netdev - RE: [PATCH net] sctp: return a one-to-one type socket when doing peeloff

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Date:   Thu, 5 Mar 2020 19:31:23 +0200 (EET)
From:   Jere Leppanen <jere.leppanen@...ia.com>
To:     David Laight <David.Laight@...LAB.COM>
cc:     Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "michael.tuexen@...chi.franken.de" <michael.tuexen@...chi.franken.de>
Subject: RE: [PATCH net] sctp: return a one-to-one type socket when doing
 peeloff

On Thu, 5 Mar 2020, David Laight wrote:

> From: Jere Leppanen
> > Sent: 04 March 2020 17:13
> > On Wed, 4 Mar 2020, Xin Long wrote:
> > 
> > > On Wed, Mar 4, 2020 at 2:38 AM Leppanen, Jere (Nokia - FI/Espoo)
> > > <jere.leppanen@...ia.com> wrote:
> > >>
> > >> On Mon, 2 Mar 2020, Xin Long wrote:
> > >>
> > >>> As it says in rfc6458#section-9.2:
> > >>>
> > >>>   The application uses the sctp_peeloff() call to branch off an
> > >>>   association into a separate socket.  (Note that the semantics are
> > >>>   somewhat changed from the traditional one-to-one style accept()
> > >>>   call.)  Note also that the new socket is a one-to-one style socket.
> > >>>   Thus, it will be confined to operations allowed for a one-to-one
> > >>>   style socket.
> > >>>
> > >>> Prior to this patch, sctp_peeloff() returned a one-to-many type socket,
> > >>> on which some operations are not allowed, like shutdown, as Jere
> > >>> reported.
> > >>>
> > >>> This patch is to change it to return a one-to-one type socket instead.
> > >>
> > >> Thanks for looking into this. I like the patch, and it fixes my simple
> > >> test case.
> > >>
> > >> But with this patch, peeled-off sockets are created by copying from a
> > >> one-to-many socket to a one-to-one socket. Are you sure that that's
> > >> not going to cause any problems? Is it possible that there was a
> > >> reason why peeloff wasn't implemented this way in the first place?
> > > I'm not sure, it's been there since very beginning, and I couldn't find
> > > any changelog about it.
> > >
> > > I guess it was trying to differentiate peeled-off socket from TCP style
> > > sockets.
> > 
> > Well, that's probably the reason for UDP_HIGH_BANDWIDTH style. And maybe
> > there is legitimate need for that differentiation in some cases, but I
> > think inventing a special socket style is not the best way to handle it.
> > 
> > But actually I meant why is a peeled-off socket created as SOCK_SEQPACKET
> > instead of SOCK_STREAM. It could be to avoid copying from SOCK_SEQPACKET
> > to SOCK_STREAM, but why would we need to avoid that?
> 
> Because you don't want all the acks and retransmissions??

I don't follow. The socket type and style have virtually no effect on the 
protocol side of things, I think.

> 
> 	David
> 
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