Skype - golemfactory/golem-rd GitHub Wiki

Network information

There is one central server for login and authentication.
For information exchange TCP is used, for resource exchange TCP and UDP.
Messages and resources exchanges use different ports.
Messages are encrypted with AES (Rijndel) and 256-bit encryption. TOo set the simmetric keys AES, 1536 or 2048-bit RSA is used. Public keys are encrypted by login server
Connection order: UDP (5 seconds time limit), TCP, TCP on port 80, TCP on port 443. There are five tries that repeat the cycle every 6 seconds.
If both clients have public IP then resources are sent via UDP (packet size 67 bytes)
If one client is behind port-restricted NAT then resources are sent via UDP through middleman (packet size 67 bytes). Bandwitch is limited.
If both clients are behind port-restricted NAT and firewall that blocks UDP and resources are sent via TCP (packet size 69 bytes).
Keep-alive msg is sent every 60 seconds

Skype uses protocol similiar to KaZaA, based on supernodes. Supernodes keep connection between themselves and normal nodes connect to one or two supernodes.
Supernode is a node with public IP, good bandwitch and high uptime.
Each connection between normal nodes goes through supernodes. Only after accepting connection the normal connection between nodes is created.
If one node is behind NAT and other is not then "connection reversal" is used. Node behind NAT always starts the connection.
If both nodes are behind NAT than "STUN-like NAT traversal" is used.
If STUN method fails than TURN-like connection is used with supernode as a middleman.
There are 250k supernodes and ~4m normal nodes.