I am using a Java application to send UDP packets to an Android device. There I have another Java application that receives these UDP packets and displays its data - very simple.
Now I am working on some routing algorithms - therefore it would be nice to know how many hops a UDP packet did since it was send. My idea is to just read out the TTL (time-to-live) value of the packet and display it. Do you know if this is possible with pure Java? The class DatagramPacket doesn't give any hints at all.
I guess that this is not possible because this information might already have been removed at a lower layer, but I just want to be sure. :-)
The TTL field is, as you know, a feature of the underlying IP protocol (when used), not of UDP. So it makes sense for it not to be visible in the DatagramPacket API. However, I think you're right; it's not normally possible to get access to the IP packets through datagram-level API:s. You probably need to look into packet capture.
For your purpose, if I get it correctly, it would be sufficient to manipulate the TTL of the sender; e.g. set TTL of the sending socket to 1,2,3,4,5 and for each send one message with content "1","2","3","4" or "5" etc. Some will likely be missing on the receiver...
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All,
Not sure whether to create some Java code or use an 'off-the-shelf' product to produce a listener / router / redirector, one which will listen to a single incoming IP stream and then 'redirect' packets based on a fixed position string variable to an appropriate TCP server.
i.e. If the incoming packet has variable = 1 redirect to a TCP server at 192.168.150.211, if a 2 then redirect to TCP server at 192.168.150.212.
Ideally the code / product should also be able to pre-launch multiple TCP servers (either at one IP address and different IP ports or at different IP addresses) on the same machine prior to the listener / router starting.
Was thinking of using an OPC server but these seem too complex / costly for a single, relatively slow, incoming stream.
Thoughts appreciated.
This depends on the depth of analyzing. If you would like to analyze each packet at TCP-level then low-level language like C will be the best choice: just listen everything that comes to eth0 (or whatever interface), search for specific string (BTW "string" definition is too wide. The times when a string could be considered as ready-to-process data are in the '80s. Nowadays string is a piece of rubbish until you know what encoding is, how lines are terminated, etc).
As already mentioned by #f1sh you can redirect traffic using Java. But Java operates with streams, it knowns nothing about network packets. Also, in Java you can listen only on certain port(s) - there is no way to filter on the whole network interface (but JNI can be real saver).
If you would like to get PoC without huge amount of coding lets consider Socat. Socat can not only transmit data from one socket to another. It can also write data to files. So you can combine Socat (don't forget about fork option), tail, grep and a bit of Bash stuff and get simple redirection server working.
I recently bought an Arduino with an LCD screen. I want to push information from my computer to the Arduino. I came across a great article, How to make a physical Gmail notifier. From what I understand, I have to send the information using Serial and read it in the C/C++ code on the Arduino. That is fine, but I want to send different information to the device.
Say I want to have one part of the LCD-screen showing the temperature outside and another part of the screen display when the next bus is coming. Is there any way to "mark" the information I send with Serial, or does everything end up in the same "channel"?
If that is the case, is there a logical, simple way to separate this information so it does not mistake bus-information for temperature and vice versa?
You need a protocol for sending information across the serial line, so that the data can be collected the other end in a way that makes sense. A simple protocol may be:
T:16.0 09.34 // Temperature, 16.0°C measured at 09.34
B:11b 11.46 // Bus, route 11b, arrives at 11.46 at your bus-stop.
M:mats#example.com 11kb 10.23 // Mail from mats#example.com, it's 11KB and arrived at 10.23
Each line contains one type of information.
Assuming the line of communication is reliable (and as long as your wire isn't several dozen feet, it should be), you don't need more than that. If the communicatio is unreliable, you need some sort of "start" and "end" markers (or a start and a length), a checksum and some way of dealing with "it went wrong". You will also need to read with a timeout, so that when you don't get enough data, the system starts over again with the next bit of information.
Is there any way to "mark" the information I send with Serial
Definitely. YOU decide how the information is sent if you have control over the information passing over the serial port on your computer.
or does everything end up in the same "channel"?
Well, the serial port is a kind of a channel I guess, since all information you wish to send to the Arduino goes over the port.
is there a logical, simple way to separate this information so it does not mistake bus-information for temperature and vice versa.
Yes. Say you want to send temperature data. Create a byte array for example in this manner: {T23.4} = Temperature data
The bracket '{' signals to the receiving code in the arduino that information is coming down the line with some data. The letter T indicates temperature. Everything after the letter 'T' up to the '}' is data. (23.4)
Bus information could be {Bxxx} where xxx is the data.
I want to write an app in java that lets two clients talk via webcam. The way it works is both clients connect to a webcam that takes pictures at a specified frame-rate (20 per second maybe) then reduced the size and resolution, then sends it to the other client via a UDP packet. My question is - should I send every picture in its own Datagram Packet? I've read that they can only hold half a kilobyte at most so should every pic be cut down that much? Or should I have it split up into several Packets?
Are you sure you want to transmit whole images, instead of using an algorithm / codec that transfers only what needs to be updated?
If you choose the second option you can take some ideas from this previous question and a already used and tested library for the purpose. I believe i'd go with VLC java bindings if i had to do it. You should evaluate what is the best codec for your specific purpose (bitrates, quality, etc).
If you nevertheless want to transmit images i'd suggest you break them down into udp datagrams, remember that they should be somehow numbered/tagged so that the client can reconstruct the image as packets come (they won't necessarily come in the same order you send them), also you need to think what the client needs to do when some of the packets fails to arrive (discard the image, request previous packet, etc.).
One last thought, the udp datagram max size might not be the best option as well, your server-client should perhaps implement a algorithm and negotiate the udp frame size depending on the speed of the transmission.
What you should be doing is encoding a video stream. Leave the network layer alone, let it do fragmenting for you.
Also, if you are sending video over UDP, you will likely want to throw in a keyframe every 2 seconds or so.
Do not send each frame as its own image. Use a video compressor.
I am using simple UDP connection.
I would like to know if by default the connection has "Carriage return" enabled or disabled, and how could I set that property?
thanks,
ray.
Eh, that's not entirely accurate. UDP isn't differentiated by virtue of sending text vs. binary. All network protocols ultimately send data as bit streams (binary). What typically differentiates it is that unlike TCP, there is no back and forth to establish sequence numbers for tracking packets, and no ACK flag to signal that a packet was received. UDP will send packets with no regard to whether or not they get to the destination.
Edit: Ray maybe you should provide a little more detail about what you're trying to do. Carriage Return is an ascii character just like any other. It has a numerical representation and occupies a byte of space just like the other ascii characters. So asking if it's "enabled" for UDP transmission isn't really a valid question. Any series of bits can be sent via UDP, or TCP, or any other protocol - which means UDP doesn't even understand what ASCII is, or the letter "b", or a carriage return. It's all just a bunch of 1's and 0's, and UDP is aware of IP addresses and Port numbers - just enough to send your bits of data somewhere. What your application does with those bits is the question.
UDP traffic is session/connection less. So you can't have a "connection" on UDP.
UDP is used to pass binary data rather than text and there is no way to disable carriage return or any other character.
UDP broadcasts binary data - if you encode \r and/or \n to bytes and add it to the message, it will be sent. No filtering, no conversion on this protocol layer.
I am trying to understand the CANopen protocol.
For now, I do not have any CAN hardware nor the CANopen stack to experiment with.
I would like to know how to write a Java program to simply interpret CANopen messages that are received at the RS-232 port.
Are there CAN interfaces that are installed as a serial port?
Will I be able to write a program to process CANopen messages? I want only to be able to receive and interpret messages. Is it as simple as creating a buffer for the input stream and then break up the transmission into separate messages according to the SOF and EOF? How do I know what is the SOF/EOF since it is only 1-bit long?
Why is there a limit on the number of PDOs of a CAN node?
How do I process the PDO to identify the node from which it is sent and the data type and value? Is the PDO a standard CAN frame?
I don't know of any CAN interface that connects to the serial port (it wouldn't be too hard to create one based on a microcontroller with CAN and serial ports). However, standard serial ports would be too slow to support the higher speeds available in CAN.
Generally, when using the API for a CAN interface, you will be able to read messages consisting of ID, Length and up to eight bytes of data. You don't need to care about SOF/EOF. Even if interfacing directly on the low level with a CAN controller (that is, if you have a CAN interface for which you need to write the driver/API yourself), you still don't need to care about those details. And you don't want to try to access the CAN bus without using a CAN controller at all...
If you want to pretend that you have a CAN interface, you may create a stub function which returns those three items: an ID, a data length and a 64-bit data buffer. This is basically what all CAN interface APIs will give you. And when transmitting CAN messages, you will use the same parameters (ID, length data).
PDOs are defined by their use of the CAN ID field. In theory the number of PDOs for a device is not really that limited, but the predefined connection set have only allocated a small number (four) of PDOs for each node.
The PDOs are standard CAN frames. As mentioned, the CAN ID identifies the PDO. In the predefined connection set (which most devices follow), the CAN ID of all messages consists of a functional part and a module-ID part (the module ID may be hard coded for the device, or configurable by dip switches for example). Bits 10-7 of the CAN ID is the function code and bit 6-0 is the module number. For example TxPDO1 from a device with module ID 0x10 would have CAN ID 0x190. The upper four bits of the 11-bit CAN ID, ((CAN_ID & 0x780) >> 7), gives you the function code (TxPDO1 = 3) and the rest of the bits,(CAN_ID & 0x7f), gives the module id (which in this example was 0x10). So if you read a message on the CAN bus with CAN ID 0x190, you would know that this was a PDO from the device with module ID 0x10.
(A simpler way to express this might be to say that TxPDO1 has CAN ID set to 0x180+<module ID>, TxPDO2 has CAN ID set to 0x280+<module ID>, etc.)
How you should interpret the data in the PDO depends on your device.
I suggest that you find a good CANopen tutorial. Unfortunately most of them make everything sound much more complicated than it really is. So look around until you find one that appears understandable.
There are many CAN interfaces that can run off a serial port - VSCOM, Vector, and many others. There are also free programs that allow you to send and receive raw CAN frames - CANhacker, etc. Google for a few of them.
What I haven't found is a free program that can do interpret CANopen - most are pay programs. The exception is Wireshark for Linux - it uses SocketCAN to pull in packets and can parse all the CANopen frames.
I run my CAN bus a 1 Mbit/s and use a VSCOM interface to monitor it on the serial port.
CANFestival is a good open source stack that ports easily to Linux as well as bare machines.