OSI Model

OSI ModelPresentation Layer ProtocolFerolino, Gil John C. B.S CpE -4

Overview- The presentation layer of the Open System Interconnection (OSI) model is responsible for how that data looks or isformatted.

2Presentation layerresponsible for the delivery and formatting of information to the application layer for further processing or displayis the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hostsAn example of a presentation service would be the conversion of anEBCDIC-coded textcomputer fileto anASCII-coded file.

This layer deals with issues of string representation - whether they use thePascalmethod (an integer length field followed by the specified amount of bytes) or theC/C++method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest.

It relieves the application layer of concern regarding syntactical differences indatarepresentation within the end-usersystems

3

EBCDIC is an 8-bit coding scheme. Valid hex values for an EBCDIC character are 00 to FF.The 16 rows in the chart below correspond to the first hex digit of an EBCDIC character (0 to F). The contents of the cells shows the ASCII value (in hex) that corresponds to that EBCDIC character.4In many widely used applications and protocols, no distinction is made between the presentation and application layers.

Example:

Hypertext Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer.The presentation layer of the Open System Interconnection (OSI) model is responsible for how that data looks or is formatted.Consider an example in which spies exchange encoded messages.The manner of passing the messages back and forth is defined by the session layer, but how the messages are encoded (or the cipher the spies used to obscure the message) is the responsibility of the presentation layer.6

Using SMTPS, the client side of the application passes the text to the presentation layer services and requests encryption.

A Transport Layer Security (TLS) component at the presentation layer receives the unencrypted message and proceeds to encrypt the message using standard TLS processes.

The encrypted message flows down through the remaining OSI layers, over the physical network to the server.

At the server, the message is sent up through all of the layers until it arrives at the presentation layer.

Now, the servers TLS processes will take over and decrypt the message so that it is clearly readable.

The clear text message is then delivered to the SMTP application layer protocol for processing.

In this case, the next step would be to deliver the message to the recipients mailbox.If the client computer is running an e-mail program and the server is the users e-mail server, then on either end of the connection (both the client and the server sides), they are likely using the Simple Mail Transfer Protocol (SMTP) application layer protocol, or rather the encrypted version, SMTPS. The data flow would be as follows:7ProtocolsOther protocols sometimes considered at this level (though perhaps not strictly adhering to the OSI model) include:

Apple Filing Protocol (AFP)Independent Computing Architecture (ICA), the Citrix system core protocolLightweight Presentation Protocol (LPP)NetWare Core Protocol (NCP)Network Data Representation (NDR)Telnet (a remote terminal access protocol)Tox, The Tox protocol is sometimes regarded as part of both the presentation and application layereXternal Data Representation (XDR)X.25 Packet Assembler/Disassembler Protocol (PAD)Apple Filing ProtocolThe Apple Filing Protocol (AFP), formerly AppleTalk Filing Protocol, is a proprietary network protocol that offers file services for Mac OS X and original Mac OS. In Mac OS X, AFP is one of several file services supported, with others including Server Message Block (SMB), Network File System (NFS), File Transfer Protocol (FTP), and WebDAV. Independent Computing ArchitectureIndependent Computing Architecture (ICA) is a proprietary protocol for an application server system, designed by Citrix Systems. The protocol lays down a specification for passing data between server and clients, but is not bound to any one platform.ICA is broadly similar in purpose to window servers such as the X Window System. It also provides for the feedback of user input from the client to the server, and a variety of means for the server to send graphical output, as well as other media such as audio, from the running application to the client.10Lightweight Presentation ProtocolUsed to provide ISO presentation services on top of TCP/IP based protocol stacks. It is defined in RFC 1085describes a mechanism for providing the ISO transport service on top of the Transmission Control Protocol (TCP) [RFC793] and Internet Protocol (IP) [RFC791]. Once this method is applied, one may implement "real" ISO applications on top of TCP/IP-based internets, by simply implementing OSI session, presentation, and application services on top of the transport service access point which is provided on top of the TCP.11NetWare Core ProtocolThe NetWare Core Protocol (NCP) is a network protocol used in some products from Novell, Inc. It is usually associated with the client-server operating system Novell NetWare which supported primarily MS-DOS client stations, but more recently a support of other platforms such as Microsoft Windows, Mac OS, Linux, Windows NT and various flavors of Unix was added.The NCP is used to access file, print, directory, clock synchronization, messaging, remote command execution and other network service functions. It originally took advantage of an easy network configuration and a little memory footprint of the IPX/SPX protocol stack. Since mid-1990s the TCP/IP implementation is available.12Network Data RepresentationNetwork Data Representation (NDR) is an implementation of the presentation layer in the OSI model.TelnetTelnet is an application protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control information in an 8-bit byte oriented data connection over the Transmission Control Protocol (TCP).Telnet was developed in 1968 beginning with RFC 15, extended in RFC 854, and standardized as Internet Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards.14Tox (software)Tox is a free and open-source, peer-to-peer, encrypted instant messaging and video calling software. The stated goal of the project is to provide secure yet easily accessible communication for everyone.[1]The core of Tox is an implementation of the Tox protocol, an example of the application layer of the OSI model and arguably the presentation layer. Implementations of the Tox protocol not done by the project exist, an example of one being Xot.[6]15External Data RepresentationExternal Data Representation (XDR) is a standard data serialization format, for uses such as computer network protocols. It allows data to be transferred between different kinds of computer systems. Converting from the local representation to XDR is called encoding. Converting from XDR to the local representation is called decoding. XDR is implemented as a software library of functions which is portable between different operating systems and is also independent of the transport layer.XDR uses a base unit of 4 bytes, serialized in big-endian order; smaller data types still occupy four bytes each after encoding. Variable-length types such as string and opaque are padded to a total divisible by four bytes. Floating-point numbers are represented in IEEE 754 format.16X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network (WAN) communication. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections or ISDN connections as physical links.X.25 is a family of protocols that was popular during the 1980s with telecommunications companies and in financial transaction systems such as automated teller machines. X.25 was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts[1] and finalized in a publication known as The Orange Book in 1976.[2]While X.25 has, to a large extent, been replaced by less complex protocols, especially the Internet protocol (IP), the service is still used and available in niche and legacy applications.17Presentation(Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.QUESTIONS1.) Describe the presentation layer protocols of the OSI protocol suite.2.) Describe the EBCDIC to ASCII Conversion Chart3.) Describe External Data Representation.4.) Describe Tox protocol.ANSWERS1. The presentation layer implementation of the OSI protocol suite consists of a presentation protocol and a presentation service. The presentation protocol enables presentation-service users (PS-users) to communicate with the presentation service.2. EBCDIC is an 8-bit coding scheme. Valid hex values for an EBCDIC character are 00 to FF..The 16 rows in the chart below correspond to the first hex digit of an EBCDIC character (0 to F). The contents of the cells shows the ASCII value (in hex) that corresponds to that EBCDIC character.3. External Data Representation (XDR) is a standard data serialization format, for uses such as computer network protocols. It allows data to be transferred between different kinds of computer systems. 4. The core of Tox is an implementation of the Tox protocol, an example of the application layer of the OSI model and arguably the presentation layer. Implementations of the Tox protocol not done by the project exist, an example of one being Xot.