The implementation will make changes in BlueZ, ConnMan and oFono. Most of the changes will be inside oFono. There, we need to use the AT command parser and the  PPP stack (recently added to the oFono repo). The work consist on integrate everything and implement the missing parts of the AT command parser and the PPP stack, and the DUN agent.

On the BlueZ part the work will be the service export for DUN Data Terminal role and and the DUN Agent server to register agents and pass the RFCOMM file descriptor. For testing purposes we can use the Serial API in the beginning. That work is very similar to what we did for the HFP this year.

The ConnMan integration: ConnMan will setup the NAT and the  Internet connection. The DUN integration on ConnMan is similar to the PAN integration(still a work in progress) , so we can reuse part of that implementation.

That’s it.  I’ll post updates here during the development of the project. Now let me code. ;-)

Also, congratulation to the others Unicamp (University of Campinas) students that were accepted on GSoC too. :-)

[1] http://google-opensource.blogspot.com/2009/09/tasty-new-google-summer-of-code-stats.html

First I did the support to send and receive I-frames (the data frames) and created the support to RR S-frames to acknowledge the received I-frames. It first worked with a txWindow=1 (which means only one packet can be send without receive acknowledge from the receiver side). The next step was support a txWindow up to 63 (the maximum value specified). To bring this support I have been using sk_buff lists from the net core implementation.

Support for Segmentation and Reassembly(SAR) of L2CAP SDUs: this feature allows upper applications to use a buffer greater than the L2CAP size packet. The buffer is break down in many packet on the transmitter side and then reassembled on the receiver side and pushed to the upper layer.

REJ frame exception: When a packet is lost we need to start the recoveries procedures. BlueZ won’t support raise of REJ exception (we support only raise of SREJ exception by now, spec says that we can choose one of them). But we need to support the receipt of a REJ-exception (i.e. receive a REJ frame). On the receipt of a REJ frame the L2CAP entity needs to stop the transmission and starts to retransmit the requested packets.  A REJ frame tells the ReqSeq(packet id sequence) and all packets up to ReqSeq – 1 shall be considered acknowledged, the others shall be retransmitted.

SREJ frame exception: BlueZ starts a SREJ exception when it detects lost packets. It sets its state to SREJ_SENT(It means that L2CAP entity is under recovery proccess) and sends a SREJ S-frame for each lost packets. The transmitter side shall resend the packets requested in the SREJ frames without stops transmission of new frames. SREJ exception saves retransmission of unneeded frames.

Retransmission And Monitor Timers: The Retransmission Timer shall be (re)started each time we send a I-frame. If it expires we send a RR-frame and start the Monitor Timer. The other side should response the RR frame immediately, if the the other side do not response and the Monitor Timer expires we resend the RR frame. A maximum value for the of Monitor Timer’s timeouts can be defined. If a L2CAP entity exceeds this value the channel shall be disconnected.

Busy condition on the remote side: If the remote side enters in a busy condition it will send a RNR frame. So, when the local side receives this frame it should mark the remote busy flag as true and stop the transmission of packets. It restarts the sending of packets only when the remote sides send a REJ, SREJ or RR frame. This says the remote busy condition was cleared.

Streaming mode: This mode of operation is useful to the Bluetooth Streaming Profiles such as A2DP and VDP(not yet implemented). It can take advantages of the Segmentation and Reassembly features.

FCS Option: It is a crc16 check for L2CAP packets. It is the default if both sides support it. If a L2CAP entity receives a packet with a broken crc16 it will drop. The receive procedure will miss it and start the recovery procedure.

Roadmap updated:

1. Implement ERTM with txWindow=1 and retransmission disabled.
   1. send I-frames
   2. receive I-frames
   3. support RR S-frames to acknowledge I-frames received
   4. acknowledge I-frames
2. Support txWindow up to 63
3. Support segmentation and reassembly of L2CAP SDUs
4. Enable retransmission
   1. support receipt REJ S-Frame
   2. support receipt SREJ S-Frame
   3. support raise of SREJ exception
5. Support busy conditions exceptions
6. Enable Retransmission Timer
7. Enable Monitor Timer
8. Implement Streaming Mode
9. Implement FCS Option
10. Support duplex channel
11. Use SOCK_STREAM as default for Enhanced Retransmission and Streaming Modes.
12. Test BlueZ against others stacks

What’s next?

Now I’m working the busy condition exception and the support for the ERTM duplex channel. Today, just one side can send data packets. Then I’ll put ERTM as default when SOCK_STREAM is selected. I expect finish these issues on the coming weeks. After, I’ll spend my time fixing the bugs I find trough the code.

ps: Sorry for my too not bad English ;-)

[1] http://padovan.org/blog/2009/07/bluez-first-patch-was-merged/

That means that ERTM implementation will look like TCP implementation[1] on Linux. So, there is no need to write ERTM from scratch, I’ve already started to code and I’m basing my code on  TCP code to get started with ERTM.

[0] http://en.wikipedia.org/wiki/Transmission_Control_Protocol
[1] http://vger.kernel.org/~davem/tcp_output.html

Bluetooth: Add configuration support for ERTM and Streaming mode

Add support to config_req and config_rsp to configure ERTM and Streaming
mode. If the remote device specifies ERTM or Streaming mode, then the
same mode is proposed. Otherwise ERTM or Basic mode is used. And in case
of a state 2 device, the remote device should propose the same mode. If
not, then the channel gets disconnected.

Signed-off-by: Gustavo F. Padovan <gustavo@las.ic.unicamp.br>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

Now I’m going to implement support to send and receive I-frames and S-frames. Some part of the code for that . First I’m going to add support to transmissions with txWindow = 1 (i.e., only one packet can be sent per time without acknowledgment from the other side) and no retransmission support and I intend to finish it during this week.

Here is a quick roadmap for my work:

1. Implement ERTM with txWindow=1 and retransmission disabled.
   1. send I-frames
   2. receive I-frames
   3. support RR S-frames to acknowledge I-frames received
   4. acknowledge I-frames
2. Support txWindow up to 63
3. Support segmentation and reassembly of L2CAP SDUs
4. Enable retransmission
   1. support REJ S-Frame
   2. support SREJ S-Frame
   3. support RNR S-Frame to indicate busy condition
   4. use RR S-frame to show that is ready to receive I-frames
5. Enable Retransmission Timer
6. Enable Monitor Timer
7. Test BlueZ against others stack that implement ERTM

I expect that until end of August finish all this topics. So, let’s hack. =)

That is the first part of my project, after  I’ll really implement ERTM.  The project will add support for two new frames types, I-frames and S-frames.  I-frames transmit all the data and S-frame are used to transmit specific protocol information – like ask to other device to resend some packet.

BlueZ:
If you wanna see all the projects accepted by BlueZ go to Vudentz’s blog, among them is my friend João Paulo Rechi Vita, also from Unicamp, with A2DP sink role.

That’s all, folks.

UPDATE: I said in the first version of this post that the first part of the project is to add ERTM to features mask. I was wrong. I can do that only when I finish all  ERTM implementation. To test ERTM Marcel has created a module parameter to enable ERTM.

ps: if you find errors in my English, please tell me, I’m learning yet. =)

About GSoC 2009 in Brazil:
Leslie Hawthorn has posted yesterday on Google Open Source Blog some statistics about GSoC 2009, and Brazil is the top 5 country for accepted student applicants , with 43 students. I’m too happy to hear that, but I’m also waiting for the statistic about students accepted per University. Last year my university (Unicamp – University of Campinas) was the 2nd in the whole world.

And I wanna congratulate my friends of University of Campinas that were accepted on GSoC 2009 too. They are João Paulo Rechi Vita, working on BlueZ with me and Vudentz is his mentor too; Bruno Cardoso working on LLVM for the third time. Helder Ribeiro on ReviewBoard; João “lvwr” Correa on NMAP and Thiago “bolaum” Abdnur.  We will hack togheter for next four months to accomplish our Google Summer of Code project.

Getting deeper on Bluetooth:
For a first read you can read [1] e [2] , if wanna more you can go to [3] e and look thought the spec.

[1] http://en.wikipedia.org/wiki/Bluetooth

[2] http://bluetooth.com/Bluetooth/Technology/Works/Overview_of_Operation.htm

[3] http://bluetooth.com/Bluetooth/Technology/Building/Specifications/

ps: if you find errors in my English, please tell me, I’m learning yet. =)