I want to write a app which will run on different computers and need all of then to communicate with each other like "utorrent" (peer to peer). This app only will send text messages.
How can I do this? I mean sending one message to remote computer on the internet?
I have a website and every app at start can send some information to it and find information of other apps on other computers (with PHP) but I do not know how address one computer through internet and send the data directly to that. I can find the ip address with PHP but it is the ip address of router (ISP).
How a message reaches a computer? I'm wondering about addressing every computer?
My brain really stuck here, I really appreciate any help. Thanks.
In a peer-to-peer network there's no centralized server for transmitting the data from one client to another, in this case the clients must be able to act as both the server and client. This means that either you'll have to be using UPnP like most modern torrent clients, which handles port forwarding in the router, or you'll have to manually forward a port to the computer in the router.
A centralized server (like a torrent tracker) is usually used to make the clients aware of each other's existence and tell them where to connect. This is where your PHP script comes in, though PHP might not offer the most effective way of doing this, assuming you're using it in combination with a webserver to serve the data though the http protocol.
As for actual text communication, you could use the Indy socket library for that. I found this example, basically which shows how to do it: http://www.ciuly.com/delphi/indy/indy-10-client-server-basic-demo/
Related
Short version: How would you recommend going about connecting a client to a server that are on the same local network, without manually entering the ip, when broadcast is disabled?
Further details: I am working on an educational multiplayer game for children. Many schools appear to be blocking broadcasting for security reasons. The children will be rather young, so it could be difficult for them and error-prone to have to enter the IP manually. They will all be in the same room and will all see the server screen. The game is made in Unity (C#).
Potential solutions: Here's what I thought about:
Connecting both the local server and clients to an external server, communicating the local server ip through the clients through the external server, then connecting directly and disconnect from the server. Not ideal because of the extra hosting costs.
Send a regular UDP message periodically to all ips on the subnet? This will probably be picked up by any decent firewall and blocked though, right?
Putting a QR code on the server that kids would take a picture of with the client app and have it connect that way? May be more of a hassle.
Having the server play random tones corresponding to numbers that the client is listening for? (Speakers may not always work though)
Sounds like the first one is the most sane and easy solution. Do you have any other ideas on what someone in this situation could try?
Is UDP multicast possible?
If yes then a common solution is that all participants join the same multicast group and the server listens on a well known port. If a client wants to know the address of the server it sends a packet to the multicast group, which is received by the client and answered with another packet, which then can be used by the client to determine the servers address.
In addition to that servers can also announce their presence in regular intervals by sending a suitable message to the multicast group.
What I can think of is an ad-hoc communication protocol between all the devices. Say you have 1 server and 10 clients. All the devices should run a service(say server-discovery) that binds to a fixed port say 9999. Now any time the client wants to connect to the server and doesn't know the IP, it starts a scan. Loops through different IPs and tries to connect to 9999. If it manages to hit, it asks for the server IP. In case it manages to hit the server it will get the IP since the server knows it's own IP and the client will maintain the server IP in a cache. If the client hits another client. It can ask for the server IP. It the other client knows the IP it will share the info else decline.
I agree there is a lot of overhead, but I think this will be robust unlike sound and would reduce cost of printing QRs everytime.
on the local network the traffic is direct from host to host.
I don't understand which devices is blocking local broadcasts.
if there aren't too many peers on the LAN ( less then 100 ), I think udp broadcasts work fine and you dont pollute the network.
to have an idea of your "pond", I suggest you to sniff your local traffic.
there are many broadcasts : arp, windows, ipp, dropbox...
I spent the last two days reading each StackOverflow questions and answers (and googling of course) about Indy TCP and UDP protocol in order to decide which one should I use in my communication method between my User Application and my Windows Service.
From what I saw so far, UDP is the easiest and the only one I managed to work to receive broadcast messages from TidUDPClient (I did not testes the response back yet). And I also noticed that TCP is a bit more complicated with it's thread loop.
But since everywhere I am told UDP is not reliable, UDP is not reliable... I begin to wonder if it's not better to use TCP anyway.
My User Application will be running on many machines, and the Service will be running in one of them, sharing one IP with a Client, or in a dedicated machine, depending on my client's funds. So, should I really be worried about UDP data loss possibilities?
I need broadcast capabilities so my server advises all clients at once about Application updates, and of course, if my the Client Application does not know in which IP the Service/Server is, it will send a broadcast call to be told where the server is. Is that applicable to TCP?
The messages I am sending are requests for users access confirmation, users privileges, and application executable file updates, since the main application can't update itself.
Those messages are encrypted like below, and they might bet bigger sometimes.
e86c6234bf117b97d6d4a0c5c317bbc75a3282dfd34b95446fc6e26d46239327f2f1db352b2f796e95dccd9f99403adf5eda7ba8
I decided to use them both!
Simple use case:
In order to communicate with TCP prococol you have to establish a connection which you can have only if you know IP and Port on both ends.
If you do not have that information when you load your Application, then you use the UDP to Broadcast your IP address and your intention to find the/a Server. You may try about 5 times before you raise the user an error telling that you did not find the Server or that the Server is down.
Sending that message in UDP will (one time or other) reach the UDP ear of the Server, which will now know the IP from the lonely Client's IP and will now begin a proper connection via TCP to be read talk about the critical messages of the Application.
What do you think of that approach?
With Delphi XE2, what is the most reliable method to detect if the computer is able to do the following things?
reach a specific website with HTTP which does not have a fixed IP address
send and receive e-mail with any local or remote e-mail client
There are too many factors involved (type of Internet connection, firewall/router rules, proxies, etc). The most reliable approach is to simply not try to determine the current state and just attempt the desired operation (perform the actual HTTP request, or the SMTP/POP3/IMAP operation, etc), and just be prepared to react to any errors. You can detect connection-related errors and prompt the user to check their Internet connection before retrying.
Use TIdHTTP.Get and try to download http://google.com.
Of course it depends on the definition of being connected to the internet. Sometimes web traffic (port 80) is blocked while other ports are open. Fortunately, nowadays most people are actually allowed to browse the web, since it also provides help with their daily activities. Google is probably one of the least firewalled websites with one of the highest uptimes.
But still, it's a lucky guess. Depending on what you need it for, you might as well just try your thing and see if it works. If not, apparently the computer was not properly connected, even if it could reach Google. :)
[edit]
Because of the discussion. InternetCheckConnection is a good alternative too, but it also checks the connection by pinging an actual server.
MSDN says
Use the InternetCheckConnection function to check the connection to
the Internet. It attempts to ping the server designated by the URL
that is passed to the function. If the FLAG_ICC_FORCE_CONNECTION flag
is set and the URL is NULL, the function checks to see if there is an
entry in the server database for the nearest server. If one exists,
the function pings that server
But since this function uses ping, it may be a bit faster than actually retrieving content. On the other hand, many firewalls actively refuse pings.
I have a custom pair of client/server sockets (TJDServerSocket and TJDClientSocket) which wrap the TServerSocket and TClientSocket in the ScktComp unit. I don't have any issues to fix, but would like to know something. I'd like to add a feature to the client side to automatically search the network for any instances of a server socket (specifically my server component).
I'm open to any suggestions, but has to be specific to the use of the ScktComp unit in Delphi 7.
Here's a link to the components of mine.
Never used the TServerSocket and TClientSocket myself, and I don't have the help files within reach, so I can't immediately see if this would work with those components.
For a project I did I needed something like that too. I ended up with using UDP to broadcast a discovery request (within the same subnet of course). The server, listening on a particular port for such a request, would reply its data back. When multiple servers would exist (a situation that though rare could occur) the client just picked the server with the required service(s) and the least load. That load was part of the data the server send back.
It worked out nice, wasn't all that difficult to write, and turned out reasonably efficient too.
The request protocol is completely up to you. The one I devised allowed clients to send a request detailing the services they need, and servers replying listing their services and the load (= connected clients in active use).
After selecting the server to talk to, a client would register itself for the services it needed, and could use them after that.
Hope this helps.
There are some standard protocols for service discovery. See for example: http://en.wikipedia.org/wiki/Zero_configuration_networking
Mailslots is a nice option here. It'll broadcast to every PC on your subnet. See Jeroen's answer to this question:
Suggestions on writing a TCP IP messaging system (Client/Server) using Delphi 2010
Searching is as easy as port scanning.
If you don't like the brute force approach, the server can register itself to a well known service application (could be a web server), and the client can connect to the service application to ask. It's quieter than broadcasting.
With more information, such as details about the network (who's it for?), I can suggest a more precise answer.
Hi
let me make my question clear. Two people using my app are connected to the internet. Both have each other's IP and they want to chat (like Y!messanger) with each other.
I think I need to use Indy components; right? Which component should I use?
Thanks in advance
Have you looked at any of the demos on Indy's website yet?
In general, you are looking to create a "Client/Server" type application. A quick Google search for "indy client server example" pulls up lots of results, including this one: http://www.devarticles.com/c/a/Delphi-Kylix/A-Real-World-Client-Server-Application-in-Delphi/
In reality, this gets a lot more complicated when you have firewalls and NATs with private IP addresses. You will have to consider how your application will either get around or through these types of technologies.
Similar to what Scott said, I think that your biggest problem is getting them talking to each other. My computers at home go through a router, which blocks all incoming connection requests (i.e. requests to start a conversation between two computers) from the Internet. My computers can send connection requests OUT, and start a conversation that way, but unless you modify the router (port forwarding) my computers can not receive connection requests.
You need a server somewhere to which both people will connect, that can then relay messages back and forth. To get really tricky, once the connection is made to the server the two computers can then be put into direct contact, but that involves UDP packets and some clever magic.
You don't have to use Indy components, you just need anything that will handle communications over the network. Any HTTP or sockets network stack will do. Indy is the defacto standard for Delphi Win32.
To do network communications, you will need to create a listener object or service on machine A and a sender object on machine B to send a network message from A to B. To send a message from B to A, you will need a reverse path as well - 4 objects total to perform bidirectional comms. Some object wrappers hide this detail internally. I don't recall offhand whether Indy hides this or not.
It would probably be easiest if you use a common TCP/IP protocol for your machine to machine communications, such as HTTP. This will make it easier to get your connections through firewalls and proxies that frequently exist between arbitrary users. To avoid conflicting with any HTTP web services that might be running on either machine, you should use a custom port number with the IP address: 192.168.1.10:12345, not the standard HTTP web server port 80. This is what most of the IM clients do.