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TCP establishes a logical connection before sending data,where LLC connections are physical and are being held just between two computers,not more.

Is this correct?

If so,why LLC=LOGICAL LINK CONTROL. Why not PHYSICAL?

Thanks!

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  • I'm not sure, but I think a "physical connection" would be a connection between a computer and the next Ethernet switch... However, layer-2 packets are routed over switches... Apr 7, 2019 at 18:23
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    – Ron Maupin
    Apr 19, 2019 at 14:50

2 Answers 2

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Why not PHYSICAL?

In my understanding, a "physical" connection is a primitive connection between two (or more) electronic devices. Only the two devices are evaluating the data transferred but the connection itself does not interpret the data at all.

When using (old) Ethernet hubs, you could argue that sending data between two computers using Ethernet is as physical connection.

But:

When using Ethernet switches, the switch will interpret some of the data sent, so you don't have a physical connection between two computers, but you have two physical connections between one of the computers and the switch. There is no physical connection between the computers.

The "link" in LLC doesn't refer to a logical connection. LLC is stateless.

I'm not absolutely sure about networking, but in other fields of computing the word "logical" means that some lower-layer software or some hardware is already providing some functionality that simulates a different behavior of the hardware to the (higher-layer) software.

In this context, it is not relevant if the connection is stateful or stateless.

You already mentioned TCP: A software using the send() socket function will send some byte stream over the internet. However, the real hardware of the internet does not support byte streams at all, but only IP packets. The TCP/IP stack simulates a network supporting byte streams.

Now let's look at an Ethernet-I frame with an LLC header sent over a setup using hubs or coaxial cable:

In such a setup the network card hardware will receive all data which is sent by other computers - independent of the destination MAC address. Note: This sometimes also happens when switches are used.

The LLC data is not evaluated by all computers in the network, but only by the computer whose MAC address is found in the "destination MAC address" field of the Ethernet frame.

This is only possible if either the network card or the driver filters out all frames with the "wrong" destination MAC address before the LLC data is evaluated.

You might also say: This is only possible if the network card simulates a hardware environment where frames intended for other computers do not reach this computer's network card.

However, if some hardware environment is simulated, we talk about a "logical connection" and not about a "physical connection".

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  • Martin, when a NIC receives a frame destined for another MAC address than its own, dropping it in hardware is the standard procedure (unless the NIC is in promiscuous mode). Anything else - frame type, Ethertype, LLC headers, ... - is managed by the NIC driver or the OS.
    – Zac67
    Apr 8, 2019 at 8:42
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Logical link control is the upper layer of layer 2. It's not part of the physical layer.

LLC provides multiplexing above the MAC layer to enable the coexistence of multiple network-layer protocols.

The physical layer provides a purely interface-to-interface link between two hardware ports (in modern times - ancient Ethernet used point-to-multipoint links).

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  • I know that it is a sublayer of layer 2.But i don't get if that connection is logical or not.The thing is:why do we need the connection oriented service of LLC,while there are TCP connections that make sure everything is error free?
    – Some1
    Apr 7, 2019 at 19:04
  • @Zach The "link" in LLC doesn't refer to a logical connection. LLC is stateless. Each layer has its purpose and the LLC's is to provide multiplex for L3 protocols. L4 provides transport for the application layer.
    – Zac67
    Apr 7, 2019 at 19:58
  • Okay i understand,but i read that LLC offers a "connection oriented service",by establishing a logical connection.What does that mean then?
    – Some1
    Apr 7, 2019 at 20:18

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