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.
TCP requires two packet transfers to set up the connection before it can send data
原文：http://www.cnblogs.com/yuanjiangw/p/6360310.html