Re: [lwip-users] STM32F4+lwIP: TCP MSS sets to 536 bytes on its own

[address@hidden]

On 21.06.2018 17:52, Jagdish wrote:
> Hi,
> Am using lwIP stack with STM32F4 series micro-controller in one of project
> board. Trying to connect this board to pc based application. I observed,
> When host sends packet of size 1088 bytes, the packet gets divided into 2.
> Please refer the packet where length column shows 590 and 552.

If you want a packet trace to be analyzed, send a packet trace, not a 
picture. In the case of a tcp transfer, be sure to include the start of 
the connection and describe as much what you think you are seeing (e.g. 
OS to IP etc.).

> In lwiopts.h, i have set TCP_MSS to 1460. Also the initial TCP handshake
> shows the MSS is 1460 from both PC and stm32f4 board.
> My understanding is that if in initial handshake the MSS is 1460. Host must
> not divide the packet in 2 segments restricting the length of packet to 536
> bytes.

No, "must not" is wrong here. A remote host can send at any small size 
it wants. Note the "M" in MSS, which stands for "maximum". There's no 
minimum defined.

> Kindly help me to understand and set this right. Is there a configuration in
> lwIP or on PC side that i am missing?

You could start by telling us which side is sending smaller segments, is 
it lwIP or the PC? Which OS is the PC running?


Simon