input boxes

vardef drawnodes(text t) =
  fixsize(t);
  forsuffixes s=t:
    fill bpath.s shifted (1pt, -1pt);
    unfill bpath.s;
    drawboxed(s);
  endfor
enddef;

def send(suffix s, d)(expr angle) =
  drawarrow s.c{(d.c-s.c) rotated angle}..d.c cutbefore bpath.s cutafter bpath.d;
enddef;

beginfig(1);
u = 3mm;

circleit.a(btex Alice etex);
circleit.n(btex $N_i$ etex);
circleit.p(btex $w_1$ etex);
circleit.q(btex $w_2$ etex);
circleit.r(btex $w_3$ etex);
circleit.m(btex $N_{i+1}$ etex);

n.c-a.c = q.c-n.c = m.c-q.c = (10u, 0);
p.c-q.c = q.c-r.c = (0, 6u);
a.e-a.c = n.e-n.c = p.e-p.c = q.e-q.c = r.e-r.c = m.e-m.c = (5mm, 0);
a.n-a.c = n.n-n.c = p.n-p.c = q.n-q.c = r.n-r.c = m.n-m.c = (0, 4mm);
drawnodes(a, n, p, q, r, m);
send(a, n, 0); %btex $M_1$ etex);
send(n, p, 12);
send(n, q, 12);
send(n, r, 0);
send(p, m, 12);
send(q, m, 0);
send(m, p, 12);
send(p, n, 12);
send(q, n, 12);

endfig;
end