"The video/voice communication through Skype is established through UDP. The trick here is that quite often, one of the users is behind a firewall or a router, hence it doesn't have a real IP address. But if both Skype clients are on real IPs, then the media traffic flows directly between them over UDP..."
"If one of the callee or both of them do not have a public IP, then they send voice traffic to another online Skype node over UDP or TCP." Can anyone explain this statement. I did not understanf how they manage to find each other?
Article Source: http://EzineArticles.com/496462
Article Source: http://EzineArticles.com/496462
The clients can communicate via Skype login server and supernodes.
More details about Skype protocol:
http://en.wikipedia.org/wiki/Skype_protocol
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I want to develop a publisher -> subscriber model with 1 publisher and many subscriber in nodejs.
Currently my idea was to use a normal websocket. The problem with this is that every subscriber needs a static ip and port forwarding if it runs over the internet. This doesnt suit the requirements.
A solution to this seems to be MQTT as it should be suited for that use case, but i saw that it also runs over websockets which should lead to the same problem or does MQTT handle it differently?
Essentially i need a solution where the publisher has a static ip and the subscriber can be anywhere on the world. Is this possible with MQTT or do i need another solution?
No, only the MQTT broker needs a fixed IP address (and preferably a DNS entry) so the clients know where to find it.
All the MQTT clients (both subscribers and publishers), be they native MQTT or MQTT over websockets connect out to the broker. This means they will work even behind a NAT router running with a dynamic IP address (they would all get disconnected when ever the IP address changed, but nearly all MQTT clients automatically reconnect).
These features make MQTT a good choice for consumer IoT devices as the situation described above is pretty much every home broadband setup.
It sounds like your subscribing devices are on local networks, and yes, you would need a static IP for the network and forwarding inside it (not to mention a firewall exception in many systems) for a local device to serve incoming requests. Regardless of protocol, your subscribers do not need to be servers. It is far safer, and ultimately easier, to have them query a central server/system. Only that system needs an IP.
WebSockets do not require port forwarding - they are often used to avoid it. The client opens a connection to a server, then keeps using it to send and receive. It no more requires port forwarding than your computer does when receiving a page from a website. If your publisher is a server or other web-exposed system, you may get the job done by configuring your subscribers to open websockets to it.
However, you may still want MQTT:
It sounds like your publisher might be something other than a webserver, and might be less suited to serving than your clients, since you asked this question. With an MQTT client, it could publish to an MQTT broker on a server, which will then pass the messages to the subscribers' clients.
Developing robust publish-subscribe functionality is extra work, and existing MQTT software will often serve better than new development.
With some extra configuration, it is even possible to make MQTT subscriptions over WebSockets, but even a regular subscription works fine for avoiding static IP, portforwarding, and inbound firewall rules.
Explore one way RPC approach. It will not need a public IP address or port forwarding.
I know p2p software like Skype is using UDP hole punching for that. But what if one of the clients is a web browser which needs to download a file from another client (TCP connection instead of UDP)? Is there any technique for such case?
I can have an intermediate public server which can marry the clients but I can't afford all the traffic between these clients go through this server. The public server can only establish the connection between the clients, like Skype does, and that's all. And this must work via TCP (more exactly, HTTP) to let the downloading client be a web browser.
Both clients must not be required to setup anything in their routers or anything like that.
I'll plan to code this in C/C++ but at the point I'm wondering if this idea is possible at all.
I previously wrote up a very consolidated rough answer on how P2P roughly works with some discussion on various protocols and corresponding open-source libraries. You can read it here.
The reliability of P2P is ultimately a result of how much you invest in it from both a client coding perspective and a service configuration (i.e. signaling servers and relays). You can settle for easy NAT traversal of UDP with no firewall support. Maybe a little more effort and you get TCP connectivity. And you can go "all the way" and have relays that have HTTPS listeners for clients behind the hardest of firewalls to traverse.
As to the answer of your question about firewalls. Depends on how the Firewall is configured. Many firewalls are just glorified NATs with security to restrict traffic to certain ports and block unsolicited incoming connections. Others are extremely restrictive and just allow HTTP/HTTPS traffic over a proxy.
The video conference apps will ultimately fallback to emulating an HTTPS connection over the PC's configured proxy server to port 443 (or 80) of a remote relay server if it can't get directly connected. (And in some cases, the remote client will try to listen on port 80 or port 443 so it can connect direct).
You are absolutely right to assume that having all the clients going through a relay will be expensive to maintain. If your goal is 100% connectivity no matter what type of firewall the clients is behind, some relay solution will have to exist. If you don't support a relay solution, you can invest heavily in getting the direct connectivity to work reliably and only have a small percentage of clients blocked.
Hope this helps.
PeerConnection, part of WebRTC solves this in modern browsers.
Under the hood it uses ICE which is an RFC for NAT hole-punching.
For older browsers, it is possible to use the P2P support in Flash.
I'm writing a piece of P2P software, which requires a direct connection to the Internet. It is decentralized, so there is no always-on server that it can contact with a request for the server to attempt to connect back to it in order to observe if the connection attempt arrives.
Is there a way to test the connection for firewall status?
I'm thinking in my dream land where wishes were horses, there would be some sort of 3rd-party, public, already existent servers to whom I could send some sort of simple command, and they would send a special ping back. Then I could simply listen to see if that arrives and know whether I'm behind a firewall.
Even if such a thing does not exist, are there any alternative routes available?
Nantucket - does your service listen on UDP or TCP?
For UDP - what you are sort of describing is something the STUN protocol was designed for. It matches your definition of "some sort of simple command, and they would send a special ping back"
STUN is a very "ping like" (UDP) protocol for a server to echo back to a client what IP and port it sees the client as. The client can then use the response from the server and compare the result with what it thinks its locally enumerated IP address is. If the server's response matches the locally enumerated IP address, the client host can self determinte that it is directly connected to the Internet. Otherwise, the client must assume it is behind a NAT - but for the majority of routers, you have just created a port mapping that can be used for other P2P connection scenarios.
Further, you can you use the RESPONSE-PORT attribute in the STUN binding request for the server to respond back to a different port. This will effectively allow you to detect if you are firewalled or not.
TCP - this gets a little tricky. STUN can partially be used to determine if you are behind a NAT. Or simply making an http request to whatismyip.com and parsing the result to see if there's a NAT. But it gets tricky, as there's no service on the internet that I know of that will test a TCP connection back to you.
With all the above in mind, the vast majority of broadband users are likely behind a NAT that also acts as a firewall. Either given by their ISP or their own wireless router device. And even if they are not, most operating systems have some sort of minimal firewall to block unsolicited traffic. So it's very limiting to have a P2P client out there than can only work on direct connections.
With that said, on Windows (and likely others), you can program your app's install package can register with the Windows firewall so your it is not blocked. But if you aren't targeting Windows, you may have to ask the user to manually fix his firewall software.
Oh shameless plug. You can use this open source STUN server and client library which supports all of the semantics described above. Follow up with me offline if you need access to a stun service.
You might find this article useful
http://msdn.microsoft.com/en-us/library/aa364726%28v=VS.85%29.aspx
I would start with each os and ask if firewall services are turned on. Secondly, I would attempt the socket connections and determine from the error codes if connections are being reset or timeout. I'm only familiar with winsock coding, so I can't really say much for Linux or mac os.
How can an application be designed such that two peers can communicate directly with each other (assuming both know each other's IPs), but without outgoing connections? That's, no ports will be opened. Bitorrent for example does it, but multiplayer games (as far as I know) require port forwarding.
I'm not sure what you mean by No Outgoing Connections, I'm going to assume like everyone else you meant no Incoming Connections (they are behind a NAT/FW/etc).
The most common one mentioned so far is UPNP, which in this context is a protocol that allows you as a computer to talk to the Gateway and say forward me this port because I want someone on the outside to be able to talk to me. UPNP is also designed for other things, but this is the common thing for home networking (Actually it's one of many definitions).
There are also more common and slightly more reliable ways if you don't own the network. The most common is called STUN but if I recall correctly there are a few variants. Basically you use a third party server that allows incoming connections to try and coordinate a communication channel. Basically, what you do is send a UDP packet to you're peer, which will open up you're NAT for a response, but gets dropped on you're peer's NAT (since no forwarding rule exists yet). Through the connection to the intermediary, they are then told to do the same, which now opens up their NAT, and matches the existing rule in you're NAT. Now the communications can proceed. Their is a variant of this which will allow a TCP/IP connection as well by sending SYN and SYN-ACK messages with some coordination.
The Wikipedia articles I've linked to has links to the relevant rfc's for these protocols on precisely how they work. Essentially it comes down to, there isn't an easy answer, as this is a very network centric problem.
You need a "meeting point" in the network somewhere: the participants "meet" at a "gateway" of some sort and the said "gateway function" takes care of the forwarding.
At least that's one way of doing it: I won't try to comment on the details of Bittorrent... I am sure you can google for links.
UPNP dealt with this mostly in the recent years, but the need to open ports is because the application has been coded to listen on a specific port for a response.
Ports beneath 1024 are called "registered" because they've been assigned a port number because a company paid for it. This doesn't mean you couldn't use port 53 for a webserver or SSH, just that most will assume when they see it that they are dealing with DNS. Ports above 1024 are unregistered, so there's no association - your web browser, be it Internet Explorer/Firefox/etc, is using an unregistered port to send the request to the StackOverflow webserver(s) on port 80. You can use:
netstat -a
..on windows hosts to see what network connections are currently established, including the port involved.
UPNP can be used to negotiate with the router to open and forward a port to your application. Even bit-torrent needs at least one of the peers to have an open port to enable p2p connections. There is no need for both peers to have an open port however, since they both communicate with the same server (tracker) that lets them negotiate and determine who has an open port.
An alternative is an echo-server / relay-server somewhere on the internet that both peers trust, and have that relay all the traffic.
The "problem" with this solution is that the echo-server needs to have lots of bandwidth to accomodate all connected peers since it relays all the traffic rather than establish p2p connections.
Check out EchoWare: http://www.echogent.com/tech.htm
I am designing a p2p application which works on port 30000. My router is not UPnP so I required to forward a port to router. But Skype a another p2p application works without port forwarding on my pc. When I analyzed it with wireshark I found its using UDP port 48980, 58544. I am using c++.
There is a library in python here which does it for Nat PnP routers. Is it possible to programmatically forward port to router irrespective of type of router and operating system. What should be the approach to do it in c++ or any other language.
Skype works in a very interesting way. From what I've read (and this is a while ago) it works as follows:
Skype server maintains a list of all
users and IPs.
Skype user A wants to
speak to user B
Skype user A sends a
network packet to user B's IP address
and waits for a response. (user B never gets this packet).
Skype user A notifies server of IP/port
combination on which it is waiting
for a response
Server notifies user B
to send a response to user A on the
specified port
A connection is made.
This is probably over simplified but last I checked, this is how it works. (Someone correct me if I'm wrong).
Edit: fixed bullet issue
UDP hole punching is (one) of ways how to traverse through the NAT.
You have to use an intermediary server, and initiate communication from the client side. As Yossarian mentioned, from that point one possibility is UDP hole punching. Depending on the type of application (and whether it's truly P2P or not) you might keep the go-between server in place the entire time.
Skype uses another peers as intermediate point when direct connection is not possible.
i.e. Peer A wants to connect to peer B, but peer B has all ports closed, then communication is initiated through peer C which has open ports.