Just getting started, there are probably some "easier" reads out there but
that book will definitely give you the goods on TCP/IP...
Regarding your question/statement, you are accurate that the "raw" Ethernet
frame format has DA, SA, EtherType, Data, and FCS - to be a valid frame it
just has to be between 64 and 1518 bytes (if we're including the 4 bytes of
the FCS in our calculations) - notice that the top end number is not 1500 -
the common "max" MTU size for Ethernet-attached devices talking IP. MTU is
a function of L3, not L2.
The IP header will indicate how many bytes of the Ethernet payload is
consumed by IP "stuff" - add this to the 14 bytes consumed by the
destination address (6), source address (6) and ethertype (2) to get the
total frame size (+ plus the trailing 4 bytes for the FCS). This statement
will be true unless adding all that up equals a number less than 60 (64
w/FCS) in which case the packet will be "padded" with 0's to make it a legal
Ethernet packet.
I think it is generally considered a good thing that packets aren't padded
to the full Ethernet size (or MTU) - it that were the spec, I'm thinking
that ATM would be a lot more popular as a LAN medium.
Hope this helps.
Ben
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
[EMAIL PROTECTED]
Sent: Friday, May 25, 2001 6:59 PM
To: [EMAIL PROTECTED]
Subject: Ethernet frame format [7:5996]
Dear Members List,
I've just started the track for CCNA and, following all the repeated advices
posted in this list, I started studing for Internetworking with TCP/IP, by
Douglas Comer.
The ethernet frame format stablishes as necessary information for the frame
as DA, SA, Type, Data Area(variable from 46 to 1500 bytes) and a trailer FCS
4 bytes.
I don't see how can we have different frame sizes correctly received, since
there is no information about the specific lenght for every single frame,
taking in account the asynchrounous nature of this communication.
I thought that the layer 3 would pad till the MTU was reached, but I saw a
trace on an ethernet network and I could see different frame sizes.
Thanks in advance,
Douglas Baltazar de Queiroz - Field Enginner
---
UOL: o melhor da Internet.
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