slow tcp acks on loopback device

David S. Miller davem at
Fri Jul 22 20:08:16 GMT 2005

From: Steve French <smfltc at>
Date: 22 Jul 2005 14:56:59 -0500

> Noticing that the loopback device (at least on RHEL4) has an unfortunate
> mtu size 16384 (which is about 50 bytes too small for SMB read
> responses), I did try increasing the MTU slightly.  Changing that to
> 18000 did avoid the fragmentation and the 40ms delay - but what puzzled
> me was why setting TCP_NODELAY after the socket was created did not
> eliminate the delay on the ack and if there is a way to avoid the huge
> tcp ack delay by either doing something else to force client acking
> immediately or to do something on the client side of the stack to get
> the server to send the whole 16K+ frame - it looks like the tcp windows
> is 32K if the value in the tcp acks in the network trace is to be
> trusted.

TCP_NODELAY does not control ACK generation, instead it modifies
the Nagle algorithm behavior when sending data packets.

Please take networking discussions to netdev at which
is where the networking developers are.

More information about the samba-technical mailing list