OSI model

OSI model
From Wikipedia,
Joris de Leeuw GTAgames Nederland

OSI Model
7 Application layer
6 Presentation layer
5 Session layer
4 Transport layer
3 Network layer
2 Data link layer
1 Physical layer

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection (OSI) initiative. It is also called the OSI seven layer model. The layers, described below, are, from top to bottom, Application, Presentation, Session, Transport, Network, Data Link, and Physical. A layer is a collection of related functions that provides services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Even though newer IETF and IEEE protocols, and indeed OSI protocol work subsequent to the publication of the original architectural standards that have largely superseded it, the OSI model is an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol they study into one of the seven basic layers. This is not always easy to do as many of the protocols in use on the Internet today were designed as part of the TCP/IP model, and may not fit cleanly into the OSI model.


History
In 1977, work on a layered model of network architecture, which was to become the OSI model, started in the American National Standards Institute (ANSI) working group on Distributed Systems (DISY).[1] With the DISY work and worldwide input, the International Organization for Standardization (ISO) began to develop its OSI networking suite.[2] According to (Bachman), the term "OSI" came into use on 12 October 1979. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model) and a set of concrete protocols. The standard documents that describe OSI are for sale and not currently available online.

Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.

In particular, Internet protocols are deliberately not as rigorously architected as the OSI model, but a common version of the TCP/IP model splits it into four layers. The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its End-to-End Layer includes the graceful close function of the OSI Session Layer as well as the Transport Layer. Its Internetwork Layer is equivalent to the OSI Network Layer, while its Interface layer includes the OSI Data Link and Physical Layers. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the Internal Organization of the Network Layer document.

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote host. Service definitions abstractly describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers inside the local host.

Create VPN Client for WindowsXP

WindowsXP VPN Client

Easy to create a WindowsXP VPN Client


1.Click to Start / Control Panel / Network Connections
2.Double Click the Create a new Connection
3.Next and Select Connect to the network at my workplace



4.Click on the Next button
5.Click on Virtual Private Network connection



6.Click on the Next button
7.Give the Connection a Name




8.Click on the Next button
9.If prompted, select whether or not you need to dial to the Internet before establishing a VPN connection.



10.Enter in the IP address of the server you want to connect to. This needs to be the external WAN IP address :Public IP Address that is being used by the VPN Server. Not the LAN IP address of the VPN server.




11.Check whether you want to have an icon placed on the desktop and click on the Finish button.

Create VPN Server for WindowsXP

Easy to create a WindowsXP VPN Server

1. Click to Start / control panel / Network Connections
2. Start the Create a New Connection



3. Click on the Next button
4. Select Set up advanced connection



5. Click on the Next button.
6. Click on Accept incoming connections



7. Click on the Next button
8. At the LPT1 page, skip it and just click on the Next button.



9. Click on Allow virtual private connection



10. Click on the Next button
11. Add user accounts that you want to be able to connect to your WindowsXP computer.



12. Click on the Next button.
13. Highlight Internet Protocol (TCP/IP) and click on Properties



14. Determine how you want the remote computers to get their IP address



***If the VPN server is behind a router, Port Mapping or port forward will need to be done on the router. Standard port usage is 1723 for PPTP and 1701 for l2tp. You might also need to configure your router for PPTP Passthrough. Port usage for IPSec is 500, 50-51. These ports will have to be forwarded to the VPN server's IP ***

http://VPNFarm.blogspot.com

OSI and TCP/IP Layering Differences

From Wikipedia
Jakub Dziwior

OSI and TCP/IP Layering Differences

The three top layers in the OSI model - the application layer, the presentation layer and the session layer - usually are lumped into one layer in the TCP/IP model. While some pure OSI protocol applications, such as X.400, also lumped them together, there is no requirement that a TCP/IP protocol stack needs to be monolithic above the transport layer. For example, the Network File System (NFS) application protocol runs over the eXternal Data Representation (XDR) presentation protocol, which, in turn, runs over a protocol with session layer functionality, Remote Procedure Call (RPC). RPC provides reliable record transmission, so it can run safely over the best-effort User Datagram Protocol (UDP) transport.

The session layer roughly corresponds to the Telnet virtual terminal functionality, which is part of text based protocols such as HTTP and SMTP TCP/IP model application layer protocols. It also corresponds to TCP and UDP port numbering, which is considered as part of the transport layer in the TCP/IP model. The presentation layer has similarities to the MIME standard, which also is used in HTTP and SMTP.





Since the IETF protocol development effort is not concerned with strict layering, some of its protocols may not appear to fit cleanly into the OSI model. These conflicts, however, are more frequent when one only looks at the original OSI model, ISO 7498, without looking at the annexes to this model (e.g., ISO 7498/4 Management Framework), or the ISO 8648 Internal Organization of the Network Layer (IONL). When the IONL and Management Framework documents are considered, the ICMP and IGMP are neatly defined as layer management protocols for the network layer. In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities" such as ARP and RARP.

IETF protocols can be applied recursively, as demonstrated by tunneling protocols such as Generic Routing Encapsulation (GRE). While basic OSI documents do not consider tunneling, there is some concept of tunneling in yet another extension to the OSI architecture, specifically the transport layer gateways within the International Standardized Profile framework [8]. The associated OSI development effort, however, has been abandoned given the real-world adoption of TCP/IP protocols.

Layers in the TCP/IP model

From Wikipedia
Alan Isherwoo


Layers in the TCP/IP model

The layers near the top are logically closer to the user application (as opposed to the human user), while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method of abstraction to isolate upper layer protocols from the nitty gritty detail of transmitting bits over, say, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol.
IP suite stack showing the physical network connection of two hosts via two routers and the corresponding layers used at each hop. The dotted line represents a virtual connection
Sample encapsulation of data within a UDP datagram within an IP packet

This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not to, provide. Again, the original OSI Reference Model was extended to include connectionless services (OSIRM CL).[5] For example, IP is not designed to be reliable and is a best effort delivery protocol. This means that all transport layers must choose whether or not to provide reliability and to what degree. UDP provides data integrity (via a checksum) but does not guarantee delivery; TCP provides both data integrity and delivery guarantee (by retransmitting until the receiver receives the packet).

This model lacks the formalism of the OSI Reference Model and associated documents, but the IETF does not use a formal model and does not consider this a limitation, as in the comment by David D. Clark, "We don't believe in kings, presidents, or voting. We believe in rough consensus and running code." Criticisms of this model, which have been made with respect to the OSI Reference Model, often do not consider ISO's later extensions to that model.

For multiaccess links with their own addressing systems (e.g. Ethernet) an address mapping protocol is needed. Such protocols can be considered to be below IP but above the existing link system. While the IETF does not use the terminology, this is a subnetwork dependent convergence facility according to an extension to the OSI model, the Internal Organization of the Network Layer (IONL) [6].
ICMP & IGMP operate on top of IP but do not transport data like UDP or TCP. Again, this functionality exists as layer management extensions to the OSI model, in its Management Framework (OSIRM MF) [7]
The SSL/TLS library operates above the transport layer (utilizes TCP) but below application protocols. Again, there was no intention, on the part of the designers of these protocols, to comply with OSI architecture.
The link is treated like a black box here. This is fine for discussing IP (since the whole point of IP is it will run over virtually anything). The IETF explicitly does not intend to discuss transmission systems, which is a less academic but practical alternative to the OSI Reference Model.

Key Architectural Principles

From Wikipedia
Alan Isherwoo
Key Architectural Principles

An early architectural document, RFC 1122, emphasizes architectural principles over layering.

End-to-End Principle: This principle has evolved over time. Its original expression put the maintenance of state and overall intelligence at the edges, and assumed the Internet that connected the edges retained no state and concentrated on speed and simplicity. Real-world needs for firewalls, network address translators, web content caches and the like have forced changes in this Principle. [3]
Robustness Principle: "Be liberal in what you accept, and conservative in what you send. Software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features".
Even when layer is examined, the assorted architectural documents -- there is no single architectural model such as ISO 7498, the OSI Reference Model -- have fewer, less rigidly defined layers than the commonly referenced OSI model, and thus provides an easier fit for real-world protocols. In point of fact, one frequently referenced document does not contain a stack of layers. The lack of emphasis on layering is a strong difference between the IETF and OSI approaches. It only refers to the existence of the "internetworking layer" and generally to "upper layers"; this document was intended as a 1996 "snapshot" of the architecture: "The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology's success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture."

No document officially specifies the model, another reason to deemphasize the emphasis on layering. Different names are given to the layers by different documents, and different numbers of layers are shown by different documents.

There are versions of this model with four layers and with five [citation needed] layers. RFC 1122 on Host Requirements makes general reference to layering, but refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer version, with the layers having names, not numbers, as

Process Layer or Application Layer: this is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
Host-To-Host (Transport) Layer: this is where flow-control and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections, ensuring that packets are in fact received.
Internet or Internetworking Layer: this layer defines IP addresses, with many routing schemes for navigating packets from one IP address to another.
Network Access Layer: this layer describes both the protocols (i.e., the OSI Data Link Layer) used to mediate access to shared media, and the physical protocols and technologies necessary for communications from individual hosts to a medium.
The Internet protocol suite (and corresponding protocol stack), and its layering model, were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model numerous times in books and classrooms. In a popular textbook, which is a secondary source,[4] but not primary IETF specifications, the model has evolved into a five-layer version that splits the network access layer into a Physical layer and a Network Access layer, corresponding to the physical layer and data link layer of the OSI model. The Internet or Internetworking layer corresponds to the OSI Network layer