about TCP header

[wanghg <wanghg@cwc.nus.edu.sg>]

Hi, all.

I am hacking the code for the TCP. For the definition of the TCP header in the kernel,
I have following questions.


struct tcphdr {
        __u16   source;
        __u16   dest;
        __u32   seq;
        __u32   ack_seq;
#if defined(__LITTLE_ENDIAN_BITFIELD)
        __u16   res1:4,

                  what is meaning of 4 here, 4 bits?
};


Thanks

Haiguang