Do all protocols based on TCP use one socket per transfer?-CodePudding

I'm studying Socket Programming HOWTO and the author at some point says that

A protocol like HTTP uses a socket for only one transfer.

Is it because of the design of the HTTP protocol itself? Or is it because it is based on TCP, so all protocols based on it (e.g. UDP) must use one socket for only one transfer?

CodePudding user response：

That statement was probably written in 1996 or earlier. Since 1997, HTTP supports persistent connections, reusing the same TCP connection and the same socket for multiple queries.

CodePudding user response：

This statement is taken out of context. The context is to point out that TCP is not a message based protocol but an unstructured byte stream. And to have a message semantic one needs to have some way to determine where a message ends.

It then takes HTTP as an example where a message might simply end with a connection close and points out the limitations - namely only a single message per connection per direction. Then it goes on to describe how protocols can be designed without this limitation, i.e. having multiple messages per connection.

HTTP still can be used like this, i.e. have a single request and end with connection close. This is the design of HTTP version 0.9, but can still be done with HTTP/1. But with HTTP/1 it can also be used for multiple messages, one after the other. And with HTTP/2 it can do multiple messages in parallel, multiplexed over a single TCP connection. And HTTP/3 does not even use TCP anymore.

Do all protocols based on TCP use one socket per transfer?

Protocols are not limited to one connection ("socket") per message ("transfer"). Depending on the design of the protocol multiple messages can be send one after the other by having some pre-known message size or a clear message delimiter. Some protocols might send multiple messages in parallel by implementing a multiplexing layer on top of TCP. Some protocols might even use multiple TCP connections in parallel to deliver a single message, i.e. distributing the message over multiple connections.