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 Certificate Authorities (CAs) and their Publication Repository server. 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 publication points. 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 Repository server
Publishers	Publishers (Certificate Authorities)
RRDP Repository	Public facing RRDP repository
Rsync Repository	Public facing rsync server

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 Repositories. It is strongly RECOMMENDED that these functions are separated from serving the repository content.

Availability 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. Dependent 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 publishing any updated RPKI objects. The most immediate impact of this is that the publisher cannot update their ROAs, ASPAs or BGPSec Router Certificates during this outage. Thus, it cannot authorise changes in its routing operations. If the outage persists for a more extended period, then the RPKI manifests and CRLs published will expire, resulting in the RPs rejecting CA publication points. For this reason, the Publication Server MUST be operated in a highly available fashion. Maintenance windows SHOULD be planned and communicated to publishers, so they can avoid - if possible - that changes in published RPKI objects are needed during these windows.

RRDP Repository In this section, we will elaborate on the following recommendations:

Use a separate hostname: do not share fate with rsync or the Publication Server.
Use a CDN if possible
Use randomized filenames for Snapshot and Delta Files
Limit the size of the Notification File
Combine deltas to limit the size of the Notification File
Timing of publication of Notification File

Unique Hostname It is RECOMMENDED that the public RRDP Repository URIs use a hostname different from both the service_uri used by publishers, and the hostname used in rsync URIs (sia_base). Using a unique hostname will allow the operator to use dedicated infrastructure and/or a Content Delivery Network for its RRDP content without interfering with the other functions.

Content Delivery Network If possible, it is strongly RECOMMENDED that a Content Delivery Network 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 back-end RRDP Repository is unavailable, in which case it is RECOMMENDED that stale files are served. When using a CDN, it will likely cache 404s for files not found on the back-end server. Because of this, the Publication Server SHOULD use randomized, unpredictable paths for Snapshot and Delta Files to avoid the CDN caching such 404s for future updates. Alternatively, the Publication Server can delay writing the notification file for this duration or clear the CDN cache for any new files it publishes.

Limit Notification File Size The size of the RRDP Notification File can significantly impact RRDP operations. If this file becomes too large, then it can easily result in significant traffic if the RRDP Repository does not use any CDN or in high costs if it does. stipulated that any deltas that, combined with all more recent delta, will result in the total size of deltas exceeding the snapshot size MUST be excluded to avoid Relying Parties downloading more data than necessary. In addition to the restriction described above, we RECOMMEND that the Notification File size is reduced by removing delta files that have been available for more than 75 minutes. As RP typically refresh their caches every 10 minutes, this will ensure that deltas are available for the vast majority of RPs, while limiting the size of the Notification File. Furthermore, we RECOMMEND that Publication Servers with many, e.g. 1000s of, Publishers ensure they 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.

Consistent load-balancing and 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. A s 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 connections to a node in the pool that is no longer active (e.g. 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 load data from another server. Depending on the exact configuration – for example, nodes behind the LB may have different RRDP sessions – this can lead to an inconsistent RRDP repository. Because of this issue, we RECOMMEND to (1) limit HTTP keepalive to a short period on the webservers 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 fail-over between RRDP sessions, where clients also risk reading a notification file for which some of the content is unavailable.

Rsync Repository 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 Repository might change the repository content while RPs transfer 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 since a failed fetch for one repository can cause the rejection of the publication point 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 (, , , 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 Repository must clean up copies of the repository; this is a trade-off between providing service to slow clients and disk space. 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 (c.f. since written). Empirical data suggests that Rsync Repositories MAY assume it is safe to do so 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 rsyncd servers as long as they each provide a consistent view. While it is RECOMMENDED that repositories are updated more frequently than the typical refresh rate for rsync repositories used by RPs to ensure that the repository continuously moves forward from a client's point of view, breaking not holding this constraint does not cause degraded behavior. It is RECOMMENDED that the Rsync Repository is load tested to ensure that it can handle the requests by all RPs in case they need to fall back from using RRDP (as is currently preferred). We RECOMMEND serving rsync repositories from local storage so 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 a single node can handle with (1) the available memory and (2) the IO performance available to be able to serve this number of connections in the time RPs allow for rsync to fetch data. Load-testing results show that machine memory is likely the limiting factor for large repositories that are not IO limited. The number of rsyncd 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 re-evaluating these parameters over time.

Single CA Repositories Some delegated CAs in the RPKI use their own dedicated Repository. Operating a small repository is much easier than operating a large one. There may not be a need to use a CDN for RRDP because the notification, snapshot and delta are relatively small. Also, the performance issues of rscynd for recursive fetches are far less of a problem for small and flat repositories. Because RPs will use cached data, short outages don't need to cause immediate issues if CAs fix their Repository before objects expire and ensure that their Publication Server () is available when there is a need to update RPKI objects such as ROAs. However, availability issues with such repositories are frequent, which can negatively impact Relying Party software. Therefore, it is strongly RECOMMENDED that CAs use a publication service provided by their RIR, NIR or other parent as much as possible. And it is RECOMMENDED that CAs that act as a parent make a Publication Service available to their children.

Acknowledgments This document is the result of many informal discussions between implementers. The authors would like to thank Job Snijders for their helpful review of this document.