2 uni-directional TCP connection good

[Robert Watson]

On Sun, 22 Mar 2009, Yoshihiro Ota wrote:

>> On Fri, 20 Mar 2009, Yoshihiro Ota wrote:
>>
>>> 1. With TCP connections, only sender side can detect some communication 
>>> issues passively if happened.  By using two connections, you lost that 
>>> ability by your self.  I agree on this one.
>>
>> Could you expand a bit on this point?  While the connection creation 
>> process (usually) asymmetric, once the connection is built it's essentially 
>> the same state machine on both sides of the connection, and socket 
>> semantics with respect to the state machine are effectively identical. 
>> Application on both sides should be able to detect disconnect, monitor 
>> connection state using TCP_INFO, etc.
>
> What I meant was that there were cases when a receiver could not tell 
> weather no data was coming or communication was interrupted.  Once 
> connection is established, a route is available between a server and a 
> client.  Let's say this route is broken for some reasons, i.e. someone 
> unplugged a cable or a firewall started dropping or rejecting between these 
> server and client, a sender may not notice as soon as it happens but at 
> least, a sender knows a massages was not delivered right.  On the other 
> hand, receiver side does not have any idea that a message delivery failure 
> has happened at all or for a while unless using heartbeat messages in upper 
> layer.  KEEP_ALIVE option seems to be implementation dependent such that you 
> cannot assure TCP connection availability for every minute.

This is generally considered a robustness property rather than a fragility 
issue, but yes: if you need a liveliness property for idle connections with 
TCP, it's something you have to implement at the application layer, and many 
protocols indeed do this.  I don't see that this is an argument for using two 
TCP connections as opposed to one, however.  If you're interested in 
alternative protocols, however, SCTP allows a number of these protocol 
behaviors to be modified, and includes support for a heartbeat.

Robert N M Watson
Computer Laboratory
University of Cambridge