Bandwidth Management

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• On a local network, delay is typically caused by:
  • Congestion
    • is where the network infrastructure:
      • is not capable of meeting the demands of peak load
      • starts to queue or drop packets
  • Contention
    • the ratio between demand for a service and its available capacity
    • e.g.,
      • if 100 video conferencing hosts each requiring 10 Mbps share a 1 Gbps link
        • the contention ratio is 1:1 (100 * 10 Mbps = 1 Gbps)
      • if 200 hosts, then 2:1
    • is a planning issue
      • don’t expect all 200 hosts to run simultaneously
• if unacceptable levels of congestion or contention, can:
  • provision higher bandwidth links and/or faster switches and routers
  • use a bandwidth management mechanism
    • e.g., QoS

Differentiated Services

Differentiated Services (DiffServ) framework classifies each packet passing through a device.

• router policies can then be used to prioritize based on packet class
• works at layer 3
• is an IP service tagging mechanism
  • uses Type of Service field in the IPv4 header
    • Traffic Class in IPv6
  • field is populated with a 6-byte DiffServe Code Point (DSCP)
    • by the sending host or router
  • packets with same DSCP and destination are referred to as behavior aggregates
    • allocated the same Per Hop Behavior (PHB) at each DiffServ-compatible router
• 3 types of DiffServ traffic classes:
  • best effort
  • assured forwarding
    • broken down into sub-levels
  • expedited forwarding
    • highest priority

IEEE 802.1p

IEEE 802.1p is used to classify and prioritize traffic passing over a switch or wireless access point.

• works at layer 2
• defines a tagging mechanism within the 802.1Q VLAN field
• referred to as 802.1Q/p
• 3-bit priority field is set to a value 0-7
• vendors map DSCP values to corresponding 802.1p values
  • e.g.,
    • 7 and 6 reserved for network control
    • 5 and 4 map to expedited forwarding levels for two-way comms
    • 3 and 2 map to assured forwarding for streaming multimedia
    • 1 and 0 for “ordinary” best-effort delivery

Info

• VLAN infrastructure is often used for traffic management on local networks
• e.g., voice traffic might be allocated to different VLAN than data traffic