gawkinet: Datagram Communications

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1 1.2 Best-effort Datagrams (Mailed Letters)
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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.
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1   1. Each document travels in its own envelope.
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1   2. Each envelope contains both the sender and the recipient address.
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1   3. Each envelope may travel a different route to its destination.
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1   4. The envelopes may arrive in a different order from the one in which
1      they were sent.
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1   5. One or more may get lost in the mail.  (Although, fortunately, this
1      does not occur very often.)
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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:
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1    * Delivery is "best effort;" the data may never get there.
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1    * Each message is self-contained, including the source and
1      destination addresses.
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1    * Delivery is _not_ sequenced; packets may arrive out of order,
1      and/or multiple times.
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1    * Unlike the phone system, overhead is considerably lower.  It is not
1      necessary to set up the call first.
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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.
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