Hi everybody,
let me share my understanding with u.
At DLL, CRC is done and this way only ethernet header
correctness is checked.
AT IP level, checksum is there to detect bad dgram.
Here checksum on the header only. Since some header
fields change (e.g., time to live), this is
recomputed and verified at each point that the
internet header is processed.
Here there is no error control on further data.
And at TCP level, checksum is calculated for the whole
data following the TCP header including header.
But now, problem may occur when something like,
control singnals stop functioning in the network
interface and the interface stores zeroes at the
places of real data.
These "all zero" messages appear to have valid
checksums. Although messages containing undetected
errors will occasionally be passed to higher levels
of protocol, it is likely that they will not make
sense at that level. In the case of TCP most such
messages will be ignored, but some could cause a
connection to be aborted.
This way its always better to have a checksum
computing routine at each layer.
-
Aneuya.
--- "Philip J. Nesser II" <phil(_at_)nesser(_dot_)com> wrote: >
Jacob,
There are many reasons for their to be multiple
checksums. Try to think of
a large network with many types of network media,
MTU sizes, and
intermediate nodes. The whole world is not
ethernet.
Here is a trivial example (happened to a client of
mine so I know that it is
not just a hypothetical example):
1. Packet goes from host A to Router 1 via
ethernet.
2. Packet is read by ethernet driver on Router 1
and the ethernet CRC is
good and the packet is accepted.
3. Router 1 has a bug which corrupts the packet in
memory.
4. Router 1 sends the packet to Host B via another
ethernet connection.
5. Host B gets the packet and the ethernet CRC is
good and the packet is
accepted.
6. The IP payload has been corrupted by the bug in
Router 1, and Host B has
no idea that the data is wrong.
---> Phil
-----Original Message-----
From: jacob mathews
[mailto:jac_mat_ietf(_at_)yahoo(_dot_)co(_dot_)in]
Sent: Saturday, September 01, 2001 12:16 AM
To: Gaurang Pandya; Chandra Shekar Reddy
Challagonda; jacob mathews;
ietf(_at_)ietf(_dot_)org
Subject: Re: TCP/IP check sum
Hi,
I understand that packet segmentation occurs at
transport layer and the fragmentation to MTUs occur
at
the network layer.To reduce software overheads the
transport layer segments the packets to sizes of
MTUs.In such a scenario each packet has a TCP and IP
checksum computed and placed in the packet.In the
data
link layer in case of ethernet MAC, CRC is computed
and appended.Now many vendors give TCP/IP checksum
offload feature in ethernet MAC device.When CRC
check
is already done on the frame (which is a more robust
error detection scheme than TCP/IP checksum!!)why
again TCP checksum has to be computed at all.The
status of the packet(good or bad)is already
known!!Then isn't the check sum offload hardware
feature redundant and adds to silicon cost!!
Expecting lot of responses!!
--jacob
--- Gaurang Pandya <gaubrig(_at_)yahoo(_dot_)com> wrote: > Hi
chandra,
All this packet segmenting and merging at
Transport
layer it self. These things never take place at
data link layer.
Dont you think so..??
Gaurang.
--- Chandra Shekar Reddy Challagonda
<chandrashekar(_dot_)challagonda(_at_)WIPRO(_dot_)COM> wrote:
Hi Jacob
CRC check is done at the Ethernet MAC layer(or
Data
Link Layer
conceptually). This error detection process is
to
detect the errors at
the MAC layer packets, which is the errors
caused
by
the physical layer
or MAC layer.
In some products single big TCP/IP packet(Jumbo
Packet) can broken into
the small packets at the MAC layer depending on
the
maximum size of the
packet that link supports(Some thing like MRU).
So
the error in the
datapackets which are broken and sent by the MAC
layer, is detected by
the CRC in the MAC layer. When sender sends the
broken data packets
each separately, at the reciever side also,
these
all packets are
joined into single packet again before passing
it
to
TCP/IP layer. To
check the errors in joined TCP/IP packet, frame
check is used at that
layer. This is to detect the errors occuring in
the
TCP/IP packets due
to breaking up and joining up.
So both has relevant significance at each layer.
Chandra
----- Original Message -----
From: jacob mathews <jac_mat_ietf(_at_)yahoo(_dot_)co(_dot_)in>
Date: Tuesday, August 28, 2001 1:35 pm
Subject: TCP/IP check sum
Hi,
On a etherent frame CRC is used for error
detection.
For TCP/IP frames frame check is done by using
16bit
1's complemet addtion.
Why is TCP/IP frame check required when the
same
is
verified by the ethernet MAC?
What is then the motivation of providing
TCP/IP
checksum overload by some products?
Waiting for responses.
thanking you,
jacob
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