  Z sources are bright neutron-star X-ray binaries, accreting at around the
Eddington limit. We analyze the 68 RXTE observations (270 ks) of Sco-like Z
source GX 17+2 made between 1999 October 3-12, covering a complete Z track. We
create and fit color-resolved spectra with a model consisting of a thermal
multicolor disk, a single-temperature-blackbody boundary layer and a weak
Comptonized component. We find that, similar to what was observed for XTE
J1701-462 in its Sco-like Z phase, the branches of GX 17+2 can be explained by
three processes operating at a constant accretion rate Mdot into the disk:
increase of Comptonization up the horizontal branch, transition from a standard
thin disk to a slim disk up the normal branch, and temporary fast decrease of
the inner disk radius up the flaring branch. We also model the Comptonization
in an empirically self-consistent way, with its seed photons tied to the
thermal disk component and corrected for to recover the pre-Comptonized thermal
disk emission. This allows us to show a constant Mdot along the entire Z track
based on the thermal disk component. We also measure the upper kHz QPO
frequency and find it to depend on the apparent inner disk radius R_in (prior
to Compton scattering) approximately as frequency \propto R_in^(-3/2),
supporting the idenfitication of it as the Keplerian frequency at R_in. The
horizontal branch oscillation is probably related to the dynamics in the inner
disk as well, as both its frequency and R_in vary significantly on the
horizontal branch but become relatively constant on the normal branch.
