Thank a ton for your explanation..Without knowing this stuff i spent quite a bit of time working on BDP trying to prove theoritically the effective throughput of a link..Of course in vague because of the reasons you gave....Anyways i can use this formula when trying to work on long fat networks with delay in it....I had a look into nistnet FAQ as well. They have given a reasonable explanation on different round trip times related to varying ping packet sizes which i will be looking into....
Regards Venkat --- On Wed, 12/31/08, Lyle Giese <[email protected]> wrote: From: Lyle Giese <[email protected]> Subject: Re: [mrtg] MRTG and bandwidth delay product To: [email protected] Cc: [email protected] Date: Wednesday, December 31, 2008, 7:19 PM In order to meet the criteria for their formula IMHO, you need a scenerio where you have time for at least three TCP packets to leave the sender before you get the ACK for the first packet back. The higher the bandwidth on the link, the sooner you hit critical mass. So I would think you need to work back from this: Size of TCP packet/speed of line * 3 = time to send three TCP packets. (You will need to know the size of the TCP packets of course. A sniffer will help here.) (speed of line is the speed of the slowest throttle point and is 4kB/s in your example.) The 'time to send three TCP packets' would need to be at least less than 1/2 the RTT, IMHO for this to kick in. In other words, if the 'time to send three TCP packets' is less than 1/2 the RTT, the three packets could leave the sender before the sender has a chance to receive the first packet ack from the receiver. Only then would waiting for packet ack start to slow down the FTP protocol. This gap between the sending of a packet and the receiption of the ack is where this limitation kicks in. Now in the old days, Zmodem protocol was well defined and you knew how many packets would be sent before the sender would stop sending and wait for an ack. I have not delved into the TCP packet protocol at this level to determine that parameter. I am using the number three to help make this explanation understandable. Plus in your setup, you seem to be limiting FTP to 4kB/s, but are measuring the bit/bye output of the NIC on the FTP server. The 4kB/s limitation is set before the overhead bits of the TCP packet is added while you are measuring the combined bit/byte output of the entire packet via the NIC. Lyle nangineni praneeth wrote: Oh!i get it now....Thanks for your explanation.But is there any way for me to show that the network throughput is being affected by latency... Regards Venkat --- On Wed, 12/31/08, Lyle Giese <[email protected]> wrote: From: Lyle Giese <[email protected]> Subject: Re: [mrtg] MRTG and bandwidth delay product To: [email protected] Cc: [email protected] Date: Wednesday, December 31, 2008, 9:34 AM nangineni praneeth wrote: 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 <[email protected]> wrote: From: Lyle Giese <[email protected]> Subject: Re: [mrtg] MRTG and bandwidth delay product To: [email protected] Cc: [email protected] 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. Ok, at the TCP packet level is where the acknowledgements are done. Now in their article you have to have a enough throughput so that the acks being sent back are not getting back fast enough, so that the sender has to wait for acknowledgements. You need to calculate how long it takes to send a TCP packet over your line. You have to enough bandwidth to send three or more packets before the acknowledgement for the first packet is received by the sender. I don't think 8kilobytes/second is fast enough to meet that criteria, in order for their formula to engage. Lyle
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