Re: add SPP_PLPMTUD_ENABLE/DISABLE flag for spp_flags

[Marcelo Ricardo Leitner <mleitner@redhat.com>]

On Thu, May 20, 2021 at 08:59:49AM +0200, tuexen@freebsd.org wrote:
> > On 20. May 2021, at 02:45, Marcelo Ricardo Leitner <mleitner@redhat.com> wrote:
> >
> > On Thu, May 20, 2021 at 01:16:38AM +0200, Michael Tuexen wrote:
> >>> On 20. May 2021, at 00:44, mleitner@redhat.com wrote:
> >>>
> >>> On Wed, May 19, 2021 at 02:44:20PM -0400, Xin Long wrote:
> >>>> On Wed, May 19, 2021 at 2:15 PM Michael Tuexen <tuexen@freebsd.org> wrote:
> >>>>>
> >>>>>> On 19. May 2021, at 18:18, Xin Long <lucien.xin@gmail.com> wrote:
> >>>>>>
> >>>>>> On Tue, May 18, 2021 at 2:33 PM 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?
> >>>>>> I just noticed that with multiple versions supported, and without extending
> >>>>>> this API, all applications will have to use the same version as it's
> >>>>>> controlled by
> >>>>>> sysctl. And when switching to another version by sysctl, all
> >>>>>> applications will be
> >>>>>> affected and have to do the switch. that seems not nice.
> >>>>> That is true, but an application can not expect any specific behaviour
> >>>>> right now when they are not disabling PMTUD.
> >>>>>
> >>>>> What about adding a sysctl variable, which defines the default
> >>>>> algorithm and a socket option, which allows to get and set
> >>>>> the algorithm being used.
> >>>> yes, that's also what I'm thinking.
> >>>
> >>> +1
> >>>
> >>>> sysctl is always used for the default value for future sockets.
> >>>> and the socket option should be added for a socket/asoc's setting.
> >>>
> >>> Speaking of inheritance, it should also use the SCTP_FUTURE_ASSOC /
> >>> SCTP_CURRENT_ASSOC / SCTP_ALL_ASSOC mechanism. Like
> >>> SCTP_PEER_ADDR_PARAMS, for example.
> >> Yepp.
> >>>
> >>> The system can provide defaults but if the application requires
> >>> something, it should have a good way of requesting it.
> >>>
> >>> Speaking of SCTP_PEER_ADDR_PARAMS, maybe reuse spp_pathmtu field?
> >>> As in, if SPP_PMTUD_ENABLE is enabled, spp_pathmtu of "1" or "2" bytes
> >>> doesn't make sense, and it could mean the algorithm used. Thing is,
> >>> the field is currently ignored, and it could lead to some unexpected
> >>> behavior change. It's probably safer to just add another sockopt, but
> >>> wanted to share the idea anyway.
> >> I leave it completely up to you what you implement in Linux. But I
> >> would prefer to use a separate socket option instead of overloading
> >> an existing one.
> >
> > Wait. Somehow I thought we were talking about extending the RFC with
> > these new definitions here, no? Or at least agreeing on a common
> > interface. It would be beneficial for the application to be able to
> > use the same API on FreeBSD or Linux.
> Hi Marcelo,

Hi!

>
> sorry for not being clear.
>
> What I wanted to say:
>
> 1. I really appreciate the discussion and I agree that it would be great

+1

>    if we can agree on a common interface allowing to write portable
>    applications.
>
> 2. I don't like the idea of overloading the spp_pathmtu.

Me neither. :D

>
> 3. I'm not in a position to put in a veto to what anyone is implementing
>    in any implementation (except maybe the FreeBSD implementation).
>
> Regarding the extension of the RFC. An RFC can't be changed. One can file
> erratas, but I think we are discussing here an extension of the socket API
> to cope with RFC 8899. So I don't think it is an errata. It would have been
> appropriate to add a socket API section to RFC 8899, but it is too late for
> that, too.
>
> So I guess we can discuss it here and come to an agreement how to extend
> the socket API for RFC 8899. I'm more that happy to do this.

Nice. Okay.

>
> I hope I expressed my view now clearer.

Yep. On the same page now, thanks. :-)

Best regards,
Marcelo

>
> Best regards
> Michael
>
>
> >
> > Thanks,
> > Marcelo
> >
> >>
> >> Best regards
> >> Michael
> >>>
> >>>>
> >>>> SCTP_PTMUD_METHOD?
> >>>
> >>> s/PTMUD/PMTUD/ :-)
> >>>
> >>>> 0: PTB one
> >>>> 1. PLPMTUD
> >>>>
> >>>>>
> >>>>> Best regards
> >>>>> Michael
> >>>>>>
> >>>>>>> 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?
> >>>>>>> As the HB for PLPMTUD probe might get lost, which we don't want to
> >>>>>>> affect the link's
> >>>>>>> connectivity.
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Best regards
> >>>>>>>> Michael
> >>>>>>>>>
> >>>>>>>>> Thanks.
> >>>>>>>>
> >>>>>
> >>>>
> >>>
> >>
> >
>