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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