Air carrier Captain reported encountering GPS jamming over the Black Sea.

Narrative

In cruise flight over the Black Sea we began receiving the EICAS message Transponder R. That is an information only message; with no checklist. We had discussed the likelihood of GPS jamming and spoofing along this route and had referenced a bulletin. We monitored the RNP/ANP on CDU (Control Display Unit) position reference page 2/4. The FMC position update status on the ND (Navigation Display) changed from GPS over IRS (Inertial Reference System) to DD (DME/DME) over IRS. Since ANP was maintaining below RNP we continued the flight; assuming the GPS status would return to normal after we left the airspace where the interference was occurring. I was in the bunk on rest approximately 4 hours later when the crew woke me up to advise they had received the UNABLE RNP EICAS message shortly after entering Chinese airspace at SARIN. The Chinese controller had questioned them about their position and had canceled their offset in response to their reply that they were unable to maintain the RNP. ATC had asked if they intended to continue along their cleared route; to which they responded affirmative. By the time I got to the flight deck the ANP was returning to the RNP; which it regained within a couple of minutes. At that point we checked the CDU position reference pages and saw their was no signal from the right GPS; but the left GPS was indicating 700 miles off the FMC generated position. We verified GPS updating was selected off. During this time the DD on the ND's FMC position update status was coming and going intermittently. We looked at the navigation requirements for the remainder of our route and cross checked with the manual limitations section to verify we were in compliance for RNAV 2 operations. I messaged dispatch to update them on the situation. We continued the rest of the flight monitoring the ANP. I wrote up the EICAS messages in the maintenance logbook as well as the ATC RIGHT status message.Cause: GPS jamming and spoofing.