Bytes with TCP socket and bit manipulation

[Little Moe]

I'm taking a computer communication summer class and this is our 2nd and last program project. The protocol for the assignment can be found here.

I've already done one project that involves a client and server connecting and playing a game of guess the number with strings being sent back and forth. For this project though we have to manipulate individual bits of bytes and send only bytes across the socket connection. How do you do this in Java? I haven't been able to find anything about forming bytes with particular bits and sending just bytes across the connection.

[NtOrZor]

I'm not sure what your project assignment is doing (I dont' have time to read it). But from my experience in Java, to read binary data you use InputStream or OutputStream to read and write binary data.

Here's an example:

Let's assume "sock" is your socket connection.

InputStream in = sock.getInputStream();
while (in.read() != -1)
{
     //do whatever you need to do with it
}

You can also look at sun's java api for more info (search for inputstream).

Hope this helps!

[Little Moe]

I've read some of the things on the input and output streams and i'll be definitely be using them. My problem comes after I've received the bytes. I have to use certain control bytes as notifiers for things:

00000001- REQuest connection.

00000010 - CONnection Granted.

ssss0011 - FILe name packet (sequence number sss = 000).

ssss0100 - DATa Packet, where sss is the sequence number of the packet.

ssss0101 - Short Data Packet(SDP), where sss is the sequence number of the packet.

ssss0110 - ACKnowledgment packet, where sss is the sequence number of the packet expected next.

ssss 0111 - list request packet (sequence number sss = 000).

0000 1111 - TERmination (a simple two-way handshake).

After I've received the bytes I don't understand how to interpret the different bits of the bytes to know what I'm dealing with.

[Andareed]

You can AND them (note this is C-style code)

if(byte == 0x01) // REQuest connection
{
}
else if(byte == 0x02) // CONnection granted
{
}
else if(byte & 0x0F == 0x03) // FILe name packet
{
    sequence = (byte & 0xF0) >> 4;
}

[Little Moe]

Thx for the info Andareed!

I've taken computer structures so I understand the idea of And'ing and Or'ing things but I don't quite understand the hex strings your using. Any extra tips on those? I tried google'ing some stuff but it didn't really help. Also, what exactly does the >> do.

Edit:

Since hex is 16 bits I'm guessing the first digit is the first 4 bits and when you check for 0x0F you're testing to see if the first 3 bits are all 1's to know it's a file packet and similarly for the others. The >> still confuses me though.

Edited June 13, 2005 by Little Moe

[Andareed]

0xF0 is 11110000b. So I'm basically isolating the upper 4 bits. The >> shifts the bits right (by 4 in my example).

[Little Moe]

Awesome, thanks a lot for the help people. I should be able to actually take on this program now.

[Little Moe]

I've run into another problem. When sending a packet with file information one of the bytes has to be a one-byte unsigned binary value. I've found a whole lot of stuff about doing unsigned bytes in Java by using ints and AND'ing stuff with 0x0FF but my I think my problem comes with actually sending the byte.

My teacher expects our clients and servers to run against his C clients and servers. He gave us an example exe of each and whenever I run my client against his it says "Number of char in file name should be between 1 and 124 - LEN field = 0" like it's not getting my byte at all. I'm thinking it's either the byte I'm using or something special in the OutputStream that might be messing up my byte? I know there is no difference between the bits of a signed or unsigned byte but something is going wrong. Does anyone have an example of sending a byte like he is asking or maybe if I'm thinking of this wrong.

[Andareed]

The range of a signed bye is -128 to 127, and his expected input is from 1 to 124, so I doubt this is a signing issue.

[Little Moe]

I am able to send and receive the connection and request packets of 00000001 and 0000010 so I don't know why all of a sudden it wouldn't be receiving stuff right. I've printed out my length value which is 10.

[Little Moe]

Here's an example of me trying something:

String fileName = "project2.c";
int fileNameLength = fileName.length();
socketOut.write(fileNameLength);

Should that work?

[Little Moe]

I ended up finishing the project. My biggest problem starting off was I was trying to send bytes individually by just writing the bytes one at a time. My teacher was expecting the bytes one right after another. I had to send a byte array for him to get everything in a packet that I wanted. Couldn't just test one byte at a time. Had to test things just by hardcoding byte array values to see what happened.