> "Richard B. Johnson" <root@analogic.com> wrote:
>
> > Well when I started in "Networking" about a thousand years ago when we
> > used puffs of smoke for communication, a "Datagram" was a single packet
>
> A lot has changed since then.
>
> > signaling. Data would go in-band. UDP (Unix Datagram Protocol) are
>
> User Datagram Protocal.
>
> > (were) datagrams with an IP Header. If the IP "layer" now allows one
> > to fragment, i.e., use datagrams that no longer consist of a single
> > packet, then the whole reason for using datagrams has disappeared.
>
> Fragmentation at lower layers has little to do with why one uses
> UDP to tranport a protocal. What happens when your "single packet"
> hits a network with a smaller MTU than yours?
Gentlemen, gentlemen! Take a look at RFC 791: Internet Protocol. On
page 2 section 1.4 (Operation):
The internet protocol implements two basic functions:
addressing and fragmentation.
...
The internet modules use fields in the internet header to
fragment and reassemble internet datagrams when necessary
for transmission through "small packet" networks.
So the total length of a UDP whateveritis can be 65,535 octets minus
epsilon, because it sits on top of IP.
And in RFC 768: User Datagram Protocol:
This protocol provides a procedure for application programs to
send messages to other programs with a minimum of protocol
mechanism. The protocol is transaction oriented, and delivery
and duplicate protection are not guaranteed.
Basically UDP provides a port namespace on top of IP and that's about
it. Whether you think that is a datagram is up to you.
Mark H. Wood, Lead System Programmer MWOOD@INDYVAX.IUPUI.EDU
Those who will not learn from history are doomed to reimplement it.