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Cloudflare was able to complete a TCP connection to the origin server, but did not receive a timely HTTP response. 525 SSL Handshake Failed Cloudflare could not negotiate a SSL/TLS handshake with the origin server. 526 Invalid SSL Certificate Cloudflare could not validate the SSL certificate on the origin web server.
After the side that sent the first FIN has responded with the final ACK, it waits for a timeout before finally closing the connection, during which time the local port is unavailable for new connections; this state lets the TCP client resend the final acknowledgment to the server in case the ACK is lost in transit.
This effect is most drastic when the TCP connection has a single outstanding segment, when it is able to avoid an RTO timeout; this is often the case for interactive connections, such as remote logins, and transactional protocols, such as HTTP requests, the conversational phase of SMTP, or SQL requests.
TCP Westwood+ is based on end-to-end bandwidth estimation to set the congestion window and slow-start threshold after a congestion episode, that is, after three duplicate acknowledgments or a timeout. The bandwidth is estimated by averaging the rate of returning acknowledgment packets.
Since TCP packets have a 40-byte header (20 bytes for TCP, 20 bytes for IPv4), this results in a 41-byte packet for 1 byte of useful information, a huge overhead. This situation often occurs in Telnet sessions, where most keypresses generate a single byte of data that is transmitted immediately.
Reasons for this message may include: the physical connection to the host does not exist (distance is infinite); the indicated protocol or port is not active; the data must be fragmented but the 'don't fragment' flag is on. Unreachable TCP ports notably respond with TCP RST rather than a destination unreachable type 3 as might be expected.
20: Yes: Assigned: Yes: File Transfer Protocol (FTP) data transfer: 21: Yes: Assigned: Yes: File Transfer Protocol (FTP) control (command) 22: Yes: Assigned: Yes: Secure Shell (SSH), secure logins, file transfers (scp, sftp) and port forwarding 23: Yes: Assigned: Telnet protocol—unencrypted text communications: 25: Yes: Assigned
The Transmission Control Protocol (TCP) detects packet loss and performs retransmissions to ensure reliable messaging. Packet loss in a TCP connection is also used to avoid congestion and thus produces an intentionally reduced throughput for the connection.
TCP reset attack, also known as a forged TCP reset or spoofed TCP reset, is a way to terminate a TCP connection by sending a forged TCP reset packet. This tampering technique can be used by a firewall or abused by a malicious attacker to interrupt Internet connections.
The Cable Guy: TCP Receive Window Auto-Tuning. The Web100 Data Bandwidth Testing. DrTCP - a utility for Microsoft Windows (prior to Vista) which can quickly alter TCP performance parameters in the registry. Information on 'Tweaking' your TCP stack, Broadband Reports. TCP/IP Analyzer, speedguide.net.