Re: bluez A2DP socket reliability

["Pali Rohár" <pali.rohar@gmail.com>]

On Monday 10 June 2019 18:06:53 Frédéric Danis wrote:
> Hi,
> 
> Le 10/06/2019 à 12:51, Pali Rohár a écrit :
> > On Friday 07 June 2019 18:23:41 Luiz Augusto von Dentz wrote:
> > > 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.
> > Fine, no retransmission is good for A2DP.
> 
> I am not sure there is no retransmission with current implementation.
> AFAIU, ERTM and automatic flush timeout are not linked. ERTM operates at
> L2CAP level while automatic flush timeout [1] operates at ACL level.
> When I read the automatic flush timeout set for an A2DP SBC connection it
> returns 0, which means that the connection uses an infinite timeout. So,
> even if higher levels set packets as flushable, the ACL policy requests
> baseband to indefinitely try to send data [3]:
>   "The default Flush Timeout shall be infinite,
>     i.e. re-transmissions are carried out until
>     physical link loss occurs. This is also
>     referred to as a 'reliable channel.'"
> 
> [1] Bluetooth core Vol 2, Part E, 6.19
> [2] Bluetooth core Vol 2, Part E, 7.3.29
> [3] Bluetooth core Vol 2, Part B, 7.6.3

Mhm... I feeling quite lost. Is A2DP socket reliable with enabled
retransmission or not?

-- 
Pali Rohár
pali.rohar@gmail.com