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The CORE API (defined in gnunet_core_service.h) is the basic messaging API used by P2P applications built using GNUnet. It provides applications the ability to send and receive encrypted messages to the peer's "directly" connected neighbours.

As CORE connections are generally "direct" connections,
applications must not assume that they can connect to arbitrary peers this way, as "direct" connections may not always be possible. Applications using CORE are notified about which peers are connected. Creating new "direct" connections must be done using the TRANSPORT API.

The CORE API provides unreliable, out-of-order delivery. While the implementation tries to ensure timely, in-order delivery, both message losses and reordering are not detected and must be tolerated by the application. Most important, the core will NOT perform retransmission if messages could not be delivered.

Note that CORE allows applications to queue one message per connected peer. The rate at which each connection operates is influenced by the preferences expressed by local application as well as restrictions imposed by the other peer. Local applications can express their preferences for particular connections using the "performance" API of the ATS service.

Applications that require more sophisticated transmission capabilities such as TCP-like behavior, or if you intend to send messages to arbitrary remote peers, should use the CADET API.

The typical use of the CORE API is to connect to the CORE service using GNUNET_CORE_connect, process events from the CORE service (such as peers connecting, peers disconnecting and incoming messages) and send messages to connected peers using GNUNET_CORE_notify_transmit_ready. Note that applications must cancel pending transmission requests if they receive a disconnect event for a peer that had a transmission pending; furthermore, queueing more than one transmission request per peer per application using the service is not permitted.

The CORE API also allows applications to monitor all communications of the peer prior to encryption (for outgoing messages) or after decryption (for incoming messages). This can be useful for debugging, diagnostics or to establish the presence of cover traffic (for anonymity). As monitoring applications are often not interested in the payload, the monitoring callbacks can be configured to only provide the message headers (including the message type and size) instead of copying the full data stream to the monitoring client.

The init callback of the GNUNET_CORE_connect function is called with the hash of the public key of the peer. This public key is used to identify the peer globally in the GNUnet network. Applications are encouraged to check that the provided hash matches the hash that they are using (as theoretically the application may be using a different configuration file with a different private key, which would result in hard to find bugs).

As with most service APIs, the CORE API isolates applications from crashes of the CORE service. If the CORE service crashes, the application will see disconnect events for all existing connections. Once the connections are re-established, the applications will be receive matching connect events.