UDP: the User Datagram Protocol in Java

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UDP: the User Datagram Protocol
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transfer service and UDP does not The answer is no, as many applications are better suited for UDP for the following reasons:
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No connection establishment As we shall discuss in Section 35, TCP uses a three-way handshake before it starts to transfer data UDP just blasts away without any formal preliminaries Thus UDP does not introduce any delay to establish a connection This is probably the principle reason why DNS runs over UDP rather than TCP -- DNS would be much slower if it ran over TCP HTTP uses TCP rather than UDP, since reliability is critical for Web pages with text But, as we briefly discussed in Section 22, the TCP connection establishment delay in HTTP is an important contributor to the "world wide wait" No connection state TCP maintains connection state in the end systems This connection state includes receive and send buffers, congestion control parameters, and sequence and acknowledgment number parameters We will see in Section 35 that this state information is needed to implement TCP's reliable data transfer service and to provide congestion control UDP, on the other hand, does not maintain connection state and does not track any of these parameters For this reason, a server devoted to a particular application can typically support many more active clients when the application runs over UDP rather than TCP Small segment header overhead The TCP segment has 20 bytes of header overhead in every segment, whereas UDP only has 8 bytes of overhead Unregulated send rate TCP has a congestion control mechanism that throttles the sender when one or more links between sender and receiver becomes excessively congested This throttling can have a severe impact on real-time applications, which can tolerate some packet loss but require a minimum send rate On the other hand, the speed at which UDP sends data is only constrained by the rate at which the application generates data, the capabilities of the source (CPU, clock rate, etc) and the access bandwidth to the Internet We should keep in mind, however, that the receiving host does not necessarily receive all the data - when the network is congested, a significant fraction of the UDP-transmitted data could be lost due to router buffer overflow Thus, the receive rate is limited by network congestion even if the sending rate is not constrained
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Table 31-1 lists popular Internet applications and the transport protocols that they use As we expect, email, remote terminal access, the Web and file transfer run over TCP -- these applications need the reliable data transfer service of TCP Nevertheless, many important applications run over UDP rather TCP UDP is used for RIP routing table updates (see 4 on the network layer), because the updates are sent periodically, so that lost updates are replaced by more up-to-date updates UDP is used to carry network management (SNMP - see 8) data UDP is preferred to TCP in this case, since network management must often run when the network is in a stressed state - precisely when reliable, congestion-controlled data transfer is difficult to achieve Also, as we mentioned earlier, DNS runs over UDP, thereby avoiding TCP's connection establishment delays
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Application-layer protocol Underlying Transport Protocol
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UDP: the User Datagram Protocol
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electronic mail Web file transfer remote file server streaming multimedia Internet telephony Routing Protocol Name Translation
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SMTP HTTP FTP NFS proprietary proprietary RIP DNS
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TCP TCP TCP TCP typically UDP typically UDP typically UDP typically UDP typically UDP typically UDP
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remote terminal access Telnet
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Figure 31-1: Popular Internet applications and their underlying transport protocols
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As shown in Figure 31-1, UDP is also commonly used today with multimedia applications, such as Internet phone, real-time video conferencing, and streaming of stored audio and video We shall take a close look at these applications in 6 We just mention now that all of these applications can tolerate a small fraction of packet loss, so that reliable data transfer is not absolutely critical for the success of the application Furthermore, interactive real-time applications, such as Internet phone and video conferencing, react very poorly to TCP's congestion control For these reasons, developers of multimedia applications often choose to run the applications over UDP instead of TCP Finally, because TCP cannot be employed with multicast, multicast applications run over UDP Although commonly done today, running multimedia applications over UDP is controversial to say the least As we mentioned above, UDP lacks any form of congestion control But congestion control is needed to prevent the network from entering a congested state in which very little useful work is done If everyone were to start streaming high bit-rate video without using any congestion control, there would be so much packet overflow at routers that no one would see anything Thus, the lack of congestion control in UDP is a potentially serious problem Many researchers have proposed new mechanisms to force all sources, including UDP sources, to perform adaptive congestion control [Mahdavi] Before discussing the UDP segment structure, we mention that it is possible for an application to have reliable data transfer when using UDP This can be done if reliability is built into the application itself (e g, by adding acknowledgement and retransmission mechanisms, such as those we shall study in the next section) But this a non-trivial task that would keep an application developer busy debugging for a long time Nevertheless, building reliability directly into the application allows the application to "have its cake and eat it too" -- that is, application processes can communicate reliably without being constrained by the transmission rate constraints imposed by TCP's congestion control mechanism Application-level
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