netdev - Re: [PATCH net-next v3] net/ipv6: Introduce accept_unsolicited_na knob to implement router-side changes for RFC9131

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Date:   Fri, 20 May 2022 20:00:23 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Arun Ajith S <aajith@...sta.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        yoshfuji@...ux-ipv6.org, kuba@...nel.org, pabeni@...hat.com,
        corbet@....net, prestwoj@...il.com, gilligan@...sta.com,
        noureddine@...sta.com, gk@...sta.com
Subject: Re: [PATCH net-next v3] net/ipv6: Introduce accept_unsolicited_na
 knob to implement router-side changes for RFC9131

On 5/20/22 1:19 AM, Arun Ajith S wrote:
> On Thu, Apr 14, 2022 at 3:37 AM David Ahern <dsahern@...nel.org> wrote:
>>
>> On 4/13/22 8:34 AM, Arun Ajith S wrote:
>>> diff --git a/tools/testing/selftests/net/ndisc_unsolicited_na_test.py b/tools/testing/selftests/net/ndisc_unsolicited_na_test.py
>>> new file mode 100755
>>> index 000000000000..f508657ee126
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/net/ndisc_unsolicited_na_test.py
>>> @@ -0,0 +1,255 @@
>>> +#!/bin/bash
>>
>> that file name suffix should be .sh since it is a bash script; not .py
>>
>> other than that looks good to me.
>>
>> Reviewed-by: David Ahern <dsahern@...nel.org>
> 
> Hi David,
> 
> It has been pointed out to me that I might have read RFC9131 in a
> narrower sense than what was intended.
> The behavior of adding a new entry in the neighbour cache on receiving
> a NA if none exists presently
> shouldn't be limited to unsolicited NAs like in my original patch,
> rather it should extend to all NAs.
> 
> I am quoting from the RFC below
> 
>    |  When a valid Neighbor Advertisement is received (either solicited
>    |  or unsolicited), the Neighbor Cache is searched for the target's
>    |  entry.  If no entry exists:
>    |
>    |  *  Hosts SHOULD silently discard the advertisement.  There is no
>    |     need to create an entry if none exists, since the recipient has
>    |     apparently not initiated any communication with the target.
>    |
>    |  *  Routers SHOULD create a new entry for the target address with
>    |     the link-layer address set to the Target Link-Layer Address
>    |     Option (if supplied).  The entry's reachability state MUST be
>    |     set to STALE.  If the received Neighbor Advertisement does not
>    |     contain the Target Link-Layer Address Option, the advertisement
>    |     SHOULD be silently discarded.
> 
> I want to fix this, but this would mean the sysctl name
> accept_unsolicited_na is no longer appropriate
> I see that the net-next window for 5.19 is still open and changing the
> sysctl name
> wouldn't mean changing an existing interface.
> I was thinking of renaming the sysctl to accept_untracked_na to
> highlight that we are accepting NAs even if there is
> no corresponding entry tracked in the neighbor cache.
> 
> Also, there's an error in my comment, where I say "pass up the stack"
> as we don't pass NAs up the stack.
> The comment can be updated as:
>         /* RFC 9131 updates original Neighbour Discovery RFC 4861.
>          * NAs with Target LL Address option without a corresponding
>          * entry in the neighbour cache can now create a STALE neighbour
>          * cache entry on routers.
>          *
>          *   entry accept  fwding  solicited        behaviour
>          * ------- ------  ------  ---------    ----------------------
>          * present      X       X         0     Set state to STALE
>          * present      X       X         1     Set state to REACHABLE
>          *  absent      0       X         X     Do nothing
>          *  absent      1       0         X     Do nothing
>          *  absent      1       1         X     Add a new STALE entry
>          */
> 
> In summary
> 1. accept=0 keeps original(5.18) behavior for all cases.
> 2. accept=1 changes original behavior for entry=asbent, fwding=1 case
> provided the NA had specified target link-layer address.
> 
> Please let me know what you think.
> 

Changes can be made until it is in a released kernel to users. This
feature has many weeks before it hits that level.