Re: How to change default MTU for Obex over L2CAP

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

Hi Sathish,

On Tue, Oct 31, 2017 at 3:24 PM, Sathish Narasimman
<nsathish41@gmail.com> wrote:
> Hi Luiz,
>
> On Tue, Oct 31, 2017 at 3:03 PM, Luiz Augusto von Dentz
> <luiz.dentz@gmail.com> wrote:
>> Hi Sathish,
>>
>> On Tue, Oct 31, 2017 at 11:27 AM, Sathish Narasimman
>> <nsathish41@gmail.com> wrote:
>>> Hi Luiz,
>>>
>>> On Tue, Oct 31, 2017 at 12:53 PM, Luiz Augusto von Dentz
>>> <luiz.dentz@gmail.com> wrote:
>>>> Hi Sathish,
>>>>
>>>> On Tue, Oct 31, 2017 at 9:05 AM, Luiz Augusto von Dentz
>>>> <luiz.dentz@gmail.com> wrote:
>>>>> Hi Sathish,
>>>>>
>>>>> On Tue, Oct 31, 2017 at 5:30 AM, Sathish Narasimman
>>>>> <nsathish41@gmail.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> I am trying to do an OPP transfer between 2 Linux bluez system.
>>>>>> It is always selecting the 672 MTU size during configuration.
>>>>>>
>>>>>> May I please know how to change the MTU size.
>>>>>
>>>>> It looks like for L2CAP we don't set the MTU properly but for RFCOMM we do:
>>>>>
>>>>> https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/obexd/client/bluetooth.c#n129
>>>>>
>>>>> I will send a patch in a moment.
>>>>
>>>> Actually, it is the other way around, the RFCOMM don't actually
>>>> support setting the MTU since it is a stream, though we should
>>>> probably adjust the socket buffer size since we are using 32k for
>>>> OBEX.
>>>
>>> Though I was trying to change the value for OBEX over L2CAP
>>>
>>> Does this have the effect of OBEX maximum packet length?
>>> In which the maximum packet length parameter is always 672.
>>>
>>> Here the Default 32K omtu is changed to 672 at this point
>>> https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/obexd/client/bluetooth.c#n468
>>
>> That should never be reached in case of RFCOMM since it is not packet
>> based, or are you really using OBEX over L2CAP aka. GOEP 2.0, that
>> should be using 32K but we should check the HCI traces if that is
>> actually what is negotiated.
>>
> Yes I am using OBEX over L2CAP(where trying to send file to another
> Linux system using obexctl tool it uses GOEP)
>
> 1. Please let me know how to change the OBEX maximum packet length
> when receiving file (i.e In OBEX response code: Success, Maximum
> packet length is 672 - Logs)
> btmon Log: https://pastebin.com/BvVERsfq
> 2. How to change the Default L2CAP MTU. In logs for configure
> response, the MTU is 672.
> 3. I am not able to see any logs in obexd when File is received.

It is requesting 32K:

< ACL Data TX: Handle 256 flags 0x00 dlen 27
            [hci0] 43.021610
      L2CAP: Configure Request (0x04) ident 7 len 19
        Destination CID: 64
        Flags: 0x0000
        Option: Maximum Transmission Unit (0x01) [mandatory]
          MTU: 32767
        Option: Retransmission and Flow Control (0x04) [mandatory]
          Mode: Enhanced retransmission (0x03)
          TX window size: 63
          Max transmit: 3
          Retransmission timeout: 2000
          Monitor timeout: 12000
          Maximum PDU size: 1009
> ACL Data RX: Handle 256 flags 0x02 dlen 23                                       [hci0] 43.023856
      L2CAP: Configure Request (0x04) ident 4 len 15
        Destination CID: 65
        Flags: 0x0000
        Option: Retransmission and Flow Control (0x04) [mandatory]
          Mode: Enhanced retransmission (0x03)
          TX window size: 63
          Max transmit: 3
          Retransmission timeout: 2000
          Monitor timeout: 12000
          Maximum PDU size: 1009

I guess because there is no MTU in the remote request we respond with 672:

< ACL Data TX: Handle 256 flags 0x00 dlen 29
            [hci0] 43.023870
      L2CAP: Configure Response (0x05) ident 4 len 21
        Source CID: 64
        Flags: 0x0000
        Result: Success (0x0000)
        Option: Maximum Transmission Unit (0x01) [mandatory]
          MTU: 672
        Option: Retransmission and Flow Control (0x04) [mandatory]
          Mode: Enhanced retransmission (0x03)
          TX window size: 63
          Max transmit: 3
          Retransmission timeout: 2000
          Monitor timeout: 12000
          Maximum PDU size: 1009
> ACL Data RX: Handle 256 flags 0x02 dlen 29                                       [hci0] 43.028883
      L2CAP: Configure Response (0x05) ident 7 len 21
        Source CID: 65
        Flags: 0x0000
        Result: Success (0x0000)
        Option: Maximum Transmission Unit (0x01) [mandatory]
          MTU: 32767
        Option: Retransmission and Flow Control (0x04) [mandatory]
          Mode: Enhanced retransmission (0x03)
          TX window size: 63
          Max transmit: 3
          Retransmission timeout: 2000
          Monitor timeout: 12000
          Maximum PDU size: 1009

I guess this will result in 32K omtu and 672 imtu, though Im not sure
if the PDU size are correct either. Have tried transfering anything to
see what is the actual payload we can carry over this channel. Btw
keep in mind OBEX channel is using ERTM the other channels like SDP
will have their own MTU, most likely 672.

> Thanks,
> Sathish N
>
>>>
>>>>https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/obexd/client/bluetooth.c#n457
>>>> --
>>>> Luiz Augusto von Dentz
>>
>>
>>
>> --
>> Luiz Augusto von Dentz



-- 
Luiz Augusto von Dentz