I would like to detect the NAT type a network interface is behind, without using an external public server, as it's done in STUN (so, without using STUN clients).
The NAT types I am interested are those as defined in http://www.ietf.org/rfc/rfc3489.txt, section 5 '5. NAT Variations'.
The bad news is you can't (I am assuming you don't have the telephone numbers of all the network administrators managing all the NATs you plan to deal with).
In order to find the type of a NAT, you must have a device poking holes from behind on the LAN to the WAN.
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I want to check NAT type and its behaviour with STUN server available (Public Stun Servers) (STUNTMAN). I have tried and found client codes (STUN CLIENT) which can communicate with STUN server to identify NAT behaviour, but all are in UDP. As per RFC-5780 we can check Mapping behaviour for UDP and TCP, Hence I want to check the same using TCP and i have implemented the same way as UDP does but both results differs. So, will behaviour/Steps to find NAT behaviour be the same as UDP or need to implement other way.
Any Sample client code will also be helpful.
STUN/TURN check the NAT type by simulating the connection of several differnt types, and step by step it can split Full Cone, Restricted NAT, Port Restricted NAT and Symmetric NAT.
We usually use UDP cause we don't need three way heandshake to establishe a connection, which may confuse the judgement of NAT types. So if you want to use TCP, you need first know how STUN check NAT types step by step for UDP, and think about what will happen for TCP three-way handshake. It can not just copy the code.
I know, how to write a C# application that works through a local network.
I mean I know, how to make my client-side application access my server-side application in a single local network.
But I wonder: How do such apps, as Skype, TeamViewer, and many other connect via global network?
I apologise, if this question is simple or obvious, but I couldn't find any information about this stuff.
Please, help me, I'll be very grateful. Any information is accepted - articles, plain info, books,and so on...
Question is very wide and I try to do short overview.
Following major difference between LAN (Local Area Network) and WAN (Wide Area Network):
Network quality:
LAN is more or less stable, WAN can be with network issues like:
Packet loss (you need use loss-tolerant transport like TCP or UDP with retransmits or packet loss concealment)
Packet jitter (interpacket intervals may differ a lot from sending part). Most common thing is packets bursts.
Packet reordering
Packet duplication
Network connectivity
WAN is less stable than LAN. So you need properly handle all things like:
Connection stale
Connection loss
Errors in the middle of the connection (if you use UDP for example)
Addresses:
In WAN you deal with different network equipment between client and server (or peers in case of peer-to-peer communication). You need to take in account:
NATs - most of the clients are behind NAT and you need to pass them through. According technics are called "NAT traversal"
Firewalls - may ISP has own rules what client can do or can't. So if you do something specific like custom transport protocol you may bump into ISP firewalls.
Routing - especially multicast and broadcast communication. In common case multicast is not possible to route. Broadcasts are never routed. So you need to avail this type of communication if you want to use WAN.
May be I forgot something. But these points are major. You can read many articles about any of them.
I started to explore the option of connecting with other using a p2p connection, so I coded a simple socket program in JAVA for android devices in which the users can share simple messages p2p (I didn't have any idea about NAT then). I got to know about NAT, so I now need to establish a TCP connection with another user which uses a server for discovery but payload is transferred p2p. I have also looked at XMPP(a very good and detailed explanation of how protocol works is here) and UPnP but I dont know how to implement them.
Another interesting question that arises is of BitTorrent because they can work on any device and even behind a NAT. I am not able to get any explanation of how BitTorrent works.
I have researched a lot but I am stuck.
My questions are:
A detailed explanation of BitTorrent(like here, not how torrents work) and how is it able to work around NAT ?
Is there a way to make a NAT entry programmatically ?
Is socket programming sufficient for p2p ?
How difficult is it to create your own protocol and how can I build one ?
If two devices D1 and D2 want to communicate p2p and they know each other's IP. D1 sends a request to D2 and that can't get through the D2's NAT, but there should be an entry created in D1's NAT. So when D2 tries to send something D1's NAT should discover an entry with D2's IP. Then why is the packet not allowed by it ?
Another interesting question that arises is of BitTorrent because they can work on any device and even behind a NAT. I am not able to get any explanation of how BitTorrent works.
This statement looks like you assume that bittorrent needs full connectivity to operate.
That is incorrect.
Behind a NAT device you will still be able to establish outgoing TCP connections. Which generally is sufficient for bittorrent as long as there are other, non-NATed (or NATed but properly port-forwarded) clients in the network that can accept incoming connnections.
NAT has no impact on the flow direction of the data because connections are bi-directional once they are established. It only is problematic for the initial connection setup.
This works perfectly fine for bittorrent because bittorent does not care from which specific node you get your data.
Although better connectivity generally does improve performance.
If the identity of the node matters or one-on-one transfers are an important use-case then other p2p protocols usually attempt NAT traversal first and if that fails rely on 3rd party nodes relaying traffic between those nodes who cannot connect to each other directly.
Additionally, IPv6 support will become essential in the future to maintain end-to-end connectivity because more and more ISPs are starting to roll out carrier-grade NAT for IPv4 while IPv6 will remain non-NATed
One thing need to be clear is that 100% P2P between all type of NAT is impossible right now. There is no practical way to establish P2P connectivity between **Symmetric and Symmetric/PRC NAT. In this scenario connection is established through a relay server called TURN.
I am answering from your 2nd question because I don't know much about the first one.
2) Yes. You can send a packet through your NAT and there will be a mapping between your internal IP:Port to your NAT's external IP:Port. You can know these external IP:Port by sending a stun request. Note that this technique doesn't work for Symmetric NAT.
3)Yes socket programming sufficient for p2p.
4)Why do you need a protocol when there already exists several. ICE protocol is the best today for NAT traversal and I don't think it was easy to create. UPnP and NAT-PMP is really vulnerable in terms of security.
5)I think what happens is usually NAT blocks unknown packets coming to it. So when D1 sends a packet to D2, its NAT blocks all packets incoming from D1s IP:Port. That is why connection establishment fails. You have to employ hole punching technique for D1 and D2 to successfully establish P2P connectivity.
**By symmetric NAT I mean symmetric NAT with random port allocation.
There is a paper on "Peer-to-Peer Communication Across Network Address Translators" which describes the UDP hole punching method and extends it to be used over TCP as well.
Of course, you will always need a relay server for the cases where hole punching is not supported.
Recent versions of BitTorrent use µTP, which is layered above UDP, not TCP. µTorrent uses a private extension (ut_holepunch) that performs UDP hole punching, most other implementations don't bother (with the notable exception of Tixati).
Some NAT routers accept port forwarding requests using either the uPNP or the PMP protocol. Whether this is supported depends on the particular brand of router and its configuration.
Yes, socket programming is enough for P2P.
Difficult to answer. I suggest that you read the wikified and annotated BitTorrent specification for a start.
Yes, this is the principle behind UDP hole punching.
I was going through the ICE/STUN specification and had a question.
If I am located behind a NAT which could be a Full Cone, Restricted or Port Restricted but not a symmetric NAT and considering I am using UDP, then in this case my SERVER REFLEXIVE and PEER REFLEXIVE addresses/candidates will be same, right? I am not able to imagine under what scenarios (for the above mentioned NAT types) they'll be different from each other? Or if they are really different then it can only mean that my NAT is symmetric one?
You are correct - it is usually not different as long as the NAT you are behind is not symmetric.
PEER reflexive addresses are identified during ICE connectivity checks. New peer addresses can be identfied in edge cases involving symmetric NAT in addition to other odd network configurations.
In the symmetric NAT scenario, the IP address between SRFLX and PRFLX will be the same, but the port is likely different.
Some other edge cases to consider:
Perhaps the ISP has implemented carrier grade NAT (double NAT configuration) among the customers it services - which is most likely going to be logically equivalent to a symmetric NAT. If two customers within the same ISP are doing ICE connectivity checks, they may discover the inner NAT public IP address.
You could probably also envision a multihomed machine scenario, something crazy with virtual machines, or an enterprise routing configuration that could influence the IP address mapping another host sees you as.
2 computers are in different subnets.
Both are Windows machines.
There are 2-5 IGMP-ready routers between them.
They can connect each other over multicast protocol (they have joined the same multicast group and they know about each other's existance).
How to establish a reliable TCP connection between them without any public server?
Programming language: C++, WinAPI
(I need a TCP connection to send some big critical data, which I can not entrust to UDP)
You haven't specified a programming language, so this whole question may be off-topic.
Subnets are not the problem. Routability is the problem. Either there is routing set up or there isn't. If they are, for example, both behind NAT boxes, then you're at the mercy of the configuration of the nat boxes. If they are merely on two different subnets of a routed network, it's the job of the network admin to have set up routing. So, each has an IP address, and either can address the other.
On one machine, you are going to create a socket, bind it to some port of your choice, and listen. On the other, you will connect to the first machine's IP + the selected port.
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I'm going to try again, but I feel like there's a giant conceptual gap here.
Once upon a time, the TCP/IP was invented. In the original conception, every item on the network has an IPV4 address, and every machine could reach every other machine, via routing, except for machines in the 'private' address space (10.x, etc).
In the very early days, the only 'subnets' were 'class A, class B, class C'. Later the idea of subdividing a network via bitmasks was added. The concept of 'subnet' is just a way of describing a piece of network in which all the hosts can deliver packets to each other by one hop over some transport or another. In a properly configured network, this is only of concern to operating system drivers. Ordinary programs just address packets over the network and they arrive.
The implementation of this connectivity was always via routing protocol. If you have a (physical) ethernet A over here, and a (physical) ethernet B over there, connected by some sort of point-to-point link, the machines on A need to know where to send packets for B. Or, to be exact, they need to know where to send 'not-A' packets, and whatever they send them needs to know where to send 'B' packets. In simple cases, this is arranged via explicit configuration: routing rules stuffed into router boxes or even computers with multiple physical interfaces. In more complex cases, routing boxes intercommunicate via protocols like EGP or BGP or IGMP to learn the network topology.
If you use the Windows 'route' command, you will see the 'default route' that the system uses to send packets that need to leave the local subnet. It is generally the address of the router box responsible for moving information from the local subnet to everywhere else.
The whole goal of this routing is to arrange that a packet sent from a.b.c.d to e.f.g.h will get there. TCP is no different than UDP, except that you can't get there by multicast or broadcast: you need to know the exact address of your correspondent.
DNS was invented to allow hosts to learn each other's IP addresses without having human being send them around in email messages.
All this stops working when people start using NAT and firewalls to turn off routing. The whole idea of NAT is that the computers behind the NAT box are not addressable at all. They all appear to have one IP address. They can send stuff out, but they can only receive stuff if the NAT box has gone to extra trouble to map them a port.
From your original message, I sort of doubt that NAT is in use here. I just don't understand your comment 'I don't have access to the network.' You say that you've sent UDP packets here and there. So how did you do that? What addresses did you use?