Re: [lwip-users] how to change MTU

[Simon Goldschmidt]

Ethernet is normally limited to 1500 bytes of payload (starting at the IP 
header). To send bigger frames, your netif driver has to support this. I think 
the defines below should then be enough, although TCP limits the size of sent 
frames to the MSS received by the remote side (included as option in SYN 
packets).

Simon


PHAM ANH THIEN wrote:
> dear all,
> 
> i want to send a packet more than 1500 bytes, in my case is 2048 bytes,
> after sending i see the below message:
> 
> Sending data...
> tcp_write(pcb=4000a718, data=400057a4, len=2048, apiflags=1)
> tcp_enqueue(pcb=4000a718, arg=400057a4, len=2048, flags=0, apiflags=1)
> tcp_enqueue: queueing 6511:7971 (0x0)
> tcp_enqueue: queueing 7971:8559 (0x0)
> tcp_output_segment: 6511:7971
>    0:.tcp_write(pcb=4000a718, data=400057a4, len=952, apiflags=1)
> tcp_enqueue(pcb=4000a718, arg=400057a4, len=952, flags=0, apiflags=1)
> tcp_enqueue: queueing 8559:9511 (0x0)
> tcp_output_segment: 7971:8559
> 
> this mean that i sent 2048 bytes successfully already, but when i use
> wireshark application (network analyzer application) i only see a packet
> with 588-byte length sent. I changed MTU (netif->mtu) from 1500 to 3000
> and
> SND_BUF, TCP_MSS, TCP_SND_BUF, TCP_WND, ETH_FRAG_SIZE, ETH_MAX_FLEN from
> 1500 to 3000 as well.
> 
> Anyone know how to do it? please advise
> 
> thank you very much.
> Thien

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