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Blockmon is software allowing construction of flexible and high performance (rates in the 10Gb range) monitoring and data analysis nodes, where a node can be for example a hardware probe or a PC. Blockmon is based around the notion of blocks, which are small units of processing (e.g. packet counting). Blocks are connected and communicate via gates, and the set of inter-connected blocks represents a composition, where compositions are expressed in terms of an XML file.

In order to control blockmon, a number of options exist:

  1. CLI: python-based, run it with python daemon/ (run it as root if for instance capturing from a network interface). At the prompt type help for a list of commands. Users are expected to write XML composition files by hand.

  2. Daemon: a json-rpc, python-based daemon. Exposes the same functionality as the CLI, but allows for programmability. If using it, make sure to first edit the daemon/config file; run it with python config (as root if needed). Users are expected to write XML composition files by hand, or to write code that generates them. Also, make sure that core/ is executable (e.g., chmod u+x core/


├── bin       blockmon executable
├── blocks    blocks
├── core      core files
├── daemon    daemon files
├── doc       documentation
├── lib       libraries
├── messages  message files
├── tests     test files
└── usr       user blocks, messages, compositions


The documentation is generated through doxygen. To generate, go to the doc directory and run:

doxygen blockmondoc.cfg

This will generate HTML documentation for all the code; the index page can be found under doc/blockmon/html/index.html

In addition to the code documentation, classes implementing blocks include additional information regarding the block's functionality. To generate this documentation, go to the doc directory and run:


The environment variable PYTHONPATH has to be set to Blockmon's daemon directory (i.e., node/daemon) for this to work.


A very simple composition file consists of a set of blocks and their connections, all in XML format. One such file is shown below:

<composition id="mysnifferctr" app_id="packet_count">
      <clock type="wall"/>
      <threadpool id="sniffer_thread" num_threads="2">
         <core number="0"/>
      <block id="sniffer" type="PcapSource" invocation="async" threadpool="sniffer_thread">
            <source type="live" name="eth0"/>
            <!--bpf_filter expression="!tcp"/-->
      <!-- NOTE: passive blocks shouldn't have a threadpool assigned to them -->
      <block id="counter" type="PacketCounter" invocation="direct">

      <connection src_block="sniffer" src_gate="source_out" dst_block="counter" dst_gate="in_pkt"/>

Compositions also allow configuration of how many threads are in a thread pool, and how a threadpool is allocated to CPU cores. Some examples:

<!-- scheduler lets OS decide how to schedule threads over the specified cores -->
<threadpool id="pool1" num_threads="10" cores="0-2,4">
<!-- core-specific thread -->
<threadpool id="pool1" num_threads="1" cores="2">
<!-- scheduler assigns to cores as it sees fit -->
<threadpool id="pool1" num_threads="10">

In addition, it is possible to modify existing compositions, updating parts of them, adding to them, or deleting from them:

<composition id="mysnifferctr">

    These sections are optional


     <!-- note: id, invocation and threadpool are MANDATORY even if they don't change -->
      <block id="sniffer" invocation="indirect" threadpool="sniffer_thread">
          <source type="live" name="wlan0" />


Note that blockmon does not support reconfiguration of connections. The user must delete and add connections to emulate the equivalent of a connection reconfiguration.


To run a composition via the CLI simply run:

sudo python daemon/

Or, in Blockmon's shell:

start [composition file]

And to stop it in Blockmon's shell just type:


Also note that to run the above, sudo is needed to access a local interface.


To create a new application, please add a subdirectory under BLOCKMON_DIR/usr with the name


Place any composition files in this directory. Also, under this directory create a further subdirectory called blocks and another one called messages.

Finally, make sure to re-run cmake . in order to include the new files into the project.


The easiest way is to start with an existing simple block such as blocks/PktCounter.[hpp|cpp] and copy it. Please make sure to document, at the top of the cpp file, what the block does; again, follow the format in PktCounter.cpp.

If you are only going to have one file for your block, please make sure it is a cpp file and not an hpp one.

For more details, please refer to the doxygen documentation of Block.hpp.

Once you're done creating the block, run

python core/ config

to let the CLI and daemon know about the new block.


Blockmon supports the ability for developers to add custom message types by putting message files under /usr/app_[your_app_name]/messages. However, note that in most cases the standard message types provided should be sufficient.

Packet: represents a packet captured from the wire and provides access to fields of the parsed packet.

Flow: represents a flow keyed by five-tuple, counting bytes and packets.

PairMsg: represents a message carrying a generic (key, value) pair.

Each block can append a Tag to an existing Message (see documentation about the TagRegistry class for details). Note that you must first register the tag before creating a message or appending the tag to an existing message.


For indirectly invoked blocks, messages are queued at the InGate. The used queues however have a finite capacity. If the messages are enqueued with a higher rate than they are consumed by the block they are by default dropped when the queue is full. This behavior can be changed on a per-block- basis. To enable this feature cmake must be run with the -DBLOCKING_QUEUE=ON parameter. One can then configure the queuing behavior in the <block> tag within the composition XML. The following queue behaviors can be specified:

  1. drop (default): The default mode is "drop", in which messages are dropped if the InGate queue has no more free slots. Example:
<block id="..." type="..." threadpool="..." invocation="indirect" blocking_mode="drop">
  1. sleep: In this mode, the enqueuing block (i.e., the thread running the block sending a message) sleeps for a configurable amount of time if the queue is full. Example:
<block id="..." type="..." threadpool="..." invocation="indirect" blocking_mode="sleep" sleep_usec="1000">
  1. yield: This mode is very similar to the "sleep" mode. However, the the enqueuing thread simply yields. Example:
<block id="..." type="..." threadpool="..." invocation="indirect" blocking_mode="yield">
  1. mutex: In this mode a mutex is used to block the enqueueing thread until a slot the queue becomes free. Example:
<block id="..." type="..." threadpool="..." invocation="indirect" blocking_mode="mutex">

The above allows you to chose the best suited queueing strategy for your application. Two important points:

  1. The queueing behavior can currently only be configured per block but not per gate.

  2. All methods (except "drop") may block the enqueueing block. This may lead to deadlocks if you are not careful with your composition design and the composition's block-to-threadpool and threads-per-threadpool configuration. E.g., assume you have two indirectly invoked blocks A and B, both assigned to the same threadpool. Also assume that the threadpool only has one thread assigned and that block A sends more messages than block B consumes. In this case the queue of block B will eventually get fully occupied. If any of the non-dropping methods above was specified, the thread will block when block A next tries to send a message. Since this thead is blocked it will not schedule any other block. Thus Blockmon's queue will never get free again, i.e., we are in a deadlock. (This limitation will be removed in a future version of blockmon)


Please follow the format used in blocks/PktCounter.[hpp|cpp]


A Modular System for Flexible, High-Performance Traffic







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