readme_how_to

常用于卫星通信方面

README for ppp_current_tx_len.

Th following application, along with gw_route_cmdr will allow
the SCPS gateway to keep the


Step 1 - Modify the PPP driver within the Linux Kernel.

The first piece of the effort is to get the current transmit queue
length of the PPP interface.  I was hoping this information would be in
bytes, however it is in packets.  The kernel mods to get this is only about
10 lines of code.

There are two file that need to be changed under the kernel source code

First edit the file ./drivers/net/ppp_generic.c

In the function ppp_net_ioctl after the SIOSGPPPSTATS: case and before
the SIOCGPPPCSTATS: case add the following


        case SIOCGPPPQLEN:
                qlen = ppp->dev->qdisc->q.qlen;
                if (copy_to_user(addr, &qlen, sizeof(qlen)))
                        break;
                err = 0;
                break;

Now edit the file ./include/linux/if_ppp.h

After the following line (approx line number 156)

#define SIOCGPPPCSTATS  (SIOCDEVPRIVATE + 2)

Add the following

#define SIOCGPPPQLEN    (SIOCDEVPRIVATE + 3)    /* PDF XXX PDF XXX */

Now compile the kernel and the modules the way you normally.

===================================================================

Step 2 -  Quick Description of ppp_current_tx_len

The ppp_current_tx_len applcation issues ioctls to get the current queue
length from the ppp interface.  This applicaton will periodically query
the kernel for this length every so often and pass this information to
SCPS. The idea here is to keep the ppp queue full enough so always has
data to send, but not too ful to cause issues with SCPS.  The notion is
to have a threshold and an increment value.  Essentially periodically
the queue length is checked against a threshold.  If less than the
threshold, SCPS will be told it can pass a certain number of bytes down
to the ppp driver.

A thought might to check the queue every second and pass up 3000 bytes
(2 full sized packet) if the current queue len is less than 5 packets. - 
These are samples and will have to be tuned based on the characteristics
of the individual links.

In addition this application will check to see if the link transitions
up or down.  If the link status transitions from UP to DOWN, then a message
will be sent to SCPS indicating you should go into Persist with a 1
second timeout until a link up indication is obtained.  NOTE:  A link up
indication may either be an ACK from the far end or an explicit signal
from this applications.  If the link status transitions from DOWN to UP,
then a mesage will be sent to SCPS indicating link is available.

To compile ppp_current_tx_len cd into utils/ppp_current_tx_len and
just type make.

===================================================================

Step 3 -  Quick Description of gw_route_cmdr

SCPS currently has a mangement interface to receive external stimuli.
This interface is a standalone SCPS currently is a standalone command line
application called 'gw_route_cmdr' and it located in the utils directory of the
SCPS RI.  The notion is for ppp_current_tx_len to issue system() commands to
execute the gw_route_cmdr application.  gw_route_cmdr will pass the information
directly to SCPS RI.  Gw_route_cmdr will need to be pathed so when system()
calls it, it will be found.

To compile gw_route_cmdr cd into utils/gw_route_cmrd and just type make


===================================================================

Step 4 -  Quick Description of the SCPS RI

4) SCPS has already been modified to augment the standard rate control technique
will a flow control token bucket mechanism.  These token are (bytes) allow to be
emitted are received from the gw_route_cmdr application.  The maximum size of the
flow control bucket is configurable via SCPS.

To compile the SCPS RI for this feature the following configuration options
need to be added to the options in the source directory

 --flow_control_thresh-yes  --gateway_ri_console=yes

In addition you need to add a directive in the SCPS resource file.

Set AIF_CC to 3 will enable the flow control feedback for A side
or
Set BIF_CC to 3 will enable the flow control feedback for B side

Setting BIF_FLOW_CONTROL_CAP to the number of flow control bytes the
gateway can accumulate on the A side
or
Setting AIF_FLOW_CONTROL_CAP to the number of flow control bytes the
gateway can accumulate on the B side


Step 5  - Change the default ppp queue length from 3 to 10

yYou may need to increaset the defaullt length on the interface for
example to increase the ppp'sinterfaces transmit queue length to 10 type

ifconfig ppp0 txqueuelen 10


Setp 6 - Run the gateway

Tyep
./gateway


Step 7 - Run the ppp_current_tx_len like the following

./ppp_current_tx_len -g 127.0.0.1 -t 5 -w 1 -i 2 -v -c -r 1 ppp0

-g is the IP address of the gateway; since they are on the same box
   the loopback address works great

-t is the ppp queue length threshold in packets.  If the threshold is
   met or exceeded no message is passed to the proxy

-w is how often in seconds to we check the queue.  Check it every second.

-i is how many packets it will signal to SCPS that is can send if we are
   below the threshold.  NOTE that SCPS works in bytes so each packet
   will correspond to 1500 bytes.

-v is used to display the current queue length on the console.

-c is used to send the messages to SCPS.  If the -c option is not present
   this application can be used just for monitoring queue size.

-r this is actually some internal number a value of 0 refers to the AIF
   interface and a value of 1 refers to the BIF interface. 

At the end you need to list add the interfaces that will be checked.

Also note that ppp_current_tx_len will call gw_route_cmdr.  Please make
sure that gw_route_cmdr is in a place the ppp_current_tx_len can find.

Step 8

Check the display of ppp_current_tx_len and see what you see.


Step 9

Start an FTP session and see what happens