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| Losses mainly occur when congested transmission devices are forced to drop packets of information, given the impossibility to forward or to dispatch them. Some protocols, as TCP, deal with this problem asking to the sender to resend the lost packets again,
previously slowing the rate of transmission. But some other protocols, less intelligent than TCP, can not deal with this situation, affecting the correct behavior of applications than use them as transport mechanism. |
| Those protocols are not more intelligent than TCP, not because their designers are not interested to improve them, but for keeping them as light and fast as possible to be used to transport real-time application data, such as voice and video. These applications demand a constant bandwidth over short periods of time, and this requirement can’t be satisfied by TCP congestion control response. |
| For example, when transmitting voice, video and multimedia traffic, most designers use UDP protocol as the transporting medium. They encapsulate multimedia frames over RTP (realtime protocol), and RTP over UDP. They select UDP because it’s a light protocol, very fast and adecuated for low latency transmissions. Using RTP, they give sequency support to multimedia frame transportation, not serviced by UDP itself (multimedia frames must be counted, frame lost must be as low as possible, and must be ordered in its original sequency before being playbacked at destination application).
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| But UDP, being simple and fast as is required for transporting realtime data, has a problem when comparing with TCP. It’s an unresponsive protocol;
this means that when congestion begins, and packet dropping is a notorious situation, it does not respond as TCP adjusting its flow of transmission for adapting itself to the receiver capacity, avoiding this way more losses, but instead it maintains its flow rate, worsening the situation.
Nevertheless, this behavior is a "must" for multimedia applications. |
| Given this explanation, we can conclude that congestion problem affect a lot more realtime traffic such as voice, video and multimedia, than other type of traffic that travel over TCP. Actually, when more and more applications are realtime applications, problem is becoming more and more complicated, being necessary that network engineers pay special attention to the "Quality of Service" premise, and try to implement QoS tools to deal with these situations. Congestion management, congestion avoidance, traffic classification and traffic prioritization is a
"must", to give individual and corporate customers and users the quality of service they are expecting from their Internet
Service Providers. |
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