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Inspecting the Retransmission Behavior of the Mosquitto MQTT Broker with GDB

GDB Debugging MQTT Mosquitto Network Protocol

Posted on Oct 6


I just wanted to actually see how the Mosquitto MQTT broker retransmit packets when a communication is interrupted and confirm the effect of the retry_interval config variable. Steps are as follows.

Environment

  • Linux 3.13.0-48-generic Ubuntu SMP 86_64 GNU/Linux

Building Mosquitto

Installing Required Packages

$ sudo apt-get install build-essential
$ sudo apt-get install git
$ sudo apt-get install gdb

Configuring and Building

$ cd $DIR_TO_HOST_MOSQUITTO_CLONE
$ mkdir mosquitto
$ git clone http://git.eclipse.org/gitroot/mosquitto/org.eclipse.mosquitto.git mosquitto
$ cd mosquitto
# Read build requirements and steps.
$ less compiling.txt
# Disable some features to reduce dependencies.
$ cp -v config.mk config.mk.orig
$ sed -e 's/^\(WITH_TLS:=\).*$/\1=no/' \
    -e 's/^\(WITH_TLS_PSK:=\).*/\1=no/' \
    -e 's/^\(WITH_SRV:=\).*/\1=no/' \
    -e 's/^\(WITH_UUID:=\).*/\1=no/' \
    config.mk.orig > config.mk
# Build.
$ make

Testing Mosquitto

$ export LD_LIBRARY_PATH=lib
$ sudo ldconfig
# Launch the broker
$ src/mosquitto -v &
# Sending PUBLISH message to the broker with topic being "topic" and QoS being 2.
#    -h hostname-to-connect
#    -q QoS-value
$ client/mosquitto_pub -h localhost -t topic -m message -q 2

Output

xxxxxxxxxx: New connection from 127.0.0.1 on port 1883.
xxxxxxxxxx: New client connected from 127.0.0.1 as mosqpub/xxx (c1, k60).
xxxxxxxxxx: Sending CONNACK to mosqpub/xxx (0, 0)
xxxxxxxxxx: Received PUBLISH from mosqpub/xxx (d0, q2, r0, m1, 'topic', ... (7 bytes))
xxxxxxxxxx: Sending PUBREC to mosqpub/xxx (Mid: 1)
xxxxxxxxxx: Received PUBREL from mosqpub/xxx (Mid: 1)
xxxxxxxxxx: Sending PUBCOMP to mosqpub/xxx (Mid: 1)
xxxxxxxxxx: Received DISCONNECT from mosqpub/xxx
xxxxxxxxxx: Client mosqpub/xxx disconnected.

Testing Retransmission using GDB

Assuming that the mosquitto broker is running background at port 1883 on localhost, we publish a retained message, launch a subscriber to receive that message and then interrupt the conversation with GDB to see the broker resend the same message.

Publishing a Retained Message

The following command publishes a message of topic "topic" with QoS 2 and makes the broker retain it for future subscribers.

# The -r option makes this message retained on the server.
$ client/mosquitto_pub -h localhost -t topic -m message -q 2 -r

Launching a Subscriber in GDB session

$ gdb client/mosquitto_sub
(gdb) # Stop the process from responding to PUBREL messages.
(gdb) b _mosquitto_handle_pubrel
(gdb) # Run the subscriber with debugging output enabled (-d) and client ID being "c1".
(gdb) run -d -h localhost -t topic -q 2 -i c1
Starting program: client/mosquitto_sub -d -h localhost -t topic -q 2 -i c1
xxxxxxxxxx: New connection from 127.0.0.1 on port 1883.
Client c1 sending CONNECT
xxxxxxxxxx: New client connected from 127.0.0.1 as c1 (c1, k60).
xxxxxxxxxx: Sending CONNACK to c1 (0, 0)
Client c1 received CONNACK
Client c1 sending SUBSCRIBE (Mid: 1, Topic: topic, QoS: 2)
xxxxxxxxxx: Received SUBSCRIBE from c1
xxxxxxxxxx:     topic (QoS 2)
xxxxxxxxxx: c1 2 topic
xxxxxxxxxx: Sending SUBACK to c1
xxxxxxxxxx: Sending PUBLISH to c1 (d0, q2, r1, m1, 'topic', ... (7 bytes))
Client c1 received SUBACK
Subscribed (mid: 1): 2
Client c1 received PUBLISH (d0, q2, r1, m1, 'topic', ... (7 bytes))
Client c1 sending PUBREC (Mid: 1)
xxxxxxxxxx: Received PUBREC from c1 (Mid: 1)
xxxxxxxxxx: Sending PUBREL to c1 (Mid: 1)

Breakpoint 1, _mosquitto_handle_pubrel (db=0x0, mosq=0x6070c0)
    at read_handle_shared.c:119

# Wait for 20 seconds ... and the server retransmit the PUBREL message.
xxxxxxxxxx: Sending PUBREL to c1 (Mid: 1)

# If we keep waiting here, the server retransmit the same message repeatedly.
# The retransmission interval can be set with `retry_interval` in a config file.
xxxxxxxxxx: Sending PUBREL to c1 (Mid: 1)
xxxxxxxxxx: Sending PUBREL to c1 (Mid: 1)

# Run the c (continue) command to make the process respond to the first PUBREL.
(gdb) c

# The breakpoint seems to hit as many times as the server has retransmitted.
...

Changing retry_interval

The config file can be supplied to the broker with the -c option.

# Here we use the default config file in the workspace.
# Change the retry_interval variable.
$ vim mosquitto.conf
# Launch the broker.
$ src/mosquitto -v -c mosquitto.conf

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