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TCP/IP – The Transport Layer The layer above the internet layer in the TCP/IP modelis now usually called the transport layer. It is designed to allow peer entities on the source anddestination hosts to carry on a conversation, the sameas the OSI transport layer. Two end-to-end protocols have been defined here: 1. TCP (Transmission Control Protocol)
is a reliable connection-
oriented protocol that allows a byte stream originating on onemachine to be delivered without errors on any other machinein the internet. It fragments the incoming byte stream intodiscrete messages and passes each one onto the internetlayer. At the destination, the receiving TCP processreassembles the received messages into the output stream.TCP also handles flow control to make sure a fast sendercannot swamp a slow receiver with more messages than itcan handle.
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TCP/IP
–^
The Transport Layer
2.^
UDP (User Datagram Protocol)
is an unreliable,
connectionless protocol for applications that do not wantTCP’s sequencing or flow control and wish to provide theirown. It is also widely used for one-shot, client-server typerequest-reply queries and applications in which fast delivery ismore important than accurate delivery, such as transmittingspeech or video. The relation of IP, TCP and UDP is shown on Fig. 4. Since thismodel was developed, IP has been implemented on many othernetworks. Fig. 4. Protocols and networks in the TCP/IP model initially.
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TCP/IP – The Application Layer^ Many other protocols have been added to these overthe years, such as: the DNS (Domain Name Service) – for mapping host namesonto their network addresses NNTP (Network News Transfer Protocol) – for moving newsarticles around HTTP (HyperText Transfer Protocol) – used for fetchingpages on the World Wide Web, and many others
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A comparison of OSI and TCP/IP^ The OSI and TCP/IP reference models have much incommon. Both are based on the concept of a stack ofindependent protocols. Also, the functionality of thelayers is roughly similar.^ Despite the fundamental similarities, the two modelsalso have many differences.^ Probably the
biggest contribution of the OSI model
is
to make the distinction between the
Services
Interfaces
, and
Protocols
explicit.
These
ideas fit very nicely with modern ideas about object-oriented programming
. An object, like a layer,
has a set of methods (operations) that can be invokedoutside the object. The semantics of these methodsdefine the set of services that the object offers. Themethods’ parameters and results from the object'sinterface. The code internal to the object is its protocoland is not visible or of any concern outside the object.
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A comparison of OSI and TCP/IP^ In summary:^ despite its problems, the OSI
model
(minus the
session and presentation layers) has proven to beexceptionally useful for discussing computer networks.In contrast, the OSI
protocols
have not become
popular. The opposite is true for TCP/IP: the
model
is
practically nonexistent, but the
protocols
are widely
used. Therefore, we would like to think of a modified OSImodel and concentrate on the TCP/IP and relatedprotocols and use a hybrid model, shown on Fig. 5. ,as a framework.
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A hybrid model Fig. 5. The hybrid reference model
hybrid
Application
Transport
Network
Data Link
Physical
