Source: https://codeberg.org/fediverse/fep/src/branch/main/feps/fep-8fcf.md (Moved from old location at git.activitypub.dev)

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authors: @Claire
status: FINAL
dateReceived: 2020-10-24
dateFinalized: 2022-02-07

FEP-8fcf: Followers collection synchronization across servers

Summary

In ActivityPub, follow relationships are established, updated and removed by
sending activities such as Follow, Accept or Reject, which are assumed to
be correctly and promptly processed upon receipt.

However, due to incompatible protocol extensions, software bugs, server crashes
or database rollbacks, the two ends of a Follow relationship may end up out of
sync.

This can be especially damaging when a remote instance has outdated information
about follow relationships that should have been revoked, as some
implementations may deliver activities addressed to the sender’s followers
collection by using the sharedInbox mechanism and letting the recipient use
the sender’s followers collection for local delivery and access control.

This proposal describes an optional mechanism for detecting discrepancies in
following relationships across instances, with minimal overhead and without loss
of privacy.

Requirements

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this
specification are to be interpreted as described in [RFC-2119].

The proposed protocol for followers collection synchronization makes a number of
assumptions that may not be suitable to every implementation or deployment.

Implementations and deployments MUST NOT implement the mechanisms described in
this proposal unless they match the following requirements:

• actors managed by an instance are required to all share the same exact URI
  scheme and authority for their id, inbox and sharedInbox URIs
• such instances are required to manage all actors using the same URI scheme and
  authority for either their id, inbox or sharedInbox URIs (that is, for
  instance, two fediverse implementations cannot implement this proposal if they
  are set up on the same exact same domain name, unless implementing an
  additional mechanism to share follower information between them, which is out
  of scope for this proposal).

The reason for those requirements is to prevent the partial followers collection
described below from missing legitimate followers, which could result in
followers being removed for no reason.

Failing to implement this proposed synchronization mechanism should not impact
compatibility with other implementations, as it is completely optional.

Partial follower collection

For efficiency and privacy purposes, we consider a subset of an actor’s
followers collection. This subset is the set of an actor’s followers whose id
shares an instance’s specific URI scheme and authority.

For instance, if https://example.org/users/1 has the following followers:

• https://example.org/users/2
• https://testing.example.org/users/1
• https://next.example.org/users/foo
• https://testing.example.org/users/2

The partial follower collection of https://example.org/users/1 for the
instance serving https://testing.example.org/users/1 is:

• https://testing.example.org/users/1
• https://testing.example.org/users/2

Partial follower collection digest

To enable quick checking of partial followers consistency across instances, a
partial follower collection digest is computed.

This digest is created by XORing together the individual SHA256 digests of each
follower’s id.

partialCollectionDigest = SHA256(follower1) XOR SHA256(follower2) XOR ... XOR SHA256(followerN)

For instance, the partial follower collection digest of
https://example.org/users/1 for the instance serving
https://testing.example.org/users/1 is:
3a06e99569547f444c352ab7f52e4bab207abec5ca6f07b0045cfdc9723f8fa9 XOR f939a1585d4a8f02ee339210dbe7315d7003476663d6095f7d996fc4bc7a49b6 = c33f48cd341ef046a206b8a72ec97af65079f9a3a9b90eef79c5920dce45c61f

The Collection-Synchronization HTTP Header

The Collection-Synchronization HTTP header provides a mechanism for quickly
checking whether the sender’s followers collection part that is relevant to the
recipient is consistent with the recipient’s knowledge.

The header field name is Collection-Synchronization and its value is a list of
parameters and values, formatted according to the signature syntax defined in
[HTTP-Signatures], Section 4.1.

Example:

Collection-Synchronization: collectionId="https://example.org/users/1/followers", url="https://example.org/users/1/followers_synchronization", digest="c33f48cd341ef046a206b8a72ec97af65079f9a3a9b90eef79c5920dce45c61f"

Collection Synchronization Header Parameters

The Collection-Synchronization header’s parameters are defined as follows:

• collectionId: this is URI of the collection that supports synchronization.
  It must be the sender’s followers collection.
• url: this is the URL of the partial followers collection intended for the
  receiving instance.
  Accessing it should require authentication from the receiving instance.
• digest: the partial follower collection digest intended for the receiving
  instance.

Synchronization procedure

On the sender end

When delivering an Activity to an inbox (or sharedInbox), an instance MAY
set a Collection-Synchronization header intended for the corresponding
instance (determined by the inbox URI scheme and authority).

When exactly to set this header is up to the sender, but it is recommended to
at least send it for any Create activity addressed specifically to the
sender’s followers collection.

On the receiving end

On the receiving end, upon receiving an Activity delivery with a
signed Collection-Synchronization header, the receiver MUST check that:

• the collectionId attribute matches the sender’s followers collection id
• the url attribute also matches the same authority (so that the instance
  cannot get tricked into requesting the followers list of a third-party
  individual)

If any of those checks fails, the receiver MUST ignore the
Collection-Synchronization header.

The receiver SHOULD then compute the partial collection digest for the sender’s
followers based on its own knowledge. If the digest does not match the digest
attribute of the header, it SHOULD then query the url, authenticating itself
to the remote server using [HTTP-Signatures] or another method.

Having fetched the up-to-date partial followers collection from the autoritative
server, the receiving end:

• SHOULD remove from its local copy of the followers collection any local actor
  not listed in the partial followers collection.
• MAY consider any pending outgoing follow listed in the partial followers
  collection as accepted.
• SHOULD send an Undo Follow for any other local follower listed in the
  partial followers collection but not known locally.

Implementations

This proposal is implemented by Mastodon since the following Pull Request: Add follower synchronization mechanism by ClearlyClaire · Pull Request #14510 · mastodon/mastodon · GitHub

References

• [RFC-2119] S. Bradner, [Key words for use in RFCs to Indicate Requirement Levels](RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels
• [HTTP-Signatures] A. Backman, J. Richer, M. Sporny, Signing HTTP Messages

Copyright

CC0 1.0 Universal (CC0 1.0) Public Domain Dedication

To the extent possible under law, the authors of this Fediverse Enhancement Proposal have waived all copyright and related or neighboring rights to this work.