What is the TCP three-way handshake?

The opening exchange of SYN, SYN-ACK and ACK in which client and server agree on starting sequence numbers before sending data.

The name for the internet's protocol suite: IP addresses and routes packets between networks, and TCP (or UDP) carries data between programs on top of it.

What is the difference between TCP and UDP?

TCP is connection-based and guarantees delivery and order by resending lost data. UDP sends independent datagrams with no handshake or resends, which is faster for real-time traffic that can tolerate loss.

HTTP, HTTPS and SOCKS5 proxies all relay TCP connections. UDP support exists in the SOCKS5 standard but is not offered by every provider.

What is a TCP fingerprint?

The pattern of TCP and IP header values a system sends, such as the initial TTL, window size and option order. It hints at the operating system and, through a proxy, belongs to the proxy server.

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What Is TCP (Transmission Control Protocol)?

TCP (Transmission Control Protocol) is the internet protocol that delivers data between two programs reliably and in order. It opens a connection with a handshake, numbers every byte, and resends anything that gets lost, which is why web pages, APIs, email and file transfers run on it.

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how it works

How a TCP connection works

TCP is specified in RFC 9293. Before any data moves, the two sides agree on starting sequence numbers in a three-way handshake. After that, every segment carries a sequence number, the receiver acknowledges what arrived, and the sender retransmits what was not acknowledged. Flow control stops a fast sender from flooding a slow receiver, and congestion control slows everyone down when the network is overloaded.

The handshake costs one round trip before any data moves, and TLS adds another on top. That is why reusing connections makes HTTP clients much faster.

StepWhat happens

StepSYNThe client asks to open a connection and sends its starting sequence number
StepSYN-ACKThe server agrees, acknowledges it and sends its own sequence number
StepACKThe client acknowledges; the connection is open and data can flow
StepFIN / ACKEither side closes its half when it has finished sending

why it matters

Why TCP matters for proxy users

Web proxies are TCP relays. An HTTP proxy, a CONNECT tunnel and a SOCKS5 proxy all accept your TCP connection and open a second one to the website. The website's TCP peer is the proxy, which has consequences beyond the IP address.

Every CleanProxies product relays TCP: HTTP(S) on all of them and SOCKS5 on static ISP proxies, with unlimited bandwidth on ISP.

TCP: what it means for you

Your TCP connection ends at the proxy, so low-level details the site sees, such as the initial TTL and TCP options, come from the proxy server's operating system, not yours.
Passive OS fingerprinting tools such as p0f compare those details with the User-Agent. A request that claims Windows but whose TCP stack looks like Linux is an inconsistency some anti-bot systems score.
Connection reuse (keep-alive, HTTP/2) through a proxy saves a handshake per request and keeps a rotating session on one exit for longer.
Proxies built for HTTP and SOCKS5 relay TCP; UDP needs separate support. See UDP.

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