On Wed, 15 Jun 2016, Alan Davey wrote:
The only case that would break is that where an application relies on the existing (documented as a bug) feature of getting an EMSGSIZE return code in the case of an over-sized packet. Applications that perform their own fragmentation would be unaffected.
If this doesn't break existing applications that are doing fragmentation in userspace on raw sockets (e.g. Quagga ospfd), that's better.
As per previous email, I'd love to be able to get rid of that code and have the kernel do it for me. However, I also don't want to have to do anything other non-trivial to that code either. :)
The issue for us is, how would we know on any given host whether the kernel will do the fragmentation or whether ospfd has to do it? We need to be able to probe for that capability, surely?
I guess "send an oversized packet and see if we get EMSGSIZE comes back or not" could be one way, though the 'man 7 raw' man page ony my system says that is returned on raw socket if the /IP/ max size (64 KiB) is exceeded - it doesn't say anything about that being returned if output MTU is exceeded. Also, that implies a failed packet send on startup on older kernels and having to rejiggle the packet we've constructed to retry.
Some kind of "I want kernel to do the fragmentation" sockopt, that was guaranteed to error with ENOPROTOOPT on non-implementing kernels, that we could try set when creating the socket, would seem simpler/nicer from my POV of my corner of user-space.
? regards, -- Paul Jakma | p...@jakma.org | @pjakma | Key ID: 0xD86BF79464A2FF6A Fortune: It would be nice to be sure of anything the way some people are of everything.
