Re: add SPP_PLPMTUD_ENABLE/DISABLE flag for spp_flags

[tuexen@freebsd.org]

> On 20. May 2021, at 04:05, Xin Long <lucien.xin@gmail.com> wrote:
> 
> 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?
Yeah, this is why I did not write a socket API section for RFC 8899. I think
when implementing it, one will find some parameters which can or should
be controlled by the application. So I guess there will be more than this
one.

Timo (CCed) is working on some algorithmic explorations, since RFC 8899, gives
you some freedom. Right now, his work is done in a simulation environment, but
once that is finished and he came to some conclusions, we will implement this.

So what about:

* Try to implement PLPMTUD and figure out what you need as user controllable
  parameters.
* Bring this up for discussion and we can agree on them and also on how
  to control them via socket options.
* Then you get the code into the Linux tree.

That way we would know what parameters are really needed in an implementation
and we would have a common interface.

Does that make sense to you and sounds like an acceptable plan?

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