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TCP and the OSI Reference Model Compared
Transcript of TCP and the OSI Reference Model Compared
Application TCP / IP OSI Application Session (Layer 5) Presentation (Layer6) Application (Layer7) The Upper Layers The Session layer permits two parties to hold ongoing communications called a session across a network.
Not found in TCP/IP model
In TCP/IP,its characteristics are provided by the TCP protocol.
(Transport Layer) The Session Layer The Presentation Layer handles data format information for networked communications. This is done by converting data into a generic format that could be understood by both sides.
Not found in TCP/IP model
In TCP/IP, this function is provided by the Application Layer.
e.g. External Data Representation Standard (XDR)
Multipurpose Internet Mail Extensions (MIME) The Presentation Layer The Application Layer is the top layer of the reference model. It provides a set of interfaces for applications to obtain access to networked services as well as access to the kinds of network services that support applications directly.
OSI - FTAM,VT,MHS,DS,CMIP
TCP/IP - FTP,SMTP,TELNET,DNS,SNMP
Although the notion of an application process is common to both, their approaches to constructing application entities is different. The Application Layer The diagram below provides an overall view on the methods use by both the OSI and TCP/IP model. Approaches use in constructing application entities Sometime called Horizontal Approach
OSI asserts that distributed applications operate over a strict hierarchy of layers and are constructed from a common tool kit of standardized application service elements.
In OSI, each distributed application service selects functions from a large common “toolbox” of application service element (ASEs) and complements these with application service elements that perform functions specific to given end-user service . ISO Approach Sometime called Vertical Approach
In TCP/IP, each application entity is composed of whatever set of function it needs beyond end to end transport to support a distributed communications service.
Most of these application processes builds on what it needs and assumes only that an underlying transport mechanism (datagram or connection) will be provided. TCP/IP Approach TCP / IP OSI
Transport (Layer 4) The functionality of the transport layer is to provide “transparent transfer of data from a source end open system to a destination end open system” (ISO / IEC 7498: 1984). Transport Layer It takes the information to be sent and breaks it into individual packets that are sent and reassembled into a complete message by the Transport Layer at the receiving node
Also provide a signaling service for the remote node so that the sending node is notified when its data is received successfully by the receiving node OSI Transport Layer Defines two standard transport protocols: TCP and UDP
TCP implements a reliable data-stream protocol
UDP implements an unreliable data-stream
connectionless TCP/IP Transport Layer The features of UDP and TCP defined at TCP/IP Transport Layer correspond to many of the requirements of the OSI Transport Layer. There is a bit of bleed over for requirements in the session layer of OSI since sequence numbers, and port values can help to allow the Operating System to keep track of sessions, but most of the TCP and UDP functions and specifications map to the OSI Transport Layer. Comparing Transport for both Models The TCP/IP and OSI architecture models both employ all connection and connectionless models at transport layer. However, the internet architecture refers to the two models in TCP/IP as simply “connections” and datagrams. But the OSI reference model, with its penchant for “precise” terminology, uses the terms connection-mode and connection-oriented for the connection model and the term connectionless-mode for the connectionless model. Comparing Transport for both Models TCP / IP OSI Internet Network (Layer 3) Like all the other OSI Layers, the network layer provides both connectionless and connection-oriented services. As for the TCP/IP architecture, the internet layer is exclusively connectionless. Network vs. Internet X.25 Packet Level Protocol – OSI’s Connection-oriented Network Protocol
The CCITT standard for X.25 defines the DTE/DCE interface standard to provide access to a packet-switched network. It is the network level interface, which specifies a virtual circuit (VC) service. A source host must establish a connection (a VC) with the destination host before data transfer can take place. The network attempts to deliver packets flowing over a VC in sequence.a Network vs. Internet Connectionless Network Service
Both OSI and TCP/IP support a connectionless network service: OSI as an alternative to network connections and TCP/IP as the only way in use.
OSI’s CLNP (ISO/IEC 8473: 1993) is functionally identical to the Internet’s IP (RPC 791). Both CLNP and IP are best-effort-delivery network protocols. Bit niggling aside, they are virtually identical. The major difference between the two is that CLNP accommodates variable-length addresses, whereas IP supports fixed, 32-bit address. Network vs. Internet Internet (IP) Addresses
The lnternet network address is more commonly called the “IP address.” It consists of 32 bits, some of which are allocated to a high-order network-number part and the remainder of which are allocated to a low-order host-number part. The distribution of bits - how many form the network number, and how many are therefore left for the host number - can be done in one of three different ways, giving three different classes of IP address Network vs. Internet OSI Network Layer Addressing
ISO/IEC and CCITT jointly administer the global network addressing domain. The initial hierarchical decomposition of the NSAP address is defined by (ISO/IEC 8348). The standard specifies the syntax and the allowable values for the high-order part of the address - the Initial Domain Part (IDP), which consists of the Authority and Format Identifier (AFI) and the Initial Domain Identifier (IDI) - but specifically eschews constraints on or recommendations concerning the syntax or semantics of the domain specific part (DSP). Network vs. Internet OSI Routing Architecture
End systems (ESs) and intermediate systems (ISs) use routing protocols to distribute (“advertise”) some or all of the information stored in their locally maintained routing information base. ESs and ISs send and receive these routing updates and use the information that they contain (and information that may be available from the local environment, such as information entered manually by an operator) to modify their routing information base. Network vs. Internet TCP/IP Routing Architecture
The TCP/IP routing architecture looks very much like the OSI routing architecture. Hosts use a discovery protocol to obtain the identification of gateways and other hosts attached to the same network (subnetwork). Gateways within autonomous systems (routing domains) operate an interior gateway protocol (intradomain IS-IS routing protocol), and between autonomous systems, they operate exterior or border gateway protocols (interdomain routing protocols). The details are different but the principles are the same. Network vs. Internet TCP / IP OSI Subnet Physical (Layer 1) Data Link (Layer 2) Data link layer
The function of the Data Link Layer is “provides for the control of the physical layer, and detects and possibly corrects errors which may occur” (IOS/IEC 7498:1984). In another words, the Data Link Layer transforms a stream of raw bits (0s and 1s) from the physical into a data frame and provides an error-free transfer from one node to another, allowing the layers above it to assume virtually error-free transmission Data link / Physical vs. Subnet Physical layer
The function of the Physical Layer is to provide “mechanical, electrical, functional, and procedural means to activate a physical connection for bit transmission” (ISO/IEC 7498:1984). Basically, this means that the typical role of the physical layer is to transform bits in a computer system into electromagnetic (or equivalent) signals for a particular transmission medium (wire, fiber, ether, etc.) Data link / Physical vs. Subnet Comparing to TCP/IP
These 2 layers of the OSI correspond directly to the subnet layer of the TCP/IP model.
Majority of the time, the lower layers below the Interface or Network layer of the TCP/IP model are seldom or rarely discussed. The TCP/IP model does nothing but to high light the fact the host has to connect to the network using some protocol so it can send IP packets over it. Because the protocol used is not defines, it will vary from host to host and network to network Data link / Physical vs. Subnet Comparing to TCP/IP
After much deliberation by organizations, it was decided that the Network Interface Layer in the TCP/IP model corresponds to a combination of the OSI Data Link Layer and network specific functions of the OSI network layer (eg IEEE 203.3).
Since these two layers deal with functions that are so inherently specific to each individual networking technology, the layering principle of grouping them together related functions is largely irrelevant. Data link / Physical vs. Subnet