Data link layer

• [3.2] Data link layer (OSI 2) transports coherent frames of bits between known participants.
• [3.2.1] Provides ordered bit sequence to physical layer below, provides frames to network layer above. [3.3] Services provided:
  • Delivers frames intact OR drops them (bits in a frame share fate).
  • Provides frame-level error detection, discarding malformed ones.
  • Addresses destinations (e.g. MAC).
  • Abstracts over physical layer (single connection \(\neq\) single physical path).
• [3.4] Low latency/jitter important as they bottleneck upper layer performance.

Ethernet frame structure

• [3.6] Preamble: alternating 1/0s to sync clocks.
• Start of frame: 0xAB.
• Header: destination/source MAC, type/length field.
• Payload: for upper layers.
• Frame Check Sequence (FCS): Cyclic Redundancy Checksum [3.9].
• Post-frame gap: idle for some time.

Link aggregation

• [3.11] Link Aggregation Control Protocol (LACP)
  • Runs at both endpoints.
  • Sends frames under one of multiple live physical links.
    • Uses same link to maintain ordering.
  • Multicast LACP frames as keepalives.

Scaling

Collisions

• [3.8] Collisions: when multiple transmissions on same medium detected, stop sending and try after random time (exponential backoff).
• [4.2] Switches split network into collision domains.
• [4.3] Source MAC learning: use source MACs to maintain address table.
  • [4.4] Timeouts necessary since networks can change.
  • Advertising frames sent upon connection.

Loops

• [4.5] Loops must be avoided as they very quickly clog network.
• [4.6] Spanning Tree Protocol to create spanning tree to use instead.
  • [4.7.1] Bridge Protocol Data Unit: payload of STP frame.
  • Root originates, and each switch forwards (with updates):
    • Root known so far, which is lowest ID (root/bridge ID fields).
    • Its distance from root (root path cost field).
  • Block all ports on each link other than min-cost port.
  • BPDUs also used as keepalives to detect failures.
  • Topology Change Notifications sent up to root then down throughout entire network.
  • Slow to converge (50s) due to Max Age + twice Forwarding Delay to change port states. (Rapid STP makes this faster.)
• [4.8] Ethernet Ring Protection Switching for ring networks.
  • Ring protection link owner and its neighbour block link inbetween.
  • Failed link blocked, originally blocked link unblocked.
  • Downtime \(<\) 50ms.