 recovering clock over the PTS network is only used upon failure of the local time source. Performance characteristics of the T-TSC-A are covered in recommenda- tion G.8273.4. Note that when the clock is either a T-TSC-P or T-TSC-A, it is an ordinary clock with either a single slave port or alternatively with multiple PTP ports, which according to IEEE 1588 definitions makes it a boundary clock. On a T-TSC with multiple PTP ports, only one port can be in a slave state at any one time, and no port can go to master state. The main features of this profile are as follows: Domain number should be within a range of 44-63 with a default value of 44. Unicast UDP over IPv4 (Annex D of 1588-2008), and optionally PTP over IPv6 (Annex E of 1588-2008), is used to transport the PTP messages. Message rates for Sync, Delay_Req, and Delay_Resp are from a minimum of 1 packet per second to 128 per second. The message rate for Announce is from a minimum of 1 packet per second to a maximum of 8 per second. Unicast message negotiation-a mechanism allowing slaves to request timing service from a master in a unicast environment (see the earlier section "Negotiation" in this chapter and clause 16.1 of 1588-2008). The default interval to request service is 300 seconds, with a range of 60- 1000 seconds. Defines an alternate BCMA (aBMCA), allowing PTP ports to be dynamically switched between master and slave states, which enables the profile to support automatic configuration. See sec- tion 6.7.1, "Alternate BMCA," and Annex B of G.8275.2 for further details on using an aB- MCA to establish the PTP hierarchy. The mechanisms of G.8275.1 and G.8275.2 are quite sim- ilar; however, the effect of using a PTS network to transport messages means that the outcome of this selection process might not be optimal, since not every upstream node is processing PTP (and the aBMCA has no visibility of that topology). One-step and two-step clocks are both allowed, with the master free to use whatever method it prefers, and the slave required to accept eithe