he destination are not able to treat the ECN mechanism (Table 12.1). When the router detects the beginning of congestion through the WRED queue management mechanism, the two bits assume the value 11 (Table 12.1). These two bits are called CE (Congestion Experienced). The ECN field only assumes the CE value if this field previously had the value ECT(0) or ECT(1) or if this field was already flagged as CE. Should the ECN field equal 00, the detection of incipient congestion would result in packet destruction. The CE value is used by the router to notify the recipient of congestion onset. The ECN mechanism further comprises two bits in the TCP header (Figure 12.6). These two bits complement the six flag bits. 3 456789012345678901234567890 CEUAPRSF Offset WCRCSSYI Window REGKHTNN Figure 12.6. ECN field in TCP header The ECE bit (ECN-Echo) is exploited by the recipient to warn the source that a beginning of congestion has been detected in the network. This bit is set when the recipient receives a packet whose ECN field in the IP header has the CE value. The CWR (Congestion Window Reduced) bit is used by the source to notify the recipient that it has indeed received the ECE flag. When the recipient receives the CWR flag, it stops transmitting the ECE flag. While the TCP connection is being opened, the client sends a TCP header with the SYN (connection opening), ECE and CWR flags set to one. The server responds with a TCP header, wherein the SYN (connection opening) and ECE flags are set to one. During that stage, the ECE and CWR bits are used to indicate the ability of terminals to treat the ECN mechanism, rather than the occurrence of congestion in the network. 226 Wi-Fi Integration to the 4G Mobile Network 12.3. MPTCP The MPTCP connection consists of a combination of several TCP connections, with each TCP connection transmitted on a path to appear as a normal connection for the different devices crossed: each TCP connection must start with the establishment procedure; each TCP connection must independently manage its