[e2e] TCP Framing

[Jon Crowcroft]

 >>The idea of "best effort" is also a hard one to support, since "best" is very
 >>much open to interpretation

this isnt rocket science.

the reliability sematics of the UDP service are not distnguisible from
the IP service that carries a UDP payload.

the checksum is optional and UDPlite work is working on amaking it
partially optional:-) so even the bit delivery sematics aren't any
better or worse than IP 

other protocols above IP add different value. SCTP and TCP and RDP and
netblt and PGM and so on all add some notion of a lower failure
probability, as well as what quaint old iso people used to call
"signalled" errors only - i.e. apart fro ma few corner cases that
stone/partidge etal idneity in the engineering noise (literally) they
attempt to reduce unsignaled errors to as close to zero as acceptable
for the application (or Upper Layer Protocol as we used to say)....

btw, we used to have several types of datagrams in other networks
other than IP ones- for example, n the cambridge distributed system
there was a Universe Datagram Protocol (i even did a gateway to ip
once for it as well as a layering of IP on it - oh, and it was run o
na sort of ATM layer, except we only had 16bit cells:-) and  in X25
nets there WAS actually a datagram service - it was called Fast
Select, and was rarely implemented. Some folks in X.25 made mistakes
about the X.25 semantics and didn't get edge-to-edge reliability
beyond the _interface_ - in this case, while pedfantically,m they were
right, what ytou actyually got was an unreliable data transfer service
withotu signaled errors -for exampl the UK academic IP on 2Mbps X.25
service in the late 80s suffered interesting performance effects from
this...

other cases -oh, look at GPRS and the "reliable" (aka window and
GBN/retransmit) link layer it offers as an option - the effect depends
on the interface spec and how long you are acutally prepared to _wait_
for a signaled error too...so its quite subtle in reality....

oh, lets not getinto fragmentation debates too :-)...

cheers
jon
who is 43