mark top;
a, b, c, d;
symbol rectangle top-bottom left at a-b +10 right at c-d -10
[fill.color=lgray, line.type=dashed, draw_time=before_entity_lines];

a->b: Message 1;
b->c: Message 2;
b->c: Message 3;
b->c: Message 4;
symbol ... center at b-c;
b->c: Message \in;
mark circletop [offset=-5];
box c--c: OK: enough;
mark circlebottom [offset=+5];
symbol arc circletop-circlebottom center at c
[fill.color=none, line.width=3, line.color=red, xsize=120];
c->d: Message \in+1;
mark bottom;