Непонятная запись Tcp заголовка

/* TCP header */
typedef struct tcp_header
{
    unsigned short source_port; // source port
    unsigned short dest_port; // destination port
    unsigned int sequence; // sequence number - 32 bits
    unsigned int acknowledge; // acknowledgement number - 32 bits

    unsigned char ns :1; //Nonce Sum Flag Added in RFC 3540.
    unsigned char reserved_part1:3; //according to rfc
    unsigned char data_offset:4; /*The number of 32-bit words in the TCP header.
    This indicates where the data begins.
    The length of the TCP header is always a multiple
    of 32 bits.*/

    unsigned char fin :1; //Finish Flag
    unsigned char syn :1; //Synchronise Flag
    unsigned char rst :1; //Reset Flag
    unsigned char psh :1; //Push Flag
    unsigned char ack :1; //Acknowledgement Flag
    unsigned char urg :1; //Urgent Flag

    unsigned char ecn :1; //ECN-Echo Flag
    unsigned char cwr :1; //Congestion Window Reduced Flag

    ////////////////////////////////

    unsigned short window; // window
    unsigned short checksum; // checksum
    unsigned short urgent_pointer; // urgent pointer
} TCP_HDR;

Что за :1?