lwIP 2.1.2 Lightweight IP stack

Here is a list of all documented struct and union fields with links to the struct/union documentation for each field:

### - r -

- r : api_msg
- rcvevent : lwip_sock
- reachability : sntp_server
- read : tftp_context
- readset : lwip_select_cb
- reass_list : lowpan6_ieee802154_data
- recv : raw_pcb , udp_pcb
- recv_arg : udp_pcb
- recv_avail : netconn
- recv_bufsize : netconn
- recv_unicast : mdns_packet
- recvmbox : netconn
- ref : pbuf
- reference : snmp_node_instance
- reference_len : snmp_node_instance
- release_instance : snmp_node_instance
- remove_callback : netif
- rs_count : netif