Frame or Packets — Make Sure You Get It Right!
August 6, 2009 by Greg Ferro · 9 Comments
It is reasonably common that someone starts to use the term “IP Frames” somewhere is your life. Asides from making it obvious that you are stupid, its really important to understand why Frame and Packets mean totally different things, even though they are really related.
OSI Model
Although the OSI model isn’t perfect (because the IETF and ITU(OSI) people don’t normally see things the same way), it does provide a very useful way of understanding how we design protocols.
Frames
As you can see, the TCP/IP model clearly defines a Network Access Layer, and Internet layer. The Network Access Layer is Ethernet, ATM, Frame Relay and so on. Anything that fits in this layer is a FRAME
Packets
The Internet Layer is where IP was defined. Anything in this layer IS A PACKET.
Closing
There is no arguing with this. When you write your documents, or discuss systems with your colleagues, make sure that you get your terms right.
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How are MPLS PDU’s called, then? Or, for example PPPoE PDU’s carried in EWS pseudowire over cell-mode MPLS, transported in SDH that has an underlying lambda-switched DWDM?
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OSI model is fine and I agree that people shouldn’t mix frames and packets, but somtims… just sometimes, there is an argument in everything
He-he, if it all was so simple.
Have a look at the IEEE 802.3as-2006, Page 6 (or clause 3.1.1 in the complete 802.3 which is newer than 2006), and you’ll see that they call “Frame” something between DA and FCS (inclusive both) and “Packet” something between Preamble and Extension (if any), inclusive both — see “Figure 3 – 1: Packet format”.
Interesting to bring that up… According to general understanding of OSI (I haven’t read actual documents, just books referring to it), L3 packet is carried in L2 frame. I.e. L3 PDU’s are considered to be payload part of L2 PDU. We call this process call data encapsulation.
Now, if you look at Figure 3 – 1 in IEE 802.3−2008 (page 49), frame is actually carried in the packet
. Confused, already? 
Yeah. At that point in time, the whle networking industry wasn’t very stable, and the ITU/IEEE hadn’t stabilised their debates. Because the OSI standard used the concept of a Protocol Data Unit (PDU) to encompass anything that was a modular data segment, it took a few more years for frames/packets to stabilise into what we use today.
I think its important to the use the same buzzwords when we communicate, that why it is accepted practice to use frames at L2, packets at L3. Even modern OSI definitions for IS-IS now use this convention.
Greg, your post reminds me of a recent interview I went to where I was filling in some basic test and corrected a couple of their questions with terms like IP Frame and Ethernet packet. When I was being interview by the guy that wrote the test the next day, he kept on saying “Ethernet packet”.
It’s a common mistake, some people just never take the time to think through the differences between frame and packets.
It’s really annoying though. Correct terminology is vital in our business, one mistake can leads to a lot of wasted money.
IMHO, the usage of IP Frame can be compromised if you think that the frame carries IP protocol.
What if it was IPx or Appletalk ?
IPX and Appletalk are Layer3 of the IETF model and therefore are packets and move across the network inside Ethernet, ATM, FrameRelay, X25 or SMDS or some other type of frame.
If you want to have an interesting discussion, what about IBM SNA traffic which uses Ethernet as a Network / Layer 3 protocol with a full signalling capability and session management layer. That gets confusing.
For IPx and Appletalk i wont mind calling a IPx Frame or an Appletalk Frame anologous to the IP Frame term.
But that was new about the IBM SNA framing that you mentioned.
Then its indeed confusing.
Appreciate deeper thoughts like that Gregg !!