[p2p-hackers] NATs reconfiguring IPs and Port Numbers

Saikat Guha saikat at cs.cornell.edu
Thu Mar 2 23:15:49 UTC 2006

On Thu, 2006-03-02 at 13:45 -0800, Lemon Obrien wrote:
> It seems port numbers change after a certain amount of time due to the
> local NAT or local ISP

Do you mean the external port number allocated by the NAT? Most NATs
will timeout idle connections; sending subsequent packets will cause a
new allocation. The length of the timeout depends on the transport
protocol; the new allocation depends on the mapping type of the NAT.

For TCP, most NATs timeout somewhere between 1-2 hours. Some (rather
aggressive) NATs will timeout within 10-15 minutes of inactivity.
Detailed numbers here: http://nutss.net/stunt-results.php?sort=-33 

Standards in the process are trying to peg the default timeout to 2h.
For UDP, the timeout is significantly less. Standards are setting the
UDP timeout to 5 minutes of inactivity. 

> , and the peer to peer application has to re-configure itself to find
> out what its new global address is
> ...and broadcast that to others so they can re-connect;

That said, the standards are also trying to ensure NATs have consistent
mapping / cone behavior / address and port independent mapping.
Consequently, the need to re-publish this information should diminish
over time.

> I'm getting up to 24 hours testing through Comcast and SBC; but i have
> to sleep so...i'm not sure on this number.

Hmmm. This should not be an ISP issue unless the ISP is putting you
behind a NAT. Also, your subject seems to suggest your IP address is
changing -- is comcast/sbc assigning you new DHCP addresses every few
days? Perhaps the DHCP lease time can hint on the value they are using.
In any event, as long as your endpoint renews your DHCP lease before it
expires, it shouldn't change IP addresses.

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