Subject: RE: TCP/IP Terms

From: "Michel Py" <[email-masked]>
Date: Sun, 29 Sep 2002 21:10:17 -0700

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.

---

From: "Bill Cunningham" <[email-masked]>
Date: Mon, 30 Sep 2002 00:34:46 -0400

http://dast.nlanr.net/Training/DCWJuly99/kai_tcpip/sld008.htm

    I looked at this page of one of the links you sent me. Notice at the
Internet and Transport levels it simply says, Application data (datagram ?),
TCP header, could this be a datagram, or maybe a packet. Then at the
Internet level, application data, TCP header and IP header. Now according to
rfc 1122, we know that -
 o application data, tcp header
 o application data, tcp header, and ip header.

Are both datagrams. Wow IMHO what a mix up. Unless you know the name of the
protocol and where you know (or think) it is in TCP/IP, you're lost.

---

From: Bill Sommerfeld <[email-masked]>
Date: Mon, 30 Sep 2002 10:05:11 -0400

I fail to see why this is such a pressing issue.  To be completely
unambiguous, you need to name the protocol.  That identifies the
layer, and then "packet" or "datagram" or what-have-you is
unambiguous.

						- Bill