Transcript Scheduling

CIS679: Scheduling, Resource Configuration
and Admission Control
 Review of Last lecture
 Scheduling
 Resource configuration
 Admission control
Review of last lecture
Scheduling Mechanisms
 Scheduling: choosing the next packet for transmission on a
link can be done following a number of policies;
 FIFO: in order of arrival to the queue; packets that arrive
to a full buffer are either discarded, or a discard policy is
used to determine which packet to discard among the arrival
and those already queued
Scheduling Policies
 Priority Queuing: classes have different priorities; class may
depend on explicit marking or other header info, eg IP
source or destination, TCP Port numbers, etc.
 Transmit a packet from the highest priority class with a
non-empty queue
 Preemptive and non-preemptive versions
Scheduling
 Scheduling:
 FIFO
 Priority Scheduling (static priority)
 Round Robin
 Weight Fair Queuing (WFQ)
Priority-driven Scheduler
 packets are transmitted according to their priorities;
within the same priority, packets are served in FIFO
order.
 Complex in terms of no provable bounded delay due to no
flow isolation
 Simple in terms of no per-flow management: SP make it
possible to decouple QoS control from the core-router.
D = ??
max
Round Robin
 Round Robin: scan class queues serving one from
each class that has a non-empty queue
WFQ
 Weighted Fair Queuing: is a generalized Round
Robin in which an attempt is made to provide a
class with a differentiated amount of service over
a given period of time
WFQ (more)
 Worst case traffic arrival: leaky-bucket-policed source
 Complex in terms of having per-flow isolation mechanism,
hence needing per-flow state maintenance and resource
reservation at per-element: WFQ couple QoS control to the
core-router.
 Simple in terms of having mathematically provable bound on
delay, which makes admission control simple.
arriving
traffic
token rate, r
bucket size, b
WFQ
per-flow
rate, R
D = b/R
max
Resource Configuration
 Traffic engineering
 QoS routing
 Resource provisioning
 Network planning
 Network design
Admission Control
 Session must first declare its QOS requirement
and characterize the traffic it will send through
the network
 R-spec: defines the QOS being requested
 T-spec: defines the traffic characteristics
 A signaling protocol is needed to carry the R-spec
and T-spec to the routers where reservation is
required; RSVP is a leading candidate for such
signaling protocol
Admission Control
 Call Admission: routers will admit calls based on
their R-spec and T-spec and base on the current
resource allocated at the routers to other calls.
Conclusion
 Scheduling:
 Decide the order of packet transmission
 Resource configuration
 Admission control