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TCP/IP the contradiction...?

In TCP/IP four layer model, The second Layer is Network Layer Which consists of protocols Like IP,ICMP..Similarly the above Layer Transport Layer consists of protocols like TCP & UDP. My Q is
  TCP is - Connection Oriented Protocol
  But the Lower Layer
  IP is - Connectionless Protocol.

 How these two layers communicates & How actually communication is handled in Tcp/Ip model ?
2 Solutions
Actually the second layer from the bottom is the Data Link layer not the Network Layer, that is the 3rd from the bottom. mmedwid has a good article there for understanding the model. If you are interested in really learning TCP/IP and OSI Modeling then stay with the Cisco based learning materials, they are far above the rest.

Saying that TCP/IP is connection-oriented means that the TCP protocol provides what appears to be a well organized, error free "pipe". You establish the pipe between two ends and data you put in on one side goes out the other in the same order and w/out errors.

In reality, TCP just hands it down to the lower layer (connection-less). The lower layer transmits the info w/out any warranty on the order of arrival. The TCP layer at the other end picks up the pieces and re-assembles them. If one is missing, it asks for retransmission. If they are out of order, it puts them back in order.

There are many layers, each using the lower layer to implement itself.

It all boils down in the end to pretty much the same layers with minor variations regardless of what "network" you are talking about.

Application layer on top of
Transport Layer on top of
Network Layer on top of
Logical Layer on top of
Physical Layer

I use "Application" very roughly here. The main interest is in the lower 4 (transport, network, logical, physical).

TCP or UDP are transport layers riding on the IP layer. Notice that TCP is connection oriented but UDP is not. Since IP is not connection oriented, then TCP must do a lot more work than UDP. This is why TCP is more "expensive" in terms of network load than UDP.

IP is the "network" layer over which TCP, UDP, ICMP, ARP, and other protocols ride. So are AppleTalk and Netware.

Ethernet is a "logical" layer over which IP rides (but not the only layer on which IP could ride).

IE 802.x (or something like that) is the "physical" layer over which the Ethernet protocol rides (but not the only physical layer on which Ethernet coul ride).

Note that most protocols fit into this layered model, even if the fit isn't perfect.

ICMP for example is used by PING. You could call PING the "application" layer and ICMP the "transport" layer, riding on IP.

I did this from memory, so there are bound to be some minor errors in the above. Yes, the second layer is "data link" not "logical" .. I knew it didn't sound right when I wrote it, but I was talking on the phone while I typed :-D

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