Is a plea agreement to forgo the right to appeal a sentence enforceable?

"[W]aivers of the right to appeal a sentence, like waivers of constitutional rights, are invalid unless they are voluntary and knowing." United States v. Ready, 82 F.3d 551, 556 (2d Cir.1996) ("Ready"). Thus, a defendant's promise in a plea agreement to forgo the right to appeal a sentence is not enforceable unless "the record `clearly demonstrates' that the waiver was both knowing (in the sense that the defendant fully understood the potential consequences of his waiver) and voluntary." Id. at 557; see, e.g., id. at 557-58 (declining to enforce waiver that was not knowing and voluntary); United States v. Chen, 127 F.3d 286, 289-90 (2d Cir.1997) (same).