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I'd like to know the general cost of creating a new connection, compared to UDP. I know TCP requires an initial exchange of packets (the 3 way handshake). What would be other costs? For instance is there some sort of magic in the kernel needed for setting up buffers etc?
The reason I'm asking is I can keep an existing connection open and reuse it as needed. However if there is little overhead reconnecting it would reduce complexity.
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TCP has a high processing overhead due to overhead in the operating system and multiple copies of data in memory. This makes it highly inefficient for high-performance and low latency networks. RDMA is widely used in high-speed, low latency networks like AI training clusters.
Because creating a new TCP connection is expensive. That's why you establish one connection and then use it for thousands of chunks of data. Also consider the new connection creation might not be limited to only the TCP handshake.
Connection initiation overhead can seriously impair common kinds of web applications that make database connections for each web request. In this common scenario, each connect-query-disconnect cycle is so slow that the architect solves the problem by increasing the number of concurrent connections.
TCP uses a three-way handshake to establish a reliable connection. The connection is full duplex, and both sides synchronize (SYN) and acknowledge (ACK) each other. The exchange of these four flags is performed in three steps: SYN, SYN-ACK, ACK, as shown in Figure 5.8.
Once a UDP packet's been dumped onto the wire, the UDP protocol stack is free to completely forget about it. With TCP, there's at bare minimum the connection details (source/dest port and source/dest IP), the sequence number, the window size for the connection etc... It's not a huge amount of data, but adds up quickly on a busy server with many connections.
And then there's the 3-way handshake as well. Some braindead (and/or malicious systems) can abuse the process (look up 'syn flood'), or just drop the connection on their end, leaving your system waiting for a response or close notice that'll never come. The plus side is that with TCP the system will do its best to make sure the packet gets where it has to. With UDP, there's no guarantees at all.
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