Computer Networks 6

congestion

situation in which too many packets demand for the same link resulting in long queues and drops of packets

resource allocation

network elements try to meet the competing demands that applications have for network resources, e. g: by limiting the bandwidth or buffer space

congestion-control

efforts made by network nodes to prevent or respond to overload conditions

Router Centric approach

each router takes responsibility for deciding when packets are forwarded and selecting which packets are to be dropped

Host Centric approach

the end hosts observe the network conditions and adjust their behaviour accordingly

Reservation Based approach

some entity asks the network for a certain amount of capacity to be allocated for a flow

Feedback Based approach

end hosts begin sending data without first reserving any capacity and then adjust their sending rate according to the feedback they receive

Window Based approach

express to the sender how much data it is allowed to transmit through advertising of a window to the sender

Rate Based approach

express to the sender how much data it is allowed to transmit through rate how many bits per second the receiver or network is able to absorb

Queuing Algorithm

allocating both bandwidth (which packets get transmitted) and buffer space (which packets get discarded)

Fair Queuing (FQ)

separate queue for each flow currently being handled by the router

Additive Increase/Multiplicative Decrease (AIMD

TCP source sets the CongestionWindow based on the level of congestion it perceives to exist in the network. Then it decreases the congestion window when the level of congestion goes up and vice versa

Slow Start Mechanism

increase the congestion window rapidly from a cold start. Slow start effectively increases the congestion window exponentially, rather than linearly

Fast Retransmit Mechanism

triggers the retransmission of a dropped packet sooner than the regular timeout mechanism

Fast Recovery Mechanism

when fast retransmit mechanism signals congestion rather than drop the congestion window all the way back to one packet and run slow start it is possible to use the ACKs that are still in the pipe to clock the sending of packets

Active Queue Management (AQM)

additional functionalities in the router to assist the end node to prevent congestion

DECbit mechanism

each router monitors the load it's experiencing and explicitly notifies the end nodes when congestion is about to occur

Random Early Detection (RED)

each router monitors its own queue length and when it detects that congestion is imminent, notify the source to adjust congestion window. However implicitly notifies the source about congestion by dropping one of its packets

Explicit Congestion Notification (ECN)

instead of dropping a packet, mark it and continue to send it along its way to the destination

Real-time applications

applications that are sensitive to the timeliness of data exchange

Non-real-time applications

applications that can work without guarantees of timely delivery of data

Integrated Services mechanism

guarantees QoS (quality of service) requirements through implementation of service classes

Guaranteed Service Classes

used for intolerant applications; these applications require that a packet never arrive late, network should guarantee that the maximum delay that any packet will experience has some specified value

Controlled Load Service

meet the needs of tolerant, adaptive applications; The goal is to emulate a lightly loaded network for those applications that request the service, even though the network as a whole may in fact be heavily loaded

adaptability

application can adapt to the amount of delay that packets experience as they traverse the network

tolerance for loss of data

data loss might occur because a packet arrived too late to be played back

Per-Hop Behaviour (PHB)

defined behaviour of individual routers

Expedited Forwarding PHB

packets marked for EF treatment should be forwarded by the router with minimal delay and loss

Assured Forwarding PHB

packets marked with AF are forwarded with higher reliability