[mrp] Make sure not all packet buffers are used for retransmissions

On constrained devices, the number of available packet
buffers can be very low. During stress testing a device
that has 8 buffers available, which is the default on
many platforms, it was observed that the device would
receive a lot of parallel read requests and then it
would use all its buffers for responses. The responses
would then be stored in the retransmission table,
awaiting an ACK, but the ACK would never arrive because
of lack of available buffers.

Configure the retransmission table size to be less than
the number of packet buffers to make sure that not all
buffers are held by the retransmission entries, in which
case the device is unable to receive an ACK and hence it
becomes unavailable until any message times out.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

src/messaging/ReliableMessageProtocolConfig.h

@@ -130,15 +130,22 @@ namespace chip {
 #if CHIP_SYSTEM_CONFIG_USE_LWIP
 
 #if !LWIP_PBUF_FROM_CUSTOM_POOLS && PBUF_POOL_SIZE != 0
-#define CHIP_CONFIG_RMP_RETRANS_TABLE_SIZE std::min(PBUF_POOL_SIZE, CHIP_CONFIG_MAX_EXCHANGE_CONTEXTS)
+// Configure the table size to be less than the number of packet buffers to make sure
+// that not all buffers are held by the retransmission entries, in which case the device
+// is unable to receive an ACK and hence becomes unavailable until a message times out.
+#define CHIP_CONFIG_RMP_RETRANS_TABLE_SIZE std::min(PBUF_POOL_SIZE - 1, CHIP_CONFIG_MAX_EXCHANGE_CONTEXTS)
 #else
 #define CHIP_CONFIG_RMP_RETRANS_TABLE_SIZE CHIP_CONFIG_MAX_EXCHANGE_CONTEXTS
 #endif // !LWIP_PBUF_FROM_CUSTOM_POOLS && PBUF_POOL_SIZE != 0
 
 #else // CHIP_SYSTEM_CONFIG_USE_LWIP
 
 #if CHIP_SYSTEM_CONFIG_PACKETBUFFER_POOL_SIZE != 0
-#define CHIP_CONFIG_RMP_RETRANS_TABLE_SIZE std::min(CHIP_SYSTEM_CONFIG_PACKETBUFFER_POOL_SIZE, CHIP_CONFIG_MAX_EXCHANGE_CONTEXTS)
+// Configure the table size to be less than the number of packet buffers to make sure
+// that not all buffers are held by the retransmission entries, in which case the device
+// is unable to receive an ACK and hence becomes unavailable until a message times out.
+#define CHIP_CONFIG_RMP_RETRANS_TABLE_SIZE                                                                                         \
+    std::min(CHIP_SYSTEM_CONFIG_PACKETBUFFER_POOL_SIZE - 1, CHIP_CONFIG_MAX_EXCHANGE_CONTEXTS)
 #else
 #define CHIP_CONFIG_RMP_RETRANS_TABLE_SIZE CHIP_CONFIG_MAX_EXCHANGE_CONTEXTS
 #endif // CHIP_SYSTEM_CONFIG_PACKETBUFFER_POOL_SIZE != 0