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Programming at different layers

Posted on 2004-03-31
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Can some one tell a novice like me, what it means when one talks about programming at different layers.
I can comprehend, writing programs which are basically application layer stuff, and using the protocols, like tcp,udp below
But suppose, one talks about the implementation of mobile ip.
I know it is the network layer, but where does one do implemnetations that follow this protocol(is it in the routers)
and how is it done.
Please explain in layman or dummy terms.

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Question by:vikasgkutty
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by:stealth188
ID: 10725320
If you don't strictly adhere to the OSI model for the purpose of this answer you could think about it like this.  TCP or UDP are protocols that operate at the network layer.  If you have particular functionality like DHCP or Multicast or Mobile IP that you want to implement you would write a program that manipulates TCP/UDP in such a way to acomplish the goal of your "application"; the "application" being Mobile IP.  This program typically runs in a networking device such as a router or L3 switch.  The distinction to be drawn between Outlook as an application and Mobile IP as an application is that the later manipulates the Network layer whereas Outlook simply uses it to send/receive email.  Heck it might even use your Mobile IP "application" in the course of it's operation.  The thing to remember about the OSI model is that each layer doesn't need to know how the ones below it or above it operate.

Let me know if you need more information.

Dan
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by:vikasgkutty
ID: 10727230
I see what you mean,
So in case of Mobile Ip(which is what I have been thinking about), there is the concept of a mobile node having a foreign agent and a home agent right.Any message received by the home agent for the mobile node, is forwarded to the new care of address of the mobile node.
So according to this, we would probably need an implementation at router/switch level on those home agent , and foriegn agent nodes of the networks rt.so, that they do the necessary translation of destination addresses or whatever.
Is tht right thinking, Dan.
Also, had a doubt, u said TCP/UDP are protocols tht operate at network layer.I thought they operate at Transport layer, and the protocol tht operates at network layer is IP
Or is it because I am not following the OSI ref model
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stealth188 earned 30 total points
ID: 10727691
You are right about TCP/UDP.  They do opeate at the transport layer.  My mistype.  I am a Cisco guy so I don't know if this applies to all implementations of Mobile IP, but I found this document that does a much better job explaining the mechanics of Mobile IP than I can so I'll just leave the URL for you.

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800c9906.shtml
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by:drelectro
drelectro earned 30 total points
ID: 10740035
Moblie IP does not fit neatly into the model.

From one point of view it's a protocol which tunnels IP (and everything above it) over IP or GRE, layer 3 (Network) gets  doubled-up if you get my drift.

You don't neceserily need to implement this at the switch/router, the MN could establish the IP or GRE tunnel with the HA directly (ie combine MN and FA).

Also theres quite a bit of Mobile IP (actually the harder bit to code) that's done at the Application layer over ICMP and UDP (Discovery and Registration) and that pretty much must be done at the MN.

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