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Reliable Transmission

TCP/IP is said to be reliable but UDP/IP is not. Reliability usually means that the protocol takes care of three properties:

Error Control
Flow Control
Sequence Control

Error Control: There are two kinds of transmission errors:

corrupted data
lost data

These can be addressed by two corresponding methods:

[Acknowledgement:] To address corrupted data, the receiver can send back acknowledgement (whether the data was good or corrupted).
[Timeout:] To address lost data, the sender can retransmit data after a timeout on not getting acknowledgement.

Now, there are several new kinds of errors:

Message OK, Acknowledgement OK.
Message OK, Acknowledgement fail.
Message fail, Acknowledgement fail.

This forces the receiver to perform the following checks:

Check if received data is OK.
Check if data is duplicated.

Flow Control: The receiver may easily be swamped with messages for a variety of reasons. If the sender ensures that this does not happen ("pacing"), then we have flow control.

Sequence Control: Received order may differ from sending order. Fragmentation can happen at each layer. In the above example, if N+4+8 bytes is larger than the MTU of the IP layer, it may have to break it up into smaller packets, and these are reassembled in the receiver IP layer. This clearly require some sequence control.

In visualization applications, it is clear that sequence control is important.

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