Are there any libraries which put a reliability layer on top of UDP broadcast?
I need to broadcast large amounts of data to a large number of machines as quickly as possible, and generally it seems like such a problem must have already been solved many times over, but I wasn't able to find anything except for the Spread toolkit, which has a somewhat viral license (you have to mention it in all materials advertising the end product, which I'm not sure our customer will be willing to do).
I was already going to write such a thing myself (because it would be extremely fun to do!) but decided to ask first.
I looked also at UDT (http://udt.sourceforge.net) but it does not seem to provide a broadcast operation.
PS I'm looking at something as lightweight as a library - no infrastructure changes.
How about UDP multicast? Have a look at the PGM protocol for which there are several commercial and open source implementations.
Disclaimer: I'm the author of OpenPGM, an open source implementation of said protocol.
Though some research has been done on reliable UDP multicasting, I haven't yet used anything like that. You should take into consideration that this might not be as trivial as it first sounds.
If you don't have a list of nodes in the target network you have no idea when and to whom to resend, even if active nodes receiving your messages can acknowledge them. Sending to a large number of nodes, expecting acks from all of them might also cause congestion problems in the network.
I'd suggest to rethink the network architecture of your application, e.g. using some kind of centralized solution, where you submit updates to a server, and it sends this message to all connected clients. Or, if the original sender node's address is known a priori, then just let clients connect to it, and let the sender push updates via these connections.
Have a look around the IETF site for RFCs on Reliable Multicast. There is an entire working group on this. Several protocols have been developed for different purposes. Also have a look around Oracle/Sun for the Java Reliable Multicast Service project (JRMS). It was a research project of Sun, never supported, but it did contain Java bindings for the TRAM and LRMS protocols.
Related
My knowledge about network programming is limited, so, all the comments are more than welcome. Essentially my question boils down to the following question:
Q1. Is there really such a thing as decentralized asynchronous cross-platform peer-to-peer communication?
Let me explain myself.
If we have two http servers running on computers with actual IP addresses, then clearly the answer is yes, assuming one writes a protocol for the interaction.
To go one step further, if one of them (or both) is (are) behind a router, then, with port forwarding the communication can still be established. However, here the problems start because if someone wants to run such a server on the background, say in a mobile phone, the app that is relying on this server really works when one is at home (we can not really expect to request port forwarding everywhere we go).
But even beyond that,
Q2. do mobile phones obtain an actual IP address from telecommunication companies when someone is not using a wi-fi?
If this is true, then clearly one can have cross-platform asynchronous peer-to-peer communication at the expense of not using wi-fi by running an http server on a smartphone. (I understand that this is not convenient, but it is certainly doable.)
Concluding, the two (perhaps there are more) relevant questions that I can think of are:
Q3. How does Skype really work?
Q4. How does Viber really work?
Based on the answer for Skype, it says: 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.
So, it appears that there is no direct communication in Skype, because they have to use a man-in-the-middle for such a scenario.
Regarding Viber, I could not find a good-thorough answer to this particular question. Do people talk to each other through a Viber centralized server, or, do they establish a direct connection? Of course if they do establish a direct connection, then I really want to know how they manage such a thing since a mobile phone may or may not have a physical address. How is a Viber message routed to my cell phone from a friend of mine even when Viber is not running and I am behind a router?
I guess the answer to Viber is really push notifications, but as far as I can understand, all the variations of push notifications rely on open connections, and then the servers of the applications send the notifications to the clients through such connection(s). So, this approach gives us the feeling that it is asynchronous, but essentially it is not. We are cheating, in the sense that there is a constantly open connection to a server, and moreover, as far as I can understand, the application server has to push the notification through that server. Schematically:
A > Central App Server > Central Server w/ open connection to my cellphone > me
So, this seems to be once again a centralized approach.
Honestly, the only approach that I can think of that is both decentralized and asynchronous (on mobile phones as well) is to run an http server on every platform/device, but this comes at the expense of not using Wi-Fi and assuming that a telecommunication company really assigns a physical IP address to every mobile phone (which I do not know if it is true, do you?).
What about WASTE, darknets, F2Fs, etc? Do they offer advantages in the sense of a more direct asynchronous communication between some interested parties? Are there real-world applications (also including mobile phones) using such approaches for communication.
Really, this is not the actual problem that I would like to work on, but I would like to know what the state of the art is so that I can figure out how I can proceed from there. So, all comments are really more than welcome. If you have references for the state of the art I would like to know about them as well, but a brief description would also be nice.
I appreciate all your time and effort in advance.
You asked many questions, here is the beginning of the answers:
Q1: Yes. For example, take BitTorrent's very successful 10 million+ node network. Aside from the bootstrapping process, the protocol is entirely decentralized and asynchronous. See here for more info.
Q2: Yes! Go to www.whatismyip.com on your mobile telephone, and you will see your assigned IP. However, you are likely to be very filtered (e.g: incoming traffic on port 80 is likely to be blocked).
Q3: It has elements of P2P and clever tricks to get around NAT issues - see here for more info.
Q4: I don't know.
I'm experimenting with my Arduino Mega. I also have an Arduino Ethernet Shield.
I need to send emails using them, without the help of a computer (or any other device; like a smartphone, etc.). Though I could find several articles, I couldn't find any acceptable solution...
How can I do it? As I'm not asking this to be used for any special application, you can
make any assumption about missing details.
From the discussion above in comments it sounds like you either need code from someone who has just done it for you or you need to take the time to learn about the components and find or make the components.
They wouldn't make an Ethernet shield for this platform if it was only useful for non-standard packets. So someone somewhere has created some level of an IP stack.
Backing up though, in order to send mail you need to learn the Simple Mail Transfer Protocol (SMTP). Almost all Internet protocol definitions are defined using something called RFCs (Request for Comments). So if you google SMTP RFC you will find RFC 2821.
IETF is Internet engineering task force. There will be many copies of these documents on many websites. And due to the age of the Internet and these protocols in many cases you will find that one RFC has been created to replace a prior one. Version numbers are not used, but it is kind of like HTML 1.0 then HTML 2.0 and so on. I recommend even though the RFC says that it completely replaces RFC xyz, go find RFC xyz and read it. I go back as far as I can find learn that one then work my way forward.
Many/most protocols that ride on top of TCP (TCP is yet another protocol defined in an RFC, more on that later) are ASCII based, makes it very easy to, for example, Telnet to learn/experiment with the protocol, you can probably use Telnet to learn SMTP.
Most protocols are some sort of a half duplex thing, make a connection and often the server sends you a string, you see that string and then you send some sort of hello string, the server responds with some sort of OKAY or fail status. For SMTP, you then do some sort of I am mailing from this email address, server says OKAY, you say I want to mail this person or this list of people, for each email address you get an okay or fail. Eventually, you tell the server you are ready to send the body of the message, you do that, end the message with the defined termination. Then either the server says okay or fail or maybe there is some more handshaking.
The protocols in general though have this back and forth. Usually you are sending strings with commands and usually the server side sends back a short okay or error. Sometimes, if they want, they send back more detail on the error, but always start with the few bytes that indicate okay or error. The protocols generally have a flow, you must do this first then this then that.
You should learn sockets programming, sometimes called Berkeley sockets. You can write programs that are mostly portable across unixes but also across to Windows using Windows sockets if that is your platform of choice. You need to learn the protocol first, and it is better on your desktop/laptop and not embedded, you can get it done faster there. You do NOT have to learn to fork or thread to use sockets. The examples may show that as it is easy to show it that way, but you can write complete applications using polling only, it is half duplex send something, wait, send something, wait. For these simple learning programs, a little time up front to learn sockets, from there, it is all learning the protocols.
Now that was the very easy part, the hard part is the TCP/IP stack. I do not recommend attempting that without gaining a lot more experience taking baby steps on your way there. For example, learn to respond to ARP first (yet another RFC protocol, address resolution protocol) then ping (ICMP echo, one subset of the ICMP protocols) then IP basics (sniffing packets) then receive and generate UDP packets. TCP is a whole other level above that, more handshaking. It is not fixed packet size, it is streaming, do not have your code operate on packets, it is a stream of bytes, like working with a serial port.
Doing your own TCP stack is very much a non-trivial thing, I don't recommend it, you need to find someone that has done a TCP/IP stack for this platform for the Ethernet shield and just use it, whatever RTOS or environment they use, use it. Then take your desktop/laptop based experience with the protocol and apply that.
From the discussion above, if you don't want to learn the protocols, etc., I think you need to google around looking at Arduino Ethernet shield examples and see if anyone has done something that sends emails.
I would like to be able to communicated with PLC controllers, so that I can send and receive custom commands on the PLC.
My idea of being able to do this was to have a TCP listener on the PLC that could read TCP incoming packets on a specific port, and execute routines based on the commands in the packets. It could also send information back via TCP/IP.
This would allow me to write software in multiple languages such as C#, PHP, JavaScript, etc. so that software can be used on any platform such as Windows, iOS, Android, etc. to issue commands to the PLC. This would also mean you do not need the PLC software (which can be costly) to view or control the PLC.
I am not a PLC programmer, so I do not know if PLC has the capability of sending and receive custom TCP packets. I would like to know that a) if it is possible b) how feasible it would be to do this and c) what exactly I should research so that I can accomplish this.
Thanks.
It sounds a bit like reinventing the wheel. You want to make something like KepServerEX?
http://www.kepware.com/kepserverex/
There are also two things to consider - one is the ability to interface with the PLC to share data (ie: for a custom HMI) and the other is to program the PLC. For the latter you still need the control software from the manufacturer unless you're willing to reverse engineer and re-write it from the ground up.
Keep in mind, also, that PLCs don't work the same way that other software does. There are no functions or procedures or classes or objects or even really any "commands", per se. A PLC is a system which executes a continuous fixed program of mostly raw logic rules and calculations. A typical interface to an HMI involves reading and writing directly to/from logic bits and word data (ie:hardware memory locations) which represent the current state of the machine. OPC already does this just fine so I'm not quite sure what you're going for.
If you're looking for a cheap/free alternative to a full commercial package, something here may work for you :
http://www.opcconnect.com/freesrv.php
If I understand correctly, when referred to "Run/Stop" you mean for the PLC to 'Start' or 'Stop' scanning the code and updating its I/O. If this is the situation, it would be perfectly suitable to add a Scan_If_On bit (which will be written by a TCP Command) in parallel connection with the "Start" bit controlled by the HMI.
This way, there will be 2 forms of "Starting" the process controlled by the PLC. HMI and TCP.
I want to build a decentralized, reddit-like system using P2P. Basically, I want to retain the basic capabilities of reddit, but make it decentralized, to make it more robust and immune to censorship. This will also allow people to develop different clients to match the way they want to browse it.
Could you recommend good p2p libraries to base my work on? They should be open-source, cross-platform, robust and easy to use. I don't care much about the language, I can adapt.
Disclaimer: warning, self-promotion here !!!
Have you considered JXTA's latest release? It is probably sufficient for what you want to do. Else, we are working on a new P2P framework called Chaupal, but it is not operational yet.
EDIT
There is also what I call the quick-and-dirty UDP solution (which is not so dirty after all, I should call it minimal).
Just implement one server with a public address and start listening for UPD.
Peers located behind NATs contact the server which can read how their private IP address has been translated into a public IP address from the received datagrams.
You send that information back to the peer who can forward it to other peers. The server can also help exchanging this information between peers.
Then peers can communicate directly (one-to-one) by sending datagrams to these translated addresses.
Simple, easy to implement, but does not cover for lost datagrams, replays, out-of-order etc... (i.e., the typical stuff that TCP solves for you at the IP stack level).
I haven't had a chance to use it, but Telehash seems to have been made for this kind of application. Peer2Peer apps have a particular challenge dealing with the restrictions of firewalls... since Telehash is based on UDP, it's well suited for hole-punching through firewalls.
EDIT for static_rtti's comment:
If code velocity is a requirement libjingle has a lot of effort going into it, but is primarily geared towards XMPP. You can port off parts of the ICE code and at least get hole-punching. See the libjingle architecture overview for details about their implementation.
Check out CouchDB. It's a decentralized web app platform that uses an HTTP API. People have used it to create "CouchApps" which are decentralized CouchDB-based applications that can spread in a viral nature to other CouchDB servers. All you need to know to write CouchApps is Javascript and learn the CouchDB API. You can read this free online book to learn more: http://guide.couchdb.org
The secret sauce to CouchDB is a Master-to-Master replication protocol that lets information spread like a virus. When I attended the first CouchConf, they demonstrated how efficient this is by throwing a "Couch Party" (which is where you have a room full of people replicating to the person next to them simulating an ad hoc network).
Also, all the code that makes a CouchApp work is public by default in special entities known as Design Documents.
P.S. I've been thinking of doing a similar project, but I don't have a lot of time to devote to it at the moment. GOD SPEED MY BOY!
I'm developing a multi-player game and I know nothing about how to connect from one client to another via a server. Where do I start? Are there any whizzy open source projects which provide the communication framework into which I can drop my message data or do I have to write a load of complicated multi-threaded sockety code? Does the picture change at all if teh clients are running on phones?
I am language agnostic, although ideally I would have a Flash or Qt front end and a Java server, but that may be being a bit greedy.
I have spent a few hours googling, but the whole topic is new to me and I'm a bit lost. I'd appreciate help of any kind - including how to tag this question.
If latency isn't a huge issue, you could just implement a few web services to do message passing. This would not be a slow as you might think, and is easy to implement across languages. The downside is the client has to poll the server to get updates. so you could be looking at a few hundred ms to get from one client to another.
You can also use the built in flex messaging interface. There are provisions there to allow client to client interactions.
Typically game engines send UDP packets because of latency. The fact is that TCP is just not fast enough and reliability is less of a concern than speed is.
Web services would compound the latency issues inherent in TCP due to additional overhead. Further, they would eat up memory depending on number of expected players. Finally, they have a large amount of payload overhead that you just don't need (xml anyone?).
There are several ways to go about this. One way is centralized messaging (client/server). This means that you would have a java server listening for udp packets from the clients. It would then rebroadcast them to any of the relevant users.
A second way is decentralized (peer to peer). A client registers with the server to state what game / world it's in. From that it gets a list of other clients in that world. The server maintains that list and notifies the other clients of people who join / drop out.
From that point forward clients broadcast udp packets directly to the other users.
If you look for communication framework with high performance try look at ACE C++ framework (it has Java bindings).
Official web-site is: http://www.cs.wustl.edu/~schmidt/ACE-overview.html
You could also look into Flash Media Interactive Server, or if you want a Java implementation, Wowsa or Red5. Those use AMF and provide native functionality for ShareObjects including synching of the ShareObjects among connected clients.
Those aren't peer to peer though (yet, it's coming soon I hear). They use centralized messaging managed by the server.
Good luck