This one post per day limit is really annoying (and apparently, doesn't really help shutting out spammers). That is how it works. Say you have two nodes that are associated to your identity, but are behind (different) firewalls doing NAT where: - There is no port forwarding set up. - uPnP is not possible. there is no way for the nodes to contact each other directly (there are other situations, such as some VPNs and DS-Lite, where relaying can kick in). Also, a thing such as: would not solve such problems. Most firewalls are set up to use connection tracking and will only allow incoming packets from associated with connection that was established from the inside to the outside. Behind such a firewall, it is by definition *not* possible for the nodes to contact each other directly. Also, if NAT is used, the node is not addressable from the outside (which is exactly the reason you needed port forwarding in the first place). The relay servers are provided for such cases. Both nodes contact the relay server (a connection initiated explicitly by the BTSync client). Say that node A contains new changes compared to node B, then node A will send the (encrypted) data to the relay server, the relay server will send the encrypted data to node B. But all packets physically go through the relay server, and this incurs bandwidth costs. This situation is pretty common. E.g. in Germany, many household connections use IPv6, and IPv4 is tunneled using DS-Lite (meaning that many consumers share one IPv4 address, ergo no port forwarding for IPv4). I have such a connection at home. At the same time, my employer blocks all incoming connections (except those permitted through session tracking). This means that when I am at work, I use a relay server (I can see this by the icon in the list of peers). Everything that is synced when I am at work, goes through Bittorrent Sync's relay servers. If you don't believe me and want it from the horse's mouth, here you have it: http://forum.bittorrent.com/topic/34509-use-over-internet-without-relay-server/?p=101168 (Of course, this does not apply when direct peer to peer connections can be established, such as on a local network or when port forwarding can be set up on one of the two ends.)