std.bitmanip help - how to combine bitfields and read

[kookman]

I am using libpcap to read from stored pcap files, and want to 
use std.bitmanip.bitfields to read TCP flags from the file, using 
a struct like:

struct TcpHeader {
     align(1):
     ushort srcPort;
     ushort dstPort;
     uint seqNo;
     uint ackNo;
     mixin(bitfields!(
         bool, "flagFin", 1,
         bool, "flagSyn", 1,
         bool, "flagRst", 1,
         bool, "flagPsh", 1,
         bool, "flagAck", 1,
         bool, "flagUrg", 1,
         bool, "flagEce", 1,
         bool, "flagCwr", 1,
         bool, "flagNs",  1,
         ubyte, "reserved", 3,
         ubyte, "dataOffsetWords", 4   // in 32bit words, min=5, 
max=15
     ));
     ushort windowSize;
     ushort checksum;
     ushort urgentPtr;

     this(inout ubyte[] bytes)
     {
         srcPort = bytes.read!ushort();
         dstPort = bytes.read!ushort();
         seqNo = bytes.read!uint();
         ackNo = bytes.read!uint();
         // now what? how to read bitfields??
     }

     const uint dataOffset()        // convenience method to get 
offset in bytes
     {
         return dataOffsetWords * 4;
     }
}

How can I get at the private field behind the bitfields to read 
into it. I'm using std.bitmanip.read because I need the network 
order (bigendian) conversion for this data.