Packed Structs with Bitfields

Hi Guys,

I'm programing my own TCP/Ip Stack. Therefore I need structs with bitfields.

For eg:

typedef struct  __attribute__ ((packed))
{
        uint16_t srcPort;
        uint16_t dstPort;
        uint32_t seq;
        uint32_t ack;
        unsigned headerlen      : 4;
        unsigned res            : 6;
        unsigned flags          : 6;
        uint16_t hlenFlags;
        uint16_t windowSize;
        uint16_t checksum;
        uint16_t urgentPtr;
}
TCP_Header

After the bitfield variables the values aren't right anymore. Can you tell me why is this so?

Thanks for your help
best regards
Robert