Re: max IP packet size = 9216 B. Why?

[Jürgen Lambrecht <J dot Lambrecht at televic dot com>]

On 06/20/2017 12:29 PM, Jürgen Lambrecht wrote:
> Hi,
>
> It is a implicit limitation of eCos: to limit the amount of RAM 
> needed, the maximum IP packet size is 9kB.
> This is hardcoded in the eCos code, so I changed it there.
> I don't remember where, but when I find it, I will mail you again.
I found it back, and it is not hard-coded.
There is a default set in 
ecos/packages/net/bsd_tcpip/current/src/sys/netinet/udp_usrreq.c.

But you can overwrite it on the socket:

  udpEchoSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
  sendBuf = 65507; //The maximum IP packet has 64KB or 65535 B. The 
maximum UDP
                   //payload is (- 20B IP  header and 8B UDP header) 65507.
  if (setsockopt(udpEchoSocket, SOL_SOCKET, SO_SNDBUF, &sendBuf, 
sizeof(sendBuf))) {
    debug_printf("<ERR> setsockopt SO_SNDBUF\n");
  }
  sendBuf = 65539; //The maximum IP packet has 64KB or 65535 B. The 
maximum UDP
  if (setsockopt(udpEchoSocket, SOL_SOCKET, SO_RCVBUF, &sendBuf, 
sizeof(sendBuf))) {
    debug_printf("<ERR> setsockopt SO_RCVBUF\n");
  }


I tried both ways (first change eCos sources, then use the ioctl calls), 
and both worked for me: I could send/receive up to 64kB IP (UDP and TCP) 
packets.

Kind regards,
Jürgen
> Kind regards,
> Jürgen
>
> On 05/04/2017 02:55 AM, lawshere@gmail.com wrote:
> > is there any patch for it,
> > no matter how i set SNDBUF, the problem is still,
> > it seems the maximum accessable size for one udp packet is the same 
> > as MTU.
> >
> > 在 2007年6月12日星期二 UTC+8下午11:49:30，Jürgen Lambrecht写道：
> >
> >     Hello,
> >
> >     I found out that the maximum IP packet size = 9216 B (0x2400).
> >     I implemented an echo program, both udp and tcp.
> >     My ecos applic can receive more that 9216B, but not send back more
> >     than
> >     that.
> >
> >     Has anybody a clue how to solve this?
> >     I need to support the maximum IP packet size of 65535 B
> >     (IP_MAXPACKET).
> >     I don't know if this is a bug in ecos, or a (hidden?) limitation
> >     of an
> >     embedded OS.
> >
> >     Thanks,
> >     Juergen Lambrecht
> >
> >     P.S.:
> >     I'm busy tracing this back in the source
> >     This comes from the socket that is allocated statically in
> >     /io/fileio/.../fd.cxx and socket is part of a cyg_file (cyg_file is
> >     defined by file in file.h). In cyg_file there is a member f_data,
> >     which
> >     is the socket.
> >     I'm continuing my search in the code at this point....
> >
> >     --     Jürgen Lambrecht
> >     R&D Engineer
> >     Televic Transport Systems
> > http://www.televic.com
> >     Televic NV / SA (main office)
> >     Leo Bekaertlaan 1
> >     B-8870 Izegem
> >     Tel: +32 (0)51 303045
> >     Fax: +32 (0)51 310670
> >
> >
> >     --     Before posting, please read the FAQ:
> > http://ecos.sourceware.org/fom/ecos
> > <http://ecos.sourceware.org/fom/ecos>
> >     and search the list archive:
> > http://ecos.sourceware.org/ml/ecos-discuss
> > <http://ecos.sourceware.org/ml/ecos-discuss>


--
Jürgen Lambrecht
R&D Associate
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Twitter: JGRLambrecht
Tel: +32 (0)51 303045    Fax: +32 (0)51 310670
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