Re: [lwip-users] netconn_write throughput stuck at 1024 bytes

[Jonathan Larmour]

DownyTif wrote:
> first, I want to say that I'm new to this lwip world. I'm trying to make a
> Ethernet speed test using the EVK1100 board from ATMEL (AVR32). The
> communication will be between the board and a PC using 2 ports, one
> dedicated to bidir requests/responses between the board and the host, the
> other dedicated to maximum speed data sending from the board to the host. I
> would like to be able to send packets of 32768 bytes.

Do you really mean "packets" there? Is this ethernet? Ethernet's normal 
MTU is around 1500 bytes, unless you're going to be using jumbo frames. 
You can send larger IP packets, but they get fragmented down to the MTU 
size, which is inefficient.

> Presently, for some reason, I can't send packets bigger than 1024 bytes.

Do you mean the data doesn't get sent at all, or it gets sent, but not all 
2048 bytes in one packet on the wire?

> Even if I write this:
> writeRet = netconn_write(pNewConnection, g_pSDRAM, 2048, NETCONN_COPY);
>
> what I get on the PC is 1024-byte packets. Tracing in the code, I found that
> my connection is correctly configured ("pcb->mss" is 2048) but for some
> reason, "pcb->mss" becomes 1460 somewhere (found by tracing in tcp_out.c). 

pcb->mss is set according to the MSS provided by the remote host when 
establishing the TCP connection. The remote host is probably setting its 
MSS option to 1460. Use a packet sniffer on the PC to see what's going on 
(tcpdump, ethereal, wireshark, etc.). You would need to configure your 
remote host's TCP stack to indicate a larger MSS.

Jifl
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