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.