Message-ID: <7627f7d5-89d5-487a-938a-5156be9d4fbd@6wind.com>
Date: Fri, 3 Nov 2023 15:21:55 +0100
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Yang Sun <sunytt@...gle.com>
Cc: Ido Schimmel <idosch@...sch.org>, davem@...emloft.net,
 dsahern@...nel.org, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
 netdev@...r.kernel.org
Subject: Re: [PATCH] net: ipmr_base: Check iif when returning a (*, G) MFC

Le 03/11/2023 à 12:05, Yang Sun a écrit :
> On Thu, Nov 2, 2023 at 10:19 PM Nicolas Dichtel
> <nicolas.dichtel@...nd.com> wrote:
>>
>> Le 02/11/2023 à 12:48, Yang Sun a écrit :
>>>> Is this a regression (doesn't seem that way)? If not, the change should
>>>> be targeted at net-next which is closed right now:
>>>
>>>> https://www.kernel.org/doc/html/latest/process/maintainer-netdev.html
>>>
>>> I see.
>>>
>>>>> - if (c->mfc_un.res.ttls[vifi] < 255)
>>>>> + if (c->mfc_parent == vifi && c->mfc_un.res.ttls[vifi] < 255)
>>>
>>>> What happens if the route doesn't have an iif (-1)? It won't match
>>>> anymore?
>>>
>>> Looks like the mfc_parent can't be -1? There is the check:
>>>     if (mfc->mf6cc_parent >= MAXMIFS)
>>>         return -ENFILE;
>>> before setting the parent:
>>>     c->_c.mfc_parent = mfc->mf6cc_parent;
>>>
>>> I wrote this patch thinking (*, G) MFCs could be per iif, similar to the
>>> (S, G) MFCs, like we can add the following MFCs to forward packets from
>>> any address with group destination ff05::aa from if1 to if2, and forward
>>> packets from any address with group destination ff05::aa from if2 to
>>> both if1 and if3.
>>>
>>> (::, ff05::aa)      Iif: if1 Oifs: if1 if2  State: resolved
>>> (::, ff05::aa)      Iif: if2 Oifs: if1 if2 if3  State: resolved
>>>
>>> But reading Nicolas's initial commit message again, it seems to me that
>>> (*, G) has to be used together with (*, *) and there should be only one
>>> (*, G) entry per group address and include all relevant interfaces in
>>> the oifs? Like the following:
>>>
>>> (::, ::)         Iif: if1 Oifs: if1 if2 if3   State: resolved
>>> (::, ff05::aa)   Iif: if1 Oifs: if1 if2 if3   State: resolved
>>>
>>> Is this how the (*, *|G) MFCs are intended to be used? which means packets
>>> to ff05::aa are forwarded from any one of the interfaces to all the other
>>> interfaces? If this is the intended way it works then my patch would break
>>> things and should be rejected.
>> Yes, this was the intend. Only one (*, G) entry was expected (per G).
>>
>>>
>>> Is there a way to achieve the use case I described above? Like having
>>> different oifs for different iif?
>> Instead of being too strict, maybe you could try to return the 'best' entry.
>>
>> #1 (::, ff05::aa)      Iif: if1 Oifs: if1 if2  State: resolved
>> #2 (::, ff05::aa)      Iif: if2 Oifs: if1 if2 if3  State: resolved
>>
>> If a packet comes from if2, returns #2, but if a packet comes from if3, returns
>> the first matching entry, ie #1 here.
>>
> 
> Thanks for your reply Nicolas!
> Here if it returns the first matching then it depends on which entry
> is returned first
> by the hash table lookup, the forwarding behavior may be indeterminate
> in that case
> it seems.
As I said, only one (*,G) entry was expected thus the 'first' one is
indeterminate if there are several entries.

> 
> If a packet has no matching (*, G) entry, then it will use the (*, *)
> entry to be forwarded
> to the upstream interface in (*, *). And with the (*, *) it means we
> won't get any nocache upcall
> for interfaces included in the static tree, right? So the (S, G) MFC
> and the static proxy MFCs
> are not meant to be used together?
Not together. With proxy multicast, the multicast tree is static, ie there is no
multicast daemon. Mcast packets received from one interface are sent to the
other interfaces that are part of the tree.


Regards,
Nicolas

> 
> I wonder how a real use case with (*, G|*) would look like, what
> interface could be an
> upstream interface. Is there an example?
> 
> Thanks,
> Yang
> 
>>
>> Regards,
>> Nicolas

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