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ACP 142 Parameters for Radio and Satellite Networks
Purpose
ACP 142 specifies a protocol for reliable multicast. It is used for message transmission over networks such as Radio and Satellite. ACP 142 defines a number of protocol parameters which are discussed in the context of different networks. The goals of this paper are to:
- Provide some understanding of the parameters and appropriate settings for different networks.
- To enable feedback from protocol experts and from deployment experience on appropriate values for the various parameters. Isode plans to share this information in updated versions of this document, and to use it in future versions of the Isode management tools, to enable easy setting of optimal parameters.
ACP 142
ACP 142 defines a reliable multicast protocol, sometimes referred to as P_Mul. ACP 142 was originally designed to support STANAG 4406 formal military messaging. ACP 142 and its use with STANAG 4406 are described in the Isode white paper Military Messaging over HF Radio and Satellite using STANAG 4406 Annex E. Use of ACP 142 to support Internet email is described in the Isode white paper Messaging Protocols for HF Radio. As well as supporting multicast, ACP 142 supports nodes that do not transmit data (EMCON or Emission Control).
ACP 142 operates over an unreliable connectionless service that will typically be provided by one of:
- UDP (User Datagram Protocol) running over IP.
- UDOP (Unreliable Datagram Oriented Protocol) running over STANAG 5066.
The connectionless nature of the bearer service requires ACP 142 to define a number of timers. Configuring these timers is the primary subject of this paper.
Network Characteristics
There are a number of network types that ACP 142 may be used for, with widely varying characteristics, that have significant impact on ACP 142
| Network | Bandwidth | Latency | Reliability | Recommended Bearer |
|---|---|---|---|---|
| Satellite | 4800 bits per second to 1 Mbit per second | 0.5 seconds physical latency for a geostationary satellite. Use of multiple satellites is common | Very high | IP |
| Radio above UHF | Fast | Low | Fair | IP |
| UHF | 9600-56,000 bits per second | Low | Fair | IP or STANAG 5066 |
| VHF | 9600 bits per second | Low | Fair | STANAG 5066. IP will degrade performance but is acceptable |
| HF (Beyond Line of Sight Radio) | 75-9600 bits per second | Bandwidth can often vary dynamically and dramatically Minimum latency is typically 5-30 seconds. Data link layer will often increase latency (up to two minutes) in order to optimize link utilization |
Fair to poor | STANAG 5066. IP performance would be unacceptable |
Isode Management Tool Goals
The current release of the Isode configuration GUIs for ACP 142 allow direct configuration of the ACP 142 parameters described in this paper. The primary reason for this is that there is relatively little operational experience, and it is likely to be important to tune some of these parameters in order to gain optimum performance.
As experience is gained, Isode plans to provide a higher level interface that will set parameters that can be easily determined (such as the type of network used) and set the ACP 142 parameters according to this. Explicit control of some parameters may be retained as an advanced option.
ACP 142 Parameters
The ACP 142 parameters are discussed in three groups:
- Parameters for general use.
- Parameters used for EMCON transmission.
- Parameters used for setting up dynamic multicast.
Each parameter is described, with notes on the effect of it being set too high or too low. Then values are suggested for a range of networks. Many of these values are suggested based on theoretical analysis of what makes sense, and practical input on these numbers would be very desirable. All times are in seconds and bandwidths in bits/second unless explicitly noted otherwise.
These STANAG 5066 values assume that a node will break transmission after 127.5 seconds and allow other nodes to transmit. Some STANAG 5066 servers will transmit until all data is sent. For such systems, it may make sense to set some values (in particular RE-TRANSMISSION_TIME, BACKOFF_FACTOR, an ACK-PDU_TIME) to larger values.
