Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 52

travel independently to the destination (potentially on a different network).
They may even arrive in a completely different order than they were sent, in
which case, it is the job of higher layers to rearrange them, if in-order deliv-
ery is desired. Note that Internet is used here in a generic sense, even though
this layer is present in the Internet.
The analogy here is with the (snail) mail system. A person can drop
a sequence of international letters into a mailbox in one country, and
with a little luck, most of them will be delivered to the correct address in
the destination country. The letters will probably travel through one or
more international mail gateways along the way, but this is transparent
to the users. Furthermore, that each country (i.e., each network) has its
own stamps, preferred envelope sizes, and delivery rules is hidden from
the users. The Internet layer defines an official packet format and pro-
tocol called IP, plus a companion protocol called ICMP (Internet Control
Message Protocol), that helps it function. The job of the Internet layer is to
deliver IP packets where they are supposed to go. Packet routing is clearly
a major issue here, as is congestion (though IP has not proven effective at
avoiding congestion).
2.2.3.2.3 Transport Layer
The layer above the Internet layer in the TCP/IP model is now usually called
the transport layer. It is designed to allow peer entities on the source and
destination hosts to carry on a conversation, just as in the OSI transport layer.
Two end-to-end transport protocols have been defined here. The first one,
TCP (Transmission Control Protocol), is a reliable connection-oriented proto-
col that allows a byte stream originating on one machine to be delivered with-
out error on any other machine in the Internet. It segments the incoming byte
stream into discrete messages and passes each one on to the Internet layer. At
the destination, the receiving TCP process reassembles the received messages
into the output stream. TCP also handles flow control to make sure a fast
sender cannot swamp a slow receiver with more messages than it can handle.
The second protocol in this layer, UDP (User Datagram Protocol), is an
unreliable, connectionless protocol for applications that do not want TCP's
sequencing or flow control and wish to provide their own. It is also widely
used for one-shot, client-server-type request-reply queries and applica-
tions in which prompt delivery is more important than accurate delivery,
such as transmitting speech or video. Since the model was developed, IP
has been implemented on many other networks.
2.2.3.2.4 Application Layer
The TCP/IP model does not have session or presentation layers. No need for
them was perceived. Instead, applications simply include any session and
presentation functions that they require. Experience with the OSI model has
proven this view correct: these layers are of little use to most applications.