Open Systems Interconnection (OSI) model


The Open Systems Interconnection (OSI) model has seven layers. The layers are stacked this way.
  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

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 defines the type of signal, i.e. electrical, optical or radio waves.


DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to another over the physical layer. Following are functions of the data link layer:
  • Physical addressing:- Adds source and destination layer 2 (MAC address on Ethernet) to header of frame.
  • Frame error detection:- Adds a Frame Check Sequence (FRC) or Cyclic Redundancy Check (CRC) to the trailer of the frame, which drops the frame if the value differs at source and destination.
Header Information:
  • Source and destination MAC address
Trailer Information:
  • Frame Check Sequence (FRC)

NETWORK LAYER

This layer provides routing technologies, creating logical paths, known as virtual circuits for transmitting data from node do node. It provides:

  • Source to destination delivery: transfers packets from source to destination.
  • Logical addressing: adds the source and destination IP address to the header.
  • Routing: selects the optimal path out of multiple paths that can be followed by packets.
  • Inter-networking: connects two or more different subnets.
Header Information:
  • Source and destination IP address
  • Time-to-Live (TTL): it specifies how long a packet is allowed to travel through the internet. Each router decrements the value of this field by one before forwarding it. When it's value reaches to 0, the packet is discarded by the router.
Protocols: IP, IPX, routing protocols (RIP, EIGRP, OSPF, BGP etc), ARP, ICMP.


TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplicates. This layer provides:

  • Service point addressing: This layer includes port address in the header, using which, the transport layer delivers the message to the correct process at destination.
  • Flow control: Transport layer assures that the sending of the data depends upon the receiving capacity of the receiver.
  • Error control: ensures that the the entire message is delivered at destination machine without any error. Segments are re-transmitted to achieve error correction.
  • Segmentation and reassembly: splits the message into smaller units (if not already small enough), and assigns a sequence number to these segments. The transport layer at the destination station reassembles the message.
Header Information:
  • Source and destination port number.
  • TCP sequence number.
  • Acknowledgement number.
  • TCP length: the length of the TCP header and data.
Protocols: TCP, UDP, SCTP


SESSION LAYER

Session layer establishes, maintains and terminates the connections between end-user application process. Session-layer services are commonly used in application environments that make use of remote procedure calls (RPCs). It provides the following services:

  • Authentication
  • Authorization
  • Session restoration (checkpointing and recovery): At this layer, each session is divided into sub-sessions by inserting checkpoints into the stream. These checkpoints allows information of different streams, perhaps originating from different sources, to be properly combined or synchronized. It also ensure that the session can be backtracked in case of errors.
Header Information: 
Information about the checkpoints.

Protocols: PPTP, L2TP, RPC, PAP


PRESENTATION LAYER

The presentation layer formats the data to be presented to the application layer. The data is encoded or translated to different formats such as .avi, .doc, .jpg and so on, so that the application layer program can recognize and work with it. The presentation layer provides:
  • Character code translation: for example, ASCII to EBCDIC.
  • 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.
Examples: File format converters, Winzip, Winrar, GPG


APPLICATION LAYER

This layer provides an interface for the user to access the information on a network.

Protocols: HTTP, FTP, SMTP, POP3, NTP, TFTP, Telnet, DNS, SNMP





Related links:
http://support.microsoft.com/kb/103884
http://en.wikipedia.org/wiki/OSI_model




Comments

Post a Comment

Popular posts from this blog

Specifying SSH port in Ansible Inventory

There may be some instances where you set a custom port for SSH on your network device. If ssh port for hosts is different than the default port 22, it can be specified in the inventory file with colon (:) after hostname. #vi inventory # Inventory file for Ansible   [XE] ios-xe-mgmt.cisco.com:8181 ios-xe-mgmt-latest.cisco.com:8181   [XR] sbx-iosxr-mgmt.cisco.com:8181
Read more

Ansible-Playbook to display output of multiple show commands (using stdout_lines with Loop)

 In this playbook, we we'll see how we can get display of multiple show commands in stdout_lines format. We can make use of loops (or with_items) for submitting multiple commands, but debug output with stdout_lines does not gives the formatted result as it would give for single command. So in case of multiple commands, we can debug the output of each command separately in stdout_lines format.
Read more

Filtering Routes in BGP using Route-maps and Prefix-list

Image
Order of preference of attributes in BGP The order of preference varies based on whether the attributes are applied for inbound updates or outbound updates. For inbound updates the order of preference is:     route-map     filter-list     prefix-list, distribute-list For outbound updates the order of preference is:     prefix-list, distribute-list     filter-list     route-map NOTE: The attributes prefix-list and distribute-list are mutually exclusive, and only one command (neighbor distribute-list or neighbor prefix-list) can be applied to each inbound or outbound direction for a particular neighbor. Scenario: We own the AS500 and advertising a network block of 192.0.2.0/24 and 180.179.179.0/16 to two different ISPs. Objectives: Configure router R1 to establish eBGP neighbor relationship with ISP1. Configure router R1 to establish eBGP neighbor relationship with ISP2. Advertise  192.0.2.0/24 ...
Read more

Ansible Playbook for Network OS Upgrade with pre and post checks

You have 100s of network switches or routers that you need to upgrade. How much time would it take for you to do the upgrades? There are a lot number of sub-tasks involved while upgrading IOS image of a Cisco router or a switch. This time of upgradation can be reduced through automation from various Enterprise Configuration Management tools that also have ability to upgrade network OS. Though these tools give an easy to use graphical interface, but this requires you to have appropriate license and also restricts you to customize your upgrade process.
Read more

VMware NSX Traffic Flow — East-West & North-South

Image
Understanding how traffic is flowing in NSX environment is an important aspect to successfully maintain and troubleshoot networks having NSX. In this post we'll understand hop-by-hop flow of traffic in East-West and North-South directions. East-West: VMs on Same Subnet, Same Host VM-1 has IP address 172.16.20.6 and VM-2 has IP address 172.16.20.7 VM-1 vNIC è Logical Switch (Segment ID 5002) è VM-2 vNIC
Read more

Bypassing Proxy Server in Google Chrome

Image
There may be times when you might want to use Google Chrome and do not wish to use the proxy settings for which you are bound to use in Internet Explorer because of your company policies. If you try to change proxy settings in Google Chrome, by default it will open the System Internet Properties dialogue box, and most of the times, change in proxy setting options is restricted by domain group policy. So here is an alternate way to remove or disable proxy settings in Google Chrome. 1. Right click the GoogleChrome.exe and create it's shortcut. 2. Open the properties of the shortcut of GoogleChrome.exe created in previous step. 3. Here you have to edit the Target of this shortcut. You have to add   --proxy-server=""  after the existing path. You are done. Now enjoy the web browsing without any restrictions.
Read more

Export or Backup Azure Network Security Groups into CSV using PowerShell

There could be many use cases where you may want to export Network Security Groups into CSV. You might have question, how to export or backup Azure Network Security Groups into CSV. Here is the PowerShell script that you can use to export Azure Network Security Groups into CSV using PowerShell script. This script will export Network Security Group along with rules of all Active subscriptions into a CSV.
Read more

Ansible-playbook for backing up running config of Cisco IOS

This ansible-playbook can be used to backup running configuration from Cisco IOS devices. You can refer to my earlier post Getting Started with your first ansible-playbook for Network Automation  to know about the parameters used in this playbook. Inventory file # Inventory file for Ansible [XE] ios-xe-mgmt.cisco.com:8181 ios-xe-mgmt-latest.cisco.com:8181 [XR] sbx-iosxr-mgmt.cisco.com:8181 [all:vars] ansible_network_os=ios Playbook --- - name: Define Parameters   hosts: XE   gather_facts: no   connection: network_cli   tasks:    - name: backup the config      ios_config:       backup: yes      register: backup_config    - name: Store the config to directory      copy:       src: "{{ backup_config.backup_path }}"       dest: "/tmp/backups/{{ inventory_hostname }}" Running the playbook [prashant@Prashant-VM01 ~]$ ansible-playbook play03.yml -i /home/prasha...
Read more