Editing Wormhole switching (section)

== Routing ==

A mix of source routing and logical routing may be used in the same wormhole-switched packet.
The value of the first byte of a [[Myrinet]] or SpaceWire packet is the address of the packet.
Each [[SpaceWire]] switch uses the address to decide how to route the packet.<ref name="Cook-Walker" >
Dr Barry M Cook; Paul Walker.
[https://web.archive.org/web/20110721012137/http://www.4links.co.uk/bibliography/Ethernet-SpaceWire-Software-Issues-Cook-Walker-4Links-IAC-2006-paper.pdf "Ethernet over SpaceWire - software issues"].{{Dead link|date=December 2023|fix-attempted=yes}}
2007.
p. 2.
</ref>

=== Source routing ===
{{Main | source routing }}

With source routing, the packet sender chooses how the packet is routed through the switch.

If the first byte of an incoming SpaceWire packet is in the range 1 to 31,
it indicates the corresponding port 1 to 31 of the Spacewire switch.
The SpaceWire switch then discards that routing character and sends the rest of the packet out that port.
This exposes the next byte of the original packet to the next SpaceWire switch.
The packet sender may choose to use [[source routing]] to explicitly specify the complete path through the network to the final destination in this fashion.<ref name="Cook-Walker" />

=== Logical routing ===

With logical routing, the Spacewire switch itself decides how to route the packet.

If the address (the first byte) of an incoming SpaceWire packet is in the range 32 to 255,
the SpaceWire switch uses that value as an index into an internal routing table that indicates which port(s) to send the packet and whether to delete or retain that first byte.<ref name="Cook-Walker" />

Address 0 is used to communicate directly with the switch, and may be used to set the routing table entries for that switch.<ref name="Cook-Walker" />