Unformatted text preview: its minimum value of 5 seconds.
When the persist timer expires, 1 byte of data is sent (segment 6). The receiving
application has read 256 bytes from the receive buffer (at time 3.99), so the byte is
accepted and acknowledged (segment 7). But the advertised window is still 0, since the
receiver does not have room for either one full-sized segment or one-half of its buffer.
This is silly window avoidance by the receiver.
The sender's persist timer is reset and goes off again 5 seconds later (at time 10.151). One
byte is again sent and acknowledged (segments 8 and 9). Again the amount of room in
the receiver's buffer (1022 bytes) forces it to advertise a window of 0.
When the sender's persist timer expires next, at time 15.151, another byte is sent and
acknowledged (segments 10 and 11). This time the receiver has 1533 bytes available in
its buffer, so a nonzero window is advertised. The sender immediately takes advantage of
the window and sends 1024 bytes (segment 12). The acknowledgment of these 1024
bytes (segment 13) advertises a window of 509 bytes. This appears to contradict what
we've seen earlier with small window advertisements.
What's happening here is that segment 11 advertised a window of 1533 bytes but the
sender only transmitted 1024 bytes. If the acknowledgment in segment 13 advertised a
window of 0, it would violate the TCP principle that a window cannot shrink by moving
the right edge of the window to the left (Section 20.3). That's why the small window of
509 bytes must be advertised.
Next we see that the sender does not immediately transmit into this small window. This
is silly window avoidance by the sender. Instead it waits for another persist timer to
expire at time 20.151, when it sends 509 bytes. Even though it ends up sending this small
segment with 509 bytes of data, it waits 5 seconds before doing so, to see if an ACK
arrives that opens up the window more. These 509 bytes of data leave only 768 bytes of
available space in the receive buffer, so the acknowledgment (segment 15) advertises a
window of 0. file:///D|/Documents%20and%20Settings/bigini/Docu...h...
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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