Michael Richardson wrote:

[PATCH] This patch introduces a mask to the fwmark test cases in the advanced
routing. This let's one test individual bits of the fwmark to determine
how things should be routed (pick a routing table). This patch retains
compatibility with tests that do not set the mask by assuming a mask
of 0 is equivalent to a mask of 0xffffffff.

One last comment: instead of looking at the value you should check if
the mask attribute is present to decide when to use the compat mask.