Bill,
Bill Cunnigham wrote:
When someone says to me 'datagram.' I don't know what level
of TCP/IP they're talking about. It could be IP datagrams at
Internet layer, or UDP datagrams at Transport layer. Datagram
only defines a connectionless protocol according to rfc 1122.
This is a good point. IMHO, "datagram" could be:
[ISO layers]
- UDP datagram (transport layer)
- ICMP datagram (network layer)
- IP datagram = a network layer animal that contains data encapsulated
from a connectionless transport protocol such as UDP.
- I think that broadcasts would fall into the datagram category, too.
And:
- IP packet = a network layer animal that contains data encapsulated
from a connection-oriented transport protocol such as TCP.
That being said, I would think that "datagram" without the "UDP" before
it is a network layer animal because this is the way it is understood in
the ISO world.
Now if someone says to me 'frame.' I think PPP first
off, not necessarily Network layer, if that's where
it is, rfc 1661 looks like it was written in accordance
with the OSI model. I'm not sure of that one.
The problem is that, IMHO, there is no such thing as a TCP/IP layered
model. Even the number of layers seems to be floating between 4 and 5.
[here are the first matches searching google for "tcp/ip model"]
http://www.pku.edu.cn/academic/research/computer-center/tc/html/TC0102.h
tml
http://www.indianest.com/computing/networking/n003.htm
http://www2.themanualpage.org/networks/networks_tcpip.php3
http://www.8052.com/tcpip/
http://www.unm.edu/~network/presentations/course/chap2/sld018.htm
http://dast.nlanr.net/Training/DCWJuly99/kai_tcpip/sld005.htm
In any case, lots of definitions seem to agree on the lower layer
encompassing the data link and physical OSI layers. This might be a
problem trying to define "frame".
This is one of the reasons I stick to the OSI model, because it is more
precise. I even like the definition of the two sub-layers that form the
data link layer:
+---+----------------------------------+--------------------+
! 4 ! Transport ! Segment !
+---+----------------------------------+--------------------+
! 3 ! Network ! Packet or Datagram !
+---+-----------+----------------------+--------------------+
! ! ! Logical Link Control ! !
! 2 + Data Link +----------------------+ Frame !
! ! ! Media Access Control ! !
+---+-----------+----------------------+--------------------+
! 1 ! Physical ! Bit !
+---+----------------------------------+--------------------+
When someone says "frame" to me, I think ESF, or SF, or 802.2, or
Ethernet_snap. And these do belong into the LLC sublayer. And frames
have nothing to do with access to the media (Token passing vs. CSMA/CD)
nor they have anything to do with Physical layer topics such as clock
frequency or wavelength.
Michel.