Jozef Stefan Institute The Enhanced Closed Swarm (ECS) protocol is a distributed access control mechanism suitable for usage in Peer-to-Peer content delivery systems. The protocol provides coarse or fine grained authorization of peers participating in content distribution. As a result, only authorized peers are allowed to communicate in a swarm. The protocol also provides data confidentiality, integrity and origin authentication for application data exchanged among peers. The protocol is simple but flexible enough to enable a range of usage scenarios. In addition to the ECS protocol, this document also describes its application in the IETF PPSPP.