PROVIDING PER-FLOW QoS (QUALITY OF SERVICE) IN PACKET-SWITCHED NETWORKS

# Specifying the QoS needs and traffic of flows

# Resource management schemes for routers to offer per-flow service.

# Connection establishment/teardown mechanisms

# Mapping end-to-end QoS to local (per-hop) QoS (admission control)
 

PICTURE:  source  --> router --> router ---> ... ---> destination
          Mapping end-to-end bw/delay to per-hop QoS bw/delay
 
 

FLOW CHARACTERIZATION

# Each flow has
  - traffic descriptor: average tx rate, max burstiness, ...
  - QoS desired: max delay, max jitter, max loss
    statistical or deterministic characterization

# Packets identify their flows by flow-id
  - source and destination IP-addresses
  - source and destination IP-addresses and ports
  - explicit flow id field (IPv6)
 

ROUTER RESOURCE-SHARING MECHANISMS

# Each router allocates resources (bw, buffer) to flows in relation to
  their traffic descriptor and QoS.
  - each flow is guaranteed a minimum prespecified amount of resources
  - resources unused by a flow can be used by other flows (otherwise this
    would be no better than TDMA)

# Buffer sharing schemes
  - each flow gets a fixed amount of buffer space
  - remaining buffe space is dynamically shared among all flows using some
    queueing discipline (fifo, priority, etc)

# Bandwidth sharing schemes
  - weighted round-robin
  - weighted fair queueing
  - etc

# Because packet sizes cannot be made very small (unlike cpu bursts),
  the resource sharing is fair only over time intervals exceeding some
  prespecifed duration.
 
 

LEAKY BUCKET

# Popular approach to characterizing and enforcing traffic descriptor.

# Parameterized by average tx rate (R) and max burst size (B).

# Implemented by a counter represnting a token pool.
  Once every 1/R seconds:
      counter := max( B, counter+1 ).
  When source sends a packet:
    if counter > 0 then
       send packet into network and decrease counter by 1
    else packet is dropped, waits, or is sent at lower priority
 
 

QoS MULTICASTING

    picture:  source   multicast tree   several destinations

# Each multicast group has a single multicast address.

# Providing QoS when destinations have varying connectivity (modem, ISDN,
  ADSL, LAN, T1, ...)
  - source data is hierarchically encoded
  - sends all layers to multicast tree (ie, to the first router)
  - each destination informs its first uptree router of its rx bw
  - these requests go back uptree, getting merged, ..

END NOTE 3