1. OSI Model And TCP\IP SUITE
By Miss Irum Matloob

2. The OSI Model
An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model
An open system is a model that allows any two different systems to communicate regardless of their underlying architecture
The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable

3. Fundamental of the OSI Model
Understanding the following fundamentals of the OSI model provides a solid basis for exploration of data communication
Layered Architecture
Peer-to-Peer Processes
Interfaces between Layers
Organization of the Layers

4. Layered Architecture
The OSI model is built of 7 ordered layers: physical, datalink, network, transport, session, presentation, and application
In developing the model, the designers distilled the process of transmitting data down to its most fundamental elements
They identified which networking functions had related uses and collected those functions into discrete groups that became the layers

5. Layered Architecture (cont.)
By defining and localizing functionality in this fashion, the designers created an architecture that is both comprehensive and flexible
Most important, the OSI model allows complete transparency between otherwise incompatible systems

6. Peer-to-Peer Processes
Within a single machine, each layer calls upon the services of the layer just below it. For example, layer 3 uses the services provided by layer 2
Between machines, a process of layer x on one machine communicate with the same process of layer x on another machine; this is called peer-to-peer processes
This communication is governed by an agreed-upon series of rules and conventions called protocols

7. Peer-to-Peer Processes (cont.)
At the higher layers, communication must move down through the layers on the sending machine over to the destining machine, and then back up through the layers
Each layer in the sending machine adds its own information (called headers or trailers) to the message it receives from the layer just above it and passes the whole package to the layer just below it
Headers are added to the message at layers 6, 5, 4, 3, and 2. A trailers is added at layer 2

8. Peer-to-Peer Processes (cont.)
At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it and then passes the rest to the process in the upper layer

9. Organization of the Layers
The 7 layers can be thought of as belonging to 3 subgroup:
Layers 1-3 are the network support layers, they deal with the physical aspects of moving data from one device to another
Layers 5-7 can be thought of as the user support layers; they allow interoperability among unrelated software system
Layer 4 ensures end-to-end reliable data transmission while layer 2 ensures reliable transmission on a single link
The upper OSI layers are almost always implemented in software; lower layers are a combination of hardware and software, except for the physical layer, which is mostly hardware

10. Interfaces between Layers
The passing of the data and network information down through the layers of the sending machine and back up through the layers of the receiving machine is made by an interface between each pair of adjacent layers
Each interface defines what information and services a layer must provide for the layer above it
As long as a layer still provides the expected services to the layer above it, the specific implementation of its functions can be modified or replaced without requiring changes to the surrounding layers

11. Functions of the Physical Layers (cont.)
The physical layer is concerned with the following:
Physical characteristics of interface and media
Representation of bits
Data rate (transmission rate)
Synchronization of bits
Line configuration
Physical topology
Transmission mode

12. Functions of the Data Link Layer
The data link layer transforms the physical layer to a reliable link and is responsible for node-to-node delivery
It makes the physical layer appear error free to the upper layer (network layer)

13. Functions of the Data Link Layer (cont.)
Specific responsibilities of the data link layer include the following:
Framing
Physical addressing
Flow control
Error control
Access control

14. Example
A node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection

15. Functions of the Network Layer
The network layer is responsible for the source-to-destination delivery of a packet possibly across multiple networks
Whereas the data link layer oversees the delivery of the packet between two systems on the same network, the network layer ensures that each packet gets from its point of origin to its final destination
Specific responsibilities of the network layer include the following:
Logical addressing
Routing

16. Functions of the Transport Layer
The transport layer is responsible for source-to-destination (end-to-end) delivery of the entire message
Whereas the network layer treats each one independently, the transport layer ensures that the whole message arrives intact and in order

17. Functions of the Transport Layer (cont.)
For added security, the transport layer may create a connection between the two end ports
A connection is a single logical path between the source and destination that is associated with all packet in a message
Creating a connection involves 3 steps: connection establishment, data transfer, and connection release

18. Functions of the Transport Layer (cont.)
Specific responsibilities of the transport layer include the following:
Service-point addressing (or port address)
Segmentation and reassembly
Connection control. There are 2 types of connection control: connectionless or connection oriented.
Flow control
Error control (damage, loss, or duplication)

19. Functions of the Session Layer
The services provided by the lowest four layers of the OSI model may be not sufficient
For example, a remote terminal access application might require a half-duplex dialogue. A transaction-processing application might require checkpoints in the data transfer stream to permit backup and recovery
Since these types of dialogue-structuring tools have widespread applicability, it make sense to organize them into a separate layer: the session layer which establishes, maintains, and synchronizes the interaction between communicating systems

20. Functions of the Session Layer (cont.)
Specific responsibilities of the session layer include the following:
Dialog control– half duplex or full duplex
Synchronization. The session layer allows a process to add checkpoints (synchronization points)

21. Functions of the Presentation Layer
The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems
Specific responsibilities of the presentation layer include the following:
Translation. Since different computers use different encoding systems, the presentation layer responsible for interoperability between these different encoding methods
Encryption. To carry sensitive information, a system must be able to assure privacy
Compression. Data compression reduces the number of bits to be transmitted

22. Functions of the Application Layer
The application layer enables application programs to access the network
It provides interfaces and support for services such as electronic mail, remote file access and transfer

23. Summary of Layer Functions
Figure 3-14
Summary of Layer Functions
WCB/McGraw-ill
 The McGraw-Hill Companies, Inc., 1998

24. Introduction TCP/IP
The Internet Protocol Suite (commonly known as TCP/IP) is the set of communications protocols used for the Internet and other similar networks.
It is named from two of the most important protocols in it:
the Transmission Control Protocol (TCP) and
the Internet Protocol (IP), which were the first two networking protocols defined in this standard.

26. Protocol Reference Model of OSI
OSI Overview
4. Data Encapsulation
a) Each protocol on the diff. layer has its own format.
b) Headers are added while a packet is going down the stack at each layer.
c) Trailers are usually added on the second layer.

29. Physical and Data Link Layers
At the physical and data link layers, TCP/IP does not define any specific protocol.
It supports all the standard and proprietary protocols.
A network in a TCP/IP internetwork can be a local-area network or a wide-area network

30. APPLICATION
This layer is comparable to the application, presentation, and session layers of the OSI model all combined into one.
 It provides a way for applications to have access to networked services. 
This layer also contains the high level protocols.  The main issue with this layer is the ability to use both TCP and UDP protocols.

31. For example TFTP uses UDP because usually on a LAN the physical links are short enough to ensure quick and reliable packet delivery without many errors.  SMTP instead uses TCP because of the error checking capabilities. 
Since we consider our important information we would like to ensure a safe delivery.

32. UDP (User Datagram Protocol) - offering a direct way to send and receive datagrams over an network with very few error recovery services.
TFTP (Trivial File Transfer Protocol) - Simplified version of the FTP protocol which has no security features.

34. TRANSPORT
This layer acts as the delivery service used by the application layer.
Again the two protocols used are TCP and UDP. 
The choice is made based on the application's transmission reliability requirements. 
The transport layer also handles all error detection and recovery. 

35. It uses checksums, acknowledgements, and timeouts to control transmissions and end to end verification. 
Unlike the OSI model, TCP/IP treats reliability as an end-to-end problem

36. User Datagram Protocol (UDP)
The User Datagram Protocol (UDP) is the simpler of the two standard TCP\IP transport
protocols.
It is a process-to-process protocol that adds only port addresses, checksum, error control, and length information to the data from the upper layer.

37. Transmission Control Protocol (TCP)
The Transmission Control Protocol (TCP) provides full transport-layer services to applications.
TCP is a reliable stream transport protocol. The term stream, in this context, means connection-oriented: A connection must be established between both ends of a transmission before either can transmit data.
At the sending end of each transmission, TCP divides a stream of data into smaller units called segments.
Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP datagrams.
At the receiving end, TCP collects each datagram as it comes in and reorders the transmission based on sequence numbers.

38. Stream Control Transmission Protocol (SCTP)
The Stream Control Transmission Protocol (SCTP) provides support for newer Applications such as voice over the Internet.
It is a transport layer protocol that combines
the best features of UDP and TCP.

39. INTERNET
This layer is known as Internet layer. The main purpose of this layer is to organize or handle the movement of data on network.
By movement of data, we generally mean routing of data over the network. The main protocol used at this layer is IP. While ICMP and IGMP are also used at this layer.

40. NETWORK This layer is also known as network interface layer
This layer normally consists of device drivers in the OS and the network interface card attached to the system.
Both the device drivers and the network interface card take care of the communication details with the media being used to transfer the data over the network.

41. In most of the cases, this media is in the form of cables.
Some of the famous protocols that are used at this layer include ARP(Address resolution protocol), PPP(Point to point protocol) etc.

42. Internetworking Protocol (IP)
The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP protocols.
It is an unreliable and connectionless protocol-a best-effort delivery service.
IP assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees.
IP transports data in the form of packets called datagram , which are transported separately.

43. Address Resolution Protocol
The Address Resolution Protocol (ARP) is used to associate a logical address with a physical address.
On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC).
ARP is used to find the physical address of the node when its Internet address is known

44. Reverse Address Resolution Protocol
The Reverse Address Resolution Protocol (RARP) allows a host to discover its Internet
address when it knows only its physical address.
It is used when a computer is connected
to a network for the first time or when a diskless computer is booted.

45. Internet Control Message Protocol
The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and Gateways to send notification of datagram problems back to the sender.
ICMP sends query and error reporting messages

46. Internet Group Message Protocol (IGMP)
The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous
transmission of a message to a group of recipients.