Acknowledgments

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Acknowledgments

Acknowledgments are a very important component necessary to ensure the reliable delivery of packets. As the receiving window receives packets, it sends acknowledgments to the sending window that the packets arrived intact. When the send window receives acknowledgments for data it has sent, it slides the window to the right so that it can send any additional data stored in memory. But it can only slide over by the number of acknowledgments it has received. By default, a receive window sends an acknowledgment for every two sequenced packets it receives. Therefore, assuming no network problems, if the send window in Figure 2.20 sends eight packets to the receive window on the other machine, four acknowledgment packets come back. An acknowledgment for packets 1 and 2, 3 and 4, 5 and 6, and 7 and 8. The sending window slides over to the next eight packets waiting to be sent and sends those out to the receiving window. In this manner, the number of acknowledgments sent over the network is reduced, and the flow of traffic is increased.

As long as the acknowledgments begin flowing back regularly from the receiving machine, data flows smoothly and efficiently. How-ever, on busy networks, packets can get lost and acknowledgments may be delayed. Because TCP guarantees delivery and reliability of traffic flow, the window cannot slide past any data that has not been acknowledged. If the window cannot slide beyond a packet of data, no more data beyond the window is transmitted, TCP eventually has to shut down the session, and the communication fails.

Each machine is therefore instructed to wait a certain amount of time before either retransmitting data or sending acknowledgments for packets that arrive out of sequence. Each window is given a timer: the send window has the Retransmit Timer and the receive window has the Delayed Acknowledgment Timer. These timers help define what to do when communication isn’t flowing very smoothly.

In the sending window, a Retransmit Timer is set for each packet, specifying how long to wait for an acknowledgment before making the assumption that the packet did not get to its destination. After this timer has expired, the send window is instructed to resend the packet and wait twice as long as the time set on the preceding timer. The default starting point for this timer is approximately 3 seconds but is usually reduced to less than a second almost immediately. Each time an acknowledgment is not received, the Retransmit Timer doubles.

For instance, if the Retransmit Timer started at approximately 1 second, the second Retransmit Timer is set for 2 seconds, the third for 4 seconds, the fourth, 8 seconds, up to a fifth attempt that waits 16 seconds. The number of attempts can be altered in the Registry, but if after these attempts an acknowledgment still cannot be received, the TCP session is closed and errors are reported to the application. Figure 2.21 illustrates the resending of data after the first Retransmit Timer has expired.

The Registry location for changing the number of times to retry a transmission is in the following subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

The Registry parameter and value is:

TcpMaxDataRetransmissions (REG_DWORD).

The default value is 5.

In the receiving window, a Delayed Acknowledgment Timer is set for those packets that arrive out of order. Remember, by default an acknowledgment is sent for every two sequenced packets, starting from the left-hand side of the window. If packets arrive out of order (if, for instance, 1 and 3 arrive but 2 is missing), an acknowledgment for two sequenced packets is not possible. When packets arrive out of order, a Delayed Acknowledgment Timer is set on the first packet in the pair.

In the parenthetical example, a Timer is set on packet number 1. The Delayed Acknowledgment Timer is hard-coded for 200 milliseconds, or 1/5 the Retransmit Timer. If packet 2 does not show up before the Delayed Acknowledgment Timer expires, an acknowledgment for packet 1, and only packet 1, is sent. No other acknowledgments are sent, including those for packets 3 through 8 that might have appeared. Until packet 2 arrives, the other packets are considered interesting, but useless. As data is acknowledged and passed to the Application layer, the receive window slides to the right, enabling more data to be received. Again though, if a packet doesn’t show up, the window is not enabled to slide past it. Figure 2.22 illustrates the Delayed Acknowledgment Timer in action.

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