TCP requires two packet transfers to set up the connection before it can send data

wHTTP重用现存连接来减少TCP建立时延。

HTTP The Definitive Guide

4.2.3 TCP Connection Handshake Delays

When you set up a new TCP connection, even before you send any data, the TCP software exchanges
a series of IP packets to negotiate the terms of the connection (see Figure 4-8). These exchanges can
significantly degrade HTTP performance if the connections are used for small data transfers.

Here are the steps in the TCP connection handshake:
1.

To request a new TCP connection, the client sends a small TCP packet (usually 40-60 bytes)
to the server. The packet has a special "SYN" flag set, which means it's a connection request.
This is shown in Figure 4-8a.
2.

If the server accepts the connection, it computes some connection parameters and sends a
TCP packet back to the client, with both the "SYN" and "ACK" flags set, indicating that the
connection request is accepted (see Figure 4-8b).

3.

Finally, the client sends an acknowledgment back to the server, letting it know that the
connection was established successfully (see Figure 4-8c). Modern TCP stacks let the client
send data in this acknowledgment packet.
The HTTP programmer never sees these packets—they are managed invisibly by the TCP/IP
software. All the HTTP programmer sees is a delay when creating a new TCP connection.
The SYN/SYN+ACK handshake (Figure 4-8a and b) creates a measurable delay when HTTP
transactions do not exchange much data, as is commonly the case. The TCP connect ACK packet
(Figure 4-8c) often is large enough to carry the entire HTTP request message,
[4]
and many HTTP
server response messages fit into a single IP packet (e.g., when the response is a small HTML file of a
decorative graphic, or a 304 Not Modified response to a browser cache request).
[4]
IP packets are usually a few hundred bytes for Internet traffic and around 1,500 bytes for local traffic.
The end result is that small HTTP transactions may spend 50% or more of their time doing TCP setup.
Later sections will discuss how HTTP allows reuse of existing connections to eliminate the impact of
this TCP setup delay.