On Thu, Jan 04, 2018 at 12:52:32PM +0800, Xin Long wrote:
> On Thu, Jan 4, 2018 at 6:59 AM, Marcelo Ricardo Leitner
> <marcelo.leitner@xxxxxxxxx> wrote:
> > Currently, if PMTU discovery is disabled on a given transport, but the
> > configured value is higher than the actual PMTU, it is likely that we
> > will get some icmp Frag Needed. The issue is, if PMTU discovery is
> > disabled, we won't update the information and will issue a
> > retransmission immediately, which may very well trigger another ICMP,
> > and another retransmission, leading to a loop.
> >
> > The fix is to simply not trigger immediate retransmissions if PMTU
> > discovery is disabled on the given transport.
> >
> > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
> > ---
> > net/sctp/input.c | 17 +++++++++++------
> > 1 file changed, 11 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/sctp/input.c b/net/sctp/input.c
> > index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..4a8e76f4834c90de9398455862423e598b8354a7 100644
> > --- a/net/sctp/input.c
> > +++ b/net/sctp/input.c
> > @@ -399,13 +399,18 @@ void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
> > return;
> > }
> >
> > - if (t->param_flags & SPP_PMTUD_ENABLE) {
> > - /* Update transports view of the MTU */
> > - sctp_transport_update_pmtu(t, pmtu);
> > + if (!(t->param_flags & SPP_PMTUD_ENABLE))
> > + /* We can't allow retransmitting in such case, as the
> > + * retransmission would be sized just as before, and thus we
> > + * would get another icmp, and retransmit again.
> > + */
> > + return;
> >
> > - /* Update association pmtu. */
> > - sctp_assoc_sync_pmtu(asoc);
> > - }
> > + /* Update transports view of the MTU */
> > + sctp_transport_update_pmtu(t, pmtu);
> > +
> > + /* Update association pmtu. */
> > + sctp_assoc_sync_pmtu(asoc);
> >
> > /* Retransmit with the new pmtu setting.
> > * Normally, if PMTU discovery is disabled, an ICMP Fragmentation
> > --
> > 2.14.3
> >
>
> commit 52ccb8e90c0ace233b8b740f2fc5de0dbd706b27
> Author: Frank Filz <ffilz@xxxxxxxxxx>
> Date: Thu Dec 22 11:36:46 2005 -0800
>
> [SCTP]: Update SCTP_PEER_ADDR_PARAMS socket option to the latest api draft.
>
> It seemed intended to move sctp_retransmit out of 'if (SPP_PMTUD_ENABLE) {}'
> on the above commit with some notes:
Good point.
>
> /* Retransmit with the new pmtu setting.
> * Normally, if PMTU discovery is disabled, an ICMP Fragmentation
> * Needed will never be sent, but if a message was sent before
> * PMTU discovery was disabled that was larger than the PMTU, it
> * would not be fragmented, so it must be re-transmitted fragmented.
> */
>
> But this patch is equivalent to move it back into 'if (SPP_PMTUD_ENABLE) {}'.
> will there be no regression caused?
I don't think this comment has been effective because the function
starts with:
void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
struct sctp_transport *t, __u32 pmtu)
{
if (!t || (t->pathmtu <= pmtu))
return;
So if the application managed to adjust pmtu after sending some data,
t->pathmtu will fit this check and nothing would be done anyway.
commit 91bd6b1e030266cf87d3f567b49f0fa60a7318ba
Author: Wei Yongjun <yjwei@xxxxxxxxxxxxxx>
Date: Thu Oct 23 00:59:52 2008 -0700
sctp: Drop ICMP packet too big message with MTU larger than
current PMTU
I guess I should have removed this comment too. WDYT?
I'll prepare a v3 meanwhile.
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