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Unformatted text preview: can also see the expected exponential backoff in each retransmission timeout: the first
appears to be 2.5 seconds, which is then multiplied by 2 (giving 5 seconds), then 4 (10
seconds), then 8 (20 seconds), then 16 (40 seconds).
We then type the third line of input ("line number 3") and see it sent on line 17, echoed on
line 18, and the echo acknowledged on line 19.
We now want to see what happens when TCP retransmits and gives up, after receiving the
ICMP host unreachable, so we take down the SLIP link again. After taking it down we type
"the last line" and see it transmitted 13 times before TCP gives up. (We have deleted lines 3043 from the output. They are additional retransmissions.)
The thing we notice, however, is the error message printed by our sock program when it
finally gives up: "No route to host." This corresponds to the Unix error associated with the
ICMP host unreachable error (Figure 6.12). This shows that TCP saves the ICMP error that it
receives on the connection, and when it finally gives up, it prints that error, instead of
"Connection timed out."
Finally, notice the different retransmission intervals in lines 22-46, compared to lines 6-14. It
appears that TCP updated its estimators when the third line we typed was sent and
acknowledged without any retransmissions in lines 17-19. The initial retransmission timeout
is now 3 seconds, giving successive values of 6, 12, 24, 48, and then the upper limit of 64. 21.11 Repacketization
When TCP times out and retransmits, it does not have to retransmit the identical segment
again. Instead, TCP is allowed to perform repacketization, sending a bigger segment, which
can increase performance. (Naturally, this bigger segment cannot exceed the MSS announced
by the other receiver.) This is allowed in the protocol because TCP identifies the data being
sent and acknowledged by its byte number, not its segment number.
We can easily see this in action. We use our sock program to connect to...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12