w control as explained above. The problem that cannot be managed by the end-to-end windowing mechanism is congestion in the interconnecting network. If the Host 1 Host 2 Application closes TCP connection, TCP sends FIN FIN, seq = X CTL=FIN,ACK Receives FIN segment, informs application ack=x+1, CTL=ACK Receives ACK Application closes FIN seq=y, TCP connection, ack=x+1, CTL=FIN,ACK TCP sends FIN Receives FIN+ACK, Sends ACK ACK, lack = y+1 CTL=ACK Both directions closed Figure 6.52 Closing the TCP connection. Introduction to Telecommunications Network Engineering destination host is able to handle arriving data, it does not reduce its window size although buffers in routers on the way may overflow. The alarm about this kind of situation may arrive to the sending host as an ICMP message or its transmission timer expires because of delayed acknowledgment. If this occurs the source host reduces its transmission rate SO that it transmits only one TCP segment and waits for its acknowledgment (this is known as a "slow start"). If the acknowledgment arrives in time the source host starts to increase its transmission rate and transmits two segments before stopping to wait for acknowledgments. The number of segments before waiting acknowledgments is increased until the maximum window size is reached or congestion occurs again. Naturally, if there is no congestion, the transmis- sion window (controlled by the receiver) is the only one that sets limits to the transmission rate. 6.6.9.3 UDP The UDP is an alternative to TCP, which is used if reliable connection- oriented service is not required. UDP offers transmission with a minimum of protocol handling. No connection is established between end points; the source host just sends separate datagrams toward the destination. The UDP header is very short, 8 bytes instead of 20 (or more) bytes of TCP header, and it is shown in Figure 6.53. Port numbers indicate the application or process for which the message is intended. Length is the number of octets in the packet including header 