UDP hole punching
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UDP hole punching is a commonly used technique employed in network address translator (NAT) applications for maintaining User Datagram Protocol (UDP) packet streams that traverse the NAT. NAT traversal techniques are typically required for client-to-client networking applications on the Internet involving hosts connected in private networks, especially in peer-to-peer and Voice over Internet Protocol (VoIP) deployments.
UDP hole punching establishes connectivity between two hosts communicating across one or more network address translators. Typically, third party hosts on the public transit network are used to establish UDP port states that may be used for direct communications between the communicating hosts. Once port state has been successfully established and the hosts are communicating, port state may be maintained by either normal communications traffic, or in the prolonged absence thereof, by so-called keep-alive packets, usually consisting of empty UDP packets or packets with minimal non-intrusive content.
UDP hole punching is a method for establishing bidirectional UDP connections between Internet hosts in private networks using network address translators. The technique is not applicable in all scenarios or with all types of NATs, as NAT operating characteristics are not standardized.
Hosts with network connectivity inside a private network connected via a NAT to the Internet typically use the STUN method or Interactive Connectivity Establishment (ICE) to determine the public address of the NAT that its communications peers require. In this process another host on the public network is used to establish port mapping and other UDP port state that is assumed to be valid for direct communication between the application hosts. Since UDP state usually expires after short periods of time in the range of tens of seconds to a few minutes, and the UDP port is closed in the process, UDP hole punching employs the transmission of periodic keep-alive packets, each renewing the life-time counters in the UDP state machine of the NAT.
UDP hole punching will not work with symmetric NAT devices (also known as bi-directional NAT) which tend to be found in large corporate networks. In symmetric NAT, the NAT's mapping associated with the connection to the well known STUN server is restricted to receiving data from the well-known server, and therefore the NAT mapping the well-known server sees is not useful information to the endpoint.
In a somewhat more elaborate approach both hosts will start sending to each other, using multiple attempts. On a Restricted Cone NAT, the first packet from the other host will be blocked. After that the NAT device has a record of having sent a packet to the other machine, and will let any packets coming from this IP address and port number through. This technique is widely used in peer-to-peer software and Voice over Internet Protocol telephony. It can also be used to assist the establishment of virtual private networks operating over UDP. The same technique is sometimes extended to Transmission Control Protocol (TCP) connections, albeit with much less success due to the fact that TCP connection streams are controlled by the host OS, not the application and sequence numbers are selected randomly; thus any NAT device that performs sequence number checking will not consider the packets to be associated with an existing connection and drop them.
Let A and B be the two hosts, each in its own private network; N1 and N2 are the two NAT devices with globally reachable IP addresses P1 and P2 respectively; S is a public server with a well-known globally reachable IP address.
- A and B each begin a UDP conversation with S; the NAT devices N1 and N2 create UDP translation states and assign temporary external port numbers
- S examines the UDP packet to see if the source port used by A and B match what is actually received (otherwise, N1 or N2 must be performing port randomization thus making hole punching more difficult)
- If ports were not randomized, A and B select ports X and Y respectively and advise S of these ports; S then tells A to send a UDP packet to P2:Y and B to connect to P1:X
- A and B contact each other's NAT devices directly.
- If ports were randomized, it may be possible for A to contact B through the source port received by S as long as port randomization is not performed by B on a per-outbound host basis.
- On some routers where port randomization is performed on a per-outbound host basis, the ports are not randomly selected, but actually sequential, making it possible to establish a conversation through guessing nearby ports.
See also 
- Peer-to-Peer Communication Across Network Address Translators, PDF - contains detailed explanation on the hole punching process
- Network Address Translation and Peer-to-Peer Applications (NATP2P)
- How Skype & Co. get round firewalls - simple explanation of how Skype uses UDP hole punching
- Use of UDP to traverse NAT gateways Use of UDP to traverse NAT gateways
- Deliver peer/p2p Web service Run HTTP over UDP and bring peer/p2p Web Service