How to Measure Peer Response Latency Using btmon ACL Data TX/RX Timestamps: A Comprehensive Guide #

Welcome to revWhiteShadow, the personal blog site of revWhiteShadow and kts. In this article, we will delve into the intricacies of using btmon to monitor Bluetooth communication and, more importantly, how to accurately measure peer response latency using the ACL data transmission and reception timestamps provided by btmon. We understand the challenges in discerning the start and end points of packet exchanges, and we aim to provide a comprehensive explanation to enable precise latency measurement.

Understanding btmon and ACL Data #

btmon is a powerful tool for capturing and analyzing Bluetooth traffic. It provides a detailed view of the Bluetooth protocol stack, including HCI (Host Controller Interface) commands, events, and ACL (Asynchronous Connection-Oriented Logical) data packets. Understanding ACL data is crucial for measuring peer response latency. ACL data packets are used for data transfer between Bluetooth devices after a connection has been established. These packets contain the actual application data being exchanged.

Decoding the btmon Log Format #

The btmon output can appear daunting at first. Let’s break down the structure of each log entry to understand the information it provides:

< ACL Data TX: Handle 3585 flags 0x01 dlen 27                                                                                                              [hci0] 20.084811
          ATT: Write Command (0x52) len 130
            Handle: 0x0013
              Data: 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001ffffffffffffffffffffffffffffffffffffff000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

• < ACL Data TX: Indicates that this log entry represents an ACL data packet being transmitted (TX) from the host to the Bluetooth controller. The > symbol indicates a received (RX) packet.

• Handle 3585: The connection handle. This is a unique identifier for the Bluetooth connection over which the data is being transmitted. Ensure you’re comparing TX and RX events with the same handle.

• flags 0x01: These flags provide information about the packet. Crucially, 0x01 (or 0x00 or 0x02) indicates that the ACL packet is part of a larger fragmented message, and specifically whether is it the first or a continuation of the previous packet.

  • 0x00: Continuation fragment of a higher layer message.
  • 0x01: First fragment of a higher layer message.
  • 0x02: Complete higher layer message or last fragment of a higher layer message.

• dlen 27: The data length of the ACL packet, in bytes.

• [hci0]: The Bluetooth interface (hci0, hci1, etc.). Useful if you have multiple Bluetooth adapters.

• 20.084811: The timestamp, in seconds, when this event occurred. This is the key to measuring latency.

• ATT: Write Command (0x52): Describes the Attribute Protocol (ATT) operation being performed. In this case, it’s a Write Command.

  • Handle: 0x0013 This is the attribute handle being written to.

• Data: ...: The actual data being transmitted, in hexadecimal format.

Identifying Send and Receive Packets #

The < ACL Data TX and > ACL Data RX prefixes clearly mark packets sent and received, respectively. However, identifying the start and end of a complete message can be trickier due to fragmentation. Bluetooth divides larger messages into smaller ACL packets, and these are sent sequentially.

Measuring Peer Response Latency: A Step-by-Step Approach #

To accurately measure peer response latency, we need to:

1. Identify the Request Packet: Locate the < ACL Data TX entry that represents the beginning of a request to the peer device. This is usually a command that triggers a response. Look for ATT operations like “Write Command” or “Read Request.” Inspect the data field to understand the command being sent. We need to identify the first packet of a higher layer message. This can be done by checking the flags. The flags should contain 0x01.
2. Identify the Corresponding Response Packet: Find the > ACL Data RX entry that represents the peer’s response to the request. This is usually a “Handle Value Notification” or a “Read Response.” Ensure that the connection handle matches the request packet’s handle. We need to identify the last packet of a higher layer message. This can be done by checking the flags. The flags should contain 0x02.
3. Account for Fragmentation: If the request or response is fragmented, you’ll see multiple < ACL Data TX or > ACL Data RX entries with the same handle and sequential timestamps. The flags will help you in determining if the packet is the first, last, or a continuation of the previous packet.
4. Calculate Latency: Subtract the timestamp of the first TX packet of the request from the timestamp of the last RX packet of the response. This difference is the peer response latency.

Example Scenario: Analyzing the Provided btmon Log #

Let’s analyze the btmon log snippet you provided to illustrate the process.

Identifying a Request #

< ACL Data TX: Handle 3585 flags 0x01 dlen 27                                                                                                              [hci0] 20.084811
          ATT: Write Command (0x52) len 130
            Handle: 0x0013
              Data: 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001ffffffffffffffffffffffffffffffffffffff000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

This is the first packet of the request. We can identify an ATT “Write Command” with handle 0x0013. The timestamp is 20.084811. Because the flags are 0x01, this is the first packet of a fragmented message. We need to find the last packet.

< ACL Data TX: Handle 3585 flags 0x00 dlen 19                                                                                                              [hci0] 20.085759
          ATT: Write Command (0x52) len 14
            Handle: 0x0013
              Data: 0000000000000000edfa5318

This is the last packet of the request, because the flags are 0x00. The timestamp is 20.085759.

Identifying a Response #

Next, we look for a corresponding response.

> ACL Data RX: Handle 3585 flags 0x02 dlen 9                                                                                                               [hci0] 20.182235
          ATT: Handle Value Notification (0x1b) len 4
            Handle: 0x0010
              Data: 0206
> ACL Data RX: Handle 3585 flags 0x02 dlen 17                                                                                                              [hci0] 20.282358
          ATT: Handle Value Notification (0x1b) len 12
            Handle: 0x0010
              Data: 000c000007177df11e96

Here, we have two ACL Data RX packets. Both have a flag of 0x02. So we need to pick the last ACL Data RX to measure the latency, in this case the second packet. This is a “Handle Value Notification” from handle 0x0010. The timestamp is 20.282358.

Calculating the Latency #

Now we can calculate the latency:

Latency = Response Timestamp - Request Timestamp

Latency = 20.282358 - 20.084811 = 0.197547 seconds = 197.547 milliseconds

Therefore, the peer response latency for this specific exchange is approximately 197.547 milliseconds.

Important Considerations for Accurate Measurement #

• Clock Synchronization: Ensure that the clocks of the host and the peer device are reasonably synchronized. Significant clock skew can lead to inaccurate latency measurements.
• Bluetooth Stack Overhead: The measured latency includes the overhead of the Bluetooth protocol stack on both the host and the peer device. This overhead can vary depending on the implementation and the system load.
• Interference: Wireless interference can affect Bluetooth communication and introduce delays. Minimize interference by conducting measurements in a controlled environment.
• Fragmentation and Reassembly: Pay close attention to packet fragmentation. The total latency is the time from the first TX fragment to the last RX fragment of the entire message.
• Multiple Connections: If multiple Bluetooth connections are active, ensure you’re analyzing the correct connection by filtering on the connection handle.
• Background Processes: Background processes on both the host and the peer device can influence latency. Try to minimize background activity during measurements.
• Averaging Multiple Measurements: To obtain a more reliable estimate of the peer response latency, perform multiple measurements and calculate the average.
• HCI Event: Number of Completed Packets: The HCI Event: Number of Completed Packets event can be useful for understanding when packets have been processed by the Bluetooth controller. However, it doesn’t directly indicate the peer’s response time. It primarily informs the host that the controller has finished transmitting or receiving a certain number of packets. This event is more relevant for flow control and buffer management within the Bluetooth stack.

Advanced Techniques for Latency Analysis #

Filtering btmon Output #

btmon allows you to filter the output to focus on specific connections, HCI events, or ACL data. This can simplify the analysis process. For example, to filter by connection handle:

btmon | grep "Handle 3585"

This command will only display log entries related to connection handle 3585.

Scripting for Automated Analysis #

For more complex analysis, consider writing a script (e.g., in Python or Bash) to parse the btmon output and automatically calculate latency values. This can be especially useful for collecting a large number of measurements.

Visualizing Latency Data #

Plotting the latency data over time can reveal patterns and trends that might not be apparent from individual measurements. Use tools like Gnuplot or Python’s Matplotlib library to create visualizations.

Where to Find Documentation on the btmon Log Format #

The most authoritative documentation for btmon is the btmon man page, which can be accessed by running man btmon in your terminal. The man page provides a detailed explanation of the command-line options and the log format. You can also find the man page online, for example, at https://manpages.debian.org/testing/bluez/btmon.1.en.html. Additionally, the source code of btmon, which is part of the BlueZ project, can provide valuable insights into the internal workings of the tool and the structure of the log messages. The BlueZ project is available on GitHub.

Conclusion #

Measuring peer response latency using btmon ACL data TX/RX timestamps requires a careful understanding of the btmon log format and the Bluetooth protocol stack. By following the steps outlined in this article, you can accurately measure latency and gain valuable insights into the performance of your Bluetooth devices. Remember to account for fragmentation, clock synchronization, and other factors that can influence latency. With practice and the right tools, you can effectively analyze Bluetooth communication and optimize the performance of your applications. We, at revWhiteShadow, hope this guide has been helpful. Good luck with your Bluetooth endeavors! This is the place to learn more about kts personal blog site, so stay tuned for more insightful content.