Howard's correct. Think about voice - do you really want voice packets
retransmitted? Due to delay and sequencing issues, likely not...
Understanding each protocol and its capabilities is the best way to
understand where retransmission occurs. Some protocols have both error
detection and error correction mechanisms as part of its design. Some
protocols have simply error detection mechanisms but lack the capability to
have the missing information resent--in such a case, protocols at higher
layers are responsible for seeing data be retransmitted. And finally, some
protocols have no error detection or correction capabilities. In my
original "not quite sure" post, I purposely asked how certain data link
layer protocols detect the loss of a "packet" because of the four listed,
each fits into one of the three above categories.
Error detection and correction begins at the lowest layers of the OSI model
and works its way upwards through the protocol stack. As Howard pointed
out, not all applications find retransmission desirable so you might not
find retransmission capabilities anywhere within the protocol set used for
transmission.
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Howard C. Berkowitz
Sent: February 9, 2001 8:06 AM
To: [EMAIL PROTECTED]
Subject: RE: not quite sure...
>In the discussion of error correction, I think an error on my part has been
>missed. I was thinking about it and I wonder if this is entirely accurate:
>
>(concerning what happens after a frame is discarded on WAN link)
>
>"The end station will respond by acking the next packet it recieves with
the
>appriopriate (lower numbered) sequence number (of the missed packet). The
>originating station will
>get this ack (with the lower sequence number) see that the end station is
>requesting a packet out of sequence and the originating station will begin
>it's next transmission with the data from that particular sequence number."
>
>Is this correct?
Emphasis: end station. You are describing what TCP does. Routers
typically are unconcerned with TCP.
And again, not all applications need reliable links, so not all
applications will have retransmission ANYWHERE in the path.
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