netdev - Re: [PATCH net-next 2/2] l2tp: add peer

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Date:   Tue, 2 Jan 2018 20:28:03 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Guillaume Nault <g.nault@...halink.fr>
Cc:     James Chapman <jchapman@...alix.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Hangbin Liu <liuhangbin@...il.com>
Subject: Re: [PATCH net-next 2/2] l2tp: add peer_offset parameter

>> > Lorenzo, is this being added to fix interoperability with another L2TPv3
>> > implementation? If so, can you share more details?
>> >
>>
>> Hi James,
>>
>> I introduced peer_offset parameter to fix a specific setup where
>> tunnel endpoints
>> running L2TPv3 would use different values for tx offset (since in
>> iproute2 there is no
>> restriction on it), not to fix a given an interoperability issue.
>>
> Yes, but was it just to test iproute2's peer_offset option? Or is there
> a plan to use it for real?
>
>> I introduced this feature since:
>>  - offset has been added for long time to L2TPv3 implementation
>>    (commit f7faffa3ff8ef6ae712ef16312b8a2aa7a1c95fe and
>>    commit 309795f4bec2d69cd507a631f82065c2198a0825) and I wanted to
>> preserve UABI
>>  - have the same degree of freedom for offset parameter we have in
>> L2TPv2 and fix the issue
>>    described above
>>
> AFAIU, the current L2TPv2 implementation never sets the offset field
> and nobody ever realised.
>

Perhaps I am little bit polarized on UABI issue, but I was rethinking
about it and maybe removing offset parameter would lead to an
interoperability issue for device running L2TPv3 since offset
parameter is there and it is not a nope.
Please consider this setup:
- 2 endpoint running L2TPv3, the first running net-next and the second
running 4.14
- both endpoint are configured using iproute2 in this way:

  - ip l2tp add tunnel local <ip0> remote <ip1> tunnel_id <id0>
peer_tunnel_id <id1> udp_sport <p0> udp_dport <p1>
  - ip l2tp add tunnel local <ip1> remote <ip0> tunnel_id <id1>
peer_tunnel_id <id0> udp_sport <p1> udp_dport <p0>
  - ip l2tp add session name l2tp0 tunnel_id <id0> session_id <s0>
peer_session_id <s1> offset 8
  - ip l2tp add session name l2tp0 tunnel_id <id1> session_id <s1>
peer_session_id <s0> offset 8

Can we assume offset is never used for L2TPv3?

Regards,
Lorenzo

>> Now what we can do I guess is:
>> - as suggested by Guillaume drop completely the offset support without removing
>>   netlink attribute in order to not break UABI
>> - fix offset support initializing properly padding bits
>>
> I'd go for the first one. I just wonder if that looks acceptable to
> David an James.