 Because of this, UDP-based applications may be sensitive to packet loss. HTTP is TCP-based. Generally, HTTP applications are not time-sensitive. When you're viewing a web page, having to wait longer for an image to load due to a dropped packet is not usually a problem. FTP is TCP-based. FTP is not a real-time protocol, nor is it time-sensitive. If packets are dropped while you're downloading a file, it's usually not a problem to wait the extra time for the packets to be resent. Telnet and SSH Telnet and SSH are both TCP-based. While they may appear to be real time, they're not. When packets are resent, the problem manifests as slow responses while you're typing. This may be annoying, but no damage is done when packets are dropped and resent. VoIP is UDP-based for the Real-Time Protocol (RTP) voice stream and TCP-based for the call-control stream. VoIP requires extreme reliability and speed, and cannot tolerate packets being delivered out of order. The use of UDP may seem odd, since UDP is not generally used for reliable packet delivery. VoIP uses UDP to avoid the processing and bandwidth overheads involved in TCP. The speed gained from using UDP is significant. Reliability issues can be resolved with QoS; in fact, VoIP is one of the main reasons that companies deploy QoS. Introduction to QoS Assuming a FIFO (first in, first out) interface for this example, packets may be sent out of a serial interface in a noncontiguous fashion. In Figure 31-3, there are three types of packets being delivered: HTTP, FTP, and voice. Voice information, in particular, is very sensitive to packets being delivered out of order. It is also very sensitive to packets being lost. Remember, voice traffic is UDP-based, and there is no reliable transport mechanism, SO if the buffer overflows and a voice packet is dropped, it's gone forever. This will result in choppy voice calls, irritated users, and executives yelling at you. Data flow Interface Voice Multiple input streams Only one output stream Figure 31-3. Packets through a serial inter