gawkinet: Datagram Communications
1
1 1.2 Best-effort Datagrams (Mailed Letters)
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1
1 Suppose you mail three different documents to your office on the other
1 side of the country on two different days. Doing so entails the
1 following.
1
1 1. Each document travels in its own envelope.
1
1 2. Each envelope contains both the sender and the recipient address.
1
1 3. Each envelope may travel a different route to its destination.
1
1 4. The envelopes may arrive in a different order from the one in which
1 they were sent.
1
1 5. One or more may get lost in the mail. (Although, fortunately, this
1 does not occur very often.)
1
1 6. In a computer network, one or more "packets" may also arrive
1 multiple times. (This doesn't happen with the postal system!)
1
1 The important characteristics of datagram communications, like those
1 of the postal system are thus:
1
1 * Delivery is "best effort;" the data may never get there.
1
1 * Each message is self-contained, including the source and
1 destination addresses.
1
1 * Delivery is _not_ sequenced; packets may arrive out of order,
1 and/or multiple times.
1
1 * Unlike the phone system, overhead is considerably lower. It is not
1 necessary to set up the call first.
1
1 The price the user pays for the lower overhead of datagram
1 communications is exactly the lower reliability; it is often necessary
1 for user-level protocols that use datagram communications to add their
1 own reliability features on top of the basic communications.
1