[ECOS] max IP packet size = 9216 B. Why?

Jürgen Lambrecht J.Lambrecht@televic.com
Wed Jun 13 14:24:00 GMT 2007

Thanks Andrew! This solves it.
So the maximum supported IP packet size for an UDP socket is a configuration option. But you must be a TCP/IP networking expert to know about the option..

For anybody's information: to send/receive maximum sized UDP packets you have to setsockopt() SO_SNDBUF & SO_RCVBUF.
The maximum IP packet size is 65535 B, so the maximum UDP payload is 65507 B.
So it has no use to set SO_SNDBUF (for an UDP socket) higher than that value (65507 B).
Mark that SO_RCVBUF must be set to 65539 B to be able to receive the 65535 B IP packets (using UDP)!

For the ping test, ecos is configured OK for 65535 B IP-ping packets

I still have to test TCP, but TCP should adapt its buffers automatically.

Andrew Lunn wrote:
>>sendto returns the error code 322 or EMSGSIZE "Message too long".
>>I forgot to tell that's because the high water level of the send socket 
>>(o->so_snd.sb_hiwat) is 9216 B (=0x2400) (in file 
>>I will try to increase the socket's high water level (after I have found 
>>where it is set ;-).
The high water level is set in bsd_tcpip/current/src/sys/netinet/udp_usrreq.c line 769.
Changing it here solves my problem.
> I think you can set it with setsockopt(). The option names are
Indeed! But I though you would then change so->so_snd.sb_mbmax in stead of so->so_snd.sb_hiwat.
With those options, I indeed changed the high water level. So the ecos patch above is wrong.

> SO_SNDBUF & SO_RCVBUF. However, there might be a system check to
> ensure they are not too large.
How can you know wat the allowed maximum is? I guess that is the value of so->so_snd.sb_mbmax that was set default to 73728 (as I saw with the debugger).
That maximum is probably related to the options CYGPKG_NET_MEM_USAGE, ...
You can of course check the return value of setsockopt().

Thank you again,
>        Andrew

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