The IP over ATM Mailing List Archive by date

[manfredi@engr04.comsys.rockwell.com (Albert E. Manfredi)]

Pat Medved:

> 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?

Actually, the CRC is for the AAL5 frame rather than a cell-by-cell check.
 
> I was wondering if most people think not doing the checksum
> would be safe.

This is very much like IP over Ethernet or FDDI. The nice thing about the 
checksum with the IP packet is that it goes with the IP frame across any 
boundary. Is that not one of IP's claims to fame (being independent of the 
underlying network)?
 
> 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.

This is true, and it's also true for IP over Ethernet or FDDI. Especially 
if the destination is within the same network, the IP checksum is pretty 
superfluous if the network has its own CRC.
 
> Any comments or reasons why this should not be done?

Consistency primarily would be my vote.

Bert
manfredi@engr05.comsys.rockwell.com