platform_network_heap is a heap that is primarily used by the network library to hold TCP segments for transmission by the MAC. If you need to allocate small control structures, you should use the system heap, platform_system_heap. TCP segments in the network heap are fleeting, being created, handed to the MAC for transmission, and freed. With a quiescent network, the network heap will most likely be entirely empty and, therefore, not fragmented.
platform_network_heap is initialized by platform_configure_nic.