Congestion Control
CONGESTION
An important issue in a packet-switched network is congestion. Congestion
in a network may occur if the load on the network-the number of packets
sent to the network-is greater than the capacity of the network-the number
of packets a network can handle. Congestion control refers to the
mechanisms and techniques to control the congestion and keep the load
below the capacity. We may ask why there is congestion on a network.
Congestion happens in any system that involves waiting. For example,
congestion happens on a freeway because any abnormality in the flow, such
as an accident during rush hour, creates blockage.
CONGESTION
Queues in a router
We need to be aware of two issues. First, if the rate of packet arrival is higher than the
packet processing rate, the input queues become longer and longer. Second, if the
packet departure rate is less than the packet processing rate, the output queues
become longer and longer.
CONGESTION
• Packet delay and throughput as functions of load
Congestion control involves two factors that measure the performance of a network:
delay and throughput.
CONGESTION CONTROL
Congestion control refers to techniques and mechanisms that
can either prevent congestion, before it happens, or remove
congestion, after it has happened. In general, we can divide
congestion control mechanisms into two broad categories:
open-loop congestion control (prevention) and closed-loop
congestion control (removal).
Congestion Control Policies:
Open-Loop Congestion Control
Closed-Loop Congestion Control
Congestion control categories
Open Loop Policies
Retransmission Policy Retransmission is sometimes unavoidable. If the sender feels
that a sent packet is lost or corrupted, the packet needs to be retransmitted.
Retransmission in general may increase congestion in the network. However, a good
retransmission policy can prevent congestion. The retransmission policy and the
retransmission timers must be designed to optimize efficiency and at the same time
prevent congestion. For example, the retransmission policy used by TCP is designed
to prevent or alleviate congestion.
Window Policy The type of window at the sender may also affect congestion. The
Selective Repeat window is better than the Go-Back-N window for congestion
control. In the Go-Back-N window, when the timer for a packet times out, several
packets may be resent, although some may have arrived safe and sound at the
receiver. This duplication may make the congestion worse. The Selective Repeat
window, on the other hand, tries to send the specific packets that have been lost or
corrupted.
Open Loop Policies
Discarding Policy A good discarding policy by the routers may prevent congestion
and at the same time may not harm the integrity of the transmission. For example,
in audio transmission, if the policy is to discard less sensitive packets when
congestion is likely to happen, the quality of sound is still preserved and congestion
is prevented or alleviated.
Acknowledgment Policy The acknowledgment policy imposed by the receiver
may also affect congestion. If the receiver does not acknowledge every packet it
receives, it may slow down the sender and help prevent congestion. Several
approaches are used in this case. A receiver may send an acknowledgment only if it
has a packet to be sent or a special timer expires. A receiver may decide to
acknowledge only N packets at a time. We need to know that the acknowledgments
are also part of the load in a network. Sending fewer acknowledgments means
imposing less load on the network.
Open Loop Policies
Admission Policy
An admission policy, which is a quality-of-service mechanism can also prevent
congestion in virtual-circuit networks. Switches in a flow first check the resource
requirement of a flow before admitting it to the network. A router can deny
establishing a virtual-circuit connection if there is congestion in the network or if
there is a possibility of future congestion.
Closed Loop Policies
Backpressure
The technique of backpressure refers to a congestion control mechanism in which a
congested node stops receiving data from the immediate upstream node or nodes.
This may cause the upstream node or nodes to become congested, and they, in turn,
reject data from their upstream node or nodes, and so on. Backpressure is a node to
node congestion control that starts with a node and propagates, in the opposite
direction of data flow, to the source. The backpressure technique can be applied
only to virtual circuit networks, in which each node knows the upstream node from
which a flow of data is coming.
Backpressure method for alleviating congestion
• Used in virtual circuits
Closed Loop Policies
Choke Packet
A choke packet is a packet sent by a node to the source to inform it of congestion.
Note the difference between the backpressure and choke-packet methods. In
backpressure, the warning is from one node to its upstream node, although the
warning may eventually reach the source station. In the choke-packet method, the
warning is from the router, which has encountered congestion, directly to the source
station. The intermediate nodes through which the packet has traveled are not
warned.
Choke packet
Closed Loop Policies
Implicit Signaling
In implicit signaling, there is no communication between the congested
node or nodes and the source. The source guesses that there is
congestion somewhere in the network from other symptoms. For
example, when a source sends several packets and there is no
acknowledgment for a while, one assumption is that the network is
congested. The delay in receiving an acknowledgment is interpreted as
congestion in the network; the source should slow down. We saw this
type of signaling in TCP congestion control.
Closed Loop Policies
Explicit Signaling
The node that experiences congestion can explicitly send a signal to the source or
destination. The explicit-signaling method, however, is different from the choke-packet
method. In the choke-packet method, a separate packet is used for this purpose; in the
explicit-signaling method, the signal is included in the packets that carry data. Explicit
signaling can occur in either the forward or the backward direction. This type of
congestion control can be seen in an ATM and Frame Relay networks. Explicit signaling,
can occur in either the forward or the backward direction.
Backward Signaling A bit can be set in a packet moving in the direction opposite to
the congestion. This bit can warn the source that there is congestion and that it needs to
slow down to avoid the discarding of packets.
Forward Signaling A bit can be set in a packet moving in the direction of the
congestion. This bit can warn the destination that there is congestion. The receiver in
this case can use policies, such as slowing down the acknowledgments, to alleviate the
congestion.
BECN : Backward Explicit Congestion Notification
BECN : Forward Explicit Congestion Notification
