IP过滤驱动问题


PF_FORWARD_ACTION
IpFilterHook(
        unsigned char   *PacketHeader,
        unsigned char   *Packet,
        unsigned int    PacketLength,
        unsigned int    RecvInterfaceIndex,
        unsigned int    SendInterfaceIndex,
        IPAddr          RecvLinkNextHop,
        IPAddr          SendLinkNextHop
)
{
  IPHeader * pHdr = ( IPHeader * )PacketHeader;
  ......
}
用参数 PacketLength 来判断包的尺寸,对于接收的包是正确的,对于本机发出的包,用此参数计算包尺寸却是错误的.该怎么获取发出数据包的尺寸?