Notification Message Size and Its Implications

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).

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.