[mrtg] MRTG and bandwidth delay product

[nangineni praneeth]

Thanks for your reply.I studied in various books and web articles that the throughput of a link is limited by Round trip time. Actually the product which i was speaking about earlier is also referred to as TCP  receive window size  which is the maximum 
amount of received data, in bytes, that can be buffered at one time on the 
receiving side of a connection. The sending host can send only that amount of 
data before waiting for an acknowledgment .

The link below explains it in much more detail
http://www.speedguide.net/faq_in_q.php?category=89&qid=185

I would be glad if anyone could show me a way which would let me prove that by limiting the  ftp server to send data at constant rates i am able to operate the network efficiently

For eg: I am limiting the server to send traffic at 4kBps hence the effective throughput is xB/s and the effecting throughput of the network is yB/s when the server is sending the traffic at 8kB/s and so on.....

Regards
Venkat


--- On Wed, 12/31/08, Lyle Giese <lyle at lcrcomputer.net> wrote:
From: Lyle Giese <lyle at lcrcomputer.net>
Subject: Re: [mrtg] MRTG and bandwidth delay product
To: nv_2030 at yahoo.com
Cc: mrtg at lists.oetiker.ch
Date: Wednesday, December 31, 2008, 8:37 AM




  
nangineni praneeth wrote:

  
    
      
        Hello everyone....

        

I am working in a lab environment...This is how configured my
network.....

        

I designed a enterprise campus network   which operates at
100mbps...The enteprise edge router in the enterprise campus network
connects to the branch office router using a frame relay connection.
The frame relay PVC operates at 8kB/s...The clients in the branch
office connect to the branch office router with links operating at
100mbps...I have setup a filezilla ftp server in the enterprise campus
network  and limited the speed at which it can send the data to the
clients at 4kB/s....

        

So i am calcuting the rtt with MRTG and monitoring network with
MRTG....MRTG calculated the max round trip time to 340ms..between the
client in the branch office and the server in the enterprise campus
network.....I wanted to theoritically prove that the link cannot carry
more than x bytes of data or in other words because of certain latency
the effective bandwidth of the link is limited to only
xbytes/second...I used bandwidth delay product to do this......

        

BDP=0.340*8kBps=2.72kB/s

        

So this is saying that the effective throughput of the link with a
latency of 340 ms is 2.72kB/s......But when i am using MRTG to graph
bandwidth the max bandwidth it was showing was 4kBps which is what we
should expect...

        

My question is how can i  theoritically prove that the effective
throughput  of the link is xbytes per second ..I think i am
misunderstanding the bandwidth delay product.....Can anyone correct me
if i am wrong....

        

I wish you all a good new year ahead..

Regards

Venkat

        
      
    
  

Delay has a heavy effect on a stop-start transfer protocol like Xmodem
or Ymodem.  But for streaming protocols(FTP is a streaming protocol),
delay has a minimal effect on throughput.  



Limiting FTP to 4k over an 8k line, you are going to get close to the
4k max in practice, because bandwidth is not a limiting factor.  FTP is
a streaming protocol and depends on the TCP protocol to ensure that
what is being sent is what will be received and there is no
acknowledgement sent back by the sender for each packet(as Xmodem or
Ymodem did over dialup) unless there is a transmission error.  Since
there is no acks required by the sender, there is no delay in waiting
for those acks.



So I am wondering why you think transmission delay would introduce a
reduction in speed, since the sender is not waiting for any responses
on a per packet basis from the receiver?  I could see that in a
protocol like Xmodem, Ymodem or Zmodem where per packet acks needed to
be returned to the sender.



Lyle Giese

LCR Computer Services, Inc.







 



      
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