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Unformatted text preview: r echoes this back in the reply field. TCP headers
containing this option will increase from the normal 20 bytes to 32 bytes.
The timestamp is a monotonically increasing value. Since the receiver echoes what it
receives, the receiver doesn't care what the timestamp units are. This option does not
require any form of clock synchronization between the two hosts. RFC 1323
recommends that the timestamp value increment by one between 1 ms and 1 second.
4.4BSD increments the timestamp clock once every 500 ms and this timestamp clock is reset to 0 on a
In Figure 24.7, if we look at the timestamp in segment 1 and the timestamp in segment II, the difference
(89 units) corresponds to 500 ms per unit for the time difference of 44.4 seconds. The specification of this option during connection establishment is handled the same way
as the window scale option in the previous section. The end doing the active open
specifies the option in its SYN. Only if it receives the option in the SYN from the other
end can the option be sent in future segments.
We've seen that a receiving TCP does not have to acknowledge every data segment that it
receives. Many implementations send an ACK for every other data segment. If the
receiver sends an ACK that acknowledges two received data segments, which received
timestamp is sent back in the timestamp echo reply field?
To minimize the amount of state maintained by either end, only a single timestamp value
is kept per connection. The algorithm to choose when to update this value is simple.
1. TCP keeps track of the timestamp value to send in the next ACK (a variable
named tsrecent) and the acknowledgment sequence number from the last ACK
that was sent (a variable named lastack). This sequence number is the next
sequence number the receiver is expecting. file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (13 of 20) [12/09/2001 14.47.34] Chapter 24. TCP Futures and Performance 2. When a segment arrives, if the segment contains the byte numbered lastack, then
the timestamp value from the segment is saved in ts...
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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