Android : InputStream on socket reads zeros

I try receive a raw binary packet on my server side. I have control over the data format and specified the first 4 bytes of a packet to describe the length of the packet (it is an int value converted to byte[] and back). This works very well, I get the correct numbers (I can also monitor and check the client side). Here is the code:

is = connectionSocket.getInputStream();
byte[] intArray = new byte[4];
//read the 4-byte int value to array
is.read(intArray, 0, 4);
int packetLength = ByteBuffer.wrap(intArray).getInt();

I then want to read the package (excluding the length-header) to a custom-fit byte array:

byte[] packet = new byte[packetLength];//make a buffer ready
is.read(packet, 0, packetLength);//read the whole packet to array

If the sent byte[] has a length of 1444 or less, everything works fine. But packets beyond this limit won't be received properly. The packet-array will be filled correctly with data at the beginning, but after the limit, there are only zeros until the end like this

Positon: ...|1440|1441|1442|1443|1444|1445|1446|1447|.........|packetLength-1
Data:    ...| 59 |-73 | 125| -3 |  0 |  0 |  0 |  0 |..zeros..| 0

From my days as a 13-years-old-wannabe-internet-tuning-master I can still remember, that there is a standard MTU (maximum transmission unit) of about 1500 bytes on Windows machines. Indeed my receiving network adapter has an MTU of 1500. That's suspiciously close to 1444, so I increased the MTU, but sadly with no results. Java should not bother me with such low-level stuff, but maybe this isn't a coincidence. btw, the client is an Android smartphone, receiver a Windows 7 machine.

Does anybody have another idea or even better a solution?