question into routing in ccnp r&s to mr/keith bogart

hello my sir


iam into lecture number 22 into your ccna videos in lecture 


you sai into L4 when data need to communicate with each other they don`t need any L2 or L3 devices for tcp connection   because they`re they connect end-to-end and pc and server into your photo i upload it don`t need any L2 or L3 devices or L4 communication 

i cannot understand that?

 we must need l2 and l3 to make l4 communicate with each others


please explain it to me again ? 



answer from mr/keith bogart

Hi Mohammed,


I’m teaching a class today so unfortunately I don’t have time to review exactly what I said in that video. However I believe what I was most likely referring to, was that each protocol (at each layer of the OSI model) creates a PDU with the intent for that PDU to be scrutinized, and interpreted by the same protocol on the receiver’s device.  So for TCP, when it creates a Segment, it is solely for the purposes of the TCP process on the receiver’s device to interpret-and-process.


Certainly TCP can’t, on its own, transport that segment directly to the receiver’s device without utilizing the services of some underlying Layer-3, and Layer-2 protocols.  But it is not up to the Layer-4 process (TCP in this case) to determine which Layer-3 and Layer-2 processes to select…nor is it the responsibility of TCP to know the exact mechanics of how those lower-level protocols will accomplish their tasks.


An analogy is when you write a letter to your mother.  The contents of that letter are meant to be read, and interpreted, only by your mother.  However if your mother resides in a different country you don’t have the ability to directly hand it to her.  Consequently you have to make use of some lower-level process (probably the Post Office) to figure out the mechanics of how that letter will be delivered to your mother.  On the actual piece of paper you’ve used to create the letter, there is nothing on that paper that is meant for the Post Office to read or interpret. However you know (as does TCP) that you must “hook into” some other process to get that paper to your mother.


When application developers create a piece of software, they must decide if that software will utilize TCP or UDP.


If TCP is selected, the developers must then decide if IPv4 or IPv6 will be used as the Layer-3 transport mechanism.  Once that it determined, they will take a pre-written piece of standard code that “connects” TCP to IPv4 (if that is selected).  So TCP will do its job of creating the segment and automatically that little snippet of code will “hook” that segment into the IPv4 process.  TCP will have no knowledge of what IPv4 is doing and it won’t care.


I hope this helped!



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