I am very new to networking and I am currently trying to get my head around the order things happen in relation to the OSI stack. I know that the Transport layer takes the data stream and converts it into segments before handing it to the Network layer where a header is added relating to the IP address creating a packet before passing this packet to the Data Link layer where a header and footer are added including the MAC addresses creating a frame before finally being passed to the Physical layer in order to be converted into bits and sent along the network.

The part I am struggling with is where in the process certain protocols are involved. For example, TCP performs the three way handshake, supposedly at layer 4 (Transport), does this mean that the SYN is sent to the destination machine at this layer before waiting for the ACK so that the data can be sent later on from the Physical layer? Does the SYN have to go down the stack to be sent by the Physical layer and in turn the ACK back up the stack of the host before the actual data is sent again by the Physical layer?

Another example is ARP, when an ARP request is sent out by the Data Link layer in order to find the MAC address of the destination machine, is this sent out before the frame is created at layer 2?

I am unsure as to whether the only communication on the network is done at the Physical layer or if each layer interacts with the network relating to different protocols, as the data moves down the stack?

I have not been able to find a good video or diagram of the actual real world order that things happen so does anyone have a suggestion?

3 Answers 3


Sometimes, I wish they'd stop teaching the OSI model. It seems to confuse more than it helps.

When we say that layers communicate with each other, we mean the data created by a particular layer (say, transport) on host A is processed by the same layer on host B. This is a logical connection.

The actual data (in this case, the segment containing the SYN flag) is encapsulated in the Network PDU (IP packet), then encapsulated in the data-link PDU (Ethernet), then finally transmitted on the Ethernet cable (Physical layer). Host B reverses this process, unencapsulating the PDU at each layer until it reaches the transport layer. The transport layer processes the SYN flag and creates a new PDU containing the SYN, ACK flags. Then it sends it to A using the same encapsulation process.

The only way data is actually sent from one host to another is via the physical wire. Layer to layer communication is just a mental construct.


First, you really need to find some good books to clear your concepts, i recommend you Computers Networks, by Tanenbaum, the best of best for me.

On the other hand, the OSI model look some like this:

Equipment1 Equipment2

Application --- Application

Presentacion -- Presentacion

Session ----- Session

Transport ----- Transport

Network ----- Network

Data Link ----- Data Link

Physical ****** Physical

The principle is that the upper layers send the data to the lower layers, until de physical layer, that is on charge to transmit the info in form of ligth, or electromagnetic waves. But, all the upper layer only spoke with it same on the other equipment. To transmit data, by eg: from the lower middle of equipment 1, segments of data are created on the transport layer, and passes to the network layer, when are added some things and the IP header, and then pakets area created with that info. Then it passes to de Data Link layer where occurred some proccess and frames is the result, that are passed to the Physical Layer where all of that are transmitted like BITS, "zeros and ones". On the other side, the invert proccess is carried out, and everyone of the Layer proccess his info.

The SYN that you are asking is send to the other machine and until is not proccesed, that machine not send it SYN,ACK to the origin machine.

Again, you can read this in one easily form on the Tanenbaum book


it seams like confusing in network layers functionality.

first of all you must know that layer 3 add information regards to both source and destination IP not destination only and Layer 2 add information regards to both source and destination mac-address not destination only.

Second when you speak about layer get some headers and footers added to be transformed to other layer you actually speaking about encapsulation , but 3 hand check which is occurred in layer 4 is actually layer to layer communication.

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third regarding your question about layer 4 TCP communication establishment which is as i told you before (layer to layer communication) . this communication contains two parts , one regarding the communication establishment (SYC,SYNACK,ACK) and other is the segment of data which need to be transferred itself and both walk the same way (encapsulating in layer 3 header and footer then layer 2 then layer 1 header and footer)

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last there are three things you must know (encapsulation ,decapsulation and layer to layer communication ) . each layer has a lot of protocols can create and understand ( ex. ARP request is layer 3 function use both layer 3 protocol IP and layer 2 protocol MAC-ADDRESS)

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