OSI model

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Originally posted here by Tiger Shark
xmeta: While there is no real need to "use it" in day to day network administration it is imperative that you understand it exists and how and why it works. If you don't know about it and how it works then you will never understand how your network is working... Then you are at a severe disadvantage.

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Exactly as Tiger said, and just to add a point taken from Cisco " The OSI reference model is considered the best tool available for teaching people about sending and receiving data on a network"

It's really just a description of networks and how they existed. The problem is, in the OSI model, some protocols commonly in use span multiple layers. IP is technically both Layers 3 (Network) and 4 (Transport), while TCP is technically Layers 4 and 5 (Session). The trouble is, how do you describe UDP/IP on the OSI Model? UDP has no session layer, so it is Layer 4, and IP is Layers 3 and 4. You end up with quite a bit of duplication attempting to explain modern networking protocol use. That's just covering network layers, when you break it down, things like ASP and PHP are capable of encapsulating layer-5 implementations inside layer 7, while skipping layer 6 entirely. To retrieve a webpage, you go through Ethernet, IP, TCP, HTTP, possibly ASP/PHP Session protocols/handlers, and an interface rendering mechanism that renders HTML/DHTML/XHTML, CSS, JavaScript/VBScript, and so on.

It worked on pre-IP networks quite well, and it still serves a purpose today.

http://en.wikipedia.org/wiki/OSI_model

I think maybe this poor chap is confised. No system in the world uses OSI per se. Its not a protocol or anything of that nature. It is a way of describing how protocols operate. Mybe thats what his professor meant when he said nobody uses OSI.

Well..My professor said OSI IS the fundamental of networking. I think that's why ppl who finish masters can't connect two computers behind the router ...even if they connect it using the instruction manual ..they will lost on the further dis connection. LOL

Master degree sucks....wondering how i will b.

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Originally posted here by chsh
It's really just a description of networks and how they existed. The problem is, in the OSI model, some protocols commonly in use span multiple layers. IP is technically both Layers 3 (Network) and 4 (Transport), while TCP is technically Layers 4 and 5 (Session). The trouble is, how do you describe UDP/IP on the OSI Model? UDP has no session layer, so it is Layer 4, and IP is Layers 3 and 4.
http://en.wikipedia.org/wiki/OSI_model

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In my little and no practical knowledge. UDP and TCP both are in the transport layer difference is in the connection oriented and connection less. I never read anywhere that IP works on the transport layer. I don't think router works on the transport layer. Actually in the M$ networks router is a gateway and it works from 4-7 layers . dunno how it works...you may b right in this way but

How can IP b layer 4 protocol?

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Originally posted here by rajunpl
In my little and no practical knowledge. UDP and TCP both are in the transport layer difference is in the connection oriented and connection less. I never read anywhere that IP works on the transport layer. I don't think router works on the transport layer. Actually in the M$ networks router is a gateway and it works from 4-7 layers . dunno how it works...you may b right in this way but
How can IP b layer 4 protocol?

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As per the wikipedia definition I linked:

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* Network layer Layer 3. : The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions. The router operates at this layer -- sending data throughout the extended network and making the Internet possible, although there are layer 3 (or IP) switches. This is a logical addressing scheme - values are chosen by the network engineer. The addressing scheme is hierarchical.
* Transport layer Layer 4. : The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer. The transport layer controls the reliability of a given link. Some protocols are stateful and connection oriented. This means that the session layer can keep track of the packets and retransmit those that fail.
* Session layer Layer 5. : The Session layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. This layer is responsible for setting up and tearing down TCP/IP sessions.

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Because IP handles both QoS and routing, it fits on Layers 3 and 4. TCP fits here as well because of the fact that it tracks sessions, and handles retransmits. TCP also fits on layer 5 because it is stateful, and thus acts as the session layer. UDP is different, and is considered "connectionless" due to the lack of session capabilities.
I think what many organizations do is simply ignore the transport layer altogether, although if you examine both IP and TCP, you begin to see that they both perform functions that Layer 4 lists: IP manages transparent data transfer (seamless protocol encapsulation) and manages QoS stuff, while TCP handles retransmission of failed packets.

Whenever anything goes wrong, I think right .. layer 1 and move on from there :) It just comes naturally

a lot of the stuff you will learn you may think i wont need to know this .. but once you do you will probably use most of it automatically when your down the track and in an IT job .. like my statement above :)