User Datagram Protocol

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User Datagram Protocol

The second protocol that lives in the Transport layer is the User Datagram Protocol, or UDP. This protocol is a nonconnection-based protocol and does not require a session to be established between two machines before data is transmitted. UDP packets are still delivered to sockets or ports, just as they are in TCP. But because UDP does not create a session between machines, it cannot guarantee that packets are delivered or that they are delivered in order or retransmitted if the packets are lost. Given the apparent unreliability of this protocol, some may wonder why a protocol such as UDP was developed. Figure 2.23 illustrates the relative simplicity of the address format of UDP compared to TCP.

Notice that sending a UDP datagram has very little overhead involved. A UDP datagram has no synchronization parameters or priority options. All that exist are the source port, destination port, the length of the data, a checksum for verifying the header, and then the data.

There are actually a number of good reasons to have a transport protocol that does not require a session to be established. For one, very little overhead is associated with UDP, such as having to keep track of sequence numbers, Retransmit Timers, Delayed Acknowledgment Timers, and retransmission of packets. UDP is quick and extremely streamlined functionally; it’s just not guaranteed. This makes UDP perfect for communications that involve broadcasts, general announcements to the network, or real-time data.

Try to visualize UDP as being similar to a postcard. In order for Shey to send a message to Kim, all Shey needs to know is Kim’s address. Shey can write his message on the postcard, put Kim’s address on it and put it in the mailbox to be sent. Shey does not have to verify that Kim is at home to send the postcard on its way. If Kim is at home when the mailman arrives, the postcard is read and the message is received. Notice that unless Kim responds back to Shey through mail or by phone, Shey can never really know whether the postcard was received. That is the nature of nonconnection-oriented protocols. Delivery is not guaranteed. If the mailman is eaten by the neighbor’s dog, or the sorting machine at the post office eats the postcard, or a tornado takes out the mail truck, Shey may never know it, and Kim may never know there was a message intended for her.

In terms of applications, the same methodology is true. For instance, the Simple Network Management Protocol (SNMP) uses UDP ports 67 and 68 for occasionally polling for data from machines on the network and for initiating traps on machines when errors occur. These polls and traps are sent as UDP broadcasts and do not require a session to be established to communicate a message. Think about how useful that is. Does it make any sense for a machine that is having a catastrophic error of some sort to have to go through the business of establishing a TCP session, just to tell you the machine is going down? No, it doesn’t. It makes perfect sense however, to let the last gasping breath of a machine be a broadcast message that it’s in serious trouble.

Another really good use for UDP is in streaming video and streaming audio. Not only does the unguaranteed delivery of packets enable more data to be transmitted (because a broadcast has little to no overhead), but the retransmission of a packet is pointless, anyway. In the case of a streaming broadcast, users are more concerned with what’s coming next than with trying to recover a packet or two that may not have made it. Compare it to listening to a music CD and a piece of dust gets stuck in one of the little grooves. In most cases, the omission of that piece is imperceptible; your ear barely notices and your brain probably filled in the gap for you anyway. Imagine instead that your CD player decides to guarantee the delivery of that one piece of data that it can’t quite get, and ends up skipping and skipping indefinitely. It can definitely ruin the listening experience. It is easier to deal with an occasional packet dropping out to have as fulfilling a listening experience as possible. Thankfully, UDP was developed for applications to utilize in this very same fashion.

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