Bandwidth meter technical
The Protocol
The protocol used by the bandwidth meter is TCP NewReno. More technical details can be found in the following documents:
Implementation of the BW meter took the TCP in-kernel module as example and some snippets of code are mostly indentical (e.g. SRTT/RTTVAR/RTO computation). In the code it is also possible to find several references to RFC sections which explain what the code is doing.
A define of note is the Receiver/Advertised Window size, which has been fixed to 2^29 bytes. Common TCP implementations instead limit this size to 2^16 bytes and can reach such high value only if using the Window Scaling option. In the latter case the window size must be less than 2^30 and this is why of the chosen value for this implementation.
This value is important because it represents the limit for the Transmit Window size and so for the maximum amount of in-flight data.
Interaction between batctl and batman-adv
The BW meter is started using batctl, in particular is started by the batman-adv module when receiving a particular message on the ICMP socket. Such socket is used for communications between batctl and batman-adv which so far were limited to ICMP Echo Requests/Replies.
To inctroduce the BW meter, a new particular ICMP packet has been introduced: the ICMP USER packet. Such packet is used to send commands from batctl to batman-adv.
For the BW Meter there are two available commands:
bw_start: initiate a new a BW measurement towards a selected node
bw_stop: halt the selected BW measurement
The following picture depict the communication between batctl and batman-adv.
The bw_stop command is used only when the user wants the test to be stopped before the predefined time elapses.
Once the test is finished, batman-adv will send back a particular packet, namely the ICMP BW RESULT, containing the time spent for the test and the amount of bytes correctly acked by the receiver. This information is then used by batctl to properly show the result to the user. The results are sent back in both the cases (test stopped by the user or test naturally ended because time is up).
The major difference between this ICMP operation and the old ones like PING or TRACEROUTE is that the packet sent by batctl is not directly forwarded to the network, but instead it is processed and consumed directly on the local node.
Performance
Theoretically we expected the performance of the bw meter to overcome iperf (when launched on the nodes themselves), but the experiments did not gave us the results we thought about. In particular we saw mostly the same throughput with both the bw meter and iperf.
In the test page it is possible to see the obtained values more in detail.