I am currently reading a paper BGPStream: A Software Framework for Live and Historical BGP Data Analysis where authors say:
Normally, a BGP session with a collector is configured as a customer-provider relationship, i.e., as if the VP was offering transit service to the collector
In a later part, authors desscribe popular projects offering route collectors:
(...) currently operate 18 and 13 collectors respectively, which in total peer with approximately 380 and 600 VPs distributed worldwide
I am not sure I understand it. VPs and collectors are peering, but they establish a customer-provider BGP session ?