Congestion Control - Advanced Computer Networks - Lecture Slides, Slides of Computer Networks

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Congestion Control

and

Resource Allocation

Definitions

• Flow control:: keep a fast sender from overrunning a slow receiver.
• Congestion control:: the efforts made by network nodes to prevent or respond to overload conditions.

Congestion control is intended to keep a fast

sender from sending data into the network due to a lack of resources in the network {e.g., available link capacity, router buffers}.

Figure 6.1 Congestion in a packet-switched

network

Router 1.5-Mbps T1 link Destination

Source 2

Source 1

100-Mbps FDDI

10-Mbps Ethernet

Flows

• flow :: a sequence of packets sent between a source/destination pair and following the same route through the network.
• Connectionless flows within the TCP/IP model:: The connection-oriented abstraction, TCP, is implemented at the transport layer while IP provides a connectionless datagram delivery service.
• With connectionless flows, there exists no state at the routers.

Figure 6.2 Multiple Flows passing through a set of

routers

Router

Source 2

Source 1

Source 3

Router

Router

Destination 2

Destination 1

Service

• Best-effort service :: The hosts are given no

opportunity to ask for guarantees on a flow’s service.

• QoS (Quality of Service) :: is a service model

that supports some type of guarantee for a flow’s service.

Congestion Control Taxonomy

• Router-Centric
  • The internal network routers take responsibility for:
    • Which packets to forward
    • Which packets to drop or mark
    • The nature of congestion notification to the hosts.
  • This includes the Queuing Algorithm to manage the buffers at the router.
• Host-Centric
  • The end hosts adjust their behavior based on observations of network conditions.
  • (e.g., TCP Congestion Control Mechanisms)

Congestion Control Taxonomy

• Reservation-Based – the hosts attempt to

reserve network capacity when the flow is established.

• The routers allocate resources to satisfy reservations or the flow is rejected.
• The reservation can be receiver-based (e.g., RSVP) or sender-based.

Evaluation Criteria

• Evaluation criteria are needed to decide how well a network effectively and fairly allocates resources.
• Effective measures – throughput, utilization, efficiency, delay, queue length, goodput and power. throughput Power = -------------- delay

Fairness

• Jain’s fairness index

For any given set of user throughputs (x 1 , x 2 ,…x n ), the fairness index to the set is defined:

f(x 1 , x 2 , …, x n ) =

 

n i

n xi 1

2

2 1

 

  

 

n i i

x

Congestion Control

(at the router)

• Some of the possible choices in queuing

algorithms:

• FIFO (FCFS) also called Drop-Tail
• Fair Queuing (FQ)
• Weighted Fair Queuing (WFQ)
• Random Early Detection (RED)
• Explicit Congestion Notification (ECN).

Drop Tail Router [FIFO]

• First packet to arrive is first to be transmitted.
• FIFO queuing mechanism that drops packets from

the tail of the queue when the queue overflows.

• Introduces global synchronization when packets are

dropped from several connections.

• FIFO is the scheduling mechanism, Drop Tail is the

policy

Priority Queuing

• Problem:: high priority packets can ‘starve’

lower priority class packets.

• Priority queuing is a simple case of

“differentiated services” [DiffServ].

• One practical use in the Internet is to

protect routing update packets by giving them a higher priority and a special queue at the router.

Fair Queuing [FQ]

• The basic problem with FIFO is that it does

not separate packets by flow.

• Another problem with FIFO :: an “ill-

behaved” flow can capture an arbitrarily large share of the network’s capacity.

Idea:: maintain a separate queue for each

flow, and Fair Queuing (FQ) services these queues in a round-robin fashion.