A number of environment variables control the packet buffer allocation on a per stack basis. Refer to Parameter Reference for a description of EF_MAX_PACKETS.
Unless explicitly configured by the user, EF_MAX_RX_PACKETS and EF_MAX_TX_PACKETS will be automatically set to 75% of the EF_MAX_PACKETS value. This ensures that sufficient buffers are available to both receive and transmit. The EF_MAX_RX_PACKETS and EF_MAX_TX_PACKETS are not typically configured by the user.
If an application requires more packet buffers than the maximum configured, then EF_MAX_PACKETS can be increased to meet demand, however it should be recognized that larger packet buffer queues increase cache footprint which can lead to reduced throughput and increased latency.
EF_MAX_PACKETS is the maximum number of packet buffers that could be used by the stack. Setting EF_MAX_RX_PACKETS to a value greater than EF_MAX_PACKETS effectively means that all packet buffers (EF_MAX_PACKETS) allocated to the stack will be used for RX - with nothing left for TX. The safest method is to only increase EF_MAX_PACKETS which keeps the RX and TX packet buffers values at 75% of this value.