Transport Layers

eCAL is able to communicate on different so called transport layers, but first of all you have to decide if you want to communicate in a network or in a local host only mode. To configure this you need to set the ecal.ini [network/network_enabled] parameter to true (network communication mode) or false (local host only communication mode).

After this you can fine tune the way of message transport for inner-process, interprocess and the interhost connections. There are different ways to configure these layers. They can be set up for a whole machine using the central configuration file (ecal.ini) or for a single eCAL process passed by command line arguments or finally for a single publish-subscribe connection using the C++ or python publisher API. Every single builtin transport layer has it’s specific communication properties.


ini parameter

Physical Layer




inner process

inner process, zeroy copy communication (pointer forwarding)



shared memory

interprocess, shared memory communication, supports N:M connections, 2 memory copies



udp multicast

interhost, topic name based dynamic multicast grouping to optimize pub/sub socket payload

Every layer can set up in 3 different activation modes. Every mode can be configured as default in the ecal.ini file and can be overwritten by the C++/Python publisher API. This is the activation logic

  • off: layer is switched off

  • on: layer is always switched on (i.e. payload will be send no matter if there is any local or network subscription)

  • auto: layer will be switched on automatically

    • inproc = 2 : layer used automatically for inner process subscribers

    • shm = 2 : layer used automatically for inter process subscribers

    • udp_mc = 2 : layer used automatically for inter host (network) subscribers

Independent from this publisher setting you can switch on/off the receiving (subscription) logic for every layer. That means you can prevent incoming payload on specific layers. This can be done in the ecal.ini file [network] section.

  • inproc_rec_enabled = true / false : enable / disable inner process subscriptions

  • shm_rec_enabled = true / false : enable / disable inter process subscriptions

  • udp_mc_rec_enabled = true / false : enable / disable inter host subscriptions