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

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Date:   Thu, 5 Mar 2020 14:01:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jere Leppanen' <jere.leppanen@...ia.com>,
        Xin Long <lucien.xin@...il.com>
CC:     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

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??

	David

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