EMCON extensions to ACP 142 to support BRASS and Data Diode (in R14.6)

February 11, 2010

This is one in a series of blog posts describing new features of Isode’s R14.6 release, scheduled for March 2010. You can see all posts relating to R14.6 by clicking this link.

EMCON (Emission Control) is a military term for Radio Silence, where nodes can receive data but not send. Isode supports messaging to EMCON recipients (both STANAG 4406 and Internet Mail) in M-Switch using ACP 142, which is a standard for multicast data transport. This is described in the whitepaper “Military Messaging over HF Radio and Satellite using STANAG 4406 Annex E“.

ACP 142 has a model of two (receiver) EMCON states: normal vs. EMCON. This two state model is insufficient and so Isode has made an extension to its ACP 142 implementation to use four receiver states, which aligns to the ACP 133 military directory specifications. The four states are:

  • Normal  (ACP 133 Enabled)
  • EMCON (ACP 133 Receive Only)
  • Radio Silence (ACP 133 Electrical/Electronic Silence)
  • Disabled (ACP 133 Disabled)

An ACP 142 sender will handle Normal and EMCON destinations as specified in ACP 142. For disabled receivers, no transmissions will be made. This saves resource and allows transmissions to other destinations. The standard ACP 142 EMCON mechanism is for the sender to transmit the message a configurable number of times at intervals. It then retains the message and waits for the destination to leave the EMCON state, when the destination is expected to acknowledge the message.

If the message delivery time limit is reached before any acknowledgement is received, the message is non-delivered. This model works well for systems that are in EMCON for a relatively short period of time, and ensures that the originator will always receive error information.

The ACP 142 model does not work well for receivers that can never send data or cannot send data for very long periods. So, Isode’s extended implementation treats receivers in Radio Silence state differently to EMCON state, and after all of the transmissions are made, the transfer is considered to have succeeded (and so no error report is sent at this point).

There are number of situations where this new mode is useful.

  1. Where a receiving system is unable to transmit for a long period of time. In most cases, repeated transmissions will ensure that messages are transferred, although the sender has no means to tell whether or not this is the case. The new mode (which treats transfers as successful) is more useful, as this reflects the majority case.
  2. When a data diode is used with one way transfer, as described in the Isode whitepaper ” Using Isode’s Messaging and Directory Applications with a Data Diode“. The transfer is very reliable and there is not possibility of getting error messages back. Here the new mode does exactly what is needed.
  3. NATO BRASS (Broadcast as Ship to Shore) is a radio setup where messages are sent to all ships by a transmitter that only broadcasts data (one to many), and messages in the reverse direction (ship to shore) are sent unicast (one to one). The new mode enables ACP 142 to be used for the broadcast portion of BRASS.

These relatively simple changes enable Isode’s ACP 142 system to be useful in a number of new types of deployment.