Re: add SPP_PLPMTUD_ENABLE/DISABLE flag for spp_flags

[Michael Tuexen <tuexen@freebsd.org>]

> 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.

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.
>>>>>> 
>>> 
>> 
>