Kernel socket structure and TCP_DIAG

sk_forward_alloc is the forward allocated memory which is the total memory currently available in the socket's quota.

sk_wmem_queued is the amount of memory used by the socket send buffer queued in the transmit queue and are either not yet sent out or not yet acknowledged.

You can learn more about TCP Memory Management in chapter 9 of TCP/IP Architecture, Design and Implementation in Linux By Sameer Seth, M. Ajaykumar Venkatesulu