HC110110002 TCPIP协议栈.pptVIP

Layer 3 of 3: In the next segment, host A sends some data. Note that the sequence number of the segment in step 3 is the same as as the ACK in step 2. Emphasize: This sequence is like two people talking. The first person wants to talk to the second, so she says, “I would like to talk with you.” (SYN.) The second person responds, “Good. I want to talk with you.” (SYN, ACK.) The first person then says, “Fine—let us talk. Here is what I have to say.” (SYN, ACK, DATA.) At this point either side can begin communicating and either side can break the connection. TCP is a peer-to-peer (balanced) communication method (no primary/secondary). Note: This figure explains TCP connection establishment. For more information regarding the three-way handshake in establishing a TCP connection, refer to RFC 793. Layer 1 of 7: The window size is the number of messages transmitted before the sender must wait for an acknowledgment. Window size was presented earlier in the course, so this slide is a review. The initial state, no messages being sent. Layer 2 of 7: Data message 1 sent. (Send 1, Receive 1) Layer 3 of 7: Acknowledgment message 2 sent. (Send ACK 2, Receive ACK 2) Layer 4 of 7: Data message 2 sent. (Send 2, Receive 2) Layer 5 of 7: ACK for message 2. (Send ACK 3, Receive ACK 3) Layer 6 of 7: Send 3, Receive 3. Layer 1 of 7: ACK for message 3. (Send ACK 4, Receive ACK 4) This sequence helps to convey the delay associated with a window size of one. Note: TCP acknowledgments are expectational and are sometimes called forward referenced, which means that they refer to the segment they are expecting to receive, not the one just sent. Acknowledgment field sizes can become an issue when transmitting data at FDDI and ATM speeds. Layer 1 of 5: This figure points out the benefit of a larger window size. Layer 1 is in the initial state, no messages being sent. Layer 2 of 5: Layer 2 illustrates how the sending device defines its window buffer as 3 and sends three bytes. Layer