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Protocol Hierarchies - analogy^ The translators have agreed on a neutral language, Dutch, so themessage is converted to “Ik hou van konijenen.” The choice oflanguage (
the layer 2 protocol) is up to the layer 2 peer processes
.
The translator then gives the message to a secretary fortransmission, by for example, fax (the layer 1 protocol). When themessage arrives, it is translated to French and passed across the2/3 interface to philosopher 2. * Note, that each protocol is completely independent of the otherones^ as long as the interfaces are not changed
. So, the translators
can switch from Dutch to say, Finnish, at will, provided that they bothagree, and neither changes his interface with layer 1 or layer 3.Similarly, the secretaries can switch from fax to email, or telephonewithout disturbing (or even informing) the other layers. Each process may add some information intended for only its peer
.
This information is
not passed upward
to the layer above.
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Protocol Hierarchies - Example^ Now, consider a more technical example: how to providecommunication to the top layer of the five-layer networkin Fig. 3.^ A message,
M , is produced by an application process running in layer 5 and given to layer 4 for transmission. Layer 4 puts a
header
in front of the message to identify
the message and passes the result to layer 3. Theheader includes control information, such as sequencenumbers, to allow layer 4 on the destination machine todeliver messages in the right order if the lower layers donot maintain sequence. In some layers, headers alsocontain sizes, times, and other control fields.
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Protocol Hierarchies - Example^ In many networks, there is no limit to the size ofmessages transmitted in the layer 4 protocol, but there isnearly always a limit imposed by the layer 3 protocol.^ Consequently, layer 3 must break up the incomingmessages into smaller units,
packets
, prepending a layer
3 header to each packet. In this example,
M^ is split into
two parts
M and^1
M.^2
Layer 3 decides which of the outgoing lines to use andpasses the packets to layer 2. Layer 2 adds not only aheader to each piece, but also a trailer, and gives theresulting unit to layer 1 for physical transmission. At the receiving machine, the message moves upward,from layer to layer, with headers being stripped off as itprogresses. (i.e.
none of the headers for layers below
n
are passed up to layer
n .)
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Protocol Hierarchies
-^ Example
The important thing to understand about Fig. 3. Is therelation between the virtual and actual communication andthe difference between protocols and interfaces. The peer processes in layer 4, for example, conceptuallythink of their communication as being “horizontal”, usingthe layer 4 protocol. Each one is likely to have a procedurecalled something like
SendToOtherSide
and
GetFromOtherSide
, even though these procedures actually communicate with lower layers across the 3/4interface, not with the other side. The^ peer process abstraction is crucial
to all network
design. Using it, the unmanageable task of designing thecomplete network can be broken into several smaller,manageable, design problems, namely the design of theindividual levels. Lower layers of a protocol hierarchy are frequentlyimplemented in hardware (completely or partially). The restis software.
