Advanced Computer Networks

Data center networking

In data centers, servers are organized in interconnected racks.

Performance metrics:

• bisection width: minimum number of links cut to divide network in half
• bisection bandwidth: minimum bandwidth of links that divide network in half
• full bisection bandwidth: one half of nodes can communicate at the same time with other half of nodes

Oversubscription: ratio (worst-case required aggregate bandwidth among end-hosts)/(total bisection bandwidth of topology)

Fat-tree topology

Example of 4-port fat-tree topology:

Allows full bisection bandwidth between core and aggregation switches.

For k-port switches:

• need (5k²/4) switches
• can have k³/4 servers

Addressing:

• pod switches: 10.pod.switch.1 (pod, switch ∈ [0, k-1])
• core switches: 10.k.j.i (i and j denote core positions)
• hosts: 10.pod.switch.id

Forwarding: two-level lookup table

• prefixes for forwarding traffic in pod
• suffixes for forwarding traffic between pods

Each host-to-host communication has single static path.

Flow-collision happens when multiple flows use same path. Solutions:

• equal-cost multi-path (ECMP): static path for each flow
• flow scheduling: centralized scheduler assigns flows to paths

Challenges & issues

• must be backward compatible with IP/Ethernet
• complex wiring
• no support for seamless VM migration (would break TCP connection)
• plug-and-play not possible, IPs have to be preassigned

PortLand

Separate node location (Pseudo MAC) from node identifier (Host IP).

Fabric manager maintains IP → PMAC mapping .

Switches self-discover location by exchanging Location Discovery Messages.