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  1. Hello Hussein.

    You are correct when you say that you cannot skip an OSI layer when communicating on the network. However, we can BEGIN our communication at layer 3 and go down to layer 1. In this case we are not skipping layers 4-7. Let me express this in an example:

    When you start an FTP file transfer from your computer, you are BEGINNING your communication at the Application layer, or layer 7. As you go down the OSI stack, you cannot skip layer 2 for example. MAC addresses must be placed in the L2 header and appropriate header information must be included. You can't get to layer 1 otherwise. All 7 layers must be traversed because you're starting from layer 7. It's like getting in an elevator on the 7th floor and you want to go to the ground floor. You can't skip floor 2!!

    However, if you happen to be on the third floor, you can enter the elevator and go to the ground floor. You're not skipping floors 7-4, you just happen to be starting at floor 3 and going down. That's what ICMP is doing.

    Similarly, ARP is a layer 3 protocol and does not know layers 4-7 exist as are all routing protocols (EIGRP, RIP, OSPF etc). CDP, VTP and LACP are examples of protocols that exist only on layer 2 and know nothing of upper layers.

    Using the elevator analogy, you can see that we are not skipping layers, we are just getting on the elevator at a different "floor" to start our journey down.

    I hope this has been helpful!


  2. Hello Hussein.

    If you have two hosts pinging the same destination, you are not creating sessions. Sessions are created by TCP on layer 4 with a three way handshake, port numbers, windowing and other functionality. Here there are no sessions. There is just a series of packets that are sent from a host to the destination (the router in your example). Packets are responded to on a first come first serve basis regardless of which host they came from.

    This detailed sequence may help:

    Host A wants to send a ping (echo request) to IP address (the router). The encapsulation process begins at layer 3 where the source IP address ( and the destination IP address ( are placed in the header. This is then encapsulated in a frame where source and destination MACs are placed in the frame header. This is then placed into the physical layer where information is converted to bits and those bits into electrical signals on the wire.

    When these electrical signals reach the destination, deencapsulation begins. The frame header is read, source and destination MAC addresses are read and the device confirms that the frame belongs to it. Deencapsulation continues where the source and destination IP addresses are read in layer 3 as well as the ICMP header information where an echo request has been recorded. The device doesn't deencapsulate any further because there are no additional headers to deencapsulate. No sessions are created as a result.

    The router then creates an ICMP echo reply placing the appropriate information in the header, places IP addresses in the IP header, encapsulates to layer two with MAC addresses and placed on the physical layer to be sent over the wire.

    The process is reversed at the host when the packet reaches its destination.

    Keep in mind that layer 3 protocols (IP essentially) are connectionless. This means no session is created. The packets are sent and "forgotten" without any mechanism of tracking each individual one.

    I hope this has been helpful!


  3. Hi again Lazaros,
    I have a small question about what you said to me before :-
    "If you have two hosts pinging the same destination, you are not creating sessions. Sessions are created by TCP on layer 4 with a three way handshake, port numbers, windowing and other functionality"
    You tell me that sessions are created by TCP on layer 4 and Rene explain in the introduction to the OSI Model lesson that session layer that holds this function, So can you please explain the relationship between what you said and what Rene said ??

    Best regards,
    Hussein samir

  4. Hello Hussein.

    That's a very good question. The different use of terms can become confusing. First of all, when using protocols such as TCP/IP, we are actually using the TCP/IP protocol stack which has four layers: Network Access, Internet, Transport and Application. However, when we speak about the layers in which protocols function, we always use the OSI model. For example, we say "layer 3" (of the OSI) for the IP protocol, even though it's "layer 2" in the TCP/IP model. For TCP and UDP, we say they function at layer 4 of the OSI even though they are at layer 3 of the TCP/IP protocol stack.

    What makes it more confusing is the fact that for the Transport layer, TCP creates what is called a TCP session. This is not to be confused with the Session layer 5 of the OSI model. The session begins with a 3-way handshake between the two hosts that are communicating to signify the beginning of the session. Data is exchanged during the session and it ends with a four way handshake that terminates the session.

    The Session layer of the OSI model isn't actually used when using TCP, because TCP/IP does not follow the OSI model. To find out more about the Session layer of the OSI model, check this out: http://www.cisco.com/cpress/cc/td/cpress/fund/ith/ith01gb.htm#xtocid1668415. Keep in mind however, that in the world of TCP/IP, this layer is non existent.

    I hope this has been helpful!


  5. Hello TAIMOOR

    First of all there is no such thing as a stupid question. Secondly, yes, you are correct. R1 and R3 must have some sort of routing (either static or dynamic) to be able to find each other's networks.

    I hope this has been helpful!


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