This is part of the heartbeat, see https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#opportunistic-grafting and https://github.com/libp2p/go-libp2p-pubsub/blob/56c0e6c5c9dfcc6cd3214e59ef19456dc0171574/gossipsub.go#L1520.