TCP/UDP activities

Once application layer send the data through layer 7/6/5 to layer 4, does the layer 4 stop that process and send some short of their own bits of data to another side of host for acknowledgement TCP connection, to make sure the it has connection and able to send the data came from above layer .... or the TCP layer, send the first segment of data comes from application layer to another host for acknowledgment to establish the connection to send the another segment of data.


  • MartinlMartinl ✭✭✭

    I tihnk Data waits untill TCP connection is established.  3-way handshake.

    then tcp features such as Flow control (windowing) and error recovery (using sequence numbers) are part of tcp headers with Data from above layers. So, once connection is established, session layer Data is placed into a Layer 4 segment.

    Most books show additonal headers appended to prevoius data in OSI model. 

  • MartinlMartinl ✭✭✭

    just noticed picture from older ccna book, it shows 3-way handshack of http web site traffic.  it shows http data is in data part of l4 segment with header in front of it; so maybe 3 way tcp handshake starts with data of previous layer, sort of saving time.  

  • JoeMJoeM ✭✭✭

    Not sure I am understanding the question.   TCP does not stop anything, but it is polite and shakes hands to establish the connection. Before an HTTP client can get data, it needs to request it with a GET command.  Then the server will send the data (and expect acks for data sent).

    Here is a packet capture.  Look at the info column to see the handshake - SYN, SYN/ACK, ACK 

    ......then the http client requests the data with an http GET command...and data begins flowing, while the client ACK's received sequenced packets.

    When the server is done sending the data, we can see it sends a FIN, ACK  followed by the clients ACK.   (END of conversation)



    Here is a really good explanation with pictures.  Good reference for review.



  • JoeMJoeM ✭✭✭



    Hopefully that helped answer your question.   You may want to take a look at the video series by Keith Bogart, Understanding TCP.   It is one of the most thourough explanations of TCP that I have ever seen.  Probably worth reviewing for everyone.  A lot of detail.


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