Re: [PATCH] sctp: Fix sending when PMTU is less than SCTP_DEFAULT_MINSEGMENT

[Jakub Kicinski]

On Fri, 6 Nov 2020 07:21:06 -0300 Marcelo Ricardo Leitner wrote:
> On Fri, Nov 06, 2020 at 10:48:24AM +0100, Petr Malat wrote:
> > On Fri, Nov 06, 2020 at 05:46:34AM -0300, Marcelo Ricardo Leitner wrote:  
> > > On Thu, Nov 05, 2020 at 11:39:47AM +0100, Petr Malat wrote:  
> > > > Function sctp_dst_mtu() never returns lower MTU than
> > > > SCTP_TRUNC4(SCTP_DEFAULT_MINSEGMENT) even when the actual MTU is less,
> > > > in which case we rely on the IP fragmentation and must enable it.  
> > > 
> > > This should be being handled at sctp_packet_will_fit():  
> > 
> > sctp_packet_will_fit() does something a little bit different, it
> > allows fragmentation, if the packet must be longer than the pathmtu
> > set in SCTP structures, which is never less than 512 (see
> > sctp_dst_mtu()) even when the actual mtu is less than 512.
> > 
> > One can test it by setting mtu of an interface to e.g. 300,
> > and sending a longer packet (e.g. 400B):  
> > >           psize = packet->size;
> > >           if (packet->transport->asoc)
> > >                   pmtu = packet->transport->asoc->pathmtu;
> > >           else
> > >                   pmtu = packet->transport->pathmtu;  
> > here the returned pmtu will be 512  
> 
> Thing is, your patch is using the same vars to check for it:
> +       pmtu = tp->asoc ? tp->asoc->pathmtu : tp->pathmtu;
> 
> >   
> > > 
> > >           /* Decide if we need to fragment or resubmit later. */
> > >           if (psize + chunk_len > pmtu) {  
> > This branch will not be taken as the packet length is less then 512  
> 
> Right, ok. While then your patch will catch it because pmtu will be
> SCTP_DEFAULT_MINSEGMENT, as it is checking with '<='.
> 
> >   
> > >            }
> > >   
> > And the whole function will return SCTP_XMIT_OK without setting
> > ipfragok.
> > 
> > I think the idea of never going bellow 512 in sctp_dst_mtu() is to
> > reduce overhead of SCTP headers, which is fine, but when we do that,
> > we must be sure to allow the IP fragmentation, which is currently
> > missing.  
> 
> Hmm. ip frag is probably just worse than higher header/payload
> overhead.
> 
> > 
> > The other option would be to keep track of the real MTU in pathmtu
> > and perform max(512, pathmtu) in sctp_packet_will_fit() function.  
> 
> I need to check where this 512 came from. I don't recall it from top
> of my head and it's from before git history. Maybe we should just drop
> this limit, if it's artificial. IPV4_MIN_MTU is 68.
> 
> > 
> > Not sure when exactly this got broken, but using MTU less than 512
> > used to work in 4.9.  
> 
> Uhh, that's a bit old already. If you could narrow it down, that would
> be nice.

I'm dropping this from patchwork, if you conclude that the patch is
good as is please repost, thanks!