Dcn Unit 3l4-Congestion

7/31/2019 Dcn Unit 3l4-Congestion

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CONGESTION

Congestion is a situation in which too manypackets are present in (a part of) the subnet,performance degrades.

Factors causing congestion:- The input traffic rate exceeds the capacity of the

output lines.

- The routers are too slow to perform bookkeepingtasks (queueing buffers, updating tables, etc.).

- The routers' buffer is too limited.


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When too much traffic is offered,congestion sets in and performance

degrades sharply.


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Policies that affect congestion.


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Congestion control is different from flow control:

1.Congestion is a global issue, involving thebehavior of all the hosts, all the routers, the

store-and-forward processing within the routers,etc.

2.Flow control relates to the point-to-point trafficbetween a given sender and a given receiver.


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General principles of congestioncontrol

Open loop solutions solve the problem by gooddesign, in essence, to make sure the problemdoes not occur in the first place.

Tools include deciding when to accept newtraffic, when to discard packets and which ones,and how to schedule packets at various points inthe network.

A common fact: they make decisions withoutregard to the current state of the network.


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Closed Loop solution

Closed loop solutions are based on theconcept of a feedback loop, which consistsof the following three parts:

Monitor the system to detect when and wherecongestion occurs.

Pass this information to places where actions

can be taken. Adjust system operation to correct the

problem


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Chief metrics for monitoring thesubnet for congestion are:-

the percentage of all packets discarded forlack of buffer space,

the average queue lengths,

the number of packets that time out andare retransmitted,

the average packet delay, and the standard deviation of packet delay.


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How to propagate the monitoredcongestion information ?

The router detecting the congestion sends aseparate warning packet to the traffic source.

A bit or field can be reserved in each packet.

When a router detects a congested state, it fillsin the field in all outgoing packets to warn theneighbors.

Hosts or routers send probe packets outperiodically to explicitly ask about congestionand to route traffic around problem areas


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How to correct the congestion

problem ?

Increase the resource: Using an additional line to temporarily increase the

bandwidth between certain points.

Splitting traffic over multiple routes.

Using spare routers.

Decrease the load: denying service to some users,

degrading service to some or all users, and having users schedule their demands in a more

predictable way


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Congestion Control in Virtual-Circuit Subnets

One technique that is widely used to

keep congestion that has already

started from getting worse isadmission control.

The idea is simple: once congestion

has been signaled, no more virtualcircuits are set up until the problem

has gone away.


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Thus, attempts to set up new transportlayer connections fail.

Letting more people in just makes mattersworse. While this approach is crude, it issimple and easy to carry out.

In the telephone system, when a switchgets overloaded, it also practicesadmission control by not giving dial tones.


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An alternative approach is to allow newvirtual circuits but carefully route all newvirtual circuits around problem areas.

Suppose that a host attached to router Awants to set up a connection to a hostattached to router B. Normally, thisconnection would pass through one of the

congested routers. Omitting the congested routers and all of

their lines. The dashed line shows apossible route for the virtual circuit that

avoids the congested routers.


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(a) A congested subnet. (b) A redrawnsubnet that eliminates the congestion. A

virtual circuit from A to B is also shown.


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Another (open loop) strategy relating to virtualcircuits is to negotiate an agreement betweenthe host and subnet, so that the subnet canreserve resources along the path when thecircuit is set up.

Since all necessary resources are guaranteed tobe available, congestion is unlikely to occur onthe new virtual circuits.

Reservation can be done all the time, or onlywhen the subnet is congested.


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Traffic shaping

One of the main causes of congestion isthat traffic is often bursty.

Another open loop method is forcing thepackets to be transmitted at a morepredictable rate.

This method is widely used in ATMnetworks and is called traffic shaping.


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Leaky bucket algorithm

Figure:(a) A leaky bucket with water. (b) Aleaky bucket with packets.


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Leaky bucket algorithm

Each host is connected to thenetwork by an interface containing aleaky bucket - a finite internal queue.

The outflow is at a constant rate whenthere is any packet in the bucket, andzero when the bucket is empty.

If a packet arrives at the bucket whenit is full, the packet is discarded.


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The token bucket algorithm

The leaky bucket algorithm enforcesa rigid output pattern at the averagerate, no matter how bursty the traffic

is. For many applications, it is better to

allow the output to speed up

somewhat when large bursts arrive,so a more flexible algorithm isneeded, preferably on that never

loses data.


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The token bucket algorithm.

The bucket holds tokens, generatedby a clock at the rate of one tokenevery sec.

For a packet to be transmitted, it mustcapture and destroy one token.

After.The token bucket algorithm

allows saving up to the maximum sizeof the bucket, , which means thatbursts of up to packets can be sent atthe maximum speed (for a certain

period of time).


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The token bucket algorithm. (a)Before. (b) After.


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Choke packet

This method can be used in both virtualcircuit and datagram subnets.

Each line is associated with a variable ,whose value (0.0 - 1.0) reflects the recentutilization:

Whenever moves above the threshold, theoutput line enters a ``warning'' state.


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Choke packet

If a newly arriving packet is outputting in aline in warning state, the router sendsa choke packet back to the source host,

giving it the destination found in thepacket.

When the source host gets the choke

packet, it is required to reduce the trafficsent to destination by certain percent.


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The host should ignore choke packetsreferring to the same destination for a

fixed time interval (why ?). If no choke packets arrive during the

listening period, the host may increase theflow again.

Some variations on this congestion controlalgorithm exist.

Choke packet


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Figure 24.7 Choke packet


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(a) A choke packet that affects only the source. (b)A choke packet that affects each hop it passes

through.


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Load shedding

When none of the above methods

make the congestion disappear,

routers can bring out the heavyartillery: load shedding, i.e, justthrowing packets away.

Which packet to discard ?

Discard any one at random.


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Load shedding

Discard the younger ones (the wine policysuitable for file transfer).

Discard the older ones (the milk policysuitable for multimedia transfer.)

Discard less important ones (thisintelligence requires the senders to mark

their packets in priority classes to indicatehow important they are).


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Jitter control

For audio and video transmission, it is importantto ensure constant transmission time.

So the agreement between the host and the

subnet might be that 99 % of the packets bedelivered with a delay in the range of 24.5 msecto 25.5 msec.

The jitter can be controlled by computing the

expected transit time for each hop along thepath:


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Jitter control

When a packet arrives at a router, therouter checks to see how much the packetis behind or ahead of its schedule.

If the packet is ahead of schedule, it isheld just long enough to get it back onschedule.

If it is behind schedule, the router tries toget it out the door quickly.


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(a) High jitter. (b) Low jitter.

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Dcn Unit 3l4-Congestion

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