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"In OSI model, each layer provides a protocol to communicate with its peer. When a packet is transmitted by a layer, a header consisting of Protocol Control Information (PCI) is added to the data to be sent. In OSI terminology, the packet data (also known as the Payload) is called a Protocol Data Unit (PDU).".

The above paragraph is from site http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/peer-to-peer.html

Can someone tell me why is this peer communication required and what type of data is passed between the peers ?

I know that data always goes from Application to physical layer of sender and then from physical to application layer of receiver. So how is the data jumping between the layers?

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  • That page seems to have PDU and SDU mixed up. The payload is the SDU and the whole packet - headers including payload is the PDU: upload.wikimedia.org/wikipedia/commons/b/bc/Pdu_and_sdu.svg
    – Zac67 ♦
    Dec 4, 2017 at 20:57
  • @Zac67 The difference between PDU and SDU is explained nicely here tcpipguide.com/free/…
    – Zephyr
    Dec 4, 2017 at 21:05
  • Yes, that's what I'm saying. "When a packet is transmitted by a layer, a header consisting of Protocol Control Information (PCI) is added to the data to be sent. In OSI terminology, the packet data (also known as the Payload) is called a Protocol Data Unit (PDU). The packet so-formed, called a Service Data Unit (SDU) is passed via a service access point to the layer below. This is sent using the service of the next lower protocol layer." is pretty much the opposite or very poorly worded.
    – Zac67 ♦
    Dec 4, 2017 at 21:19

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Remember that the OSI model is just a model, and nothing in the real world actually adheres to it.

I believe what this is trying to get across to you is that the application in one host is peering with the application in the other host. Also, the transport protocol in one host is peering with the transport protocol in the other host, the network protocol in one host is peering with the network protocol in the other host, and the data-link protocol in one host is peering with the data-link protocol in the other host*.

The data that one application sends to the other application ends up in the destination application unchanged. Yes, as the data moves down the network stack in the sending host, it gets headers from the various network layers attached to it, but as it travels up the network stack in the destination host, those headers are stripped off, leaving the original data from the source unchanged.

Each network layer in the source host adds a header for the corresponding network layer in the destination host, and the corresponding network layer in the destination host will strip off the header, leaving the PDU for the next layer unchanged form the source.

*This hold true for the data-link layer only if both hosts are are the same data-link LAN. If the network packet must cross to another LAN, each router in the path will strip off the data-link header, replacing it with its own data-link header for the next network through which it will forward the network packet.

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  • The communication happens between both the layers but the data always moves from top to bottom in sender and from bottom to up in receiver right ?The diagram was bit misleading for me.
    – Zephyr
    Dec 4, 2017 at 20:20
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    Really the different layers in the network stack simply do not care what is in the other layers. A lower layer simply knows that what is from the layer above is payload for it, and an upper layer doesn't care what layer is below it. That facilitates things like IP being carried on ethernet, Wi-Fi, token ring, etc., and IP simply doesn't know or care what data-link protocol is carrying it. Conversely, a data-link protocol can carry IPv4, IPX, IPv6, AppleTalk, etc., and it simply doesn't care.
    – Ron Maupin ♦
    Dec 4, 2017 at 20:34
  • Ok, thanks for your answer. Can you answer this question please ? networkengineering.stackexchange.com/questions/46114/…
    – Zephyr
    Dec 4, 2017 at 20:35

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