Re: [e2e] query reg improving TCP performane

Greetings,

On 05/07/07, query <query.cdac@gmail.com> wrote:
>
> I was doing some Bandwidth measurement test on a 100 mbs link with a RTT
> of about 70ms.
> Based on that, I calculated the BDP as follows.
>
> BDP = Bandwidth * RTT
> = 921600 bytes
> I did the following adjustments. I increased the above calculated BDP by
> nearly
> half of the value . The TCP settings now look like this.
>
> /proc/sys/net/core/rmem_max 175636
> /proc/sys/net/core/wmem_max 175636
> /proc/sys/net/ipv4/tcp_rmem 4096 87380
> 175636
> /proc/sys/net/ipv4/tcp_wmem 4096 87380
> 175636
>
> After these settings , I find the link utilisation to be nearly 95 mbs.
>
> According to many papers that I read , I found that the BDP should be
> equal
> to the product of Bandwidth * RTT .

The papers probably said that *router* buffers need to equal the bandwidth*RTT. You are adjusting the sender/receiver buffers. These need to be significantly larger, as you have found.

In order to allow retransmissions, the sender buffer needs to be able to store all "packets in flight", which include both those in the in the router buffers and those "on the wire" (that is, in the nominal RTT of the link).

In order to be able to provide in-order delivery, the receiver buffer needs to be able to hold even more packets. If a packet is lost, it will receive an entire RTT (plus router buffer) worth of data before the first retransmission of that packet will arrive. If the first retransmission is also lost, then it will need to store yet another RTT worth of data.

The general rule-of-thumb for Reno is that the send buffer should be at least twice the bandwidth*RTT. For BIC is is probably reduced to about 120% of the BDP (because it reduces its window by a smaller factor when there is a loss). The receive buffer should still be at least equal to the BDP plus the router buffer.

I hope this help,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
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