Re: add SPP_PLPMTUD_ENABLE/DISABLE flag for spp_flags

[Michael Tuexen <tuexen@freebsd.org>]

> On 19. May 2021, at 20:44, Xin Long <lucien.xin@gmail.com> 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.
> sysctl is always used for the default value for future sockets.
> and the socket option should be added for a socket/asoc's setting.
> 
> SCTP_PTMUD_METHOD?
OK.
> 0: PTB one
I don't know what the above would mean. Not sure anything is really specified.
> 1. PLPMTUD
I guess you would need an struct sctp_assoc_value here.

Maybe some constants such as:
SCTP_PMTU_CLASSIC
SCTP_PMTU_NEW
or
SCTP_PMTU_RFC_8899

Best regards
Michael
> 
>> 
>> 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.
>>>>> 
>>