2 uni-directional TCP connection good?

[Yoshihiro Ota]

On Fri, 20 Mar 2009 01:56:56 -0700
Michael David Crawford <mdc at prgmr.com> wrote:

> Yoshihiro Ota wrote:
> 
> > I saw a program that opens 2 TCP connections.
> > One connection is only used for server to client messaging only
> > and the other connection is used only for client to server messaging.
> 
> 
> > 2. He also said that it would also waste network bandwidth.
> 
> You have a two-way communication no matter what you do.  But if you 
> don't actually use inbound direction, all it gets used for is the 
> receipt of ACK packets.
> 
> That is, the inbound connection is used to make the data transfer reliable.
> 
> If you don't have any payload data on the inbound connection, then the 
> outbound connection won't have any ACK packets.
> 
> If you're sending payload data, the ACK info can "hitchhike" along with 
> the payload packets, thus saving bandwidth.  But if you're not sending 
> any payload data at all, there will be packets transmitted which contain 
> the ACKs and nothing else.
> 
> The extra network overhead will be modest if you're sending a lot of 
> data all at once, say transferring a large file.  But if very little 
> data is sent per packet, say individual characters in a telnet 
> connection, the overhead would be very high.

So far until this, this was what I had though and learned about TCP connections.

> If you have a single connection with payload data in both directions, 
> then the ACKs will almost always ride along with some payload data.  The 
> only time a packet will contain nothing but an ACK will be when some 
> data was transmitted, but none is to be received at the time.
> 
> Mike

However, I had forgotten this case.  This can explain he even said that
using 2 TCP connection would cut the bandwidth into half.  This sounds
like the case he was referring to.

Thanks,
Hiro