Routing

The ISL (Inter-Satellite Link) Routing Protocol provides point-to-point message delivery across a satellite constellation organized as a 2D torus topology.

Topology

Satellites are arranged in a grid where:

• Each satellite has an (orbit_id, satellite_id) address
• Orbits wrap around (orbit 0 is adjacent to orbit N-1)
• Satellites within an orbit wrap around (sat 0 is adjacent to sat M-1)

This forms a 2D torus with four neighbors per satellite:

• North: Previous orbit (orbit_id - 1)
• South: Next orbit (orbit_id + 1)
• East: Next satellite in orbit (satellite_id + 1)
• West: Previous satellite in orbit (satellite_id - 1)

Packet Structure

ISL Routing Packet (10 + N bytes)

SPP Primary Header (6 bytes)

Version3 bitsPacket version (0)

Type1 bit1 = Telecommand

Sec Hdr Flag1 bit1 = Present

APID11 bitsDestination application (0–2047)

Seq Flags2 bitsSegmentation control

Seq Count14 bitsSequence number (0–16383)

Data Length16 bitsRemaining length − 1

cFE Secondary Header (2 bytes)

Function Code8 bitsCommand code

Checksum8 bitsXOR of entire packet

ISL Routing Header (2 bytes)

Target Address16 bitsDestination (orb, sat) or ground

Application Payload (variable)

The SPP and cFE headers are shared across all packet types. The routing layer adds:

• Target Address — the final destination satellite or ground station. The router at each hop reads this field, computes the next hop on the torus, and forwards the packet. The address is not modified in transit — every hop sees the same destination.

Address Encoding

The address wire format (2 bytes):

ground_or_orbit	station_or_sat	Meaning
0	N	Ground station N
K (1-255)	0	Service area for orbit K-1
K (1-255)	M (1-255)	Satellite (orbit=K-1, sat=M)

Routing Algorithm

The router determines the next hop based on the shortest path through the torus:

1. Parse the target address from the packet header
2. Convert source and destination to torus points
3. Calculate Manhattan distance in each direction (accounting for wraparound)
4. Choose the direction that minimizes total distance
5. Forward to the appropriate neighbor link

Direction Selection

if target == self:
    deliver to Local
else:
    calculate shortest path on torus
    forward to North, South, East, or West

Router Operation

The router manages six interfaces:

• north, south, east, west: Inter-satellite links
• ground: Earth communication link
• local: Application interface

Main Loop

loop {
    packet = recv from any interface
    target = parse packet header
    direction = next_hop(target)
    forward packet to direction
}

Error Handling

Condition	Behavior
Invalid packet format	Drop silently
Link send failure	Log error, continue
Unknown target	Route based on torus algorithm

Configuration

The router requires:

• Own address (orbit_id, satellite_id)
• Constellation dimensions (max_orb, max_sat)
• Routing algorithm implementation