# How to define the frame size in FreeRTOS+TCP? **URL:** \ **Category:** Kernel\ **Created:** [May 24, 2019, 1:51pm UTC](https://forums.freertos.org/t/how-to-define-the-frame-size-in-freertos-tcp/7877 "2019-05-24T13:51:18Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\ **Post date:** [May 24, 2019, 1:51pm UTC](https://forums.freertos.org/t/how-to-define-the-frame-size-in-freertos-tcp/7877/1 "2019-05-24T13:51:18Z")
**[edeviser](https://sourceforge.net/u/edeviser/)** wrote on [Friday, May 24, 2019](https://sourceforge.net/p/freertos/discussion/382005/thread/60e9ef1774/#9d3e): I am running FreeRTOS+TCP and I would like to decrease the MTU. Inside the `stm32f4x7_eth_conf.h` i find this: ```auto #define ETH_RX_BUF_SIZE ETH_MAX_PACKET_SIZE /* buffer size for receive */ ``` `ETH_MAX_PACKET_SIZE` is defined in `stm32f4x7_eth.h` to 1536. Further more in `FreeRTOSIPConfig.h` i found this: ```auto #define ipconfigNETWORK_MTU 1500 ``` What is the difference between `ETH_RX_BUF_SIZE` and `ipconfigNETWORK_MTU`? Where should I reduce the frame size? ---
**Author:** ![htibosch](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/htibosch/32/62_2.png) [@htibosch](https://forums.freertos.org/u/htibosch)\ **Post date:** [May 24, 2019, 3:16pm UTC](https://forums.freertos.org/t/how-to-define-the-frame-size-in-freertos-tcp/7877/2 "2019-05-24T15:16:54Z")
**[heinbali01](https://sourceforge.net/u/heinbali01/)** wrote on [Friday, May 24, 2019](https://sourceforge.net/p/freertos/discussion/382005/thread/60e9ef1774/#6ae8): > I am running FreeRTOS+TCP and I would like to decrease the MTU. When you are running low on RAM, it is indeed a good idea to decrease MTU. But please make sure that the peripheral knows that you can not handle packets longer than a certain MTU. The MTU does not include the size of the Ethernet header, which is 14 bytes long. So an MTU 0f 1000 bytes will need 1000+14 = 1014 bytes. Sometimes the peripheral will also write a 4-byte Frame Check Sequence into the DMA buffer, so that would make it 1018 bytes. [Here](https://en.wikipedia.org/wiki/Frame_check_sequence#/media/File:Ethernet_Frame.png) is a very clear picture of a complete Ethernet packet. > ETH\_MAX\_PACKET\_SIZE is defined in stm32f4x7\_eth.h to 1536. The size of ‘1536’ is favourite because it is super well aligned: it is divisible by 32. Which is important when the DMA buffers are cached. A cache line is often 32 bytes long. > Further more in FreeRTOSIPConfig.h i found this: > `#define ipconfigNETWORK_MTU 1500` Normally 1500 is the maximum MTU on a LAN. There is a possibility to send Jumbo packets, which may become much longer. Generally, on the Internet, the supported MTU is only 1400 bytes. FreeRTOS+TCP will decrease the MSS (Maximum Segment Size) when the peer is located on the Internet. > What is the difference between ETH\_RX\_BUF\_SIZE and ipconfigNETWORK\_MTU? That should be clear by now: MTU is the nett packet size, without the Ethernet header and FCS. `ETH_RX_BUF_SIZE` is used to define the size of the DMA buffers. > Where should I reduce the frame size? So in 3 places: - Adapt `ipconfigNETWORK_MTU` - Adapt `ETH_MAX_PACKET_SIZE` - Inform the peripheral about the maximum Maximum transmission unit ---
**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\ **Post date:** [May 27, 2019, 7:05am UTC](https://forums.freertos.org/t/how-to-define-the-frame-size-in-freertos-tcp/7877/3 "2019-05-27T07:05:06Z")
**[edeviser](https://sourceforge.net/u/edeviser/)** wrote on [Monday, May 27, 2019](https://sourceforge.net/p/freertos/discussion/382005/thread/60e9ef1774/#f61b): Hello Hein! Great to hear from you and thank you for your advices and explanations! I going to test this out, soon!