Load Shedding In Networking

When none of the above methods make the congestion disappear, routers can

bring out the heavy artillery: load shedding. Load shedding is a fancy way of

saying that when routers are being inundated by packets that they cannot handle,

they just throw them away. The term comes from the world of electrical power

generation where it refers to the practice of utilities intentionally blacking out certain areas to save the entire grid from collapsing on hot summer days when the demand for electricity greatly exceeds the supply.

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A router drowning in packets can just pick packets at random to drop. but usually it can do better than that, Which packet to discard may depend 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 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.

A step above this in intelligence requires cooperation from the senders. For

many applications. some packets are more important than others. For example,

certain algorithms for compressing video periodically transmit an entire frame and

end subsequent frames as differences from the last full frame. In this case,

dropping a packet that is part of a difference is preferable to dropping one that is

part of a full frame As another example, consider transmitting a document con

Training ASCII text and pictures. Losing a line of pixels in some image is far less

damaging than losing a line of readable text.

To implement an intelligent discard policy, applications must mark their packets in priority ciasses io indicate how important they are. if they do this. When packets  move to be discarded, routers can first drop packets from the low

then the net lowest class so on Of course, uni is there is a significant

Incentive o mark puckets anything other than VERY IMPORTANT-NEVER

EVER DISCARD. notty will do i

The incentive might be in the form of money with the low-priority packets

heing cheaper to send than the light-penits One Alternatively, priority classes

could be coupled with traffic shaping For example, there might be a rule saying

thar when the token bucket algorithm is being used and a packet arrives at a

moment when na token is available, it may still be sent provided that it ik marked

45 the lowest possible priorities, and thus subject to discard the instant trouble

appears. Under conditions af litht load, users might be happy to operate in this

way, but as the increases and packets actually begin to be dicarded, they

might cut bock and only send packets when tokens are available

Another ouons o milow how to exceed the limits specified in the agreement negotiated when the virtual circuit was set up e ase a higher bandwidth

than allowed). but subject to the condition that all access traffic be marked as low

ions Such a strategy is actually not a bad idea because it makes more efficient use of ile resource allowing hustus the long nobody else is

interested. but without establish to them when times get tough

Marking packets by class requires one or more header bits in which to put the

priority ATM cells have 1 bit reserved in the header for this purpose o cry

ATM cell is labeled either as low priority or high priority. ATM switches indeed

use this bit when making discard decisions

In some network packets are grouped together into later units that are used

for retransmission purposes. For example. in ATM networks. what we have been

calling nackers are fixed-length cells These cells are just fragments of mes

aces. When a cell is dropped, ultimately the entire message" will be

retransmitted not the missing cell. Under these condition. a router that drops

a cell might as well drop all the rest of the cells in that message, since transmitting

them out bandwidth and wins nothing can they get through they will still

be retransmitted later

Simulation russhow that when a router senses trouble on the horizon, it is

better off staying to discard packets only rather than wait until it becomes com

piety clogged up Food and Jacobson. 1993. Romansana Flova100

Doing so hay even the congestion from texting a foothold.