QoS in UDP: trade-offs

In IntServ-like mechanisms.

• An application specifies traffic and QoS parameters.

• A resource reservation protocol estimates and reserves sufficient resources at each node on the path.

In a multi-hop wireless network, however:

• It is hard to estimate available resources.

  • Shared medium.

    • All traffic in the transmission range reduces available bandwidth.

  • Dynamic bandwidth due to node mobility and contention.

• It is hard to do resource reservations.

  • Shared medium reservation requires global coordination.

  • Violations can occur as bandwidth fluctuates.

• Resource reservation is pinned to a route.

  • Must be redone whenever the route changes.

  • This might NOT be a good idea.

    • Time for recovering a route and reserving resources in the new route.

In DiffServ-like mechanisms.

• An application chooses a class of service.

• The network needs admission control to avoid class overload.

• Examples.

  • Assured Forwarding Per-hop behavior assures per-hop throughput.

  • Expedited Forwarding Per-hop behavior assures per-hop low delay.

In a multi-hop wireless network, however.

• It is hard to do admission control.

  • Flows do not go through common ingress nodes.

• It is hard to maintain assurances.

  • Flow distribution varies as routes change.

  • Bandwidth fluctuates.

QoS Routing

Routing is an essential component of QoS.

It can inform a source node of the bandwidth and QoS availability of a destination node and of the path to the destination node.

Add QoS requirements in routing metrics.

• Difficult to route maintenance.

• The overhead of QoS routing.

• Reserved resources may not be guaranteed.

• Needs to be responsive to nodes mobility.

QoS for AODV

Add extensions to the route messages (RREQ, RREP).

The node that receives an RREQ + QoS extension must be able to meet the service requirement in order to rebroadcast the RREQ (if not in the cache).

In order to handle the QoS extensions, some changes need to be done to the routing tables.

AODV current fields.

• The destination sequence number, Interface, Hop count, Next hop, and List of precursors.

AODV new fields (4 new fields).

• Maximum delay.

• Minimum available bandwidth.

• List of sources requesting delay guarantees.

• List of sources requesting bandwidth guarantees.

Delay

Handling Delay.

• Maximum delay extension.

• List of sources requesting delay guarantees.

• Used during the route discovery process.

Losing QoS

Losing QoS parameters.

• If, after establishment, a node detects that the QoS cannot be maintained anymore, it originates an ICMP QoS_LOST message to all depending nodes.

  • Reason to keep a List of Sources requesting delay/bandwidth guarantees.

Reasons for losing QoS parameters.

• The increased load of a node.

• Why would a node take over more jobs than it can handle?