At 2:40 PM -0400 4/27/02, "Tom Scott" wrote:
>"Howard C. Berkowitz" wrote:
>
>> Not everything neatly falls into connectionless or
>> connection-oriented, and, indeed, different functions in ICMP have
>> different characteristics.
>
>It would be an interesting exercise to list the characteristics of
>connection-oriented connections (did I say that?). I'm looking for
>the title of the book in which the author defined all communication
>as connection oriented. He compared it to the empty set being a set.
>Will post when I find it.
>
>-- TT
It occurs to me that protocols might be a little simpler to
understand if people know some real theoretical principles, rather
than some of the oversimplifications that often are taught. Such
oversimplifications tend to lead to futile efforts to coerce
protocols into OSI, when the basic OSI model never planned for such
features.
A more useful and lower-level concept idea than that of connections
is state. State is the property that a host, router, etc., remembers
something about a packet, message, physical link, etc., after it has
transmitted the "something."
Consider a basic case of DNS query, assuming the server has the
information to answer the query. The client sends a DNS query
request over UDP, and remembers that it has sent it, so it can retry
if it gets no answer. In other words, the client retains state even
though it is using a connectionless protocol.
The server, on the other hand, is both connectionless and stateless.
Assuming pure DNS (no logging, etc.), the DNS server receives the
query, looks it up, responds, and then keeps no recollection that it
ever received the query.
From the standpoint of the server, this is completely connectionless
and stateless. From the standpoint of the client, it is
connectionless and stateful.
RSVP runs over connectionless UDP, but all routers and hosts in the
path are "soft state." In other words, they retain state as long as
they keep receiving RSVP messages before a timeout or receiving an
explicit teardown.
Soft state is blurry between connection-orientation and
connectionlessness. A general definition of connection orientation
includes:
1. There are explicit connection, information transfer, and
disconnection
phases.
2. Between the connection and disconnection phases, resources are
reserved
for the particular connection instance.
In RSVP, resources are reserved, but connection and disconnection
equivalents are generally implicit. Teardown messages are explicit
disconnects.
I can't think of an example of something that is connection-oriented
and stateless, but there certainly can be stateful and stateless
connectionless protocol. Further, not all hosts and routers in a
connectionless communication may or may not need to retain state.
Flow-based protocols are stateful but may or may not be
connection-oriented. The state involved is remembering that there is
an association between source and destination, but there may not be
explicit resource reservation. There may be reservation for a pool
of flows that share a common fate.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=42757&t=42757
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