Congestion control refers to the techniques used to control or prevent congestion. Congestion control techniques can be broadly classified into two categories:
Open Loop
Congestion Control
Open loop
congestion control policies are applied to prevent congestion before it
happens. The congestion control is handled either by the source or the
destination.
Policies
adopted by open loop congestion control –
1. Re transmission Policy:
·
It
is the policy in which retransmission of the packets are taken care. If the
sender feels that a sent packet is lost or corrupted, the packet needs to be re-transmitted. This transmission may increase the congestion in the network.
·
To
prevent congestion, retransmission timers must be designed to prevent
congestion and also able to optimize efficiency.
2. Window
Policy:
The type of
window at the sender side may also affect the congestion. Several packets in
the Go-back-n window are resent, although some packets may be received
successfully at the receiver side. This duplication may increase the congestion
in the network and making it worse.
Therefore,
Selective repeat window should be adopted as it sends the specific packet that
may have been lost.
3. Discarding
Policy:
A good
discarding policy adopted by the routers is that the routers may prevent
congestion and at the same time partially discards the corrupted or less
sensitive package and also able to maintain the quality of a message.
In case of
audio file transmission, routers can discard fewer sensitive packets to prevent
congestion and also maintain the quality of the audio file.
4.
Acknowledgment Policy:
Since
acknowledgement are also the part of the load in network, the acknowledgment
policy imposed by the receiver may also affect congestion. Several approaches
can be used to prevent congestion related to acknowledgment. The receiver
should send acknowledgement for N packets rather than sending acknowledgement
for a single packet. The receiver should send a acknowledgment only if it has
to send a packet or a timer expires.
5. Admission Policy:
In admission policy
a mechanism should be used to prevent congestion. Switches in a flow should
first check the resource requirement of a network flow before transmitting it
further. If there is a chance of a congestion or there is a congestion in the
network, router should deny establishing a virtual network connection to
prevent further congestion.
Closed Loop
Congestion Control
Closed loop
congestion control technique is used to treat or alleviate congestion after it
happens. Several techniques are used by different protocols; some of them are:
1.
Back pressure:
Back pressure is
a technique in which a congested node stop receiving packet from upstream node.
This may cause the upstream node or nodes to become congested and rejects
receiving data from above nodes. Backpressure is a node-to-node congestion
control technique that propagate in the opposite direction of data flow. The
backpressure technique can be applied only to virtual circuit where each node
has information of its above upstream node.
2. Choke Packet
Technique:
Choke packet
technique is applicable to both virtual networks as well as datagram subnets. A
choke packet is a packet sent by a node to the source to inform it of
congestion. Each router monitors its resources and the utilization at each of
its output lines. whenever the resource utilization exceeds the threshold value
which is set by the administrator, the router directly sends a choke packet to
the source giving it a feedback to reduce the traffic. The intermediate nodes
through which the packets have traveled are not warned about congestion.
3. Implicit
Signaling:
In implicit
signaling, there is no communication between the congested nodes and the
source. The source guesses that there is congestion in a network. For example,
when sender sends several packets and there is no acknowledgment for a while,
one assumption is that there is a congestion.
4. Explicit
Signaling:
In explicit
signaling, if a node experiences congestion it can explicitly sends a packet to
the source or destination to inform about congestion. The difference between
choke packet and explicit signaling is that the signal is included in the
packets that carry data rather than creating different packet as in case of
choke packet technique.
Explicit
signaling can occur in either forward or backward direction.
·
Forward
Signaling: In forward
signaling signal is sent in the direction of the congestion. The destination is
warned about congestion. The receiver in this case adopt policies to prevent
further congestion.
· Backward Signaling: In backward signaling signal is sent in the opposite direction of the congestion. The source is warned about congestion and it needs to slow down.
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