Culler Chapter 10

   Network topologies
    Fully connected
    Linear Arrays  (SCI, FDDI, FiberChannel)
    Multidimensional Meshes and Tori  (J-Machine)
    Trees
    Butterflies  (Connection Machine)
    Hypercube (SGI Origin, IBM SP-2)
    Arbitrary  (Myrinet, others)

   Routing Issues
    Where to put the routing information?
      Dimention ordered  (match first digit, then second, ...)
      Source Based  (Packet contains switch decisions)
      Table driven  (Lookup table in switch)
     
    Deterministic vs Adaptive

    Deadlock problems.
      Don't allow messages to form gridlock.
      Fix with:
        Virtual channels.
        Up-Down Routing (building a DAG)     
        Turn-Model Routing (Once turned from x dir to y, no more turns)
          (Examples: West-first, North-last, Negative-first)
      
  Switch Design
    Input Buffering:  Buffer on input port
      Problem: head-of-line blocking.
    Other alternatives:
      Output Buffering
      Shared Pool
      Virtual Channels buffering

    Scheduling:
      When to handle a packet.

  Flow Control:
    Ethernet: Fail to transmit, backoff
    Link-Level:  
      ACKS and NACKS
      READY and STOP
      Multiple inflight messages
      Slack Buffer  (Low mark/high mark)

  Case Studies:
    Cray T3D:  3D Torus
    IBM SP-2:  Locally Butterfly. External Variant of a butterfly 
    Scalable Coherent Interface (SCI):  
	     Rings
	     Numa-Q / HP Convex Exemplar
    SGI Origin:  SPIDER network  
	Hypercube
    Myrinet:  Source base switched arbitrary.