Re: add SPP_PLPMTUD_ENABLE/DISABLE flag for spp_flags

[Xin Long <lucien.xin@gmail.com>]

On Wed, May 19, 2021 at 6:24 PM <mleitner@redhat.com> wrote:
>
> On Tue, May 18, 2021 at 09:19:21PM +0200, Michael Tuexen wrote:
> >
> >
> > > On 18. May 2021, at 20:33, Xin Long <lucien.xin@gmail.com> wrote:
> > >
> > > On Tue, May 18, 2021 at 1:38 PM Michael Tuexen <tuexen@freebsd.org> wrote:
> > >>
> > >>> On 18. May 2021, at 18:43, Xin Long <lucien.xin@gmail.com> wrote:
> > >>>
> > >>> Hi, Michael,
> > >>>
> > >>> We're implementing RFC8899 (PLPMTUD) on Linux SCTP recently,
> > >>> and to make this be controlled by setsockopt with
> > >>> SCTP_PEER_ADDR_PARAMS, as in
> > >>>
> > >>> https://datatracker.ietf.org/doc/html/rfc6458#section-8.1.12:
> > >>>
> > >>> we need another two flags to add for spp_flags:
> > >>>
> > >>> SPP_PLPMTUD_ENABLE
> > >>> SPP_PLPMTUD_DISABLE
> > >>>
> > >>> Do you think it makes sense? if yes, does the RFC6458 need to update?
> > >>> if not, do you have a better suggestion for it?
> > >> It is great new that you want to implement RFC 8899. I plan to do the
> > >> same for the FreeBSD stack.
> > >>
> > >> In my view, RFC 8899 is the right way to implement PMTU discovery.
> > >> So I will just use the SPP_PMTUD_ENABLE and SPP_PMTUD_DISABLE. I don't
> > >> think that the user needs to control which method is used.
> > >> I you want to support multiple versions, I would make that
> > >> controllable via a sysctl variable. But I think for FreeBSD, support
> > >> for RFC 8899 will be the only way of doing PMTU discovery. There
> > >> might be multiple choices for details like how to do the searching,
> > >> how long to wait for some events. These will be controllable via
> > >> sysctl.
> > >>
> > >> So in my view, there is no need to extend the socket API. What do you think?
> > > OK, that makes sense to me.
> > >
> > > Another thing I want to know your opinion on is:  do you think the HB
> > > should be created
> > > separately for PLPMTUD probe, instead of reusing the old HB that
> > > checks the link connectivity?
> > Yes. I think testing for connectivity is conceptually different
> > from testing a particular PMTU. When testing for PMTU, I think
> > about sending probe packets. Not that they consist of a HB chunk
> > bundled with a PAD chunk.
> > > As the HB for PLPMTUD probe might get lost, which we don't want to
> > > affect the link's
> > > connectivity.
> > Yes, I agree completely.
>
> With this, Xin, seems we should have a separate timer for the
> PROBE_TIMER, other than the heartbeat one.
>
> Otherwise, converging the two logics into one single timer is not
> worth the hassle for saving a timer. For example, we would have to
> have it fire on the active transport but to send only the probe.
> Also, considering they can and (AFAIU the RFC) should have different
> expire timeouts from time to time.
>
> With a separate timer, we won't have issues converging the
> user-selectable heartbeat interval to the recommended 600s
> PMTU_RAISE_TIMER, for example.
>
> Maybe I am missing something. But it seems the hassle for reusing the
> timer here is just not worth it. Thoughts?
You're right, when I was doing testing, I had to set the hb_interval to 5s.
the common value of hb_interval really doesn't fit in the probe_interval.
I will try adding a new timer, but at the same time a probe_interval
by sysctl/sockopt may be needed, or we can use a value equal to
(hb_interval / n) for it?

>
> Best,
> Marcelo
>
> >
> > Best regards
> > Michael
> > >
> > >>
> > >> Best regards
> > >> Michael
> > >>>
> > >>> Thanks.
> > >>
> >
>