switch: draft 1

switch.md

@@ -1,3 +1,54 @@
-# Switch
+# The FC Switch
 
-TODO: add skeleton
+The Switch provides multihop message passing.
+It receives packets from several Peering Connections,
+and passes them on to the respective next hop,
+as defined by the packet's Switch Label.
+
+The Switch operates below the Routing layer and above the Peering layer.
+The Switch is to FC, what Layer 2 is to an IP/Ethernet network.
+
+## Overview
+
+- Wire
+- Switch label
+  - Encoding schemes
+    - Fixed-width vs. variable-width
+  - Label operations
+- Packets
+- PeerLink
+  - Congestion Control
+  - Multipath Peering
+
+The Router adds a Switch Label to the packet,
+which is used by the Switch to pass the packet on
+to the right connection maintained by the Peering Controller.
+
+An fc00 node doesn't neccessarily run the layers above the Switch.
+It can just as well merely provide transit to its peers.
+(how to look past the switch?)
+
+The Peering Controller opens, maintains, and closes Peering Connections.
+Every connection is registered with the Switch, which assigns it a Director.
+Multiple connections might be registered per peer,
+and will result in separate Directors, e.g. for Wifi and cable.
+In addition to outgoing and incoming connections,
+a "self connection" is registered, which is not backed by a transport.
+The self connection is used by the router to transmit and receive packets.
+
+## Packet Layout
+
+```
+                     1               2               3
+     0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  0 |                                                               |
+    +                         Switch Label                          +
+  4 |                                                               |
+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+  8 |   Congest   |S| V |labelShift |            Penalty            |
+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ 12 |                                                               |
+    +                 Data Packet or Control Packet                 +
+    |
+```