I am having a tough time understanding the network layer or layer 3 in the OSI model. Could anyone please help me with my questions
1)correct me if I am wrong, from my understanding the basic task of the network layer is to allow communication between two end devices that are on different networks and for that to be possible, the network layer has to send data via the internet.
2)If the answer to question 1 is YES then could it mean that if two computers within the same network wish to exchange data then there is no point in the network layer interfering?
3)Is there a difference between layer 3 of the OSI model and the internet layer of the TCP/IP model?
Any help is greatly appreciated, thank you very much in advance!
the basic task of the network layer is to allow communication between two end devices that are on different networks and for that to be possible, the network layer has to send data via the internet.
The end nodes may be in different networks (connected by gateways) but they don't need to. Traffic only crosses the Internet when it's routed across, using public addressing.
could it mean that if two computers within the same network wish to exchange data then there is no point in the network layer interfering?
The network layer (IP) is required by common protocols (e.g. HTTP requires TCP which requires IP). It's perfectly possible to communicate within a local network using the data link layer alone (Ethernet usually), but it's very uncommon and may be complicated using modern operating systems, especially for an unprivileged application.
Is there a difference between layer 3 of the OSI model and the internet layer of the TCP/IP model?
The internet layer in the TCP/IP model doesn't entirely adhere to the OSI concept of network layer, but it's very much the same thing.
I have heard, Many companies like facebook are using UDP to fetch data from memcached. I have a doubt, How they make sure there is NO packet loss and order of received packet is per requirement.As we know tcp provide such facility but udp does not.
OSI Model has 7 layers which are:
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Splitting things to layers is very good approach to solve problems but it doesn't mean you have to do all the network operations in network layer.
As you have mentioned, TCP provides feedback to end systems when UDP doesn't but UDP has it's own advantages. First of UDP's datagram is simpler than TCP's one. And also most of huge systems like Facebook uses UDP because using TCP for these kind systems would not be very clever since all the data senders would have to keep track of sending rate, retransmissions rate for many many receivers. So if they've used TCP, their network layer would be under very very big presure.
So they make flow control in the application layer to reduce network traffic.
I'm trying to understand Network layers. There are five layers, application, transport, network, link and physical layer. I have studied from various sources but I couldn't understand clearly, especially transport, network and link layers. Can you explain those layers for a dummy person like me? Thanks.
So I think the hardest part people have with the OSI model is that they look at some of the layers and forget its talking about communication... application layer specifically people get confused as to what its talking about.
Application references the protocols applications use to communicate.
Examples being ftp, http, etc.
As stated above, the original OSI model (which is really just theory, nothing actually implements it exactly as it would be too inefficient) has 2 more layers in between application and transport (which are most likely wrapped into the application layer). These are Presentation and Session.
Presentation is responsible for things like HTTP's Accept-Encoding: gzip, deflate. encryption and character encoding are said to be Presentation layer, so SSL/TLS falls under this category (the s in https).
HTTP Sessions is literally would be a literal example of the Session layer (anything that keep your application state alive across connections). If a protocol is connection oriented, it might not have a session layer, which is why HTTP is probably the only example I can think of at the moment.
As you can see HTTP was the answer to all 3 of these top layers, which is why they have all been combined into Application layer in newer versions of the model. Cisco still uses the 7 layer model, but Microsoft uses the 5 layer you're using.
Transport is TCP... it contains information about reordering packets and can adjust the amount of packets per window, allows the computer to know if anything was lost and ask for re-transmission, etc.
UDP is also an example of the transport layer, however its a lot simpler of a protocol, no re-transmission of packets are done on UDP.
Network is the IP protocol (also IPX/SPX from the old netware days, and ICMP (pings) and IGMP (routers)) this allows for addressing computers that rely outside the a collision domain (things separated by switches or routers).
Link or DataLink layer is ethernet, (or ATM, or FDDI) which addresses computers physically connected to a hub or directly with a network cable to each other.
This layer adds the MAC addressing in ethernet and the frame part of a packet is the header it uses.
Physical layer (in original OSI model) is just your cables and network equipment.
Pretty much the only people who still talk about OSI is network techs.
They may still say Layer 2 or Layer 3 switches, etc which is in reference to OSI.. Layer 2 is an ethernet switch, a layer 3 switch adds routing.
The best way to see how this is used is to load up wireshark and snoop your own network traffic. It will actually show you the parts of a packet that are responsible for most of the layers.
Knowing the OSI model is not really overly useful, but it can help you organize the stages of network communication in your mind and help you troubleshoot.
Knowing the protocols and how they interact is extremely useful, learn how tcp negotiates connections, IP addressing and subnet masking, HTTP and Ethernet can help you whether you are a developer or a server admin, or network admin or even a DBA. There is nothing like a bad network card to ruin your weekend or even a whole month if no one thinks to check it, and the only way to tell is use tcpdump, and wireshark to see errors in the Ethernet frames.
The OSI reference model
The OSI model is used to connect to the open systems—these are the systems that are open and communicate with other systems. By using this model, we do not depend on an operating system anymore, so we are allowed to communicate with any operating system on any computer. This model contains seven layers, where each layer has a specific function and defines the way data is handled on certain different layers. The seven layers that are contained in this model are the Physical layer, Data Link layer, Network layer, Transport layer, Session layer, Presentation layer, and the Application layer.
THE PHYSICAL LAYER
This is the first layer in the OSI model and contains a definition of the network's physical specification, including the physical media (cables and connectors) and basic devices (repeaters and hubs). The layer is responsible for the input raw bits transmission data stream into zeros and for the ones that are on the communication channel. It then places the data onto the physical media. It is concerned with data transmission integrity and makes sure that the bits that are sent from one device are exactly the same as the data that is received by the other device
THE DATA LINK LAYER
The main role of the Data Link layer is to provide a link for raw data transmission. Before the data is transmitted, it is broken up into data frames, and the Data Link layer transmits them consecutively. The receiver will send back an acknowledge frame for each frame that has been sent if the service is reliable.
This layer consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The LLC sublayer is responsible for transmission error checking and deals with frame transmission, while the MAC sublayer defines how to retrieve data from the physical media or store data in the physical media.
We can also find the MAC address, also called as the physical address, in this layer. The MAC address is used to identify every device that connects to the network because it is unique for each device.
The MAC address contains twelve hexadecimal characters, where two digits are paired with each other. The first six digits represent the organizationally unique identifier and the remaining digits represent the manufacturer serial number. If you are really curious to know what this number means, you can go to www.macvendorlookup.com and fill the text box with our MAC address to know more about it.
THE NETWORK LAYER
The Network layer is responsible for defining the best way to route the packets from a source to the destination device. It will generate routing tables using Internet Protocol (IP) as the routing protocol, and the IP address is used to make sure that the data gets its route to the required destination. There are two versions of IP nowadays: IPv4 and IPv6. In IPv4, we use 32-bit addresses to address the protocol and we use 128-bit addresses in IPv6. You are going to learn more about Internet Protocol, IPv4, and IPv6 in the next topic.
THE TRANSPORT LAYER
The Transport layer is responsible for transferring data from a source to destination. It will split up the data into smaller parts, or in this case segments, and then will join all the segments to restore the data to its initial form in the destination.
There are two main protocols that work in this layer: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
TCP supplies the delivery of data by establishing a session. The data will not be transmitted until a session is established. TCP is also known as the connection-oriented protocol, which means that the session has to be established before transmitting the data.
UDP is a method of delivering data with the best efforts, but does not give a guaranteed delivery because it does not establish a session. Therefore, UDP is also known as the connection-less protocol. In-depth explanation about TCP and UDP can be found in the next topic.
THE SESSION LAYER
The Session layer is responsible for the establishment, maintenance, and termination of the session. We can analogize the session like a connection between two devices on the network. For example, if we want to send a file from a computer to another, this layer will establish the connection first before the file can be sent. This layer will then make sure that the connection is still up until the file is sent completely. Finally, this layer will terminate the connection if it is no longer needed. The connection we talk about is the session.
This layer also makes sure that the data from a different application is not interchanged. For example, if we run the Internet browser, chat application, and download manager at the same time, this layer will be responsible for establishing the session for every single application and ensure that they remain separated from other applications.
There are three communication methods that are used by this layer: the simplex, half-duplex, or full-duplex method.
In the simplex method, data can only be transferred by one party, so the other cannot transfer any data. This method is no longer common in use, since we need applications that can interact with each other.
In the half-duplex method, any data can be transferred to all the involved devices, but only one device can transfer the data in the time, after it completes the sending process. Then, the others can also send and transfer data.
The full-duplex method can transfer data to all the devices at the same time. To send and receive data, this method uses different paths.
THE PRESENTATION LAYER
The Presentation layer role is used to determine the data that has been sent, to translate the data into the appropriate format, and then to present it. For example, we send an MP3 file over the network and the file is split up into several segments. Then, using the header information on the segment, this layer will construct the file by translating the segments.
Moreover, this layer is responsible for data compression and decompression because all the data transmitted over the Internet is compressed to save the bandwidth. This layer is also responsible for data encryption and decryption in order to secure communication between two devices.
THE APPLICATION LAYER
The Application layer deals with the computer application that is used by a user. Only the application that connects to a network will connect to this layer. This layer contains several protocols that are needed by a user, which are as follows:
The Domain Name System (DNS): This protocol is the one that finds the hostname of an IP address. With this system, we do not need to memorize every IP address any longer, just the hostname. We can easily remember a word in the hostname instead of a bunch of numbers in the IP address.
The Hypertext Transfer Protocol (HTTP): This protocol is the one that transmits data over the Internet on web pages. We also have the HTTPS format that is used to send encrypted data for security issues.
The File Transfer Protocol (FTP): This protocol is the one that is used to transfer files from or to an FTP server.
The Trivial FTP (TFTP): This protocol is similar to FTP, which is used to send smaller files.
The Dynamic Host Configuration Protocol (DHCP): This protocol is a method that is used to assign the TCP/IP configuration dynamically.
The Post Office Protocol (POP3): This protocol is an electronic mail protocol used to get back e-mails from POP3 servers. The server is usually hosted by an Internet Service Provider (ISP).
The Simple Mail Transfer Protocol (SMTP): This protocol is in contrast with POP3 and is used to send electronic mails.
The Internet Message Access Protocol (IMAP): This protocol is used to receive e-mail messages. With this protocol, users can save their e-mail messages on their folder on a local computer.
The Simple Network Management Protocol (SNMP): This protocol is used to manage network devices (routers and switches) and detect problems to report them before they become significant.
The Server Message Block (SMB): This protocol is an FTP that is used on Microsoft networks primarily for file and printer sharing.
This layer also decides whether enough network resources are available for network access. For instance, if you want to surf the Internet using an Internet browser, the Application layer decides whether access to the Internet is available using HTTP.
We can divide all the seven layers into two section layers: the Upper Layer and Lower Layer. The upper layer is responsible for interacting with the user and is less concerned about the low-level details, whereas the lower layer is responsible for transferring data over the network, such as formatting and encoding.
There are FIVE LAYERS in TCP/IP Model and SEVEN LAYERS in OSI Reference Model.
The primary difference between our five-layer model and the seven-layer OSI model is that the OSI model abstracts the application layer into three layers total.
The physical layer represents the physical devices that interconnect computers. This includes the specifications for the networking cables and the connectors that join devices together along with specifications describing how signals are sent over these connections. The physical layer is all about cabling, connectors and sending signals.
The second layer in our model is known as the data link layer. Also called the network interface or the network access layer. The data link layer is responsible for defining a common way of interpreting the signals, so network devices can communicate. Lots of protocols exist at the data link layer, but the most common is known as Ethernet.
The third layer, the network layer is also sometimes called the Internet layer. It's this layer that allows different networks to communicate with each other through devices known as routers.
The most common protocol used at this layer is known as IP or Internet Protocol. IP is the heart of the Internet and most small networks around the world.
While the data link layer is responsible for getting data across a
single link, the network layer is responsible for getting data
delivered across a collection of networks.
While the network layer delivers data between two individual nodes, the transport layer sorts out which client and server programs are supposed to get that data. The transport layer Protocol is known as TCP or Transmission Control Protocol and UDP or User Datagram Protocol.
The big difference between the two is that TCP provides mechanisms to
ensure that data is reliably delivered while UDP does not.
The network layer, in our case IP, is responsible for getting data
from one node to another. The transport layer, mostly TCP and UDP, is
responsible for ensuring that data gets to the right applications
running on those nodes.
The fifth layer is known as the application layer. There are lots of different protocols at this layer, and as you might have guessed from the name, they are application-specific. Protocols used to allow you to browse the web or send receive emails are some common ones. Application Layer Protocols are HTTP, SMTP, etc.
You can think of layers like different aspects of a package being
delivered. The physical layer is the delivery truck and the
roads. The data link layer is how the delivery trucks get from
one intersection to the next over and over. The network layer
identifies which roads need to be taken to get from address A to
address B. The transport layer ensures that the delivery
driver knows how to knock on your door to tell you your package has
arrived. And the application layer is the contents of the
package itself.
Transport layer and datalink layer of OSI model provide similar functionality. If one of said was already there, why was the other needed
They do provide some similar functionality, but at a different level. The link layer provides for communications for MAC addresses on the same LAN; the transport layer provides for communication between any devices anywhere.
A good design typically includes the concept of separation of concerns. That is, the data link layer need only be concerned with how to get packets to other hosts on that specific link. Remember that Ethernet is not the only link type in the world. You might need to get packets to the other side of a PPP link over an analog modem. Since the network layer is separate, you can use a different data link type and your network layer packets can remain the same.
You may refer to an image here for reference.
Physical: Only knows about bits. Handled byires and hubs
Data Link: Knows about MAC addresses. Handled by layer 2 switches
Network: Knows about IP addresses. Handled by routers or layer 3 switches
Transport: TCP comes here
I am curious to know how the incoming bits at the physical layer are properly framed and sent to the data link layer. How the OS deal with this process.
It would be grateful if you explained it in detail or give me some links/pdf.
I am interested to know in depth about Layer 1 and 2 operations.
Advance Thanks.
The physical layer depends on your hardware. You're probably connected via ethernet, see here. The operating system doesn't do a lot here, it's mostly left up to the network card and the device drivers written by the card's manufacturer.
The good thing about today, is you seldom have to remember anything but Google.
http://en.wikipedia.org/wiki/Ethernet
However, the question is NOT about the physical layer as that is really the wires and interfaces for the wires; hence physical connections. His questions addresses the Data-Link layer in OSI Model terms.
As Ethernet Does not match the OSI model and the fact that the OSI model is a strategic rather than a tactical model, in Ethernet terms you are interested in the 802.3 MAC Frame. But then as Ethernet is old, there are actually a several of different EtherTypes.
For sharing the data, some protocol is needed as human .so here 7 layers are given .
Physical layer direct connect with the hardware( when pulse sends in Bit format ).
It deals with mechanical and electrical specification of the interface and transmission medium.
OS have only authority to run the application not any participation in any software .
If you want detail then better read SIGNAL AND SYSTEM.