It is not necessary to realize timeout handling in send function or to disconnect from AP if lwIP packet buffer is exhausted. Waiting that the frame allocated in lwIP packet buffer is freed by MAC layer led to the complete blockage of send function on heavy network load. Disconnecting from AP is counterproductive since reconnecting usually fails on heavy network load.
Disconnecting from the AP in the send function if the lwIP packet buffer is exhausted is counterproductive since reconnecting usually fails on heavy network load. A better strategy is to slow down the sending of MAC frames from netif a bit to wait for flushing the buffer in the MAC layer.
Defines a number of lwIP functions that are required as symbols by Espressif's SDK libraries. These functions are only dummies without real functionality. Using these functions instead of real lwIP functions provided with the SDK saves arround 4 kBytes of RAM.
Since _esp_wifi_recv_cb is not executed in interrupt context but in the context of the `ets` thread, it is not necessary to pass the`NETDEV_EVENT_ISR` event first. Instead, the receive function can be called directly which result in much faster handling, a less frame lost rate and more robustness.
Since _esp_wifi_recv_cb is not executed in interrupt context but in the context of the `ets` thread, the receive function can be called directly. There is no need for a mutex anymore to synchronize the access to the receive buffer between _esp_wifi_recv_cb and _recv function.
Once the packet buffer is full on heavy network load, gnrc_netif_hdr_build may return NULL. In that case, the following unchecked access to hdr->data leads to a crash.
`gnrc_sixlowpan_frag` internally derives the offset value directly
from the fragment header, so for normal usage within GNRC this
assertion is redundant, but to make the tests of `rbuf_add` 100%
water-tide I added it.
Currently the loop just continues to run after a viable type is found.
In #10851 this lead to a crash of the tests, when the dependency of
`gnrc_sixlowpan` to `gnrc_ipv6` was removed.
When a new queue entry is tried to be allocated for a neighbor who's
address is currently tried to be resolved there was no error case
before. The packet that was tried to be put in the queue was thus not
released and stayed in the packet buffer for ever.
