Congestion management refers to the strategies and techniques used by a network to handle and control traffic when congestion occurs. One of the simplest methods is the FIFO (First In, First Out) queue, which does not classify packets based on their type or importance. Instead, it processes packets in the order they arrive at the interface. This approach follows a best-effort forwarding model, where all packets are treated equally, regardless of their content or urgency.
In contrast, priority queuing (PQ) is designed for critical applications that require guaranteed performance during congestion. PQ classifies packets based on various criteria such as protocol type, source/destination addresses, port numbers, or specific fields like the ToS or EXP bits. These packets are then assigned to one of four priority queues—high, medium, normal, or low. During data transmission, the router always sends packets from the highest priority queue first, ensuring that time-sensitive services like VoIP receive preferential treatment.
Traffic classification and marking play a key role in effective congestion management. While packets can be classified using any field, common marking practices involve the CoS field in 802.1Q Ethernet frames, the ToS field in IP packets, and the EXP field in MPLS packets. These marks help other network devices understand the packet’s service class and apply appropriate handling policies. For example, VoIP data might be marked as EF (Expedited Forwarding), while VoIP signaling could be marked as AF (Assured Forwarding), ensuring different levels of service quality.
At intermediate network nodes, these marks allow for differentiated traffic handling. EF traffic guarantees low delay and minimal jitter, often with traffic policing mechanisms, while AF traffic ensures a minimum bandwidth even during congestion. This helps maintain quality of service across the network.
Understanding congestion management principles is essential for designing efficient networks. When LANs communicate over a slower WAN link, routers may experience congestion if the incoming data rate exceeds the outgoing capacity. This can lead to packet loss and poor performance, especially for real-time applications. Proper congestion control techniques, such as queue management and traffic shaping, help mitigate these issues and ensure smoother communication.
The FIFO queue, while simple and resource-efficient, lacks the ability to prioritize certain types of traffic, making it unsuitable for environments where real-time performance is critical. On the other hand, PQ offers better prioritization but can lead to starvation of lower-priority traffic if higher queues remain constantly busy. Therefore, choosing the right congestion management technique depends on the specific needs of the network and the applications it supports.
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