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The isarithmic congestion control method 

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What is a Isarithmic control?

This method attempts to tackle the fundamental reason for congestion within the subnet. Proposed by Davies in 1972, this method says that if congestion is caused by there being too many packets in the subnet, then it can be solved by setting an upper limit on the number allowed to be present at any given time.

He approaches this by introducing a system of permits within the subnet. If an IMP wishes to transmit a packet, it must first obtain and destroy one of the permits which are circulating within the system. Once the packet has been delivered to its destination, the receiving IMP regenerates the permit back into the subnet.

The isarithmic congestion control method is however a long way from being ideal. Firstly the method does nothing about the fact that individual IMPs can become congested. Even though there are a limited number of packets allowed in the subnet, all of these could conceivably be going to the same IMP, causing congestion. Also, the procedure for dealing with the permits themselves is not very good. A fair way of distributing the permits throughout the subnet, and also allowing for a situation where a permit is accidently destroyed, are both very difficult to do efficiently and effectively.

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The main idea in isarithmic flow is controlling the congestion by limiting the total
number of packets existing in the network at any instant of time. This restriction
is achieved by having a fixed number of "permits" in the whole network, stored
at the nodes and travelling with packets. In order to enter the subnet, a packet
must capture one of the permits stored in the source-node, otherwise it waits outside
the network. After arriving at the destination, the permit is released by the packet
and can be returned to any node.

Problems. The particular problems appeared at isarithmic flow control are listed
below:
1) Although the global congestion is prevented, it does not guarantee that there will
not be an accumulation of permits anywhere, leading to congestion in that point
of the network.
2) It is difficult to find a good algorithm for the permits distribution. If they are
returned randomly, it is certain that every node will have some of them and a new
packet will not suffer a large delay before capturing a permit and leaving the
source-node. This random distribution may cause problems, when the arrival rate
packets for a Virtual Circuit (VC) is large (e.g. file transfer) related to another Virtual Circuit. Then, there will not exist enough permits for the service of the first
VC (permit starvation).
3) The disappearance of generation of permits must be avoided.

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The main idea in isarithmic flow is controlling the congestion by limiting the total
number of packets existing in the network at any instant of time. This restriction
is achieved by having a fixed number of "permits" in the whole network, stored
at the nodes and travelling with packets. In order to enter the subnet, a packet
must capture one of the permits stored in the source-node, otherwise it waits outside
the network. After arriving at the destination, the permit is released by the packet
and can be returned to any node.

Problems. The particular problems appeared at isarithmic flow control are listed
below:
1) Although the global congestion is prevented, it does not guarantee that there will
not be an accumulation of permits anywhere, leading to congestion in that point
of the network.
2) It is difficult to find a good algorithm for the permits distribution. If they are
returned randomly, it is certain that every node will have some of them and a new
packet will not suffer a large delay before capturing a permit and leaving the
source-node. This random distribution may cause problems, when the arrival rate
packets for a Virtual Circuit (VC) is large (e.g. file transfer) related to another Virtual Circuit. Then, there will not exist enough permits for the service of the first
VC (permit starvation).
3) The disappearance of generation of permits must be avoided.

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Isarithmic Congestion Control

This method attempts to tackle the fundamental reason for congestion within the subnet. Proposed by Davies in 1972, this method says that if congestion is caused by there being too many packets in the subnet, then it can be solved by setting an upper limit on the number allowed to be present at any given time.

He approaches this by introducing a system of permits within the subnet. If an IMP wishes to transmit a packet, it must first obtain and destroy one of the permits which are circulating within the system. Once the packet has been delivered to its destination, the receiving IMP regenerates the permit back into the subnet.

The isarithmic congestion control method is however a long way from being ideal. Firstly the method does nothing about the fact that individual IMPs can become congested. Even though there are a limited number of packets allowed in the subnet, all of these could conceivably be going to the same IMP, causing congestion. Also, the procedure for dealing with the permits themselves is not very good. A fair way of distributing the permits throughout the subnet, and also allowing for a situation where a permit is accidently destroyed, are both very difficult to do efficiently and effectively.

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An approach to congestion avoidance is to limit the total number flow of packets in the subnet at any one time. The idea is similar to the token ring:
1. When an router accepts a packet from a host, it must obtain a permit before sending the packet into the subnet.
2. Obtaining a permit is analogous to “seizing the token”, but there can be many permits in the subnet. When an router obtains a permit, it destroys it.
3. The destination router regenerates the permit when it passes the packet to the destination host.

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Isarithmic control can be a flow that is  controlling the congestion by limiting the total number of packets existing in the network at any instant of time. This restriction is achieved by having a fixed number of "permits" in the whole network, stored at the nodes and travelling with packets.

The major problem associated with isarithmic control are:

1.      Although the global congestion is prevented, it does not guarantee that there will not be an accumulation of permits anywhere, leading to congestion in that point of the network.

2.      It is difficult to find a good algorithm for the permits distribution. If they are returned randomly, it is certain that every node will have some of them and a new packet will not suffer a large delay before capturing a permit and leaving the source-node. This random distribution may cause problems, when the arrival rate packets for a Virtual Circuit (VC) is large (e.g. file transfer) related to another Virtual Circuit. Then, there will not exist enough permits for the service of the first VC (permit starvation).

3.      The disappearance of generation of permits must be avoided

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The isarithmic congestion control method is however a long way from being ideal. Firstly the method does nothing about the fact that individual IMPs can become congested. Even though there are a limited number of packets allowed in the subnet, all of these could conceivably be going to the same IMP, causing congestion. Also, the procedure for dealing with the permits themselves is not very good. A fair way of distributing the permits throughout the subnet, and also allowing for a situation where a permit is accidently destroyed, are both very difficult to do efficiently and effectively.

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The isarithmic method is an algorithm for network access level flow control , that allows packets enter the subnet only if a free "permit" exists at the source-node

The main idea in isarithmic flow is controlling the congestion by limiting the total number of packets existing in the network at any instant of time

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The main idea in isarithmic flow is controlling the congestion by limiting the total
number of packets existing in the network at any instant of time. This restriction
is achieved by having a fixed number of "permits" in the whole network, stored
at the nodes and travelling with packets. In order to enter the subnet, a packet
must capture one of the permits stored in the source-node, otherwise it waits outside
the network. After arriving at the destination, the permit is released by the packet
and can be returned to any node.

Problems. The particular problems appeared at isarithmic flow control are listed
below:
1) Although the global congestion is prevented, it does not guarantee that there will
not be an accumulation of permits anywhere, leading to congestion in that point
of the network.
2) It is difficult to find a good algorithm for the permits distribution. If they are
returned randomly, it is certain that every node will have some of them and a new
packet will not suffer a large delay before capturing a permit and leaving the
source-node. This random distribution may cause problems, when the arrival rate
packets for a Virtual Circuit (VC) is large (e.g. file transfer) related to another Virtual Circuit. Then, there will not exist enough permits for the service of the first
VC (permit starvation).
3) The disappearance of generation of permits must be avoided.

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When one part of the subnet (e.g. one or more routers in an area) becomes overloaded, congestion results. Because routers are receiving packets faster than they can forward them, one of two things must happen:
1. The subnet must prevent additional packets from entering the congested region until those already present can be processed.
2. The congested routers can discard queued packets to make room for those that are arriving.

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This method attempts to tackle the fundamental reason for congestion within the subnet.

Disadvantage:

It does not guarantee that a given IMP will not suddenly be swamped with packets.

If permits ever get destroyed for any reason the carrying capacity of the network will be forever reduced.

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Regenarating lost permits is difficult because no single nodes know how much permits currently in subnets.

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The main idea in isarithmic flow is controlling the congestion by limiting the total number of packets existing in the network at any instant of time. This restriction is achieved by having a fixed number of "permits" in the whole network, stored at the nodes and travelling with packets. In order to enter the subnet, a packet must capture one of the permits stored in the source-node, otherwise it waits outside the network. After arriving at the destination, the permit is released by the packet and can be returned to any node. Problems. The particular problems appeared at isarithmic flow control are listed below: 1) Although the global congestion is prevented, it does not guarantee that there will not be an accumulation of permits anywhere, leading to congestion in that point of the network. 2) It is difficult to find a good algorithm for the permits distribution. If they are returned randomly, it is certain that every node will have some of them and a new packet will not suffer a large delay before capturing a permit and leaving the source-node. This random distribution may cause problems, when the arrival rate packets for a Virtual Circuit (VC) is large (e.g. file transfer) related to another Virtual Circuit. Then, there will not exist enough permits for the service of the first VC (permit starvation). 3) The disappearance of generation of permits must be avoided.

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it keeps the number of packets constant there exist permits,which circulate about within the subnet, but it has some disadvantages.
Firstly the method does nothing about the fact that individual IMPs can become congested. Even though there are a limited number of packets allowed in the subnet, all of these could conceivably be going to the same IMP, causing congestion. Also, the procedure for dealing with the permits themselves is not very good. A fair way of distributing the permits throughout the subnet, and also allowing for a situation where a permit is accidently destroyed, are both very difficult to do efficiently and effectively.

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This method attempts to tackle the reason for congestion within the subnet. This method says that if congestion is caused by there being too many packets in the subnet, then it can be solved by setting an upper limit on the number allowed to be present at any given time.
It use a system of permits within the subnet. If an IMP wishes to transmit a packet, it must first obtain and destroy one of the permits which are circulating within the system. Once the packet has been delivered to its destination, the receiving IMP regenerates the permit back into the subnet.

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Principle • Limiting the number of packets in the network by assigning "permits“
Amount of "permits" in the network
A "permit" is required for sending •
When sending "permit" is destroyed
When receiving: "permit" is generated
Problems • Parts of the network may be overloaded
• Equal distribution of the "permits" is difficult •
Additional bandwidth for the transfer of "permits" necessary
• Bad for transmitting large data amounts (e.g. file transfer)
• Loss of "permits" hard to detect

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Isarithmic flow is controlling the congestion by limiting the total number of packets existing in the network at any instant of time. This restriction is achieved by having a fixed number of "permits" in the whole network, stored at the nodes and travelling with packets. In order to enter the subnet, a packet must capture one of the permits stored in the source-node, otherwise it waits outside the network. After arriving at the destination, the permit is released by the packet and can be returned to any node.

PROBLEMS IN ISARITHMIC CONTROL: 1. Although the global congestion is prevented, it does not guarantee that there will not be an accumulation of permits anywhere, leading to congestion in that point of the network.
2. It is difficult to find a good algorithm for the permits distribution. If they are returned randomly, it is certain that every node will have some of them and a new packet will not suffer a large delay before capturing a permit and leaving the source-node. This random distribution may cause problems, when the arrival rate packets for a Virtual Circuit (VC) is large (e.g. file transfer) related to another 473 Virtual Circuit. Then, there will not exist enough permits for the service of the first VC (permit starvation).
3. The disappearance of generation of permits must be avoided.

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Awadh

It's a method that tries to solve the fundamental reason of congestion in subnets. Congestion is caused by having too many packets in the subnet. It's usually solved by setting an upper limit to the number allowed at a certain time.