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TOPIC: NAT

NAT 11 years 11 months ago #5540

  • sLz
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I must first point out that I've not got this actual problem, however I have been asked several times over the past year or so of the following problem:

Take a game, for example Counter Strike, you currently have a LAN setup with S/DNAT technology configured on the gateway correctly. Now all is fine so as long as you don't play Counter Strike on exactly the same server as another person on your network is, if you do so then you encounter extreme laggy conditions or the game simply prevents you from joining. You can play Counter Strike online with normal latency on a different Counter Strike server to the other person on your network.

I understand the concepts of NAT, it's table indexing for connections in and out et cetera.. on the specific inbound and outbound ports. My question is... what would cause such a problem, does this problem reside at the application level, i.e Counter Strike or is the problem much more profound. I understand that in the design of IPv4 the problems that NAT patches were never a problem back in the days of IPv4 design, so maybe the lag is solely down to NAT? I'm quite positive that Counter Strike picks an inbound port of n+1 for each connection, so maybe NAT gets muddled up somewhere along the line, perhaps on inbound packets. Another possibility is that NAT can only add 1 entry per inbound port, though this would stop the other PC from connecting at all.

Any information would be great, no matter how much information you can give me on this, i'm sure we can get a good answer to explaining this from patching together all the responses.

Thanks in advance. Adam.
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Re: NAT 11 years 11 months ago #5543

  • FallenZer0
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-There Is A Foolish Corner In The Brain Of The Wisest Man- Aristotle
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Re: NAT 11 years 11 months ago #5550

  • sLz
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Morning, and thanks for the links, however, previously stated I am not having this problem, when i asked the question it was more targeted at NAT. Basically what in the NAT architecture may cause such a problem? This is not just a Counter Strike problem, it's other games too such as Battle.net, so I believe it is a problem with NAT, you do not need any understanding of Counter Strike apart from that it's an online FPS, that when 2 people on the same network connect to exactly the same server (different servers at the same time is fine, so it's not a general bandwidth issue) the lag increases immensly or you simply cannot connect.

Counter Strike is usually played in a Windows environment yet it can be played on Linux using such tools as X Windows. A router is usually in place as the gateway with an S/DNAT implementation, though i've heard people encounter the same problem using ICS on Windows. Ipmasq may also encounter such a problem.

I just find it very strange that if NAT is conflicting somewhere then I'd have thought it would have simply not allowed another PC to connect (Bypassing the NAT using the same internal port) - this would be quite easy to deduce, though this seems not to be the case, it's as if NAT realises that 2 people are using the same port and requests the data again, thus causing the lag.

Any theories on why NAT would do this? Thank you. Thanks once again for the links FallenZer0. Adam.
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Re: NAT 11 years 11 months ago #5553

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A router is usually in place as the gateway with an S/DNAT implementation, though i've heard people encounter the same problem using ICS on Windows. Ipmasq may also encounter such a problem.

it's as if NAT realises that 2 people are using the same port and requests the data again, thus causing the lag.

Any theories on why NAT would do this? Thank you. Thanks once again for the links FallenZer0. Adam.

--sLz:

As far as SNAT is concerned, NAT enabled devices should NOT even worry about ports as there is a 1:1 mapping for every private IP address on the network to public IP.

As far as DNAT, you have a pool of internet routable IP addresses.

I don't see why you would have a Lag using S/DNAT.

IPMasquerading: Even if the Source Ports are the same from multiple different machines, NAT enabled devices *WILL* assign new port numbers, so it knows where to send the replys. So, you technically shouldn't see any lag.

My theoretically educated guess would be, may be it's not NAT. It is something else that is causing the Lag. These are just my ideas. May be someone knows better. Hope that someone can shed some light on the issue.
-There Is A Foolish Corner In The Brain Of The Wisest Man- Aristotle
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Re: NAT 11 years 11 months ago #5558

  • sLz
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Thanks for your replies FallenZer0. Absolutley any input will be greatful, if only random stab in the darks or educated guesses, it all builds up and leaves things open to consideration and interpretation. This question was never meant to be straight forward. I do not know the answer and I'll be greatly pleased with all who may contribute.

I'm sure our Chris will have some good points to throw onto the topic...
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