Thread: RE: [GD-General] NAT Negotiation

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If I've understand this correctly you have two clients (named C1 and C2)
behind NAT connected to a server (named S) and you want to establish a
connection between the two clients without reconfiguring the NAT server.
I've thought about this problem and I've come up with a solution that may
work.

1. C1 decides that it want to establish a connection between itself and C2
so it send that request to S. 2. S starts listening to two new ports and
sends one of the numbers to C1 and the other C2. 3. C1 and C2 respond to
this packet. 4. S now have the current port numbers of the two clients NAT.
Assuming that the NAT will give the next "connection" a port one step higher
then this it sends out C2 port number+1 to C1 and vice versa. 5. C1 and C2
starts sending each other packets (about 3-5 with 100ms delay should be
enough) on the assigned ports from the server until they get a response.

Now the C1 and C2 can communicate with each other. I don't know if this
works because of the assumption at point 4. You could perhaps use some other
algorithm to predict what port will be the next one (don't have PP installed
so I can't look at the .ppt file). Another problem is if the NAT is under a
lot of pressure a lot of new entries will be made in the dynamic routing
table so the assumption fails, but the procedure can be repeated until
success. Also, if the NAT is smart it can refuse to use a port that has data
that has been transmitted to it recently. This means that at point 5 the
client must send the packets at the same time within one half of a RTT. A
lot of problems but this are the only way I found to solve the problem.

Please try it if you want. I don't know if it works. Perhaps there are
better ways to solve it.

:)

.magnus auvinen

-----Original Message-----
From: Brett Bibby [mailto:res...@ga...] 
Sent: den 12 januari 2004 09:18
To: Gam...@li...
Subject: [GD-General] NAT Negotiation

This thread is being transferred form the GD-Algorithms list as Brian
pointed out it may be OT for the algorithms list. I apologize if this
attempt to transfer doesn't go well :-)

**Original Post**

Yet another network post <sigh>...

I searched the archives for NAT and came up with 14 posts. Some say NAT is a
problem, others say it's no problem especially when using a third server
connected directly to the internet. Google results in a number of old pages,
many with outdated links. My problem is that I need to come up with some
sort of internal NAT solution (if it's really needed?) because we need to
support multiple platforms, and I cannot find any third party solution
(including Gamespy or open source) that covers all the platforms or
compilable for them.

Let's say I have two machines, both behind NATs. They both think their
address is 192.168.1.100, but to the outside world it's 123.123.123.123 and
124.124.124.124 (whatever). The only way for these two machines to ever find
each other is through some third party, and they are at 125.125.125.125.

They each get their address locally (192.168.1.100) and send that to the
server in a packet it can understand. The server reads the packet and also
notes the ip address of the received packet, so now the server has both the
public and private address of a given client. It then sends each of the
clients the other's address(es) and they try to contact each other.

Is this correct so far? Does it work because the NAT assigned different
internal port numbers to the outgoing requests so it uses that to route the
incoming packets back to the requestor? If so, this seems deceptively
simple. Why does Gamespy mention they have to do other things too?

http://www.igda.org/oc/IGDAGamespyNAT.ppt

In a slide near the end they say they do some port guessing based upon
common patterns. Why would they need this if a third party server is used?

Also, Brian Hook mentions in one of his posts in the archive about using the
lower 16 bits of the ip address as a unique identifier. Why would I need to
do this?

http://sourceforge.net/mailarchive/message.php?msg_id=3667950

There is also a mention that you shouldn't hard code port numbers. Is there
a recommended way to generate one?

Finally, what if I am playing a game where two of us are behind a NAT on a
LAN but connected to the same game on the internet. It's likely that I would
recieve an address from the server of some other player's private address
that would coincide with the address of my friend. When I contact them at
that address they reply and I get the wrong connection. Is this what Brian
was using the low 16 bits as an identifier for in order to distinguish
between people using the same private address accidentally contacting
another machine on their LAN?

Thanks
Brett

** End Original Post **
** Reply by Brian Hook**

This is precariously on the border of being OT and should probably be 
handled on gd-general, but I don't think it's flagrant (but it's Tom's 
list, so if he disagrees, we can take it to gd-general)

> I searched the archives for NAT and came up with 14 posts. Some say
> NAT is a problem, others say it's no problem especially when using

NAT is not a problem if you know what you're doing.  That's not meant 
to be flippant or derogatory, but only to point out that A.) it's a 
hairy problem to the uninitiated and B.) it's a solvable problem.

> They each get their address locally (192.168.1.100) and send that
> to the server in a packet it can understand. The server reads the
> packet and also notes the ip address of the received packet, so now
> the server has both the public and private address of a given
> client. It then sends each of the clients the other's address(es)
> and they try to contact each other.

Sounds about right.

> Is this correct so far? Does it work because the NAT assigned
> different internal port numbers to the outgoing requests so it uses
> that to route the incoming packets back to the requestor?

Yes, but you can't rely on this.  UDP is a connectionless protocol, so 
in _theory_ (but rarely in practice, at least in the past 5 years or 
so), the reverse port mapping can change on a per packet basis.  This 
is described in a bit more detail in my article on multiplayer 
programming you can find at bookofhook.com

> In a slide near the end they say they do some port guessing based
> upon common patterns. Why would they need this if a third party
> server is used?

What they're trying to do is allow two players to connect to each 
other _without opening up any ports_.  At least, that's my 
understanding.  This avoids the entire "explaining to users how NAT 
works and why they can't host a server" problem.

> Also, Brian Hook mentions in one of his posts in the archive about
> using the lower 16 bits of the ip address as a unique identifier.
> Why would I need to do this?

To resolve clients behind a NAT.

> There is also a mention that you shouldn't hard code port numbers.
> Is there a recommended way to generate one?

Which port numbers?  For your server, you should hardcode them (so 
that you can tell users what ports to open/forward) but you should 
also allow them to be relocated in case there's a conflict.

> Finally, what if I am playing a game where two of us are behind a
> NAT on a LAN but connected to the same game on the internet. It's
> likely that I would recieve an address from the server of some
> other player's private address that would coincide with the address
> of my friend. 

No, you'll receive the other person's _public_ address and port 
mapping (ideally) that allows you to communicate with them directly.  
At least, that's what I'm assuming they're trying to do.  If you 
received 192.168.1.100, well, that's not too helpful since it'll just 
route to your own internal network (on a router that uses that 
mapping).

Brian

**End Brian Hook's Reply**
**Begin Second Post**

> This is precariously on the border of being OT and should probably be 
> handled on gd-general, but I don't think it's flagrant (but it's Tom's 
> list, so if he disagrees, we can take it to gd-general)

I posted it here since there was discussion before on this list with no
mention of OT at that time. Let me know if you want it moved to GD-General
and I will do so immediately.

> What they're trying to do is allow two players to connect to each 
> other _without opening up any ports_.  At least, that's my 
> understanding.  This avoids the entire "explaining to users how NAT 
> works and why they can't host a server" problem.

Couldn't they host a server as long as the third party matchmaking server
game out thier public ip? Or is the rarely changing port number the problem
here too? There isn't much difference between a client and a server once the
third party matchmaking knows the correct ip and port number is there?

> 
> > Also, Brian Hook mentions in one of his posts in the archive about
> > using the lower 16 bits of the ip address as a unique identifier.
> > Why would I need to do this?
> 
> To resolve clients behind a NAT.
> 

So this number would be used to identify players internally rather than
ip/port, and when I want to send the packet I lookup the ip/port based upon
this?

> > Finally, what if I am playing a game where two of us are behind a
> > NAT on a LAN but connected to the same game on the internet. It's
> > likely that I would recieve an address from the server of some
> > other player's private address that would coincide with the address
> > of my friend. 
> 
> No, you'll receive the other person's _public_ address and port 
> mapping (ideally) that allows you to communicate with them directly.  
> At least, that's what I'm assuming they're trying to do.  If you 
> received 192.168.1.100, well, that's not too helpful since it'll just 
> route to your own internal network (on a router that uses that 
> mapping).
> 

Is this correct? So if I'm behind the same NAT as my friend, will the router
detect the outgoing public ip address and port and reroute it back inside
directly to them?

Brett

** End Second Post**

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