Cell Relay Archive

[qiwei@ecn.purdue.edu (Qiwei Xiao)]

In article <870276108.4917@dejanews.com>, manfredi@arl.bna.boeing.com writes:
   
   [ long text deleted ]

|> 
|> Note the definition of "connection" in Douglas Comer's book
|> _Internetworking with TCP/IP_:
|> 
|> connection: The path between two protocol modules that provides reliable
|> stream delivery service. In a TCP/IP internet, a connection extends from
|> a TCP module on one machine to a TCP module on the other.
|> 
|> ... ATM per se is not connection oriented,
|> according to Comer's definition. But SSCOP, TCP, or TCP/IP running _over_
|> ATM provide connection oriented service.


   Since Prof. Comer was primarily talking about TCP/IP and UDP/IP in his book,
so the definition of connection-oriented is limited to transport layer. In that 
case, connection-oriented usually implies reliability. (I am one of Prof. 
Comer's most faithful students, BTW). 

   Most people define connection-oriented as this: before a protocol data 
unit(PDU) containing payload from higher layer is sent, one or more PDUs 
*in the same layer* must be sent prior to that to "establish a connection". 
In some sense, connectionless is "stateless" meaning that in any time, it
(sender or receiver) is in one and only one state. While for connection-oriented, 
it is "stateful" meaning that incoming PDUs may trigger state changes and it 
must be in some certain state to do some certain operations. For example, it 
must be in "connect" state when you send a payload PDU and it must be in 
"disconnect" state when you can send a "connect" signaling PDU, which in turn
triggers a state change.

   In general, connection-oriented and connectionless have little to do with 
the duration or reliability of data streams, and they can be seen in any layer.
However, since the upmost layer an application sits on is the transport layer, 
(L5 and L6 are irrelevant here), people primarily concern the connection 
property of that layer and pay little attention to others because they have 
less impact to applications than L4.

Regrads,
Qiwei Xiao
ECE, Purdue University