Sockets .. IO controlling TCP frame length and number of packets

Question

Here's the deal, I'm trying to develop a proxy for this application, an application wich is registred and can only be used from a specific host. I want to do it from another location, so I am writing a proxy.
This should be trivial. But it has proven not to be!
1. I am using java.net.Socet and ServerSocket
2. I am using DataInput/OutputStream writeByte/readByte
3. flushing after each byte OR not, same result
It is not working, I GET the data just fine, and transmit it just fine, but the server doesnt respond.
With a network sniffer, the only difference I can see is that with the original application (C++) only ONE TCP packet is transmitted over the network length = 52
With my proxy TWO TCP packets is transmitted first LEN=1 and second LEN=51 for a grand total of the exact data ..
So all other things being equal, im thinking that this must be my issue!
Anyway I can deal with this ?
Any ideas welcome!
ps. tried .setBuffer .noTcpDelay etc. and gotten the same results .... perhaps nio gives more options ?
Thanks.

Answer 1

Your network "sniffer" operates at a lower level than you application. The differences in Ethernet frame that you are detecting with the network sniffer should not change the behavior of your TCP/IP application because, at the sockets programming level, the TCP stream has no boundaries.
I have noticed the behavior in Java that you mention. I think I have seen it in the implementation of java.io.DataOutputStream.writeBytes(String), where they write the bytes one-byte-at-a-time and your get this side-effect. The source for that method looks like:
    public final void writeBytes(String s) throws IOException {
        int len = s.length();
        for (int i = 0 ; i < len ; i++) {
            out.write((byte)s.charAt(i));
        incCount(len);
    }What happens is that in that first call to out.write(), the something decides to send the one byte right away, and only on the second and third bytes does something (not sure if it is Java or TCP/IP) see that it is piling up and attempt to bunch up the bytes into a single frame.
If you, the the Java programmer, wanted to avoid this, you would have to avoid all use of writeBytes(String s) and convert your String's to byte arrays explicitly, and then only call write(byte[] b, int off, int len), but all this should change it what you see in your network sniffer, not how the application behaves.