Unformatted text preview: he T1 network is 5.211 seconds. If we throw
more bandwidth at the problem, a T3 network (45,000,000 bits/sec), the total time
decreases to 0.208 seconds. Increasing the bandwidth by a factor of 29 reduces the total
time by a factor of 25.
With the gigabit network the total time to transfer the file is 0.038 seconds: the 30-ms
latency plus the 8 ms for the actual file transfer. Assuming we could double the
bandwidth to 2 gigabits/sec, we only reduce the total time to 0.034 seconds: the same 30ms latency plus 4 ms to transfer the file. Doubling the bandwidth now decreases the total
time by only 10%. At gigabit speeds we are latency limited, not bandwidth limited.
The latency is caused by the speed of light and can't be decreased (unless Einstein was
wrong). The effect of this fixed latency becomes worse when we consider the packets
required to establish and terminate a connection. Gigabit networks will cause several file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (9 of 20) [12/09/2001 14.47.34] Chapter 24. TCP Futures and Performance networking issues to be looked at differently. 24.4 Window Scale Option
The window scale option increases the definition of the TCP window from 16 to 32 bits.
Instead of changing the TCP header to accommodate the larger window, the header still
holds a 16-bit value, and an option is defined that applies a scaling operation to the 16-bit
value. TCP then maintains the "real" window size internally as a 32-bit value.
We saw an example of this option in Figure 18.20. The 1-byte shift count is between 0
(no scaling performed) and 14. This maximum value of 14 is a window of 1,073,725,440
bytes (65535 x 214).
This option can only appear in a SYN segment; therefore the scale factor is fixed in each
direction when the connection is established. To enable window scaling, both ends must
send the option in their SYN segments. The end doing the active open sends the option in
its SYN, but the end doing the passive open can send the option only if the received SYN
specifies the option....
View Full Document
- Spring '12