[Sis-SCPS-INTEREST] SCPS + WAN Compression

[Frank Murray]

Hi all,

We are using SCPS in gateway mode to accelerate TCP traffic running over a
satellite link. Native IP is used as the network protocol. For congestion
control we are using Vegas. I've got a question about compressing data on a
per packet basis after it has left SCPS on the WAN side. The setup looks
something like this:

LAN INTERFACE
|
SCPS
|
COMPRESS/DECOMPRESS DATA
|
WAN INTERFACE
|
SAT LINK
|
WAN INTERFACE
|
COMPRESS/DECOMPRESS DATA
|
SCPS
|
LAN INTERFACE

At the moment the WAN bandwidth is limited by the throughout of the
uncompressed data. I am trying to find a way to inform the SCPS system that
instead of logging the transmit of x bytes for a particular segment, an
amount of x/[compression ratio] should in fact be logged. The current
compression ratio is available in real-time as each packet is transmitted.
Our theory then is that SCPS should be able to increase the rate at which it
transmits data over the WAN interface as it would then have a correct
reading of the amount of unack'ed data in the network.

Up to now I hold the following assumptions:

sWAN->snd_cwnd:		WAN Congestion Window. This is decremented when new
unack'ed data is introduced into the network and incremented when the data
is ack'ed.

sWAN->sndwin:		WAN Send Window. This seems to behave as
sWAN->snd_cwnd. I presume I'm wrong here.

sWAN->snd_awnd:		WAN Receive Window Of Remote Peer. Again this is
decremented when new unack'ed data is introduced into the network and
incremented when the data is ack'ed.

All these variables are decremented in tp_iovCoalesce. I have tried updating
this change proportionally using the current compression ratio. However the
transfer still seems to be limited by the BIF_RATE and BIF_BUF parameters
set at SCPS startup. If I set BIF_RATE and BIF_BUF to be much greater at
startup, the WAN throughput increases as expected. This however isn't an
option as it is not dynamic and causes problems if compression is turned off
or it's affects are negligible.

I also assume that these variables are incremented as the data is ack'ed and
these too would have to be changed proportionally based on the compression
ratio.

Finally I'm not sure what part the sWAN->rt_route structure plays in the
limiting of throughput on the WAN interface. I have noticed though that the
current_credit variable is updated in a similar way to the variables listed
above. The range of values seen here though seem to be lower.

Let me know if you need any further information. I'd greatly appreciate any
help forthcoming.

Best regards,

Frank Lynam