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Sunday, January 29, 2017

Open Systems Interconnection model (OSI model)

 

for simpler definitions  check in bottom of page

 
 

OSI MODEL

 
The open connection model describes about the networking process and their flow
It consists of some seven layers describes about working functions of networking.
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORKING
DATALINK
PHYSICAL LAYER
 The top three layers are the operating system related layers and the bottom four are lower layers
In normally the end to end networking depends on the three address [Physical address(MAC  address)+logical address(IP address)+ Port address]

APPLICATION LAYER


The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
  • Resource sharing and device redirection
  • Remote file access
  • Remote printer access
  • Inter-process communication
  • Network management
  • Directory services
  • Electronic messaging (such as mail)
  • Network virtual terminals
protocols used:
other protocols in application layer
 

PRESENTATION LAYER

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

The presentation layer provides:
  • Character code translation: for example, ASCII to EBCDIC.
  • Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
  • Data compression: reduces the number of bits that need to be transmitted on the network.
  • Data encryption: encrypt data for security purposes. For example, password encryption.
protocols used:

SESSION LAYER

The session layer allows session establishment between processes running on different stations. It provides:
  • Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
  • Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.
protocols used:
 

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.
The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.
The transport layer provides:
  • Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
  • Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
  • Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
  • Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.
The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.
End-to-end layers
Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.
Common TCP/IP Protocols and Ports
Protocol TCP/UDP Port Number Description
File Transfer Protocol (FTP)
(RFC 959)		 TCP 20/21 FTP is one of the most commonly used file transfer protocols on the Internet and within private networks. An FTP server can easily be set up with little networking knowledge and provides the ability to easily relocate files from one system to another. FTP control is handled on TCP port 21 and its data transfer can use TCP port 20 as well as dynamic ports depending on the specific configuration.
Secure Shell (SSH)
(RFC 4250-4256)		 TCP 22 SSH is the primary method used to manage network devices securely at the command level. It is typically used as a secure alternative to Telnet which does not support secure connections.
Telnet
(RFC 854)		 TCP 23 Telnet is the primary method used to manage network devices at the command level. Unlike SSH which provides a secure connection, Telnet does not, it simply provides a basic unsecured connection. Many lower level network devices support Telnet and not SSH as it required some additional processing. Caution should be used when connecting to a device using Telnet over a public network as the login credentials will be transmitted in the clear.
Simple Mail Transfer Protocol (SMTP)
(RFC 5321)		 TCP 25 SMTP is used for two primary functions, it is used to transfer mail (email) from source to destination between mail servers and it is used by end users to send email to a mail system.
Domain Name System (DNS)
(RFC 1034-1035)		 TCP/UDP 53 The DNS is used widely on the public internet and on private networks to translate domain names into IP addresses, typically for network routing. DNS is hieratical with main root servers that contain databases that list the managers of high level Top Level Domains (TLD) (such as .com). These different TLD managers then contain information for the second level domains that are typically used by individual users (for example, cisco.com). A DNS server can also be set up within a private network to private naming services between the hosts of the internal network without being part of the global system.
Dynamic Host Configuration Protocol (DHCP)
(RFC 2131)		 UDP 67/68 DHCP is used on networks that do not use static IP address assignment (almost all of them). A DHCP server can be set up by an administrator or engineer with a poll of addresses that are available for assignment. When a client device is turned on it can request an IP address from the local DHCP server, if there is an available address in the pool it can be assigned to the device. This assignment is not permanent and expires at a configurable interval; if an address renewal is not requested and the lease expires the address will be put back into the poll for assignment.
Trivial File Transfer Protocol (TFTP)
(RFC 1350)		 UDP 69 TFTP offers a method of file transfer without the session establishment requirements that FTP uses. Because TFTP uses UDP instead of TCP it has no way of ensuring the file has been properly transferred, the end device must be able to check the file to ensure proper transfer. TFTP is typically used by devices to upgrade software and firmware; this includes Cisco and other network vendors’ equipment.
Hypertext Transfer Protocol (HTTP)
(RFC 2616)		 TCP 80 HTTP is one of the most commonly used protocols on most networks. HTTP is the main protocol that is used by web browsers and is thus used by any client that uses files located on these servers.
Post Office Protocol (POP) version 3
(RFC 1939)		 TCP 110 POP version 3 is one of the two main protocols used to retrieve mail from a server. POP was designed to be very simple by allowing a client to retrieve the complete contents of a server mailbox and then deleting the contents from the server.
Network Time Protocol (NTP)
(RFC 5905)		 UDP 123 One of the most overlooked protocols is NTP. NTP is used to synchronize the devices on the Internet. Even most modern operating systems support NTP as a basis for keeping an accurate clock. The use of NTP is vital on networking systems as it provides an ability to easily interrelate troubles from one device to another as the clocks are precisely accurate.
NetBIOS
(RFC 1001-1002)		 TCP/UDP 137/138/139 NetBIOS itself is not a protocol but is typically used in combination with IP with the NetBIOS over TCP/IP (NBT) protocol. NBT has long been the central protocol used to interconnect Microsoft Windows machines.
Internet Message Access Protocol (IMAP)
(RFC 3501)		 TCP 143 IMAP version3 is the second of the main protocols used to retrieve mail from a server. While POP has wider support, IMAP supports a wider array of remote mailbox operations which can be helpful to users.
Simple Network Management Protocol (SNMP)
(RFC 1901-1908, 3411-3418)		 TCP/UDP 161/162 SNMP is used by network administrators as a method of network management. SNMP has a number of different abilities including the ability to monitor, configure and control network devices. SNMP traps can also be configured on network devices to notify a central server when specific actions are occurring. Typically, these are configured to be used when an alerting condition is happening. In this situation, the device will send a trap to network management stating that an event has occurred and that the device should be looked at further for a source to the event.
Border Gateway Protocol (BGP)
(RFC 4271)		 TCP 179 BGP version 4 is widely used on the public internet and by Internet Service Providers (ISP) to maintain very large routing tables and traffic processing. BGP is one of the few protocols that have been designed to deal with the astronomically large routing tables that must exist on the public Internet.
Lightweight Directory Access Protocol (LDAP)
(RFC 4510)		 TCP/UDP 389 LDAP provides a mechanism of accessing and maintaining distributed directory information. LDAP is based on the ITU-T X.500 standard but has been simplified and altered to work over TCP/IP networks.
Hypertext Transfer Protocol over SSL/TLS (HTTPS)
(RFC 2818)		 TCP 443 HTTPS is used in conjunction with HTTP to provide the same services but doing it using a secure connection which is provided by either SSL or TLS.
Lightweight Directory Access Protocol over TLS/SSL (LDAPS)
(RFC 4513)		 TCP/UDP 636 Just like HTTPS, LDAPS provides the same function as LDAP but over a secure connection which is provided by either SSL or TLS.
FTP over TLS/SSL
(RFC 4217)		 TCP 989/990 Again, just like the previous two entries, FTP over TLS/SSL uses the FTP protocol which is then secured using either SSL or TLS.

protocols used
 

NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
  • Routing: routes frames among networks.
  • Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
  • Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
  • Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
  • Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.
Communications Subnet
The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet).
In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.
protocols used

DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:
  • Link establishment and termination: establishes and terminates the logical link between two nodes.
  • Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.
  • Frame sequencing: transmits/receives frames sequentially.
  • Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • Frame delimiting: creates and recognizes frame boundaries.
  • Frame error checking: checks received frames for integrity.
  • Media access management: determines when the node "has the right" to use the physical medium.
protocols used

PHYSICAL LAYER

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
  • Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

    • What signal state represents a binary 1
    • How the receiving station knows when a "bit-time" starts
    • How the receiving station delimits a frame
  • Physical medium attachment, accommodating various possibilities in the medium:
    • Will an external transceiver (MAU) be used to connect to the medium?
    • How many pins do the connectors have and what is each pin used for?
  • Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.
  • Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:
    • What physical medium options can be used
    • How many volts/db should be used to represent a given signal state, using a given physical medium
Technologies used
The following technologies provide physical layer services:

.
.
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for simpler definitions

OSI MODEL
The open connection model describes about the networking process and their flow
It consists of some seven layers describes about working functions of networking.
 
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORKING
DATALINK
PHYSICAL LAYER
 
 The top three layers are the operating system related layers and the bottom four are lower layers
In normally the end to end networking depends on the three address [Physical address(MAC  address)+logical address(IP address)+ Port address]
APPLICATION LAYER
The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
  • Resource sharing and device redirection
  • Remote file access
  • Remote printer access
  • Inter-process communication
  • Network management
  • Directory services
  • Electronic messaging (such as mail)
  • Network virtual terminals
PRESENTATION LAYER
The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

The presentation layer provides:
  • Character code translation: for example, ASCII to EBCDIC.
  • Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
  • Data compression: reduces the number of bits that need to be transmitted on the network.
  • Data encryption: encrypt data for security purposes. For example, password encryption.
SESSION LAYER
The session layer allows session establishment between processes running on different stations. It provides:
  • Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
  • Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.


The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.
The transport layer provides:
  • Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
  • Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
  • Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
  • Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.
The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

End-to-end layers

Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.

NETWORK LAYER

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
  • Routing: routes frames among networks.
  • Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
  • Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
  • Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
  • Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

Communications Subnet

The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet).
In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.
DATA LINK LAYER
The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:
  • Link establishment and termination: establishes and terminates the logical link between two nodes.
  • Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available.
  • Frame sequencing: transmits/receives frames sequentially.
  • Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
  • Frame delimiting: creates and recognizes frame boundaries.
  • Frame error checking: checks received frames for integrity.
  • Media access management: determines when the node "has the right" to use the physical medium.
PHYSICAL LAYER
The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
  • Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:
  • What signal state represents a binary 1
  • How the receiving station knows when a "bit-time" starts
  • How the receiving station delimits a frame
  • Physical medium attachment, accommodating various possibilities in the medium:
  • Will an external transceiver (MAU) be used to connect to the medium?
  • How many pins do the connectors have and what is each pin used for?
  • Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.
  • Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:
  • What physical medium options can be used
  • How many volts/db should be used to represent a given signal state, using a given physical medium
 
 
 



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