This limit of 100 TCBs in the `SYN Rcvd` state was chosen to protect the TCP entity from the risk of overloading its memory with too many TCBs in the `SYN Rcvd` state. However, it was also the reason for a new type of Denial of Service (DoS) attack :rfc:`4987`. A DoS attack is defined as an attack where an attacker can render a resource unavailable in the network. For example, an attacker may cause a DoS attack on a 2 Mbps link used by a company by sending more than 2 Mbps of packets through this link. In this case, the DoS attack was more subtle. As a TCP entity discards all received `SYN` segments as soon as it has 100 TCBs in the `SYN Rcvd` state, an attacker simply had to send a few 100 `SYN` segments every second to a server and never reply to the received `SYN+ACK` segments. To avoid being caught, attackers were of course sending these `SYN` segments with a different address than their own IP address [#fspoofing]_. On most TCP implementations, once a TCB entered the `SYN Rcvd` state, it remained in this state for several seconds, waiting for a retransmission of the initial `SYN` segment. This attack was later called a `SYN flood` attack and the servers of the ISP named panix were among the first to `be affected <http://memex.org/meme2-12.html>`_ by this attack.
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