1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

TCP Three-Way Handshake. Query Regarding SYN

Discussion in 'General Cisco Certifications' started by culley, Dec 1, 2008.

  1. culley

    culley New Member

    6
    0
    10
    Hi,

    I've just been reading through Chapter 4 OSI Transport Layer of Cisco Fundamentals and i have a question that i can't answer, It's regarding the Three-Way Handshake that TCP use's when making a connection between client and server.

    I understand the process, its very simple its just in my book is says something that i don't understand.

    In part 2 of the Three-Way Handshake in my book it reads;

    "The acknowledgment value is 1 greater than the sequence number because there is no data contained to be acknowledged."

    I just dont get this sentence, does that mean when there is data the value will be greater then x or what.

    Thanks for any help in advance

    Sam Culley
     
    WIP: A+
  2. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

    13,493
    179
    287
    See if this helps:

    http://www.unix.com.ua/orelly/networking_2ndEd/tcp/ch01_06.htm

     
    Certifications: A+ and Network+
  3. hbroomhall

    hbroomhall Petabyte Poster Gold Member

    6,623
    115
    224
    That sounds a bit confusing. (Edit: The quote from culley - not trip!)

    Have a look at the page (and pages following) here from my favourite (currently) book on TCP/IP and see if an alternate explanation helps.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  4. culley

    culley New Member

    6
    0
    10
    Here's the whole paragraph to give you some more light on it and to put it in more context.

    "The server responds with a segment containing an acknowlegdment value equal to the recieved sequence value plus 1, plus its own synchronizing sequence value. The acknowledgment value is 1 greater than the sequence number because there is no data contained to be acknowledged. This acknowledgment value enables the client to tie the respone back to the original segment that it sent to the server."
     
    WIP: A+
  5. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

    13,493
    179
    287
    Boiling it all down, the ACK is just the SYN plus 1. The single bit is added to let the TCP client know that the SYN was received by sending the same SYN back plus the bit added by the server.

    Think of it this way. You are sending a letter and you want to make sure it's received on the other end. You request a verification of the letter being received. At least in the US postal service, when I send a letter Certified, I fill out a card with my return address on it and that card is attached to the outside of the letter I'm sending.

    When the person at the other end receives the letter, they are required to detach the card, sign it (adding the single bit) and the postal worker then puts it back into the mail to be sent to me. My letter is the SYN and the card I filled out plus the receiver's signature is the ACK.
     
    Certifications: A+ and Network+
  6. culley

    culley New Member

    6
    0
    10
    Thanks i understand the functions of the Three-Way handshake now.

    It's just i didn't understand what Cisco ment when they wrote that sentence in my book, (its an offical Cisco Press book) but after reading the link you gave me and reading more in my book i have come to the conclusion that what they ment is that the example there were using didn't have any data so the ACK would be the SYN number + 1.

    In the link you gave me its says the ISN for the SYN is 0 and that the recieving host has recieved 2000 bytes of data so the ACK would be 2001.

    Basically the ACK = SYN + 1 + the data i.e 0+1+2000 = 2001

    I'm pretty sure i have got that correct if you have trouble understand the way i've expressed it i'm sorry in advanced.
     
    WIP: A+
  7. Obinna Osobalu

    Obinna Osobalu Banned

    539
    7
    0
    The quanity of data segments(measured in bytes) the transmiting machine is allowed to send without receiving an acknowledgment for them is called a WINDOW. assume that there are two window sizes—one set to 1, and one set to 3, when configured a window size of 1, the sending machine waits for an acknowledgment for each data segment it transmits before it transmits another. If configured a window size of 3, its allowed to transmit three data segments before it must receive an acknowledgment. This is my explanation based on my understanding of the above thats if i properly understood that... let me know though if it doesnt really clear you up
     
    Certifications: MCITP:SA,MCTS(x5),MCSE2K3;MCSA2K3:M;MCP
    WIP: EDA7,70-652,Project+,MSP(70-632)
  8. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

    13,493
    179
    287
    Looks like you've got it. :)
     
    Certifications: A+ and Network+

Share This Page

Loading...