What is Load Shedding in Congestion Control?

When congestion cannot be controlled by using any of the algorithms then routers will opt out for load shedding. When the routers are inundated by packets that they cannot tackle, packets will be thrown. Routers drowning in packets will just pick randomly to drop. Which packets to be dropped are dependent on the applications running.For file transfer, an old packet is worth more than a new one because dropping packet 6 and keeping packets 7 through 10 will cause a gap at the receiver that may force packets 6 through 10 to be retransmitted (if the receiver routinely discards out-of-order packets). In a 12-packet file, dropping 6 may require 7 through 12 to be retransmitted, whereas dropping 10 may require only 10 through 12 to be retransmitted. In contrast, for multimedia, a new packet is more important than an old one. The former policy (old is better than new) is often called wine and the latter (new is better than old) is often called milk.

In this, its very much required for sender to cooperate. For many applications, some packets are more than others. For example: some algorithm for compressing a video, after a specific interval transmits the complete frame and then subsequent frames  as differences from the last frame. In this case, dropping a packet which contains the difference is preferable then dropping one that is part of the full frame.

Application must mark their packets in priority classes to tell how important those are. With this, router will be pick the packets which are marked as lowest class for discarding. Another option is to allow hosts to exceed the limits specified in the agreement negotiated when the virtual circuit was set upbut subject to the condition that all excess traffic be marked as low priority. Such a strategy is actually not a bad idea, because it makes more efficient use of idle resources.

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How hop–by–hop choke packets are sent to source host?

Sending a choke packet to the source hosts does not work well at high speeds or over long distances because the reaction is so slow.Let’s take an example that host existing in San Francisco and is sending traffic to host at New York.The choke packet propagation is shown as the second, third, and fourth steps in figure (a). In those 30 msec, another 4.6 megabits will have been sent. Even if the host in San Francisco completely shuts down immediately, the 4.6 megabits in the pipe will continue to pour in and have to be dealt with.

Another approach is to have the choke packet take effect at every hop it passes through, as shown in the sequence of figure (b). Here, F is required to reduce the flow to D as soon as the choke packet reaches F. Doing so will require F to devote more buffers to the flow. In the next step, the choke packet reaches E, which tells to reduce the flow to F. This action puts a greater demand on E‘s buffers but gives F immediate relief. Finally, the choke packet reaches and the flow genuinely slows down.

How hop–by–hop choke packets are sent to source host

Reduced flow and maximum flow of packets

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Congestion Prevention Policies in Transport, Network and Data link layer

Congestion Prevention Policies in Transport, Network and Data link layer

We will first consider the open Loop systems. These systems are not designed to prevent congestion; rather they are designed to minimize congestion in the first place. They try to achieve their goal by using appropriate policies at various … [Continue reading]

What are the General Principles of Congestion Control?

Various problems in complex system, such as computer network, can be viewed from control theory point of view, with this approach all the solutions can be divided into two groups: Open Loop and Closed Loop. Open Loop is just like being proactive … [Continue reading]

What are Congestion Control Algorithms?

What are Congestion Control Algorithms

When too many packets are available in the subnet, performance degrades which is called as Congestion. Figure shows the symptom. When the number of packets dumped into the subnet by the hosts is within its carrying capacity, they are all delivered … [Continue reading]

Routing in Ad-hoc networks: Route Discovery algorithm

Routing in Ad-hoc networks Route Discovery algorithm

Routing in Ad-hoc network is different from wired network because all the rules pertaining to fixed topologies, fixed and known neighbors, fixed relationship with IP address and location does not holds good. In ad-hoc network, routers can come and go … [Continue reading]

What is Multi Cast Routing?

Multi Cast Routing

There are few applications that need the involvement of far-away separated process to work together in groups. Better example is that of process involving group of different data base distributed in different systems. In such cases its required for … [Continue reading]

What is Broadcast Routing?

Reverse path forwarding

Sometimes, hosts need to send messages to many or all other hosts. Sending a packet to all destinations simultaneously is called broadcasting. Various methods have been proposed for broadcasting: One of the broadcasting methods simply send a … [Continue reading]

What is Shortest Past Routing?

Shortest Past Routing

The concept here is to build a graph of the subnet, with each node of the graph representing a router and each arc of the graph representing a communication line called a link. To select a route between a given pair of routers, the algorithm … [Continue reading]

What is Routing Algorithms and different type of Routing Algorithm?

Main function of network layer is to route packets from Source to Destination. In most subnets, to make a journey complete, multiple hops are required. An algorithm that chooses the routes and data structures that is used is major area of … [Continue reading]