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Design Issues for the Layers Some of the key design issues that occur incomputer networking are present in severallayers. Next, we will briefly mention some of theimportant ones. Every layer, needs a mechanism for
identifying
senders and receivers
. Since a network normally
has many computers, some of which havemultiple processes, a means is needed for aprocess on one machine to specify with whom itwants to talk. As a consequence of havingmultiple destinations,
some form of addressing
is
needed in order to specify a particulardestination.
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Design Issues for the Layers Another set of design decisions concerns the
rules for data
transfer
. In some systems, data only travel in one direction (simplex
communication). In others they can travel in either direction, but not simultaneously (
half-duplex
communication).
In still others they travel in both directions at once (
full-duplex
communication). The protocol must also determine how manylogical channels the connection corresponds to, and whattheir priorities are. Many networks provide at least two logicalchannels per connection, one for normal data and one forurgent data. Error control
is an important issue because physical
communication circuits are not perfect. Many error-detectingand error-correcting codes are known, but both ends of theconnection must agree on which one is being used. Inaddition, the receiver must have some way of telling thesender which messages have been correctly received andwhich have not.
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Design Issues for the Layers Another problem that must be solved at several levels is the inability of all processes to accept arbitrary long messages
This property leads to mechanisms for disassembling,transmitting, and then reassembling messages. A relatedissue is what to do when processes insist upon transmittingdata in units that are so small that sending each oneseparately is inefficient. Here the solution is to gather togetherseveral small messages heading toward a commondestination into a single large message and dismember thelarge message at the other side. When it is inconvenient or expensive to set up a separateconnection for each pair of communicating processes, theunderlying layer may decide to
use the same connection for
multiple, unrelated conversations
. As long as this multiplexing
and demultiplexing is done transparently, it can be used byany layer. Multiplexing is needed in the physical layer, forexample, where all the traffic for all connections has to besent over at most a few physical circuits.
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Design Issues for the Layers Finally, when there are
multiple paths between
source and destination, a route must be chosen
Sometimes this decision must be split over twoor more layers. For example, to send data fromLondon to Rome, a high-level decision mighthave to made to go via France or Germanybased on their respective privacy laws, and alow-level decision might have to be made tochoose one of the many available circuits basedon the current traffic load.
