Introduction to the OSI Model

Hi,

I was wondering what can be the issues, if we just dont use the MAC address.
What if all devices have only IP addresses. What do you think would happen then.

Here is what I feel:

  1. Every device would have to look up all the way till layer 3 to decide how to forward packets/frames, not just L2.
  2. Using MAC addresses to identify devices on a local network enables us to use any upper layer L3 protocol we want to use, if we used only IP,then we would be stuck with only IP or IPX or AppleTalk etc. We cant use any other way.

I am not satisfied by my thoughts !
Can you give some more suggestions ?

@Mansi,
Without MAC addresses you would have more problems to solve:

  1. Convenience would be a problem. DHCP would no longer be possible, because until you have an IP, you can’t communicate on a network. This means that every time you move a computer or device to a new network, you would have to configure it manually.

  2. Management would be a problem. Without setting up a global registry to ensure every IP address is unique from the factory (which would be impossible with IPv4 since there aren’t enough addresses), how would you ever be able to track down a mis-configured machine that has a conflicting IP address?

  3. Security would be a problem. How would you stop someone from plugging in a rogue machine to your network and spoofing an IP address without the possibility of layer-2 level security?

  4. Resiliency would be a problem. There are many services, such as GLBP, HSRP, and VRRP, that depend upon a separation of IP addresses from MAC addresses to provide highly available services.

I am sure there are plenty of other issues that I haven’t thought of yet :slight_smile: …

Thanks Andrew.
However I have some queries about your first and second point.

  1. MAC addresses wont be anyways needed since DHCP server is going to broadcast the offers anyways.
  2. As far as gratuitous ARP is an issue, some other method can surely be developed to understand conflicts: eg send out a broadcast ie a L3 Broadcast…if the receiver sees the source IP as its own IP, then it will know of an IP conflict. So yes even this issue can be solved.

Thanks for the answers Andrew.

  1. How is a machine supposed to receive a broadcast? You are assuming it already has an IP address to get it. Even if a broadcast somehow worked without a transport protocol (it wouldn’t), you would have to have a DHCP server on every possible broadcast segment on a network. Most businesses don’t work that way–they use centralized DHCP servers with IP Helpers that forward DHCP requests.

Hi,

Very nice explanation. I have a basic query, similar to the above ones, but other way. What all problems if we dont have IP address at all, and we have only MAC addresses for communication?

Regards,
Durga Prasad

Durga,
The Internet would break :slight_smile:

Seriously, it would, and here’s why: Let’s say we tried to use MAC addresses as a layer 3 identity for communicating between devices at a distance (not on the same local network). MAC addresses are burned into the network cards by the manufacturer, and each card has a unique MAC (in theory). The problem comes when you are trying to keep track of who is where. The reason that the Internet is able to function now is primary due to a protocol called BGP that figures out the best way to get from one IP address to another.

Even though there are millions and millions of connected devices, BGP is able to function by using groups of addresses–most BGP providers will reject routes for any network smaller than a /24. Being able to group large numbers of IP addresses together and treat them the same is critical so the memory and processing power required to run BGP wouldn’t overwhelm a router’s resources. In the case of try to track individual addresses on a per machine level (like we are thinking about doing with MAC addresses), this would force BGP to deal with the equivalent of /32 routes. Since devices can easily move from one location to another, there just isn’t a way to “lump” groups of them together. There would just be too many objects to keep track of within BGP!

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Hi Andrew,

Thanks for clearing me.

Regards,
Durga Prasad

19 posts were merged into an existing topic: Introduction to the OSI Model

Hi,
Very valuable explanation . I would like to confirm one point . The first piece of information (The frame which is displayed before Ethernet II) has been added by Wireshark. I believe it is not part of OSI and here in this case OSI layers will be starting from Ethernet(II) as layer 1 (voltage signals ) we couldn’t capture for analyse . Please correct me if I am wrong.

Hi Sreenath,

That is correct, this information is added by Wireshark and not part of the OSI model. What we see in Wireshark is layer two (data link) and above.

Hi Rene,

Sonet \SDH belongs to which layer in OSI ? I believe it belongs to layer-1 , if so next concern is which layer 2 protocol it use for encapsulation like Ethernet interface use Ethernet frame (layer-2) frame to encapsulate , will it use sonet/sdh frame in layer-2 ? If possible please demonstrate a scenario about the packet flow of traffic from Ethernet source to Ethernet destination and if we have sonet\sdh networks in between it.

Thanks,
Sreejith

Hi Sreejith,

I would say SONET/SDH belongs to layer 1. It specifies the physical layer and you can run different L2 protocols (including Ethernet) on top of it. It’s not like frame-relay which has a clear specification of L1 + L2.

The main difference is that the physical layer is different between SONET on one end and Ethernet on a LAN on the other side. When you receive something from the SONET side, it goes from the physical layer to the data link layer and you end up with an Ethernet frame. The outgoing interface is selected, the frame goes from the data link layer to the physical layer and is then transmitted.

Rene

I have the same doubt. I´ve read L2 (eth frames) doesn’t perform error CORRECTION but error DETECTION.

Error Correction (Retransmission) is only on L4 TCP.

Is that correct ?

Hi Rene,
Thanks for your reply . I thought STS frames are the layer-2 protocol for Sonet/SDH interface(layer-1) like Ethernet frame for Ethernet interface (layer-1). Once a packet reaches from an Ethernet interface on a router and if it’s destination points to Sonet interface on the same router , how the router will forward it ? I believed the router will de-encapsulate the Ethernet frame , take the IP packet , do some routing \ARP table look ups, encapsulate the IP packet in STS frame and forward towards the Sonet interface (OC signal : Optical link). Please correct me if I am wrong :slight_smile:
Thanks,
Sreejith

Hi Juan,

That’s right. Ethernet frames have a FCS (Frame Check Sequence) so a device can validate if the frame is OK or not, but there is no error correction.

TCP does have error correction, it uses retransmissions.

Rene

Hi Sreejith,

It’s not really layer 2 but it’s more like layer 1.5…it doesn’t really fit in perfect with the OSI model but after all, the OSI model is just a reference model.

You are right about how the router does encapsulation / de-encapsulation. On one interface, the router receives some Ethernet frame. It de-encapsulates it and ends up with an IP packet. It checks the destination of the outgoing IP packet in the routing table and finds out that the outgoing interface is the Sonet interface.

It then encapsulates the IP packet in a new Ethernet frame, encapsulates it in an STS frame and forwards it out of the Sonet interface.

Rene

When I learned the OSI Model years ago. I’ve always like this saying.

Please, Don’t, Never, Throw, Sausage, Pizza, Away

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Hello Rene,

I just wanted to clarify the application, presentation, and session layer. Basically when you go onto your browser (Firefox which is the application) and put www.google.com your computer sends a request to the webserver to request the webpage by using HTTP protocol. The presentation layer makes sure the data at the application layer is in a readable format. The session layer takes care of the sessions meaning your not the only one making a request to www.google.com. Could you please confirm if I’m correct!

Cheers!

Hello Rene,

Can you please explain the following.

  1. How is data is formatted in the correct way
  2. How error detection is done
  3. How does it make sure data is delivered reliably

Cheers!

Hello Iynkaran

First of all, the application layer is not where the actual applications on your computer function. These software applications sit on top of the OSI model and are not actually part of it. The application layer is the layer where protocols such as HTTP/HTTPS, FTP, SMTP, IMAP and others function. These protocols are then leveraged by software applications. This is what makes software applications “network aware” if you will.

From a practical standpoint, the model that is primarily used in today’s networks is the TCP/IP model. This model incorporates parts of the session and presentation layers into the transport and application layers resulting in a model with fewer layers.

So in your example of a web page, the web browser would use the HTTP protocol (Application layer) to communicate between the client (web browser) and the server (Web server). HTTP contains within its mechanisms the functionality of the presentation layer, so we don’t actually see the presentation layer in the Wireshark packet capture. The presentation functionalities essentially allow the information that is received from lower layers to be presented in a manner that the HTTP protocol, and the client can understand and display.

Similarly, the session layer functionalities are incorporated into the transport layer. The sessions that are being referred to here are those between the host and server, that is between the web browser and the web server and do not involve the sessions of any other hosts.

For more information about the TCP/IP model as compared to the OSI model, take a look at this lesson:

I hope this has been helpful!

Laz

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