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Is a protocol for dynamic realtime networking in embedded systems. A fully meshed network with minimum hardware overhead. Increased determinism in the face of network congestion, and reduced message delivery times.
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## To be Addressed
### What is our Objective Cost Function
##### Packet Delivery Times in the face of Increasing Network Traffic
Packet Delivery Time is the critical metric in Networked Control Systems - small amounts of information (sensor, motor data) needs to be delivered quickly.
In Switched Ethernet, because a Minimum Spanning Tree is created, nodes in a particular layer compete for link-time on the layer above. Message delay time increases linearly with the probability that peers are transmitting at the same time, and with the number of peers on that layer.
~ plot w/ x-axis is # nodes, transmitting at some % of time, y axis is per-packet delivery time ~
~ graph is single branch / spanning tree
Critically, adding multi-path to the graph above decreases the slope of this plot. Twice the total link-time on the layer above is available, or Nx, where N is the number of nodes added to the next.
* state of the art for this? there is lit review to do here - look up TRILL and SPB (shortest path bridging). Much of it comes from Data Center Networks.
- https://datacenteroverlords.com/2011/08/19/multi-path-ethernet-the-flying-cars-of-the-data-center/
- http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6888840
- https://en.wikipedia.org/wiki/Equal-cost_multi-path_routing
- https://infoscience.epfl.ch/record/231114/files/main_nr.pdf
Also critically, *it seems that, in a cursory overview* many multi-path techniques in use today basically do dynamic spanning-tree rebuilds. These spanning-tree rebuilds have long convergence times (162ms is excitingly fast).
However, to increase determinism routes need to be chosen dymanically - so that *very short timescale* changes can be made to a packet, mid-network-traverse, to route around busy switches. Of interest is finding evidence that this is not done in any existing system.
### What are our Constraints
##### Minimum or No Hardware Overhead
System should be arbitrarily implemented on microcontrollers, in software, with system-designer defined hardware and network parameters. No big jacks, no small bits, no proprietary IC's or black box IC's. Open Source Network for Realtime Control.
Network Switching should take place in the same processor as computation for the application - I.E. chips that do motor control should also do network switching. This allows network implementation to shrink into micro-robotics, where singular processors are available at each joint / DOF, and these processors perform network tasks as well as application (control) computation.
### Key Contribution
A real-time cost function, using next-hop buffer size (i.e. business metric) as well as historical hop-count for per-packet dynamic re-routing, that increases packet delivery-time determinism.
A software-defined network architecture for arbitrary implementation in any embedded system, where computing, physical space, and time is limited.
### Direct Application
##### Open Source Reconfigurable Hardware Systems
In particular, hardware design for embedded systems in the open source (i.e. non-expert) realm. We want to offer a networking solution that allows open source designers to easily integrate their systems. We take the example of a proliferation of 3D Printer Control systems, none of which interop, and the interfaces between are a PITA. Expand on this.
# TinyNet Protocol & Architecture
We develop a switch, protocol and implementation of a software-defined network that:
- Does Realtime Route Selection
- Does Automatic, Convergence-free Route Discovery and Optimization
- Can be arbitrarily implemented in software on numerous microcontrollers
- 10-bit address (1024 Unique in System)
- Addresses are assigned in software (Ethernet: Hardware Addresses)
- Can be location-based (e.g. first five MSBs correspond to x, last five correspond to y)
| Type | 8 Bits | 10 Bits | 6 Bits | 10 Bits | 6 Bits | N Bytes | CRC |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Standard: | 255 | Dest. | Hop Count | Src. | # Bytes | Payload … | CRC |
| ACK: | 253 | Dest. | Hop Count | Src. | x | x | CRC |
| Buffer Length: | [0 - 251] | x | x | x | x | x | x |
* previously-flooded Standard Packets have start delimiter 254
* previously-flooded Acks have start delimiter 252
if hop count > max:
kill packet
increment hop count
if packet is standard:
if LUT does not already have source address:
add entry to LUT
if I am destination:
process data in packet
else:
if LUT has destination address:
send packet to port which minimizes C(hops, buffer) = hops + \lambda*buffer over all ports
else:
send packet to all ports as standard flood
elseif packet is ack:
if LUT does not already have source address:
add entry to LUT
if I am destination:
else:
increment hop count
if LUT has destination address:
send packet to port which minimizes C(hops, buffer) = hops + \lambda*buffer over all ports
else:
send packet to all ports as ack flood
elseif packet is standard flood:
if LUT does not already have source address:
add entry to LUT
if I am destination:
process data in packet
open window for duplicate packet elimination
check previous duplicates
else:
if LUT has destination address:
send packet to port which minimizes C(hops, buffer) = hops + \lambda*buffer over all ports
else:
send packet to all ports
elseif packet is ack flood
if LUT does not already have source address:
add entry to LUT
if I am destination:
process acknowledgement, increment window
open timer for duplicate ack elimination
check previous duplicates
else:
if LUT has destination address:
send packet to port which minimizes C(hops, buffer) = hops + \lambda*buffer over all ports
else:
send packet to all ports
The routing table (or lookup table, LUT) consists of rows of:
| Destination | Seen on Ports | Min. Hopcount Recorded on Port | Port buffer size |
This is different from standard Ethernet routing tables:
| Ports | Destinations seen on Port |
Because it includes the current port buffer size and number of hops as part of the table entry, allowing for a more robust cost function for use in the path planning algorithm.
## Buffer Depth Updates
Send buffer depth on all ports every q seconds, and every time a packet leaves or arrives
## Announcements
New arrivals to network do not announce, they simply begin transmitting. Their addresses are recoreded in surrounding switches' tables on their first packet-out.
## Withdrawals
Buffer Depth Updates are Periodic as well as event-based (on buffer-depth change). When no BDU is heard within a 250ms (or other setting) window, the node is considered withdrawn.
See /circuit
See /embedded
# Reading
Ethernet in Networked Control, advantages and drawbacks.
See especially
**/litreview/papers/network-control-systems/survey-on-realtime-via-ethernet**
**/litreview/papers/network-control-systems/the-emergence-of-networked-controls**
#### Robotics
Papers on the particular applications of distributed control in a robotics context
#### ALA-APA-ATP
Prior work from our lab on networking, aligning hardware with software, etc ...