What is the rhyming and metric structure of Robert Frost's poem, "Stopping by Woods on a Snowy Evening?"

"Stopping by Woods on a Snowy Evening," is one of Robert
Frost's best known and best loved short poems. The simple narrative of a wintertime
traveler on horseback who stops for a moment to watch snow fall in his neighbour's dusky
woods has entranced readers for generations. A large part of that pleasure has to be in
the unique - for poetry written in English - rhyming and metric structure on which the
poet builds his recollected scenes. It is the usual practice in a poem constructed of
four line stanzas of four feet (a quatrain) to rhyme the first with the third line and
the second with the fourth. Even that much rhyme in English poems (the language being
generally poor in rhymes) was too much for many poets; many of the ancient balladeers,
for example, ceased their labours after rhyming the first and third lines. However,
Frost who once defined freedom (poetic as well as moral) as "moving easy in
harness," decided to write quatrains of three rhymes (aaba). Then, as if engaged in some
kind of New England-style dare, set himself the truly herculean task of picking up the
'odd' rhyme in the following quatrain: Thus: bbcb and ccdc. Having set himself this
difficult task, Frost was faced with having to 'tie up' the rhyming loose end in the
final quatrain. He might have elected purely mathematical symmetry by rhyming the final
'odd' line with the dominant rhyme of the first quatrain, and thus confusing the reader
with an anomalous rhyme. Instead, in a compositional act that can only be described as
inspired, Frost repeated the penultimate line and rhyme of the
poem. Working brilliantly within this self-imposed structure, Frost elucidated the deep
meaning of the poem concealed behind its simple story.