Outbound circuit list. This is not a selection
criteria. If non-null, The octet string contains one
or more 2-tuple, each representing a circuit out of
which mulicast packets whose (s, g) matche this rule
will be forwarded.
Each tuple is in the following format:
first 2-octet: circuit number
following 2 octets: ttl threshold
A null string means those packets should not be
forwarded at all.