If you attempt to set TCP_SACKPERMITTED with setsockopt() on a socket that already has SACK enabled, NDK_setsockopt() will set fderror to NDK_EALREADY. 

However if you call setsockopt() from NS it will go through NDK's slnetifndk layer. Here the NDK_EALREADY error is not properly translated and sent to NS. 

To fix this getBSDSlNetErr() in slnetifndk.c needs to add the following code 

         case NDK_EALREADY:
            ret = SLNETERR_BSD_EALREADY_ENABLED;
            break;