Uncorrect reception of a variable length datagrams using TCP/IP connection

There is the problem I couldn't solve myself.
I need to receive а datagram with variable length (up to 4 Kb) using
TCP/IP connection from remote controller to SIMATIC S7 400.
The datagram is sent by the remote controller after a certain event.
SIMATIC gets the data using the function FC60 "AG_LRECV", where
the input parameter RECV is, for example, "P#DB1.DBX0.0 BYTE 4096".
Everything is OK if the lenght of datagram is equal to the length of
the parameter RECV. But (this case is cardinal) if the lengh of
datagram is less then the parameter RECV only the
first portion of the datagram (divisible by 400 bytes) is written into
DB1. The rest of the datagram is written into DB1 only when the next
datagram comes. My aim is to recieve the whole datagram at once.

With regards, Eugene.