediate issues for the CA, provided that the service is restored before their manifest and CRL expire. This may be acceptable to the CA operator; however, because this can negatively impact RPs, it is RECOMMENDED that these CAs instead use a Publication Server that is provided as a service, e.g. by their RIR or NIR.

Limit Notification File Size Nowadays, most RPs use conditional requests for notification files, which reduces the traffic for repositories that do not often update relative to the resynchronisation frequency of RPs. On the other hand, for repositories that update frequently, the underlying snapshot and delta content accounts for most of the traffic. For example, for a large repository in January 2024, with a notification file with 144 deltas covering 14 hours, the requests for the notification file accounted for 251GB of traffic out of a total of 55.5TB (i.e. less than 0.5% of the total traffic during that period). However, for some servers, this ratio may be different. stipulates that the sum of the size of deltas MUST not exceed the snapshot size, in order to avoid RPs downloading more data than necessary. However, this does not account for the size of the notification file that all RPs download. Keeping many deltas present may allow RPs to recover more efficiently if they are significantly out of sync. Still, including all such deltas can also increase the total data transfer, because it increases the size of the notification file. In order to mitigate potential problems here, the notification file size SHOULD be reduced by removing from the notification file delta files that have been available for a long time. Because some RPs will only update every 1-2 hours (in 2024), the Publication Server SHOULD include deltas for at least 4 hours. Furthermore, it is RECOMMENDED that Publication Servers do not produce delta files more frequently than once per minute. A possible approach for this is that the Publication Server SHOULD publish changes at a regular (one minute) interval. The Publication Server then publishes the updates received from all Publishers in this interval in a single RRDP delta file. While the latter may not reduce the amount of data due to changed objects, this will result in shorter notification files, and will reduce the number of delta files that RPs need to fetch and process.

Manifest and CRL Update Times The manifest and CRL nextUpdate times and validity periods are determined by the issuing CA rather than the Publication Server. From the CA's point of view, longer validity periods mean that there is more time to resolve unforeseen operational issues, since the current RPKI objects will remain valid for longer. On the other hand, longer validity periods also increase the risk of a successful replay attack. From the Publication Server's point of view, shorter update times result in more data churn due to manifest and CRL reissuance. While the choice is made by the CAs, in certain modes of operation (e.g. hosted RPKI services) it may be possible to adjust the timing of manifest and CRL reissuance. One large repository has found that increasing the reissuance cycle from once every 24 hours to once every 48 hours (still deemed acceptable) reduced the data usage by approximately 50%, as most changes in the system are due to reissuance of manifests and CRLs, rather than e.g. ROA changes.

Consistent Load-Balancing

Notification File Timing Notification Files MUST NOT be available to RPs before the referenced snapshot and delta files are available. As a result, when using a load-balancing setup, care SHOULD be taken to ensure that RPs that make multiple subsequent requests receive content from the same node (e.g. consistent hashing). This way, clients view the timeline on one node where the referenced snapshot and delta files are available. Alternatively, publication infrastructure SHOULD ensure a particular ordering of the visibility of the snapshot plus delta and notification file. All nodes should receive the new snapshot and delta files before any node receives the new notification file. When using a load-balancing setup with multiple backends, each backend MUST provide a consistent view and MUST update more frequently than the typical refresh rate for rsync repositories used by RPs. When these conditions hold, RPs observe the same RRDP session with the serial monotonically increasing. Unfortunately, does not specify RP behavior if the serial regresses. As a result, some RPs download the snapshot to re-sync if they observe a serial regression.

L4 Load-Balancing If an RRDP repository uses L4 load-balancing, some load balancer implementations will keep in the pool connections to a node that is no longer active (e.g. one that is disabled because of maintenance). Due to HTTP keepalive, requests from an RP (or CDN) may continue to use the disabled node for an extended period. This issue is especially prominent with CDNs that use HTTP proxies internally when connecting to the origin while also load-balancing over multiple proxies. As a result, some requests may use a connection to the disabled server and retrieve stale content, while other connections retrieve data from another server. Depending on the exact configuration - for example, nodes behind the load balancer may have different RRDP sessions - this can lead to clients observing inconsistent RRDP repository state. Because of this issue, it is RECOMMENDED to (1) limit HTTP keepalive to a short period on the servers in the pool and (2) limit the number of HTTP requests per connection. When applying these recommendations, this issue is limited (and effectively less impactful when using a CDN due to caching) to a failover between RRDP sessions, where clients also risk reading a notification file for which some of the content is unavailable.

Rsync Server In this section, we will elaborate on the following recommendations:

Use symlinks to provide consistent content
Use deterministic timestamps for files
Load balancing and testing

Consistent Content A naive implementation of the Rsync Server might change the repository content while RPs are transferring files. Even when the repository is consistent from the repository server's point of view, clients may read an inconsistent set of files. Clients may get a combination of newer and older files. This "phantom read" can lead to unpredictable and unreliable results. While modern RPs will treat such inconsistencies as a "Failed Fetch" (), it is best to avoid this situation altogether, since a failed fetch for one repository can cause the rejection of delegated certificates and/or RPKI signed objects for a sub-CA when resources change. One way to ensure that rsyncd serves connected clients (RPs) with a consistent view of the repository is by configuring the rsyncd 'module' path to a path that contains a symlink that the repository-writing process updates for every repository publication. Following this process, when an update is published:

write the complete updated repository into a new directory
fix the timestamps of files (see next section)
change the symlink to point to the new directory

Multiple implementations implement this behavior (, , , the rpki.apnic.net repositories, a supporting shellscript ). Because rsyncd resolves this symlink when it chdirs into the module directory when a client connects, any connected RPs can read a consistent state. To limit the amount of disk space a repository uses, a Rsync Server must clean up copies of the repository; the timing of these removal operations involves balancing the provision of service to slow clients against the additional disk space required to support those clients. A repository can safely remove old directories when no RP fetching at a reasonable rate is reading that data. Since the last moment an RP can start reading from a copy is when it last "current", the time a client has to read a copy begins when it was last current (cf. the time when it was originally written). Empirical data suggests that Rsync Servers MAY assume it is safe to remove old instances of repositories after one hour. We recommend monitoring for "file has vanished" lines in the rsync log file to detect how many clients are affected by this cleanup process.

Deterministic Timestamps By default, rsync uses the modification time and file size to determine if it should transfer a file. Therefore, throughout a file's lifetime, the modification time SHOULD NOT change unless the file's content changes. We RECOMMEND the following deterministic heuristics for objects' timestamps when written to disk. These heuristics assume that a CA is compliant with and uses "one-time-use" EE certificates:

For CRLs, use the value of thisUpdate.
For RPKI Signed Objects, use the CMS signing-time (see ()).
For CA and BGPsec Router Certificates, use the value of notBefore.
For directories, use any constant value.

Load Balancing and Testing To increase availability during both regular maintenance and exceptional situations, a rsync repository that strives for high availability should be deployed on multiple nodes load-balanced by an L4 load balancer. Because Rsync sessions use a single TCP connection per session, there is no need for consistent load-balancing between multiple rsync servers as long as they each provide a consistent view. It is RECOMMENDED that the Rsync Server is load tested to ensure that it can handle simultaneous requests from all RPs, in case those RPs need to fall back from using RRDP (as is currently preferred). We RECOMMEND serving rsync repositories from local storage, so that the host operating system can optimally use its I/O cache. Using network storage is NOT RECOMMENDED, because it may not benefit from this cache. For example, when using NFS, the operating system cannot cache the directory listing(s) of the repository. We RECOMMENDED setting the "max connections" to a value that allows a single node to handle simultaneous resynchronisation by that number of RPs, taking into account the amount of time that RP implementations usually allow for rsync resychronisation. Load-testing results show that machine memory is likely the limiting factor for large repositories that are not IO limited. The number of rsync servers needed depends on the number of RPs, their refresh rate, and the "max connections" used. These values are subject to change over time, so we cannot give clear recommendations here except to restate that we RECOMMEND load-testing rsync and reevaluating these parameters over time.

Acknowledgments This document is the result of many informal discussions between implementers. The authors wish to thank Mike Hollyman for editorial suggestions.