Re: bluez A2DP socket reliability

From: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
To: "Pali Rohár" <pali.rohar@gmail.com>
Cc: "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
Subject: Re: bluez A2DP socket reliability
Date: Fri, 7 Jun 2019 18:23:41 +0300	[thread overview]
Message-ID: <CABBYNZKQ0WYTH4Oa_eFv11Ok8=42oY_GPFzN-D6UqfeNsvaXug@mail.gmail.com> (raw)
In-Reply-To: <20190607130021.ntd3dfd6nzmuy3m3@pali>

Hi Pali,

On Fri, Jun 7, 2019 at 4:00 PM Pali Rohár <pali.rohar@gmail.com> wrote:
>
> On Sunday 19 May 2019 14:22:23 Pali Rohár wrote:
> > On Sunday 19 May 2019 11:13:09 Luiz Augusto von Dentz wrote:
> > > Hi Pali,
> > >
> > > On Sat, May 18, 2019 at 11:12 PM Pali Rohár <pali.rohar@gmail.com> wrote:
> > > >
> > > > Hello! How is L2CAP layer of bluetooth socket used for A2DP audio
> > > > transfer configured in bluez? It is reliable with big/infinite
> > > > retransmit count? Or in best-effort manner and some packets may be
> > > > dropped? And it is possible to change between these two modes for
> > > > application which uses bluez DBUS API? I'm asking because some A2DP
> > > > audio codecs are designed to deal with packet loss and for those codecs
> > > > it would be probably better to configure L2CAP socket to unreliable
> > > > mode.
> > >
> > > We don't use ERTM with AVDTP, both signaling and transport sockets are
> > > using basic mode which don't support retransmissions, there the
> > > concept of flush timeout which iirc we don't currently it.
> >
> > On bluez.org site there is no information how to use bluez sockets and
> > the only documentation/tutorial which I found on internet was this:
> >
> >   https://people.csail.mit.edu/albert/bluez-intro/x559.html
> >
> > I do not know how up-to-date it is, but seems that by default bluez
> > L2CAP sockets are reliable and to change them to unreliable mode it is
> > needed to issue OGF_HOST_CTL / OCF_WRITE_AUTOMATIC_FLUSH_TIMEOUT (0x28)
> > request. As default is zero = infinity = reliable connection.
> >
> > I do not understand low level bluetooth details, but is ERTM related to
> > OCF_WRITE_AUTOMATIC_FLUSH_TIMEOUT?
> >
> > So what are default settings for L2CAP socket used by AVDTP/A2DP
> > profiles which are transferred to user application via DBUS?
>
> Hi! Do you have any idea about OCF_WRITE_AUTOMATIC_FLUSH_TIMEOUT? It is
> related to ERTM or not?

The OCF usually describes an HCI command which may affect the entire
ACL connection, ERTM is a L2CAP channel mode that includes
retransmissions. The A2DP stream transport doesn't ERTM so no
retransmissions shall take place.

-- 
Luiz Augusto von Dentz