(Layer 5)
conversations (called sessions). This includes the control
and management of multiple bidirectional messages so
that the application can be notified if only some of a
series of messages are completed. This allows the
presentation layer to have a seamless view of an
incoming stream of data. The presentation layer can be
presented with data if all flows occur in some cases. For
example, an automated teller machine transaction in
which you withdraw cash from your checking account
should not debit your account, and then fail, before
handing you the cash, recording the transaction even
though you did not receive money. The session layer
creates ways to imply which flows are part of the same
session and which flows must complete before any are
considered complete.
NetBios names,
AppleTalk ASP, DECnet
SCP
(Layer 4)
do not provide error recovery. Multiplexing of incoming
data for different flows to applications on the same
host (for example, TCP sockets) is also performed.
Reordering of the incoming data stream when packets
arrive out of order is included.
(Layer 3)
accomplish this, the network layer defines logical
addressing so that any endpoint can be identified. It also
defines how routing works and how routes are learned so
that the packets can be delivered. The network layer also
defines how to fragment a packet into smaller packets to
accommodate media with smaller maximum transmission
unit sizes. (Note: Not all Layer 3 protocols use
fragmentation.) The network layer of OSI defines most of
the details that a Cisco router considers when routing. For
example, IP running in a Cisco router is responsible for
examining the destination IP address of a packet,
comparing that address to the IP routing table,
fragmenting the packet if the outgoing interface requires
smaller packets, and queuing the packet to be sent out to
the interface.