Hot answers tagged

42

There are two important facts about the OSI model to remember: It is a conceptual model. That means it describes an idealized, abstract, theoretical group of networking functions. It does not describe anything that someone actually built (at least nothing that is in use today). It is not the only model. There are other models, most notably the TCP/IP ...


17

A Layer 2 switch learns most of its information about the location of other endpoints via "listening" to ingressing frames, and when it is not aware of the location, it uses floodingand will learn from the answer. Lets say the topology is: (Host A) <--> (Switch A) <--> (Switch B) <-->(Host B). Also important to note, a L2 Switch forwards, it ...


15

tl;dr Many different devices can encounter multiple OSI levels. Whichever end point is requesting something from a layer 7 protocol (like HTTP), will use all 7 layers before putting it on the wire. Intermediate nodes, like routers and switches might only use up to the first 3 layers, firewalls or WAN accelerators can affect layer 4, load balancers do ...


13

Since firewalls essentially filter data, the answer depends on what kind of filtering you do. If you filter based on IP address (for example), you can say that your firewall is filtering at layer 3. If you filter specific ports, you can say you're filtering at layer 4. If your firewall inspects specific protocol states or data, you can say it operates at ...


13

You have to remember that models like OSI are just that, models. They are theoretical. The real world doesn't fall neatly into these models. For the most part, routing is a layer-3 function, but, as you pointed out, BGP uses a layer-4 protocol to communicate with other BGP speakers in order to do what is normally considered a layer-3 function. Many network ...


13

You need to understand the concept of layers. An application will send data to the Transport Layer. The Transport Layer protocol will encapsulate the data inside headers for the Transport Layer protocol, and pass those to the Network Layer. The Network Layer will encapsulate the datagrams it receives inside Network Layer headers, and those are called ...


13

It's been done. I used to have a 300-baud acoustic layer 1 device.


12

The network layers provide a framework to structure the complex functions for sending data over a network - as byte stream, in dialogue, telegram style datagrams, ... On the very top, the application uses a lower layer (very often the transport layer) to do its job. It doesn't have to worry about routing, network interfaces, MAC addresses, line codes etc - ...


11

Managed switches are in simple terms switches that can be "managed." Managed means that they can provide information/statistics about their operation and usually that they can be configured. While the vast majority of managed switches can be configured for IP (and this includes all the more capable devices with full feature sets), there are some that can be ...


10

It's very much possible. Even excluding the old acoustically coupled modems that eventually evolved into a direct connection to a phone line, there are also programs that will let you use a sound card as a modem (I've used some before for out-of-band communications when debugging an Ethernet driver, though I used direct audio cabling instead of actual ...


9

TCP or UDP ports are defined in either layer 4 of the OSI model or layer 3 of the TCP/IP model, both are defined as the 'transport' layer. OSI layer 5 'session layer' uses the ports defined in layer 4 to create sockets and sessions between communicating devices/programs/etc. The result I got from Google that actually mentioned 'Ports Layer' is a completely ...


9

The original RFC for ICMP, RFC777 state that: ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP, and must be implemented by every IP module. This statement is also present in RFC792 which obsoletes RFC777.


9

I have to disagree with @ronroyston here. The OSI model is an idealized abstract model, and there are no protocols in use today that follow it. HTTP was created without regard to the OSI model, so there's no point to trying to make it fit. The TCP model is a little closer, because it lumps everything above transport into "application." As the Wikipedia ...


9

In general, forwarding refers to a device sending a datagram to the next device in the path to the destination, switching refers to moving a datagram from one interface to another within a device, and routing refers to the process a layer-3 device uses to decide on what to do with a layer-3 packet. A host sending data to another host through an ethernet ...


9

Bluetooth devices are required to have a unique device address, assigned from the same registry as Ethernet and Wifi MAC addresses. Quoting the Bluetooth specification version 5.0 volume 1: Each Bluetooth device shall be allocated a unique 48-bit Bluetooth device address (BD_ADDR). The address shall be a 48-bit extended unique identifier (EUI-48) ...


9

The OSI Model is seven layer model of how you can build a networking system. The TCP/IP networks that we all use does not follow this model and roughly ends at the transport layer with higher-level concerns left to individual applications. So - there is no in-built standard encryption, character-set translation etc. For FTP you need some way of: ...


8

If Layer-2 has a checksum, shouldn't this validation be enough? Why we are having checksum validation in different OSI Layers? Simply put, different layers of the OSI model have checksums so you can assign blame appropriately. Suppose there is a webserver running on some system (assume TCP port 80, i.e. OSI Layer 4) Suppose there is a software error ...


8

BGP is on top of TCP, so it would be Internet layer 4, OSI layer 7. Usually external BGP is done only between 2 directly connected peers, enforced by setting TTL flag on IP header, which is located at layer 3.


8

Here's a simple answer. Consider a file of 5 bytes abcde. You can send that data over the network, sure. But how would you tell the other side hey, I've done sending my file in order to send next one? Or where does metadata (e.g. filename) fit in? And how to distinguish it from the file content? You do that by introducing a protocol. For example you send ...


8

Anything that can transport information can be used as physical layer - sound waves as well as pigeons. Some air-gap attacks use (ultra)sound to communicate across the air gap. However, since the frequencies even for ultrasound are rather low (some kHz), the data rate would be low as well (some kbit/s). Also, the reach of sound waves limits the use to a ...


7

Layer 4 - Transport layer. This is the layer charged with handling end-to-end communication; be it reliable or unreliable. You'll frequently find that most networking concepts don't fit neatly into the OSI model.


7

Because there is no "state " in the network layer nor signaling for connection setup. Packets are forwarded based on destination address and each packet is forwarded irrespective of previous traffic.


7

UDP and TCP are completely separate Transport Layer (Layer 4 in the OSI model) protocols. There are also other layer-4 protocols, but TCP and UDP are the most commonly used. UDP was first, and it is a connectionless, unreliable, fire-and-forget protocol. TCP was added later to provide connections and reliability to layer 4. Layer-4 protocols have their own ...


7

Data isn't stored in a network, it is transported. User data is transported as payload by the application-layer protocol (L7) - HTTP, FTP, SMTP, ... For instance, if you write some text and send that per email it can be encapsulated in an RFC2822 structure (L6), transmitted per SMTP (L7), encapsulated in TCP (L4), in IPv4 (L3), Ethernet frame (L2), ...


6

Of course you can directly interact with the IP layer. The question is not about "why you can't", but about "why you wouldn't want/need to". Since you mention higher level languages, the reason why they exist is the same as the reason the transport layer exists. A higher level language makes things easier by providing abstraction. You can code your ...


6

PPTP, PPPoE, and L2TP all provide OSI Layer 2 services. That is, the user of these protocols (usually, a network layer protocol suite) thinks it's running over a "normal" link layer. However, each of these protocols provides the link layer service by transporting packets over another service, rather than over the physical layer. PPTP provides PPP (link ...


6

ISIS works quite differently than most of the protocols you know. In the original OSI networking, ISIS would make adjacencies with it's neighbors, and tell them about the adresses of any ES it knows. IS means intermediate system, so besically a router. ES means end system, so any host. ISIS has different logic with different levels of routing. The levels ...


6

First a word about: I understand word persistent as following : no one else can occupy wire at the time of persistent connection. So this connection is only one-to-one. But this is not right. Persistent connection can be established while other packets are going through the same wire. This is circuit-switched network vs packet-switching network ...


6

I know that in the data link layer that is responsible ensuring that frames has been transmitted successfully by sending acknowledgement That is incorrect. There are very few data-link protocols that do this. The transport protocol is normally where acknowledgement takes place, but not all transport protocols do that. TCP does, but UDP does not. With a ...


6

DNS resolution sits at the application layer: a host communicates over DNS to a server, which is encapsulated in (usually) UDP (the transport layer), which is encapsulated in an IP packet (the internet layer), which is typically wrapped in an ethernet frame (the link layer) and sent to either a local resolver or a local router for forwarding to a resolver. ...


Only top voted, non community-wiki answers of a minimum length are eligible