Re: [ns] how to specify the byte rate of tcp sender

[Brian Lee Bowers <bowersb@lanl.gov>]

On Wednesday 25 July 2001 03:35, Neundorf Alexander wrote:
> Hi,
>
> I wanted to do something like a cbr tcp source, I took the Telnet
> class as example. I did the following:

Little clarification, please.  Are you trying to make a TCP Agent 
that sends at a fixed rate?  Are you trying to make an Application 
that generates data at a fixed rate to be sent using TCP?

> mainly I changed this function:
>
> void MyTestApp::timeout()
> {
>    if (running_)
>    {
>       int sendNow=bitrate_/80;
>       agent_->sendmsg(sendNow);
>       timer_.resched(0.1);
> };

OK.  You are creating a new Application, not a TCP Agent.
>
> Now my tcp source generates packets 10 times per second, but their
> size is *always* 1000 bytes. If I set bitrate_=64000, it should
> send 800 bytes at once. So my received bitrate is 80.000 bit per
> second (10 x 1000 bytes per second, x 8 bit per byte) instead of
> the expected 64000. I already found that TCPAgent::output() uses
> hdr_cmn->size, which is 1000. What am I doing wrong ?

TCP has a field, size_, that gets assigned the value 1000.  size_, 
through a number of function calls, gets put into the datalen_ field 
of the common header.  hdr_cmn::access(p)->size() accesses datalen_ 
to tell how many bytes of data are carried in the packet payload.

I assume that you aren't changing the value, so you are getting the 
default size for a TCP packet - 1000 bytes of data per packet.

There are several things you are doing wrong:
1)	you are getting the terminology mixed up.  You are developing an
	Application.  Just because you want the Application to send its data
	through TCP doesn't make it a TCP source.
2)	you are using an awful lot of magic numbers in your code.  Why not
	make the interval a constant - say MY_INTERVAL.  With the constant,
	you can then toss the magic number 80 away, making the line read
	something like	int sendNow = bitrate_ / (8 / MY_INTERVAL);.  Change
	your mind about how often to send packets, make a single change my
	way; make (at least) two changes your way.
3)	you are doing something creative that has exposed a flaw in ns :)
	sendmsg takes a number of bytes to be sent, but doesn't really use
	that number anywhere useful.  If sendmsg is going to transmit a
	packet, seems like it could use the size it is given to properly
	initialize the packet.