What will be the speed of one photon with respect to another if they are travelling in opposite direction?

Two photons are travelling in opposite directions at speed of light. What will be the speed of one photon with respect to the other photon? Why? and How? I mean how will time and space behave or change according to relativity to satisfy both the observations that with respect to ground they both are travelling at speed of light in opposite directions but with respect to each other they again are travelling with same speed of light.