Bird's Hole Punching Protocol


Hole punch - a mechanism to bypass a firewall
Server     - a third party which is not behind a firewall
Client     - anything contacting the server
Magic      - the four bytes 00 52 EB 11
Address    - an IPv4 address


                  0 1 2 3 4 5 6 7 8  9
                 +-------+-------+----+
                 | Magic |Address|Port|
                 +-------+-------+----+
                      Relay Packet


A client that expects to be the target of a hole punch
must contact the server frequently, to keep a UDP
"connection" open, so that the server may relay hole
punching requests to them.

A client makes a hole punching request to another client
by sending a Relay Packet to the server. The server then
sends another Relay Packet to the client described by the
first packet. The second packet is filled with the source
address and port of the first packet.

Once a client receives a Relay Packet, this protocol's
purpose is fulfilled and the client is aware that another
client requests a hole punch.


vim: noai