Date:   Mon, 21 May 2018 14:16:56 +0200
From:   Michael Tuexen <tuexen@...muenster.de>
To:     Neil Horman <nhorman@...driver.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH net-next] sctp: add support for SCTP_REUSE_PORT sockopt

> On 21. May 2018, at 13:39, Neil Horman <nhorman@...driver.com> wrote:
> 
> On Sun, May 20, 2018 at 10:54:04PM -0300, Marcelo Ricardo Leitner wrote:
>> On Sun, May 20, 2018 at 08:50:59PM -0400, Neil Horman wrote:
>>> On Sat, May 19, 2018 at 03:44:40PM +0800, Xin Long wrote:
>>>> This feature is actually already supported by sk->sk_reuse which can be
>>>> set by SO_REUSEADDR. But it's not working exactly as RFC6458 demands in
>>>> section 8.1.27, like:
>>>> 
>>>>  - This option only supports one-to-one style SCTP sockets
>>>>  - This socket option must not be used after calling bind()
>>>>    or sctp_bindx().
>>>> 
>>>> Besides, SCTP_REUSE_PORT sockopt should be provided for user's programs.
>>>> Otherwise, the programs with SCTP_REUSE_PORT from other systems will not
>>>> work in linux.
>>>> 
>>>> This patch reuses sk->sk_reuse and works pretty much as SO_REUSEADDR,
>>>> just with some extra setup limitations that are neeeded when it is being
>>>> enabled.
>>>> 
>>>> "It should be noted that the behavior of the socket-level socket option
>>>> to reuse ports and/or addresses for SCTP sockets is unspecified", so it
>>>> leaves SO_REUSEADDR as is for the compatibility.
>>>> 
>>>> Signed-off-by: Xin Long <lucien.xin@...il.com>
>>>> ---
>>>> include/uapi/linux/sctp.h |  1 +
>>>> net/sctp/socket.c         | 48 +++++++++++++++++++++++++++++++++++++++++++++++
>>>> 2 files changed, 49 insertions(+)
>>>> 
>>> A few things:
>>> 
>>> 1) I agree with Tom, this feature is a complete duplication of the SK_REUSEPORT
>>> socket option.  I understand that this is an implementation of the option in the
>>> RFC, but its definately a duplication of a feature, which makes several things
>>> really messy.
>>> 
>>> 2) The overloading of the sk_reuse opeion is a bad idea, for several reasons.
>>> Chief among them is the behavioral interference between this patch and the
>>> SO_REUSEADDR socket level option, that also sets this feature.  If you set
>>> sk_reuse via SO_REUSEADDR, you will set the SCTP port reuse feature regardless
>>> of the bind or 1:1/1:m state of the socket.  Vice versa, if you set this socket
>>> option via the SCTP_PORT_REUSE option you will inadvertently turn on address
>>> reuse for the socket.  We can't do that.
>> 
>> Given your comments, going a bit further here, one other big
>> implication is that a port would never be able to be considered to
>> fully meet SCTP standards regarding reuse because a rogue application
>> may always abuse of the socket level opt to gain access to the port.
>> 
>> IOW, the patch allows the application to use such restrictions against
>> itself and nothing else, which undermines the patch idea.
>> 
> Agreed.
> 
>> I lack the knowledge on why the SCTP option was proposed in the RFC. I
>> guess they had a good reason to add the restriction on 1:1/1:m style.
>> Does the usage of the current imply in any risk to SCTP sockets? If
>> yes, that would give some grounds for going forward with the SCTP
>> option.
>> 
> I'm also not privy to why the sctp option was proposed, though I expect that the
> lack of standardization of SO_REUSEPORT probably had something to do with it.
> As for the reasoning behind restriction to only 1:1 sockets, if I had to guess,
> I would say it likely because it creates ordering difficulty at the application
> level.
> 
> CC-ing Michael Tuxen, who I believe had some input on this RFC.  Hopefully he
> can shed some light on this.
Dear all,

the reason this was added is to have a specified way to allow a system to
behave like a client and server making use of the INIT collision.

For 1-to-many style sockets you can do this by creating a socket, binding it,
calling listen on it and trying to connect to the peer.

For 1-to-1 style sockets you need two sockets for it. One listener and one
you use to connect (and close it in case of failure, open a new one...).

It was not clear if one can achieve this with SO_REUSEPORT and/or SO_REUSEADDR
on all platforms. We left that unspecified.

I hope this makes the intention clearer.

Best regards
Michael
> 
> Neil
> 
>>> 
>>> Its a bit frustrating, since SO_REUSEPORT is widely available on multiple
>>> operating systems, but isn't standard (AFAIK).  I would say however, given the
>>> prevalence of the socket level option, we should likely advocate for the removal
>>> of the sctp specific option, or at the least implement and document it as being
>> 
>> Is it possible, to remove/deprecate an option once it is published on a RFC?
>> 
>>> an alias for SO_REUSEPORT
>>> 
>>> 
>>> As this stands however, its a NACK from me.
>>> 
>>> Neil
>>> 
>> 
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