* Help requered with TCP windowing question *

Can anyone could provide a better explanation to the answer of the question below ?
As you can see I selected B and C as opposed to B and E

Option C. seems correct in as much as ACK 3100 = I ACKnowledge the last sequence of 2600, send 3100 next

Question

Host 1 sends three consecutive TCP segments to Host 2, with sequence numbers 1600, 2100, and 2600, respectively. All three segments have a source port of 80 and a destination port of 1600. Assuming that Host 2 responds to acknowledge successful receipt of all three segments, which of the following are true ?

Select the best 2 answers

A. The segment that is sent from Host 2 to Host 1 has an acknowledgment field set to 3 more than the acknowledgment field of the previous segment that Host 2 had sent to Host 1.
B. The segment that is from Host 2 to Host 1 has a destination port of 80.
C. The segment that is sent from Host 2 to Host 1 has an acknowledgment field of 3100
D. The segment that is sent from Host 2 to Host 1 has a source port of 80
E. The segment that is sent from Host 2 to Host 1 has a acknowledgment field larger than 2600

The Answers were B and E, I picked B and C

Explanation:
Each TCP segment might be a different length. Nothing in the question states the number of bytes of data in the third packet, so it is impossible to know which number Host 2’s field has been set.


Kind regards

Damien2008

It's really simple. Your somehow misunderstanding how the sequence works.

Host one sends:

SYN 1600, Source port 80, Destination port 1600
SYN 2100, Source port 80, Destination port 1600
SYN 2600, Source port 80, Destination port 1600

Host two replies with:

ACK 1601, Source port 1600, Destination port 80
ACK 2101, Source port 1600, Destination port 80
ACK 2601, Source port 1600, Destination port 80


C shouldn't be an answer because all Acknowledgements are simply SYN + 1 and there was no SYN of 3100.

E is a correct answer because 2601 is larger than 2600


Are these the boson exam questions I recall seeing that one before?

Hi Nevins

Host two replies with:

ACK 1601, Source port 1600, Destination port 80
ACK 2101, Source port 1600, Destination port 80
ACK 2601, Source port 1600, Destination port 80

Host 2 would not reply with the above.....
The question states that Host 1 sends three consecutive segments, Host 2 responds with successful receipt of all three segments.

Host 2 would not send three separate ACKs, it would send one ACK plus a new Window size ?


Host 1 is waiting for ACK from Host 2 as the window is exhausted, Host 2 will send a new larger window size so that Host 1 can send more than 3 segments next time...

See example below



I now agree that E is correct, but for other reasons, what do you think ?

Seeing that you have that picture I assume you have:

CCENT/CCNA ICND1
Official Exam Certification Guide,
Second Edition
by Wendell Odom

The image you are showing off is figure 6-7 on page 142. Admittedly I can see how this could be confusing. But any example I've ever seen aside from this one does it simply by adding +1 not +1000.



In the image a lot of detail you would see in an actual packet is left out such as source port and destination port. Here is fig 6-8 on page 143 done in a more detailed manner.

If someone could spread some light on why sequence appears to have 2 sets of rules in this example please do.

Your pic of fig 6-8 on page 143 is not displaying but you are correct in assuming I have the ICND 1 Wendell Odom book.

I have watched the videos on you tube, indeed the ACK numbers are the SEQ + 1. The answer of E is much clearer in my mind now....

One thing I would still query in your answer..........

The SYN flag is only set during initiating the 3 way handshake, it seems that Host 1 would send

SEQ 1600, Source port 80, Destination port 1600
SEQ 2100, Source port 80, Destination port 1600
SEQ 2600, Source port 80, Destination port 1600

And not

SYN 1600, Source port 80, Destination port 1600
SYN 2100, Source port 80, Destination port 1600
SYN 2600, Source port 80, Destination port 1600

Because the initial 3 way connection has already taken place......

Yeah your right for some reason I was thinking SYN instead of SEQ and that it was the inital connection. I plan on doing some re-reading on the subject because frankly it's been a while.

Anyways I would avoid opening any power points on the subject I got a nasty virus that I had to fix. Sure it only took me 2 hours to fix but thats because I knew what to do right away and I've dealt with viruses before.

The virus:

locked out task manager
locked out msconfig
set Chrome and Internet explorer to proxy the loopback address
added some files and attempted to read my hard drive for information
Also some other stuff that got stopped by me pulling the ethernet cord and rebooting to safemode and running AV's

Fortunately any useful files I always keep on a thumb drive which is only attached physically when I need it to be and I noticed the issue right away and pulled the ethernet cord and did a restart really quick.-