Missing ProtectionLevel property on ServiceContractAttribute

I have installed the Nuget package `System.ServiceModel.NetTcp` , trying to invoke an existing WCF operation from a .Net Core 2.1 project.

The service's contract (that i do not cotrol) specifies `ProtectionLevel.EncryptAndSign` at the contract level.
I can see the `ProtectionLevel` property in the source code of `ServiceContractAttribute` in this repo, and also in the decomplied assembly on my hard drive, but the compiler reports the classic

> "The Type or namespace 'ProtectionLevel' could not be found"

The Enum `ProtectionLevel` itself is known to the compiler.

Using VS 2017 (15.9).