Re: AX.25 Kernel - problem - ax25_sendmsg returns EMSGSIZE !

[Hamish Moffatt <hamish@cloud.net.au>]

On Mon, Oct 08, 2007 at 01:10:45PM +0200, Tihomir Heidelberg - 9a4gl wrote:
> Hi,
> 
> Hamish Moffatt wrote:
> > On Sun, Oct 07, 2007 at 09:58:06PM +0200, Tihomir Heidelberg - 9a4gl wrote:
> >   
> >> Using kernel 2.6.21.6 here. If you write to AX.25 socket bytes more then
> >> MTU, write will return -1 and errno will be set to 90 (EMSGSIZE =
> >> [Message too long]).
> >>     
> > Is it sensible to fragment raw AX.25 packets? I think that would depend
> > on what the next layer protocol is. 
> > For APRS, each packet is significant (ie it's datagram rather than stream oriented) so fragmenting a packet would not be correct. 
> >   
> No, only SOCK_SEQPACKET should be fragmented. APRS is using SOCK_DGRAM
> and in most cases SOCK_DGRAM should not be fragmented, but I think we

socket(7) doesn't make any distinction between SOCK_DGRAM and
SOCK_SEQPACKET with regard to fragmentation. SOCK_SEQPACKET just adds
reliability and order.

> Hm, you mean I should use SOCK_STREAM ? As I see that kind of socket is
> not supported in AX.25 stack, right ? When it would be, then it makes
> sense to fragment for SOCK_STREAM and return EMSGSIZE for SOCK_SEQPACKET.

Yes, except that it doesn't exactly make sense to have streams on a raw
socket (which is I guess why they are not supported for AX.25). Streams
would be implemented by the transport layer and above, which is above
what a raw socket provides.

> Hamish Moffatt wrote:
> > The change seems to be requested here:
> > http://oss.sgi.com/archives/netdev/2004-01/msg00097.html
> >
> > with the rationale that there is no fragmentation logic, as I suggested
> > in my other followup (which hasn't arrived back here yet...)
> But, we do have fragmentation logic in ax25_output.

So I see. I could not turn up a standard for this on Google. Perhaps I
am wrong.. Ralf seems to agree with you and so I defer to his judgement.


Hamish
-- 
Hamish Moffatt VK3SB <hamish@debian.org> <hamish@cloud.net.au>