Are signal fronts in a beam not at rest to each other? I'd like to investigate how the notion of "mutual rest" might be applied consistently, but distinctively, in the following thought experiment:
Consider a light source ("$A$") which directs a beam towards a receiver ("$B$"). Source $A$ is modulating the beam, emitting discrete recognizable signals in rapid succession (i.e. with short durations from stating one particular recognizable signal modulation until stating the next; compared to $A$'s duration from stating one particular recognizable signal modulation until observing the corresponding echo from receiver $B$).
Is there some definite, operational sense of "mutual rest" whereby in this case it could be said 


*

*that source $A$ and receiver $B$ had been "at rest to each other" ("throughout the experiment", if applicable); but 

*that a successive pair of signal fronts had not been "at rest to each other" ("while both were on their way")      
? 
 A: In relativity, in order for something to be at rest with respect to something else, both of them must be particles with nonzero mass in their respective rest frames, which are represented by time-like four-vectors. Saying that two photons are at rest one w.r.t another does not make sense, because there exists no inertial frame in which a photon can be at rest, photons are represented by null-like four-vectors. An inertial frame is the coordinate system of an inertial observer, who is time-like by definition. This is a reformulation of the axiom that the maximum speed of a particle is the speed of light.
Wave fronts are not particles, they are the parts of a wave with equal phase, traveling at the speed of light. As such, saying one is at rest w.r.t another does not make sense at all. You could say that they move at the same speed of propagation, though.
A: A necessary condition for mutual rest is mutual rigidity; and a necessary condition for mutual rigidity is according to Synge (Relativity: The General Theory, p. 115) operationally ("chrono-geometrically") defined as

 [...] sending photons from one [...] to another, and receiving back the scattered or reflected photons. The criterion for rigidity is that the elapsed time [duration of the signal source] from emission to return [...] should be constant. 

Any two distinct signal fronts in a beam are however not exchanging any photons (as signal fronts) between each other. Consequently they cannot be said having been rigid to each other; nor therefore having been at rest to each other.
Remark:
Synge's more complete, sufficient definition refers to:         

 [...] sending photons from one timelike curve to another [...] 

