The IP over ATM Mailing List Archive by date

[Jim Forster <forster@cisco.com>]

> The AAL5 specification calls for AAL5 to do a CRC for each cell.
> With this being done, and the fact that lines today
> are better than when TCP was first implemented, is there
> still a need to do a checksum in TCP when running over ATM?

Yes!

Actually, the AAL5 CRC is over the entire AAL5 PDU (packet), not on each
cell, but that doesn't materially change the issue.

Recently, a certain large network encountered a terrible problems during
trials.  TCP checksums detected the problems and prevented hosts from
receiving corrupt data.  It seems there was a SAR function in certain boxes
in the net (not ours!) which was incorrectly reassembling packets from
incomplete set of cells, and then putting a valid CRC on the resulting
frame and transmitting it on a serial line.  TCP Checksums can detect this
failure mode (usually!).


  -- Jim


> I was wondering if most people think not doing the checksum
> would be safe.
> 
> I could see an option to TCP where as part of
> negotiation, we negotiate whether or not to do checksums.
> This would save on one of the biggest CPU users in
> TCP/IP processing.
> 
> Any comments or reasons why this should not be done?
> 
> 
> Pat Medved
> 
> medved@npr.legent.com
> 
> 
>