Tail drop occurs when a packet needs to be added to a queue, but the queue is full. Yes, tail drop is indeed that simple. However, tail drop results in some interesting behavior in real networks, particularly when most traffic is TCP based, but with some UDP traffic. Of course, the Internet today delivers mostly TCP traffic, because web traffic uses HTTP, and HTTP uses TCP.
The preceding section described the behavior of a single TCP connection after a single packet loss. Now imagine an Internet router, with 100,000 or more TCP connections running their traffic out of a high-speed interface. The amount of traffic in the combined TCP connections finally exceeds the output line rate, causing the output queue on the interface to fill, which in turn causes tail drop.
What happens to those 100,000 TCP connections after many of them have at least one packet dropped? The TCP connections reduce their CWND; the congestion in the queue abates; the various CWND values increase with slow start, and then with congestion avoidance. Eventually, however, as the CWND values of the collective TCP connections approach the previous CWND value, the congestion occurs again, and the process is repeated. When a large number of TCP connections experience near simultaneous packet loss, the lowering and growth of CWND at about the same time causes the TCP connections to synchronize. The result is called global synchronization. The graph in Figure 6-3 shows this behavior.
Figure 6-3 Graph of Global Synchronization
The graph shows the results of global synchronization. The router never fully utilizes the bandwidth on the link because the offered rate keeps dropping as a result of synchronization. Note that the overall rate does not drop to almost nothing because not all TCP connections happen to have packets drop when tail drop occurs, and some traffic uses UDP, which does not slow down in reaction to lost packets.
Weighted RED (WRED), when applied to the interface that was tail dropping packets, significantly reduces global synchronization. WRED allows the average output rates to approach line rate, with even more significant throughput improvements, because avoiding congestion and tail drops decreases the overall number of lost packets. Figure 6-4 shows an example graph of the same interface, after WRED was applied.
Figure 6-4 Graph of Traffic Rates After the Application of WRED
Average Rate _ Before WRED
-Actual Rate with WRED
Another problem can occur if UDP traffic competes with TCP for bandwidth and queue space. Although UDP traffic consumes a much lower percentage of Internet bandwidth than TCP does, UDP can get a disproportionate amount of bandwidth as a result of TCP's reaction to packet loss. Imagine that on the same Internet router, 20 percent of the offered packets were UDP, and 80 percent TCP. Tail drop causes some TCP and UDP packets to be dropped; however, because the TCP senders slow down, and the UDP senders do not, additional UDP streams from the UDP senders can consume more and more bandwidth during congestion.
Taking the same concept a little deeper, imagine that several people crank up some UDP-based audio or video streaming applications, and that traffic also happens to need to exit this same congested interface. The interface output queue on this Internet router could fill with UDP packets. If a few high-bandwidth UDP applications fill the queue, a larger percentage of TCP packets might get tail dropped—resulting in further reduction of TCP windows, and less TCP traffic relative to the amount of UDP traffic.
The term "TCP starvation" describes the phenomena of the output queue being filled with larger volumes of UDP, causing TCP connections to have packets tail dropped. Tail drop does not distinguish between packets in any way, including whether they are TCP or UDP, or whether the flow uses a lot of bandwidth or just a little bandwidth. TCP connections can be starved for bandwidth because the UDP flows behave poorly in terms of congestion control. Flow-Based WRED (FRED), which is also based on RED, specifically addresses the issues related to TCP starvation, as discussed later in the chapter.
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