Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
  ?d=R2-4-015&et=core.rd-group&rt=light

Res: 2.05 Content
Payload:
</rd/501>;ep=grp_R2-4-015;et=core.rd-group;
          base="coap://[ff05::1]";rt=core.rd-ep