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We find that notification messages are small. Specifically, all messages
contain less than 256 bytes. We show the message size distribution
in Figure 4 to illustrate this point.
Consequently, it is important for the delivery protocol to handle small messages
efficiently. For example, if the protocol creates a new TCP connection for
every notification message, the overhead can be high. In particular, the
connection establishment may increase the user-perceived latency by a factor
of 3 (i.e., from one half round-trip time to one and a half round-trip
time). Assuming the average notification message size to be 128 bytes, the
connection setup and tear-down increases the bandwidth usage from 168 bytes
per message to 448 bytes per message (i.e., 7 additional packets: 3 packets
in the three-way handshake connection setup, and 4 packets in the connection
teardown).
Figure 4:
Size distribution of notification messages (including duplicates).
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One suggestion for reducing the overhead of connection setup and teardown is to use
persistent connections [13], i.e., reuse a TCP connection for multiple
transfers. In our case, the servers sending the notification messages can
maintain persistent connections with the gateways of the wireless ISPs and then
send all messages on this connection.
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Lili Qiu
2002-04-17