def transfer(topo, p_out, p_in):
"""Build constraint for moving p_out to p_in across an edge."""
options = []
for s1, s2 in topo.edges():
p1 = topo.node[s1]["ports"][s2]
p2 = topo.node[s2]["ports"][s1]
# Need both directions because topo.edges() only gives one direction for
# undirected graphs.
constraint1 = And(And(switch(p_out) == s1, port(p_out) == p1), And(switch(p_in) == s2, port(p_in) == p2))
constraint2 = And(And(switch(p_out) == s2, port(p_out) == p2), And(switch(p_in) == s1, port(p_in) == p1))
options.append(constraint1)
options.append(constraint2)
forward = nary_or(options)
# We also need to ensure that the rest of the packet is the same. Without
# this, packet properties can change in flight.
header_constraints = []
for f in HEADERS:
if f is not "switch" and f is not "port":
header_constraints.append(HEADER_INDEX[f](p_out) == HEADER_INDEX[f](p_in))
# header_constraints is never empty
return And(forward, nary_and(header_constraints))
def one_per_edge(topo, pol, field="vlan"):
"""Determine if pol only uses one value of field on each internal edge.
We don't care about external edges because they never have the problem of
preventing tiling of two-node connectivity since this slice never reads
packets sent to them.
"""
p, pp, q, qq = Consts("p pp q qq", Packet)
r, rr, s, ss = Consts("r rr s ss", Packet)
solv = Solver()
solv.add(forwards(pol, p, pp))
solv.add(transfer(topo, pp, r))
solv.add(forwards(pol, r, rr))
solv.add(forwards(pol, q, qq))
solv.add(switch(pp) == switch(qq))
solv.add(port(pp) == port(qq))
solv.add(HEADER_INDEX[field](pp) != HEADER_INDEX[field](qq))
if solv.check() == unsat:
return None
else:
return solv.model(), (p, pp), HEADER_INDEX