Who can tell me why it says as below when I was compiling a kernel module:
WARNING: "tcp_send_ack" [/root/test/test.ko] undefined!
WARNING: "__tcp_push_pending_frames" [/root/test/test.ko] undefined!
WARNING: "tcp_current_mss" [/root/test/test.ko] undefined!
since I've checked that these functions are declared in ``/usr/src/linux-2.6.17.11/include/net/tcp.h''.

I invoke make as below
make -C /lib/modules/2.6.17.11/build M=/root/test modules

And the first few lines of my source code are:
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <net/netlink.h>
#include <net/ipv6.h>
#include <net/tcp.h>