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Home > Archive > CCNA > March 2004 > TCP Question
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| JennyJennyCCNA 2004-03-19, 1:24 am |
| Our instructor asked us to match these up, I have my own opinions but wanted your's ... thanks guys!
A. Ports
B. Sequencing
C. Window
D. PAR
E. 3 way handshake
1. Provides reliability by requiring recipient acknowledgements of a group of segments before a time expires (PAR?)
2. Initiates communication by establishing an initial sequence # and window size (Sequencing? duh ... )
3. Amount of data that can be sent before an acknowledgement is required (Window? duh ...) | |
| nethead 2004-03-19, 8:37 am |
| I'm not familiar with the term PAR, can someone explain? | |
| dmaftei 2004-03-19, 10:03 am |
| 1 - closest is b; nothing matches exactly
2 - e
3 - c
What's "PAR"?! | |
| Joe Dali 2004-03-19, 10:50 am |
| See my posts from when I failed, this is on the Exam ... PAR is positive acknowledgement retransmission.
This exactly what confused me on the test, this is covered in Lammle and Cisco.com article on TCP-IP.
Thanks Jenny! Post more! | |
| dmaftei 2004-03-19, 11:30 am |
| quote:
Originally posted by Joe Dali
positive acknowledgement retransmission
Well, then PAR fits perfectly for 1. | |
| Joe Dali 2004-03-19, 10:04 pm |
| TCP Connection Establishment
To use reliable transport services, TCP hosts must establish a connection-oriented session with one another. Connection establishment is performed by using a "three-way handshake" mechanism.
A three-way handshake synchronizes both ends of a connection by allowing both sides to agree upon initial sequence numbers. This mechanism also guarantees that both sides are ready to transmit data and know that the other side is ready to transmit as well. This is necessary so that packets are not transmitted or retransmitted during session establishment or after session termination.
Each host randomly chooses a sequence number used to track bytes within the stream it is sending and receiving. Then, the three-way handshake proceeds in the following manner:
The first host (Host A) initiates a connection by sending a packet with the initial sequence number (X) and SYN bit set to indicate a connection request. The second host (Host B) receives the SYN, records the sequence number X, and replies by acknowledging the SYN (with an ACK = X + 1). Host B includes its own initial sequence number (SEQ = Y). An ACK = 20 means the host has received bytes 0 through 19 and expects byte 20 next. This technique is called forward acknowledgment. Host A then acknowledges all bytes Host B sent with a forward acknowledgment indicating the next byte Host A expects to receive (ACK = Y + 1). Data transfer then can begin.
Positive Acknowledgment and Retransmission (PAR)
A simple transport protocol might implement a reliability-and-flow-control technique where the source sends one packet, starts a timer, and waits for an acknowledgment before sending a new packet. If the acknowledgment is not received before the timer expires, the source retransmits the packet. Such a technique is called positive acknowledgment and retransmission (PAR).
By assigning each packet a sequence number, PAR enables hosts to track lost or duplicate packets caused by network delays that result in premature retransmission. The sequence numbers are sent back in the acknowledgments so that the acknowledgments can be tracked.
PAR is an inefficient use of bandwidth, however, because a host must wait for an acknowledgment before sending a new packet, and only one packet can be sent at a time.
TCP Sliding Window
A TCP sliding window provides more efficient use of network bandwidth than PAR because it enables hosts to send multiple bytes or packets before waiting for an acknowledgment.
In TCP, the receiver specifies the current window size in every packet. Because TCP provides a byte-stream connection, window sizes are expressed in bytes. This means that a window is the number of data bytes that the sender is allowed to send before waiting for an acknowledgment. Initial window sizes are indicated at connection setup, but might vary throughout the data transfer to provide flow control. A window size of zero, for instance, means "Send no data."
In a TCP sliding-window operation, for example, the sender might have a sequence of bytes to send (numbered 1 to 10) to a receiver who has a window size of five. The sender then would place a window around the first five bytes and transmit them together. It would then wait for an acknowledgment.
The receiver would respond with an ACK = 6, indicating that it has received bytes 1 to 5 and is expecting byte 6 next. In the same packet, the receiver would indicate that its window size is 5. The sender then would move the sliding window five bytes to the right and transmit bytes 6 to 10. The receiver would respond with an ACK = 11, indicating that it is expecting sequenced byte 11 next. In this packet, the receiver might indicate that its window size is 0 (because, for example, its internal buffers are full). At this point, the sender cannot send any more bytes until the receiver sends another packet with a window size greater than 0. | |
| worrywarm 2004-03-20, 12:08 am |
| quote:
Originally posted by dmaftei
Well, then PAR fits perfectly for 1.
but I thought PAR is to ack each packet before transmit next. And here
1. Provides reliability by requiring recipient acknowledgements of a group of segments before a time expires
?? | |
| Joe Dali 2004-03-20, 12:15 am |
| That is exactly why I prolly got this one wrong on zee testee. | |
| dmaftei 2004-03-20, 9:22 am |
| quote:
Originally posted by worrywarm
??
Hmm...
A. Ports don't provides reliability (they're addresses).
B. Sequencing doesn't provides reliability (provides order).
C. Window doesn't provides reliability (provides flow control).
E. 3 way handshake deson't provides reliability (provides connection params).
That leaves only PAR. Maybe that doesn't make for a "perfect fit", but seems to be the only one that makes sense.
I think there is some confusion here... I think PAR is the general technique of acknowledging PDUs. But the question asks about acknowledging segments, while Joe Dali, in that two miles long post of his, talks about acknowledging packets. Meanwhile, TCP acknowledges octets (groups of octets, not individual octets)...
Now, regardless of how you call the acknowledgments (PAR or whatever), TCP does use them to achieve reliability, so the answer 1 - d still stands. |
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