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for assuring that data sent across a network is delivered to one of a number of other
end systems is the network layer. This argues for inclusion of a network layer.
In fact, the OSI layer 2 is split into two sublayers. The lower sublayer is
concerned with medium access control (MAC), assuring that only one end system
at a time transmits; the MAC sublayer is also responsible for addressing other end
systems across the LAN. The upper sublayer is called Logical Link Control (LLC).
LLC performs traditional link control functions. With the MAC/LLC combination,
no network layer is needed (but an internet layer may be needed).
4.6 The internet protocol can be defined as a separate layer. The functions performed
by IP are clearly distinct from those performed at a network layer and those
performed at a transport layer, so this would make good sense. This results in 8
layers.
The session and transport layer both are involved in providing an end-to-end
service to the OSI user, and could easily be combined. This has been done in
TCP/IP, which provides a direct application interface to TCP. This results in 6
layers
4.7 a. No. This would violate the principle of separation of layers. To layer (N – 1),
the N-level PDU is simply data. The (N – 1) entity does not know about the
internal format of the N-level PDU. It breaks that PDU into fragments and
reassembles them in the proper order.
b. Each N-level PDU must retain its own header, for the same reason given in (a).
4.8 Data plus transport header plus internet header equals 1820 bits. This data is
delivered in a sequence of packets, each of which contains 24 bits of network
header and up to 776 bits of higher-layer headers and/or data. Three network
packets are needed. Total bits delivered = 1820 + 3 24 = 1892 bits.
4.9 UDP provides the source and destination port addresses and a checksum that
covers the data field. These functions would not normally be performed by
protocols above the transport layer. Applications can't provide port addresses (they
aren't really addresses). Port-ids have to be unambiguous between the application
and the layer (or OS). If they were provided by the application then the same port-
id could be assigned by different applications and layer (or OS) couldn't distinguish
them. UDP therefore is a necessity. Thus UDP provides a useful, though limited,
service.
4.10 In the case of IP and UDP, these are unreliable protocols that do not guarantee
delivery, so they do not notify the source. TCP does guarantee delivery. However,
the technique that is used is a timeout. If the source does not receive an
acknowledgment to data within a given period of time, the source retransmits.
4.11 UDP has a fixed-sized header. The header in TCP is of variable length.
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