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TCP/IP Stack Network Buffers Allocation Schemes
and their implication on simplicity, CPU load, and throughput performance

Network Data Buffers

Data being sent to the network or received from the network is placed in network buffers. Network buffer descriptors hold information about network buffers. The descriptors are pre-allocated, whereas the network buffers themselves are allocated as they are needed.

The total number of descriptors is set by the ipconfigNUM_NETWORK_BUFFER_DESCRIPTORS constant in FreeRTOSIPConfig.h. Pre-allocating the descriptors allows the application writer to limit the maximum number of network buffers that can exist at any one time in order to prevent memory exhaustion.

Different buffer allocation schemes suite different embedded applications, so FreeRTOS+TCP keeps the buffer allocation schemes as part of the TCP/IP stack's portable layer. At the time of writing, two example buffer allocation schemes are provided - each with different trade offs between simplicity, RAM usage efficiency, and performance. The two schemes are described on this page.


The C source files that implement the buffer allocation schemes are located in the FreeRTOS-Plus/FreeRTOS_Plus_TCP/portable/BufferManagement directory. Only one scheme can be used at a time.


Buffer Allocation Schemes

Scheme 1: Implemented by BufferAllocation_1.c

Description  
  • Ethernet buffers are statically allocated by the embedded Ethernet peripheral driver (at compile time). This ensures the buffers can be aligned as required by the specific Ethernet hardware.

    BufferAllocation_1.c calls vNetworkInterfaceAllocateRAMToBuffers(), which must be provided by the peripheral driver. Information detailing the requirements of this function are provided in the Functions That Must Be Provided By The Port Layer section of the Embedded Ethernet Driver Porting documentation page.

Attributes  
  • Fast run time performance.
  • Ethernet buffers can be allocated and freed from interrupts, allowing for more efficient embedded Ethernet peripheral drivers.
  • Inefficient use of RAM - all the buffers are the same size making BufferAllocation_1.c unsuitable for some RAM constrained embedded systems.
  • More complex to configure and tune than the scheme implemented by BufferAllocation_2.
  • Simpler to achieve any special buffer alignment requirements imposed by the embedded Ethernet peripheral DMA.
  • Requires support from the network interface driver (see the description bullet points above).

Usage  
  • The ipconfigNUM_NETWORK_BUFFER_DESCRIPTORS constant in FreeRTOSIPConfig.h) defines both the total number of descriptors and the total number of buffers.
  • The ipconfigNETWORK_MTU constant (defined in FreeRTOSIPConfig.h) defines the size of each Ethernet buffer (the total size being the defined MTU size plus the number of bytes needed by the Ethernet header).


Scheme 2: Implemented by BufferAllocation_2.c

Description  
  • Ethernet buffers of exactly the required size are dynamically allocated and freed as required. This requires a fast memory allocation scheme that does not suffer from fragmentation - at the time or writing it is recommended that heap_4.c or heap_5.c is used.

Attributes  
  • Extremely easy to use.
  • Dynamic allocation results in slower run time performance when compared with the scheme implemented by BufferAllocation_1.c.
  • Ethernet buffers cannot be allocated and freed from interrupts, necessitating the use of deferred interrupt handling tasks in embedded Ethernet peripheral drivers.
  • Very efficient RAM usage - only the exact amount of RAM required is allocated making BufferAllocation_2.c particularly suited for RAM constrained small embedded systems.

Usage  
  • Ethernet buffers are allocated from the FreeRTOS heap. To avoid memory fragmentation problems, BufferAllocation_2.c can only be used reliably with a memory allocation scheme that combines adjacent free blocks of heap memory (a coalescence algorithm). The FreeRTOS memory allocation schemes implemented in heap_4.c and heap_5.c are suitable.
  • The TCP/IP stack will recover from a failed attempt to allocate a network buffer, however, as the standard heap implementation is used such a failure will result in the malloc failed hook being called (if configUSE_MALLOC_FAILED_HOOK is set to 1 in FreeRTOSConfig.h).


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