On Sat, 2007-01-20 at 09:47 +0100, Patrick McHardy wrote:
> Russell Stuart wrote:
> > b. There is no compatibility problem.
>
> Again, (b). You seem to have something in mind, it would be
> easier if you would just explain exactly where you think there
> is a problem.
I though I had :(.
Consider:
Line speed is 256 K bits/sec.
Protocol: ADSL/ATM (PPPoE VC/LLC) (Overhead is 42 bytes + cell pad).
Kernel HZ is 1000.
cell_log = 8.
Below is a table which shows the RTAB that would be sent
to a pre-STAB kernel:
IP Datagram Packet Size Packet Size Ticks to
Packet Size Seen by Kernel On the Wire Send packet
RTAB[0]=2 -14..-7 0..7 53..53 1.656
RTAB[1]=2 -6..1 8..15 53..53 1.656
RTAB[2]=3 2..9 16..23 53..106 3.313
RTAB[3]=3 10..17 24..31 106..106 3.313
...
RTAB[9]=5 58..63 72..79 106..159 4.968
RTAB[10]=5 64..71 80..87 159..159 4.968
Below is the same thing for a post-STAB kernel:
IP Datagram Packet Size Packet Size Ticks to
Packet Size Seen by Kernel On the Wire Send packet
RTAB[0]=0 - Undefined as no STAB entry is 0.
RTAB[1]=0 - Undefined as no STAB entry is 0.
...
RTAB[5]=0 - Undefined as no STAB entry is 0.
RTAB[6]=2 -14..-7 0..7 53..53 1.656
RTAB[7]=2 -6..1 8..15 53..53 1.656
RTAB[8]=3 2..9 16..23 53..106 3.313
RTAB[9]=3 10..17 24..31 106..106 3.313
...
RTAB[15]=5 58..63 72..79 106..159 4.968
RTAB[16]=5 64..71 80..87 159..159 4.968
The two RTAB's are different. Thus you must send
different RTAB's to pre-STAB and post-STAB kernels.
How is "tc" to decide which one to send? I did add
code that checked uname once to solve a very
similar problem in "tc", but that got my wrist
slapped.
Replacing RTAB with STAB would solve the problem, BTW,
as the post-STAB kernel would ignore the RTAB.
It would also solve another problem. The granularity
of RTAB sucks for VOIP (my area of interest). Eg on
a 2 M bit link, one ATM cell takes 0.0848 ticks to
send, two cells 0.170 ticks, three cells 0.2544 ticks.
RTP voice packets are typically two or three cells.
RTAB only holds an integral number of ticks of course,
making the current traffic control engine useless for
VOIP links with speeds of around 2.5M bit or above.
This could be fixed in an STAB implementation.
-
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
