comp.protocols.tcp-ip
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Re: Keeping UDP port open

Date : Le 03 mai 2005
From : glen herrmannsfeldt
Sujet : Re: Keeping UDP port open

Bjoern wrote:

> I'm designing an application which transmits voice in UDP packets. To
> keep the connection alive (like if users don't talk for a while) I send
> UDP keepalive packets every 30 seconds in order to keep the UDP ports
> open on routers. It seems, however, that some routers still close the
> UDP ports although I send these keepalive packets. Does anyone know
> some tricks to make sure that routers won't close the UDP ports?

UDP is a connectionless protocol but...

NAT requires that the router keep some state to know where to
send the replies. Note, though, that there is no requirement
in UDP that the replies have a port number related to the request,
or that the source address be the same as the destination address
of the request.

It seems that many NAT implementations don't even recognize TCP
keepalive (zero data length) packets to keep a TCP connection open.
I would call it a defective NAT implementation, and search for
router companies that don't have this bug.

-- glen

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