congestion prevention policies

Let us begin our study of methods to control congestion by looking at open loop systems.
These systems are designed to minimize congestion in the first place, rather than letting it
happen and reacting after the fact. They try to achieve their goal by using appropriate policies
at various levels.
policies that affect congestion

Let us start at the data link layer and work our way upward. The retransmission policy is
concerned with how fast a sender times out and what it transmits upon timeout. A jumpy
sender that times out quickly and retransmits all outstanding packets using go back n will put
a heavier load on the system than will a leisurely sender that uses selective repeat. Closely
related to this is the buffering policy. If receivers routinely discard all out-of-order packets,
these packets will have to be transmitted again later, creating extra load. With respect to
congestion control, selective repeat is clearly better than go back n.
Acknowledgement policy also affects congestion. If each packet is acknowledged immediately,
the acknowledgement packets generate extra traffic. However, if acknowledgements are saved
up to piggyback onto reverse traffic, extra timeouts and retransmissions may result. A tight
flow control scheme (e.g., a small window) reduces the data rate and thus helps fight
congestion.
At the network layer, the choice between using virtual circuits and using datagrams affects
congestion since many congestion control algorithms work only with virtual-circuit subnets.
Packet queueing and service policy relates to whether routers have one queue per input line,
one queue per output line, or both. It also relates to the order in which packets are processed

(e.g., round robin or priority based). Discard policy is the rule telling which packet is dropped
when there is no space. A good policy can help alleviate congestion and a bad one can make it
worse.
A good routing algorithm can help avoid congestion by spreading the traffic over all the lines,
whereas a bad one can send too much traffic over already congested lines. Finally, packet
lifetime management deals with how long a packet may live before being discarded. If it is too
long, lost packets may clog up the works for a long time, but if it is too short, packets may
sometimes time out before reaching their destination, thus inducing retransmissions.