The index allocation attribute is the sequence of index buffers. Each index buffer is 800 bytes (2K). The structure of an index buffer is 
offset(length)         Description
0(4)                   Magic number 'INDX'
4(2)                   Offset to fixup (usually 28)
6(2)                   Number of fixups including first word
10(8)                  VCN of buffer
18(2)                  Header size
1C(4)                  in-use length of buffer
20(4)                  total length of buffer
24(4)                  1 if not leaf block
28(A)                  Fixup
18+header size         Entry list
The entry list is just a sequence of index entries. The fixup works just as it does for MFT records. Because the record size is larger, there are four numbers to check and modify.

What is in the index header, other than the fixup data?