explanation required 3

Answered Question
Jun 25th, 2007

Please explain the following questions:


Which of the TCP connection-establishment flows sets both the SYN and ACK flags in the TCP header?

a. First segment

b. Second segment

c. Third segment

d. Fourth segment

e. Fifth segment


Answer:B


Which of the following protocols are reliable?

a. Frame Relay

b. TCP

c. IP

d. UDP

e. Ethernet


Answer:B (why not A it is also connection oriented)

Correct Answer by jasonnzcisco about 9 years 8 months ago

Hello Neerav,


I'm really not to sure about your first question.


But looking at the second question. We know TCP is reliable and works at layer 4 of the OSI. We also know that Frame-relay works at layer 2 and is just a method of encapsulating data. There are also different encapsulation types such as IETF and Cisco.


Remember the initial virtual circuit is predetermined at layer 4. Where it can be reliable or not.


Hope this makes sense.


Regards J



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Correct Answer
jasonnzcisco Mon, 06/25/2007 - 16:30

Hello Neerav,


I'm really not to sure about your first question.


But looking at the second question. We know TCP is reliable and works at layer 4 of the OSI. We also know that Frame-relay works at layer 2 and is just a method of encapsulating data. There are also different encapsulation types such as IETF and Cisco.


Remember the initial virtual circuit is predetermined at layer 4. Where it can be reliable or not.


Hope this makes sense.


Regards J



srue Mon, 06/25/2007 - 17:41

With frame-relay, the frames will have a frame check sequence (FCS) which help indicate frame problems across the link. If a receiving router decides the frame has been corrupted through one means or another, the router will simply discard the frame (w/o any notification to the side that sent the frame - think of it as a silent discard). It will leave it up to higher layer protocols (like TCP) to ask for a retransmit.

neerav_kumar Mon, 06/25/2007 - 17:47

What if the layer 4 protocol used by the connection (frame relay) is UDP?

Then the connection wouldn't be reliable. So frame relay is only reliable if the layer 4 protocol is TCP. Is this statement correct?

srue Mon, 06/25/2007 - 17:59

frame-relay in and of itself is never reliable...TCP (or other high layer protocols/applications) can add reliability.

with the case of UDP, you are correct that in that case neither frame-relay nor UDP provide reliability so reliability would have to be built in to an even higher layer, like TFTP, or NFS. make sense?

neerav_kumar Mon, 06/25/2007 - 20:16

I got the answer to the first question.

Here they are talking about the three way handshake and

SYN: synchronize the sequence number

ACK: the acknowledgment field is valid


Now in the three way connection the following steps exist:

1. The client sends the sequence number to the server.

2. The server acknowledges it by sending its sequence number and acknowledgment number.

3. The client replies back with an acknowledgment number.


So you see in step 2 both sequence and acknowledgment fields are set and hence the answer is Second Segment

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