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UDP
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... TCP connections through NAT. It allows
incoming UDP packets through NAT, but only through a subset of
existing NAT ...
... existing NAT types. In particular, STUN does not enable incoming UDP
packets through symmetric NATs (defined below), which are common in
...
... STUN's discovery procedures are based on
assumptions on NAT treatment of UDP; such assumptions may prove
invalid down the road as new NAT devices are deployed. STUN ...
... meet the needs of many applications.
The protocol described here, Simple Traversal of UDP Through NAT
(STUN ...
... NATs. It has been
observed that NAT treatment of UDP varies among implementations. The
four treatments observed in implementations are:
...
... mapping is used. Furthermore, only the external host that
receives a packet can send a UDP packet back to the internal host.
...
... a server, and the server returns a response. There are two types of
requests - Binding Requests, sent over UDP, and Shared Secret
Requests, sent over TLS ...
... NATs. The client sends a Binding Request to the server, over UDP.
The server examines the source IP address and port ...
... NAT. Binding Requests are sent to the STUN server using UDP.
When a Binding Request arrives at the STUN ...
... connection the request was received on. If the Shared Secret
Request was receive over UDP, the Shared Secret Error Response is
...
... transport failure of
some sort (generally, due to fatal ICMP errors in UDP or connection
failures in TCP ...
...
o Firewall that allows UDP out, and responses have to come back to
the source of the request (like a symmetric NAT, but no
...
... the source of the request (like a symmetric NAT, but no
translation. We call this a symmetric UDP Firewall)
...
... client begins by initiating test I. If this test yields no
response, the client knows right away that it is not capable of UDP
connectivity. If the test produces a response, the client examines
...
... NAT, but without the translation). If no response
is received, the client knows its behind a symmetric UDP firewall.
...
... /\ /\
N / \ Y / \ Y +--------+
UDP <-------/Resp\--------->/ IP \------------->| Test |
Blocked \ ? / \Same/ | II |
...
... NAT, for the express purpose of
receiving incoming UDP traffic from another host, targeted to that
...
...
o The discovery process assumes a certain classification of devices
based on their treatment of UDP. There could be other types of
NATs that are deployed that would not fit into one of these molds.
...