mastodon/FEDERATION.md

Federation

Supported federation protocols and standards

• ActivityPub (Server-to-Server)
• WebFinger
• Http Signatures
• NodeInfo

Supported FEPs

• FEP-67ff: FEDERATION.md
• FEP-f1d5: NodeInfo in Fediverse Software
• FEP-8fcf: Followers collection synchronization across servers
• FEP-5feb: Search indexing consent for actors
• FEP-044f: Consent-respecting quote posts: partial support for incoming quote-posts

ActivityPub in Mastodon

Mastodon largely follows the ActivityPub server-to-server specification but it makes uses of some non-standard extensions, some of which are required for interacting with Mastodon at all.

• Supported ActivityPub vocabulary

Required extensions

WebFinger

In Mastodon, users are identified by a username and domain pair (e.g., Gargron@mastodon.social). This is used both for discovery and for unambiguously mentioning users across the fediverse. Furthermore, this is part of Mastodon's database design from its very beginnings.

As a result, Mastodon requires that each ActivityPub actor uniquely maps back to an acct: URI that can be resolved via WebFinger.

• WebFinger information and examples

HTTP Signatures

In order to authenticate activities, Mastodon relies on HTTP Signatures, signing every POST and GET request to other ActivityPub implementations on behalf of the user authoring an activity (for POST requests) or an actor representing the Mastodon server itself (for most GET requests).

Mastodon requires all POST requests to be signed, and MAY require GET requests to be signed, depending on the configuration of the Mastodon server.

• HTTP Signatures information and examples

Optional extensions

• Linked-Data Signatures
• Bearcaps

Additional documentation

• Mastodon documentation

Size limits

Mastodon imposes a few hard limits on federated content. These limits are intended to be very generous and way above what the Mastodon user experience is optimized for, so as to accommodate future changes and unusual or unforeseen usage patterns, while still providing some limits for performance reasons. The following table summarizes those limits.

Limited property	Size limit	Consequence of exceeding the limit
Serialized JSON-LD	1MB	Activity is rejected/dropped
Profile fields (actor PropertyValue attachments) name/value	2047	Field name/value is truncated
Number of profile fields (actor PropertyValue attachments)	50	Fields list is truncated
Poll options (number of anyOf/oneOf in a Question)	500	Items list is truncated
Account username (actor preferredUsername) length	2048	Actor will be rejected
Account display name (actor name) length	2048	Display name will be truncated
Account note (actor summary) length	20kB	Account note will be truncated
Account attributionDomains	256	List will be truncated
Account aliases (actor alsoKnownAs)	256	List will be truncated
Custom emoji shortcode (Emoji name)	2048	Emoji will be rejected