Tail Drop, or Drop Tail, is a simple queue management algorithm used by Internet routers to decide when to drop packets. In contrast to the more complex algorithms like RED and WRED, in Tail Drop all the traffic is not differentiated. Each packet is treated identically. With tail drop, when the queue is filled to its maximum capacity, the newly arriving packets are dropped until the queue has enough room to accept incoming traffic.
The name arises from the effect of the policy on incoming datagrams. Once a queue has been filled, the router begins discarding all additional datagrams, thus dropping the tail of the sequence of datagrams. The loss of datagrams causes the TCP sender to enter slow-start, which reduces throughput in that TCP session until the sender begins to receive acknowledgements again and increases its congestion window. A more severe problem occurs when datagrams from multiple TCP connections are dropped, causing global synchronization; i.e., all of the involved TCP senders enter slow-start. This happens because, instead of discarding many segments from one connection, the router would tend to discard one segment from each connection.
Further reading 
- Comer, Douglas E. (2005-07-10). Internetworking with TCP/IP (5 ed.). Prentice Hall. ISBN 978-0131876712..
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