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 <[EMAIL PROTECTED]> 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
> <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>
> > 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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